PROTEIDS. 9 



(c.) Bruise some of the solid boiled white of egg, diffuse 

 it in water, and test it with Millon's reagent. 



(d.) For the effect of the digestive juices see " Digestion." 



10. VI. Peptones are exceedingly 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 excess of absolute alcohol. 



Preparation (see "Digestion"). For applying the tests, dis- 

 solve a small quantity of Darby's Fluid Meat in water, and 

 filter, or dissolve some pure peptone in water. The latter can 

 be bought as a commercial product. 



(a.) Boil a portion, it is not coagulated. 



(b.) To another portion add strong nitric acid; and boil 

 = a faint yellowish colour ; allow it to cool, and add strong 

 ammonia = orange colour (Xanthoproteic Reaction). 



(c.) Acidify a third portion strongly with acetic acid, 

 and add ferrocyanide of potassium = no precipitate. 



(d.) Test separate portions with tannic acid, mercuric 

 chloride, picric acid, and lead acetate. Each of these causes 

 a precipitate. In the case of picric acid the precipitate 

 disappears on heating, and partly reappears on cooling. 



(e.) To another portion add a few drops of very dilute 

 solution of cupric -sulphate, and then caustic soda (or potash) 

 =a rose colour ; on adding more cupric sulphate, it changes 

 to a violet (Biuret Reaction). 



(f.) To another portion add a drop or two of Fehling's 

 solution = a rose colour ; on adding more Fehling's solution 

 it changes to violet (Biuret Reaction). 



(g.) Neutralise another portion = no precipitate. 



(h.) To another portion add an excess of absolute alcohol 

 = a precipitate of peptone, but not in a coagulated form. 



(i.) Precipitate a portion with ferric acetate. 



