94 PHYSIOLOGICAL CHEMISTRY. 



protein substances with water under pressure. But the names, 

 in practice, are usually restricted to the products of enzymic 

 formation. 



Of the exact nature of the reactions by which these sub- 

 stances are reached little is known. They represent the very 

 last stages in the process of breaking down complex native pro- 

 tein bodies which still give the characteristic protein tests. 

 Further disintegration leads to bodies which are no longer pro- 

 teins, but which, like the amino acids, are simply constituent 

 groups of the complex protein molecule. The peptone sub- 

 stances represent a more advanced stage of modification than 

 do the albumoses. In both groups of bodies we find the reac- 

 tions with the alkaloid reagents and with the precipitating 

 metallic solutions in most cases still marked; the biuret reac- 

 tion is also still present. But for the peptones we find lacking 

 the. property on which the salting out processes depend. By 

 adding plenty of ammonium sulphate or zinc sulphate it is 

 possible to throw the albumoses out of solution ; the peptones 

 do not respond to this treatment and in other points also they 

 are further removed from the original proteins than are the 

 albumoses. 



But it must not be understood that the distinction between 

 the two groups is perfectly simple and clear. Unfortunately 

 much confusion still prevails in the literature of the subject 

 and an elementary presentation which is satisfactory and con- 

 sistent is not yet possible. In this chapter only a brief outline 

 of the relations now generally accepted among chemists and 

 physiologists will be attempted, while in a following chapter 

 on digestion some of the more practical details will receive 

 consideration. 



Basis of Classification. The general classification of these 

 substances commonly recognized is that of Kiihne, which was 

 elaborated mainly in conjunction with Chittenden. The 

 scheme has been enlarged and modified somewhat by other 

 workers but in its important features the ideas of Kiihne still 

 hold the first place. In the weak acid as well as in the enzymic 



