252 Mr. Donovan on the nature and combinations 
a slight odour of the gas, but even this is destroyed by expo- 
sure to the air for a few days. 
Vauquelin observes, that malic acid thus obtained is nearly 
colourless: his was therefore diluted. I have found that it 
becomes perfectly brown by concentration : and I have de- 
composed and recomposed malate of lead several times, using- 
each time the same specimen of malic acid, yet so obstinately 
did the colouring matter adhere, that it was always found in 
the resulting acid. Thus, as far as we know, this acid can- 
not be procured free from colour ; and the nearest approxi- 
mation is that obtained by Vauquelin’s process. 
Suggestions concerning the state in which acids may previously 
have existed in vegetables. 
I have sometimes indulged in the supposition, that the vege- 
table acids are not primarily formed by the immediate union 
of their elements, but that they may have previously existed 
in a definite combination, called the bitter principle. It is 
possible that this principle may be a compound basis, which 
by uniting to oxygen, or by undergoing more complicated 
processes, might change its nature so far as to become an 
acid. The whole is a mere conjecture, and perhaps deserv- 
ing of little consideration ; the facts, however, which sug- 
gested it may be noticed. 
The sweetness of any vegetable juice, has been generally 
attributed to a sweet principle called sugar. In the same 
manner it has been lately supposed, that bitterness depends 
on a bitter principle, which, although variously disguised, 
is always identical. Dr. Thomson has shown, that when 
