OIL. 
pKcation of heat, and merely by pressure, is 
■the purest ; but the quantity which seeds in 
general yield is comparatively small, and 
some seeds which contain a considerable 
portion of oil scarcely afford any when 
treated in this way. It therefore becomes 
necessary for extracting the oil from seeds 
of the latter description, and to have it in 
greater abundance from all seeds, to em- 
ploy heat to facilitate the separation of the 
oil from the mucilage, or other matters 
with which it is combined. For this pur- 
pose heat is applied, either to the apparatus 
which is employed in pressing out tlie oil, or 
the bruised seeds are exposed to the vapour 
of water, and sometimes they are boiled in 
the water itself; by which means those sub- 
stances which are soluble in water are sepa- 
rated, and thus the oily part which adhered 
to these substances is disengaged. The oils 
which are obtained in this manner are very 
impure. They are mixed with mucilage, 
and other parts of the substances from which 
they have been extracted. Many of these 
matters separate from the oils when they 
are left at rest. They are sometimes me- 
chanically purified by .filtration through 
coarse cloths, by which means the grosser 
parts are separated. Different oils too, it is 
said, undergo different kinds of purification 
by different manufacturers, but these pro- 
cesses are kept secret. After they have re- 
mained at rest for some time, they are fil- 
tered and agitated with water, by which 
the parts that are soluble in this fluid are 
separated from the oil. Sometimes they 
are gently heated, for a shorter or longer 
time, according to the nature of the sub- 
stances with which the oil is contaminated. 
Acids diluted with water are employed to 
separate the mucilage; lime and the alkalies 
are also used to combine with an acid which 
holds this mucilage in solution, and thus to 
favour its precipitation. Alum, chalk, clay, 
and ashes, are also employed in the purifi- 
cation of oils. 
Fixed oils are generally liquid, but of a 
thick, viscid consistence, and in general 
they are lighter than water. The specific 
gravity varies from 0.91, which is that of 
olive oil, to 0.94, that of linseed oil. The 
boiling point of the fixed oils is not under 
the temperature of 600°. When exposed to 
cold they congeal, and even crystallize. 
There is, however, a considerable variety in 
this respect among fixed oils: some become 
solid at the temperature of a few degrees 
above the freezing point of water; while 
Athers, on the contrary, require a degree of 
cold = 5°; and some remain fluid when ex^ 
posed to the greatest cold. Those oils, it 
has been observed, which most readily be- 
come solid, such as olive oil, are least sub- 
ject to change; while those which congeal 
with difficulty have a greater tendency to 
spoil and become rancid'. When fixed oil is 
exposed to heat it does not evaporate, till 
it is raised to the temperature of boiling, or 
600 ; but when it is thus raised in vapour 
its properties are changed. It is decom- 
posed by the separation of some of its prin- 
ciples. The part that is volatilized has a 
greater proportion of hydrogen ; charcoal 
is deposited, and water and sebacic acid 
are formed, while carbonated hydrogen gas 
is disengaged. When oil is exposed to the 
open air, and a burning body is brought in 
contact with it, it readily takes fire' and 
burns rapidly, with a yellowish white flame. 
It is on this conversion of oil into vapour, 
and the inflammation of this vapour, that the 
application of oil in lamps and candles de- 
pends.^ The oil is gradually and in small 
quantities brought in contact with the 
burning part of the wick ; it is converted 
into vapour, which is immediately inflamed, 
and continues to burn till new portions are 
supplied to undergo the same change, and 
thus keep up a constant and uniform light 
and heat. According to the analysis of 
ohve oil by Lavoisier, it is composed'of by- 
drogen and carbon, viz. 
Carbon 78.93 
Hydrogen ^l.OS 
100.00 
The fixed oils are insoluble in w'ater.Whcn 
it is necessary to combine them with this 
liquid, it is by means of mucilaginous sub- 
stances, in which case the mixture is known 
under the name of emulsion, or with alka- 
line substances, when it is distinguished bv 
the name of soap. Some of the.se oils be- 
come thick, opaque, white, granulated, and 
are analogous in appearance to tallow. Oils 
subject to this change are called fat oils ; 
such, for instance, is olive oil, almond oil, 
and rape-seed oil. This change is more or 
less rapid in different circumstances. If a 
thin layer of oil be spread on the surface of 
water, and exposed to the air, it takes place 
in a few days, and this effect is owing to the 
absorption of oxygen, which combines with 
the oils. But other oils, when they are ex- 
posed to the air, dry altogether, yet have 
the property of retaining their transparency. 
Oils which have this peculiar property are 
