ARTIFICIAL ILLUMINATION. 159 
What I have just given you may be called the popular description 
of the theory of the gas flame. It is not by any means a full, or even 
strictly accurate description. What really goes on in a gas flame is still 
a subject of scientific controversy. The modern.and more accurate 
theory, for the development and publication of which we are largely 
indebted to my colleague, Professor Vivian Lewes, may be roughly 
stated thus :—On the gas issuing from the burner the hydrogen, due to 
its lighter density and greater diffusive power, burns at first at the 
edges of the non-luminous zone, evolving considerable heat as it does 
so, while the central body of that zone contains gases as yet unburnt. 
As the temperature of the flame rises, as we go higher up, the heavier 
hydro-carbons decompose into acetyline. Higher up still in the flame, 
and under the influence of the greater heat, the acetyline decomposes 
into carbon and hydrogen, and the solid carbon particles are rendered 
incandescent. This incandescence is the more intense in consequence 
of a curious property of acetyline. Most substances, on decomposition, 
absorb heat, or are exothermic, as it is called, while acetyline appears 
to be endo-thermic, that is to say, it gives out heat when it is decom- 
posing. Thus the temperature of the carbon particles, set free by the 
decomposition of the acetyline, is further raised by the heat which is 
given out at the instant of their being evolved, and the brilliance of 
the illuminating zone is in this way enhanced. Higher up still in the 
flame the carbon itself is being burnt, and ceases to exist as solid par- 
ticles capable of radiating light, and, although the heat here is intense, 
there is no illuminating effect. 
Now we will turn to the Bunsen or atmospheric burner. It differs 
from the ordinary burner in that the requisite amount of air, as 
you know, is previously mixed with the gas before it comes to the 
point of combustion ; the carbon does not have to wait, so to speak, 
till the hydrogen is satisfied, but each burns at once, having plenty 
of oxygen in its neighbourhood. ‘There are in the Bunsen flame no 
solid carbon particles unburnt and which are being rendered in- 
candescent. ‘hat is the reason why, as you see, it gives us no 
light. If I supply this deficiency of solid unburnt particles by putting 
into the flame a refractory substance, such, for example, as a delicate 
Welsbach mantle, we have now something which can be rendered in- 
candescent by the heat of the flame, and you see the brilliant light with 
which we are, most of us, now familiar. I may notice in passing that 
the total heat evolved by the burning of a certain quantity of gas in 
the Bunsen burner is precisely the same as that which is given out 
when the same quantity of gas is completely burnt in an ordinary 
burner. This is not always clearly understood. 
Now let us consider some of the principal causes which affect the 
brightness of the illumination obtained from our gas burners. 
(1.) There is the question of size. This little burner, a common 
No. 1 “Bray fish-tail”’ is a-very inefficient one, for, while it is burning 
nearly 3 feet of gas per hour, it is giving a light of only about 14 
candles, or a miserable half-candle power for every cubic foot of gas 
consumed. The explanation of this inefficiency is a simple one. You 
know that if we make two holes in a water pipe, one a very small one 
