60 On the Ufe of the Oxyds of Iron in dyeing Cotton. 
then it is brought to the ftate of black oxyd, as has been 
obferved by our colleague Berthollet. It is poffible alfo to 
embrown thefe colours, and to give them a variety of tints, 
from the bright grey to the deep black, by merely paffing the 
cottons impregnated with the aftringent principle through 
a folution of iron. The oxyd is then precipitated itfelf by 
the principle which is fixed on the ftuff. . 
An obfervation, which. may become of the utmoft value 
for the art of dyeing, is, that the moft ufual aftringent ve- 
getables. all furnith a yellow colour, which has not much 
brilliancy, but which has fufficient fixity to be employed 
with advantage. This yellow colour is brightened in the 
feries of vegetables, in proportion as the aftringent principle 
is diminifhed, and the vivacity of the colour is augmented 
in the fame proportion. It is difficult then to obtain yellow 
colours which are at the fame time durable and brilliant. 
Thefe two valuable qualities are to each other in an mverfe 
ratio; but it is poflible to unite the colouring principles in 
fuch a manner as to combine fplendour with fixity. Green 
oak bark unites perfectly with yellow weed, and fumach 
with green citron. It is by this mixture that we may be able 
to combine with the oxyd of iron vegetable colours, the 
fplendour of which is equal to their durability. 
I fhall conclude thefe reflections with an obfervation in 
regard to the employment of aftringents in the dyeing of 
cotton. It has been pretended, that by increafing the pro- 
portions of fumach and the bark of alder tree or oak, thefe 
fubftances might fupply the place of gall-nuts in dyeing 
cotton red. I fhould have received the more pleafure from 
this difcovery, as galls tend to render our colours confiderably 
dearer, and as I could have procured fumach at a very low 
price, fince it grows almoft every where in the dry parts of 
our fouthern diftricts. But I can fafely affert, that it is im- 
poffible to employ thefe as fubftitutes, in whatever dofes they 
may be ufed. The colour is always much paler, poorer, 
and lefs fixed. I know that the cafe is not the fame in re- 
gard 
