CONSISTENCY OF COLLOIDS 67 



approaches its original gelatinous state; such gels are called inelastic 

 gels. One of the most important operations of microscopical technic 

 is the hardening of elastic gels. (See Chapter XXIII.) Their ability 

 to swell in water is destroyed by the chemical action of formalin, 

 chromic acid, mercuric chlorid, etc. We must consider that in the 

 organism, the inelastic gels, i.e., connective tissue and also the cell 

 pellicle, etc., arise from elastic gels by chemical changes with con- 

 sequent loss of water or drying, as may be observed at the surface of 

 any wound. In this connection, we may revert to the formation 

 of surface films (see p. 33), whose formation is certainly more than 

 merely analogous to that of organized membranes, skin, etc. 



Classical studies on the swelling and shrinking of slightly elastic 

 gels were made by J. M. VAN BEMMELEN in the case of silicic acid 

 gel and amplified by O. BUTSCHLI. So many difficulties are unfor- 

 tunately offered to the application by analogy of these properties 

 to organized inelastic gels, that we must confine our attention to 

 the most important ones. The evaporation of water from a silicic 

 acid gel proceeds at first as it would from a solution. When the gel 

 reaches a certain consistency a turbidity appears, that is, hollow 

 spaces of about 5 /*/* form between the supporting walls of the gel 

 which become filled with air. Upon losing still more water, the tur- 

 bidity disappears and the gel becomes glassy. In this latter respect 

 the inelastic gel of silicic acid differs very materially from the elastic 

 gel of gelatin, which does not become turbid. This is likewise the 

 case in the reabsorption of water. Though gelatin shows a similar 

 curve both on swelling and shrinking, silicic acid gel and indeed we 

 may say all inelastic gels show entirely different curves. That is, the 

 swelling of elastic gels is practically completely reversible, whereas 

 with inelastic gels this is not the case. 



The changes a gel undergoes on freezing and thawing are very 

 similar to those of shrinking and swelling. The crystallization of ice 

 from a gel containing water indicates a withdrawal of water, whereas 

 upon thawing, water becomes again available for swelling (H. W. 

 FISCHER, 0. BOBERTAG and C. FEIST*). There are consequently 

 substances which after freezing and thawing revert almost completely 

 to their original state, e.g., soluble starches, fish glue, whereas others, 

 e.g., silicic acid hydrosol and albumin, undergo changes which are 

 more or less irreversible. 



The influence of electrolytes on the swelling of gelatin, agar, pig's 

 bladder, cartilage and fibrin, is very considerable. It has been in- 

 vestigated especially by F. HOFMEISTER,* Wo. PAULI,** K. SPIRO,* 

 Wo. OsTWALD,* 2 and MARTIN H. FISCHER.* It may, in general, be 

 stated that acids and alkalis increase the swelling capacity to an 



