Gas-Burners 5 and on the Illuminating Power of the Gases . 15 
namely, the proportionally superior light given out by the gases at high than 
at low elevations of flame. The supply of air does not increase in proportion 
to the height of the column of flame. On the one hand, its quantity is not 
proportional ; on the other, its quality is impaired as it rises along the flame ; 
and hence a larger proportion of gas undergoes decomposition before being 
burnt. 
If these views be entertained of the cause of the light of the gases, it will 
be necessary to modify somewhat the principle laid down above, as to the most 
economical mode of burning the gases for the purpose of illumination. We 
have there said, that the intensity of the combustion should not be greater 
than is necessary to its being complete. Now, charcoal always gives a strong- 
er white light, the greater the rapidity with which it is consumed. Hence it 
would be advisable to enliven the combustion as much as possible, provided 
the means employed for that end do not also cause the gas to be burnt with- 
out undergoing previous decomposition. So far as we have yet tried, how- 
ever, the same means which are useful on the former, are apt to be injurious 
on the latter account, when the intensity of the combustion surpasses the li- 
mit above mentioned. Accordingly, when it is desired to increase the intensity 
of a light, which can be done only by increasing the supply of air, means must 
be taken to counteract its tendency to burn the gas before decomposition, 
otherwise there will be a loss of light in relation to the expenditure. Thus, 
as will afterwards be seen, when the central supply of air in a burner is in- 
creased, either by enlarging the central air-hole or contracting the diameter of 
the chimney, the increased tendency of the gas to be consumed without de- 
composition must be prevented by multiplying the jet-holes. 
The circumstances which, through the operation of the foregoing prin- 
ciple, affect the light given out by the gases, may be arranged under three 
heads, as they respect the flame itself, the construction of the burner, and the 
shape of the glass-chimney. 
1. The only point relative to the flame itself which calls for consideration, 
is its length. The relative length of the flame has a most important influence 
on its light : For, as the flame is lengthened, its light increases in a much 
greater ratio than the expenditure. The fact holds true both with regard to 
single jets and Argand burners. 
First, With regard to single jets, it is very well shown by the following 
series of experiments, in which the standard of comparison was a 3-inch jet of 
coal-gas (Spec. Grav. 602.) and the varying jet, also of the same coal-gas, was 
gradually lengthened from 2 to 6 inches. The light and expenditure of the 
standard being each supposed equal to 100, the numbers for the different 
lengths of flame were as follows, 
2 -inch. 
3 -inch. 
4. -inch. 
5-inch. 
6-inch . 
Light, 
55.6 
100 
150.6 
197.8 
247.4 
Expenditure, 
- 60.5 
101.4 
126.3 
143.7 
182.2 
Consequently the light given out by equal expenditures for each length of 
flame, is in the following proportions, neglecting decimals, 
100 
109 
131 
150 
150 
