128 
THE CAUSE OF THE LUMINOSITY OF FLAME. 
are supposed to communicate to the ethereal medium impulses that produce waves of 
light of all lengths; in other words, they are believed to give rise to the light emitted 
by the flame, which light forms a continuous spectrum, with no break in it. Various 
experiments have been adduced in proof of this theory. One of them consists in 
depressing upon the gas flame a piece of wire gauze. The flame, as you see, is cut down 
by the cooling action of the gauze, and the so-called solid particles become visible as 
smoke. [This and the subsequent experiments the lecturer exhibited as he described 
them.l We get a like result by exposing a piece of white porcelain to the flame, when 
soot is deposited upon the porcelain. As a further proof, we have the fact adduced that 
if the gas be mixed with air before its combustion, it burns with scarcely any light; 
and when the wire gauze or the porcelain is applied to such a flame we get no trace of 
smoke or soot. If we take the lightless flame, produced by means of a Bunsen burner, 
and sift particles of carbon into it, we obtain a considerable amount of light. It is fur¬ 
ther known that those gases contained in coal-gas, and which do not deposit sooty 
matter on burning, give no light. The flame of hydrogen, for instance, does not give 
more light than that of coal-gas mixed with air; but when we sift carbon into it, it 
does give a certain amount of light. The same is the case with carbonic oxide, which, 
when burnt alone, gives practically no light. If I increase the temperature of a hydro¬ 
gen flame by passing oxygen into the interior, we get but little more light than from 
hydrogen alone ; but if I now sift into the flame particles of carbon, a luminosity is ob¬ 
tained much more brilliant than before, because the particles are more highly ignited 
than in the previous experiment. And though the light is very different from that of 
the gas-flame, yet these experiments appear at first sight to be conclusive in proof of 
the theory that the light arises from the solid particles of carbon, which are supposed 
to be produced by the decomposition of the hydrocarbons contained in coal-gas. Lastly, 
the continuous spectrum which the gas-flame affords is brought forward to prove that 
the light must be derived from incandescent solid matter. 
“Nevertheless, I have, during recent experiments connected with this course of lec¬ 
tures, been led to entertain great doubts as to the truth of this theory ; and I propose 
to lay before you the grounds upon which these doubts rest. Let us return to the 
flame of hydrogen, burning in free air. It possesses the very high temperature of 3776° F. 
If it be fed with oxygen, instead of atmospheric air, we get a temperature of 7364°; 
and, although it will fuse platinum and dissipate iron wire, still the luminosity 
is not greatly increased. If we ignite a mixture of oxygen and hydrogen with which 
we have blown the soap-bubbles contained in this dish, we get a very loud and sharp 
report, but scarcely any light. In this experiment we have an almost instantaneous 
combination of oxygen with hydrogen. At the moment of the explosion the resulting 
product expands to about ten times the volume of the mixed gases, and thus causes an 
immense wave of air to rush through the theatre, which produces the loud noise that 
assails your ears. When cool, the product of the combination of oxygen and hy¬ 
drogen occupies less space than the gases filled before the explosion. The sudden 
expansion that occurs on combination is simply the effect of the great heat produced. 
Suppose the combination of a cubic inch of the two gases to be effected at the bottom 
of a tube, one inch square and ten inches high, the expansion would be sufficient 
to fill the tube—thus lifting fifteen pounds of atmospheric air to a height of nine 
inches. That is the work the gases have to perform in expanding to ten times 
their original volume in the soap-bubbles; and in order to do this, a certain amount of 
heat must be expended. By the researches of Messrs. Joule and Meyer, we know that 
amount to be capable of raising the temperature of two-thirds of a cubic inch of water 
through ’592° F. But if this amount of heat were imparted to two-thirds of a 
cubic inch of steam, it would raise its temperature to 2121° F. If we prevent the 
expansion of the combining gases, the heat produced increases their temperature 
from 7364° to 9485° F. Let us see what effect these altered conditions have upon 
the light produced during the conbination of the two gases. This glass vessel [a 
Cavendish’s eudiometer], which is strong enough to resist the expansive force of the 
combining gases, contains the mixture that produced so much noise and so little light 
when we exploded it in the soap-bubbles. We now ignite the mixed gases, thus con¬ 
fined, by means of the electric spark. [The result was a brilliant flash of light.] We 
had no noise, because the gases were not allowed to expand; but we got an intense 
luminosity. It will be said, as an obvious explanation, that the light was caused by 
