125 
thollet produced the change of colour from blue to 
red *. Hence, therefore, we learn, that vegetable 
infusions, like the solutions before mentioned (367.)? 
form carbonic acid, when they are placed in contact 
with oxygen gas ; and that this acid, when thus 
formed, is able to discharge their colour. 
373. Thus, then, it appears, that the phenomena 
exhibited by solutions of the colourable parts of ve- 
getables in alcohol and in water very nearly agree ; 
that both are rendered respectively red or green by 
the predominance of acid or alkaline matter ; and 
that, according to the proportions in which these in- 
gredients exist, various intermediate tints are produ- 
ced. Even the pale liquor, obtained by digesting 
etiolated leaves in alcohol, was found by M. Senebier 
to become green by the addition of alkali t ; so that 
these leaves contain resinous or colourable matter si- 
milar to that which green leaves afford, and the chief 
difference between the two solutions is in their pro- 
portion of alkaline matter. 
374. But acids and alkalis not only change the 
colourable matter of vegetables, when it is extracted 
by alcohol or water, but they act also on the entire 
leaf. Sulphurous acid quickly discharges the colour 
of green leaves, and when these leaves are plunged 
in the vapour of nitric, muriatic, or sulphuric acids, 
they pass rapidly from a green to a yellow colour. 
Etiolated leaves suffer at first no change, but at length 
become whiter. Similar effects are produced in green 
* Berthollet's Elem. de la Teinturc, vol. i. p, 56. 
f Mem. Phys, torn. ii. p, 152. 
