GENERAL PROPERTIES AND REACTIONS OE PROTEIDS. 47 
Third group— The indol group, of which indol, skatol, and skatol- 
earbonic acid are the most important members. 
We can now proceed to the consideration of the proteid colour 
reactions. 
1. The xanthoproteic reaction. — This is characterised l.y the yellow- 
colour given by boiling with nitric acid, turned orange by ammonia. 
O. Loew : considered thai the yellow material was a mixture of oxynitro-, 
trinitro-, and hexanitro-albumin ; but these substances are very doubtful 
as chemical individuals. Salkowski found the reaction to be given by all 
the members of his first and third groups of aromatic substances. 
Pickering 2 found that salicylic acid, and salicylsulphonic acid, cholesterin, 
cholalic acid, and taurocholic acid also give the test. A large number 
of other organic substances which were tested did not give the same 
result. It was noticed that bodies with a benzene nucleus with one 
hydrogen replaced by hydroxyl, give the xanthoproteic reaction, whereas 
substances which contain a benzene nucleus without the hydroxyl, as 
phenylacetic and benzoic acids, do not. 
Milloris reaction. — A brick-red coloration occurs when proteid matter 
is boiled with Mil Ion's reagent (a mixture of the nitrates of mercury 
with excess of nitric acid) ; the reaction was thought by Kiihne 3 to 
be due to tyrosine. Salkowski also took this view, as the reaction is 
given by the substances in his first group, the most prominent member 
of which is tyrosine. Those in the second and thh'd groups do not give 
the test. Xasse, 4 however, demonstrated that Millon's reaction is due 
to benzene derivatives, in which one hydrogen atom has been replaced 
by hydroxyl (hydroxy benzene nucleus) and not to tyrosine. That 
Nasse's view is correct is shown by the following considerations : — 
Ktibne and Chittenden 5 have found that certain anti-products of diges- 
tion, which yield neither leucine nor tp'osine on further digestion, or on 
decomposition with sulphuric acid, do not give the reaction. Schiitzen- 
berger 6 found that tyrosine is absent from the putrefaction products of 
gelatin. Xow, Salkowski stated that gelatin does not react with Millon's 
reagent. But Chittenden and Solley 7 have found that the products of 
gelatin digestion give a characteristic reaction, and Pickering that pure 
gelatin and gelatinoses give it in a marked manner ; thus confirming the 
statement made by Millon 8 in his original memoir. Gelatin, therefore, 
owes this property to something which is not tyrosine, but which, like 
tyrosine, contains a hydroxybenzene nucleus. 
Adamhieiuicz reaction. 9 — If glacial acetic acid in excess and then 
concentrated sulphuric acid are added to proteid, a violet colour with 
feeble fluorescence is produced. The test is by no means a certain one, 
and is given by proteoses and peptones in concentrated solutions only. 
It is not given by gelatin (Hammarsten). 
This test is only given by the aromatic substances of Salkowski's 
third (indol) group. The strong reagents added are likely to produce 
1 Journ. f. prakt. Chem., Leipzig, X.F., Bd. iii. S. 180. 
2 Journ. Physiol., Cambridge and London, 1893, vol. xiv. p. 372. 
= Ztschr. f. d. ges. Naturw., Halle, Bd. xxix. S. 506 : Virchoirfs Archiv, Bd. xxxix. S. 130. 
4 Chem. Centr.-Bl., Leipzig, 1879, Bd. x. 
5 Ztschr. f. Biol., Miinchen, Bd. xxii. S. 423. 
6 Article in Wurtz' "Diet, de chim.," 1886, Suppl. 1 A, p. 58. 
7 Journ. Physiol., Cambridge and London, vol. xii. p. 23. 
Compt. rend. Acad. d. sc, Paris, tome xxviii. p. 40. 
9 Bcr. d. deutsch. chem. Gesellsch., Berlin, Bd. viii. S. 761. See also Wurster, Chem. 
Ztg., Cbthen, Bd. xi. S. 187. 
