On Colouring Matter. 
161 
Doctor Bancroft affirms, that in madder there are two kinds of colouring' matter, 
red and brown; I should interpret the facts differently. All colouring matter derived 
from roots and woods is likely, and that from various causes, to be united with 
■ oxygen in varying proportions ; which will of course alter their properties without 
effecting their essence : thus indigo, is indigo whether in the form of a blue concrete, 
or a green solution ; its properties arc changed, it essence is unaltered. 
Ail the phenomena mentioned by Doctor Bancroft, convince me, that the difference 
between the red and fawn coloured particles of the colouring matter of madder, is 
owing to their being combined with different proportions of oxygen. 
Thus, in an account which he gives of a pink, bran is used to disoxygenate the 
colouring matter ; and he observes, if the proportion of bran is too large, the colour 
will be weak, i. e. too much disoxygenated ;) and if it be too small, the colour will be 
less rosy, (viz. not sufficiently disoxygenated.) Again he observes : “ Glue, with 
madder, manifestly obstructed the combination of colouring matter with alum, since 
only a kind of salmon colour was produced.” In my view of the case, the glue was 
too disoxygenating, or used in too large a proportion. Further he says : “ Boiling 
extracts the fawn coloured particles that is, promotes oxygenation : “ acids 
prevent their extraction i. c. disoxygenate them. The colour of madder infusion 
or decoction is destroyed by sulphuretted hydrogen gas. 
The red colour of this dye, by disoxygenation, is first rendered yellow ; then co- 
lourless ; and finally annihilated. What is here observed respecting madder, will 
apply to other woods and roots, yielding colouring matter ; but ns Brazil wood and 
logwood present a few anomalies, I shall as shortly as I can explain them. 
Acids produce precipitates ill decoctions of Brazil wood; an addition of acid, or a 
stronger acid, increases the colours. Brazil wood contains glim, which takes oxygen 
from the acid, and is thereby precipitated, carrying with it much colouring matter ; 
an addition of acid, ora stronger acid, separates and dissolves the colouringmatter, 
which deepens the colour of the decoction. If the colouring matter existed in combi- 
nation with different proportions of oxygen, the most oxygenated would be precipi- 
tated, and its solution would add much colour to the decoction. 
The other anomaly is presented by logwood. Solution of tin, instead of lightening, 
deepens its colour. Logwood, besides its colouring matter, possesses an astringent 
principle, on which account it is employed in medicine; to this last is owing the 
anomalyin question, Its power with different agents in producing dark colours, and its 
nse in the black dye arise from the same cause. 
This colour, as well as that of Brazil wood, is destroyed by sulphuretted hydrogen 
gas. Litmus tincture and violet syrup, secluded from air, lose their colour; which 
oxygen, timely administered, will restore*. 
Doctor Bancroft insists chiefly on his experiments onBuceinum, or the Tyrian pur- 
ple, to prove that colours are sometimes owing to an abstraction of oxygen. If my 
opinion, respecting the agency on colouring matter of alkalis and acids, he well 
founded, the better half of his experiments would prove deserters and range them- 
selves under the banner of the enemy. But waving any advantage derivable from 
this source, is it not probable that phosphorous acid (which the Doctor says is given 
out by the Buceinum while it gains its colour) muy, by its disoxygenating effects, 
render the colour latent ? 
The principal fact, however, which is brought forward in proof of the truth of the 
opinion that the Tyrian purple owes its colour to an abstraction of oxygen, is, that the 
white mucous matter which is the subject of this colour acquires its colour equally, 
whether it be in vacuo, or in the open air, provided only that it be exposed (o the 
%ht. I think an experiment on Prussian blue, which I shall afterwards have occasion 
to adduce, will prove highly illustrativeof this matter. That Buceinum does not differ 
with regard to its colouring principle from other colouring matter may, I think, be 
inferred, both from its passing when acquiring its colour from the lightest shades or 
colours to the darkest, and from its colour being, as all other colours are, destroyed 
hy chlorinef. 
* Vegetable juices are strongly disposed to undergo decomposition ; hence perhaps 
bus loss of colour. Litmus, it is said, retains its colour longer in spirits than in 
water ; the one checks, the other promotes decomposition. The colour obtained from 
dry red cabbage leaves is much more permanent, than that extracted from the same 
plant when recent. 
t Chlorine, in a diluted form, causes the same changes in the shades of colours 
as are effected by other deoxygenating agents ; if it be undiluted, the effect may be 
too sudden and powerful to admit of these changes being observed ; in the same 
