Mr. Lavoisier's explanation of Combustion 
spends upon the two laws discovered by 
imself and Dr. Black. When a combustiole 
ody is raised to a certain temperature, it 
legins to combine with the oxygen of the 
Itmos'phere, and this oxygen during its cora- 
lination lets go the caloric and light with 
Lv high it was combined while in the gaseous 
State. Hence their appearance during every 
combustion. Hence also the change which 
the combustible undergoes in consequence 
bf combustion. 
| Thus Lavoisier explained combustion with- 
out having recourse to phlogiston ; a prin- 
ciple merely supposed to exist, because 
[combustion could not be explained without 
it. No chemist had been able to exhibit 
phlogiston in a separate state, or to give any 
proof of its existence, excepting only its 
fcouveniency in explaining combustion. I he 
proof of its existence consisted entirely in 
the impossibility of explaining combustion 
[without it. Mr. Lavoi tier therefore, by 
gi ving a satisfactory explanation of combus- 
tion without having recourse to phlogiston, 
proved that there was no reason for suppos- 
ing any such principle at all to exist. 
But the hypothesis of Mr. Kirwan, who 
made phlogiston the same with hydrogen, 
was not overturned by this explanation, be- 
I cause there could be no doubt that such a 
Isubstance as hydrogen actually exists. But 
hydrogen, if it is phlogiston, must constitute 
a component part ot every combustible, and 
lit must separate from the combustible in 
'every case of combustion. These were 
j points accordingly, which Mr. Kirwan un- 
j dertook to prove. It he. failed, or il the 
I very contrary of his suppositions holds in 
fact, his hypothesis of course falls to the 
ground. 
Lavoisier and his associates saw at once 
the important uses which might be made of 
Mr. Kir wan’s essay. By refuting an hypo- 
| thesis which had been embraced by the most 
respectable chemists in Europe, their cause 
1 would receiv e an eclat which would make it 
i irresistible. Accordingly, the essay was 
translated into French, and each of the sec- 
j tions into which it was divided was accompa- 
| nied by a refutation. Four of the sections 
were refuted by Lavoisier, three by Ber- 
thollet, three by FoUrcroy, two by Morveau, 
I and one by Monge ; and, to do the French 
j chemists justice, there never, was a refutation 
-j more complete. Mr. Kirwan himself, with 
I ,that candour which distinguishes superior 
: 'minds, gave up his opinion as untenable, and 
declared himself a convert to the opinion of 
i Lavoisier. 
Thus Mr. Lavoisier destroyed the exist- 
jj ence of phlogiston altogether, and established 
a theory of combustion almost precisely si- 
ll rnilar to that which had been proposed long 
ago by Dr. Hooke. The theory of Hooke 
, is only expressed in general terms, that of 
Lavoisier is much more particular. The 
i first was an hypothesis, or fortunate conjecture 
which the infant state of the science did not 
I enable him to verify ; whereas Lavoisier was 
led to his conclusions by accurate expe- 
riments, and a train of ingenious and mas- 
terly deductions. 
According to the theory of Lavoisier, 
which is now almost generally received, and 
considered by chemists as a full explanation 
of tbe phenomena, combustion consists in 
COMBUSTION. 
two things: first, a decomposition; second, a 
combination. The oxygen of the atmo- 
sphere being in the state of gas, is combined 
with caloric and light. During combustion 
this gas is decomposed : its caloric and light 
escape, while its base combines with the 
combustible, and forms the product. This 
product is incombustible ; because its base, 
being already saturated with oxygen, cannot 
combine with any more. Such is a short 
historical detail of the improvements which 
have been gradually introduced into this 
interesting part of the science of chemistry. 
Let us now take more particular view of the 
subject. 
By combustion is meant a total change in 
the nature of combustible bodies, accompa- 
nied by the copious emission of heat and light. 
Every theory of combustion must account 
for these two things ; namely, the change 
which the body undergoes, and the emission 
of heat and light which accompanies, this 
change. 
Mr. Lavoisier explained completely the 
first of these phenomena, by demonstrating, 
that in all cases oxygen combines with the 
burning body ; and that the substance which 
remains behind, after combustion, is the 
compound formed of the combustible body 
and oxygen. But he did not succeed so 
well in accounting for the heat and the light 
which are evolved during combustion. In- 
deed this part of the subject was in a great 
measure overlooked by him. The combina- 
tion of oxygen was considered as the import- 
ant and essential part of the process. Hence 
his followers considered the terms oxygenize- 
ment and combustion as synonymous; but 
this was improper, because oxygen often 
unites to bodies without any extrication of 
heat or light. In this way it' unites to azote, 
muriatic acid, and mercury ; but the extrica- 
tion of heat and light is considered as essen- 
tial to combustion in common language. The 
union of oxygen without that extrication is 
verv different from its union when accompa- 
nied by it, both in the phenomena and in 
the product; they ought therefore to be dis- 
tinguished. 
To account for the emission of heat and 
light, which constitutes a part of combustion, 
Mr. Lavoisier had recourse to the theory of 
Dr. Crawford. The heat and the light were 
combined with the oxygen gas, and sepa- 
rated from it when that gas united to the 
combustible body. But this explanation, 
though it answers pretty well in common 
cases, fails altogether in others. Heat and 
light were supposed to be combined with the 
oxygen of the atmosphere, because it is in a 
gaseous state ; and to separate from it, be- 
cause it loses its gaseous state. But as vio- 
lent combustions take place when the ox- 
ygen employed is solid or liquid, as when 
it" is in the state of a gas. Thus if nitric 
acid be poured upon linseed-oil, or oil of tur- 
pentine, a very rapid combustion takes place, 
and abundance of caloric and light is emitted. 
Here the oxygen forms a part of the liquid 
nitric acid, and is already combined with 
azote ; or, according to the language of the 
French chemists, the azote lias undergone 
combustion. Now in this case the oxygen 
is not only in a liquid state, but it has also 
undergone the change produced by combus- 
tion. So that oxygen is capable of giving 
out caloric and light, not only when liquid, 
399 
but even after combustion, which is directly 
contrary to the theory. 
Farther: gunpowder, when kindled, burns 
with great rapidity in close vessels, or under 
an exhausted receiver. This substance is 
composed of nitre, charcoal, and sulphur ; 
the two last of which ingredients are combus- 
tible; the first supplies the oxygen, being 
composed of nitric acid and potash. Here 
the oxygen is not only already combined 
with azote, but forms a component part of a 
solid; yet a great quantity of caloric and 
light is emitted during the combustion, and 
almost the whole product of the combustion 
is in the state 'of : gas. This appears doubly 
inconsistent with the theory ; tor the caloric 
and light must be supposed to be emitted 
from a solid body during its conversion into 
gas, which ought to require more caloric and 
light for its existence in the gaseous state 
than the solid itself contained. 
Mr. Brugnatelli, the celebrated professor 
of chemistry at Pavia, seems to have been 
the first who saw this objection in its proper 
light. He has endeavoured to obviate it in 
the following manner. According to this 
very acute philosopher, the substance com- 
monly called oxygen combines with bodies 
in two states: 1. Retaining the greatest part 
of the caloric and light with which it is com- 
bined when in the state of gas. 2. After hav- 
ing let go all the caloric and light with which 
it was combined. In the first state, he gives 
it the name of thermoxygen, in the second of 
oxygen. Thermoxygen exists as a compo- 
nent part, not only of gaseous bodies, but 
also of several liquids and solids. It is only 
in those cases where thermoxygen is a com- 
ponent part of liquids or solids, that caloric 
and light are emitted. All metals, according 
to him, combine with thermoxygen ; those 
substances, on the contrary, which by com- 
bustion are converted into acids, combine 
with oxygen. This ingenious theory obvi- 
ates the objection completely, provided its 
truth can be established in a satisfactory 
manner. But as the evidence for it rests 
almost entirely upon its convenience in ex- 
plaining several difficult points in the phe- 
nomena of combustion, we must consider it 
rather in the light of an ingenious conjecture 
than as a theory fully established. 
It was long the general opinion of chemists, 
that light exists in a fixed state in all combus- 
tible bodies. The discoveries of Lavoisier 
induced the greater number of them to give 
up this opinion, on the supposition that com- 
bu.st i oh could be explained in a satisfactory 
manner without it. Indeed the followers of 
that illustrious philosopher considered it as 
incumbent upon them to oppose it w r ith all 
their might, because the fixed light, which 
had been supposed to constitute a part of 
combustibles, had been unfortunately deno- 
minated phlogiston, a term which they con- 
sidered as incompatible with, truth. The 
hypothesis, however, was occasionally re- 
vived;' first by Richter and Delametherie, 
and afterwards in a more formal, manner by 
Gren. But ljttle attention has been paid to 
it in this country till lately. The very cu- 
rious phenomena observed bv Mr. Chenevix 
in his experiments on the Jiyperoxymuriatic 
acid, induced him to incline to the same 
opinion. 
That the light exists combined with the. 
combustible, will appear exceedingly pro- 
