192 
But if light is nbt a substance emanated from luminous 
bodies, but a vibration and undulation in ether, analagous to 
the undulation in air, which is the immediate cause of 
sound, as a great number of distinguished philosophers have 
thought; in this case, we ought to seek the cause of the 
light diffused in a flame, in the very high temperature of 
the different particles of matter which compose this flame ; 
these particles would be luminous in the same manner as 
red hot iron is luminous, and would disappear at the instant 
when they became so cold', that their light (or the vibrations 
which they excite in ether) should cease to be visible. 
In adopting this hypothesis, we must not expect to find 
the quantities of light bearing a constant proportion to the 
quantities of inflammable matter burnt; much less ought 
we to expect the existence of such a proportion, as. a demon- 
Strative proof of the falsity of the hypothesis, for this reason 
—The volume and form of a flame ought necessarily to 
have a great influence on the celerity of its cooling ; and if 
it shall be found that these circumstances have no influence 
on the quantity of light which is diffused, it will be suffi¬ 
ciently proved, that this light does not depend on the pre¬ 
servation of the heat of the particles which compose this 
flame. 
But, if it shall be found by the result of decisive experi¬ 
ments, that the light diffused in the perfect combustion of 
the same quantity of inflammable matter is variable ; it 
will be impossible, it appears to me, not to perceive that 
this light cannot be a chemical product of combustion, and 
that the hypothesis which supposes light a substance ema¬ 
nated from luminous bodies, would become more and more 
difficult to maintain. 
If this was the agitation of a question merely speculative, 
and which in its nature could have no real influence either 
on the progress of the sciences, or on the useful arts which 
depend on them, I should be the first to condemn its discus¬ 
sion, not only on account of its inutility, but also, and par- 
