ON COLOURING MATTERS. 71 
by the action of sulphuric and muriatic acid on xanthin, has the following 
_ properties :—When dry it has a dark olive colour. It burns with a flame 
_ and a smell like burning wood, leaving a large quantity of charcoal, which 
however burns away without any fixed residue. It is decomposed by boiling 
dilute nitric acid, and changed into a yellow flocculjent substance. It is in- 
soluble in concentrated sulphuric acid, and also in boiling aleohol. When 
treated with caustic potash, a part dissolves with a dark brown colour, and 
is re-precipitated by acids as a dark brown powder, while the other part re- 
_ mains undissolved as a black powder. 
__. Mordanted cloth acquires no colour in a boiling solution of xanthin, if the 
_ latter is in its yellow unoxidized state ; but if the solution has become brown 
_ by contact with the air, then both the alumina and the iron mordant acquire 
| in the boiling solution a brown colour, while the unmordanted parts, which 
_ should remain white, become of a brown tint. Hence it follows that xanthin 
is injurious in madder-dyeing, and must contribute, together with the two 
_ resins, in impairing the purity of the colours, and sullying the whiteness of 
_ those parts which should attract no colour. To get rid of the xanthin is one 
| object of changing madder into garancin. 
Se a eee 
It remains for me to say a few words in regard to the part which the dif- 
_ ferent substances described above play in the process of madder-dyeing. I 
_ regret to say that in my last report there are contained some views on this 
head, which I have found, on more exact investigation, to be erroneous. 
_ The two principal points to be determined are, which is the substance that 
_ produces the chief effect in dyeing with madder, and why is a certain pro- 
_ portion of lime, either in the plant or in the dye-bath, necessary for the pro- 
_ duction of fine and durable colours. In my last report I stated it as my 
_ opinion, that both alizarin and rubiacin take part in the process, that rubiacin 
_ alone produces no effect, but that when it is in combination with an alkali or 
_an alkaline earth, it forms double compounds with the alizarin compounds 
_ of aJumina and peroxide of iron, and thus increases the intensity of colour in 
the latter. I have since found that this opinion cannot be sustained, since 
ubiacin, whether free or combined, produces no beneficial effect in the pro- 
ess of dyeing, and is therefore no true colouring matter, as the following 
experiments will show. 
Since the brown precipitate produced by acids in a watery extract of 
_ madder contains all the free colouring matter of the root, and acts in dyeing 
in the same way as madder itself, it was evident that by trying the constitu- 
ents of this precipitate in conjunction with one another, both in a free state 
and in combination with lime, a correct view of the part performed by each 
would be arrived at. Having therefore taken a piece of calico on which 
hree mordants had been printed, one for red, one for purple, and one for 
lack, in alternate stripes, each stripe being one quarter of an inch broad, and 
aving intervals between them of the same width, it was divided into pieces 
if six inches by three, and one of these pieces was taken for each of the fol- 
owing experiments. As the tinctorial power of alizarin is very great, so 
eat that one quarter of a grain was enough to over-dye one of these pieces, 
T took one or two grains of crystallized alizarin, dissolved it in a measured 
quantity of water, to which a little caustic alkali had been added, and was 
n able to divide the solution into portions corresponding to quarters, 
shths, and sixteenths of a grain, so that by precipitating one of these por- 
_ tions with muriatic acid, filtering and carefully washing, I obtained small 
_ quantities in a state very well adapted for dyeing. By treating one of these 
