32« 



NATURE 



[November 4, 1920 



developments. The ensuing discussion was divided 

 into me following tive sections: (i) "Emulsions and 

 iimulsincation," opened by i'rol. F. G. Uonnan; 

 (2) •• ihe l^nysical I'ropcrties ot Elastic Gels," openeU 

 by Mr. E. Hatscliek and froi. H. K. Procter; 

 l3) ••Glass and Pyrosols," opened by Sir Herbert 

 jackson; (4) ■.Non-aqueous Systems," opened by Sir 

 Kobert Robertson ; and (5) " i^recipitation in Disperse 

 Systems : Cataplioresis and Electro-endosmose," 

 opened by Prof. \. V\'. Porter. 



In section (i) the discussion centred mainly arourid 

 the important problem of the reversal of phases in 

 emulsions produced by electrolytes. The results of 

 experiments on soap emulsions carried out by Mr. 

 S. S. Bhatnagar at University College, London, 

 were given by the author, who concluded that 

 there was a strict parallel between the reversal 

 of phases in emulsions and the precipitation of sus- 

 pensions by electrolytes. Apart from the considerable 

 ftieoretical'interest attaching to the subject, the matter 

 is of practical importance in that it is closely asso- 

 ciated with the action of soaps as protective colloids, 

 to which property it appears probable their detergent 

 nature is due. The speakers were unanimous 

 in affirming the efficiency of the electrical method, 

 first suggested by Clayton (Brit. Assoc. Colloid 

 Reports, No. 2, p. 114, 1918), of ascertaining the 

 point at which phase reversal takes place. A 

 xnatter of considerable importance referred to by Prof. 

 W. C. McC. Lewis, and agreed to by other speakers, 

 was the necessity when studying the effects of electro- 

 lytes on colloids of employing solutions possessing 

 the same conductivity rather than those of identical 

 molar concentrations. 



The discussion on the physical properties of elastic 

 gels revealed how scanty is the present state of our 

 knowledge of the mechanism of gel formation and 

 the importance of further research in this direction. 

 Mr. Hatschek emphasised the importance of an ex- 

 tended study of the mechanical properties of gels, 

 which hitherto had been confined prectically to gelatin, 

 in which chemical complications may arise. A much 

 more promising field presented itself in non-aqueous 

 systems, e.g. vulcanised rubber in benzene. In an 

 investigation into the mechanical properties of gelatin 

 Mr. Hatschek had obtained the astonishing result that, 

 after straining a rectangular prism of 10 per cent, 

 gelatin gel for five days, not only had the stress prac- 

 tically disappeared, so that on removing the constraint 

 the strain remained, but the optical anisotropy 

 remained after removal of the stress. 



Prof. Procter, in advocating his well-known solid- 

 solution theory — which, he explained, differed from 

 the sponge-like structure theory mainly on the ques- 

 tion of size — pointed out that the difference was not 

 unimportant, since microscopic size of network ex- 

 cludes or complicates the simple chemical causes 

 which are sufficient for the solid-solution view. The 

 opposite view involves a mechanical structure which 

 itself demands explanation. Critics of the solid- 

 solution theory, however, found it difficult to believe 

 that a I per cent, agar gel is a solid solution, and 

 Prof. Procter admitted that the case of agar presented 

 great difficulties. 



Some very important work relating to gel structure 

 has been carried out in the laboratories of Prof. 

 McBain at Bristol on soap solutions, which in Prof. 

 McBain's opinion excludes the cellular-structure 

 theory. It has been shown in his laboratory that a 

 half-normal solution of sodium oleate could be made 

 to exist in any one of three forms : (a) Transparent 

 liquid (sol), W) a jelly (gel), and (c) a curd. The sol 

 . and gel are absolutely identical in every respect except 

 in mechanical properties. They display identity of 

 NO. 2662, VOL. 106] 



Na-ion content, refractive index, osmotic pressure, 

 and conductivity, so that it appears that the particles 

 present are identical in the sol and gel. On the 

 contrary, in the curd form some of the soap separates 

 out into fibrils and the conductivity disappears, this 

 process being analogous to crystallisation. Prof. 

 •McBain considered these results to bear out the view 

 of Zsigmondy and others that the particles in both 

 sol and gel exist in micellar form and are linked 

 together in some way analogous to the structure of 

 liquid crystals. An example of the extraordinary 

 character of the sol-gel transformation was brought 

 to notice by Prof. Svedberg. A non-aqueous gel was 

 formed of cadmium in alcohol having a cadmium 

 concentration of only 01 per cent., m which the 

 slightest vibration was sufficient to break down the 

 whole structure and change it to the sol state. 



Sir Herbert Jackson, in opening up the subject of 

 glass and pyrosols, expressed doubt, except perhaps 

 in some eases of colouring, as to whether glass came 

 within the domain of colloids. With good glasses 

 ordinarv methods of illumination failed to reveal the 

 Tyndall phenomenon, this being visible only with very 

 strong illumination. The figures obtained by etching 

 glasses he considered to be merely surface-tension 

 effects, and afforded no evidence whatever of the 

 colloidal nature of glass. In regard to the colouring 

 of glasses much research is needed into the condi- 

 tions under which various colours are produced. 

 Evidence was adduced by Sir Herbert which makes 

 it appear probable that the colouring substances have 

 specific effects, although pure diffraction effects 

 depending on the sizes of particles undoubtedly 

 exist. 



Non-aqueous colloid systems were dealt with under 

 three headings : (a) Nitro-cellulose, (b) Celluloid, and 

 fc) Rubber. Sir Robert Robertson dealt with the col- 

 loidal properties of nitro-cellulose gelatinised by 

 means of suitable solvents, which properties have an 

 important bearing on the manufacture of propellants. 

 L^seful relationships had been established between the 

 viscosities of solutions of cellulose, those of the result- 

 ing solutions of nitro-cellulose, and the mechanical 

 properties of the final dried material. By con- 

 trolling the viscosities of the solutions the required 

 mechanical properties of the resultant dried nitro- 

 cellulose mixtures were assured. In connection with 

 celluloid the discussion centred largely around the 

 solvent propertv of binary mixtures, such as ether- 

 alcohol, which is very different from that of the con- 

 stituents separately. The ether-alcohol complex 

 theorv originally put forward by Baker was largely 

 criticised. The discussion on the colloidal properties 

 of rubber was confined practically to a communica- 

 tion by Mr. B. D. Porritt, who, in describing the 

 effects of light on rubber, emphasised the important 

 part played in rubber deterioration by oxygen, both 

 as a catalyst and by direct chemical action. The 

 inclusion of a dye to absorb ultra-violet light helps 

 to prevent the deterioration. Experiments on the 

 sol-gel transformation produced in rubber solutions 

 bv light and oxygen were described. 



Perhaps the most important paper of the whole 

 discussion, in that it represented a distinct ad- 

 vance in theory, was that by Mr. J. N. Mukherjee 

 in section (5).' Starting with the view that the 

 charge on a "suspensoid particle is due to adsoqjtion, 

 arising from chemical forces, of the ion the_ particle 

 has in common with the peptising or stabilising elec- 

 trolyte. Mr. Mukheriee has deduced a relation between 

 the '' electrical adsorbabilitv "of the oppositely charged 

 ion of the precioitating electrolyte and its valency 

 and mobility. This theory not only results in the 

 same series of cations arranged in order of adsorb- 



