Gas-Burners , and on the Illuminating Power of the Gases. 37 
faint smell, but we never found it altogether inodorous. The 
best oil-gas appears to have the least smell. The odour of coal- 
gas is of a mixed kind, being in part empyreumatic like oil-gas, 
and partly of an exceedingly offensive nature, like that of sul- 
phuretted hydrogen. In Edinburgh coal-gas we have generally 
observed the empyreuma alone ; but frequently the other is per- 
ceptible also, and sometimes it prevails to an insufferable degree. 
The most serious objection to coal-gas arises from the pre- 
sence of impurities. These are, a black matter like tar, and 
compounds of sulphur,— all derived from the coal itself, and 
therefore necessarily present originally in every description of 
coal-gas. Without purification, therefore, coal-gas could scarcely 
be used at all ; and it becomes a question of importance to deter- 
mine, whether or not the noxious ingredients may be wholly re- 
moved from it. The greater part of the tar is deposited at the 
works in the proper vessels, but a minute portion does common- 
ly pass over with the gas. It tends to clog the apertures of the 
burner, and of course soils substances upon which it is deposited. 
In common shops, where a free current of air is preserved, the 
effect is hardly noticed ; but we suspect that a part of the in- 
convenience found by jewellers to attend the use of coal-gas 
arises from this cause. 
The most formidable of the compounds of sulphur present in 
coal-gas, is sulphuretted hydrogen. The presence of this gas is 
hurtful in two ways. If it escape unburnt, it offends by 
its insupportable odour, and attacks silver, and paint, with 
great readiness. When consumed, it forms sulphurous and sul- 
phuric acids, which may injure the health, if habitually in- 
spired, and act chemically on various substances, as on iron and 
steel. Hence the necessity of removing it entirely from coal gas. 
On this subject two important questions naturally occur, to both 
of which we can give a decisive answer. 1st, Can sulphuretted 
hydrogen be wholly separated from coal-gas ? and, Qdly, when it 
is removed, Can coal-gas be regarded as perfectly free of sulphur ? 
We are satisfied that sulphuretted hydrogen may be wholly 
removed ; for we have repeatedly examined the Edinburgh coal- 
gas by the most delicate tests, without detecting a trace of it. 
Of course we do not vouch that it is always equally pure, be- 
cause the least neglect, on the part of the workmen, must inevi- 
tably cause some sulphuretted hydrogen to escape into the pipes. 
