169 
433. The muriate of silver, which is thus acted on 
by light, is composed of 15 parts oxide of silver, 18 
muriatic acid, and seven of water. Its discoloura- 
tion has been ascribed to a partial reduction of the 
oxide ; but, by operating upon the salt under water, 
Scheele and Berthollet found, that muriatic acid was 
liberated ; and to the disengagement of this acid a- 
lone, the discolouration seems to be owing *. In 
conformity with this explanation, Dr Bancroft ob- 
serves, that, if this salt be covered with muriatic acid, 
instead of water, it experiences no change, though 
exposed, for many days, to the direct rays of the 
sunf. 
434. The action of light on vegetables, seems to 
resemble, in many respects, its operation on inanimate 
bodies. We have already seen, that light, in its un~ 
decomposed state, causes the expulsion of oxygen 
gas, and gives rise, at the same time, to the green co- 
lour of plants. To discover the manner in which 
it was more immediately concerned in these opera- 
tions, M. Senebier sowed different quantities of let- 
tuce seeds in several small cups. One of these cups 
he left exposed to the light and air ; another he 
placed in darkness ; a third he confined under a 
large glass vessel, whose bottom was thrust so far up 
into its body, as to leave a hollow space, nine or ten 
inches in height/ and four or five in width ; this ves- 
sel was then filled with water, through which the 
light, that fell on the seeds beneath, necessarily pass- 
* Murray's System of Chemistry, vol. iii. p. 1<20. 
t On Permanent Colours, p. 35. 
