546 THE RELATION OF COLLOIDS AND LIPOIDS TO IMMUNITY 



solution it is structureless; but such solutions may become solid as the 

 result of changes of temperature and other physical means and from 

 admixture with certain chemical fixing agents. The structure of the 

 coagula varies according to the concentration of the colloidal solution 

 and the nature of the coagulant, but in general the figures obtained in 

 the solidification of protein solutions by such fixing agents as mercury 

 bichlorid and formalin bear a striking resemblance to the finer structure 

 of protoplasm as described by cytologists. These facts, no doubt, have 

 an important bearing upon the various "foam," "reticular," and 

 "pseudo-alveolar" structures of the protoplasm of cells described by 

 Biitschli, Fromann, Arnold, Reinke, and others, and may indicate the 

 effect of fixatives upon colloid solutions, explaining the usual time-worn 

 objections to theories of protoplasmic structure as based upon arti- 

 ficial conditions not present in the normal living cell, and variously 

 interpreted according to the fixative employed. 



Studies in the size of colloidal particles have been greatly facilitated 

 by dark field illumination of the microscopic field or the so-called ul- 

 tramicroscope devised by Siedentopf and Zsigmondy. Various other 

 methods have been devised for studying the size of colloidal particles, 

 as ultrafiltration by Bechhod 1 ; the weight of a dispersed substance in a 

 given volume by chemical or other analysis; by studies in the density 

 of the dispersed substance and other methods. These studies have 

 indicated that colloidal particles are usually round or at times ovoid, as 

 indicating beginning crystallization; that the main difference between 

 suspensions and colloidal solutions is one of dispersion and not relative 

 size of particles, and that in a true colloidal solution particles of widely 

 differing sizes may be found side by side. 



9. Colloids may be precipitated by electrolytes of opposite sign, as well 

 as by colloids. In a colloidal solution surface tension constantly tends 

 to make the particles of colloid approach one another, so that the surface 

 may become as small as possible, and in this manner brings about pre- 

 cipitation or coagulation. 



In a stable solution this action is counterbalanced by a force of 

 electric repulsion. Pure colloids do not carry an electric charge and 

 are not conveyed by an electric current; their apparent charge depends 

 upon the nature of electrolytes that may be present. Traces of acid 

 and of acid salts give it a positive charge, whereas alkalis and alkaline 

 salts do the opposite (Pauli). 



The process of coagulation of proteins, therefore, must depend upon 



1 Ztsch. f . Chem. Ind. Koll., 1907, 2, 3. 



