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Rigidity and other anomalies in colloidal solutions
Aug., t928. ]
CURRENT TOPICS.
265
is often v e r y s m a l l - - i n one case five separate responses were m e a s u r e d between 4,260 and 4,640." Reflection of sounds of high pitch was shown across a room by m e a n s of metal mirrors. T h e creeping of sound waves a r o u n d a whispering gallery was d e m o n s t r a t e d . W h e n waves are reflected from a plane surface those going t o w a r d the surface interfere with those coming from it and s t a t i o n a r y waves are formed. It is such s t a t i o n a r y light-waves t h a t Otto Wiener fixed photographically a b o u t I889. Mr. H u m b y explored the space occupied b y the standing waves by the use of two types of sensitive flames. One kind showed least disturbance at the loops and the other at both loops and nodes. " I t is interesting to hear the variations of loudness of sound t h r o u g h o u t a room when the note is switched on. T h e room is found to be filled with s t a t i o n a r y nodes and loops, for, as the ear is moved, the sound swells out and diminishes in a fascinating way. I t was found t h a t with the one-valve circuit used and 6o to 20o volts high tension the stationary waves could be located b y a sensitive flame at distances up to I5 feet from the source." T h e waves were reflected at almost grazing incidence from a plane surface and c a m e to interference with other waves travelling in almost the same direction t h a t had not been reflected. A sensitive flame showed definitely this acoustical analogy of Lloyd's mirror fringes. In addition fringes from the interference of waves coming from two different sources and other interference effects like those due to the Heaviside layer obtained in wireless transmission were d e m o n s t r a t e d . Diffraction effects were d e t e c t e d for a slit a n d for a straight edge as well as with a grating. W i t h a zone plate m a r k e d alternate increases and decreases in i n t e n s i t y were found as the successive zones were removed. G. F. S.
Rigidity and other Anomalies in Colloidal Solutions. EMIL HATSCHEK. (Proc. Royal Inst. Great Britain, No. I2I.) Let a cube of elastic solid be held between two planes one of which is fixed while the other is m o v e d to one side b y a force acting parallel to itself. A shear is produced and owing to its rigidity the deformed solid will r e t u r n to its original form upon the disappearance of the distorting force. If the cube is t r a n s p a r e n t and isotropic it becomes doubly refracting when sheared. This " a c c i d e n t a l double refract i o n " affords a sensitive test for the existence of strains
266
CURRENT TOPICS.
[J. F. I.
and has been been used to study experimentally their extent and a m o u n t in cases t h a t present difficulties to computation from theory. The modulus of rigidity, the force producing unit deformation on a cube of unit dimensions, is some tons per sq. cm. for steel and about IOO g./sq, cm. for gelatin jelly. All true solids possess rigidity b u t no normal liquid has it. If the space between two planes be filled with a liquid and a tangential force be applied to one plane, the other being fixed, the plane continues to move as long as the force acts. T h e force requisite to maintain the plane in motion is proportional to the area of the planes, to the velocity gradient between the fixed and the moving plane and to the coefficient of viscosity of the liquid, which is the force per unit area requisite to keep up unit velocity in the moving plane when the planes are a unit distance apart. In all liquids as in the case of molasses the viscosity increases in cold weather. T h e coefficient is remarkably independent of the velocity with which the moving plane travels. " T h e viscosity of water has been measured by a n u m b e r of observers over a range of velocity gradient varying in the ratio of I to IO,OOO and has been found quite constant." A study of the viscosity of colloidal liquids m a d e clear that they manifested bewildering anomalies. Order was introduced by the use of an instrument in which the liquid was contained between two coaxial cylinders one of which was rotated. T h e results for gelatine solutions are characteristic. As the angular velocity of the rotating cylinder increases the coefficient of viscosity grows less and tends to become constant for high velocities. At the smallest angular velocities the coefficient is two or three times as large as at high velocities. M a n y colloidal solutions of very different kinds have been studied and all show the same general c o m p o r t m e n t in respect to viscosity. To explain this m a r k e d divergence from the behavior of normal liquids Freundlich has suggested t h a t colloidal solutions possess not only viscosity b u t also rigidity. " I f this assumption is true, the liquid--to p u t the m a t t e r colloquially but picturesquely-will have to be stretched as well as sheared. It can be shown mathematically t h a t the result will be w h a t has been found experimentally, viz., t h a t the viscosity decreases with increasing shear." T h e author has discovered a colloidal solution that displays rigidity to a remarkable e x t e n t - " a very dilute, about one-quarter per cent. solution of a m m o n i u m oleate, a soap familiar as material for soap
Aug., 1928.]
CURRENT TOPICS.
267
b u b b l e s . " T h e space b e t w e e n t w o cylinders was occupied b y this solution. T h e p i v o t e d inner cylinder was given a spin. H a d it been s u r r o u n d e d b y ordinary liquid it would have slowed up and finally come to rest, b u t with t h e amm o n i u m oleate it came to rest and then s t a r t e d to r o t a t e in the opposite direction s o m e w h a t as if it h a d been rotating in c o n t a c t with a hollow cylinder of elastic rubber. T h e moduli of rigidity of several colloidal solutions h a v e been determined. All h a v e no rigidity a b o v e 4 °0 C. T h e viscosity of a m m o n i u m oleate is v e r y t e m p e r a m e n t a l changing c o n s t a n t l y and over long periods of time. F o r colloidal solutions the moduli of rigidity are m e a s u r e d in milligrams per sq. cm. instead of in grams as is the case for gelatine jellies. F u r t h e r m o r e the m o d u l u s increases with the age of the solution. F o r example in the case of a solution of mercury-sulphosalicylie acid its m o d u l u s was eight times as great after 23 d a y s as it was after 20 hours. In respect to the manifestation of rigidity the colloidal solutions examined showed m u c h diversity. One shows rigidity if prepared h o t b u t n o t if m a d e at room t e m p e r a t u r e . Some, when h e a t e d to the t e m p e r a t u r e at which rigidity disappears regain u p o n cooling t h a t p r o p e r t y as it existed w h e n the solution was old while others upon cooling b e h a v e as freshly m a d e solutions. T h e p h e n o m e n o n of double refraction occurs in m o s t colloidal solutions. G. F. S. A New Mass-Spectrograph and the Whole Number Rule. F. W. ASTOr,. (Proc. Roy. Soc., A 772.) The first mass-spectrograph, set up in the Cavendish Laboratory in I919, could separate lines derived from masses differing by about I in I3o. Its accuracy of measurement was I in I,OOOapproximately. "These capabilities sufficed to determine with fair certainty the isotopic constitution of over 5° elements, and to demonstrate that, with the exception of hydrogen, the masses of all atoms could be expressed as integers on the scale o = I6 to one or two parts in a thousand." It was not till I925 that this instrument which had rendered such iconoclastic service was dismantled. A need was felt for an instrument that would better separate the mass lines of the heavier elements and furnish more accurate information on divergences from integral mass numbers. A study of these divergences is of great value because they throw light on the structure of atomic nuclei. Costa in Paris, using an instrument with an accuracy of I in 3,000, in I925 published excellent measurements on this matter. Now
CURRENT TOPICS.
265
is often v e r y s m a l l - - i n one case five separate responses were m e a s u r e d between 4,260 and 4,640." Reflection of sounds of high pitch was shown across a room by m e a n s of metal mirrors. T h e creeping of sound waves a r o u n d a whispering gallery was d e m o n s t r a t e d . W h e n waves are reflected from a plane surface those going t o w a r d the surface interfere with those coming from it and s t a t i o n a r y waves are formed. It is such s t a t i o n a r y light-waves t h a t Otto Wiener fixed photographically a b o u t I889. Mr. H u m b y explored the space occupied b y the standing waves by the use of two types of sensitive flames. One kind showed least disturbance at the loops and the other at both loops and nodes. " I t is interesting to hear the variations of loudness of sound t h r o u g h o u t a room when the note is switched on. T h e room is found to be filled with s t a t i o n a r y nodes and loops, for, as the ear is moved, the sound swells out and diminishes in a fascinating way. I t was found t h a t with the one-valve circuit used and 6o to 20o volts high tension the stationary waves could be located b y a sensitive flame at distances up to I5 feet from the source." T h e waves were reflected at almost grazing incidence from a plane surface and c a m e to interference with other waves travelling in almost the same direction t h a t had not been reflected. A sensitive flame showed definitely this acoustical analogy of Lloyd's mirror fringes. In addition fringes from the interference of waves coming from two different sources and other interference effects like those due to the Heaviside layer obtained in wireless transmission were d e m o n s t r a t e d . Diffraction effects were d e t e c t e d for a slit a n d for a straight edge as well as with a grating. W i t h a zone plate m a r k e d alternate increases and decreases in i n t e n s i t y were found as the successive zones were removed. G. F. S.
Rigidity and other Anomalies in Colloidal Solutions. EMIL HATSCHEK. (Proc. Royal Inst. Great Britain, No. I2I.) Let a cube of elastic solid be held between two planes one of which is fixed while the other is m o v e d to one side b y a force acting parallel to itself. A shear is produced and owing to its rigidity the deformed solid will r e t u r n to its original form upon the disappearance of the distorting force. If the cube is t r a n s p a r e n t and isotropic it becomes doubly refracting when sheared. This " a c c i d e n t a l double refract i o n " affords a sensitive test for the existence of strains
266
CURRENT TOPICS.
[J. F. I.
and has been been used to study experimentally their extent and a m o u n t in cases t h a t present difficulties to computation from theory. The modulus of rigidity, the force producing unit deformation on a cube of unit dimensions, is some tons per sq. cm. for steel and about IOO g./sq, cm. for gelatin jelly. All true solids possess rigidity b u t no normal liquid has it. If the space between two planes be filled with a liquid and a tangential force be applied to one plane, the other being fixed, the plane continues to move as long as the force acts. T h e force requisite to maintain the plane in motion is proportional to the area of the planes, to the velocity gradient between the fixed and the moving plane and to the coefficient of viscosity of the liquid, which is the force per unit area requisite to keep up unit velocity in the moving plane when the planes are a unit distance apart. In all liquids as in the case of molasses the viscosity increases in cold weather. T h e coefficient is remarkably independent of the velocity with which the moving plane travels. " T h e viscosity of water has been measured by a n u m b e r of observers over a range of velocity gradient varying in the ratio of I to IO,OOO and has been found quite constant." A study of the viscosity of colloidal liquids m a d e clear that they manifested bewildering anomalies. Order was introduced by the use of an instrument in which the liquid was contained between two coaxial cylinders one of which was rotated. T h e results for gelatine solutions are characteristic. As the angular velocity of the rotating cylinder increases the coefficient of viscosity grows less and tends to become constant for high velocities. At the smallest angular velocities the coefficient is two or three times as large as at high velocities. M a n y colloidal solutions of very different kinds have been studied and all show the same general c o m p o r t m e n t in respect to viscosity. To explain this m a r k e d divergence from the behavior of normal liquids Freundlich has suggested t h a t colloidal solutions possess not only viscosity b u t also rigidity. " I f this assumption is true, the liquid--to p u t the m a t t e r colloquially but picturesquely-will have to be stretched as well as sheared. It can be shown mathematically t h a t the result will be w h a t has been found experimentally, viz., t h a t the viscosity decreases with increasing shear." T h e author has discovered a colloidal solution that displays rigidity to a remarkable e x t e n t - " a very dilute, about one-quarter per cent. solution of a m m o n i u m oleate, a soap familiar as material for soap
Aug., 1928.]
CURRENT TOPICS.
267
b u b b l e s . " T h e space b e t w e e n t w o cylinders was occupied b y this solution. T h e p i v o t e d inner cylinder was given a spin. H a d it been s u r r o u n d e d b y ordinary liquid it would have slowed up and finally come to rest, b u t with t h e amm o n i u m oleate it came to rest and then s t a r t e d to r o t a t e in the opposite direction s o m e w h a t as if it h a d been rotating in c o n t a c t with a hollow cylinder of elastic rubber. T h e moduli of rigidity of several colloidal solutions h a v e been determined. All h a v e no rigidity a b o v e 4 °0 C. T h e viscosity of a m m o n i u m oleate is v e r y t e m p e r a m e n t a l changing c o n s t a n t l y and over long periods of time. F o r colloidal solutions the moduli of rigidity are m e a s u r e d in milligrams per sq. cm. instead of in grams as is the case for gelatine jellies. F u r t h e r m o r e the m o d u l u s increases with the age of the solution. F o r example in the case of a solution of mercury-sulphosalicylie acid its m o d u l u s was eight times as great after 23 d a y s as it was after 20 hours. In respect to the manifestation of rigidity the colloidal solutions examined showed m u c h diversity. One shows rigidity if prepared h o t b u t n o t if m a d e at room t e m p e r a t u r e . Some, when h e a t e d to the t e m p e r a t u r e at which rigidity disappears regain u p o n cooling t h a t p r o p e r t y as it existed w h e n the solution was old while others upon cooling b e h a v e as freshly m a d e solutions. T h e p h e n o m e n o n of double refraction occurs in m o s t colloidal solutions. G. F. S. A New Mass-Spectrograph and the Whole Number Rule. F. W. ASTOr,. (Proc. Roy. Soc., A 772.) The first mass-spectrograph, set up in the Cavendish Laboratory in I919, could separate lines derived from masses differing by about I in I3o. Its accuracy of measurement was I in I,OOOapproximately. "These capabilities sufficed to determine with fair certainty the isotopic constitution of over 5° elements, and to demonstrate that, with the exception of hydrogen, the masses of all atoms could be expressed as integers on the scale o = I6 to one or two parts in a thousand." It was not till I925 that this instrument which had rendered such iconoclastic service was dismantled. A need was felt for an instrument that would better separate the mass lines of the heavier elements and furnish more accurate information on divergences from integral mass numbers. A study of these divergences is of great value because they throw light on the structure of atomic nuclei. Costa in Paris, using an instrument with an accuracy of I in 3,000, in I925 published excellent measurements on this matter. Now