173 
sun's rays, says he, that the production of ve- 
getable colours is owing. Light, also, disengages 
oxygen from nitric and oxy-muriatic acids, from some 
metallic oxides, and from plants in a state of vegeta- 
tion. In these examples, he continues, effects oppo- 
site to those of combustion are produced ; but when 
it contributes to the destruction of colours, it com- 
bines oxygen, and produces a kind of combustion. 
What, however, are the circumstances, and what the 
affinities, which determine sometimes one effect, and 
sometimes the other, he does not know ; but both, 
he adds, are equally proven *. Dr Bancroft, also, 
refers to different instances, in which light acts some- 
times by separating, and sometimes by combining, 
oxygen with bodies ; which he ascribes to the varied 
operation of an affinity exerted between them f. These 
various, and apparently contradictory effects of light, 
may, perhaps, receive illustration from a more minute 
inquiry into the nature and constitution of that sub- 
tile matter. , The researches of modern philosophers 
furnish many new and important facts to aid our in- 
vestigation ; and it is by collecting and comparing 
these that we are led to indulge the hope of being 
able to penetrate somewhat farther into the secrets 
of its chemical action. 
440. THE colours of the primary rays of light, 
and their different degrees of refrangibility and re- 
* Elem. de la Teinture, torn. i. p. 58. 
| On Permanent Colours, p. 46, 
