ON COLOURING MATTERS. 
137 
hot water. The whole of the alizarin dissolved in the boiling water is de¬ 
posited on cooling, but mingled with fat, which probably accompanies it 
throngh the filter in a state of suspension produced by the heat of the boil¬ 
ing fluid. This fat disguises its properties very much, and has been the cause, 
in all previous investigations, of alizarin never having been obtaitu d in a state 
of purity except through the agency of heat, which has always left it doubt¬ 
ful whether it existed in the plant as such, or was fnnned by the action of 
heat from some other substance. I hope to establish satisfactorily its exist¬ 
ence as a constituent of the madder root, and also the fact of its being a pure 
colouring matter, a eircumstaoce which has likewisi* been doubled. In order 
to obtain it in a state of purity, the red ilucks wliich are deposiUtd by the boil¬ 
ing watery solution are seftarated by filtration and dissolved in boiling alco¬ 
hol. To the boiling aoluiiou, which has a brownish-yellow colour, hydrate 
of alumina is added, and the boiling is continued for some lime. Tim aliza¬ 
rin combines with the alumina, fonning a dark-red compound, while the fluid 
loses its colour. Fresh alumina is added, until no mure colouring matter can 
be thereby separated. A great part of tlic fat remains dissolved in the alco¬ 
hol, while a part combines with the alumina. The coloured alumina is sepa¬ 
rated by filtration ami washed with alcohol for some time. It is then treated 
with a weak boiling solution uf caustic potash, which dissolves the excess of 
^uraina and all the fat which may have combined with the alumina, but 
leaves the compound of alizarin and alumina iindissolvcd, merely changing 
Its colour from red to dark purple. This process is repeate<l several times, 
until the alkaline fluid is no longer red, but of a pure purple colour. The 
rwidiie is treated with muriatic acid, which dissolves the alumina and leaves 
the alizarin behind in crystalUua flocks of an orango colour, which are washed 
uilh water to remove the acid and then dissolved in alcohol. Tlie alcoholic 
solution on evaporation gives shining, prismatic, orange-coloured crystals of 
Miz^in, which may, if necessary, bt; purified by a second crystallization from 
mass left undissolvcd by boiling water consists of rubiacin ami two 
tuBbnet kinds of fat. I have only boon able to discover one methotl of ex¬ 
tracting the rubiacin from this mixture. This method is founded on the 
Bombflity of rubiacin in pcrchloridti and peruilrato of iron. It is inimaterial 
w <«h oi these two salts be taken. Persulphate of iron would not answer 
the purpose. If the mixture of rubiacin with the two fats bo treated with a 
sotnewhat concentrated boiling .solution of perchloride or pernitrate of iron, a 
Boution of a deep reddish-brown colour is obtained, while a brown residue 
ten^Dj insoluble in an excess of the iron salt solution, and consisting of one 
V fc combination with oxide of iron. The fluid is filtered, and 
® the addition to it of muriatic acid, a yellow flocculent precipitate is ob- 
neo, which is separated by filtration anrl washed until all tiie iron salt and 
e excess of acid are removed. This precipitate conaiste of rubiacin, the 
of the two fats, and of a new body which I shall call ruhueric acid. 
Ate latursubstance dues not exist ready-formed in madder, but is produced 
j e action of the persalt of iron on rubiacin. This action consists in the 
>acm taking up a certain number of atoms of oxygen from the persalt 
omir***’ which is thus formed combines with peroxide ot iron, 
Lm soluble in water with a reddish-brown colour, and de- 
mposabi^e by any strong acid. Part of the rubiacin however escapes this 
and IS precipitated together with the rubiacic acid and fat on the addi- 
dissf r 'I'h® precipitate w treated with boiling alcohol, which 
shatw'^ rubiacin and the fat, and leaves behind the rubiacic acid in the 
P« 01 a yellow powder. This process Is repeated until nothing more is 
