90 PHYSIOLOGICAL CHEMISTRY. 



thrown out of solution by the addition of a sufficient quantity of 

 a mineral acid, whereas a weak acidity produces a slight precipitate 

 which dissolves upon agitating the solution. Metallic salts also 

 possess the property of precipitating albumins, some of the precipi- 

 tates being soluble in excess of the reagent whereas others are in- 

 soluble in such an excess. Of those proteins which occur native 

 the albumins contain the highest percentage of sulphur, ranging 

 from 1.6 to 2.5 per cent. Some albumins have been obtained in 

 crystalline form, notably egg albumin, serum albumin and lactal- 

 bumin but the fact that they may be obtained in crystalline form 

 does not necessarily prove them to be chemical individuals. 



GENERAL COLOR REACTIONS OF PROTEINS. 



These color reactions are due to a reaction between some one or 

 more of the constituent radicals or groups of the complex protein 

 molecule and the chemical reagent or reagents used in any given 

 test. Not all proteins contain the same groups and for this reason 

 the various color tests will yield reactions varying in intensity of 

 color according to the nature of the groups contained in the par- 

 ticular protein under examination. Various substances not pro- 

 teins respond to certain of these color reactions and it is therefore 

 essential to submit the material under examination to several tests 

 before concluding definitely regarding its nature. 



TECHNIQUE OF THE COLOR REACTIONS. 



i. Millon's Reaction. To 5 c.c. of a dilute solution of egg 

 albumin in a test-tube add a few drops of Millon's reagent. A 

 white precipitate forms which turns red when heated. This test 

 is a particularly satisfactory one for use on solid proteins, in 

 which case the reagent is added directly to the solid substance and 

 heat applied, which causes the substance to assume a red color. 

 Such proteins as are not precipitated by mineral acids, for example 

 certain of the proteoses and peptones, yield a red solution instead 

 of a red precipitate. 



The reaction is due to the presence of the hydroxy-phenyl group, 

 C 6 H 4 OH, in the protein molecule and certain non-proteins such 

 as tyrosine, phenol (carbolic acid) and thymol also respond to 

 the reaction. Inasmuch as the tyrosine grouping is the only hy- 

 droxy-phenyl grouping which has definitely been proven to be 



