8 CHEMISTRY OF THE PROTEIDS 



CHAP. 



4. The Lead Sulphide Reaction of Vogel 



If albumins are boiled with a lead salt and soda solution there is 

 formed either a black precipitate or at least a brown or blackish dis- 

 coloration of the fluid. The reaction depends on the splitting off of the 

 sulpho-hydryl group SH, derived from the cystein radical (Hofmeister), 

 and the subsequent formation of lead sulphide. Instead of a lead salt, 

 the salt of any other metal may be employed, the sulphide of which is 

 black or darkly coloured. All albumins except protamin, peptones, 

 and perhaps histone, contain sulphur, and therefore give this reaction. 

 Vogel l first noticed that decomposing serum exhales a gas which has 

 the property of blackening lead acetate. 



5. The Reaction of Molisch 



By the addition of a few drops of an alcoholic solution of a-naphthol 

 to an albumin solution, followed by the addition of some strong, 

 sulphuric acid, there is produced a violet colour, which on the addition 

 of alcohol, ether, or potash solution turns yellow. If thymol be taken 

 instead of a-naphthol, a carmine-red colour results, which by the addi- 

 tion of water is turned green. 



Molisch 2 considered this reaction to be characteristic of carbo- 

 hydrates, but Seegen 3 showed later that albumins also give it. The 

 reaction depends on the strong sulphuric acid converting carbohydrate 

 radicals, wherever met with, into furfurol, which latter gives the 

 colour reactions with a-naphthol or thymol. This reaction is there- 

 fore identical with Pettenkofer's bile acid reaction and belongs to the 

 so-called furfurol reactions, 4 and is a sure index as to whether carbo- 

 hydrate groups are present or absent in any given proteid. 



6. The Reaction of Adamkiewicz, Hopkins and Cole 



Adamkiewicz 5 described the following reaction : On dissolving 

 dry, defatted albumin in glacial acetic acid and subsequently adding 

 concentrated sulphuric acid, there are formed at the junction of the 

 two fluids red, green, and violet rings. On shaking, the whole solu- 

 tion becomes coloured. On spectroscopic -examination a broad band 

 is seen stretching from the blue to the yellow. The explanation of 



1 Vogel, Ann. Chim. 87. 215 (1813). 



2 H. Molisch, Monatsheftef. Chem. 7. 198 (1888). 



3 J. Seegen, Zentralbl. f. d. media/in. Wiss. 1886, pp. 785 and 801. 



* 4 F. Mylius, Zeitschr. /. physiol. Chem. 11. 492 (1887) ; L. v. Udranszky, ibid. 12 

 389 (1888). / 



5 A. Adamkiewicz, Pfliiger's Arch. /. d. ges. Physiol. 9. 156 (1874), and Btr. d. 

 deutsch. chem. Ges. 8. I. 161 (1875). 



