1853.] OF THE ROYAL INSTITUTION. 321 
solids, such as lime, emit light of the most dazzling splendour when 
they are heated in this flame. Hence, those gases and vapours 
only can illuminate, which produce or deposit solid or liquid matter 
during their combustion. This dependence of light upon the pro- 
duction of solid matter is strikingly seen in the case of phosphorus, 
which when burnt in chlorine produces a light scarcely visible ; 
but, when consumed in air or oxygen, emits light of intense bril- 
liancy: in the former case the vapour of chloride of phosphorus is 
produced, in the latter solid phosphoric acid. 
Several gases and vapours possess this property of depositing 
solid matter during combustion, but a few of the combinations of 
carbon and hydrogen are the only ones capable of practical applica- 
tion: these latter compounds evolve during combustion only the 
same products as those generated in the respiratory process of 
animals, viz. carbonic acid and water. The solid particles of carbon 
which they deposit in the interior of the flame, and which are the 
source of light, are entirely consumed on arriving at its outer 
boundary; their use as sources of artificial light under proper 
regulations is therefore quite compatible with the most stringent 
sanatory rules. 
In the usual process of gas manufacture there are generated in 
addition to these illuminating hydrocarbons two other classes of 
gaseous constituents, viz. impurities and diluents. With the ex- 
ception of bisulphuret of carbon and some organic compounds 
containing sulphur, all the impurities are removed in the usual 
processes of purification, which have now been brought to great 
perfection; but the presence of these sulphur compounds in coal gas 
is very objectionable, and constitutes the chief barrier to the universal 
employment of gas in dwelling-houses. The attention of the manu- 
facturer ought therefore now to be earnestly directed to the dis- 
covery of means for preventing the formation of these compounds, 
as it will probably be found impossible to remove them from the 
gas when once they have been formed. 
In addition to traces of these sulphur compounds, purified coal gas 
contains only the following ingredients. i 
Formula. 
i g ( Olefiant Gas - - - Cy wy 
& 5 Propylene? = - . - C3 es 
g % Butylene ? - - s C, Hy 
aS Other Hydrocarbons - - unknown 
a Light carburetted hydrogen - Cr “i 
g Hydrogen - - - H 
& | Carbonic Oxide - - Cig G 
* This gas has usually been described as possessing a certain amount of 
