it is converted into an insipid inert substance, no longer soluble in 
water. By the oxygenated mariatie acid, it is also changed into an 
insoluble concrete of a yellow color. These poperties seem suffi- 
cient to characterize it as a distinct principle; but there is still — 
some obscurity respecting the mode of its existence in vegetable pro- 
ducts. According to the French chemists, it is the principle upon 
which the virtues of many vegetable medicines depend, though it is 
difficult to obtain it unmixed with the other principles. - By exposure 
to heat, it affords an empyreumatic acid and oil, and a small quantity 
of ammonia. Its elements, therefore, are carbon, hydrogen, azot, 
and oxygen. se 
‘Ou is a very common proximate principle of vegetable matte 
it is of two kinds, cxpeinannin petiionscat and distilled, volatile or 
essential oil. These have some common qualities, they are inflam- 
mable, insoluble in water, and unctuous, to the touch ; but they are 
also possessed of peculiar properties by which they are distinguished. 
The first class, the expressed, fat, or fixed oils, are thick and unc- 
tuous, nearly insipid and inodorous; they congeal on exposure to 
cold, are soluble in water or in alcohol; they combine with the alka- 
lies, forming soaps; they are not volatilized at the temperature of pe 
212° ; they are decomposed in close vessels, and burn when the at- 
mospheric air is admitted, the products of 0 
water and carbonic acid. At a natural temperature they 
sorb oxygen, and by long exposure to the air they are thicl 
and at length become concrete. They consist of carbon and 
gen in the proportion nearly of 12 of the former to 3 of the latter. 
<a 
Expressed oils are*ger contained in the seeds and fruit of 
certain vegetables. They are extracted by expression or decocti 
with water, and are thus frequently in part impregnated with the ex- 
tractive, mucilaginous, or resinous particles, whence in some instan- - 
ces they derive taste, odor, and even some medicinal virtues. In 
general, however, they are insipid, and are used in medicine princi- 
pally for their lubricating quality ; they are diffused in water by the 
medium of gum or sugar, or by the addition of a: 
‘of the alkalies. ~ et 
slate; essential, or distilled oils, differ fromthe 
several of their properties. They are volatile at a low 
are entirely and quickly converted into vapor, at the hea 
water, without being decomposed ; they are soluble ‘in a small) 
portion in water, and are more abundantly soluble in alcohol ; the 
do not combine with the alkalies with facility ; they are more highly 
inflammable than the fixed oils,.and, on exposure to the atmosphere, 
slowly absorb oxygen, are ened, and are at length converted into 
substances of a resinous nature. In their. composition, they contain 
more hydrogen proportioned to their carbon than the fixed oils do. — 
The essential oils are in general highly odorous, pungent, and 
often very acrid; they exist in greatest quantity in the aromati 
=. and are usually obtained by distillation ; the vegetable be 
‘Reated to 212° along with water, and the oil being volatilized | 
the aqueous vapor. As medicines, they possess a highly stimu 
SOW ee ae : es 
ee 
ss there exists a natural com: 
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