General Refleciions on Heat. 33 
lt now ranks only as one among a class of agents, which by 
their combinations can produce that phenomenon. By the 
discovery of other similar or analogous bodies, oxygen, which 
before stood at the head of a genus, stands at present only at 
the head ofa species, although the cases of combustion in- 
cluded under it are far more numerous than those of all the 
other species. 
But Lavoisier, having observed that oxygen was absorbed 
in every process of combustion that fell under his notice, and 
that, in general, the vigour of the process was proportioned 
to the rapidity of the absorption, was justified in his infer- 
ence, that combustion is universally owing to a combination 
of oxygen with an inflammable base. Though we cannot 
now employ the term universally, yet the proposition is still 
true in all those instances adduced by Lavoisier. 
If we are asked, however, why the absorption of oxygen 
should produce light and heat, and communicate to the in- 
flammable body the power of spontaneous and vehement con- 
sumption,—circumstances which chiefly characterize this 
process,—we may fairly reply, that we cannottell, and that we 
are not bound to tell. It is demanding of us to explain the 
connexion between the cause and the effect, which we arc 
generally unable to do. ‘To call upon us to say why the ab- 
sorption of oxygen produces light aad heat, is like calling 
upon us to say why the action of light upon the optic nerve 
produces vision, or why the vibrations of the air upon the or- 
gans of hearing produce sound. Lavoisier’s definition of 
combustion was the best that could possibly have been de- 
rived at the time when it was given, though subsequent dis- 
coveries have shown, that oxygen is not the only agent in 
combustion. If we demand that the cause assigned for this 
phenomenon shall explain the reason of the heat and light 
exhibited, this, and probably every other explanation of the 
same process. will be found inadequate ; but if we deem ii 
sufficient, in accounting for a physical fact, to specify its inva- 
riable antecedent, then we must admit that Lavoisier reduced 
by far the greater number of cases of combustion to a spe- 
cies under their proper head: that is, he accounted for all 
those cases of combustion which depend on the agencies of 
oxygen gas, though he failed to assign a cause which embraces 
not oniy these, but every other possible case in which this 
process occurs. In the present state of our knowledge, the 
phenomena of combustion admit of a higher classification, as 
