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weight of the substance, the remainder will be the 
weight of the gaseous matter. 
The acetous and mucous acids, and the ammo- 
nia formed, are usually in very small quantities ; and 
by comparing the proportions of water and charcoal 
with the quantity of the gases, taking into account 
their qualities, a general idea may be formed of the 
composition of the substance. The proportions of 
the elements in the greater number of the veget- 
able substances which can be used as food, have 
been already ascertained by philosophical chemists, 
and have been stated in the preceding pages ; the 
analysis by distillation may, however, in some cases, 
be useful in estimating the powers of manures, in 
a manner that will be explained in a future Lecture. 
The statements of the composition of vegetable 
substances, quoted from MM, Gay Lussac and 
Thenard, were obtained by these philosophers by 
exposing the substances to the action of heated 
hyper-oxymuriate of potassa ; a body that consists 
of potassium, chlorine, and oxygene ; and which 
afforded oxygene to the carbon and the hydrogene. 
Their experiments were made in a peculiar appa- 
ratus, and required great caution, and were of a 
very delicate nature. It will not therefore be ne- 
cessary to enter upon any details of them. 
It is evident from the whole tenor of the state - 
ments which have been made, that the most essen- 
tial vegetable substances consist of hydrogene, 
carbon, and oxygene in different proportions, gene- 
rally alone, but in some few cases combined with 
azote. The acids, alkalies, earths, metallic oxides, 
and saline compounds, though necessary in the ve- 
