124 
however, is not generally true; for we found the 
blue infusion of red cabbage, which had been first 
rendered green by an alkali, to acquire a red colour 
by exposure to the air. Indeed, it is reasonable to 
expect, that different plants should furnish colourable 
juices which are variously affected by chemical 
agents, since not only do vegetables vary in their 
properties from each other, but their colourable mat- 
ter is often extracted from different parts of the plant. 
These colourable matters not only possess peculiar 
properties, but differ essentially from each other, says 
Dr Bancroft, and must therefore be applied in different 
ways, and with very different means, to produce per- 
manent colours in other matters* Many species, 
however, of these colourable matters suffer, he adds, 
nearly similar changes from the action of acids, alka- 
lies, and other chemical agents ; from which it may 
be presumed, that there is something of a common 
or similar nature in many of them *, 
372. That the discharge of colour in the forego- 
ing examples, proceeded from the formation of an 
acid, seems to follow from the experiments of M. 
Berthollet. He placed portions of tincture of turn- 
sole in contact with oxygen gas, over mercury, both 
in darkness and in the light of the sun ; the first 
portion continued a long time without alteration, and 
without producing any diminution in the air, but the 
second lost its colour and became red ; the oxygen 
gas had in great part disappeared and a little carbo- 
nic acid was formed, which, doubtless, says M. Ber- 
* On Perm. Colours, p. 72. 3. 
