PROTEIN COMPOUNDS. 485 



either spontaneously or by the action of heat or acids. 

 In the soluble form they are contained in plants and 

 animal fluids, and can, for the greater part, be obtained 

 by evaporating at a temperature below 50. It is, 

 however, exceedingly difficult and scarcely possible to 

 thoroughly purify them of all foreign substances. In this 

 condition they form translucent masses, similar to gum 

 Arabic; inodorous and tasteless; soluble in water, in- 

 soluble in alcohol and ether. In the insoluble, coagu- 

 lated condition they are white, amorphous, principally 

 flocculent or clotted masses ; insoluble in ordinary 

 solvents. A few of them are soluble in dilute mineral 

 acids. Concentrated acetic acid dissolves them all 

 with the aid of heat, some rapidly, others slowly. 

 Dilute potassa also dissolves them all after a time, 

 when kept at a temperature of 60, forming potassium 

 sulphydrate and other decomposition-products. In a 

 weak solution in acetic acid, potassium ferrocyanide 

 or ferricyanide, and potassium platinocyanide give 

 white amorphous precipitates. When gently heated 

 with a solution of mercury nitrate (containing nitrous 

 acid),* they turn beautiful red with a slight tinge of 

 violet. Hydrochloric acid dissolves them all with the 

 aid of heat, and this solution, when boiled for a long 

 time, becomes a beautiful and very deep violet. 

 When a" solution of sugar and concentrated sulphuric 

 acid is carefully poured upon them, they turn first red, 

 then dark violet, the colors being the more beautiful, 

 the more freely the air has access. 



Decompositions. At a high temperature they are 

 decomposed, yielding ammonium carbonate and nume- 

 rous other products. When kept in a moist condition 

 they easily undergo putrefaction and yield ammonia, 

 ammonium sulphide, acids of the acetic acid series, 

 leucine (p. 98), tyrosin (p. 350), and other bodies, with 

 which we are only very imperfectly acquainted. Leu- 

 cine and tyrosin are also produced from them when 



* Milton's reagent. Prepared by dissolving 1 part mercury in 1 part 

 concentrated nitric acid, and diluting the solution with double the 

 volume of water. 



41* 



