98 THE PROTEIN SUBSTANCES. 



4. Precipitation by Metallic Salts such as copper sulphate, ferric 

 chloride, neutral and basic lead acetate (in small amounts), mercuric 

 chloride and others. On this is based the use of proteids as antidotes 

 in poisoning with metallic salts. 



5. Precipitation by Mineral Acids at Ordinary Temperatures. The 

 proteids are precipitated by the three ordinary mineral acids in proper 

 amounts, but not by orthophosphoric acid. If nitric acid be placed in a 

 test-tube and the proteid solution be allowed to flow gently thereon, a 

 white opaque ring of precipitated proteid will form where the two 

 liquids meet (HELLER'S albumin test). 



6. Pr-ecipitation by the so-called Alkaloid Reagents. To these belong 

 the precipitation by metaphosphoric acid and by hydroferrocyanic acid, 

 which is carried out by the aid of potassium ferrocyanide in a 

 liquid containing acetic acid; precipitation by phosphotungstic acid or 

 phosphomolybdic acid in the presence of free mineral acids; precipita- 

 tion by potassium-mercuric iodide or potassium-bismuth iodide in solu- 

 tions acidified with hydrochloric acid; precipitation by tannic acid in 

 acetic acid solutions. The absence of neutral salts or the presence of 

 free mineral acids may prevent the appearance of the precipitate, but 

 after the addition of a sufficient quantity of sodium acetate the precipitate 

 will in both cases appear; precipitation by picric acid in solutions acid- 

 i'fied by organic acids. Proteids are also precipitated by trichloracetic 

 acid in 2-5 per cent solutions, by phenol, salicyl sulphonic acid, nucleic acid, 

 taurocholic acid and by chondroitin sulphuric acid in acid solutions. 



Color Reactions for Proteid Bodies. 



1. Millon's Reaction. 1 A solution of mercury in nitric acid contain- 

 ing some nitrous acid gives a precipitate with proteid solutions which 

 at the ordinary temperature is slowly, but at the boiling-point more 

 quickly, colored red; and the solution may also be colored a feeble or 

 bright red. Solid albuminous bodies, when treated by this reagent, 

 give the same coloration. This reaction, which depends on the presence 

 of the aromatic group in the proteid, is also given by tyrosine and other 

 monohydroxyl benzene derivatives. According to O. NASSE 2 it is best 

 to use a solution of mercuric acetate which is treated with a few drops 

 of a 1 per cent solution of potassium or sodium nitrite; previous to use 



1 The reagent is prepared in the following way: 1 pt. mercury is dissolved in 2 pts. 

 nitric acid (of sp.gr. 1.42), first cold and then warmed. After complete solution of 

 the mercury add 1 volume of the solution to 2 volumes of water. Allow this to stand 

 a few hours and decant the supernatant liquid. 



2 See O. Nasse, Sitzungsb. d. Naturforsch. Gesellsch. zu Halle, 1879, and Pfliiger's 

 Arch., 83; see also Vaubel and Blum, Journ. f. prakt. Chem. (N. F.), 57. 



