form a thin layer on its surface, and is in 
that manner exposed to the atmosphere ; 
these changes are produced much sooner. 
BertholLet, who lirst examined these phe- 
nomena with attention, ascribed them to the 
action of light : but Sennebier observed that 
lio" such change was produced on the oil 
though ever so tong exposed to the light, 
I provided atmospherical air was excluded; 
but that it took, place on the admission ot 
oxygen gas, whether the oil was exposed to 
the light or not. It cannot be doubted, then, 
that it is owing to the action of oxygen. It is 
supposed at present to be the consequence 
of the simple absorption of oxygen and its 
combination with the oils. 
3. Both these classes of oils, when expos- 
ed in considerable quantity to the action of 
the atmosphere, undergo another change, 
well known under the name of rancidity. But 
the fat oils become rancid much more rea- 
dily than the drying oils. Rancid oils are 
thick, have usually a brown colour, convert 
vegetable blues to red, and have the smell 
and taste of sebacic acid. During the change 
which they undergo, some drops of water 
also appear on their surface. The rancidity 
of oils then is owing to the formation of a 
quantity of acid in them. This, together 
with the water, is evidently the consequence 
of a partial decomposition. 
Fixed oils readily dissolve sulphur when 
assisted by heat. 'The solution assumes a 
reddish colour. When distilled, there comes 
over a great quantity ofsulpluireted hydrogen 
gas. When the solution is allowed to cool, 
the sulphur is deposited in chrystals. By 
this process Pelletier obtained sulphur in re- 
gular octahedrons. 
They likewise dissolve a small proportion 
of phosphorus when assisted by heat. These 
oily phosphurets emit the odour of pbosplui- 
ret’ed hydrogen, and yield, when distilled, a 
portion of that gas. When rubbed in the 
open air, or v hen spread upon the surface 
of other bodies, they appear luminous in con- 
sequence of the combustion of the phospho- 
rus. When hot oils saturated with phospho- 
rus are allowed to cool, the phosphorus chrys- 
tallizes in octahedrons, as Pelletier ascer- 
tained. 
Charcoal has no sensible action on fixed 
oils; but when they are filtred through 
charcoal-powder, they are rendered purer, 
the charcoal retaining their impurities. Nei- 
ther hydrogen nor azotic gas has any action 
on fixed oils. 
Fixed oils have scarcely any action upon 
metals ; but they combine with several me- 
tallic oxides, and form compounds known by 
the name of plasters. See Piaster. 
They combine likewise with alkalies and 
earths, and form w ith them compounds called 
soaps. The fat oils enter into these combi- 
nations much more readily than the drying 
oils. See Soap. 
Fixed oils absorb nitrous gas in consider- 
able quantities, and at the same time become 
much thicker and specifically heavier than 
before. 
Sulphuric acid decomposes fixed oils, at 
least when concentrated. It renders them 
first thick and of a brown colour; then water 
is formed, charcoal precipitated, and an acid 
formed. Nitric acid renders them thick and 
viscid. When nitrous acid is poured upon 
the drying oils, it inflames them without ad- 
VoL. II. 
OILS. 
dition ; but it does not produce that effect 
upon the fat oils, unless it is mixed with a 
portion of sulphuric acid. 
Plie affinities of fixed oils aje as follows : 
Lime, Ammonia, 
Barytes, Oxide of mercury, 
Fixed alkalies, Other metallic oxides, 
Magnesia, Alumina. 
The importance of fixed oils is w'ell known. 
Some them are employed as seasoners of 
food; some are burnt in lamps ; some form 
the basis of soap ; not to mention their utility 
in painting, and the many other important 
purposes which they serve. 
Oils, volatile, called also essential oils, 
are distinguished by the following properties: 
1 . Liquid ; often almost as liquid as water ; 
sometimes viscid. 
2. Very combustible. 
3. An acrid taste and a strong fragrant 
odour. 
4. Boiling point not higher than 212°. 
5. Soluble in alcohol ; and imperfectly in 
water. 
6. Evaporate without leaving any stain on 
paper. 
By this last test it is easy to discover whe- 
ther they have been adulterated with any of 
the fixed oils. Let a drop of the volatile oil 
fall upon a sheet of writing-paper, and then 
apply a gentle heat to it. If it evaporates 
without leaving any stain upon the paper, 
the oil is pure; but if it leaves a stain, it lias 
been contaminated with some fixed oil or 
other. , 
Volatile oils are almost all obtained froffi 
vegetables, and they exist in every part of 
plants ; the root, the bark, the wood, the 
leaves, the flower, and even the fruit : though 
they are never found in the substance of the 
cotyledons ; whereas the fixed oils, on the con- 
trary, are almost always contained in these 
bodies. 
When the volatile oils are contained in 
great abundance in plants, they are some- 
times obtained by simple expression. This 
is the case w ith the oil of oranges, of lemons, 
and of bergamot; but in general they can 
only be obtained by distillation. The part 
of the plant containing the oil is put into a 
still with a quantity of water, which is dis- 
tilled off by the application of a moderate 
heat. Hie oil comes over along with the 
w'ater, and swims upon its surface in the 
receiver. By this process are obtained the 
oils of peppermint, thyme, lavender, and a 
great many others, which are prepared and 
employed by the perfumer. Others are pro- 
cured by the distillation of resinous bodies. 
This is the case in particular with oil of tur- 
pentine, which is obtained by distilling a kind 
of resinous juice, called turpentine, that 
exudes from the juniper. 
The greater number of volatile oils are 
liquid, and some of them are as transparent 
and colourless as water. This is the case with 
the oil of turpentine; but for the most part 
they are coloured. Some of them are yel- 
low, as the oil of lavender; some brown, as 
the oil of rhodium ; some blue, as the oil of 
camomile ; but the greater number of vola- 
tile oils are yellow or reddish-brown. 
Their odours are so various as to defy all 
description. It is sufficient to say, that all 
the fragrance of the vegetable kingdom re- 
sides in the volatile oils. Their taste is al- 
O o 
299 
ways acrid, hot, and exceedingly unpleasant. 
Their specific gravity is for the most part 
less than that of water ; but some volatile 
oils, as those of canella and sassafras, are hea- 
vier than water. The specific gravity of the 
volatile oils varies from 0.8697 to 1.0439. 
Water dissolves a small portion ot volatile 
oils, and acquires the odour and the taste ot 
the oil which it holds in solution. 
When heated, they evaporate very readily 
and without alteration. They are much 
more combustible than fixed oils, owing to 
their greater volatility. They bum with .i 
fine bright white flame, exhale a great deal 
of smoke, deposit much soot, and consume 
a greater proportion of the oxygen of the 
atmosphere than fixed oiis. The products 
of their combustion are water and carbonic 
acid gas. From these facts it lias been con- 
cluded that they are composed of the same 
ingredients as the fixed oils, but that they 
contain a greater proportion of hydrogen. 
When exposed to the action of coid they 
congeal like the fixed oils ; but the tempe- 
rature necessary to produce this effect, varies 
according to the oil. Some of them, as oil 
of anise and of fennel, become solid at the 
temperature of 50 1 ; frozen oil of bergamot 
and of canella become liquid at 23 ’ ; oil of 
turpentine at 1 4°. Margueron exposed seve- 
ral volatile oils to a cold of 17 J . They con- 
gealed or rather chrystallized partially, and 
at the same time emitted an elastic fluid. 
These chrystals consisted partly of the oils 
themselves, partly of other substances. Some 
of them had the properties of benzoic acid. 
Volatile oils, when exposed to the 
action of light inclose vessels, and excluded 
from common air, undergo very singular 
changes. Their colour becomes deeper, 
they acquire a great deal of consistency, and 
their specific gravity is considerably increas- 
ed. The cause of these changes is but im- 
perfectly known. Tingrv, to w hom we are 
indebted for these interesting researches, 
has proved that light is a necessary agent. 
It was supposed formerly that they were oc- 
casioned by the absorption of oxygen; and 
w hen oxygen is present, it has been ascer- 
tained that it is absorbed; but Tingrv has 
proved that the same changes go on when 
oxygen is excluded. This philosopher as- 
cribes them ta the fixation of light. If this 
is the real cause, the quantity of light fixed 
must be enormous; for as the specific gravity 
of the oils is increased considerably while the 
bulk continues the same, it is evident that 
the absolute w eight must be increased pro- 
portionally. - One circumstance, however, 
renders this conclusion somewhat doubtful, 
at least in its full extent; and that is, that 
the quantity of change was always propor- 
tional to the quantity of the oil and, the quan- 
tity of air contained in the vessel. 
When exposed to the open air their co- 
lour becomes gradually deeper, and they ac- 
quire consistency, while they exhale at the 
same time a very strong odour. The air 
around, as Priestley first ascertained, is de- 
prived of its oxygen, a quantity of water is 
formed, and the oils at last, for the most part, 
assume the form of resins. 
Volatile oiis dissolve sulphur and phos- 
phorus, and the solutions have nearly the 
same properties as those made by means of 
fixed oils. 
They have no action on the metals, and 
