floats on spirit of wine, and on every fluid yet known; its weight to that of water being as seven and a half to ten. It is so volatile, that it immediately evaporates, and spreads itself all through the surrounding atmosphere.

As it evaporates very soon in this spontaneous way, so it very soon arrives at its boiling point, which is 100 degrees Fahr. But its boiling point in vacuo appears from some experiments of Dr. Cullen far below frost; wherefore if the pressure of the air did not keep its particles together in all the variations that happen here, ether would never appear in any other form than that of an elastic fluid, like air.

After having shown that the evaporation of fluids is attended with considerable cold, Dr. Cullen repeated his experiments in vacuo. Among other fluids he tried ether, in a phial surrounded by water. On exhausting the air the ether boiled violently, and the water was converted into ice. Upon the evaporation of the ether, the sensible heat was converted into latent heat, which necessarily produces cold sufficient to freeze water.

Nitrous ether is made by gently distilling spirit of wine with one half its weight of strong nitric acid. The mixture thus distilled has a peculiar fragrance. At first the nitrous ether was only considered as a composition of spirit of wine and nitric acid, till a French chemist obtained ether from a mixture of this kind. He mixed

the spirit and acid in the ordinary way, only allowing more acid. He set the mixture by, well corked, and after some time found the ether swimming at the top, with a watery fluid at bottom. This process is disagreeable, on account of the vapours and the danger of bursting the vessel. There are also a muriatic and acetic ether.

3. OILS are combustible substances formed by nature in animal and vegetable bodies. They may be distinguished into two principal kinds; the volatile and the fixed.

1st. The volatile aromatic or essential oils affect the tongue generally with great pungency and heat, and have all more or less odour, generally strong. They have no smoothness, slipperiness, or unctuosity when felt between the fingers, a quality that belongs to the fixed or unctuous, which are particularly bland to the taste and smell; and these two qualities distinguish them, viz. want of taste and of smell. What have been called the empyreumatic resemble so much the aromatic, that they are only distinguished by their origin, being produced by means of fire.

Volatile oils, as well as the fixed, are composed chiefly of two ingredients, hydrogen and carbon, but they contain a larger proportion of hydrogen than the fixed oils, from which they may easily be distinguished, as when dropped on paper and evaporated they leave no mark or stain

behind.-The same test will of course determine whether they have been adulterated with any of the fixed oils. They are very numerous, and differ in fragrance according to the vegetable which has produced them; the most common are oil of turpentine, of lavender, and camo-` mile.

Camphor, according to some chemists, belongs to the class of essential oils, on account of its strong and diffusive smell, being very inflammable, and burning with great obstinacy, even sometimes upon the surface of water. Its flame is very white and bright. It impregnates water with its taste and odour, and dissolves very readily in spirit of wine, from which it may be separated by water. In all these qualities it agrees with the aromatic oils, but differs from them in appearing always in a solid form.

When exposed to heat, it leaves no feculencies behind, and cannot, like the rest, be resolved into an acid water and charcoal. Instead of being liquefied by heat, it totally evaporates before it becomes liquid: for it is one of those bodies which have their evaporating point below that of their fluidity, while under the pressure of the atmosphere; but if close corked up, it melts. into a transparent fluid.

To this class may perhaps also be referred balsams and resins, which differ from the oils chiefly in their degree of consistence. Some of the balsams approach in fluidity and tenacity the

volatile oils, and on examination show themselves to be natural compounds of the same materials. Some contain one-half their weight of oil, and the matter left behind is resinous, which may be restored to its former consistence by the restitution of the oil. Many of the aromatic oils, when distilled, leave behind them a resinous substance; and the resins, when first exuded, are almost as thin as aromatic oils.

Balsams and resins are usually obtained by wounding the bark, &c. of the vegetables which contain them, so as to let the juices flow out. These juices at first are fluid, but upon exposure to the air they become thicker by losing some of their water and more volatile parts. In some few cases they are separated by spirit of wine.

2. FIXED OILS are liquid, unctuous to the feel, very combustible, of a mild taste, their boiling point under 600°. They are insoluble in water and alcohol, and leave a greasy stain upon paper. They are obtained partly from animals, and partly from vegetables, and are often called expressed oils, from the manner of preparing or obtaining them. They are produced and contained in particular cells in considerable quantities, so that mechanical pressure is often sufficient for obtaining them. In some cases it is done by heating the vegetable or animal matter, and then exposing it to a strong pressure. It is thus linseed oil is prepared. In other cases the oily matter is boiled a considerable time in water,

as is sometimes practised with olives, and more especially with the animal oils. The water in this case dissolves the matter of which the cells are composed, and sets the oil at liberty.

"It deserves attention," Dr. Thomson remarks, "that the only part of vegetables in which fixed oils are found is the seeds of bicotyledinous plants." In animals they are usually deposited in the most copious quantity in the adipose membrane, as in the lard of hogs, the tallow of cattle, and the blubber of the whale.

Fixed oils are at present considered as composed only of hydrogen and carbon. By the analysis of Lavoisier 100 parts of olive oil consisted of about seventy-nine parts carbon, and twenty-one hydrogen.

Some of these oils have the property of drying easily without losing their transparency, such as linseed oil, nut oil, poppy oil, and hempseed oil. This property is increased by the addition of litharge (an oxide of lead) which is partly reduced by the process, whence it follows that this property is increased by the absorption of oxygen, or by taking away part of the hydrogen.

The mild alkalies, carbonate of potass, soda, &c., have little effect upon these oils; but the caustic, or pure alkali, has a considerable attraction for them, and unites with them into a compound called soap. The caustic alkali gives the soap an attraction for water, and penetrates the oily matter, which we want to wash out.