162 
On Colouring Matter. 
The facts here mentioned maybe augmented ad infinitum, and I think they prove, 
that the principle of colour is the same in all colouring matter, and that colours are 
darkened or liahtencd in proportion as they combine with more or less ot this prin- 
ciple ; that agents which are known to be powerfully disoxygenating, lighten colours 
and ultimately destroy them ; that therefore the principle of colour in colouring mat- 
ter is oxygen ; and, judging from analogy, that chlorine destroys colours by depnv- 
'"The lid proposition, that oxygen, in proportion as it combines with colouring 
matter, increases the depth of its colour, and diminishes its solubility, has received 
much support from the facts already exhibited. Lac dye is much darker than lac 
colouring matter in stick lac, and it is not soluble by means ot an alkali and lodh, 
as is the latter. In the red and yellow baths from woods or roots, if the colouring 
matter be cquallv oxygenated, any cotton or woollen stuff immersed in them, will 
remove the particles most highly oxydated ; those particles being insoluble, attach 
themselves to the cloth and are of a much darker and more degraded colour than 
the other less oxygenated colouring matter in solution. Indigo is a powerful example 
of the truth of this proposition, hut as its phenomena have already been mentioned it 
is not necessary to repeat them. It maybe proper to notice in this place an ambiguity, 
accompanying many resinous colouring matters ; an addition of oxygen appears to 
destroy the colour, and reduce the resin, which is generally of a brownish yellow 
colour, to a solid state. As these colours arc destroyed, and acted upon by disoxyge- 
nating agents in the same manner as other colours, I do not suspect that they differ 
from them at all in their nature. But I conclude that the resin with which they are 
combined attracts oxygen, by means of which (as will be explained and illustrated 
when examining the VUtb proposition) it deprives the colouring matter of its oxygen, 
and thus decomposes it. With this apparent exception, the rule here laid down applies 
to all colouring matter. 
The Illrd proposition maybe permitted to rest upon the facts which have been or 
maybe brought forward in the course of the essay* ; I will now pass to the consider- 
ation of the IVth general principle. 
IVth. The fact stated in this proposition is of very considerable importance, 
especially as relates to the preparation of dyes. Such experiments as I have had an 
opportunity of instituting convince me, that all dyes suffer an irreparable injury by 
combining with a large portion of oxygen ; and this may probably he owing to some 
derangement which this excess of oxygen may occasion in the relation and proportions 
of their several constituents. 1 shall coniine myself here to the adduction of two 
instances. 
1st. I took a portion of dried indigo leaves, and a piece of the finest indigo 
I could procure, and having made from them two separate solutions, I dyed a bit of 
calico in each ; the colour obtained from the leaves was incomparably stronger, 
fresher, and more vivid, than that produced by the indigo: the experiment was 
many times repeated, and always with the same result. 
2d. I took some fresh stick lac and some lac dye, and dyed with the colouring 
matter of each a bit of flannel; the woollen dyed with the fresh stick lac colouring 
matter was fully equal to the finest cochineal scarlet, and many degrees superior to 
that produced by the lac dye. The inferiority of lac dye to cochineal, I am disposed 
to attribute to this cause : perhaps age may in this manner work its ill effects on 
colouring matter. The practical utility of this fact it is foreign from the object of this 
paper to discuss. 
manner as the copious and speedy union of oxygen with indigo solution or 
the Tyrian purple basis, will produce a full purple or blue colour without the 
intermediate shades being perceptible. Charcoal, on many occasions, exhibits effects 
similar to chlorine, and charcoal is known to be powerfully disoxygenating. 
I attribute to this its antiseptic property ; its power of destroying colour and 
smell, &c. Sulphur when precipitated from alkalis is white ; as I conceive by the 
loss of oxygen : by heat or melting, the colour is restored ; in my opinion by regain- 
ing its oxygen. Sulphur at a certain heat changes its consistency, and is formed 
into a substance used for taking the impressions of medals, &c.: this has been attri- 
buted, and as I think justly, to an acquisition of oxygen. Charcoal under certain 
management is dissolved, and combined with sulphur into a fluid, which has been 
called carburetted sulphur. I imagine the charcoal must have been deprived of i ts 
oxygen by the sulphur previous to their combination taking place. 1 should like 
to ascertain whether chlorine would produce any change on charcoal, and what ? 
* The characteristics of alkalis and acids are, their effects on colouring mat- 
ter, and these effects proceed from their oxygenating or disoxygenating properties- 
