COMBUSTION. 
S? bbsen'able tliat, according to the nature 
of the body itself, the heat is either conduct- 
ed away, and nothing farther happens, or 
else it continues and even increases so as to 
spread by coniinunication through every 
part of tlie body, and produce a change in 
its nature. Thus, if one corner or extre- 
mity of a thin piece of stone or glass be 
made red hot, it will soon become cold 
again, and no farther effect will follow ; but 
if the corner of a piece of paper or wood 
be heated in like manner, it will not, in 
common circumstances, become cold again 
without alteration, but the heat will be com- 
municated to the whole mass, and will con- 
tinue until the body shall have undergone 
a remarkable change. This phenomenon is 
called combustion or burning ; the bodies 
which ai'e liable to it are called combusti- 
ble ; and after they have undergone this 
process they are said to have been burned. 
Tiiere are scarcely any chemical changes 
by which heat is produced, sufficient to ex- 
hibit the appearance of light, unless oxygen 
be in the act of entering into combination 
with a combustible body. One of the em- 
liest observations respecting ordinary com- 
bustion must have been, that it cannot take 
place without common air, and that it is ex- 
tinguished by shutting out the air. It is 
now well known, that the air acts only by 
means of its oxygen, which unites with and 
changes the combustible body. 
The earlier doctrines respecting heat and 
fire are scarcely entitled to notice ; and cer- 
tainly must not occupy our pages. It will 
be sufficient for us to remark that the hy- 
pothesis of an element called fire, which 
was supposed to escape from burning 
bodies and ascend to a sphere above, was 
modified by Beecher and Stahl, by the sup- 
position of a general principle, assumed to 
exist in all combustible bodies, and deno- 
minated phlogiston ; capable of passing in 
combination from one body to another, or 
of flying off with a violent agitation, in 
which the heat was imagined to consist. 
As this theory was established upon the 
observation of a nun.ber of striking che- 
mical facts, it was for a long time univer- 
sally received. Various modifications were, 
however, proposed by different chemists, 
as discoveries came to be made ; particu- 
larly with regard to the agency and combi- 
nation of air in bodies, and afterwards those 
of the existence of oxygen, and the laws by 
which heat, or the cause of temperature, is 
governed. These advances led to the re- 
jection of phlogiston altogether j a change 
of theory, which was more rapidly effected 
by the patronage, exertions, and scienri-' 
fic labours of Lavoisier ; who devoted the 
influence of an elevated situation, the ex-: 
tent of his fortune, and the powers of an 
uncommonly clear and comprehensive in- 
tellect, to this object. It is to be regretted 
that, with claims so well founded and so 
great, this philosopher should have sought 
for more ; but it is certainly true that 
he himself gave support to the powerful cry 
of that party which has proclaimed him the 
author of the modern theory of combustion ; 
whereas if they had continued to do justice 
to Rey, Hooke, Mayow, Hales, Bayen, 
Priestley,and others, there would have been 
little, of absolute facts left for Lavoisier to 
claim in the way of original discovery; 
though it would be difficult to find adequate 
terms to express tlie obligation under which 
the scientific world is placed with regard to 
him, for his ample and accurate repetition 
of experimental investigations, and the very 
luminous and able manner in which he has 
digested and stated the whole mass of facts, 
and applied them to theoretical results. 
Combustion, as understood by modern 
chemists, is the rapid combination of oxy- 
gen with a body, which is attended with 
increase of temperature and the emission 
of light. The burned body is therefore an 
oxygenated compound. Thus we may form 
a notion of combustion by burning a piece 
of iron wire. If the diameter of the wire 
be very small, such, for example, as half 
the thickness of a hair, and it be made up 
into a tuft like wool; it may be lighted by a 
candle, and will burn like other more readily 
combustible bodies until it has received a 
certain portion of oxygen, after Wliich the 
combustion wall cease. If the same iron 
had been exposed to the atmosphere with- 
out additional heat, it would also have at- 
tracted 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 combustion. 
Though the modern theory of combustion 
is simplified by rejecting phlogiston, and 
rendered more accurate by comprehending 
facts foimeriy unknown, yet it must not be 
disguised, tliat it is inadequate to account 
for the great and most striking fact, namely, 
the increase of temperature, otherwise than 
by hypothesis. Heat, or elevation of tem- 
perature, seems in the opinions of all philo- 
sophers to consist in the agitation of the 
particles of some thing, whether we suppose 
that thing to be the body itself, or a pecu- 
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