kept stirred in the flame of a furnace, is converted into red lead. Dissolved in vinegar, this metal forms acetat or sugar of lead, a dreadful poison, sometimes, however, used by the dealers in liquors to recover them when sour.

White lead, or cerusse, is formed by exposing plates of lead to the vapours of vinegar, which corrode the surface. The white lead thus formed. is scraped off, and afterwards ground and washed, to be used as a paint. This manufacture proves fatal to the workmen in about three years, and ought only to be carried on by condemned criminals.

9th and 10th. Of Nickel nothing is to be said deserving your attention. The oxides of zinc are used in surgery, particularly in complaints of the eyes, or rather of the eyelids, and in that well known plaster Turner's cerate. When zinc is heated to a strong red heat in an open vessel, it burns with a bright flame, and at the same time emits very light white flakes, which were formerly called lana philosophica, or flowers of zinc. They are merely an oxide of the metal.

It has been already mentioned that an alloy of zinc with copper forms all the different varieties of brass, pinchbeck, &c.

LECTURE XXXIII.

CHEMISTRY.

WATER, AND MINERAL WATERS.

SOME of the antient philosophers supposed that all things were originally derived from water. To form this judgment they must have taken no cursory view of the operations of nature. We see how it produces dews, clouds, rain, snow, and other meteors, and we cannot help observing how every vegetable, every animal, in a manner, seems to rise out of it, and be nourished by it.

Modern philosopl:y, however, which has introduced us to a new world of wonders, while it lessens those of the antient world, has shown that water itself, I mean the purest water, is really a compound substance. If into a glass receiver, or other close vessel, reversed in a quantity of quicksilver, we put about three parts of oxygen gas, and one part of hydrogen gas, and cause an electric spark to pass through them (for additional caloric is necessary to promote their union), we shall see the two gasses inflame, the mercury will rise in the receiver, proving that the gasses are condensed, and pure water will be formed. This water may again be de

composed by bringing it in contact with any heated combustible substance, as in making it pass through a gun-barrel heated red-hot, when the oxygen will be abstracted by the combustible or oxidizable matter (the iron), and the hydrogen comes off in the form of hydrogen gas. Water is indeed formed wherever there is a combustion of any matter containing hydrogen, as may be seen by fixing a glass receiver, of a globular form, over one of Argand's lamps, which in a little time will be found to contain water. Thus, like most other matters on the surface of the globe, water is continually changing

its nature.

While water was considered as a pure elementary substance, some were of opinion that the sea was continually diminishing, so that the parts which are now covered with it would in time be dry land. A number of proofs have been offered in support of this opinion; but those in the Swedish Memoirs, concerning the Baltic Sea, are most curious. We are told that a number of rocks, well known, have, in the memory of many persons now living, become more prominent above the surface of the sea. Solid rocks are the best proof of any opinion of this kind, for land might be washed away, or have additions made to it; but a solid rock, which is composed of materials not so liable to decay, must necessarily show if the surface of the sea is lower.

There were chemical experiments instituted at the same time to account for the supposed gradual diminution of the water of the sea. Van Helmont hinted that water, by repeated distillation, might be converted wholly into earth. Mr. Boyle repeated the distillation of the same water fifty times in the clearest glass vessels, and obtained at each distillation a quantity of earth. He says, a friend of his obtained at least threefourths of an ounce of earth from one ounce of water. Boerhaave suggested many doubts and suspicions, and endeavoured to invalidate the credit of these experiments. He says they were not performed with accuracy, that water was entirely a pure substance, but that it was capable of being rendered impure by a variety of mixtures, especially of air, which he says is a chaos; so that if any other bodies were found in water, it was only accidental: hence he supposes, and with justice, that the earth in these experiments proceeded either from dust in the air contained within the distilling vessels, or from some other cause which introduced it previous to the experiments. These experiments, therefore, only serve to show the imperfection of all chemical analysis.

Though water cannot be regarded as a simple elementary substance, yet there are not different sorts of pure water; for water obtained in any manner whatever, either attracted from the air, by deliquescent salts, or from salts themselves,

from oils, from animal or vegetable matters, is still the same when rendered pure by chemical means. It is undoubtedly of consequence to consider by what means water may be altered from its purity; but it is not difficult to obtain it in that state in which the most rigid experiment will find no admixture. Distil two-thirds of inodorous, insipid, clear stream, rain, or snow water, in glass vessels, to one-fourth, and the water will be perfectly pure. This operation is always necessary when water is to be used in accurate experiments, for no natural water is perfectly pure. Rain water is, indeed, much of the same purity, when collected in clean vessels at a distance from towns, and answers equally well in experiments with distilled water; it is, in fact, water distilled by nature. Those springs which flow from high and rocky grounds, where we can discover no mineral or metallic substance, are the next in purity to rain water; they are rain water collected on the hills, and filtered through ground, in which there are no soluble materials; but this is seldom the case, for most soils contain lime, &c, but in so small a quantity, that these spring waters are often as useful as the purest. The water of rivers and lakes is never so pure; indeed, it has often a sensible taste and smell.

But, besides the waters of rivers, which usually derive their impurities from a mixture of animal and vegetable substances, there are other waters,