166 
of its colour. Because air is necessary to the vege- 
tation of plants, M. Fourcroy, indeed, ascribed the 
different shades of vegetable colours to the different 
degrees in which he supposed oxygen to combine 
with them * ; but it has been sufficiently shewn, 
that, during vegetation, the oxygenous portion of 
the air never combines with plants ; and that, as far 
as external agents are concerned, light, and not air, 
is the ostensible cause of their colouration. The or- 
dinary effects of acids in changing colours, must not 
be attributed to the oxygen they contain, but to 
their action as acids ; for muriatic acid, which does 
not yield its oxygen to any known substance, acts, 
like other acids, upon vegetable infusions ; and oxy- 
muriatic acid, which, of ail substances, most readily 
parts with its oxygen, entirely destroys all vegetable 
colours !. In whatever light, therefore, this subject 
is viewed, no good reason appears for considering 
the element of oxygen as essentially concerned in 
producing the colouration of natural bodies. 
430. Since, then, neither the mechanical doctrine 
of density, nor the supposition of phlogiston, nor the 
actual combination of oxygen with bodies, seems 
sufficient to explain those affections of light, from 
which the diversity of their permanent colours pro- 
ceeds, we must seek out some other mode of ac- 
* Mem. de 1'Acad. an. 1789, p. 335. 
f In our opinion, the late experiments of Mr Murray seem to 
establish the compound nature of oxy-muriatic gas, in opposi- 
tion to the view of Mr Davy, who regards it as a simple sub- 
stance *. 
* Nicholson's Journal, Feb. 1811. 
