I.] THE PROTEIDS. 9 



peptone, and much albumose. ]|)issolve some of this body in warm 

 water, or preferably in lo per cent, sodium chloride. 



(a.) They are soluble in water; not coagulated by heat; and 

 are precipitated by saturation with neutral ammonium sulphate. 

 The precipitate with (NH^)^S04 partly disappears on heating, and 

 reappears on cooUng. They are precipitated but not coagulated 

 by alcohol. 



(b.) Add nitric acid = a white precipitate which dissolves witli 

 heat (yellow fluid) and reappears on coohng. Run tap water on 

 the tube, the precipitate reappears. This is a characteristic re- 

 action, and occurs best in the presence of NaCl. 



(c.) It, like peptone, gives a rosy -pink with Piotrowski's test. 



{d.) It is precipitated by acetic acid and ferrocyanide of potas- 

 sium, but the precipitate disappears on heating, and reappears on 

 coohng. 



(e.) It is precipitated by acetic acid and saturation with 

 NaCl. The precipitate disappears on heating, and reappears on 

 cooling. 



10. VI. Peptones are hydrated proteids, and are usually produced 

 by the action of proteolytic ferments on proteids. They are exceed- 

 ingly soluble in water. Their solutions are not precipitated by 

 sodic chloride, acids, or alkalies, nor are they coagulated by heat. 

 They are precipitated by tannic acid, and with difficulty by a large 

 excess of absolute alcohol. !N"ot precipitated by (NH^)^SO^. 



Preparation (see " Digestion "). — For applying the tests dissolve 

 a small quantity of Darby's fluid meat or commercial peptone in 

 warm water. Commercial peptone contains only a small amount 

 of peptone, and much albumose. 



{a.) Boil a portion ; it is not coagrdated. 



{h.) Xanthoproteic Reaction. — Add nitric acid, and boil = a 

 faint yellowish colour, and rarely any previous precipitate ; cool, 

 and add ammonia = orange colour. 



(c.) Acidify strongly with acetic acid, and add ferrocyanide of 

 potassium = no precipitate. 



{(1.) Test separate portions with tannic acid; potassio-mercuric 

 iodide ; mercuric chloride ; picric acid (saturated solution) ; and 

 lead acetate. Each of these causes a precipitate. In the case of 

 picric acid the precipitate disappears on heating, and reappears on 

 cooling. 



(e.) Biuret Reaction. — Add excess of caustic sodn, and then a 

 few drops of very dilute solution of copper sulphate = a rose colour ; 

 on adding more copper sulphate, it changes to a violet. 



(/.) Add a drop or two of Fehling's solution = a rose colour; add 

 more I^ehHng's solution it changes to violet. 



{(/.) Neutralise another portion = no precipitate. 



