38 THE CHEMISTRY AND PHYSICS OF THE CELL 



As the former type resembles in many details the true solutions, while 

 the latter approaches more closely to the suspensions, it has been 

 proposed to distinguish them by the terms "colloidal solution" and 

 "colloidal suspension."-'^ Of the two types, the colloidal solutions 

 are by far the more important in biological considerations, since the 

 colloidal suspensions are usually prepared artificially and seldom occur 

 in nature, e. g., Bredig's colloidal suspensions of the noble metals. 



The colloidal solutions of proteins are of two tj^pes — one, such as 

 albumin, forms a coagulum when heated, which under ordinary 

 conditions is not reversible; that is, it does not again go into solution. 

 Gelatin, however, becomes more fluid when heated, and when cooled 

 it forms a gel which is readily reversible to the soluble form under the 

 influence of heat. Within the cell, as far as we know, occur only the 

 first type, the proteins that form non-reversible coagula. 



An extensive study of the physical structure of the colloids has 

 been made by Hardy. ^^ As long as the colloid is in solution it is 

 structureless, although, as before mentioned, the existence of free 

 solid particles can be demonstrated by certain optical methods. The 

 solution is homogeneous, and although perhaps viscid, still it is a typ- 

 ical solution. Such solutions can become solid, either by the effect of 

 temperature, of certain chemical fixing agents, or physical means. 

 It was found by Hardy that in undergoing this solidification there oc-' 

 curs a separation of the solid from the liquid, the solid particles 

 adhering to form a framework holding the liquid within its interstices. 

 Heat-reversible gels show no structure until they are made irreversible 

 by hardening agents, etc.; e. g., a jelly of gelatin appears structui'e'ess, 

 but when treated with formalin or other fixing agent, the structural 

 appearances described below appear. The figures formed by the 

 framework vary according to the nature and concentration of the 

 colloid and of the solvent, and also with the fixing agent used, the 

 temperature, and the presence or absence of extraneous substances. 

 In general, however, the figures obtained in the solidification of pro- 

 tein solutions by fixing agents, such as bichloride of mercury or 

 formalin, bear a striking resemblance to the finer structures of protoplasm 

 as described by cytologists. There is produced an open network 

 structure with spherical masses at the nodal points, or minute vesicles 

 hollowed out in a solid mass, or a honej'comb appearance, or, when the 

 concentration of the colloid is very slight, perhaps there is only a 

 precipitation of fine granules of protein such as we often see in histo- 

 logical preparations of edematous cells and tissues. All these forms 

 seem to depend chiefly uj^on the concentration of the colloid. The 

 important fact is that when the chemicals ordinarily used as fixatives 

 of cells for histological purposes act upon solutions of colloids that 

 are perfectly homogeneous, they produce very constant and charac- 



^* Noyes, American Cliemical Journal, 1905 (27), 85. 

 2» Journal of Physiology, 1899 (24), 158. 



