CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 393 



remarkable rapidity converted into acid-albumin. Thus simple 

 dilute hydrochloric acid of the same degree of acidity as gastric 

 juice merely converts these proteids into acid-albumin, the rapidity 

 of the change differing with the different proteids. This at least is 

 the broad result, though by prolonged action of the acid, especially 

 at high temperatures, other changes may be brought about. 



If the same white of egg or serum-albumin or other proteid in 

 solution be treated with gastric juice instead of simple dilute 

 hydrochloric acid, it will be found, when the action is completed, 

 that the proteid is not simply converted into acid-albumin but 

 has undergone a more profound change ; it is now neither coagu- 

 lated by heat nor precipitated on neutralisation ; it has become 

 the same extremely soluble form of proteid which results from 

 the solution of solid proteids by gastric juice. Thus gastric juice 

 converts all proteids, whether solid or in solution, into a form of 

 proteid soluble by itself in water, without the addition of acids or 

 alkalis, the solution not being coagulated by heat. 



Such is the result when the action is complete and the gastric 

 juice is adequately potent. At the beginning of the action 

 neutralisation will throw down a certain quantity of what appears 

 to be ordinary acid-albumin, but this in time disappears. Later 

 on, in many cases at all events, neutralisation, or the cautious 

 addition of alkali, will throw down a precipitate which has certain 

 resemblances to ordinary acid-albumin, but which differs from it 

 and has been called parapeptone. Indeed we have reason to 

 believe that the action of the gastric juice is a complex one and 

 that various intermediate or by-products, of which parapeptone is 

 one, may make their appearance according to circumstances. But 

 these we may leave on one side for the present; we may here 

 consider the main product of the action of gastric juice to be the 

 soluble form of proteid spoken of above. 



203. We find however upon examination that this main 

 product, this soluble proteid matter, consists of at- least two dis- 

 tinct proteid bodies. If the solution be saturated with neutral 

 ammonium sulphate, part of the proteid matter is precipitated 

 while part is still left in solution. The body which is not thrown 

 down by ammonium sulphate is called peptone. In addition to 

 being soluble in water, and to its solutions not being coagulated 

 by heat, it has the character of being diffusible, for it will pass 

 through membranes. The diffusion is not nearly so rapid as that 

 of salts, sugar, and other similar substances ; indeed solutions 

 of peptones may be freed from salts by dialysis. But it is very 

 marked as compared with that of other proteids ; these pass 

 through membranes with the greatest difficulty, if at all. Peptone 

 is insoluble in alcohol, and may be precipitated from its solutions 

 by the addition of an* adequate .quantity of this reagent ; but for 

 this purpose a very large excess of alcohol is needed, otherwise 

 much of the peptone remains in solution. It may be kept under 



F. ii. 26 



