120 
therefore, of considering the green colour, in Sene- 
fcier's experiments, to have been destroyed by com- 
bustion, he supposes it to have been effected by the 
operation of oxygen, and conceives the precipitate 
that was formed to have been the colourable matter 
saturated with oxygen *. 
365. This opinion, that the discharge of colour 
depends on the simple addition or combination of 
oxygen, appears to us not less gratuitous than that 
of M. Senebier, which considers it to arise from the 
escape of carbon ; for, though it be granted that 
oxygen disappears, yet we are not told into what 
combination it enters, nor in what way it acts on this 
colourable matter. Dr Bancroft himself, in his ob- 
servations on Berthollet's doctrine, remarks, that the 
muriatic acid discharges colours in the same manner 
as other acids, although it has never been proved to 
contain oxygen ; or, if it do, this oxygen is united 
by an affinity too powerful to be overcome by any 
known substance or means. When, therefore, says 
he, this acid changes or destroys colours, it must be 
by producing effects different from those of combus- 
tion ; and as these changes are in most cases similar 
to those produced by the other acids, which contain 
oxygen, it seems reasonable to conclude, that these also 
act upon colours by producing other effects than those 
of combustion f. If, however, this reasoning be va- 
lid against the hypothesis of combustion, because the 
muriatic acid cannot afford oxygen to carry on that 
process, it must, for similar reasons, apply equally 
Against Dr Bancroft's hypothesis, which ascribes the 
* Pbilos. of Perm. Colours, vol. i. p. 43. f Ibid. p. 51. 
