CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 369 



portion of the proteid originally present in the white of egg or 

 serum-albumin can thus be regained by precipitation. Though 

 the neutralisation be carried out with the greatest care it will be 

 found, on filtering off the neutralisation precipitate, that is the 

 acid-albumin, that the filtrate, as shewn on employing the various 

 tests for proteid (see 15) or on adding an adequate quantity of 

 strong alcohol, still contains a very considerable quantity of proteid 

 matter ; and, on the whole, the longer the digestion is carried on, 

 the greater is the proportion borne by the proteid remaining in 

 solution to the precipitate thrown down on neutralisation ; indeed, 

 in some cases at all events, all the proteid matter originally 

 present remains in solution, and there is no neutralisation 

 precipitation at all, or at most a wholly insignificant one. 



203. The proteid matter, thus remaining in solution after 

 neutralisation, differs from all the proteids which we have hitherto 

 studied in as much as, though existing in a neutral solution, it is not 

 coagulated by heat, like the egg-albumin or serum-albumin from 

 which it has been produced ; the solution, after the neutralisation 

 precipitate has been filtered off, remains quite clear when boiled. 

 The only other solutions of proteids which do not coagulate on 

 boiling are solutions of acid or alkali-albumin ; but these solutions 

 must be acid or alkali respectively ; the acid-albumin or alkali - 

 albumin is insoluble in a neutral solution, and when simply 

 suspended in water is readily coagulated at -a temperature of 75. 

 This new proteid matter of which we are speaking is soluble/in 

 neutral solutions, indeed in distilled water, and can under no 

 circumstances be coagulated by heat. 



Upon examination we find that the new proteid matter thus 

 left in solution consists of at least two distinct proteid bodies. If 

 to the solution ammonium sulphate be added, part of the proteid 

 matter is precipitated while part is still left in solution. The 

 proteid body thus thrown down is called albumose (there are 

 several varieties of albumose but these need not now detain us). It 

 approaches albumin in nature by reason of the fact that it will not 

 diffuse through membranes ; that it differs however widely from 

 that proteid is shewn by its solutions not coagulating on boiling. 

 The body which is not thrown down by ammonium sulphate is 

 called peptone ; it differs from albumose in 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 solu- 

 tions 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 

 alcohol for a long time without undergoing change, whereas other 



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