42 THE CHEMISTRY AXD PHYSICS OF THE CELL 



solutions are by far the more important in l)iological considerations, 

 since the colloidal suspensions are usuall}" prepared artifieiaUy and 

 seldom occur in nature, e. g., Bredig's colloidal suspensions of the 

 noble metals. 



The colloidal solutions of proteins, which constitute the chief 

 part of every cell, are of two types — 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, how- 

 ever, becomes more fluid when heated, and when cooled, it forms 

 a gel which is readily reversible to the soluble form under the influ- 

 ence of heat. Agar is another familiar example of this heat-reversible 

 type. Within the cell, so 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 h\ Hardy that in undergoing this solidification there oc- 

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

 adhering to form a framework holding the liquid within its intei^tices. 

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

 ble by hardening agents, etc. ; e. g., a jelly of gelatin appears struc- 

 tureless, 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 protein 

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

 hear a striking reseniblance to the finer structures of protoplasm as 

 described by cytologists. There is produced an open network struc- 

 ture with spherical masses at the nodal points, or minute vesicles hol- 

 lowed out in a solid mass, or a honeycomb 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 cliiefly u])on the concentration of the colloid. Tlie 

 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- 

 teristic formations wliicli recall at once the structures found in the 

 protoplasm of hai-deiied cells. IMoreover, the use of different fixing 



23.|,,nriial nf riiysiolo^ry, ISOO (2-1). loS. 



