52 PHYSICAL CHEMISTRY IN MEDICINE. 



chromic acid, alcohol, or ether, when their optical recog- 

 nition is made much easier. Any one can at any time 

 prove to himself that a thin layer of gelatine when treated 

 with dilute chromic acid according to BUTSCHLI' s instruc- 

 tions becomes opaque and white, and shows under the 

 microscope a very regular, finely chambered structure 

 entirely identical with the picture found in true coagula- 

 tions of colloids. But we are supposed to deal here not 

 with a true coagulation, but, as BUTSCHLI expresses it 

 in a by no means clear and unequivocal way, with a 

 "kind of coagulation" through which a preformed struc- 

 ture becomes visible. 



That structures which have been produced through the 

 action of alcohol disappear when put into water, to 

 reappear in exactly the same form when subjected a 

 second time to the action of alcohol, does not argue at 

 all in favor of the primary nature of the structure. The 

 explanation of this phenomenon is easily found in the 

 well-known properties of the changes in state which 

 colloids suffer. The change which is brought about in 

 gelatine through alcohol is, in contrast to that produced 

 through chromic acid for example, simply reversible. 

 Since, however, changes in state take place only very 

 slowly in gels, these are, if at all, only gradually and 

 scarcely completely reversible. It is readily intelligible, 

 therefore, why under these circumstances structures 

 which have once been produced and which are not 

 entirely destroyed even through remelting of the gelatine 

 reappear in their old form. 



That a fluid condition of the colloid is necessary for 

 the production of coagulation structures, as BUTSCHLI 

 believes, and that these structures must be present 



