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EXPERIMENTAL WORK.
xvi Some SiO2 is always present in combination with The object in view was to find some method of A1203, probably in suspension as very finely divided measuring the suspended impurity as distinct from clay. The mean salinity of the rain-water was that deposited from the air. The method should 9’29 mg. per litre, and the annual rainfall was permit, if possible, of ascertaining the nature of the suspended matter. Two general methods were considered :(1) A method which will permit of measuring and analysing the nature of suspended impurity, or,
88’95 in. The constituents in solution are subject to great variation ; for instance, in the quarterly rainfalls there have been the following variations :— Rainfall ............
Maximum. 30’00 (762 mm.)
failing this, (2) A measurement only in,
Minimum. 13’99 (356 mm.)
say, mg. per cubic
metre of air.
Milligrammes per litre. Ca
Mg K
............
............
............
Na
............
A1203 ............ Fe2O3 Cl2 S04 C03 NO3 NH4............ SiO2 SiOz (clay held in .........
............
............
1-279 0-490 0-507
4-468 0-380 0.380 9-027 1-983 1’100 3500 0-045 0-501
0-514 0-025 0060 0489 Nil.
Nil. 1-500 0868 0330 0.186 0’001 Nil.
METHODS AIMING
AT
WEIGHING
AND
ANALYSING.
To get an idea of the problem the following is important :In experiments made by Mr. Clark for the calibration of the Owens Filter, in London, we found that the amount of suspended impurity varied from
about 15 to 30 mg. per 1000 cub. ft. If
we assume-
errors of weighing will not exceed, plus or 1 minus, mg. ; That the total error permissible in the estimation is 10 per cent. ; this would call for a volume of 667 cub. ft., assuming the presence of 15 mg. per 1000 cub. ft. 0-850 0-325 suspension) ... But as this refers to London and the method is to The mean composition of the saline constituents be applicable to the country also, we might of the coastal sea-water was determined to be as assume that the quantity of air to be dealt with would sometimes be at least three times this follows :amount. 54.510 Ca 1-344 Cl We may then lay down one condition; that is, 0-200 Br Mg ... ... ... ... 3.748 , that the apparatus must be capable of handling the K 1-278 SO4... ... ... ... 7.723 ;above volume of air, say, 2000 cub. f t. 1-017 Na 30046 HCO3 As the element of time is important, owing to NH;4............ Trace. NO3 ... ... ... ... 0.070 rapid changes in the state of the atmosphere, especially during fogs, we may then proceed to fix Si02............ 0030 99966 a maximum time which would be considered Read with the average composition of the saline suitable for taking an observation. These two constituents of rain-water these show that 55 per considerations should fix the scale of the cent. of the solids in solution in the rainfall are apparatus. POSSIBLE METHODS CONSIDERED. cyclic sea salts, whilst 45 per cent. must have been derived from atmospheric, in part at least cosmic, (1) High tension electric current with electrodes: sources. fixed in a light detachable glass vessel, through The composition of the rain-water solids is not which air might be passed in a continuous stream solely dependent on the amount of rainfall-that through some form of meter. The glass vessel is, it is not constant and is subject to markedmight be removed and weighed and the deposited variation. For instance, the content of potassium matter separated for analysis. increased from 2’05 per cent. of the solids in the Whether this method would be possible or not rain during the first quarter of 1916 to 5’55 inseems to be a matter for experiment, but as a test that of the third quarter, but has since steadilyof the of this or any of the following efficiency fallen, and was only 0’28 per cent. in the rain of]methods the air, after being dealt with, might be the first quarter of 1918. The high potassiumpassed through the Owens filter to ascertain if content on several samples of the rain-water1there was any deposit. which had been specially collected in metal and (2) Some method of precipitation, by fog produced in wooden receptacles was most carefully checked,in saturated air, or by a spray of distilled water, or Potassium but always with the same result. steam. This method alone could hardly be possibly largely in excess of that derivable from cyclic seaapplied to a continuous current of air, as it presalts was a characteristic feature of the rainfall supposes a settling chamber, but it might possibly of January to September, 1916. be combined with Method (1). (3) The air might be filtered through a soluble filter, which by suitable treatment might permit of EXPERIMENTAL WORK. weighing and analysis. Reference was made in the last Report to the (4) Filter through filter paper and weigh, then research which the Committee had in hand relative aim at ascertaining only the tarry matter soluble in to the best method of measuring continuously the CS2 and matter soluble in water, which might suspended impurity in the air. A short account of possibly be done by simply washing the paper. the work done in this connexion is given here. (5) A modification of the small standard Owens. The general problem of measuring suspended filter, described in last Report and adapted to give: impurity was considered. continuous records, might be used. ............
...
...
...
...
...
...
...
...
...
...
...
...
That
............
............
il
.........
............
............
. ,
xvii METHODS
AIMING AT WEIGHING ONLY WITHOUT I ASCERTAINING THE COMPOSITION. I
was
utilised, and 16 different records taken on the disc, the time required for taking each record
same
being noted as well as the difference in head on to the large volume of air to be filtered each side of the filter paper. The latter was in order to get a weighable quantity, the only measured by connecting a pressure gauge between feasible methods for taking observations in a the filter paper and the outlet A of the tube (see reasonable time appeared to be those dependent Fig. 4). In taking each record the time at which and the water was started flowing from the upper upon filtration through a white paper, bottle and the time when 2000 c.cm. had flowed out comparison of the resulting records by shade. (1) The single record Owens filter already were noted. The maximum difference of pressure described.-This, while permitting an observation on each side of the filter paper was in every case to be taken in about 10 or 15 min., has the following attained aftet 10 secs. When 2000 c.cm. had flowed objections : (a) It is not automatic nor continuous ; out, aspiration was stopped and the cork removed b) it does not take account of different coloured from the -upper bottle so as to admit air; thus the actual volume of air in each case was somewhat deposits. 2000 c.cm. owing to the reduction in (2) Thomson’s continuous filter.*-This is operated less thansince the volume was measured under a pressure, taking records on white paper at regular of about 4 ft. of water. But as the conditions pull intervals, automatically and continuously. It is were the same in each experiment the effect was questionable whether the seal surrounding the merely to reduce the volume aspirated, leaving it record is sufficiently sound to prevent error due to the same in each case. leakage. The Committee have been unable to The time required for each record gradually obtain one of these filters for experiment, but from No. 1 to No. 16. increased No. 1 required are in Journal Chemical the given of particulars 16 4 5 45 secs. The whereas No. took mins. mins., Technology, Vol. II., No. 4. head of water, H on Fig. 4, was 4 ft. 6 in., and the (3) A simple method might be devised in which difference on each side of the filter paper air is aspirated automatically through white filter pressure varied from 4 ft. in No. 1 run to 4 ft. 1 3/4 in. in paper by means of water. No. 15; thus the pressure difference may be With regard to the method of estimating the regarded as practically constant. On plotting these amount of impurity on a record by its shade, it results it was found that the relation of time may be remarked that owing to the fact that the required for aspiration to number of volumes deposit may not be strictly the same colour in all filtered was practically a straight-line one. (See districts, thus introducing some error when Fig. 3.) observed by reflected light and compared with a FIG. FIG. 3. scale of shades, it was thought to be worth investigating whether greater accuracy could not be obtained by comparing such records by transmitted light, as the amount of light obstructed by a record should be some function of the amount of
Owing
electrically,
-
deposit. At the end of the last Report reference was made to an experiment on the effect of obstruction of the filter paper upon the rate of filtration. Particulars of this experiment are given below as bearing upon the subject in hand.
The rate of aspiration through filter paper is HP VOLUMES FILTERED 20pOG.C EACH altered as the pores become clogged with impurity it and that this clogging might trapped, appeared be made to measure the amount of impurity present The figures obtained are given below :in the air. The pores in the filtering disc may be Experiment showing Degree of Obstruction of Pores of Filter regarded as giving an orifice of a certain area, Paper Resulting from Impurities Trapped from the Air. which is gradually reduced in size as they become clogged by impurity. If then an arrangement be adopted in which there was a constant difference in pressure maintained on each side of the filtering disc, the now of air through this disc will vary directly as the orifice or area of the pores in the disc, and as the latter are gradually clogged up it would seem that the time required for filtering a given volume should have a direct relationship to the amount of impurity in the air. If we could obtain a measure of the impurity which does not depend upon the shade of the record, it would obviously be unaffected by the colour of the impurity trapped, and so have the advantage that it would be equally applicable to all places and presumably to all forms of solid OF
impurity. In view of the above, were made in which the *
series of experiments ordinary standard filter
a
Designed by Mr. William Thomson, F.R.S. Edin., F.I C.
Note.-Each succeeding volume was drawn through the paper disc. Condition of air as to impurity appeared to be fairly steady during experiment. Head was measured by manometer just behind filter paper. The head of water in outlet tube from bottle was 54 in. same
xviii
I Whipple, experi- retard
The effect of surface tension is, therefore, to the outflow when the water is allowed to On the suggestion of Mr. F. J. W. in drops. ments were made to ascertain if the single record The amount A test run of 24 hours was made. Owens filter could be made to spread its record ! and the rate of of water 1940 delivered was c.cm., over a period of, say, 24 hours instead of 10 mins., with a view to obtaining an average for the longer delivery at the end of the run was exactly the same The as at the start-i.e., 7’3 c.cm. in 5 minutes. period. Referring to Fig. 4was 60° F. temperature FIG. 4. In order to test the effect of temperature on the rate of now the upper bottle was filled with water at 100° F., and this allowed to trickle through the capillary tube, the head being as before. The water was somewhat cooled in the tube and delivered at about 75° F. The quantity delivered in this case was 8’5 c.cm. in 5 minutes. Owing to the cooling in the delivery tube a water-jacket was fitted to the capillary tube and filled with water at 144° F. so as to warm the water at delivery only. This arrangement delivered 10 c.cm. in 5 minutes. The temperature of the water-jacket at the end of the test was 107° F., and the temperature of the water delivered as takenin the measure-glass was 87° F. A control run was then made without the waterjacket, the water being at a temperature of 600 F. ; otherwise all the conditions were the same. The quantity delivered was 7’2 c.cm. in 5 minutes. Thus it is clear that the temperature has a considerable effect upon the rate of flow of the water by altering its viscosity. In order further to elucidate this another test was made, the conditions being as follows :Capillary tube used in outlet, 0’02 of an inch bore, length 4 in., head H 9 in., rate of flow adjusted to give about 7’35 c.cm. in 5 minutes at 60° F. In this case, instead of measuring the amount delivered in five minutes, which was somewhat cumbersome, the temperature of the air was taken at intervals and the time required for the fall of 20 drops from the capillary tube noted. The-following table gives the results of this run :INTEGRATING FILTER.
fall
In the first experiment a filter was fitted up in which the inlet tube reached almost to the bottom of the upper bottle, and the -outlet for The water reached to about the same level. rubber outlet tube terminated in a capillary tube of 3 in. long and a bore of 0’02 of an inch. The head was adjusted to give an outnow of water equal to 7’3 c.cm. in 5 minutes. The water dropped in large drops from the end of the tube. This rate gives practically 2000 c.cm. in 24 hours. To find the effect of surface tension upon the rate of discharge 3 tests were made; in the first the water was allowed to drop in large drops, and the rate of discharge was 7’3 c.cm. in 5 minutes; in the second the water was caused to drop in smaller drops, and the rate of discharge was 7’7 c.cm. in 5 minutes; while in the third the end of the capillary tube was allowed to touch the edge of the measuring vessel, so that no drops formed but a steady trickle took place. This arrangement delivered 9’3 c.cm. in 5 minutes.
Taking flow at 7-2 A.M. Flow at 11 A.M.
was
as
=
=
100. 117.
’
The range of temperature was from 52° to 620 F., and the rate of flow from the capillary tube was 17 per cent. higher at 62° than 52°. The time covered by the test was from 11 A.M. on the 26th to 7.20 A.M. on the 27th, or 19 hours 20 mirrutes, and the amount of water delivered from the capillary tube was 1725 c.cm., which is equivalent to a flow of 2140 c.cm. in 24 hours. It should be noted that in these experiments the rate of flow of the water was measured; but it does not follow that -the aspiration of air through the filter paper was at the’same rate, since the
xix. latter is affected also by the expansion or contraction of the air imprisoned in the upper bottle due to the changes of temperature. Expansion of air following a rise of temperature causes a reduced rate of aspiration through the filter paper, since the volume which would normally be aspirated during the rise of temperature is supplied by expansion of the imprisoned air. Thus there may be an automatic compensation for the increased rate of aspiration due to the alteration in the i viscosity of the water.
volume of air aspirated during change of the flrst, second, and nth minutes. The condition for steady working is that Q = W. If the temperature now rises C degrees per min. Let W increase each minute by an amount = x, owing to change of viscosity of the water, Let V be increased each minute by an amount =y, owing to expansion. Assuming a free inflow of air and that pressure of V remains constant, then the condition for continued aspiration is that the rate of fiow of water must exceed the rate of increase of volume of V due to expansion. Correction for Cl1ange of Water Level in 1’ilter as ! The now of water is increased approximately by 3 per cent. per 1° C. rise of temperature ; if, therefore, the rise is Affecting Head. C. degrees per minute, the flow will be increased per minute to the When a flow Referring figure : steady 3 C is established the water level in the tube A D will by 3 W+ and after n minutes will be
Qi, Q2- Qn temperature in
=
i
from
stand at A and the pressure on the water surface is reduced from atmospheric by the head h. Thus-
WC
n WC.
=
Taking normal rate of flow
1.39 c.cm. per minute and 1000 C = 5-7 C.
=
1000 C
assuming V = 1000, y will be atmospheric pressure in feet of water, in p = pressure bottle If the rise is such that 3-7 C. 1-39, or C 0-38, aspiration then p=P-h,orP=p-h. will cease momentarily, but commence again slowly as The head producing flow is H + h + p - P, but since P p + h W increases. I this becomes H, thus the head causing flow is independent Suppose the rise is greater than 0.38° say 0.5°, aspiration of h, and change of level of the water is automatically will stop, and will recommence after n minutes when corrected for. if P
=
=
"
"
"
=
=
=
=
=
flow of
Effect of Change of Temperature. This operates in three ways :(1) The rate of flow of water from C is altered due to change of viscosity. (2) The imprisoned air in the bottle has its ’, -
volume V altered. (3) The density of the air as measured is altered. Considering (1) : The change of viscosity will cause an increase of rate of flow from the tube C of about 3 per cent. per degree C. of rise of pressure p
water becomes
= 3.7 C., when, if C The maximum
Considering (2): Suppose
the temperature to rise. The pressure p is maintained constant as long as the water level stands at A. (a) If rise is rapid so that volume V increases more rapidly than water flows out, p will rise and the, water in A D will rise and aspiration cease. As the water rises in A D the head causing flow increases and a balance may be struck between rate of flow from C under increased head and rate of increase of V. (b) If the rise of temperature is gradual, and such that the rate of increase of volume V due to the rise is less than the rate of flow from C. the water-level will stand at A, but the rate of aspiration will be reduced by an amount equal to the rate of change of volume of V under constant pressure. For when filter is started the head producing flow of water is H z- h and as the water falls gradually in DA the pressure p is gradually reduced until it is diminished by an amount h, the flow of air increasing as the difference of pressure on each side of the paper increases, while the flow of water from C is reduced. Very soon a steady state is reached when p becomes constant and, assuming no change of temperature, the flow of air through the filter becomes equal to the flow of water from C. This condition continues in the absence of any disturbing factor. Let Q = volume of air aspirated per min. in this steady ,
=
state. Let W = volume of water state.
flowing
per min. in this
steady ’
3-7 C.
0.5, W
=
probable
=
n
when W +
or
1.39,
rise is
1000 when y becomes = 273 x 30 The rate of aspiration after
or
temperature.
=
=
n =
3 n100WC
22 minutes. 1°
about 1°/30
C. per
minute,
0-124. minutes will be
:—
-0.124. 3n W W+nW1000’
W + 30x 100
Aspiration Aspiration
after 1 min.= 1.267-i.e., slightly reduced. will gradually approach the normal
W increases, and when
nW
become normal and will normal after 90 minutes.
0.124
=
then
or n
=
as
90 minutes it will
commence
to exceed
the
Considering what error in total volume aspirated will result after 90 minutes, taking C 1000 (and neglecting change of V due to aspiration).
= ,V=
Since x is the acceleration of W. Volume of water passed after n minutes Wn +½ Xn2 (1). -124 n Increase of V (2). Volume of air aspirated (1) (2) W n + ½ x n2 - -. 124 n. And when W=1’39 Volume aspirated after 90 m. ==119-5 c.c. The volume which should have been aspirated = 1-39 x 90 125 c.cm., therefore error = 5-5 0.cm. in 125. =4.4 per cent. =
=
=
This is
-
=
extreme case, and since after 90 m. aspiration m. have an error of opposite sign, it seems that the viscosity change will with this condition of the apparatus give a rough compensation for the expansion of imprisoned air. an
would be normal again and the next 90
Referring to the experiments already mentioned Whipple made the following comment :-
Mr.
" In
the
important experiment you aspirated from 11 to 9 and 865 c.c. from 9 to 7.20. The 860 c.c. of air if cooled from 62° to 52° would have been reduced in volume to 845 c.c. so that the quantity taken in during the latter period may have been nearly 880 c.c. most
apparently 860
c.c.
The rate from 11 to 9 "
"
"
9 to
The difference does not
was
7.20 " seem
to
8-6
c.c.
8-5
c.c.
me
of
(at 62°) per hour (at 52°) much importance." "
At his suggestion the following experiment was. ) made to try if in actual working the result obtained
xx
by the slow working filter over a long period gave a fair average of the impurity in suspension during
period. Two similar Owens filters were fitted up on Oct. 18th, 1917, at 47, Victoria-street, S.W. One of these was arranged to run very slowly so This as to deliver about 2000 c.cm. in 12 hours. 5.30 was started at 10.40 A.M. and at stopped after 1000 c.cm. had been aspirated through the paper. The other filter was used to take hourly records that
2000 c.cm., so that eight such records were obtained during the time the slow operating filter was working. Calling the slow filter (A) and the filter (B) :hourly
using
If the shade numbers obtained by (B) when added and averaged gave a number = the shade of the record got by (A), corrected for volume, the inference would be that (A) gave a satisfactory result. Suppose NA shade number obtained by (A) and NB average shade obtained by (B), then for satisfactory working=
NA should
be
=
NB x
1000
2000 or
NB
= NA
or
2
NB = 22 NA. NB N A. =
The actual records from (B) gave a uniform series of shade 4. Thus there was little change in the amount of impurity in the air during the test. The shade
NA was
=
2.
Thus
NB was= 2 NAshowing
AUTOMATIC FILTER. In this apparatus an aspirating vessel is provided into which water is admitted through a regulating cock at the bottom. A syphon is fixed inside the vessel, which causes the water-level to oscillate at regular intervals between two fixed levels in the vessel; thus, while the water is rising air is driven out of the vessel, and while it is falling air is aspirated into the vessel. On the top of the instrument an entrance for airis provided of a fixed diameter, over which revolver a disc of filter paper. Provision is made by which this disc is locked between two plugs, on the lines of the single-record filter, during the time air is being drawn into the vessel. The disc is caused to revolve by a weight and string, but the rate of its revolution is regulated by a stop which follows the hour hand of a small clock placed above the disc. The force for clamping the disc is obtained by a pressure operated flexible diaphragm acting through a lever. The records are given upon a disc of paper upon which the hours have already been marked similarly to a clock-face, and each record is placed automatically opposite the time at which it is taken. Thus the interval between records is unimportant, although, as will be shown below, this is easily adjustable. The present instrument takes a 12-hour disc, but there is no reason why the time should not be longer. ,
The following is a detailed description of the apparatus, which is illustrated in Fig. 5. The experiment was repeated on October 19th. Water- supply.-Water is admitted to the aspiratSeven hourly records were obtained from (B) ing vessel through a regulating cock, A, at the and one record from (A) running from 11 till bottom. This cock is connected with the main, and 4.25 P.M. by adjusting it the interval between records is The (B) shades were-6, 6, 6, 4, 4, 4, 4. NB was varied. Fixed inside the vessel and immediately cock is a small valve, B, of the or 5 as near as possible. NA was between above the regulating therefore, This is operated by a central piston-valve type. 2 and 3, taken as 2, so that again rod passing vertically upwards through the axis of a
good result.
=
.
34, 7
NB
=
2
NA.
In order to verify the above conclusions the following observations were made :-
The filter was set up and adjusted so as to run at rate which would cause it to aspirate 2 litres in 24 hours. Instead of counting the drops of water falling from the end of the capillary tube the interval between the bubbles rising in the upper bottle was measured; the temperature of the air at this time was 61° F., the interval between bubbles was 22 seconds, counted from a distance of four or five yards from the filter. There was about 1500 c.cm. of air imprisoned in the upper bottle over the water Dr. Owens, who conducted the experisurface. ments, then came close up to the filter and, without touching it, counted the interval again. It had altered to 32 seconds. a
He next placed his hand upon the upper bottle for two minutes and then removed it. The result of this was to stop aspiration for a period of 5 min. 50 sec., the water being driven up the tube A D to a height of about 2 in. This observation indicates the importance of the temperature
the main vessel and actuated as described later. The lifting of this rod through a short travel of about 1/8 in. opens the water-supply, and lowering the rod closes it, but not completely, as a small leakage is allowed past this valve for the purpose to be described later. Air entrance.-The entrance for the air is at L on the diagram; a sleeve is screwed into the top plate of the vessel and contains two plugs, the lower one of which is fixed and the upper capable of a sliding movement sufficient to clamp the filter paper, H, which is placed between the plugs through a slot in the side of the sleeve, in a similar manner to that adopted in the single-record Owens filter. The upper or sliding plug is operated by a lever, K, which is attached to the centre rod, C, the weight of which is balanced by a sliding weight, N. Thus, when the lever is pulled down the upper sliding plug clamps the paper in position over an aperture, the lifting of the lever releases the paper and leaves it free to revolve.
Syphon.-The syphon, F, consists of a long leg communicating with the outer air and a shorter leg of larger diameter inside the vessel, the length of which determines the volume of air aspirated. error. After this investigation it appeared best to try a This shorter wide leg terminates at the bottom in a different form of apparatus. A design for a filter sliding piece which has a V-shaped notch cut in its which would operate automatically and give records lower edge. This sliding piece permits the length at short intervals over a period of 12 or 24 hours of the leg to be adjusted. was therefore worked out and an experimental It was found that the operation of the syphon was liable to be interfered with in certain ways. apparatus constructed by Dr. Owens.
[xxi FIG. 5.
xxii
downwards
If the rate of inflow of water to the vessel was veryI small and the long leg of the syphon was, say, of 5/16 in. diameter, it was possible to get a trickling of water over the bend of the syphon which kept pace exactly with the inflow ; thus the syphon would fail to act. To prevent this trickling over,
the diameter of the long leg was reduced to about 3/16 in., the object being to force the water to pass over the bend as a solid plug, since the surface tension in the small tube would make it impossible for it to pass otherwise. With a tube of this diameter it was found that the difficulty mentioned was entirely overcome, and the water, if it passed over the. bend, went over in a solid plug and the syphon acted.
difficulty arose from the fact that the the syphon when very small in diameter. as has been shown to be necessary, failed to empty itself completely of water, a plug being . retained in the lower end which caused an erratic action of the syphon on the next filling, this plug being held up by the capillarity of the tube and the surface tension In order to overcome this diffiacross the mouth. culty the short leg of the syphon was made of much larger diameter than the long, so that it contained, after the syphon broke, a volume of air several times greater than the long leg, and this air- was driven out as the water rose again in the vessel, thas scavenging the long leg and blowing out the plug of water. Again, great care had to be taken that no positive pressure was produced inside the aspirating vessel as the water level rose therein. For example, several times it was found that the syphon acted too soon, owing to the fact that the paper disc was revolving in contact with the mouth of the bottom plug, through which the air had to pass out as the water entered. This was remedied very simply by lifting the disc clear of the mouth. The action of the syphon then is as follows. As the water rises in the main vessel it also rises in the short leg of the syphon until it reaches the bend, when it flows over and the syphon commences to operate. It continues to operate until the water almost reaches the bottom of the short leg, in which, it will be remembered, a V-shaped slot is cut. This V-shaped slot remains inoperative or closed by the surface tension of the liquid until, as the water level falls, the pressure against the surface skin becomes too great for it to resist without rupture : at this moment the skin closing the V breaks and the short leg of the syphon is emptied into the main vessel, air taking the place of water. As the water from the short leg flows out it raises the water in the main vessel slightly, but not enough to cover the V-shaped slot; thus the tube can completely empty itself, which would not be the case if no slot were provided. Immediately the syphon " breaks the water level in the main vessel begins to rise slowly owing to the slight leakage in the water valve, B, already mentioned, and when the surface has risen a little this valve is opened fully by means to be described, and the water again rises steadily in the main vessel until it again flows over the bend of the syphon, .thus completing Another
long leg of
"
a
cycle.
Filter disc.-The filter disc, H, is about 3 in. in diameter and fixed upon a small turntable opposite the slot in the sleeve, already mentioned. It is caused to revolve by means of a weight and string, but the rate of its revolution is regulated by means of a c.ock, J. This clock is placed face
I
above the paper disc and the hour hand engages a stop on the disc. The effect of the weight and string is to keep the stop always up against the hour hand when the disc is free to move; when, however, the disc is clamped between the filtering plugs and is not free to move the hour -hand of the clock moves away from the stop. to be subsequently overtaken again by the disc when it is released. Thus the interval between two records is made up of (a) the period during which the disc is locked and the record being taken, and (b) the period during which the disc is free and following the clock-hand round. By setting the clock when the instrument is started the records are marked automatically opposite the time at which they are taken, as shown on the disc; thus whatever the interval between the records may be is comparatively unimportant, since actual time of taking is shown opposite each
the
record.
Diaphragm.-In the
centre of the upper plate the main vessel is fixed a flexible covering diaphragm of leather, E, penetrating the centre of which is the central rod, C. This rod is attached at the upper end to the lever, K, and its lower end This diaphragm serves forms the water valve, B. the double purpose of permitting the movement of the rod when acted on by the bell to be next described, and also it exerts a pull upon the lever, K, when the pressure has fallen inside the main vessel. Bell.-A bell, D, is fixed on the central rod, C, so that it is just completely immersed at the highest level of the water, when the syphon begins to act. The balance weight is adjusted so that when the bell is empty and not immersed the weight keeps the upper plug lifted clear of the filter paper. The bell is formed as a short cylinder about 3 in. diameter and 1 in. deep, open below and closed above. In the side is cut a narrow vertical slot extending the full depth of the bell and 1/32 in. wide. The effect of the slot is to permit the air inside the bell to escape as the water rises, but when the syphon acts and the water level falls the slot remains closed by the surface tension of the water until the water has fallen sufficiently to rupture the closing film, and until then the bell remains full of water and acts as a weight on the rod, C, pulling it down and giving time for the diaphragm to act. When the water has fallen sufficiently the film closing the slot ruptures and the water falls out, leaving the bell empty and capable of being lifted by the balance weight, N, as soon as the diaphragm ceases to pull on the rod, C. In this arrangement the surface tension.is used as a valve and there are no parts to get out of order or vary in their action. Further, if for any cause the syphon acted too soon the instrument would refuse to take a record since the bell would not operate to pull down the lever, K, unless the slot in its side bad been previously immersed to form the closing film. This arrangement was found to work satisfac-
torily. Manometer cock.-A small cock, N, is provided in the top of the vessel for the purpose of calibrating. In designing this instrument it was decided to operate with the same volume-i.e., 2 litres-and the same sized filter disc-i.e., 1/8 in.-as in the single-record instrument. In order to fix the volume, attach a manometer to the cock, N, and observe the pressure inside the main vessel. When
xxiii the water level in the main vessel begins to fall inside falls, and the diaphragm; E, comes into air is drawn in through the filtering plugs, but not operation exerting a pull upon the lever, K, which through the filter paper, until the latter is forced forces the sliding plug harder down upon the down by the action of the diaphragm and bell. filter paper, making a joint around the small The moment that the air commences to pass ( disc. As the rod, C, is pushed down, the waterthrough the paper is indicated by a fall of pres- supply is cut on by the valve, B, and remains lifted. Air is now so until the rod is again sure inside the main vessel; mark the upper water level when the pressure begins to fall. drawn through the filter paper into the vessel level has been found to be perfectly until the water reaches the bottom of the This definite and easily observed. To fix the lower syphon, when the syphon breaks and aspiration level of the water observe the rise of pressure ceases; the water again commencing to rise after the syphon breaks ; the pressure rises and the cycle described above is again repeated. vile It was found that some gradually i n s i a e FIG. 6. vessel until it becomes of the earlier records were surrounded by a atmospheric, and at this moment the level of the ring, showing slight water should be marked. leakage beside the filter The volume of water plug. This indicates between these two levels that the pressure on the is then measured and plug was not quite adjusted by altering the enough, a point of some length of the short leg of importance ; the head on the syphon to make this delivery end of syphon volume equal to 2 litres. was about 3 ft. To inThe rise of pressure in crease the pressure upon the the vessel after the plug we may either increase the length of syphon broke caused the the pressure to become equal long leg of the to atmospheric after the syphon to, say, 4 ft. or 5 ft., water had risen about or increase the effective area of the diaphragm. 1/4 in. in all experiments made, and this level was Using a head of 4 ft. on always quite definite, so syphon delivery the that there appears to pressure of plug on filter be no great room for paper is about 6 lb., and the end of plug in variation in the volume as of air aspirated when contact with paper is an annular ring 1/16 in. once the levels are fixed. The of film actual records consist a uniform of the wide surrounding the deposit, Operation.-The operashading in the diagram giving an idea of the variations 1/8 in. hole the pressure tion of the instrument is of intensity. as follows : Water enters per square inch is actually not too great, as it is in the main vessel. The 180 This is the and rises about lb. cock, A, through balance weight keeps the rod, C, lifted, the diaphragm necessary so to compress the paper surrounding 1/8 in. disc, which is operating as the filter, being out of action, and the exit of air through the hole, L, is permitted. The water continues to riseas to prevent leakage of air. until it flows over the bend of the syphon, immersing I To obtain this pressure without an inconveniently I head the diaphragm, E, is made to rest on a and filling the bell, D, when the syphon commences great to act. As soon as the syphon acts the water level in metal disc of slightly smaller diameter. This conthe main vessel begins to fall; the bell, D, also falls verts the diaphragm into what is practically a owing to the weight of its contained water and piston, and greatly increases the effective pressure pulls down the lever K, bringing the upper filter produced. The instrument works satisfactorily and with plug in contact with the paper which is pressed down over the air entrance, L. There is thus a practically no attention. An illustration of the resistance to air entering the vessel, the kind of records obtained is shown in Fig. 6.
I the
pressure ’
88’95 in. The constituents in solution are subject to great variation ; for instance, in the quarterly rainfalls there have been the following variations :— Rainfall ............
Maximum. 30’00 (762 mm.)
failing this, (2) A measurement only in,
Minimum. 13’99 (356 mm.)
say, mg. per cubic
metre of air.
Milligrammes per litre. Ca
Mg K
............
............
............
Na
............
A1203 ............ Fe2O3 Cl2 S04 C03 NO3 NH4............ SiO2 SiOz (clay held in .........
............
............
1-279 0-490 0-507
4-468 0-380 0.380 9-027 1-983 1’100 3500 0-045 0-501
0-514 0-025 0060 0489 Nil.
Nil. 1-500 0868 0330 0.186 0’001 Nil.
METHODS AIMING
AT
WEIGHING
AND
ANALYSING.
To get an idea of the problem the following is important :In experiments made by Mr. Clark for the calibration of the Owens Filter, in London, we found that the amount of suspended impurity varied from
about 15 to 30 mg. per 1000 cub. ft. If
we assume-
errors of weighing will not exceed, plus or 1 minus, mg. ; That the total error permissible in the estimation is 10 per cent. ; this would call for a volume of 667 cub. ft., assuming the presence of 15 mg. per 1000 cub. ft. 0-850 0-325 suspension) ... But as this refers to London and the method is to The mean composition of the saline constituents be applicable to the country also, we might of the coastal sea-water was determined to be as assume that the quantity of air to be dealt with would sometimes be at least three times this follows :amount. 54.510 Ca 1-344 Cl We may then lay down one condition; that is, 0-200 Br Mg ... ... ... ... 3.748 , that the apparatus must be capable of handling the K 1-278 SO4... ... ... ... 7.723 ;above volume of air, say, 2000 cub. f t. 1-017 Na 30046 HCO3 As the element of time is important, owing to NH;4............ Trace. NO3 ... ... ... ... 0.070 rapid changes in the state of the atmosphere, especially during fogs, we may then proceed to fix Si02............ 0030 99966 a maximum time which would be considered Read with the average composition of the saline suitable for taking an observation. These two constituents of rain-water these show that 55 per considerations should fix the scale of the cent. of the solids in solution in the rainfall are apparatus. POSSIBLE METHODS CONSIDERED. cyclic sea salts, whilst 45 per cent. must have been derived from atmospheric, in part at least cosmic, (1) High tension electric current with electrodes: sources. fixed in a light detachable glass vessel, through The composition of the rain-water solids is not which air might be passed in a continuous stream solely dependent on the amount of rainfall-that through some form of meter. The glass vessel is, it is not constant and is subject to markedmight be removed and weighed and the deposited variation. For instance, the content of potassium matter separated for analysis. increased from 2’05 per cent. of the solids in the Whether this method would be possible or not rain during the first quarter of 1916 to 5’55 inseems to be a matter for experiment, but as a test that of the third quarter, but has since steadilyof the of this or any of the following efficiency fallen, and was only 0’28 per cent. in the rain of]methods the air, after being dealt with, might be the first quarter of 1918. The high potassiumpassed through the Owens filter to ascertain if content on several samples of the rain-water1there was any deposit. which had been specially collected in metal and (2) Some method of precipitation, by fog produced in wooden receptacles was most carefully checked,in saturated air, or by a spray of distilled water, or Potassium but always with the same result. steam. This method alone could hardly be possibly largely in excess of that derivable from cyclic seaapplied to a continuous current of air, as it presalts was a characteristic feature of the rainfall supposes a settling chamber, but it might possibly of January to September, 1916. be combined with Method (1). (3) The air might be filtered through a soluble filter, which by suitable treatment might permit of EXPERIMENTAL WORK. weighing and analysis. Reference was made in the last Report to the (4) Filter through filter paper and weigh, then research which the Committee had in hand relative aim at ascertaining only the tarry matter soluble in to the best method of measuring continuously the CS2 and matter soluble in water, which might suspended impurity in the air. A short account of possibly be done by simply washing the paper. the work done in this connexion is given here. (5) A modification of the small standard Owens. The general problem of measuring suspended filter, described in last Report and adapted to give: impurity was considered. continuous records, might be used. ............
...
...
...
...
...
...
...
...
...
...
...
...
That
............
............
il
.........
............
............
. ,
xvii METHODS
AIMING AT WEIGHING ONLY WITHOUT I ASCERTAINING THE COMPOSITION. I
was
utilised, and 16 different records taken on the disc, the time required for taking each record
same
being noted as well as the difference in head on to the large volume of air to be filtered each side of the filter paper. The latter was in order to get a weighable quantity, the only measured by connecting a pressure gauge between feasible methods for taking observations in a the filter paper and the outlet A of the tube (see reasonable time appeared to be those dependent Fig. 4). In taking each record the time at which and the water was started flowing from the upper upon filtration through a white paper, bottle and the time when 2000 c.cm. had flowed out comparison of the resulting records by shade. (1) The single record Owens filter already were noted. The maximum difference of pressure described.-This, while permitting an observation on each side of the filter paper was in every case to be taken in about 10 or 15 min., has the following attained aftet 10 secs. When 2000 c.cm. had flowed objections : (a) It is not automatic nor continuous ; out, aspiration was stopped and the cork removed b) it does not take account of different coloured from the -upper bottle so as to admit air; thus the actual volume of air in each case was somewhat deposits. 2000 c.cm. owing to the reduction in (2) Thomson’s continuous filter.*-This is operated less thansince the volume was measured under a pressure, taking records on white paper at regular of about 4 ft. of water. But as the conditions pull intervals, automatically and continuously. It is were the same in each experiment the effect was questionable whether the seal surrounding the merely to reduce the volume aspirated, leaving it record is sufficiently sound to prevent error due to the same in each case. leakage. The Committee have been unable to The time required for each record gradually obtain one of these filters for experiment, but from No. 1 to No. 16. increased No. 1 required are in Journal Chemical the given of particulars 16 4 5 45 secs. The whereas No. took mins. mins., Technology, Vol. II., No. 4. head of water, H on Fig. 4, was 4 ft. 6 in., and the (3) A simple method might be devised in which difference on each side of the filter paper air is aspirated automatically through white filter pressure varied from 4 ft. in No. 1 run to 4 ft. 1 3/4 in. in paper by means of water. No. 15; thus the pressure difference may be With regard to the method of estimating the regarded as practically constant. On plotting these amount of impurity on a record by its shade, it results it was found that the relation of time may be remarked that owing to the fact that the required for aspiration to number of volumes deposit may not be strictly the same colour in all filtered was practically a straight-line one. (See districts, thus introducing some error when Fig. 3.) observed by reflected light and compared with a FIG. FIG. 3. scale of shades, it was thought to be worth investigating whether greater accuracy could not be obtained by comparing such records by transmitted light, as the amount of light obstructed by a record should be some function of the amount of
Owing
electrically,
-
deposit. At the end of the last Report reference was made to an experiment on the effect of obstruction of the filter paper upon the rate of filtration. Particulars of this experiment are given below as bearing upon the subject in hand.
The rate of aspiration through filter paper is HP VOLUMES FILTERED 20pOG.C EACH altered as the pores become clogged with impurity it and that this clogging might trapped, appeared be made to measure the amount of impurity present The figures obtained are given below :in the air. The pores in the filtering disc may be Experiment showing Degree of Obstruction of Pores of Filter regarded as giving an orifice of a certain area, Paper Resulting from Impurities Trapped from the Air. which is gradually reduced in size as they become clogged by impurity. If then an arrangement be adopted in which there was a constant difference in pressure maintained on each side of the filtering disc, the now of air through this disc will vary directly as the orifice or area of the pores in the disc, and as the latter are gradually clogged up it would seem that the time required for filtering a given volume should have a direct relationship to the amount of impurity in the air. If we could obtain a measure of the impurity which does not depend upon the shade of the record, it would obviously be unaffected by the colour of the impurity trapped, and so have the advantage that it would be equally applicable to all places and presumably to all forms of solid OF
impurity. In view of the above, were made in which the *
series of experiments ordinary standard filter
a
Designed by Mr. William Thomson, F.R.S. Edin., F.I C.
Note.-Each succeeding volume was drawn through the paper disc. Condition of air as to impurity appeared to be fairly steady during experiment. Head was measured by manometer just behind filter paper. The head of water in outlet tube from bottle was 54 in. same
xviii
I Whipple, experi- retard
The effect of surface tension is, therefore, to the outflow when the water is allowed to On the suggestion of Mr. F. J. W. in drops. ments were made to ascertain if the single record The amount A test run of 24 hours was made. Owens filter could be made to spread its record ! and the rate of of water 1940 delivered was c.cm., over a period of, say, 24 hours instead of 10 mins., with a view to obtaining an average for the longer delivery at the end of the run was exactly the same The as at the start-i.e., 7’3 c.cm. in 5 minutes. period. Referring to Fig. 4was 60° F. temperature FIG. 4. In order to test the effect of temperature on the rate of now the upper bottle was filled with water at 100° F., and this allowed to trickle through the capillary tube, the head being as before. The water was somewhat cooled in the tube and delivered at about 75° F. The quantity delivered in this case was 8’5 c.cm. in 5 minutes. Owing to the cooling in the delivery tube a water-jacket was fitted to the capillary tube and filled with water at 144° F. so as to warm the water at delivery only. This arrangement delivered 10 c.cm. in 5 minutes. The temperature of the water-jacket at the end of the test was 107° F., and the temperature of the water delivered as takenin the measure-glass was 87° F. A control run was then made without the waterjacket, the water being at a temperature of 600 F. ; otherwise all the conditions were the same. The quantity delivered was 7’2 c.cm. in 5 minutes. Thus it is clear that the temperature has a considerable effect upon the rate of flow of the water by altering its viscosity. In order further to elucidate this another test was made, the conditions being as follows :Capillary tube used in outlet, 0’02 of an inch bore, length 4 in., head H 9 in., rate of flow adjusted to give about 7’35 c.cm. in 5 minutes at 60° F. In this case, instead of measuring the amount delivered in five minutes, which was somewhat cumbersome, the temperature of the air was taken at intervals and the time required for the fall of 20 drops from the capillary tube noted. The-following table gives the results of this run :INTEGRATING FILTER.
fall
In the first experiment a filter was fitted up in which the inlet tube reached almost to the bottom of the upper bottle, and the -outlet for The water reached to about the same level. rubber outlet tube terminated in a capillary tube of 3 in. long and a bore of 0’02 of an inch. The head was adjusted to give an outnow of water equal to 7’3 c.cm. in 5 minutes. The water dropped in large drops from the end of the tube. This rate gives practically 2000 c.cm. in 24 hours. To find the effect of surface tension upon the rate of discharge 3 tests were made; in the first the water was allowed to drop in large drops, and the rate of discharge was 7’3 c.cm. in 5 minutes; in the second the water was caused to drop in smaller drops, and the rate of discharge was 7’7 c.cm. in 5 minutes; while in the third the end of the capillary tube was allowed to touch the edge of the measuring vessel, so that no drops formed but a steady trickle took place. This arrangement delivered 9’3 c.cm. in 5 minutes.
Taking flow at 7-2 A.M. Flow at 11 A.M.
was
as
=
=
100. 117.
’
The range of temperature was from 52° to 620 F., and the rate of flow from the capillary tube was 17 per cent. higher at 62° than 52°. The time covered by the test was from 11 A.M. on the 26th to 7.20 A.M. on the 27th, or 19 hours 20 mirrutes, and the amount of water delivered from the capillary tube was 1725 c.cm., which is equivalent to a flow of 2140 c.cm. in 24 hours. It should be noted that in these experiments the rate of flow of the water was measured; but it does not follow that -the aspiration of air through the filter paper was at the’same rate, since the
xix. latter is affected also by the expansion or contraction of the air imprisoned in the upper bottle due to the changes of temperature. Expansion of air following a rise of temperature causes a reduced rate of aspiration through the filter paper, since the volume which would normally be aspirated during the rise of temperature is supplied by expansion of the imprisoned air. Thus there may be an automatic compensation for the increased rate of aspiration due to the alteration in the i viscosity of the water.
volume of air aspirated during change of the flrst, second, and nth minutes. The condition for steady working is that Q = W. If the temperature now rises C degrees per min. Let W increase each minute by an amount = x, owing to change of viscosity of the water, Let V be increased each minute by an amount =y, owing to expansion. Assuming a free inflow of air and that pressure of V remains constant, then the condition for continued aspiration is that the rate of fiow of water must exceed the rate of increase of volume of V due to expansion. Correction for Cl1ange of Water Level in 1’ilter as ! The now of water is increased approximately by 3 per cent. per 1° C. rise of temperature ; if, therefore, the rise is Affecting Head. C. degrees per minute, the flow will be increased per minute to the When a flow Referring figure : steady 3 C is established the water level in the tube A D will by 3 W+ and after n minutes will be
Qi, Q2- Qn temperature in
=
i
from
stand at A and the pressure on the water surface is reduced from atmospheric by the head h. Thus-
WC
n WC.
=
Taking normal rate of flow
1.39 c.cm. per minute and 1000 C = 5-7 C.
=
1000 C
assuming V = 1000, y will be atmospheric pressure in feet of water, in p = pressure bottle If the rise is such that 3-7 C. 1-39, or C 0-38, aspiration then p=P-h,orP=p-h. will cease momentarily, but commence again slowly as The head producing flow is H + h + p - P, but since P p + h W increases. I this becomes H, thus the head causing flow is independent Suppose the rise is greater than 0.38° say 0.5°, aspiration of h, and change of level of the water is automatically will stop, and will recommence after n minutes when corrected for. if P
=
=
"
"
"
=
=
=
=
=
flow of
Effect of Change of Temperature. This operates in three ways :(1) The rate of flow of water from C is altered due to change of viscosity. (2) The imprisoned air in the bottle has its ’, -
volume V altered. (3) The density of the air as measured is altered. Considering (1) : The change of viscosity will cause an increase of rate of flow from the tube C of about 3 per cent. per degree C. of rise of pressure p
water becomes
= 3.7 C., when, if C The maximum
Considering (2): Suppose
the temperature to rise. The pressure p is maintained constant as long as the water level stands at A. (a) If rise is rapid so that volume V increases more rapidly than water flows out, p will rise and the, water in A D will rise and aspiration cease. As the water rises in A D the head causing flow increases and a balance may be struck between rate of flow from C under increased head and rate of increase of V. (b) If the rise of temperature is gradual, and such that the rate of increase of volume V due to the rise is less than the rate of flow from C. the water-level will stand at A, but the rate of aspiration will be reduced by an amount equal to the rate of change of volume of V under constant pressure. For when filter is started the head producing flow of water is H z- h and as the water falls gradually in DA the pressure p is gradually reduced until it is diminished by an amount h, the flow of air increasing as the difference of pressure on each side of the paper increases, while the flow of water from C is reduced. Very soon a steady state is reached when p becomes constant and, assuming no change of temperature, the flow of air through the filter becomes equal to the flow of water from C. This condition continues in the absence of any disturbing factor. Let Q = volume of air aspirated per min. in this steady ,
=
state. Let W = volume of water state.
flowing
per min. in this
steady ’
3-7 C.
0.5, W
=
probable
=
n
when W +
or
1.39,
rise is
1000 when y becomes = 273 x 30 The rate of aspiration after
or
temperature.
=
=
n =
3 n100WC
22 minutes. 1°
about 1°/30
C. per
minute,
0-124. minutes will be
:—
-0.124. 3n W W+nW1000’
W + 30x 100
Aspiration Aspiration
after 1 min.= 1.267-i.e., slightly reduced. will gradually approach the normal
W increases, and when
nW
become normal and will normal after 90 minutes.
0.124
=
then
or n
=
as
90 minutes it will
commence
to exceed
the
Considering what error in total volume aspirated will result after 90 minutes, taking C 1000 (and neglecting change of V due to aspiration).
= ,V=
Since x is the acceleration of W. Volume of water passed after n minutes Wn +½ Xn2 (1). -124 n Increase of V (2). Volume of air aspirated (1) (2) W n + ½ x n2 - -. 124 n. And when W=1’39 Volume aspirated after 90 m. ==119-5 c.c. The volume which should have been aspirated = 1-39 x 90 125 c.cm., therefore error = 5-5 0.cm. in 125. =4.4 per cent. =
=
=
This is
-
=
extreme case, and since after 90 m. aspiration m. have an error of opposite sign, it seems that the viscosity change will with this condition of the apparatus give a rough compensation for the expansion of imprisoned air. an
would be normal again and the next 90
Referring to the experiments already mentioned Whipple made the following comment :-
Mr.
" In
the
important experiment you aspirated from 11 to 9 and 865 c.c. from 9 to 7.20. The 860 c.c. of air if cooled from 62° to 52° would have been reduced in volume to 845 c.c. so that the quantity taken in during the latter period may have been nearly 880 c.c. most
apparently 860
c.c.
The rate from 11 to 9 "
"
"
9 to
The difference does not
was
7.20 " seem
to
8-6
c.c.
8-5
c.c.
me
of
(at 62°) per hour (at 52°) much importance." "
At his suggestion the following experiment was. ) made to try if in actual working the result obtained
xx
by the slow working filter over a long period gave a fair average of the impurity in suspension during
period. Two similar Owens filters were fitted up on Oct. 18th, 1917, at 47, Victoria-street, S.W. One of these was arranged to run very slowly so This as to deliver about 2000 c.cm. in 12 hours. 5.30 was started at 10.40 A.M. and at stopped after 1000 c.cm. had been aspirated through the paper. The other filter was used to take hourly records that
2000 c.cm., so that eight such records were obtained during the time the slow operating filter was working. Calling the slow filter (A) and the filter (B) :hourly
using
If the shade numbers obtained by (B) when added and averaged gave a number = the shade of the record got by (A), corrected for volume, the inference would be that (A) gave a satisfactory result. Suppose NA shade number obtained by (A) and NB average shade obtained by (B), then for satisfactory working=
NA should
be
=
NB x
1000
2000 or
NB
= NA
or
2
NB = 22 NA. NB N A. =
The actual records from (B) gave a uniform series of shade 4. Thus there was little change in the amount of impurity in the air during the test. The shade
NA was
=
2.
Thus
NB was= 2 NAshowing
AUTOMATIC FILTER. In this apparatus an aspirating vessel is provided into which water is admitted through a regulating cock at the bottom. A syphon is fixed inside the vessel, which causes the water-level to oscillate at regular intervals between two fixed levels in the vessel; thus, while the water is rising air is driven out of the vessel, and while it is falling air is aspirated into the vessel. On the top of the instrument an entrance for airis provided of a fixed diameter, over which revolver a disc of filter paper. Provision is made by which this disc is locked between two plugs, on the lines of the single-record filter, during the time air is being drawn into the vessel. The disc is caused to revolve by a weight and string, but the rate of its revolution is regulated by a stop which follows the hour hand of a small clock placed above the disc. The force for clamping the disc is obtained by a pressure operated flexible diaphragm acting through a lever. The records are given upon a disc of paper upon which the hours have already been marked similarly to a clock-face, and each record is placed automatically opposite the time at which it is taken. Thus the interval between records is unimportant, although, as will be shown below, this is easily adjustable. The present instrument takes a 12-hour disc, but there is no reason why the time should not be longer. ,
The following is a detailed description of the apparatus, which is illustrated in Fig. 5. The experiment was repeated on October 19th. Water- supply.-Water is admitted to the aspiratSeven hourly records were obtained from (B) ing vessel through a regulating cock, A, at the and one record from (A) running from 11 till bottom. This cock is connected with the main, and 4.25 P.M. by adjusting it the interval between records is The (B) shades were-6, 6, 6, 4, 4, 4, 4. NB was varied. Fixed inside the vessel and immediately cock is a small valve, B, of the or 5 as near as possible. NA was between above the regulating therefore, This is operated by a central piston-valve type. 2 and 3, taken as 2, so that again rod passing vertically upwards through the axis of a
good result.
=
.
34, 7
NB
=
2
NA.
In order to verify the above conclusions the following observations were made :-
The filter was set up and adjusted so as to run at rate which would cause it to aspirate 2 litres in 24 hours. Instead of counting the drops of water falling from the end of the capillary tube the interval between the bubbles rising in the upper bottle was measured; the temperature of the air at this time was 61° F., the interval between bubbles was 22 seconds, counted from a distance of four or five yards from the filter. There was about 1500 c.cm. of air imprisoned in the upper bottle over the water Dr. Owens, who conducted the experisurface. ments, then came close up to the filter and, without touching it, counted the interval again. It had altered to 32 seconds. a
He next placed his hand upon the upper bottle for two minutes and then removed it. The result of this was to stop aspiration for a period of 5 min. 50 sec., the water being driven up the tube A D to a height of about 2 in. This observation indicates the importance of the temperature
the main vessel and actuated as described later. The lifting of this rod through a short travel of about 1/8 in. opens the water-supply, and lowering the rod closes it, but not completely, as a small leakage is allowed past this valve for the purpose to be described later. Air entrance.-The entrance for the air is at L on the diagram; a sleeve is screwed into the top plate of the vessel and contains two plugs, the lower one of which is fixed and the upper capable of a sliding movement sufficient to clamp the filter paper, H, which is placed between the plugs through a slot in the side of the sleeve, in a similar manner to that adopted in the single-record Owens filter. The upper or sliding plug is operated by a lever, K, which is attached to the centre rod, C, the weight of which is balanced by a sliding weight, N. Thus, when the lever is pulled down the upper sliding plug clamps the paper in position over an aperture, the lifting of the lever releases the paper and leaves it free to revolve.
Syphon.-The syphon, F, consists of a long leg communicating with the outer air and a shorter leg of larger diameter inside the vessel, the length of which determines the volume of air aspirated. error. After this investigation it appeared best to try a This shorter wide leg terminates at the bottom in a different form of apparatus. A design for a filter sliding piece which has a V-shaped notch cut in its which would operate automatically and give records lower edge. This sliding piece permits the length at short intervals over a period of 12 or 24 hours of the leg to be adjusted. was therefore worked out and an experimental It was found that the operation of the syphon was liable to be interfered with in certain ways. apparatus constructed by Dr. Owens.
[xxi FIG. 5.
xxii
downwards
If the rate of inflow of water to the vessel was veryI small and the long leg of the syphon was, say, of 5/16 in. diameter, it was possible to get a trickling of water over the bend of the syphon which kept pace exactly with the inflow ; thus the syphon would fail to act. To prevent this trickling over,
the diameter of the long leg was reduced to about 3/16 in., the object being to force the water to pass over the bend as a solid plug, since the surface tension in the small tube would make it impossible for it to pass otherwise. With a tube of this diameter it was found that the difficulty mentioned was entirely overcome, and the water, if it passed over the. bend, went over in a solid plug and the syphon acted.
difficulty arose from the fact that the the syphon when very small in diameter. as has been shown to be necessary, failed to empty itself completely of water, a plug being . retained in the lower end which caused an erratic action of the syphon on the next filling, this plug being held up by the capillarity of the tube and the surface tension In order to overcome this diffiacross the mouth. culty the short leg of the syphon was made of much larger diameter than the long, so that it contained, after the syphon broke, a volume of air several times greater than the long leg, and this air- was driven out as the water rose again in the vessel, thas scavenging the long leg and blowing out the plug of water. Again, great care had to be taken that no positive pressure was produced inside the aspirating vessel as the water level rose therein. For example, several times it was found that the syphon acted too soon, owing to the fact that the paper disc was revolving in contact with the mouth of the bottom plug, through which the air had to pass out as the water entered. This was remedied very simply by lifting the disc clear of the mouth. The action of the syphon then is as follows. As the water rises in the main vessel it also rises in the short leg of the syphon until it reaches the bend, when it flows over and the syphon commences to operate. It continues to operate until the water almost reaches the bottom of the short leg, in which, it will be remembered, a V-shaped slot is cut. This V-shaped slot remains inoperative or closed by the surface tension of the liquid until, as the water level falls, the pressure against the surface skin becomes too great for it to resist without rupture : at this moment the skin closing the V breaks and the short leg of the syphon is emptied into the main vessel, air taking the place of water. As the water from the short leg flows out it raises the water in the main vessel slightly, but not enough to cover the V-shaped slot; thus the tube can completely empty itself, which would not be the case if no slot were provided. Immediately the syphon " breaks the water level in the main vessel begins to rise slowly owing to the slight leakage in the water valve, B, already mentioned, and when the surface has risen a little this valve is opened fully by means to be described, and the water again rises steadily in the main vessel until it again flows over the bend of the syphon, .thus completing Another
long leg of
"
a
cycle.
Filter disc.-The filter disc, H, is about 3 in. in diameter and fixed upon a small turntable opposite the slot in the sleeve, already mentioned. It is caused to revolve by means of a weight and string, but the rate of its revolution is regulated by means of a c.ock, J. This clock is placed face
I
above the paper disc and the hour hand engages a stop on the disc. The effect of the weight and string is to keep the stop always up against the hour hand when the disc is free to move; when, however, the disc is clamped between the filtering plugs and is not free to move the hour -hand of the clock moves away from the stop. to be subsequently overtaken again by the disc when it is released. Thus the interval between two records is made up of (a) the period during which the disc is locked and the record being taken, and (b) the period during which the disc is free and following the clock-hand round. By setting the clock when the instrument is started the records are marked automatically opposite the time at which they are taken, as shown on the disc; thus whatever the interval between the records may be is comparatively unimportant, since actual time of taking is shown opposite each
the
record.
Diaphragm.-In the
centre of the upper plate the main vessel is fixed a flexible covering diaphragm of leather, E, penetrating the centre of which is the central rod, C. This rod is attached at the upper end to the lever, K, and its lower end This diaphragm serves forms the water valve, B. the double purpose of permitting the movement of the rod when acted on by the bell to be next described, and also it exerts a pull upon the lever, K, when the pressure has fallen inside the main vessel. Bell.-A bell, D, is fixed on the central rod, C, so that it is just completely immersed at the highest level of the water, when the syphon begins to act. The balance weight is adjusted so that when the bell is empty and not immersed the weight keeps the upper plug lifted clear of the filter paper. The bell is formed as a short cylinder about 3 in. diameter and 1 in. deep, open below and closed above. In the side is cut a narrow vertical slot extending the full depth of the bell and 1/32 in. wide. The effect of the slot is to permit the air inside the bell to escape as the water rises, but when the syphon acts and the water level falls the slot remains closed by the surface tension of the water until the water has fallen sufficiently to rupture the closing film, and until then the bell remains full of water and acts as a weight on the rod, C, pulling it down and giving time for the diaphragm to act. When the water has fallen sufficiently the film closing the slot ruptures and the water falls out, leaving the bell empty and capable of being lifted by the balance weight, N, as soon as the diaphragm ceases to pull on the rod, C. In this arrangement the surface tension.is used as a valve and there are no parts to get out of order or vary in their action. Further, if for any cause the syphon acted too soon the instrument would refuse to take a record since the bell would not operate to pull down the lever, K, unless the slot in its side bad been previously immersed to form the closing film. This arrangement was found to work satisfac-
torily. Manometer cock.-A small cock, N, is provided in the top of the vessel for the purpose of calibrating. In designing this instrument it was decided to operate with the same volume-i.e., 2 litres-and the same sized filter disc-i.e., 1/8 in.-as in the single-record instrument. In order to fix the volume, attach a manometer to the cock, N, and observe the pressure inside the main vessel. When
xxiii the water level in the main vessel begins to fall inside falls, and the diaphragm; E, comes into air is drawn in through the filtering plugs, but not operation exerting a pull upon the lever, K, which through the filter paper, until the latter is forced forces the sliding plug harder down upon the down by the action of the diaphragm and bell. filter paper, making a joint around the small The moment that the air commences to pass ( disc. As the rod, C, is pushed down, the waterthrough the paper is indicated by a fall of pres- supply is cut on by the valve, B, and remains lifted. Air is now so until the rod is again sure inside the main vessel; mark the upper water level when the pressure begins to fall. drawn through the filter paper into the vessel level has been found to be perfectly until the water reaches the bottom of the This definite and easily observed. To fix the lower syphon, when the syphon breaks and aspiration level of the water observe the rise of pressure ceases; the water again commencing to rise after the syphon breaks ; the pressure rises and the cycle described above is again repeated. vile It was found that some gradually i n s i a e FIG. 6. vessel until it becomes of the earlier records were surrounded by a atmospheric, and at this moment the level of the ring, showing slight water should be marked. leakage beside the filter The volume of water plug. This indicates between these two levels that the pressure on the is then measured and plug was not quite adjusted by altering the enough, a point of some length of the short leg of importance ; the head on the syphon to make this delivery end of syphon volume equal to 2 litres. was about 3 ft. To inThe rise of pressure in crease the pressure upon the the vessel after the plug we may either increase the length of syphon broke caused the the pressure to become equal long leg of the to atmospheric after the syphon to, say, 4 ft. or 5 ft., water had risen about or increase the effective area of the diaphragm. 1/4 in. in all experiments made, and this level was Using a head of 4 ft. on always quite definite, so syphon delivery the that there appears to pressure of plug on filter be no great room for paper is about 6 lb., and the end of plug in variation in the volume as of air aspirated when contact with paper is an annular ring 1/16 in. once the levels are fixed. The of film actual records consist a uniform of the wide surrounding the deposit, Operation.-The operashading in the diagram giving an idea of the variations 1/8 in. hole the pressure tion of the instrument is of intensity. as follows : Water enters per square inch is actually not too great, as it is in the main vessel. The 180 This is the and rises about lb. cock, A, through balance weight keeps the rod, C, lifted, the diaphragm necessary so to compress the paper surrounding 1/8 in. disc, which is operating as the filter, being out of action, and the exit of air through the hole, L, is permitted. The water continues to riseas to prevent leakage of air. until it flows over the bend of the syphon, immersing I To obtain this pressure without an inconveniently I head the diaphragm, E, is made to rest on a and filling the bell, D, when the syphon commences great to act. As soon as the syphon acts the water level in metal disc of slightly smaller diameter. This conthe main vessel begins to fall; the bell, D, also falls verts the diaphragm into what is practically a owing to the weight of its contained water and piston, and greatly increases the effective pressure pulls down the lever K, bringing the upper filter produced. The instrument works satisfactorily and with plug in contact with the paper which is pressed down over the air entrance, L. There is thus a practically no attention. An illustration of the resistance to air entering the vessel, the kind of records obtained is shown in Fig. 6.
I the
pressure ’