known volatility of these oils, or on the probability of part of them 
being dissolved by the sonia air. : “ a ee 
After this enumeration of the proximate principles of veg 
it. may be proposed as a question of some importance, whether j 
xist mthe vegetable in a state of chemical combination, or whether 
they are merely mixed together ? 
The latter seems most probable. These principles can often be 
distinguished as existing separately from each ot er, and even as 
placed in separate cells; they can in many cases be separated by 
mechanical means ; and even where they are more intimately mixed, 
that change of properties does not take place, which we must have 
expected were they chemically united, the virtues of each principle 
being discernible in the entire mixture, weakened, but not ¢ ; 
It seems to follow, therefore, that the virtues of vegetable substances 
do not depend on chemical combinations of their proximate princi- 
ples, but rather on the peculiar ultimate composition of one or other 
of these principles. Hence also it is evident, that in’separating the 
proximate principles of any vegetable, we cannot expect to alter or 
improve its virtues, farther than in concentrating them by a separa- 
tion from what is inert, or in separating principles which are posses- 
sed of different, or even opposite powers. The attainment even of 
these ends, however, is, in innumerable cases, of importance in their 
exhibition as medicines, : +9 aE eas 
From this enumeration of the proximate principles of vegetables, 
we may perceive the reasons for those pharmaceutic processes to 
which plants are usually subjected. ; 
- By infusion in water, we impregnate the fluid with the gum, sugar, 
extract, tannin, saline substances, part of the essential oil, and part 
also of the resinous principle. The aroma of the plant is generally 
first taken up: by longer infusion the water is loaded with the co- 
loring astringent and gummy parts: these are also most abundant- 
ly dissolved when the temperature is high. Hence an infusion 
differs according as the water has stood longer or shorter on the 
materials, and according as it has been promoted or not by heat. An 
infusion made in the cold is in general more grateful, while one made 
with heat, or by keeping the fluid Jong upon the materials, is more’ 
strongly impregnated with active matter. - 
By decoction or boiling, the solvent power of the water is still far- 
ther increased ; and hence the liquor always appears darker colored, 
and is, in fact, more loaded with the principles of the vegetable 
which it can hold dissolved. The volatile parts, however, particu- 
larly the essential oil, are entirely dissipated ; and therefore it is an 
improper process for those vegetables whose virtues depend, wholly 
er partially, on these parts. - Even the fixed principles of vegetables, 
at least some of them, are injured by long decoction. The extrac- 
tive matter, for instance, gradually absorbs oxygen from the atmos- 
phere, and is converted into a substance nearly insipid and inert. 
ium, Peruvian bark, and many other vegetables, are injured in 
this manner by decoction, especially if the atmospheric air is freely 
adn ; and these two circumstances, the dissipation of the vola- 
tile matter, and the oxygenation of the extractive, considerably limit 
the application of this process: It is still used, however, with advan- 
