OIL. 
called drying oils. The oil of poppies, hemp- 
seed oil, and particularly linseed oil, are 
possessed of this property. The nature of 
the change which takes place in these dry- 
ing oils is supposed to depend on the ab- 
sorption of oxygen ; and this oxygen com- 
bining with the hydrogen of the oil forms 
water. This opinion is supported by the 
practice which is followed to increase the 
drying property of linseed oil. It is usu- 
ally boiled with litharge, before it is em- 
ployed by painters. The litharge in this 
case is partly reduced to the metallic 
state, by being deprived of its oxygen, 
which is supposed to combine with the oil, 
Phosplwrus combines with oils, with the as- 
sistance of heat. A small portion of the 
phosphorus is dissolved, which communi- 
cates a luminous property to the oils, so that 
when they are spread upon any surface they 
shine in the dark. Hence some twenty years 
ago a person exhibited in . London, as the 
everlasting lamp of the ancients, a vessel 
containing phosphorus immersed in oil. 
The various purposes to which fixed oils 
are applied, are too well known to require 
particular enumeration. They arfe employ- 
ed in domestic economy, either as articles 
of food, and for this purpose are used alone, 
or in combination with other substances; 
or they are employed for giving light, by 
being burnt in lamps. They are used in 
medicine, either on account of the proper- 
ties which peculiar oils possess, or on ac- 
count of the properties they communicate 
to other substances with which they are 
combined. In this state the use of oils is 
well known in the form of unguents, plas- 
ters, and liniments. In the arts, fixed oils 
are of the most extensive utility. They are 
employed in the fabrication of soaps, for 
mixing colours in painting, for some kinds 
of varnish, and for defending substanees 
from the action of air and moisture. 
Volatile oils are distinguished from the 
fixed oils by their volatility, fragrance, and 
acrid taste. They are also known under 
the name of aromatic oils, from their odour ; 
or essential oils, or simply essences, from 
being supposed to constitute the essence or 
the existence of the vegetable matters which 
furnish them. Volatile oils are not limited 
to particular parts of plants, but are found 
to exist in every part of the plant, excepting 
in the seed, which furnishes the fixed oils. 
A great number of roots, which are gene- 
rally distinguished by an aromatic odour, 
and have more or less of an acrid taste, 
aft'ord volatile oils. They are furnished also 
by many woods, such as those of the pine 
and fir tribe, and by many of those which 
are natives of warm climates. The leaves 
of a great nuirrber of plants belonging to 
the Didynamia class also afford volatile oil, 
as well as many of the umbelliferous plants. 
It is obtained also from many flowers of 
vegetables, and also from the covering of 
marry fruits, as the skin of oranges and 
lemons. It is likewise obtained fr om a great 
nitmber of seeds ; but it is never found in 
the cotyledons or lobes themselves, but on- 
ly in the external covering, The quantity 
of volatile oil which is obtained from vege- 
tables, varies according to the age, the soil 
in which they grow, and the state of the 
plant, Some plants while green furnish it 
in greatest abutrdance, while others yield 
most when they are drjq There are two 
processes by which volatile oil may be ob- 
tained. When it exists in plarrts in great 
abundance, and in vesicles in a fluid state, 
it may be separated by mechanical means, 
Thus, by sinrple expression, the volatile oils 
are extracted from nrany plants, as, for in- 
stance, from the frrrit of the orange and the 
lemon. From the outer rind of these fruits, 
when they are fresh, the volatile oil is ob- 
tained in the liquid form ; but in general 
the volatile oils of plants are neither so 
abundant, nor do they exist in that state of 
fluidity by which they can be procured by 
so simple a process, In most cases they are 
subjected to the process of distillation; 
and for this purpose they are macerated for 
some hours in water. They are then intro- 
duced into a still with the water; a mode- 
rate heat is applied and continued till the 
fluid boil, when a great quantity of vapour 
of water, mixed with the volatile oil, passes 
over, and is received in proper vessels. The 
oil collects on the surface of the water, from 
which it may be easily separated. The 
water itself is of a milky colour, on account 
of a small quantity of oil suspended in it ; 
and even after the water becomes transpa- 
rent by the particles of the oil separating 
from it, and rising to the top, it is still load- 
ed with the peculiar odour of the plant. 
The volatile oils are particularly distin- 
guished by their fragrance, which varies 
in the oils extracted from different plants. 
The consistence of the volatile oils also va- 
ries considerably. Sometimes they are as 
fluid as water, which is the case with those 
oils obtained by expression. Some are thick 
and viscid, as those generally are which 
are extracted from woods, roots, barks, and 
fruits of the warmer regions. Some con- 
