8 PROTEIDS. 



The following reactions are specially used for freeing solutions from 

 all proteids by precipitation. 



6. Acidulate faintly with acetic acid and add tannic acid. 



7. Acidulate with hydrochloric acid and add the double iodide of 

 mercury and potassium. (Brucke's reagent 1 .) 



8. Add hydrochloric acid until the reaction is strongly acid ; then 

 add phosphotungstic acid. 



The following methods are often additionally useful for freeing solutions from 

 all proteids. 



i. Precipitate by excess of absolute alcohol, having previously made the solution 

 neutral or faintly acid. 



ii. Prepare a solution of ferric acetate by saturating acetic acid with freshly 

 precipitated ferric oxide, avoiding all excess of free acid. Add this to the solution 

 and boil ; the whole of the proteids are precipitated together with the iron ; the 

 latter as a basic salt 2 . In some cases a mixture of ferric chloride and an excess of 

 sodium acetate is employed 3 . 



iii. Boil the solution for a few minutes with a little hydrated oxide of lead in 

 presence of a little lead acetate 4 . 



In recent years various neutral salts, more particularly neutral 

 ammonium sulphate 5 , have been largely employed for effecting the 

 precipitation and separation of the several proteids. 



All proteids yield a characteristic violet colouration with simul- 

 taneous slight fluorescence upon treatment with glacial acetic acid and 

 strong sulphuric acid (Adamkiewicz' reaction). The reaction is best 

 obtained by adding to the suspected solution or substance a mixture of 

 one volume of strong sulphuric acid and two volumes of glacial acetic 

 acid and boiling 6 . The violet-coloured solution observed if proteids are 

 present gives an absorption band between the lines b and F in the solar 

 spectrum. 



No general method can be given for the quantitative estimation of 

 the various proteids. For this some special manuals should be con- 

 sulted and use made of the reactions which are specifically characteristic 

 of each proteid as given below. 



Solutions of different proteids rise to different heights in capillary tubes. It is 

 possible that this fact may be of use in detecting and estimating their approximate 

 relative amounts 7 . 



1 Sitzb. d. Wien. Akad. LXIII. 2 (1871), Feb. Hft. 



2 Hoppe-Seyler, Hdbch. S. 264. 



3 Seegen, Pfluger's Arch. Bd. xxxiv. (1884), S. 391. 



4 Hofmeister, Zt. /. physiol. Chem. Bd. n. (1878), S. 288. 



5 Wenz, Zt. f. Biol Bd. xxn. (1886), S. 10. Kiihne, Verhand. d. Naturhist.-Med. 

 Ver. Heidelb. N. F. Bd. in. 1885, S. 286. See also Halliburton, Jl. of Physiol. Vol. v. 

 (1883), p. 172. 



6 Hammarsten, Pfliiger's Arch. Bd. xxxvi. (1885), S. 389. 



7 Bodlander and Traube, Ber. d. deutsch. chem. Gesell. Bd. xix. (1886), S. 1871. 



