5S On the Ufe of the Oxyds of Iron 
indigo and the oxyds of iron. I obtained only a dirty earthy 
green of different fhades, and exceedingly poor. The oxyd 
ef iron combines on the other hand very eafily with 
the red of madder, and produces a bright violet or plum 
€prunsaz) colour, the ufe of which is as extenfive as bene- 
ficial im the cotton manufactory. But if we thould confine 
ourfelves to apply thefe two colours to cotton, without hav-~ 
mg employed a mordant capable of fixing the latter, the 
eolour would not only remain dull and difagreeable by the 
mmpoflibility of brightening it, but it would ftill be attended 
with the great inconvenience of not refitting leys. We 
muft begin then by preparing the cotton, as if to difpofe it 
for veceiving the Adrianople red; and when it has been 
brought to the operation of galling, it is to be paffed 
through a folution of iron, more or lefs charged according 
to the nature of the violet required: it is then to be care- 
fully wafhed, twice maddered, and brightened in a bath of 
foap- 
When a real velvety rich violet is required, it is not to be 
paffed through the folution of iron till it has been previoufly 
galled ; the iron is then precipitated in a blueifh oxyd, which 
combined with the red of madder gives a moft brilliant 
purple, more or lefs dark according to the ftrength of the 
galling and of the ferruginous folution. Ht is very difficult 
_ to obtain an equal colour by this procefs, and in manufac- 
tories am equal violet is confidered as a mafter-piece of art. 
It is generally believed that-it is only by well-direéted mani- 
pulations that it is poffible to refolve this problem, of fo 
much importance in dyeing. But I am convinced that the 
ereat caufe of the inequality in this dye is, that the iron de- 
pofited om the cottom receives an oxydation merely by ex- 
pofure to the air, which varies in different parts of it. The 
threads which are on the eutfide of the hank are ftrongly 
oxydated, while thofe in the infide, removed from the aétion 
ef the air, experience no change. It thence follows, that 
the infide of the hank prefents a weak fhade, while the 
exterior 
stitial 
