in dyeing Cotton. 59 
exterior part exhibits a violet almoft black. The means to 
remedy this inconvenience is, to wafh the cotton when it is 
taken from the folution of iron, and to expofe it to the 
madder moift. The colour will become more equal and 
velvety. The folvents of iron are almoft the fame for this 
colour as for the yellow colour already mentioned. 
I fupprefs here every thing that regards manipulation, that 
T may attend only to chemical relations; and on that account 
T fhall mention an obfervation which may ferve to guide the 
artift in brightening the violet on his cotton. The red of 
madder and the oxyd of iron depofited on the ftuff determine 
the violet colour. This colour becomes red or blue, ac- 
cording as either of the two principles predominates. The 
dyer knows by experience how difficult it is to obtain a com- 
bination which produces the tone of colour defired, efpe- 
cially when it is required to be very full, lively and durable. 
This object, however, may be obtained, not only by varying 
the proportions of the two colouring principles, but alfo by 
varying the procefs of brightening. The only point is, to 
be acquainted with the two following facts; that the foda 
deftroys the iron, while the foap, by ftrong ebullition, feizes 
in preference the red of the madder. Hence it is, that the 
colour may be inclined to red or blue, according as you 
brighten with one or the other of thefe mordants. Thus, 
cotton taken from the madder dye, when wafhen and boiled 
in the brightening liquor with 30,00 of foap, will give a 
fuperb violet; whereas you will obtain only a plum colour 
in treating it with foda. ° 
The oxyd of iron precipitated on any ftuff unites alfo 
very advantageoufly with the fawn colour furnifhed by aftrin- 
gents, and by varying the ftrength of mordants an infinity 
of fhades may be produced. In this cafe, it is lefs a combi- 
nation or folution of principles, than the fimple mixture or 
juxta-pofition of the colouring bodies on the ftuff. By means 
of a boiling heat, we may combine in a more intimate 
manner the oxyd of iron with the aftringent principle ; and 
thes 
