LECTURES ON CHEMISTRY, BY PROFESSOR BRANDE.

166

the decease face on one side of the pelvis, and the occiof the foetus as certain. That the mother put to the other. Again, the arm presenthave not felt her child for weeks together, is ing as here ; the head is situated on the one no decided proof. This mobility of the bones os innominatum, the body on the other; the alone deserves no reliance whatever. Cuti- abdomen in front, and the back in the poscular desquamation itself (possible in conse- terior part of the uterus. So, then, to drop quence of cutaneous disease) is an ambi- amore extended exemplification, in the acguous indication ; the total dissolution and curacy of obstetric language, by presentabreaking up of the bony structure of the cra- tion we mean that part of the child which is nium is the best, and perhaps the only cer- lying over the centre of the pelvis; by tain sign of death. Many a child, rashly situation, the place which the child holds pronounced to be dead, breathes and cries with respect to surrounding bones. immediately on leaving the vagina ; and the When we meet again, 1 shall resume. recollection of these acknowledged truths may, I trust, hereafter paralyse some prurient murderous hand, too eager for the per-

pulpy brain,) you may look on

forator. I shall conclude my remarks upon LECTURES ON CHEMISTRY, fostal heads which deviate from the standard, BY by observing that we sometimes meet with heads without brains, and I here show a specimen, unique, no doubt, and the only PROFESSOR BRANDE. one to be met with in this theatre. (Laughter.) By the Gexznans, in consequence of Delivered at the Royal Institution of Great their resemblance, these crania are called Britain. cat’s head, a denomination by no means inappropriate. The bones of the occiput, front, sides, and summit, are wanting, while those LECTURE V. which form the basis cranii are perfect enough. This defective organisation I the On the Thermometer, and the Nature of Heat.

those

rather notice, because where it occurs, and where the accoucheur is not in full possession of the confidence of the family, it leads

GENTLEMEN,—The last Lecture

was oc,

cupied by endeavouring to make you undersometimes to an ill-grounded suspicion, that stand the action of heat, in producing the the cranium has been laid open. Observe, expansion of bodies ; at present, we shall therefore, the specimen here submitted to consider the apparatus usually employed for inspection, and recollect that this is nothing the admeasurement of the degrees of heatmore than a particular variety of monstro- namely, the thermometer. thermometer, in itself, is a sity, on the whole not unfrequent. Within Now, the circle of my own obstetric acquaintance, very simple instrument; a certain quantity four or five examples of this brainless mon- of a fluid is enclosed in a vessel, allowing ster have occurred, and in two instances its expansion or contraction to take place gave rise to unpleasant and unjust sur- with facility, and it is generally found, that mises.-Et hœc huctenus. if we mix equal quantities of the same fluid, Presentation and Situation.—Before I enter at different temperatures, the cold portion on the next important topic, I mean the pas- will expand as much as the hot portion consage of the fall grown foetus through the pel- tracts, and that the resulting temperature

the

vis, it may

not be amiss that I should ex- will be the mean ; so that, it appears, that you the meaning of two obstetric as much heat is gained by one portion as is terms of frequent use—presentation, I mean, lost by the other. Upon this principle, therand situation. By presentation, the accoucheur, mometers are constructed. It is assumed, accurate in his language, understands that that if 10° of heat be taken from a cold, and part of the child which is found lying added to a heated body, that the expansion over the centre of the pelvis. Thus, in of the one will be equal to the contraction the illustrations I here give you, the arms, of simply supposing, that the the face, the breech, the legs, and so on, increase of bulk is equal to the increase constitute the presentations, for these are of heat, between 32° and 2120, which is lying successively over the centre of the the ordinary range of the thermometrical pelvis. By the situatiorc of the child, when scale. Without taking you far into the hisspeaking of its passage through the pelvis, tory of the thermometer, I may just obwe mean the place of it, with respect to the serve, that the first which was constructed surrounding bones. Thus, in the illustra- upon truly scientific principles, was that tion here given, the vertex of the child pre-. invented by Sanctorio, of Padua. He conof air in tubes, which, by senting one ear is situated on the symphysis fined a the its contraction and expansion, was used to pubis, and the other on the

plain

to

the other ;

sacrum;

portion

167

exhibit the alternations of heat and cold. come ; and Mr. Boyle has the merit of hav. This instrument was very imperfect, and ing suggested that point, at which water extremely inconvenient, from the rapid and is converted into ice, as the zero; and great dilatability of air, by moderate changes Martin gives Dr. Halley the merit of fixing of temperature ; but principally on account upon the boiling point of water as a standard of the pressure of the atmosphere on the of graduation, which elevates the quicksilver column of fluid at different altitudes. The in the thermometer tube always to a given Florentine academicians were the first to point, under given barometrical pressure, employ a solid body instead of air, the ad- and the mean pressure of thirty inches is vantage of which is very evident ; but, generally understood to be that at which our upon the whole, it was an useless instru- thermometers are graduated. Now, the three thermometric scales at ment, if we speak of it in comparison with those now constructed. This instrument present most in use in Europe, are those of was a bulb containing spirit of wine, and Fahrenheit, Reaumur, and the centigrade. closed so as to exclude the influence of air, Fahrenheit’s scale commences at the temand to prevent the co-operation of this fluid. peratnre produced by mixing snow and salt, Now, this served to show, very well, the being at 32° below the freezing of water, amount of increase or decrease of tempera-i which point, therefore, is marked 32°, and ture in a given body, but it did not allow of the boiling point’212°, the intermediate space any accurate comparison being made with being divided into 172°. In Reaumur’s scale, for there being no fixed point at the freezing point is zero, and 80° the boileither extremity of the scale, the graduation I ing ; and in the centigrade, the space between Now it was arbitrary, and no two instruments, these two is graduated into 100°. will frequently happen, that during your when placed in the same temperature, cated the same degree of heat. You will chemical studies, you will find temperatures find, in the proceedings of the Florentine mentioned, reckoned according to one or Academicians, drawings of various instru- other of these scales ; therefore, it is very ments made on this principle, but they were necessary that you should know how to of no use, except by direct comparison. compare these, so as to ascertain the equivaAll these, however, have been far sur- lent degree on the one scale, for any given i passed, by contrivances of a later date ; so degree on another. There are some therthat, now, thermometers are constructed mometers, on which the degrees are marked with an extreme degree of nicety. The off, according to these three scales ; but if thermometer, as now constructed, consists you should not be in possession of such, you of a glass tube, having a bulb at one ex- may easily reduce Fahrenheit to Reautremity, for the fluid to be employed, and mur, if you recollect that each degree of hermetically sealed at the other. In select- the former is equal to four-ninths of a de. ing a tube for the manufacture of a ther- gree of the latter; and that, if you take any mometer, care should be taken to see that it degree of Fahrenheit, subtract 3io, multiply is perfectly cylindrical and perfectly clean ; ’ by four, and divide by nine, and the result and if the column of mercury inserted, be will give you the corresponding degree of equal in all parts of the length of the tube, Reaumur. To reduce Reaumur to Fahrenit shows that it is quite straight. Then a i heit, you multiply the degree by nine, aittt bulb is blown by heating the end of the divide by four ; to the product, 32° must be tube, and by forcing air into it from a added. Each degree of Fahrenheit is equal caoutchouc bottle, attached to the other ex- to five-ninths of a degree on the centigrade li scale ; so that, if you multiply any degree tremity of the tube, until the bulb of a sufficient size. The caoutchouc is ne- of Fahrenheit, having first substracted 32°, cessary, because the moisture of the breath , then multiply by five, and divide by nine, would destroy the tube for the purpose in- you will obtain the correspondingdegree on tended. The next point is to get the fluid the centigrade scale-and vice versa. You will understand then now, upon into the bulb, which is intended to be used to show the degrees of expansion, which what principles the thermometers are grawill be hereafter explained. Now two duated, and how far they are to be depended fluids are, at present, usually employed for upon as measures of heat. Mercury being this purpose, alcohol and mercury, the very equal in its expansion, is for general former being very well suited for the indi- purposes preferred ; and spirit, as it is never cation of extreme cold, from the fact of its frozen, being well adapted for the admeanot freezing ; and mercury, as it boils only surement of low temperatures. Linseed oil at a very high temperature, is a very good was employed by Sir Isaac Newton to fill substance for showing great degrees of heat. his thermo metrical tubes, and he assumed The main difficulty to be overcome in esta- the boilingand the freezing of water as the blishing what states of temperature should fixed points; it is a liquid, however, very be considered as the fixed points from which inconvenient on account of its viscidity, and to reckon the degrees, remained to be over- from the irregularity of its expansion. A

others;

indi-

becomes

168

variety of thermometers are in use, will be bent upwards, and in hot weather in but for common purposes, you cannot have the other direction. This principle has been a better than the spirit thermometer, which applied to the manufacture of the penduis a very simple instrument; the bulb is lum of clocks, and also to the balances cf and and when we usually protected by a brass shield, but for the great minuteness of watch-work, measuring the heat of fluids, the bulb should be bare. You may have the scale marked we cannot but feel surprised at the great de. upon the tube, or you may have the scale gree of perfection at which this compensainserted into the tube, containing the spirit tion balance, as it is called, has been or mercury. (Mr. Brande then showed the brought. The chronometer taken by Capt. class a great variety of these instruments, Parry to the North Pole, varied a very few marking the gradual improvement that had seconds only during the voyage, on account taken place in their construction.) of the compensation of the balance by the Now of course the indications of the different degrees of expansion and contracthermometer become unavailing, when we tion of the metals of which it was composed. arrive at temperatures at which mercury These things belong, however, more to the boils, which is about 500° ; the thermome- mechanical than to the chemical philoso. ter is unavailable, because the tube con- pher. taining the mercury would burst, and we Here is an instrument, which has been must have recourse to another mode of an apple of discord in the philosophical ascertaining the increase of temperature world, especially between Count that point. Such a mode was long and Mr. Leslie ; Count Rumford said, he since suggested by Mr. Wedgewood, who, had discovered it, and he called it a therobserving the differences produced in the moscope, and VIr. Leslie called it a differential bulk of clay on exposure to a strong heat, thermometer. Van Helmont, however, has took a piece of clay, measured it, and then described a similar instrument, and has submitted it to a powerful heat; and by made the same application of it as these comparative experiments of this kind, he two philosophers have done. It has been endeavoured to establish, by the degree of called a differential thermometer, on acshrinkage of a given bulk on exposure to a count of its showing the difference of temgiven heat, the absolute heat of any furnace perature between the two balls. Now, rebeyond that point by the comparative shrink- member that this thermometer merely indi. age. But as Sir James Hall has correctly cates the degree of heat given off by bodies, observed, the shrinkage of the clay is the and thus becomes a measurer of that quansame when submitted to a red heat for a tity of heat, which bodies are capable of quarter of an hour, as when exposed to a absorbing or communicating under ordinary white heat for five minutes; and hence a circumstances ; but it is by no means a great door was opened at once to fallacy in measurer of the quantity of heat contained arriving at any conclusions fiom this test in bodies. It consists of two large glass of heat. The shrinkage, as it will be seen, bulbs containing air, united by a tube twice depending rather upon time, than upon the bent at right angles, containing coloured degree of heat to which the clay was sub- sulphuric acid. When a hot body apmitted. The metals are not, however, liable proaches one of the bulbs, it drives the to that objection; and here is an instru- fluid towards to other. Its principal adment invented by Mr. Daniel, which I will vantage is, that in making delicate experiexplain to you. A platinum bar is inserted ments, the general changes of the atmosin the centre of a cylinder of black lead pheredo not affect it ; but you must rememware, as it is called, which bears a great de- ber, that it only indicates the difference between the two balls. gree of heat, and from the top of the pla- of tinum bar, a wire was passed over the axis Sometimes a simpler form of air tliermomeof an index. He established by various ex- ter is used, consisting merely of a bulb periments a sort of accordance between a with a tube at one extremity, which is scale which he attached to the index and plunged into coloured water, that is made the scale of Fahrenheit, and he considers to stand at any convenient height in the that the boiling point of mercury would be tube, by expelling a portion of air by the about 92° upon his pyrometer, or equal to application of heat. As the air in the bulb 644° of Fahrenheit; that the point of fu- may be heated or cooled, the fluid will rise sion of tin would be about 630, or equal to or fall in the tube, and such variations may 4410 of Fahrenheit; bismuth 66°, or equal be marked by affixing a scale on the instiato 462° ; lead 870, or equal to 609°; cast ment. iron 497°, or equal to S477° of Fahrenheit. Speaking of the quantity of heat, it must If you take a bar of brass and steel, rivet be borne in mind, that there must be twice them together, and lay them on a flat sur- as much heat in a quart of water as in a face, the brass contracts more rapidly than pint, yet the thermometer will only indithe steel, so that in cold weather the steel cate the same degree of heat; the quantities

great

watches

chronometers ;

consider

Rumford

beyond

temperature

169 of lieat which bodies, in the same state, require to raise them to the same theimcmetric temperature, is called their specificheat, and those bodies requiring most heat for that purpose, are said to have a greater ca-

and this is the

case

with

regard

to

solids,

liquids, and gases. On condensation, solids lose their capacity for heat; by hammering

iron on an anvil you squeeze out the heat from it, to use a vulgar phrase; but if you put yacity for heat. Dr. Black, of Glasgow, it into the fire it again expands, and if allowed first showed. in his lectures in 1762, that to cool gradually it again retains, when cold, the quantity of heat in different bodies of the same degree of heat as before it was hammered. Now if you mix together two the same temperature, was different. It has been said, in proof of the accuracy so that by their admixture their density as by mixing together oil of the thermometer, that the mixture of a quantity of heat will and the fluid at volumes same different of water, equal temperatures, will give the arithmetical be evolved, or the capacity of the bodies for On mean of the temperature of the two ; and heat, in other words, is diminished. if you suddenly expand or rarefy that a mixture of a pint of hot and cold the water will afford the mean temperature of air, it becomes cold; the air in expanding the two, as indicated by the thermometer has its capacity for heat increased, and previous to mixture ; but if you take a whence is it to get its supply of heat but quart of oil, and a quart of boiling water, from itself’! It is, therefore, clear, that if as indicated by the thermometer at 212°, you compress air you produce proportionably of heat, and with a syringe of and mix them, you will find if you attend a kind this that mixto what takes the by a small stroke ofthe piston you carefully place, ture will not possess the arithmetical mean may set free so much heat from the air of the two, but that there will be a certain contained in the cylinder as to set fire to a quantity of heat absorbed. Now, it was to piece of tinder attached to the piston. Now it is easy to see how these facts bear determine how this happened, that Lavoisier and La Place commenced a series ofupon natural temperature ’as the air ascends interesting experiments. They took a ves- it becomes rarefied, and deposits its sel properly constructed, into which they moisture on the mountain tops ; and on the put a quantity of ice ; and into the centre other hand, as it approaches the earth, it beof this vessel, they put a given quantity of comes more dense, its capacity for heat is boiling water ; the whole was then’covered greater, it absorbs moisture from the earth’s up, and so contrived, that the quantity of surface, which being conveyed in its turn ice thawed in the cooling of the water, from to the upper regions of the atmosphere, is the boiling to the freezing point, could be again deposited in the form of rain and ascertained; and it was found, that twelve dew, or snow and hail, as the temperature ounces of ice were thawed. They then of the medium through which it has to pass took a quantity of oil heated to the same may be hot or cold. The temperature of temperature as the boiling water, 212°, and mines may be easily accounted for by the inthey found that, on cooling the mass down creased pressure to which the atmospheie to 320, only six ounces of water ran out of of the mine must be subjected by the weight the ice; so that they inferred, that the of the column of air above it. next lecture the various conducting quantity of heat in the water, to that of the In the ice, was as2 to 1. There are experiments, powers of- bodies in regard to heat will be which might be made to show the same considered, and also the nature of latent heat. thing, namely, that the degree of heat, shown to belongto bodies by the tlxermometer, differs from the absolute quantity LECTURE VI. of heat they contain. You will see, from this table, (pointing to one behind him,) On Heat. the resulting temperatures of a mixture of bodies at different temperatures. Thus two GENTLEMEN,—It happens, that’.’.’hen different bodies are exposed to the same source portions of water mixed together, at of heat, they allow it to pass through them with unequal velocity, and the body is said a afford mean of 75°. to have a greater or less conducting power in proportion to the celerity or tardiness with Water at 50°, a mean which it becomes cooled down to the temof 70°. Mercury at perature of the surrounding atmosphere. A pint of water at Among the solid bodies, down, feathers, mean aa mean of of 80°, o wool, cotton, and other light and porous arA pint of oil at 100°, ticles of clothing, are bad conductors of The capacity of bodies for heat is very heat, and are therefore chosen for that purdifferent, and when you change the form of pose ; next to them we may rank light bodies you change their capacity for heat ; earthy bodies and wood, and from some ex-

fluids,

shall be increased, of vitriol contrary,

quantity

cooling,

as

50° 10Uo

&}

temperature

10 °, }}a

50°, }

170

periments of Professor Mayer,

of Erlangen, it appears that the conducting power of different woods is in proportion to their density, as you will see by the table behind me. Thus, assuming water to have the specific gravity of 1.000, and its conducting power 10, the conducting power of ebony wood will be 21.7, and its specific gravity 1.054; apple tree 27.4 and 0.639; ash 30.8 and 0.631 ; beech 32.1 and 0.692, and so on. Next to them we may name silicious and hard stony bodies in general ; then glass, the topaz, and the diamond. But of all solid bodies, the metals are best conductors, and silver, gold, and copper are better conductors than platinum, iron, and

with extreme caution, and taking into ac. count the conducting power of the sides of the vessel, that heat does ultimately make its way downwards through the liquid ; but it must be still considered that fluids have a very weak conducting power. Air is an equally bad conductor as water, and indeed worse ; you may make substances red hot in the air contained in the upper part of a vessel, yet the air below will remain cool ; this experiment, however, is difficult to show in a class room. We arrive, then, at this general conclusion, that heat passes through solid bodies with various degrees of retardation of its progress, and that it passes through them equally in a-11 directions. We find that different solids retard the heat very differently, as shown in their different conducting powers ; and in regard to liquids, we find that they are all very bad and very imperfect conductors of heat, the exact conducting power of each not having been determined, and that air is I may refer a still worse conductor of heat. you to the experiments -of Count Rumford, described in the Philosophical Transactions of 1792, respecting the conducting power of different materials used as clothing. He heated the thermometer to 100°, and having enveloped the bulb in different kinds of material, he measured the time it required to come down to the temperature of the atmosphere, and he found that that time was

lead.

Liquids are bad conductors, and on this subject we may advert to the experiments of Count Rumford. He took a large glass jar of this kind, having filled it with warm water, he put at the bottom of it a lump of ice ; the water about the ice formed a stratum of ice-cold water, as was shown by immersing a thermometer into it, and it required a long time to make that water warm. But there is a number of simple experiments to show the same thing. In the process

of distillation, you know that the

steam

enters the worm tub at the upper surface of the water, and the water in the upper part

of the tub will become almost boiling hot, whilst the water in the bottom of the tub remains quite cold ; therefore, when we directly according to the conducting power of wish to heat a mass of water, we do not ap- the substance in which the thermometer was ply the heat at the top, which would be an enveloped. The relative conductingpowers endless job, but we apply to the bottom of of wood, metal, and other substances, are the vessel containing it, and the ascent and shown in a number of familiar applications, descent of the heated and cold portions of as when wooden handles are fixed to metallic water constitute the phenomena of boiling. tea-pots. The experiments made by the Now here is an apparatus with which the late Lord Stanhope may be mentioned as bad conducting power of water may be easily showing the different conducting powers of shown : we have an air thermometer im- wood and the metals. He placed a piece of mersed in a vessel of water, and you will paper around a metallic cylinder, and held it find, that although we apply heat to the over the flame of a candle. (Mr. Brande resurface of this liquid, we shall in vain look peated the experiment.) It will be long befor any elevation of temperature in the wa- fore the paper will burn, and the reason is ter beneath. The water about the fire be- very easy to see, the heat is diffused over comes very hot, and that stratum of the the surface of the metal, ::’.s fast almost, as it water will remain hot without communicat- is communicated to the paper ; but if you ing the heat downwards, and consequently envelop a piece of wood with paper, and without producing any effect upon the air expose it in the same way to the flame of a thermometer below. I might show you a lamp, the heat is not rapidly distributed, number of experiments to prove, that if and therefore the paper speedily burns. We now proceed to another very importfluids do conduct heat, thev conduct it very ant part of the subject of heat, namely, that the of boil surface You might imperfectly. water in a vessel in which ice was at the i which relates to its effect upon the state or bottom. Another mode of determining the form of bodies. same fact is this ; you take a glass cylinder, I We purposely avoid entering here into the in which an air thermometer has i state of heat, as connected with decomposiplaced, and you pour some boilingoil on, tion, or chemical change. It is very genethe surface of the water ; the thermometer rally known, that when solids are heated il does not rise, although it may be held very I they have a tendency to become liquid, and near to the surface of the water. It has liquids to be converted into gaseous or aeribeen found, however, by experiments made form bodies; cool a liquid and it becomes

I

been:

I

171 the case, and you was only 52°; so that, you observe, a large have a familiar illustration of this fact inquantity of thermometric heat disappeared the case of water and ice. Heat water and in order to melt the ice. He then found, you convert it into steam ; the steam, as it that when he took ice at 32° and water at was a quantity of ice-cold cools, becomes converted into water, and 172, the the water, on further cooling, into ice. We water at32. Here the water was cooled are indebted to the late Dr. Black, of Glas- 140°, while the temperature of the ice was gow, for an important series of observations unaltered, that is, 140° of heat disappeared, upon these subjects, and his researches de- the effect being not to increase temuerature. You see what an serve to be ranked among the most admira- but produce fluidity. immense loss of heat there is in the converble efforts of experimental philsophy. shall endeavour to give you an abstract only sion of ice into water ; and you observe that of these experiments, because to enter into the thawing of ice must, under ordinary cirdetail would require too much time. In cumstances, be only a slow process, and prosecuting his investigations, Dr. Black hence it is that natural temperatures are was the extreme slow- changed by very slow degrees; you have struck ness with which ice thaws. If, for instance, not a sudden transition from heat to cold, you bring ice into a warm room, it is a long but a gradual and slow change of temperatime before it begins to run down into the ture. liquid state; and there is a still more re- Having made these experiments to demarkable fact, which is this, if you put the termine the quantity of heat, or the therbulb of the thermometer into the ice, it will mometic quantity of heat in water, it next fall to 32°, and will remain there until the occurred to Dr. Black, that the liquifacwhole of the ice is melted. Now it is clear tion of other solids ought to be obedient to that heat is communicated to the ice from similar laws. He took a piece of lead, and the surrounding atmosphere of the room, he found that, until all the lead was melted, and yet the effect of that heat is not to he could not raise the temperature of the elevate the thermometer, but to melt melted lead beyond a certain point; but as ice. Suppose you make the experiment soon as the whole mass was melted, he could with water instead of ice ; you take a por- convert it into vaponr by increasing the tion of water at 32°, and put into it a piece temperature. There is, of course, a diffeof ice ; you bring the vessel, containing the rent degree of heat necessary for the liqulwater and the ice, into a room, the tempe- faction different substances. In the case rature of which is at 60° ; you will find that you have just seen, of mixing a quantity of the temperature of the water will ascend powdered ice with hot water at 172°, you gradually to 60°, but the temperature of the perceive that there is a sudden loss of 140 ice will not ascend until the whole is melted, degrees of heat; but whenever you liquify and then it rises. The Doctor called the a body, it must of course produce cold, and heat, absorbed duringthe meltingof the if you set out with bodies already very cold, ice, the heat of fluidity, and havingobserv- you will still lower their temperature, and ed it to affect the state of the body without we often produce artificial cold of great raisingthe thermometer, lie called it latent intensity by the rapid solution of certain heat, or the quantity of heat conducing to saline substances in water. Mix together make the ice liquid. He then proceeded common salt and ice ; in consequence of the a new series of experiments, to deter- action of the salt upon the ice, a quantity of mine the quantity of heat that became latent water will be immediately produced, and a during the conversion of ice into water, and portion of the salt be dissolved, forming he found it very considerable. He did this brine ; if we examine the temperature of in two ways: in the first, he brought the the brine, we shall find that it is many deice into a warm room, and ascertained the grees below the previous heat of the ice, time required to thaw it; he then estimated many degrees of heat having been taken the quantity of heat which had entered the from the ice by the liquifaction of the salt; ’ ice in that time. and this is the common process of obtainIn another suite of experiments, he mixed ing a large degree of cold, artificially, by a quantity of snow and finely powdered ice purging mixtures, and so on. If you diswith a quantity of boiling water, and he ob- solve a salt in water, you will find that there served that a quantity of heat suddenly dis- is a great lowering of its temperature. Here appeared to liquify the ice. Here are the is a mixture of sal-ammoniac and nitre, and results of one or two of his experiments, set if you dissolve them in water, especially if down on these tables. He mixed water at the water be cold, there is a sudden lower320 with water at 213°, and he founn, as he ingof the temperature, in consequence of suspected, that the mean was 1220; but on the sudden liquifaction of the salts, and you making the experiment under the same cir- may, without difficulty, freeze small portions cumstances, but taking ice at 32° and waterof water in this way. Here is nitrate of at 21x°, he found that the mean temperature. ammonia, which dissolves rapidly in water,

solid; this’is generally

I

forcibly

result

with

the

of

by

172

produces a considerable degree of cold. experiments, that you should use the salts Take equal portions of nitrate of ammonia recently crystallised and finely powdered.

and

and water, the cold produced on the dissolvingof the salt is very considerable, and of course the colder the vessel the more immediate the effect would be. I should observe here, however, in making these

This table shows the results of some expe. riments made on this subject by Mr. Wal. ker ; he found that, on dissolving the salts, here set down in water, the thermometer sinks as indicated.

THERMOMETER SINKS.

MIXTURES.

Parts

Muriate of ammonia ..... 5 Nitre Water

165}*)

..................

..................

Nitrate of ammonia........ Water .....

1 1}

From 50°

to

40.

Sulphate of soda..........

Dilutedsulphuricacid ..... 45

From 50° to 3°.

snow

From 32° to 0°.

.....

Common salt .............1}

’

From 50° to 10°.

Thus you see that considerable degrees into solids, and solids into liquids, then of cold may be artificially produced, and by proceeded to examine the effects of heat mixing together some substances, in diffe- i upon liquids themselves; and here there rent proportions, you may carry the tempe- is a circumstance closely corresponding rature down to 90° below zero, as bv mix- to that already pointed out, in regard to ing ten parts of diluted sulphuric acid with the thawing of ice in a warm room; if you eight parts of snow, the thermometer sinks put a portion of water over a lamp, and imfrom 680 to 910. merse a thermometer in it, you wiil find that Now it becomes necessary to show you the thermometer will go on rising until the the converse changes, namely, to show you water boils, but the moment it boils, the that heat would be evolved when fluids are thermometer shows no further increase of converted into solids. When you freeze temperature. Dr. Black proceeded to exwater, you must let out a certain quantity amine the cause of this interesting pheof heat, and whenever you condense a liquid, nomenon. There is an increase of heat apor bring a liquid into a solid state, there is plied to the vessel, but there is no increase I need only to in the temperature of the water, the effect a great evolution of heat. present you with a single instance : if you of that surplus heat being to convert the condense water by pouringit upon water into steam, which contains a large lime, it enters into composition with the quantity of latent heat, but condense the lime, and in consequence of this rapid con- steam into water, and a large quantity of densation, you know that a great degree of heat is given out. Before I state to you heat is evolved, often sufficient to set fire Dr. Black’s experiments, to determine the to shavings of wood, and other combustible quantity of heat thus believed to be consumed in the conversion of water into steam, bodies, which may happen to be. near. it will be well to advert to the boiling of crystallisation of Glauber’s salt is with a sudden evolution of heat, and this ’ liquids generally. Now they boil at differhappens in all cases where you convert nI ent temperatures, and yet the boiling point liquid into a solid; congelation, therefore, @f water, for example, is often spoken of, as is to the surrounding bodies a heating pro- ’, if that were an invariable point of tempecess, and liquifaction a cooling process. ’, rature, whereas nothing can be more variDr. Black having made out these facts,’! able ; it depends very much upon the presin regard to the conversion of liquids suie to which they are subject; and when

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say that water boils at 2H°, we mean state. We use the term boiling, therefore, that it does so under the pressure of an to express the conversion of a fluid into atmosphere equal to a column of thirty vapour. There is an experiment called the chemiinches of mercury, or thereabouts, equal to about 14 or 15 pounds upon each square cal paradox ; because the manner in which inch. Of course, if you diminish the pres- the experiment is made, and its effect, are sure, you diminish the temperature of the very paradoxical. Here is a Florence flask, boiling point; and if you increase the pres- to which is appended a stop cock ; you put sure, and put two atmospheres upon the some water into it, and cause it to boil ; water instead of one, the boiling point will you have then an atmosphere of steam upon be increased. Accordingly, as you find that the water which is invisible, you turn the the atmosphere is always varying in its stop-cock, and the boiling ceases. You pressure on our earth, the boiling point of plunge the vessel into cold water, the water water is always changing, sometimes being will again boil, because the steam has been at 2080, sometimes at 210°, sometimes at condensed, and the pressure of the atlnos212°, and so on. Another cause which in- phere of steam is taken off from the surface fluences the boiling point, is the nature of! of the water. the vessel in which the liquid may be There are a few facts remaining to be noheated. In glass vessels, liquids boil at two ticed, which must be deferred to the next or three degrees higher temperature than in lecture, when the circumstances under which metallic vessels. because slass is a more heat disappears will.be pointed out, and the irregular, and a worse conductor of heat subject concluded. than metal. But if you introduce a bad conductor into the vessel, as a few shavings of wood, you will find that the boiling point will be lessened, that is to say, under the mean atmospheric pressure. If then you ascend a mountain, of course you have a WESTMINSTER MEDICAL SOCIETY. less atmospheric pressure, and the water will boil at a less temperature. Saussure accordH. MAYO, Esq. in the Chair. ingly found, that on the summit of Mont Blanc, water boiled at 187°; and, on the AFTER the minutes of the preceding’ contrary, in a deep mine, it will boil at a high temperature, because then the atmus- meeting had been read, and some private pheric pressure is greater, in proportion to its business transacted, depth. The Rev. Mr. Wollaston constructed Mr. DUNCAN rose, to give an account of a thermometer so nicely, that he could tell the highly interesting case of aneurismal the difference of temperature in the differ- disease in the temporal arteries, at present ent stories of a house, and he proposed to in the Hospital of Surgery, and which has determine the heights of mountains, by been already so fully detailed in the preforming a scale from it. The thermometer vious Numbers of THE LANCET, in which he is so delicate, that it is seldom employed made a very full, clear, and candid statein barometrical experiments generally re- ment of the appearances of the disease, and of the history and progress of the patient; sorted to. Now, to show you that pressure influences and after relating Pelletan’s opinions and the boiling point with extreme facility, we practice, in a similar instance, proposed, may introduce under the receiver of an air- as questions of great importance for the conpump a small quantity of warm water, and sideration of the Society ; lst, What are the ? we shall find that the water will boil at a true anatomical characters of this tumour much less temperature, than when exposed M, Does it consist of mere dilatation of arto ordinary atmospheric pressure. Again, terial branches, of a varicose state of the if we increase the pressure, it will boil ataa temporal veins, or like n2eviis maternus, is greater temperature ; and it is of importance there aay distinct parenchyma through to us to kuow, that the temperature of the which the blood circulates 1 3d, Is the loss steam is always the temperature of the of the eye referrible to the operation, or to liquid, at the time of boiling; and we shall the disease of the brain? and 4th, What is be able, by and by, to show you that water the plan of treatment most advisable at boils at a temperature below the freezing present, the tumour having again increased point. Other liquids, such as the spirit of considerably in size ? To these interesting vitriolic sether, and so on, will boil at the points, Mr. Dunuan begged to call the atcommon temperatures of the atmosphere, tention of the Society, hoping that the when placed in an exhausted receiver of an members would be able to throw some light air-pump ; and were it not for the pressure on them, which might be of use to the paof the atmosphere, we should have all tient, at the same time acknowledging his liquids converted into a gaseous or aeriform own inability for the task. we

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