over mercury; and the two invisible, elastic fluids form a white saline solid.

522. If salt be mixed in water, it is said to be in solution, and the water is called the menstruum. If no more salt will dissolve, the water is said to be saturated.

If we would extract the salt, we must evaporate the water by heat; and if the vapour from the retort pass through a spiral tube or worm, to the receiver, we shall have distilled water, and the extract, or residuum of salt will remain in the still.

523. All mineral waters are formed by the solution, or mixture in them, of oxygen and nitrogen gases, of acids, alkalies, and neutral salts.

Sulphurous acid is found in some mineral waters; soda, in others; and salts, as sulphats, nitrats, muriats, and carbonats of soda, or lime; and in chalybeate waters, or carbonat of iron.

Obs. 1.-The test of the presence of carbonic acid in any mineral water is an infusion of litmus, which will be turned red by water containing it; and this acid also gives the briskness of champaign into whatever it enters, and an acidulated flavour to water. Any acid contained in any water may be detected by its turning the infusion of violets, red. Alkalies in water may, in like manner, be detected, by turning the infusion of violets green. The infusion of dry violets, or paper stained with them, answers best. The infusion of turmeric, or paper stained with turmeric, is rendered brown by alkalies; or reddish brown, if the quantity is minute. When the change is temporary, it is volatile alkali. Sulphur and bitumen may be detected, by the smell and taste. Iron, in mineral water, may be detected by Prussian alkali, which will precipitate it, and tinge it blue. The solution of galls also is an exquisite test of the presence of iron. When there is copper in water, it will shew itself on

the surface of any piece of bright iron put into it. If arsenic, the residuum will tinge copper white.

2.-Chemistry is an unsettled, but interesting science; and new discoveries, and further decompositions of bodies, deemed elementary, are constantly taking place.

XXI. Electricity and Galvanism.

524. If a piece of glass, or sealing wax, be rubbed on a piece of dry woollen cloth, or silk, and instantly held over any small pieces of paper, they will be attracted towards it, raised on an end, and otherwise put in motion.

The power thus excited is called electric; and if the experiment be made in the dark, the glass and the wax will exhibit faint signs of light; which light is called the electric fire or fluid.

525. If the glass be of larger dimensions, and turned rapidly round by a wince and a wheel, instead of being rubbed backward and forward with the hand; and be provided with a piece of silk to rub against it during its rotation, streams and large sparks of fluid fire will be elicited; which will fly round the glass, attract light bodies, and produce a pungent sensation, if the hand be held to it.

526. This glass, its cushion of silk, wheel, &c. are called an electrical machine. The fluid, or power produced by it, is one of the most wonderful in nature.

It is found, that it will pass along some bodies, and not along others; that it may be received and diffused by sharp points; that a superabundance of it, in one place, acts as a repellent, in the parts immediately adjoining; and that it has

a constant, and violent tendency to restore its own equilibrium in all bodies.

527. The bodies, over which it passes freely, are all animals, most animal and vegetable substances, water, &c.; all which are called conductors of electricity.

But it will not pass over glass, sulphur, charcoal, silk, baked woods, or dry woollen substances; nor through air, except by force in sparks, to short distances.

All these bodies, therefore, are called non-conductors.

528. The power of exciting it, receiving it on points, and confining it to bodies, over which it freely passes, by placing these on bodies, over which it will not pass, gives rise to all the phenomena of practical electricity,

Hence a metallic conductor, provided with brass-points, and elevated on glass-legs, is placed opposite the revolving glass-cylinder, to receive by its points the electric power, which is con

densed on the cylinder, but unable to escape on account of its being surrounded only by air, and supported by glass-legs, both which are non-conductors.

529. If the hand, or a metallic knob, be held within three or four inches from this metallic or main conductor, a large spark will escape, which in the dark will be forked, and of the colour of lightning.

There will also be a snapping noise; which, increased by larger quantities, would be likely to produce the noise of thunder.

In fact, lightning and thunder are effects of electricity in the clouds.

A flash of lightning is simply a stream of the electric power passing from the clouds to the earth; from the earth to the clouds; or from one cloud to another cloud; and thunder is the report, and the echoes of the report, between the clouds and the earth.

530. But the most wonderful effect of the electric fluid, is its power of suddenly contracting the muscles of animals, when it violently passes through them, from one place to another, to restore its equilibrium.

It will not pass through glass; if, therefore, a plate of glass, in the form of a jar, or otherwise, be coated on both sides, with either gold, silver, or tinfoil, and one side be brought into contact with the main conductor, the other side will instantly part with its electricity, and the plate of glass be said to be charged.

531. If one hand be put to the under or outer side of the said charged plate, and the other hand

be brought into contact with the other, or charged side, the equilibrium of the two sides will be restored through the body; and a violent contraction, or blow of the muscles will be felt, producing a shock peculiar to this operation.

The severity of the shock, is proportioned to the size of the plate or jar. When many jars are joined together, and charged in this way, they are called a battery; and some batteries have been made so powerful, as to kill an ox, melt gold, and produce all the surprising phenomena of real lightning,

532. Philosophers amused themselves, for a century, with experiments on the electrical apparatus; but a new mode of exciting this power, was discovered by Galvani; and the experiments made in his way, are called Galvanism.

It is found, that there are two classes of conductors-perfect, as the metals; and imperfect, as water and the mineral acids; and that if these are laid alternately, two perfect and one imperfect, or two imperfect and one perfect, the two ends or sides, will constantly produce an electric shock.

Instead, therefore, of the glass-cylinder, conductor, coated jar, &c., used in electrical experiments, the Voltaic pile, or trough, is now preferred; and is so called, from Volta, its inventor.

Obs. The common exhibition of electrical effects, is in attractions and repulsions, in which masses of matter are concerned; but there are other effects, in which, the changes that take place, operate, in a manner, in small spaces of time imperceptibly; and in which, the effects are produced upon the chemical arrangement of bodies. If a piece of zinc and a piece of copper be brought in