ON COLOURING MATTERS. 73 
When alizarin and pectic acid are present together in the dye-bath, the 
‘pectic acid having most affinity for bases, combines with the alumina and 
peroxide of iron, ‘and. the alizarin crystallizes out when the bath cools, as I 
noticed in performing the experiment No. 8. The same is without doubt the 
_ ease when using rubiacin or the resins. ‘The alumina and peroxide of iron 
‘combine with these substances to the exclusion of the alizarin; and these 
_ compounds are either colourless, or have a poor and unsightly colour. The 
use of lime is therefore easily explained ; it serves, not to increase the tinc- 
torial power of the colouring matter, but to combine with and render harm- 
less the substances which are injurious in a free state. Now if we treat 
_ madder with muriatic or sulphuric acid, we remove all the lime and magnesia 
_ from it; the pectic acid, the rubiacin and the resins become free; and if we 
_ wash with water, the muriatic or sulphuric acid is certainly removed; but 
_ those substances being but little soluble in cold water, remain and destroy the 
- effect of the alizarin in dyeing. But if previous to dyeing we add lime, the 
pectic acid, the rubiacin and the resins being more electro-negative than 
_ alizarin, combine with the strongest base, which is the lime ; and the alizarin, 
_ which is less electro-negative, combines with the weakest bases, viz. the 
_ alumina and peroxide of iron. If we add an excess of lime, then of course 
_ the alizarin also combines with the lime, and the alumina and peroxide of 
iron having no free body to combine with, remain colourless. The process 
is thus brought into harmony with our previous knowledge of the relative 
affinity of acids and bases. It is probable that lime is not absolutely neces- 
sary for the success of the operation, and that it might be replaced by potash, 
_ soda, magnesia or baryta; but as lime is the cheapest substance that can be 
used for the purpose, it would be of no practical importance to find a substi- 
tute for it. 
__ [have in the preceding remarks left xanthin out of consideration. During 
the process of madder-dyeing this substance no doubt becomes oxidized, 
and deposits the brown substance mentioned above, on all parts of the cloth. 
This substance, together with the pectic acid, the rubiacin and the resins, are 
removed afterwards by passing the cloth through a boiling solution of soap. 
The alkali of the soap dissolves these substances, which have more affinity 
for alkalies than alizarin, while the fat acid remains on “= cloth in combina- 
In order to prove analytically that alizarin is the ai tunes which produces 
madder colours, | took several yards of cloth which had been dyed purple 
th madder, but not soaped, and treakedl it with muriatic acid, which re- 
s ‘thrown ona filter ee well- washed with cold water: it was then treated 
th boiling alcohol. |The alcoholic liquid, which was dark yellow, was spon- 
neously evaporated, and gave crystals of alizarin mixed with a powder re- 
mbling beta-resin, and a few yellow micaceous plates, which were probably 
jacin. There remained a brown residue insoluble in alcohol, part of 
hich dissolved in boiling water, and proved to be pectic acid. On treating 
me cloth which had been dyed with madder, and then soaped, with muriatic 
as before, and then with caustic alkali, I obtained a purple solution, in 
ch acids produced a yellow precipitate. This precipitate was treated with 
ing alcohol like the other ; it gave a yellow liquid, which on evaporation 
