70 PROTOPLASMIC ACTION AND NERVOUS ACTION 



The subject of the physical chemistry of emulsions 

 forms a part of the now extensively developed field of 

 colloid chemistry, and cannot be considered here in 

 any detail. Some of the more general facts relating to 

 the structure and properties of emulsions must, however, 

 be discussed briefly, since a clear conception of the physi- 

 cal conditions existing in these systems is necessary 

 before proceeding to the consideration of the more 

 complex types of structure and behavior which have 

 evolved in living matter, apparently with emulsion- 

 systems of a relatively simple kind as a basis. 



EMULSIONS'^ 



Emulsions and foam structures are essentially similar 

 systems, with the difference (as usually defined) that in a 

 foam the disperse or discontinuous phase is a gas, in an 

 emulsion a liquid. Jellies or gels also resemble these 

 systems in constitution in many cases. It is now known 

 that various different types of gel structure exist; 

 many jellies, however, are essentially dense emulsions; 

 thus stiff* foams of air with a soap solution (or solutions 

 of albumin, saponin, or other surface-active colloidal 

 substances) have many of the characters of solids or 

 semi- solids; i.e., permanence of form, elasticity, high 

 viscosity; and all transitions between liquid and solid 

 systems of the emulsion type are known — a fact familiar 

 to anyone who makes a lather of soap solution. The 

 same is true of an emulsion of one liquid in another; 

 thus an emulsion of oil in a soap solution may be made 



^For a general review cf. Bancroft's series of articles on '"The 

 Theory of Emulsification " in Journal of Physical Chemistry, XVI-XIX 

 (191 2-15); also the recent book of Clayton, The Theory of Emulsions 

 and Emulsification. 



