Experiments on the Gas from Coal. 421 
supposed,* though within the period men- 
tioned, and- in such narrow tubes, it does 
not occasion a sensible diminution of bulk. 
The method described may, therefore, be 
considered as affording a tolerably near ap- 
proximation to the proportion of olefiant 
gas; and as all the varieties of coal gas were 
subjected to the test under precisely the same 
circumstances, the errors must bave been of 
nearly the same amount inall cases, and can- 
not materially interfere with the fair compari- 
son of the differentspecimens of coal gas, so far 
as respects their proportion of olefiant gas. 
3. To ascertain the quantity of combustible 
matter in gas which had been deprived only 
of sulphuretted hydrogen and carbonic acid, 
a mixture of the gas with a due proportion 
of oxygen gas was fired by the electric spark 
over mercury. This method I preferred to 
slow combustion, carried on with the appa- 
ratus, which I have described in the Philo- 
sophical Transactions for 1808, solely because, 
when a great number of experiments are ne- 
cessary, as in this enquiry, the method of 
* While this sheet was passing through the press, I 
have noticed a passage in Mr. Brande’s Manual of Che- 
mistry (page 156 n.), from which it appears that the 
speedy action of chlorine on carburetted hydrogen had bee 
observed by Mr. Faraday, 
