PROTOPLASMIC STRUCTURE 75 



capable of holding one liquid finely dispersed in another 

 in a permanent state of emulsion. We may infer that in 

 at least some forms of protoplasmic emulsion-structure 

 the interfacial films are of molecular thickness, a con- 

 sideration of much importance in relation to the proper- 

 ties of irritability and transmissivity (or propagation of 

 excitation-states), as will be seen below. 



ADSORPTION 



It will be clear from the above that in the formation 

 of emulsions — and hence of living protoplasm as a system 

 based upon the emulsion type of structure — the con- 

 ditions determining the formation of interfacial films 

 are of primary importance. Adsorption, the process 

 by which material collects or concentrates at boundary 

 surfaces, is thus a fundamental factor in the formation 

 and behavior of emulsion systems and of colloidal sys- 

 tems in general. The physics and chemistry of adsorp- 

 tion processes have recently been discussed fully in 

 several excellent textbooks,^ so that it is unnecessary here 

 to give any detailed account. One general fact, however, 

 which may be emphasized as especially important from 

 the physiological point of view, is that adsorbed sub- 

 stances are typically more subject to chemical change 

 than substances uniformly distributed in a solution. 

 Both the increase of concentration and the presence of 

 surface factors are concerned in the increase of reactivity, 

 the catalytic action of many finely divided materials 

 (charcoal, platinum, etc.) is usually referred to the 

 increased concentration of the chemically altered material 



^ Hober, Physikalische Chemie der Zelle und der Gewehe (1914); 

 Freundlich, Kapillar chemie, Leipzig (1909); Bayliss, Principles of Gen- 

 eral Physiology; Bancroft, Applied Colloid Chemistry. 



