734: APPENDIX. 



Physical Properties. Proteids are all amorphous and non-crystalliz- 

 able, so that they possess as a rule no power (or scarcely any) of passing 

 through animal membranes. They are soluble, but undergo alteration 

 in composition in strong acids and alkalies ; some are soluble in water, 

 others in neutral saline solutions, some in dilute acids and alkalies, few 

 in alcohol or ether. Their solutions exercise a left-handed action on 

 polarized light. 



Chemical Properties. Certain general reactions are given for pro- 

 teids. They are a little varied in each particular case: 



i. Xantho-Proteic Reaction. A solution boiled with strong 

 nitric acid, becomes yellow, and the color is darkened on addi- 

 tion of ammonia. 



ii. Biuret Reaction. With a trace of copper sulphate and an 

 excess of potassium or sodium hydrate they give a purple col- 

 oration. 



iii. Millon's Reaction. With Millon's reagent (a solution of me- 

 tallic mercury in strong nitric acid), they give a white or pink- 

 ish clotted precipitate, becoming more pink on boiling. 



iv. They are, with the exception of peptone, entirely precipitated 

 from their solutions by saturation with ammonium sulphate. 



Many of the proteids give, in addition, the following tests: 



v. With excess of acetic acid, and potassium ferro-cyanide, a white 



precipitate. 



vi. With excess of acetic acid and a saturated solution of sodium 



sulphate, on boiling, a white precipitate. This test is often 



used to get rid of all traces of proteids, except peptones, from 



solutions. 



vii. Boiled with strong hydrochloric acid, they give a violet red 



coloration, 

 viii. With cane sugar and strong sulphuric acid, on heating, they 



give a purplish coloration, 

 ix. They are precipitated on addition of 



Citric or acetic acid, and picric acid ; or, 

 Citric or acetic acid, and sodium tungstate ; or 

 Citric or acetic acid, and potassio-mecuric iodide. 



Varieties. Proteids are divided into seven classes, chiefly on the 

 basis of their solubilities in various reagents. Each class, however, if 

 it contains more than one substance, may often be distinguished by 

 other properties common to its members. 



(1.) Native- Albumins. These substances are soluble in water and 

 in saline solutions, and are coagulated, i. e., turned into coagulated pro- 

 teid, on heating. 



(2.) Derived- Albumins. These are soluble in acids or alkalies, but 

 insoluble in saline solutions and in water, and are not coagulated on 

 heating. 



(3.) Globulins. These are soluble in strong or in weak saline solu- 

 tions, in dilute acids and alkalies, and insoluble in water. They are co- 

 agulated on heating. 



