163 
transparent plates, the chemists were generally indu- 
ced to make all colour depend on the principle of in- 
flammability or phlogiston, which, until very lately, 
was supposed to exist in metals and other substances, 
where there certainly is no evidence or appearance 
of it." 
426. " But since the existence of phlogiston in bo- 
dies has been denied by the pneumatic chemists," 
continues Dr Bancroft, " they have, in most cases, at- 
tributed the origin and changes of colours to the 
application or combination of different gases, and 
particularly of oxygen, in different proportions ; and 
it has been supposed that these gases possessed consi- 
derable refractive powers, and were thereby enabled 
to produce effects on colours, like those which the 
followers of Stahl had imputed to phlogiston *." 
" Hence," he adds, " M. Berthollet, in his work on 
dyeing f, intimates that many important observations 
remain for those who would follow the steps of the 
great Newton, and compare the refractive powers of 
the different gases, and of other substances, the con- 
stituent principles of which are now known." 
427. This opinion concerning the influence of oxy- 
gen, in producing the colouration of bodies, seems 
to have arisen from the well-known changes of 
colour which certain metals exhibit during their oxi- 
dation. If, however, oxygen were the cause of these 
changes of colour, some uniformity of effect might 
be expected to attend its different degrees of combina- 
* Philos. of Perm. Colours, p. 27- 
t Elem. de la Teinture, torn, i, p. 5, 
