106 GENERAL PHYSIOLOGY 



that they arise by the hydrolytic cleavage of the original proteid 

 molecule, so that the peptones represent the hydrates of the 

 original proteids. Important conclusions follow from this fact. 

 Since the proteid molecule, which was originally not diffusible 

 on account of its enormous size, is split up in the peptonising 

 process into the peptone molecules, which are much smaller and 

 therefore diffusible, but which have the chemical characteristics of 

 proteids, it follows that the proteid molecule is not simple but 

 polymeric, i.e., it consists of a chain-like combination of many similar 

 groups of atoms. In the transition to the peptone condition the 

 proteid molecule is broken up with hydration into these single, 

 similar atomic groups, all of which, however, have the chemical 

 characteristics of proteids, but represent much smaller molecules. 

 The inability of proteids to diffuse through membranes depends, 

 therefore, solely upon their polymerism. Wholly analogous cases 

 occur in inorganic nature ; e.g., certain forms of silicic acid are 

 unable to diffuse through membranes because of their polymerism. 

 Hence it is evident that no fundamental difference exists between 

 solutions of simple molecules, as found in peptones, and those of 

 polymeric molecules, as in ordinary albumin. 



A further physical property, which is perhaps connected with 

 the polymerism of the ordinary proteid molecule, and which belongs 

 to almost all proteids with the exception of their hydrates, the 

 peptones, is their capacity of clotting, or coagulating. Coagula- 

 tion consists in the passing of the substance from the dissolved to 

 the solid condition within the solvent medium. Boiling is a method 

 that causes coagulation in almost all proteids. In the fresh hen's 

 egg the proteid is present in a thick clear viscous solution. In 

 the boiled egg it has become a solid white opaque mass ; it is coagu- 

 lated. By boiling, proteid can be separated out of thin solutions in 

 the form of fine curdled flakes. Other methods, such as the use 

 of inorganic acids and alcohol, also cause proteid in solution to 

 be coagulated and precipitated, the result being indicated by a 

 clouding of the liquid. That the power of coagulation is in some 

 way connected with polymerism is indicated by the fact that in- 

 organic polymeric molecules, such as the above-mentioned silicic 

 acid, in aqueous solution can likewise be coagulated into a jelly. 

 If, e.g., hydrochloric acid be added to a solution of sodium silicate, 

 free silicic acid and sodium chloride are produced ; the silicic acid 

 may then be separated from the salt by dialysis, since, in contrast 

 to the salt, the former, like a polymeric body possessing very large 

 molecules, does not diffuse through membranes. By the addition 

 of a few bubbles of carbonic acid this solution of silicic acid may 

 be changed at once into a coagulated jelly-like mass. Since our 

 knowledge of the chemical composition of proteids is at present 

 very incomplete, it is not easy to produce definite chemical reactions 

 with them. Nevertheless, a number of tests have been empirically 



