116 
360. The heat that exists in the solar beam, is not, 
ays M. Senebier, the cause of this change of colour; 
for, when light is excluded, the green tincture pre- 
serves its colour for many months, although it be 
exposed to a considerable degree of heat. Even at 
the temperature of 60 Reaum. the colour suffers no 
change, if the solution be kept in darkness ; so that 
light, not heat, is the agent concerned in producing 
this change. But though light thus accelerates this 
change, yet its direct influence is not essential to its 
production ; for the green colour will be gradually, 
but much more slowly, discharged in the shade *. 
361. Besides light, however, air also is necessary 
to the success of the experiment j for vessels, which 
are quite filled with green solution, and closely stop- 
ped, suffer no change of colour for many months, e- 
ven in sunshine : and those portions of solution, which 
present the greatest surface to the air, or which are 
contained in phials with the largest quantity of air, 
experience the greatest change ; so that the disco- 
louration is al \vays proportional to the quantity of air 
that is present ! It is, however, only the oxygenous 
part of the air that contributes to these changes ; for 
when confined in phials which contain only nitrogen 
gas, the solution experiences no change, although it 
be exposed to the sun ; but when oxygen gas is pre- 
sent, the discolouration is then more rapidly effected, 
than by common air alone *. It follows from these 
facts, that the presence of oxygen gas, and the agen- 
*Mem. Phys. Cbum. t. iii. p. g. p. 13. 
t Ibid* p. 15. 16. I Ibid. p. 18, 
