I.] THE PROTEIDS. g 



peptone, and much albumose. Dissolve some of this body in warm 

 water, or preferably in 10 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 4 ) 2 S0 4 partly disappears on heating, and 

 reappears on cooling. They are precipitated but not coagulated 

 by alcohol. 



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

 heat (yellow fluid) and reappears on cooling. Eun 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 

 cooling. 



(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 proteoly tic 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. Not precipitated by (NH 4 ) 2 S0 4 . 



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 coagulated. 



(b.) 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. 



(d.) 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 soda, 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 Feh ling's solution = a rose colour ; add 

 more Fehling's solution it changes to violet. 



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



