166 
On Colouring Matter. 
in a stronger degree than the pure acid ; 2d, this acid seems to possess the same 
property which I hare attributed to acids generally, namely, that of disoxygenating and 
increasing the solubility of colouring matter, since in dyeing printed goods, it tends 
to keep the grounds clean, and prevents them from being much discoloured*. 
I cannot therefore help being of opinion, that in fact-f, there exist two distinct sub- 
stances — gallic acid, and the principle which with highly oxydated salts of iron forms* 
black, and generally adds to the depth and fixity of adjective colours. M. Berthollet 
is said further to have affirmed, that tannin in combination with salts of iron, much 
oxydated, dyed silks and cotton black ; if this be true, (and it could not be owing to 
the presence of gallic acid, for gallic acid alone would not do it, and had this acid 
existed in a large proportion in the tannin, it would not have escaped the notice of 
this eminent chemist,) it evidently carries the opinion towards certainty ; and as it 
also proves that this principle is not necessarily a constituent nor in inseparable 
union with gallic acid, so it affords reasonable grounds for expecting that it may be 
obtained in a separate form, and may admit of being accurately examined and ana- 
lyzed. 
The VTIIth proposition has been established whilst considering the Vllth : it has 
been shown that sugar renders tin in solution less disoxygenating, and that the two 
produce an effect the very reverse of what would be the effect of their joint forces. 
Spirits of wine, soap, and water, I found to produce good red solution with lac dye ; 
hut lodh did not serve to cooperate with these more powerful agents. 
The IXth proposition might have been included in some of the preceding ones. 
If colouring matter be exposed to the sun and air, in a divided state, (as when ap- 
plied to stuffs as a dye), the sun and air disoxygenate it, producing a fading, and 
ultimately, a destruction of the colour. 
Whilst on the other hand, colouring matter in masses, especially if it be moist, 
acquires, instead of losing, oxygen by exposure to the sun and airj . 
I perhaps ought to notice one other circumstance, which seems to be deserving of 
some attention from those who would desire, in acting upon these principles, (o 
avoid a very obvious source of error. If colouring matter, in parting with oxygen, 
loses colour, it is evident that other matter may acquire colour by gaining oxygen, 
and that in some eases of this kind the disoxygenating agents, by combining’ with 
the colouring matter, may degrade the colour, as much as by its disoxygenating 
power it had improved it. 
My apology for having written this memoir is derived from mv opinion of its 
utility and necessity. Hitherto (as it appears to me) science has shed little or no light 
on those arts, whose subject, is colour. Doctor Bancroft (who I believe is good au- 
thority, and whose work is the only one on the subject, with which I am acquainted,) 
describes very faithfully the general dyeing processes, and 1 dare say gives the best 
known instructions for producing and fixing the various colours from the several 
if we except indigo, the rationale of a single process is 
not, that I recollect, unfolded. He indeed tells us, that old fustic produced a muddy 
* The circumstance of adjective colours being deepened and fixed, and as regards 
the grounds of printed stuffs, the colouring matter being lightened and removed, 
i = f a h!l° g0U . S8 “ bStanCeS ’ ? ? ms t0 be wrongly in favour of the opinion, that 
s m bt ° af ?T nts hav ? n « even opposite properties, in order to occa- 
sion these apparently contradictory effects. 
A X,?o Ct0 ,‘ has promulgated the same idea, and conceives gallic acid to be 
I Sink manT if nTIil'" fu : ? Ws d 1 0 . e8 not soera P ossible 5 is sublimeable, and 
+ ’ a - C,ds hl4vm e VVl Property, produce marked effects on iron. 
concerns the rel»W,n S ,°. ™ me . nse > and bad aln >» s t said so appalling as that which 
concerns the relation which substances hear towards oxygen ; almost all nhvsical 
seera to me to proceed from it— electricity.' chemical affinity meteoric 
i^TrCsS^he USti0n % &C - 1 • 1 'r tbat /ex“ 
even been dhSSswWt ’ y COnver * ,n * k iu t<» chlorine. I have 
according to thetl^lTon?n i 7 - . r ' t . a " h ' ! " -Halved by acids 
and partly escapJbi^fet i s ? lutIon combined with hydrogen 
metals existing in the atmr>snh#v §aS a n0t f t ? 11S . fact evince the possibility of 
neratedorpreLitated?rZh » R ! f a . n r Tr c “’ t <“ r > circumstances of being rege- 
ceeding far beyond my depth. 1 1 ^ 1 am de I Mrtm g horn my subject, and pro- 
