394 PEPTONE. [BOOK n. 



alcohol for a long time without undergoing change, whereas other 

 proteids are more or less slowly coagulated by alcohol. A useful 

 test for peptone is furnished by the fact that a solution of peptone, 

 mixed with a strong solution of caustic potash, gives on addition 

 of a mere trace of cupric sulphate in the cold a pink colour, 

 whereas other proteids give a violet colour. In applying however 

 this test, which is often spoken of as the " biuret " test, care must 

 be taken not to add too much cupric sulphate, since in that case a 

 violet colour, deepening on boiling, that is the ordinary proteid 

 reaction (see 15), is obtained. 



There are reasons for thinking that there are several kinds 

 or at least more than one kind of peptone ; but we may for the 

 present regard the substance as one. 



The substance which is thrown down by ammonium sulphate 

 is called albumose. It resembles peptone in being soluble in 

 water (though not quite so soluble as peptone), in its solutions 

 not being coagulated by heat and in giving the pink, " biuret " 

 reaction with caustic potash and cupric sulphate. It differs in 

 being much less diffusible than peptone. We have reason to 

 think that there is more than one kind of albumose ; but we may 

 for the present speak of it as one body. The amount of albumose 

 appearing in a digestion experiment, relative to the amount of 

 true peptone, depends on the activity of the juice, and other 

 circumstances. We may regard albumose as a stage of gastric diges- 

 tion just short of the final stage of peptone ; indeed by the further 

 action of gastric juice albumose may be converted into peptone. 

 For a long time albumose was confounded with peptone, and many 

 of the commercial forms of " peptone " consist largely of albumose. 



Milk when treated with gastric juice is first of all " curdled." 

 This is the result partly of the action of the free acid but chiefly 

 of the special action of a particular constituent of gastric juice of 

 which we shall speak hereafter. The curd consists of a particular 

 proteid matter mixed with fat ; and this proteid matter is sub- 

 sequently dissolved, being converted into peptone, with the same 

 appearance of albumose and other by-products as in the case of 

 other proteids. 



We may say then that in the case of all proteids the effect of 

 the action of the gastric juice is to change the less soluble proteid 

 into a more soluble form, the change being either completed up to 

 the stage of peptone, the most soluble of all proteids, or being left 

 in part incomplete. This will be seen from the following tabular 

 arrangement of proteids according to their solubilities. 



Soluble in distilled water. 



Aqueous solutions not coagulated on boiling. 



Diffusible Peptone. 



Diffusible with difficulty . . . Albumose. 



Aqueous solutions coagulated on boiling . Albumin. 





