CHAP. II.] REACTIONS OF PEPTONES. 139 



Reagents Peptones are precipitated by the following re- 



which precipi-. agents : tannic acid, mercuric chloride, mercuric and 

 tate peptones, mercurous nitrates, Millon's reagent, potassio-mercuric 

 iodide, and especially in feebly acid solutions by phospho-molybdic 

 acid and phospho-tungstic acid. These two reagents furnish us with 

 the means of separating peptones and have of late been much em- 

 ployed in research. 



Preparation of phospho-tungstic acid. This reagent, which is of special 

 value in the search for and separation of the peptones, is prepared as 

 follows. Commercial tungstate of soda is dissolved in boiling water and 

 phosphoric acid added to the solution until the mixture exhibits an acid 

 reaction. It is then allowed to cool, rendered strongly acid with hydro- 

 chloric acid, and filtered after standing for twenty-four hours (Huppert) *. 



Preparation of phospho-molybdic acid. This reagent is made by digest- 

 ing moist molybdic trioxide with solution of phosphoric acid. With a 

 small quantity of the acid a lemon-yellow salt insoluble in water is formed. 

 With a larger proportion of acid, the yellow salt dissolves on the application 

 of heat, yielding a colourless liquid which, on evaporation, leaves a 

 tenacious, non-crystalline, mass, very soluble in water and alcohol. 



The action of Millon's reagent serves, as Kiihne and Chittenden 

 have shewn, to distinguish ampho-peptones from antipeptone (as 

 obtained by the action of trypsin upon fibrin). When added to 

 ampho-peptones the reagent at first throws down an abundant white 

 precipitate which, on being warmed, assumes a beautiful red colour. 

 With antipeptone, it gives a white precipitate, changing on heating 

 to a dirty yellow or reddish colour. It is therefore obvious, that the 

 source of the red reaction with ampho-peptone is to be sought for 

 in the hemipeptone which forms a moiety of the ampho-peptone. To 

 this matter, reference will again be made. 



Colour re- 1. Solutions of peptones exhibit when boiled with 



actions of the Millon's solution the characteristic proteid reaction, 

 peptones. 2. Their most marked reaction is the so-called 



biuret reaction 2 . It is obtained by adding to a solution of peptone, 

 a drop or two of a weak solution of copper sulphate, and then a large 

 excess of sodium or potassium hydrate, when a fine, more or less deep, 

 rose colour is obtained, with no violet tint. If the quantity of copper 

 added is in excess, then a violet colour is produced. 



As has already been stated, the biuret reaction is common to 

 albumoses as well as peptones, though not given with equal intensity 

 by all. 



Peptones also exhibit the other colour reactions of solutions of the pro- 

 teids, viz. the xantho-proteic reaction and the so-called ' Adamkiewicz's 

 reaction.' 



Adamkiewicz's Reaction. Any solution of a proteid or of a peptone 

 when treated first of all with glacial acetic acid and then with concentrated 



1 Quoted by v. Jacksch, ' Clinical Diagnosis,' p. 219. 



2 So-called because it is also obtained in a very characteristic manner with biuret, 

 CoHgNgOo , a body obtained by the action of heat on urea. 



