130 
THE CAUSE OF THE LUMINOSITY OF FLAME. 
temperature is produced, the carbon and sulphur have, of course, a much better chance 
of becoming completely oxidized, and are consequently still less likely to be precipitated 
in the solid form within the flame. Yet we shall find that the combustion of the bisul¬ 
phide in oxygen is attended with great luminosity. [A jet of oxygen was projected 
into the flame of the bisulphide.] The light is of a peculiar character, but is very in¬ 
tense. If we substitute nitric oxide for oxygen, we shall get an equally brilliant light; 
in fact, one of the most powerful lights that can be obtained by purely chemical means. 
We will produce this light in two ways. First, we will burn a jet of nitric oxide,—a 
gas which is usually said to be non-combustible,—in the vapour of bisulphide of carbon. 
We place a small quantity of the liquid in a large test-tube, and heat it till its vapour 
fills the tube. We ignite the vapour as it issues from the mouth of the tube, and now 
pass through the flame a glass tube which is delivering a jet of nitric oxide. [The 
experiment was performed as described.] We thus get a very brilliant flame, which 
might be employed instead of the magnesium light in taking photographic pictures of 
dark interiors, mines, and caverns. This dazzling flame, produced on burning a jet of 
nitric oxide in bisulphide of carbon vapour, certainly contains no solid particles ; it con¬ 
sists entirely of gases and vapours. We will now burn the vapour of bisulphide of 
carbon in nitric oxide gas. Into this jar of nitric oxide I drop a large glass bulb, con¬ 
taining bisulphide of carbon, and, after replacing the cover, I agitate the jar, so as to 
break the bulb and get the gas well charged with the vapour of the volatile compound. 
I now remove the ground-glass cover, and apply a light to the mouth of the jar. The 
mixture burns, producing a light which is almost insupportable, and which has a blind¬ 
ing effect upon the eye when near. Yet no solid particles are concerned in this extra¬ 
ordinary evolution of light. The products of the combustion, as well as the bodies 
burned, are gaseous. Hence it is evident that the presence of solid particles in the 
flame is not an essential condition of luminosity. 
“ The argument founded on the continuous spectrum of the gas flame affords no 
substantial support to the theory of incandescent particles, for the flame of nitric oxide 
and bisulphide of carbon, and the flame of arsenic, also give continuous spectra. It is 
therefore evident that the light produced by ignited gases and vapours may resemble 
that which proceeds from an incandescent solid, and give an unbroken spectrum. 
“ Does the luminosity of flame depend, then, upon temperature ? There can be no 
doubt that temperature is one of the factors involved in luminous combustion—that it 
has considerable influence upon the character and the intensity of the light produced. 
You have seen the kind of light which results from the combustion of sulphur in atmo¬ 
spheric air,—a light somewhat greater than that of hydrogen; and you know that 
when sulphur is burned in oxygen the light produced is much more intense. In the 
latter case it is probably from twenty to fifty times as intense as in the former. Yet, 
in substituting oxygen for atmospheric air, all that is done is to raise the temperature of 
the flame. Neither in the one case nor the other are there any solid particles present 
in the flame. The elevation of temperature depends upon the circumstance that in 
oxygen there is nothing that will not combine with the sulphur; whereas in atmospheric 
air there is a large proportion of nitrogen which does not take part in the combustion, 
but which shares the heat with the active elements, and thus reduces the temperature of 
the flame. Clearly, then, temperature has something to do with the luminosity of 
flames. Another instance, perhaps not quite so striking, is afforded by the combustion 
of phosphorus. When this body is burnt in atmospheric air we get a very brilliant 
light; indeed, phosphorus is one of the most luminiferous substances known. But if 
we now burn our phosphorus in pure oxygen [experiment performed] we get a much 
more intense light, simply because there is no nitrogen present to abstract the heat of 
the combining bodies. In both cases the phosphorus unites with oxygen ; but in the 
former case a large proportion of the heat resulting from the chemical action is expended 
in raising the temperature of a considerable volume of inert nitrogen. The different 
illuminating powers of the phosphorus flames in the two experiments seem to me to 
depend entirely upon their different temperatures. I specially direct your attention to 
phosphorus, because it is commonly supposed that the remarkable luminosity of its 
flame is due to incandescent solid particles of anhydrous phosphoric acid. But this sub¬ 
stance is really volatile. It volatilizes below white heat; and as the temperature of the 
flame of phosphorus in oxygen is much higher than white heat, I believe that the expe¬ 
riment which we have just made is a pure and clear example of the evolution of light 
