146 CHEMICAL STATICS 



Hence, on adding free picric, molybdic, tungstic, phospho- 

 tungstic, tannic, stearic or chromic acids to a neutral solution of 

 protein a precipitate or coagulum is, as a rule, formed immedi- 

 ately. But if these acids be added in the form of their (neutral) 

 salts then no precipitate results until the reaction of the solution 

 is rendered acid, when the protein at once combines with the 

 acid radical of the salt. Similarly, neutral lead acetate will 

 not, as a rule, precipitate protein (egg-albumin) from neutral 

 solutions, but it will precipitate it readily from faintly alkaline 

 solutions (77) (solutions, that is,, in which the protein is in the 

 form of an ionized compound with a base). 



Upon these facts probably depend certain phenomena which 

 are encountered in the tanning of leather. As is well known, 

 chromium unites firmly with the hide-substance only in the 

 form of its cation Cr +++ , and not when it is united with oxygen 

 to form an anion as in chromic acid. Now it is observed that 

 firm union only occurs when the chromium salt is "basic" (such 

 as Cr 2 Cl 3 (OH) 3 ), i.e., when the salt is neutral in reaction or even, 

 at the moment of union, faintly alkaline (97). Further addition 

 of alkali results in a displacement of the chromium by the added 

 base. On the other hand, tannic acid unites with the hide sub- 

 stance in a faintly acid medium. 



Similar principles apply to the union of dyes with proteins (77) 

 (39). Proteins which are about equally acid and basic in charac- 

 ter, such as gelatin and egg-albumin, combine with basic dyes only 

 in faintly alkaline solutions. Predominantly acid proteins, such 

 as casein or the nucleins, will combine with basic dyes even in 

 neutral or faintly acid solutions (80) since (ionized) salts of these 

 proteins with bases can exist even in solutions acid to litmus 

 (Cf . Chap. V) ; on the other hand, they combine with acid dyes 

 with great difficulty. Lilienfeld has shown that so long as nucleic 

 acid is not saturated with albumins to form neutral nuclein an 

 insoluble compound with methyl green is formed when this dye 

 is added to a solution of the nuclein, but after saturation with 

 protein the nucleins show a greater affinity for acid dyes (71). 



An interesting compound of azolitmin and mucoid has been 

 obtained by Rosenbloom and Gies (109). I have found that in 

 faintly alkaline solution trypsin, or some constituent of Gruebler ; s 

 trypsin (nach Spateholz), forms an insoluble compound with 

 safranin (99) (44). 



