160 
On Colouring Matter. 
I. That oxygen is the principle of colour in colouring matter, is I think establish- 
ed upon universal experience ; upon the whole mass of facts which nature thruusli- 
out time has accumulated, and made to hear upon this point. Almost all vegetabk 
exudations are primarily of a white colour or transparent, and of a fluid consistence, 
till they acquire colour and solidity by exposure to the air*. The external partsof 
these substances will he found to be much harder and darker than the internal, and 
those which combine most intimately with, and with the largest portion of oxygen, 
can only be dissolved by agents the most powerfully disoxvgenating. In short, after 
having carefully examined the whole of the phenomena recorded of colouring mat- 
ter, I declare that I have not found them to yield a single exception or a solid objec- 
tion, which can clash with the universality of this proposition. 1 shall, therefore, 
endeavour to reduce the proofs to as small a space as possible ; although the receiv- 
ed opinion respecting the modus operandi of chlorine on colour, and my inability, 
from having no chlorine in my possession, to refute it by experiments, will neces- 
sarily expand the argument, by obliging me to show by indirect means, that chlorine 
destroys colours by depriving them of oxygen. 
i st. I shall assume that as chlorine acts alike upon all colours, all colours depend 
upon some common principle. 
2nd. I shall endeavour to show that all colours are weakened by a partial loss, 
and destroyed by a total loss of oxygen. 
3rd. I shall infer, that if this effect, brought about by other agents, proceeds from 
privation of oxygen, that chlorine acts in a similar manner ; or, that the same effect 
is attributable to the same cause. 
Assuming then that the principle of colour is one and the same, in all colouring 
matter, I now proceed to show, by as large an assemblage of facts as is compatible 
with the limits of an essay, that all colours are weakened by a partial loss, and de- 
stroyed by a total privation of oxygenf. The action of the rays of the sun upon mi- 
nute particles of colouring matter is disoxygenating 5 so also is the action of the sul- 
phurous acid gas ; and to this property Doctor Bancroft ascribes the effects of these 
agents in bleaching. Here is one instance, if it be admitted, wherein agents, pro- 
ducing the same effect as that which is now produced by chlorine, accomplish it by 
depriving colouring matter of its oxygen. Indigo (at what, for the sake of distinction, 
I would call the maximum of oxydation) is an insoluble blue concrete : in propor- 
tion as it is deprived of oxygen, it loses its solid form; and passes from the darkest 
shade of green to the lightest ; from thence to different shades of yellow ; till at 
last it becomes colourless, and, as proved by an experiment of Doctor Bancroft’s, is 
irrecoverably decomposed!. 
Lac dye is an insoluble, dark, dingy blue concrete ; when the lac colouring matter 
is obtained from fresh stick-lac, it may be formed by means of an alkali and iodhf 
into a beautiful red solution ; if sulpherreted hydrogen gas be mixed with this solu- 
tion, it will soon destroy its colours. 
For the sake of analogy it might be mentioned, that considering lac dye as a 
peroxyde, the changes of colour by gradual disoxygenation are from blackish blue 
to purple ; from purple to bluish red ; from bluish red to red ; and from thence 
through all the shades of red to yellow ; til! at last it becomes colourless, aud the 
essence of the colour is irreparably destroyed||. 
Lac dye kept for some days in a highly oxydized solution of tin, will be deprived 
of its colour; and arguing a fortiori , other colours would experience the same fate. 
Doctor Bancroft, states that the outer bark of the quercitron is full of colouriug 
matter; but on account of its dark din^y colour he recommends it to be thrown 
away. 1 imagine that the only difference between this and the inner bark, is that 
the former is more highly oxygenated. 
-f- AA here an assertion is hazarded without proof, the proof must be sought for, in 
the sum ol the evidence taken together. Thus if J say that the rays of the sun are 
Oisoxygeuating, and do not prove it; but it appears that disoxygenation from other 
causes produces the same effect: it must be takeu as evidence that the rays of the 
son are disoxygenating. 
+ Oxygen seems to be the bond of union between the constituents of the basis of 
colouring matter, since the slightest portion, appears to be sufficient to prevent their 
separation, and to hinder total decompositions; but as soon as oxygen is totally re- 
moved, the constituents of the basis enter into new combinations" and the colour 
can no longer be regenerated. 
J The bark of a tree which in India is used to produce the same effects in dying 
as in Europe, are obtained by tin solution. 
|j Combustible matter generally, if gradually carbonized, passes through a series 
of brown colour., to black, passing through the lightest shades to the darkest? 
