COMBUSTION. 



ble bodies, until it has received a cer- 

 tain portion of oxygen, after which the 

 combustion will cease. If the same iron 

 had been exposed to the atmosphere with- 

 out additional heat, it would also have 

 attracted oxygen, but in a longer time ; 

 and though the result might have been 

 the same, we should not have called 

 this slow process by the name of com- 

 bustion. 



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 opi nions 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 furfrom 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 a 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 differ- 

 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 they^must 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. Buee, 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, become 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 bodi'. s 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- 



