158 ARTIFICIAL ILLUMINATION. 
salt. Notice how different the colours appear from what you saw a 
few moments ago. Here are two colours which are apparently so 
nearly alike that they might be considered a good match. But now let 
me illuminate them, as I proceed to do, by a burning magnesium wire, 
the pure brilliant white light of which comes from the incandescent 
oxide of magnesium which is being formed. How brilliant and true 
the colours are now! One of these two colours which you just now 
thought were so nearly alike is really, as you see, a beautiful blue, and 
the other is a brilliant green. After this you will understand how 
necessary it is, for instance, when ladies visit the draper’s, that the 
coloured fabrics which they are selecting should be illuminated by as 
pure a white light as possible—the comparatively yellow ordinary gas- 
light giving a very imperfect rendering of the various tints. 
And now, with a view to studying what really goes on in a gas 
flame and how the light is produced, let us first consider what are the 
constituents of which coal gas is composed. Ordinary London gas 
generally contains about 52 per cent., or a little more than half its 
bulk, of hydrogen ; 34 per cent., or a little more than a third of its 
bulk of marsh gas, or methane, as it is called—a light hydro-carbon ; 
and about 4 per cent. of heavy hydro-carbons to which, though so 
small in quantity, the illuminating effect is almost entirely due; the 
remaining 10 per cent. being composed of carbonic oxide, carbonic 
acid, nitrogen and oxygen, all more or less objectionable components 
of an illuminating gas. The hydrogen and methane, which together 
compose about 86 per cent. of the bulk of the gas have high thermal 
values, but practically no intrinsic effect as illuminants; their chief 
function seems to be to raise heat by their combustion sufficient to 
decompose the small proportion of heavy hydro-carbons which are 
present and to render incandescent the carbon which these latter 
contain. 
Now I think it is time to give a little attention to the structure of 
an ordinary gas or candle flame. Let us look at this flat flame issuing 
from an ordinary No. 6 “Bray” burner. There are evidently two 
visible zones into which the flame may be divided. The lower zone 
nearest the burner, and which gives little or no light, ig usually called 
the zone of no combustion—in it the hydrogen is only just beginning to 
burn. In the light-giving zone above it we have the hydrogen burning 
with the oxygen of the air with which it has come into contact, and the 
heat thus evolved is rendering the carbon particles incandescent or white 
hot. The carbon is not yet burnt because the hydrogen has a greater 
affinity for the oxygen and has, as it were, to be accommodated first. 
Higher up there is a third zone which is invisible. By the time the 
constituents of the gas have reached this zone the hydrogen has been 
satisfied with its proper amount of oxygen, and the carbon is now being 
accommodated and is burning. This invisible zone, in fact, 1s generally 
the hottest part of the fame. Let me show you that in the luminous 
zone the carbon is really unburnt. [ plunge this cold white saucer into 
it and bring the unburnt carbon out as soot. Moreover, I could burn this 
soot off again by plunging it into the invisible zone at the top of the 
flame where there is a very high temperature and an excess of oxygen. 
