On Colouring Matter . 
165 
the colour, and because a large portion of acid of tin (at a low degree of oxydation) 
with sugar, added to the same decoction, did not destroy the colour, although, as the 
Doctor affirms, they would have destroyed the colour of indigo. 
Lodh, as I have before mentioned, will disoxygenate lac colouring matter in re- 
cent stick-lac, but it will produce no such effect on lac dye, which is much more 
oxydated. 
This introduces some degree of perplexity into the subject, and prevents the esta- 
blishing of any general rule with regard to disoxygenating agents. , Doctor Ban- 
croft directed his attention towards the extraction and consolidation of colouring 
matter, but failed, probably from this propensity of colouring matter to combine, 
whilst in any bulk acquiring a dry solid form, with oxygen, and not, when using 
his preparations, adverting to this circumstance, and taking proper means to obviate 
it Should it be necessary to concentrate colouring matter by means of heat, it 
should be done in vacuo. 
It is astonishing how many circumstances may change the relations of oxygenat- 
ing and disoxygenating as applied to the same substances. I will here transcribe 
an account from Nicholson’s Chemical Dictionary, which will not only confirm the 
remark, but I think also throw considerable light on the phenomena displayed by 
the colouring matter of the Buccinuin. “ A French colonrman having mixed some 
Prussian blue and white lead with neat oil, and setit by for some time covered with 
water, he found the surface only blue and all the rest white. On pouring it out on the 
stone, and beginning to grind it afresh, with intention to add more Prussian blue, 
he found the colour gradually returning of itself ; here it might be supposed the oxyde 
of the Prussiate had parted with oxygen to the oil, or to the lead, or to cloth, thus 
becoming white, except that in the surface, which was supplied with oxygen from 
the superincumbent water ; and that it recovered its colour by attracting oxygen 
from the air: but on this supposition, it would seem that light must contain oxygen, 
since the colour of this paint, spread on wood or paper, returned by exposure to 
light in vacuo as well as in the open air.” 
Here I imagine light enables the colouring matter to disoxygenate the oil, where- 
as, in the absence of light, the oil disoxygenated the colouring matter. 
I suspect experiments would prove that the changes in the colour of the blood 
proceed from this change in the relation of substances towards each other, with re- 
gard to their affinity for oxygen. 
Thus* it seems not improbable that the animal matter of the blood, by absorbing 
oxygen, disoxygenates the colouring matter; and that in the course of circulation, by 
parting with oxygen, its disoxygenating power on the colouring matter is gradually 
lost; by which means the colouring matter regains its oxygen. 
by what I have said, it will easily be understood that any attempt at classing 
oxygenating substances in the present state of our knowledge, must be attended with 
insuperable difficulties. Alkalis totally deprived of oxygen, take it from every 
thing : perhaps the disoxygenating property of chlorine 1* likewise is owing to a si- 
milar cause. On the other hand, many substances retain oxygen more strongly, the 
more they possess of it, and as they acquire oxygen have their disoxygenating power 
increased. 
I will here subjoin a list of a few of the acids in the order in which they appeared 
to me to act in disoxygenating lac dye. Mineral acids; nitric; phosphoric; sulphuric; 
muriatic; vegetable "acids ; oxalic ; a fruit supposed to contain oxalic, malic, and 
citric; citric; tartarous; malic ; aceto -citric ; lactic (whatever that may be) ; a- 
cetous. The gallic acid presents considerable difficulty, in other words, the pheno- 
mena which it occasions have not (so far as my knowledge extends) been examined 
'nth that care and attention, which they merit. The property of striking a black 
with highly oxydated salts of iron, is always (I believe) evinced by the gallic acid. M. 
nerthoUet affirms that no black can be dyed in silk or cotton by means of this acid ; 
and that this property is distinct, from the acid nature of this substance, appears to 
me to be proved by these two circumstances : 1st, gallates possess this property 
51 The colouring matter in fresh stick lac contains much animal matter, and al- 
kalis seem to disoxygenise it: upon, as I conceive, this principle old stick lac has 
the animal matter changed and diminished. 
t AU anti putrescent substances are disoxygenating ; the most powerful agent of 
this class is^chlorine. Putrescent miasmata, animal and vegetable, would seem to owe 
their activity to oxygen. This is no place in which to discuss this point, but any 
one acquainted with the history of these substances will at once recognize both the 
truth and importance of this fact. 
