— —— 
of a newly-discovered vegetable Acid. 19 
off, and the liquor should be boiled in a vessel freely exposed, 
until paper moistened with acetate of lead is no longer blackened 
by the discharged vapour. This acid is the purest that can be 
obtained ; it retains a slight odour of the gas, but even this is 
destroyed by exposure to the air for a few days. 
Vauquelin observes, that malic acid thus obtained is near ly co- 
lourless: his was therefore diluted. I have found that it be- 
comes perfectly brown by concentration: and I have decomposed 
and recomposed malate of lead several tines, using each time 
the same specimen of malic acid; yet so obstinately did the co- 
louring matter adhere, that it was always found in the resulting 
acid. Thus, as far as we know, this acid cannot be procured 
free from colour ; and the nearest approximation 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 deserving of little consideration ; 
the facts, however, which suggested it may be noticed. 
The sweetness of any vegetable juice has been generally at- 
tributed 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 w hen water is digested over 
quassia, and afterwards evaporated to dryness, a ‘transparent 
substance is obtained, which differs in its properties from all 
other vegetable principles: this he considers as the bitter prin- 
ciple, and, I believe, with very great justice. I found that the 
liquor obtained by digestion, although slightly coloured, was 
transparent even to the end of the evaporation. The resulting 
mass was nearly transparent, and minute in quantity, considering 
the proportion of quassia employed ; and such was its bitterness, 
that a particle plactd on the tongue, which could not have ex- 
ceeded oth of a grain, diffused an intense bitterness over the 
whole mouth and fauces. 
This matter was heated with nitric acid; it dissolved with 
effervescence, and the bitterness was no longer sensible. The 
remaining substance formed a precipitate in acetate of lead, 
which possessed all. the properties of malate of lead: and it 
B2 appeared 
