the metal, earth, or alkali, occupies the place of the combustible. The first furnishes caloric, the second light, while the base of each combines together. Hence we see that the base of sulphurets and phosphurets resembles the base of products in being destitute of light, the formation of these bodies exhibiting the separation of fire like combustion; but the product differing from a product of combustion in being destitute of oxygen, Dr. Thomson distinguishes the process by the title of semi-combustion; indicating by the term, that it possesses one half of the characteristic marks of combustion, but is destitute of the other half.

The only part of this theory which requires proof is, that light is a component part of the earths and alkalies. But as potash and lime are the only bodies of that nature which we are certain to be capable of exhibiting the phenomena of semi-combustion, the proofs must of necessity be confined to them. That lime contains light as a component part has been long known. Meyer and Pelletier observed long ago, that when water is poured upon lime, not only heat but light is emitted. Light is emitted also abundantly when sulphuric acid is poured upon magnesia, or upon lime, potash, or soda, freed from the water of crystallization. In all these cases a semi-combustion takes place. The water and the acid being solidified give out caloric, while the lime or potash gives out light.

That lime during its burning combines with light, and that light is a component part of lime, is demonstrated by the following experiment, for which we are indebted to Scheele.

It has been mentioned already that fluor spar (fluate of lime) has the property of phosphorescing strongly when heated, but the experiment does not succeed twice with the same specimen. After it has been once heated sufficiently, no subsequent heat will cause it to phosphoresce. Now phosphorescence is merely the en.ission of light; light of course is a component part of fluor spar, and heat has the property of separating it. But the phosphorescing quality of the spar may be again recovered to it, or which is the same thing, the light which the spar had lost may be restored by the following process:

Decompose the fluate of lime by sulphuric acid, and preserve the fluoric acid separate. Boil the sulphate of Îime thus formed with a sufficient quantity of carbonate of soda; a double decomposition takes place; sulphate of soda remains in solution; and carbonate of lime precipitates. Ignite this precipitate in a crucible till it is reduced to lime, and combine it with the fluoric acid to which it was formerly united. The fluor spar thus regenerated phosphoresces as at first. Hence the lime during its ignition must have combined with light.

That potash contains light, may be proved in the same manner as the existence of that body in lime. Now as potash is deprived of its carbonic acid by lime, the doctor supposes that the process must be a double decomposition; namely, that the base of the lime combines with carbonic acid, while its light combines with the potash.

These remarks on semi-combustion might easily be much enlarged upon. For it is obvious that whenever a liquid combines with a solid containing light, and the product is a solid body, something analogous to semicombustion must take place. Hence the reason why water increases the violence of combustion when thrown sparingly into a common fire, &c.

Such is the theory of Dr. Thomson, against which a few objections have been advanced by an anonymous writer (Nicholson's Journal, July 1802, p. 206.) but there is little doubt that these objections will soon be reconciled, when the ingenious investigations of Dr. Thomson have been the subject of further consideration.

END OF VOLUME I.

APPENDIX.

PAGE 11. NO. 1.

Stahlean Theory. It is necessary to explain what is the Stahlean Theo ry; according to which, there is only one substance in nature capable of being burnt or inflamed. Combustion therefore according to Stahl, consisted in the disunion of this principle of inflammability called by him PHLOGISTON, from the inflammable body: during combustion the phlogiston combines with heat, and is exhibited when so combined in the form of fire or flame: when all the phlogiston is disunited from the inflammable body, this latter is no longer inflammable. According to this theory, metals are peculiar earths united chemically to phlogiston; when by exposure to strong heat in the open air, or by solution in acids the phlogiston is separated, the metals lose their metallic properties, and become calcined; the term calx, or metallic calx, meaning under that theory, what oxyd means now. Those metals which would not part with their phlogiston by calcination in the open air, such as gold and silver, and whose calces could regain it without the addition of inflammable matter, were the perfect metals: the rest were called imperfect metals. When by means of heat, the uninflammable calx of a metal was treated with any inflammable substance, as charcoal, the phlogiston of the charcoal left the charcoal, and united with the metallic earth or calx: in this case, the charcoal became an incombustible substance, and the metallic earth was reduced (i. e. brought back to a metalline state) or revived; and again acquired the property of being combustible.

So when sulphur was burnt in open air, it became converted into the sulphuric or vitriolic acid (so called because this acid used formerly to be procured from green vitriol.) Here then, the phlogiston of the sulphur When being burnt away, the sulphur became an incombustible acid. this acid was treated with phlogistic or inflammable substances, it became sulphur again, as when sulphate of soda was fused with charcoal in a strong heat. So when the combination of potash and sulphur called liver of sulphur, was exposed to heat in open air, the produce was sulphate of potash. The inference seemed clear that combustion consisted in nothing more than the separation, by means of heat, of phlogiston, or the one common principle of inflammability, from the bodies with which it was united; that bodies united with or containing phlogiston, as wood, wax, oils, resina, charcoal, coal, &c. were combustible and inflammable, in consequence of this union alone; when disunited were no longer so.

VOL. I

50

and

But John Rey, a French chemist, had noticed that metallic earths or calces acquired weight during their calcination; and Lavoisier by a train of most ingenious experiments, showed that the weight thus acquired was owing to the oxygen or pure part of atmospheric air imbibed during the process of calcination [oxydation.] That sulphur, also, on being burnt, absorbed and combined with, the oxygen of the atmosphere, and an acid was produced heavier than the sulphur employed, by the whole weight of pure or oxygen air which the atmospheric air had lost during the process.

Stahl's theory, therefore, could not be true, which explained combustion by means of the loss of one of the principles of the inflamed body; for instead of gaining in weight, it ought, according to that theory, to lose in weight. Kirwan, Keir, and Priestley long opposed this reasoning. Phlogiston, they said, was the inflammable air (hydrogen as it is now called) which was separated from metals during their solution in acids, and which formed a part of all combustible bodies. Still the accession of weight, where there ought to be a loss of weight, was not accounted for.

The French chemists then went further, and showed that the inflammable air came from the water, and neither from the metal nor the acid. They decomposed water into inflammable air and pure air, (hydrogen and oxygen) they composed water, by burning together, gradually, streams of hydrogen and oxygen: they showed that sulphur and oxygen formed sulphuric acid; that metals and oxygen formed metallic earths, calces or oxyds. They showed that when the oxygen was driven off, the metal was revived, without the addition of any principle of inflammability for the purpose. For the Stahlean Theory therefore, which made combustion to consist in the extrication of phlogiston by means of heat, they substituted the Lavoiserian or French Theory, which makes it to consist in the union of oxygen to an inflammable body. That theory has long reigned triumphant; but facts seem to indicate nevertheless, that there is such a substance as phlogiston, a principle of inflammability common to all bodies capable of being inflamed; and when united to caloric, forming hydrogen gas; and producing heat, and sometimes flame when it suddenly and violently unites to oxygen; and that combustion is nothing more than the union of oxygen and phlogiston, during which the latent heat of the two bodies thus combining, is set free; this heat is more violent in proportion to the quantity of the two substances combined, and the suddenness of their combination.

PAGE 31. NO. 2.

Dr. Hutton's Theory has been very ably defended and illustrated by Professor Playfair of Edinburgh: and Dr. James Hall in his papers on the Effects of Heat Modified by Compression (13 and 14 Nicholson's Journal) has shown that many appearances of carbonate of lime, which it was supposed nothing but watery solution could occasion, may be produced by heat, under a pressure which prevents the escape of the carbonic acid. His experiments also go far to show that the coal formations have probably been produced in the same way.

PAGE 39. NO. 3.

The facts hitherto observed, show, that all the transition and secon dary mountains have at one time or other been covered with water; for the vegetable and animal remains found in them, can no otherwise be accounted for. But facts hitherto observed, do not show the same as to the granite and porphyry ridges which form the highest points of the highest mountains. There is no satisfactory evidence of any such remains being found in these primitive formations; which are so denominated and classed, from the fact of their containing no such remains.

PAGE 42. NO. 4.

Vegetables are fixed by the root, but they have loco-motion in their stem, branches, and leaves. The sun-flower has been observed to have its disk mostly turned to the sun. The hop winds round the pole in a similar direction. Let light be admitted, only through a hole, into an apartment, and a plant placed there will shoot in that direction. The roots of plants growing on walls, as house-leek, and of trees growing amid rocks, will turn out of their way to seek for nourishment, in joints and crevices. The contraction of the various species of mimosa or sensitive plant, of the dionoa muscipula, or Venus's fly-trap, of South Carolina, the hedasgrum gyrans-the bending of the stamina of the water-lily, to facilitate the process of impregnation, and many other facts collected by Dr. Percival, of Manchester, the bishop of Landaff, and Dr. Smith, of Litchfield, tend to make it rather probable than otherwise, that vegetables have the faculty of sensation, if not of voluntarity; and that the living vegetable, like the living animal fibre, is, in all cases, endowed with the property of contractility, on the application of appropriate stimuli.

PAGE 63. NO. 5.

The only two essential properties of matter are extension and impenetrability, frequently, though improperly, termed solidity; for the opposite to solidity, is fluidity.

These two properties of matter, are resolvable into the properties of attraction and repulsion, which appertain to every particle of matter at given distances; and as the planetary system has, so also has each particle of matter, centripetal and centrifugal forces. Matter is not inactive and inert, but is incessantly repelling or attracting other matter. A few observations on this subject, not usually noticed in elementary books of chemistry or natural philosophy, may not be out of place.

Father Boscovick, in his Theoria Philosophie Naturalis, Mr. Mitchell, in the Philosophical Transactions, and Dr. Priestly, in his Treatise on Matter and Spirit, particularly the latter, have suggested that all matter may be permeable to other matter; and that mere physical points,