Dr. Schunck on the Colouring Matters of Madder. 47 



zarin. Alizarin is doubtless the most interesting and the most 

 definite in its nature of all the substances contained in madder. It 

 also presents itself the most easily to the observer even on the most 

 superficial examination. If we heat madder spread out in a thin 

 layer on a metal plate without carrying the heat far enough to char 

 the woody parts of the root, we shall in the course of a few hours 

 find its surface covered with small red or orange-coloured crystals, 

 which consist of alizarin. In the same way any extract of madder, 

 whether with water, alcohol or alkalies, evaporated to dryness and 

 gently heated, gives a crystalline sublimate of alizarin, which is va- 

 riously coloured from a light yellow to a dark red or brown. Now 

 one of the first points to be ascertained in regard to this body was 

 whether it exists as such in the root, or whether it is formed by the 

 process of sublimation. Robiquet, the discoverer, states that it pre- 

 exists in the plant. He considered alizarin as the colouring principle 

 of madder, and merely subjected it to sublimation for the purpose 

 of purifying it. But his investigation presents us with no convincing 

 proof of this opinion, for the extract of madder with water, alcohol, 

 &c., from which he prepares his alizarin by sublimation, shovvs no 

 trace of anything crystalline; and many chemists have asserted in 

 consequence that it is u product of decomposition, being formed by 

 the action of heat in the same way as pyrogallic, pyrotartaric acitl, 

 and many other bodies. I have however no hesitation in affirming 

 that it exists in the plant as such, having in more than one way ob- 

 tained it in a crystallized state without the intervention of heat. 

 If we make an extract of madder with cold water, we obtain a brown 

 fluid which produces no reaction on test-paper. After being ex- 

 posed however to the action of the atmosphere for some hours, it 

 acquires a distinctly acid reaction ; and if it be now examined care- 

 fully, there will be found floating about in it a number of long hair- 

 like shining crystals : these crystals are alizarin. If the fluid be still 

 further exposed to the influence of the atmosphere, a yellow amor- 

 phous substance begins to separate, which I shall mention afterwards. 

 This is succeeded by a gelatinous substance, and after some days a 

 complete state of putrefaction ensues. It seems as if the alizarin in 

 madder, or at all events that part which dissolves in the water, exists 

 in combination with lime. On exposure to the atmosphere, there 

 is formed, from some constituent of the root dissolved in the fluid 

 through the instrumentality of the oxygen, some acid, which seizes 

 hold of the lime in the solution and separates the bodies which are 

 combined with the lime. Now the alizarin, being a body of very 

 slightly acid properties, is separated first, and the other substances 

 follow in succession. The fresher the madder is, the purer will be 

 the alizarin, which separates on exposure to the atmosphere ; in some 

 instances it forms on the surface of the fluid a thick light yellow 

 scum ; but in most cases it is mixed with brown or red substances, 

 from which it is separated with difficulty. It is therefore most 

 advisable to separate the crystals which are deposited after twelve 

 hours' standing, by filtration. These crystals are then washed from 

 the filter and boiled -with very dilute nitric acid until they have be- 



