THE CHEMISTRY AND PHYSICS OF THE CELL 51 



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. Agar is another 

 familiar example of this heat-reversible type. Within the 

 cell, so far as we know, occur only the first type, the proteids 

 that form non-reversible coagula. 



An extensive study of the physical structure of the colloids 

 has been made by Hardy. 1 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 typical 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 occurred 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. y a jelly of gelatin 

 appears structureless, 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 upon the fixing agent used, the temperature, and the 

 presence or absence of extraneous substances. In general, how- 

 ever, the figures obtained in the solidification of proteid solu- 

 tions 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 net- 

 work structure with spherical masses at the nodal points, or 

 minute vesicles hollowed 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 

 proteid such as we often see in histological preparations of 

 edematous cells and tissues. All these forms seem to depend 

 chiefly upon 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 

 characteristic formations which recall at once the structures 

 found in the protoplasm of hardened cells. Moreover, the use 

 of different fixing agents, such as osmic acid, formalin, and 

 bichloride of mercury, produce just the same differences in the 

 1 Journal of Physiology, 1899 (24), 158. 



