OIL 
geal, or assume a granulated solid consis- 
tence at different temperatures. Of these 
last some are always found to be in the con- 
crete state. Several of the volatile oils are 
susceptible of crystallization, depositing in 
the remaining portion of the oil, which con- 
tinues liquid, transparent polyhedrons, more 
or less of a yellow colour, which are found 
to be pure oil. This last change is probably 
owing to an incipient oxydation ; for it never 
takes place unless oils have been kept for 
some time. There is great variety of co- 
lour among volatile oils. Some indeed are 
nearly colourless, as the oil of turpentine ; 
but in general they are of different shades 
of colour. Some are yellow, as the oil of 
lavender; some are of a reddish yellow or 
brown, as the oil of cinnamon or of rhodium ; 
some are blue, as the oil of chamomile; and 
some are green, as that of parsley. But 
the most prevailing colour among volatile 
oils is yellow or reddish. 
Volatile oils have almost always an acrid, 
hot, and even burning taste. It is observed 
that the most acrid vegetable matters do 
not yield an oil possessed of this quality. 
The specific gravity of volatile oils is gene- 
rally less than that of water. Some volatile 
oils, however, as those of sassafras and 
canella, have a greater specific gravity. 
The specific gravity of oils varies from 0.87" 
to 0.99, in those which are lighter than 
water; but those which are heavier are from 
1.03 to 1.40. When volatile oils are exposed 
to the light, the colour becomes considerably 
deeper; they become tliicker, and increase 
in specific gravity. ^V^hen volatile oils are 
exposed to heat, they evaporate very 
readily. They are much more combus- 
tible than the fixed oils; and in burning 
give out a great quantity of smoke, a very 
bright white flame, and a good deal of heat. 
They require a greater proportion of oxy- 
gen than the fixed oils, and yield a greater 
quantity of water. This arises from a 
greater proportion of hydrogen, and a smal- 
ler quantity of carbon, which they contain. 
The volatile oils are in some degree soluble 
in water. When they are agitated with this 
liquid they combine with it, and communi- 
cate a very strong odour, and a slightly acrid 
taste. Phosphorus and sulphur are soluble 
in volatile oils. With phosphorus the solu- 
tion is luminous in the dark, is extremely 
fetid, and gives out by the force of heat 
phosphorated hydrogen gas. Some of these 
oils are employed in medicine. They are 
used also for the solution of those substances 
■which are to be employed as varnishes ; 
OLD 
and many of them are used in perfumery. 
As many of the volatile oils are produced 
but in small quantity, they are consequent- 
ly high priced. There is therefore some 
temptation to adulterate them with fixed 
oils, with cheaper volatile oils, or with other 
substances, to increase the quantity. It is 
therefore of some importance to be able to 
detect such frauds. When a volatile oil is 
adulterated with a fixed oil, there is a very 
easy test to discover it. Let a single drop 
of the oil tliat is suspected fall on clean pa- 
per, and expose it to a gentle heat. If the 
oil is pure, the whole will be evaporated, 
and no trace remain on the paper ; but if it 
has been mixed ■tt'ith a fixed oil, a greasy 
spot remains behind. Volatile oils are fre- 
quently adulterated with oil of turpentine ; 
but this can only be detected by its peculiar 
odour, which continues for a longer time 
than most of the other volatile oils. When 
they are adulterated with alcohol, it is easily 
detected by mixing a little of the oil witlj 
water, which immediately produces a milki- 
ness, by the abstraction of the alcohol from 
the oil, and its combination with the water. 
There is another class of oils, known under 
the name of empyreumatic oils, which have 
different properties from those which have 
been described. These oils are acrid and 
stimulating, with a strongly fetid and disa- 
greeable odour. It would appear that 
these properties are owing to a partial de- 
composition of other oils. These oils are 
produced, as the name imports, by the ac- 
tion of fire. They are obtained when oils 
are forced to rise in vapour, and pass over 
in common distillation, with a greater de- 
gree of heat than that of boiling water, or 
by the application of a strong heat to sub- 
stances from which no oil ■w'as previously 
extracted. These empyreumatic oils agree 
in some of their properties with the volatile 
oils. They combine in small proportion 
with water, and they are soluble in alcohol ; 
and probably any difference that exists be- 
tween them is owing to a partial decompo- 
sition; for when they are distilled, the oil 
is restored to a state of purity, and the car- 
bonaceous matter which had been separated 
remains behind. See Thomson’s Chemistry, 
OINTMENT. See Pharmacy. 
OLAX, in botany, a genus of the Trian- 
dria Monogynia class and order. Natural 
order of Sapotae, Jussieu. Calyx entire; 
corolla funnel-form, trifid ; nectarium four ; 
berry three-celled, many-seeded. There is 
but one species, viz. O. Zeylanica, a native 
of Ceylon, 
