posed to be the universal cause of acidity; the peculiar power of which is to turn all vegetable blues into reds, and give the flavour called sour.

There is also another property in nature, called alkaline, which is distinguished from the acid by a burning and urinous taste: and it has the distinct property of converting vegetable blues into greens.

The only alkalies are potash, soda, and the volatile alkali called ammonia.

518. Alkalies and acids have the property, when combined, of simplifying or neutralizing each other; and hence acids (which, it must again be remembered, form one of the constituents of all kinds of salts ;) when combined with alkalies, form what are called neutral salts.

Alkalies, mixed with fats, make soap; and, when melted with silex, glass.

Potash is chiefly obtained by burning vegetables, and hence called pot-ashes; but Soda, the other alkali, is obtained from sea-salt, the ashes of sea-weeds, from natron beds, and, even from mines.

Ammonia is obtained by the distillation of bones, horns, hoofs and other animal substances.

519. The principal acids obtained by the union of oxygen with other substances, are oxygen and sulphur, called sulphuric acid, or oil of vitriol.

Oxygen and nitrogen, forms nitric acid, or aqua fortis, which dissolves silver, and other metals.

Oxygen, and an unknown base, or radical, forms. muriatic acid, or acid of sea-salt ;-when united to soda, called muriat of soda, it is the common salt of the table.

Oxygenated muriatic acid, or chlorine, is used for bleeching, cleaning paper, and taking out inkspots.

Nitro-muriatic acid, or aqua regia, dissolves gold and platina.

Oxygen, and an unknown radical obtained from Derbyshire-spar, called fluoric acid, is employed for etching on glass.

There are fifty other acids known and used in the arts.

520. Affinity is the apparent preference which one elementary body has for another; and the various degrees of this preference create most of the chemical phenomena of nature.

Oxygen has a strong affinity for all bodies; therefore, to oxygenate any body, it is necessary to weaken the affinity between its parts by heat or motion; and, if this be done in the open air, the oxygen of the atmosphere will leave its azote, and combine with the new body, forming, according to its degree, an oxyde or an acid.

Obs. When olive-oil and water are agitated together, they refuse to act upon each other, and separate according to the order of their densities, the oil swimming above the water. Oil and water will not mix intimately; they will not combine; and they are said to have no chemical attraction or affinity for each other. But if oil and soap-lees, or solution of potassa in water, be mixed, the oil and the solution blend together, and a species of soap will be formed, which may be procured in a soft solid substance by evaporating a part of the water. This is an instance of combination; and solution of potassa and oil are said to attract each other chemically, or to have an affinity for each other.-DAVY.

521. The chemical properties of bodies arise in a great measure, from their varied affinities, propensities, or preferences for one another, by

which they leave one, and combine with another; and this varied power of affinity, leads to most of the combinations of nature, and affords to chemists their powers of analyzing bodies.

In passing from a fluid to a solid state, the substance unites into very curious and regular forms, called Chrystals.

When bodies unite, and form a new substance, they are said to be chemically combined; and when they are separated again by the action of any re-agent, they are said to be decomposed.

Obs-Oil is almost insiped; but the solution of potassa is a caustic substance, which corrodes the skin, and has a strong taste.-The body resulting from their union, differs both from the oil and the alkali in taste, smell, colour, and in all its sensible qualities; and it is a general character of chemical combination, that it changes the sensible qualities of bodies. Corrosive and pungent substances often become mild and tasteless by their union ; as is the case with sulphuric acid and quicklime, which form gypsum, or sulphat of lime. Bodies possessed of little taste or smell, often gain these qualities in a high degree by combination. Thus, sulphur, when inflamed in oxygen or in common air, dissolves and forms an elastic fluid of a most penetrating and disagreeable odour and peculiar flavour. The forms of bodies, or their densities, likewise usually alter; solids become fluids; and solids and fluids, gases; and gases are often converted into fluids or solids. Thus, sugar, or salt, or isinglass, dissolve in water. The consumption of charcoal in our fires, depends upon its uniting with a part of the air, with which it forms an invisible elastic fluid; mercury is rendered solid, by being heated with half its weight of tin; and a substance of this kind is used for silvering mirrors. The gas produced by the combustion of charcoal, is condensed by another gas procured from quicklime and sal ammoniac, when they are mixed

over mercury; and the two invisible, elastic fluids form a white saline, called carbonate of ammonia.

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; others contain sulphat, nitrat or muriat of soda, magnesia or lime, and chalybeate waters, contain carbonate 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 unto 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, 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 in it. If arsenic, the residuum will tinge copper white.

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