There are other acids, such as the uric (extracted from calculi), the rosacic, the amniotic, and the lactic, or acid of milk, &c.

The arrangement of the compound or neutral salts under the new chemistry is peculiarly easy. The barbarous jargon of the old school, which meant nothing, such as sal digestivus, 'sal mirabilis, sal polychresti, is banished, and the name now discovers the materials of which the salt actually consists. Thus, when we hear of nitrat of potass, or muriat of soda, no person who knows any thing of chemistry can mistake: but must understand that these mean compounds of the nitric and muriatic acids with potass and soda.

The different genera of compound salts are denominated from the acids. Thus, we have sulphats, nitrats, muriats, borats, fluats, tartrats, acetats, phosphats, &c. But it is necessary to remind the student, that where the salt is compounded of the acid, when not fully saturated with oxygen, the compound is dis stinguished by the termination ite. Thus, the sulphurous acid produces sulphits; the sulphuric acid sulphats.

The species of compound salts are distinguished by subjoining the base; as sulphat of soda, which distinguishes the salt from sulphat of lime, or any other sulphat; and as some of the acids will unite with two bases, the triple salt is described by adding both bases, as tartrat of

potass and soda. Where there is an excess of acid in the salt, it is expressed by prefixing the preposition super.-Thus sulphat of potass expresses the combination of the sulphuric acid with potass, and super-sulphat of potass denotes that there is a redundancy of acid. On the contrary, the preposition sub implies that there is an excess of base; thus sub-sulphat of potass is the salt before described, but with a deficiency of the acid, and a redundancy of the potass or base.

Salts have also been divided into the alkaline, the earthy, and the metallic, according to the nature of their respective bases. But it is unnecessary to enter into any particular description of them here, since the nomenclature itself defines the nature of the salts. Dr. Thomson calculates that there may be about 2000 different species of compound or neutral salts; and he remarks that “ some idea may be formed of the progress which this branch of chemistry has made, by recollecting that forty years ago not more than thirty salts in all were known."

VOL. II.

CHEMISTRY.

EARTHS.

WE are led next, by the arrangement we have adopted, to consider earthy and stony substances. The common definition of these substances is, that they are bodies not soluble in water, not inflammable, and whose specific weight to that of water is not more than four to one. This definition, like that of salts, is not so precise as to be out of the reach of criticism, because there are some earths perfectly soluble in water, and many that are met with in the bowels of the earth that afford the greatest probability that they have been in a dissolved state. But the distinction, though not strictly just, may be proper in a looser sense; for there is a great difference of solubility between earths and salts, for a few grains of earth are sufficient to saturate a large quantity of water, and even these dissolve less perfectly than salts. There are two other circumstances, not included in the definition, that make part of the idea of an earthy substance. 1. A great degree of fixedness. 2. A disposition to assume the form of a glassy concretion when melted. By the first of these they are distinguished from all the other objects of chemistry, for we cannot in general convert a pure earth

into vapour. The second characteristic is indispensable to an earthy substance, and in a great measure peculiar to this class. The bodies which come under the class of earths are nine in number, viz.

Lime, Barytes, and Strontian have been shown by Sir Humphry Davy to have metallic bases, and most of the other earths have been analogically considered to be the oxides of peculiar metals.

Some of these earths are much more plentiful than others, and the great mass of this globe may be said to be composed of lime, flint, and clay, variously mixed and disposed in the dif ferent species of stony bodies.

The surface of the earth evidently consists of a confused mixture of decayed animal and vegetable substances, and earths rudely united together; but when we have got below the surface, we find the materials of the globe arranged in a regular manner. Sometimes, indeed, we find heaps of stone, which do not consist of layers, but are confused masses of unequal thickness; these are called rocks. The strata are in general extended through a whole country, and perhaps through the globe itself. Thus, in digging for coal, the workmen first come to a bed of clay, next to a bed of sand, for the thickness of

which there is no particular rule; then perhaps to a bed of freestone; and, lastly, to the coal. After they have wrought the coal, they are obliged to sink another pit; and in this they find the same strata varying in thickness as above. These extensive bodies are found most regular when the country is flat, being generally parallel to the horizon, though frequently dipping downwards in a certain angle. In many places the beds have a wave, as where the country consists of gently rising hills and vales: here too they generally dip. In riding a mile, we perhaps pass through ground composed mostly of sand, in another mile, perhaps of clay; and this is occasioned by the edges of the different strata lying with an obliquity to the horizon. The projection of the edges of other strata is called by the miners cropping, and where the edge shows itself, the crop. By the same kind of projection, mountains, or ridges of mountains, are produced, and the form of them regulated. Mountains are said to have a back and a face; the latter smoother, and the former more rugged. We generally find too on one side of the mountain a more gradual ascent than on the other, which is occasioned by the cropping of the strata. The back of the mountain shows the obliquity with which the strata sinks into the ground; the abrupt edge of the strata, however, becomes more sloping as time, producing a decay, draws rubbish from above. Where the face of the country is irregular, this appearance depends on the different