Though the modern theory of combus tion is simplified by rejecting phlogiston, and rendered more accurate by compre hending facts formerly unknown, yet it must not be disguised, that it is inade quate to account for the great and most striking fact, namely, the increase of temperature, otherwise than by hypo thesis. Heat, or elevation of tempera ture, seems, in the opinions of all philoso phers, to consist in the agitation of the particles of something, whether we sup pose that thing to be the body itself, or a peculiar element called caloric. Accord ing to those philosophers who assert the existence of this last principle, the combination of oxygen and the combusti ble body does emit or give out caloric, either because there is less room for it in the new compound, of which the capa city is changed, according to Dr. Irvine's doctrine; or because a portion of caloric, which was before latent, or combined in one or both of the component parts, is, according to Black, given out in conse quence of the resulting attraction of the new compound for it being less than be fore. They, who are disposed to see this subject treated at length, may consult the system of the ingenious Fourcroy, where they will find the modern caloric affording the same general services to chemical hypothesis, as were formerly obtained from its predecessor, phlogis ton.
Notwithstanding the truly valuable and numerous discoveries of facts by Black, Irvine, Crawford, and other modern philo sophers, we are far from being in posses sion of proof, that elevation of tempera ture is universally occasioned by diminu tion of capacity, or the extrication of latent heat. But, as we arc upon the whole more habituated to consider bodies themselves, than their properties in the abstract, a preference has been given to the method of ascribing events to pecu liar additional substances, rather than to motions or modifications of the bodies in which they may take place. Many emi nent philosophers have,nevertheless,con sidered heat as s motion in the particles themselves ; but it is not so easy to specu late upon the principles of motion among a system of particles, as it is to assert the combination and disengagement of a chemical element, though this assertion does not remove the difficulty, but only places it a step farther off.
If we admit that the particles of a body do not touch each other; as ap pears to be established from the ent degrees of inertia and of weight, as well as from the expansions and contrac tions occasioned, by change of tempera ture, and other causes ;' and if we like wise consider the particles as attracting each other,—it appears to follow by ana logy, from what we know of the rest of the universe, that theymust be kept asun der by motion. From this inference we shall be led to consider natural masses as distinct systems of revolving particles; comparable with those nebulae which oc cupy the celestial spaces, and of which the parts are, no doubt, governed by cometary and planetary revolutions. It
is much to be regretted, that the mathe matical consideration of this subject by Mr. Buile, in a work announced in Nicholson's Journal, vol. iii. p.234, quar to series, has not yet been laid before the public.
The ordinary appearances of bodies in a state of combustion may be explained, in a general way, by attending to the state of the bodies which undergo it. If the parts of an ignited body, such as that of a piece of charcoal, bocome oxygena ted, previous to or at the very instant of their separation from the mass, there will be no appearance of light but at the surface of the burning body ; but if small parts of the body be separated from the general mass, during the very process of combustion, and before it is completed, as happens mechanically when the par ticles of iron are torn off' by the action of a dry grindstone, or chemically when the particles of fat rise in vapour from the wick of a lighted candle, a burning mass will be seen, variable in its figure, which, in the latter case, is called flame. And that this explanation accounts for the flame of burning bodies is manifest ed, from the little difference between the two phenomena here mentioned, and the still less difference between the results, namely flame, which are produced by projecting the dust of rosin, or a stream of hydrogen, through the flame of a candle.
According to the theory which sup poses caloric to be an independent sub stance, combustion must be a rapid union of oxygen with a combustible body; and the heat has been supposed to be given out from the oxygen during a condensa tion of this last, which, it is imagined, takes place universally in this process. This, however, has not been proved.
Dr. Thomson, considering caloric and light as distinct substances, has adduced many facts and observations, to prove, that as caloric abounds in oxygen, so light is a component part of every com bustible. And thence, according to his doctrine, while the base of oxygen combines with the base of the combusti ble, the caloric of the one and the light of the other unite in the form of fire. From this theory he shows why, in the transitions of oxygen from one combusti ble base to another, the act of combustion does not take place ; namely, because the caloric of the oxygen has no light pre sented to it to combine with. The whole tlogrine, though undoubtedly requiring further developement and proof, is enti tled to the greatest attention of chemists. See CALORIC, CAPACITY, CHEMISTRY, HEAT.