385 
of simple alimentary substances, &c. 
colour, and seemed to have suffered decomposition, though it 
lost in weight only 2*6 per cent. more. Hence, gum pro- 
bably parts with the whole of the water not essential to its 
composition at the temperature of 21 2°, provided it be exposed 
for a sufficient time to this degree of heat. 
Substances belonging to this series appear in general to be 
of a weak or low kind, though they are probably very numer- 
ous. They may be readily distinguished by being converted 
into saclactic acid by the action of nitric acid. 
The vegetable Acids. 
Oxalic acid. Many years ago I ascertained that this acid 
in the crystallized state consists of 
Carbon 19- 04 
Water 42*85 
Oxygen 38*11, 
a composition assigned to it long since by other chemists, and 
now I believe generally admitted, except by Dr. Thomson, 
who informs us that he has met with a specimen containing 
as much as half its weight of water.* I have examined a 
great many specimens with the view of verifying this result, 
but hitherto have not been successful. 
Citric acid. This and all the following acids, except the 
malic, were analysed at the same period as the oxalic acid 
above mentioned, and the results have been recently verified. 
I find the crystals of citric acid to consist of 
Carbon 34*28 
Water 42*85 
Oxygen 22*87. 
* Attempt to establish the first principles of chemistry by experiment, ii. 103. 
MDCCCXXVII. 3 D 
