ARTIFICIAL LIGHT. 
87 
certain quantity of coke, consisting of the carbon that has not 
combined with oxygen and hydrogen, mixed with the earthy 
impurities of the coal ; but by various processes several liquid 
and solid substances, of more or less utility, become condensed 
on the other side, before the gases are entirely set free. The 
gases intended for brnming require to be purified, so as to get 
rid more especially of the sulphur compounds and carbonic 
acid, an operation in which slaked lime is especially useful, as 
it absorbs large quantities of the most objectionable substances. 
The gas being set free in a tolerably pure state, yields, 
within certain definite limits, a quantity of light greater in 
proportion to the carbon it contains. For this purpose, the 
poor and rich gases require to be mixed, the pure light car- 
buretted hydrogen giving very little light at the ordinary 
temperature at which combustion is effected, and gases with 
too much carbon giving off smoke while burning. The mixture 
being made, the maximum fight is obtained by a nice arrange- 
ment of the quantity of gas allowed to escape, and the draught 
of an admitted or forced to pass through the flame. 
It is unnecessary to describe the ordinary contrivances used as 
gas-burners, although some of them are much more ingenious 
than others, and better adapted to give fight. On a large scale, 
however, and in public buildings, the method of lighting that 
is adopted has such enormous influence on the health and 
comfort of those exposed to the atmosphere of the place, that 
it becomes a matter of the most serious consideration. 
There cannot be a doubt that a large proportion of the head- 
aches, sleepiness, and general discomfort felt in public buildings 
hghted with gas, where no special means are adopted for re- 
moving the products of combustion, are due to the accumula- 
tion of carbonic acid and other poisonous gases given off during 
combustion. While gas is binning, it removes from the atmo- 
sphere a large quantity of oxygen ; and as this is also the result 
of breathing, the effect is soon felt where a large number of 
I human beings are together. There is but one way of remov- 
ing this great evil, but fortunately that method is fully adequate. 
It consists in the use of a ventilating burner, either resembling 
in its principles of action the burner originally contrived by 
Faraday, or of a still more simple arrangement, the whole of 
the jets being connected with an air-chamber and chimney, so 
placed that the draught carries off at once into the open air 
every particle of matter produced during combustion. Faraday’s 
burner is an ordinary argand burner, of large size, with a 
chimney, surrounded by a wider and taller chimney, closed at 
the top, and opening at the bottom into another tube, that 
carries away the products of combustion. The star method of 
illumination involves the use of numerous groups of small jets 
