148 
part itself, as it may affect the reflexion of trans- 
mission of light. 
404. That the red colours, at least of the damask 
rose, arise from the action of an acid substance > may be 
collected from some experiments of M. Senebier. He 
remarks, that the petals of this flower are rendered 
white by digestion in alcohol ; but their former colour 
is restored if they be exposed to light and air. If, how- 
ever, they be confined only in nitrogen gas, they do not 
recover their fine colour, but have only a yellow hue. 
The sun's light gradually accelerates the restoration 
of their colour, when they are confined in common 
ah*j but it is not indispensably necessary ; for, where 
the air has free access, these petals resume their co- 
lour in obscurity, but more slowly than when exposed 
to light. When the colour is thus recovered, it is 
again given out to alcohol, and these operations may 
be repeated several times, until the colourable matter 
is exhausted. Various other flowers afford the same 
phenomena, but not in so marked a degree *. From 
these facts, we collect, that, though the sun accele- 
rates the restoration of the colour, yet that air also is 
necessary 5 and since it is the oxygenous part of the 
air which alone seems to produce the effect, we may, 
from the analogy of similar changes on the colour- 
able matter (372.) of plants, conclude, that carbonic 
acid is produced, and by its action restores the co- 
lour. This conclusion is supported by an obser- 
vation of M. de la Folie, who remarks, that roses, 
which, have been whitened by the vapour of burning 
Mem. Ph}S< Chim. torn. iii. p< 120. et seq. 
