242 SCIENCE PROGRESS 



coagulation does not take place at the iso-electric point, when 

 the potential difference between particle and medium is nil, 

 as might have been expected, but there is a critical value of 

 potential difference, dbo'os volt, above which the emulsion 

 is stable. He has more recently shown ^ that this is also true 

 of colloidal AS2S3. If these results are of general application, 

 it is an observation of considerable importance. 



It has been found that the adsorption of oppositely charged 

 ions is specific, both for the ions and different colloids. The 

 present writer has, however, obtained an indication that for 

 the same colloid the coagulative power of the ions is propor- 

 tional to their atomic number, but this observation requires 

 verification. It is possible that the kinetic energy of the ion 

 plays a part in coagulation. 



A close contact between particles of the dispersoid phase 

 being effected, a large positive surface tension will cause 

 coagulation. It has, however, been noticed that coagulation 

 may be brought about in some cases by substances which lower 

 the interfacial tension. This would apparently tend to the 

 production of an opposite effect, i.e. passage of the dispersoid 

 into the molecular disperse state. This action has not been 

 discussed, and its meaning at present remains obscure. In 

 connection with the type of adsorption effected by coagula- 

 tive ions, it has been suggested that a loose chemical union takes 

 place. This hypothesis is based on the fact that the coagu- 

 lating electrolyte is very difficult to wash out of the coagulated 

 substance, and it has some experimental grounds for belief. 



Conclusion. — The chemistry of colloids is a comparatively 

 new branch of science, but, owing to its importance, a con- 

 siderable amount of work has been done upon it. In an article 

 of this length, it is impossible to touch upon all its points of 

 attack — the phenomena of peptisation, of viscosity changes, 

 and so on — but it is hoped that the more important theoretical 

 considerations have been presented as fully as space would 

 permit. As has been mentioned, the phenomena of coagula- 

 tion have provided us with our deepest insight into dispersoid 

 systems, and it is from this direction that our future knowledge 

 will probably come. 



1 Journ. Chem. Soc, vol. cix, 734, 1916. 



