117 
cy of light, are conjointly required to produce these 
changes of colour in the green solution : and, in con- 
formity with his hypothesis, concerning the cause of 
the green colour of plants, M. Senebier supposes the 
oxygen to be necessary to carry off the carbon, on 
the presence of which substance, he believes this co- 
lour to depend *. 
362. The observations of Senebier were repeated by 
M. Berthollet, who remarks that the green colour of 
the solution disappears rapidly in sunshine, more slow- 
ly in the shade, and not at all, or very slowly indeed, 
in perfect darkness. To discover the effects produced 
in the air, he inverted a vessel, half filled with green 
solution, over mercury, and exposed it to the light 
of the sun. When the colour was discharged, the 
mercury was found to have risen into the vessel : 
and, consequently, says M. Berthollet, the oxygen 
had combined with the colourable parts of the solu- 
tion f. He did not observe the precipitation which 
M. Senebier mentions, but the solution continued 
transparent, and of a clear yellow colour. When 
* Physiol Veg, torn, iii, p 1/5. 
f Authors, in general, have denominated the matter which, in 
these experiments, affords colour, the colouring parts of the vege- 
table. This matter, however, is not itself coloured, but is only 
capable of exhibiting colours, by the addition of other matters : 
and hence we have ventured to call it the colourable^ rather than 
the colouring parts of the plant, by which we merely indicate its 
property of becoming coloured, but not its actual possession of 
colour. When, by the addition of other matter, it is made to as- 
sume colour, and is, in that state, employed as a dye or a pig- 
ment, it may then be called colouring matter. 
