THE CAUSE OF THE LUMINOSITY OF FLAME. 
131 
of great brilliancy from gaseous matter alone. This, however, may be disputed, for it 
may be said that in the outlying parts of the flame the anhydrous phosphoric acid passes 
into the solid condition. I will not, therefore, insist on the absence of solid particles in 
the phosphorus flame. 
“ Though it is clear that the luminosity of certain flames increases with their tempera¬ 
ture, we must not forget that hydrogen burns in oxygen with an intensely hot, but 
almost lightless flame. The hydrogen flame in air has a temperature of 3776° F., but in 
oxygen it rises to 7364° F. The great increase in temperature is, however, attended 
with but a very slight increase in the luminosity of the flame. 
“ What additional condition, then, comes into play when the flames of oxygen and 
hydrogen, carbonic oxide and oxygen, and hydrogen and chlorine, were made to give a 
dazzling liglft ? You will remember that we confined the mixed gases so that they 
could not expand ; and in consequence of this the density of the vapour or gas produced 
by the combustion of each mixture was greatly increased during the ignition. In the 
case of oxygen and hydrogen the product had for a moment ten times the density of the 
product of the combustion of the unconfined gases. Now, if we review our experiments 
on luminous combustion, we shall find that those bodies from which we obtained the 
most light are those which yield dense gases or vapours. In fact, the relative degrees 
of luminosity exhibited by the flames we have produced by burning different substances 
may be said to be roughly proportional to the relative densities to the different products 
of combustion. The densities of some of these products are expressed in this table, the 
density of hydrogen being taken as the unit:— 
“ Relative Densities of Gases and Vapours. 
Hydrogen. 1 
Water . 9 
Hydrochloric acid . 18£ 
Carbonic acid . 22 
Sulphurous acid . 32 
Terchloride of arsenic . 90f 
Anhyd. Phosphoric acid ...71 or 142 ? 
Metallic arsenic. 150 
Arsenious acid .198 
The vapour of water which is the product of the non-luminous combustion of hydrogen 
in oxygen or atmospheric air has a density only nine times as great as that of hydrogen. 
When hydrogen was burned in chlorine it formed hydrochloric acid gas, the density of 
which is eighteen and a quarter times that of hydrogen; and you remember that the 
luminosity of the flame, though very low, was perceptibly greater than that of the oxy- 
hydrogen flame. Now, let us look into the case of metallic arsenic, which burned in 
oxygen with a flame of dazzling brilliancy. Arsenic is a peculiar body, having what is 
called an abnormal vapour density. Its vapour is squeezed into half the space which, 
according to the analogy of other vapours, it ought to occupy. It has a density of 150, 
hydrogen being taken as 1. But on burning in oxygen, arsenic produces arsenious acid, 
which has also an abnormal vapour density, no less than 198 times as great as hydrogen. 
Hence, in this remarkable case, we have to do, in the first place, with a volatile metal, 
which itself has a remarkably high density when in the state of vapour ; and, secondly, 
with a volatile compound of the metal and oxygen—volatile far below a red heat— 
which in the state of vapour is 198 times as dense as hydrogen, and more than twenty 
times as dense as steam. In the combustion of phosphorus in oxygen, which is attended 
with so much light, we get anhydrous phosphoric acid, the density of which in the 
state of vapour has unfortunately not been determined; but as phosphorus vapour has 
also an abnormal density it is not improbable that it may be as high as 142, for the 
normal density would be 71. Although there are some exceptions, yet, taking the 
whole series of experiments on luminous combustion, we see that the density of the 
vapour has in some way to do with the intensity of the light. If this be so, any flame 
which is producing light by the virtue of the density of the vapour present in it ought 
to give less light if we cause the vapour to expand by reducing the pressure on the 
flame. If we cause arsenic, for instance, to burn in oxygen under a greatly reduced 
pressure, so that the vapour of arsenious acid may expand and become much lighter, 
the amount of luminosity from the flame ought to be materially diminished, and this 
we shall find to be the case. I first burn the arsenic at the ordinary pressure, and then 
gradually reduce the pressure by means of an air-pump until the arsenic burns under a 
pressure one-half that of the external air, and you pei ceive we get less light from the 
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