NATURE AND PROPERTIES OF COLLOIDS 513 



colloid fluids possess the force that strives to reduce its free surface to a 

 minimum. As partial expressions of this force, the formation of emul- 

 sions when oil and water are mixed and the ameboid movements of the 

 ameba and leukocytes may be mentioned as examples. 



6. Colloids do not separate freely into ions when dissolved, and accord- 

 ingly do not conduct electricity to an appreciable extent. When an electric 

 current is passed through a colloidal fluid, most of the colloids move 

 toward the anode; this phenomenon, known as cataphoresis, is also 

 generally exhibited by suspensions, and in this particular the colloids 

 resemble suspensions. 



7. Colloids are usually easily predpitable and coagulable, and this is 

 readily understood when the slender margin that exists between many of 

 the colloids and the suspensions is borne in mind. Relatively slight 

 changes, such as exposure, gentle heat, the presence of large quantities 

 of crystalloids, the action of enzymes, etc., may throw an organic colloid 

 out of solution, and when once precipitated, it is often incapable of again 

 dissolving in the same solvent. Colloids are also precipitated by many 

 electrolytes, apparently through the formation of true ion compounds. 



8. The physical structure of colloids. This subject has been studied 

 extensively by Hardy. 1 Cells contain but one type of colloids, the 

 proteins that form non-reversible coagula. So long as a colloid is in 

 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 

 Btitschli, 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. 



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



1 A good general outline of the subject of colloids may be found in Pauli's " Physi- 

 cal Chemistry in the Service of Medicine," 1907, translated by Fischer (Chapman 

 and Hall). 



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