Chap, i.] TISSUES AND MECHANISMS OE DIGESTION. 325 



found, on filtering off' the neutralisation precipitate, that the fil- 

 trate, as shewn on employing the various tests for proteid (see 

 8 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. 



§ 181. 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 neutrali- 

 sation precipitate has been filtered off, remains quite clear when 

 boiled. The only other solutions of proteids which do not coagu- 

 late 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 neutral ammonium sulphate be added to 

 saturation, part of the proteid matter is precipitated while part 

 is still left in solution. The proteid body thus thrown down is 

 called albumose. The body which is not thrown down by ammo- 

 nium sulphate is called peptone. Now peptone is characterised 

 by being diffusible; it will pass through membranes. The diffu- 

 sion is not nearly so rapid as that of salts, sugar, and other simi- 

 lar 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 w T ith the great- 

 est 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 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 sul- 

 phate in the cold a pink colour, whereas other proteids give a 

 violet colour. In applying this test, known as the ' biuret ' test, 

 however, care must be taken not to add too much cupric sulphate 



