OF THE MANUFACTURE OP GAS. 



81 



watery vapour; the proper correction was afterwards made 

 for this, and the per centage numbers given in the following 

 analysis invariably refer to the gases free from watery vapour. 

 The carbonic acid was determined by caustic potash, the 

 oxygen by Liebig's new method, viz., by absorption with a 

 solution of pyrogallic acid in caustic potash, the illuminating 

 hydrocarbons by strongly fuming sulphuric acid, and the rest 

 of the gases by explosion with excess of oxygen, in which 

 the amount of oxygen consumed and carbonic acid generated 

 were estimated, and the respective volumes of light carburetted 

 hydrogen, carbonic oxide, hydrogen and nitrogen, calculated 

 from the numbers thus obtained. 



Various attempts have been made to estimate the illu- 

 minating power of coal and other gases from the analytical 

 results yielded by them, but hitherto no certain method of 

 accomplishing this has been established. Dr. Henry regarded 

 the consumption of oxygen by a given volume of the gas to 

 be a rough estimate of its illuminating power; but it is 

 evident that although generally those gases which have the 

 highest illuminating power consume the largest amount of 

 oxygen in relation to their volume, yet this is not always the 

 case, for a gas containing 10 per cent, of olefiant gas, 20 

 per cent, of light carburetted hydrogen, and 70 per cent, of 

 hydrogen, would consume much less oxygen during com- 

 bustion than one containing only 5 per cent, of olefiant gas, 

 and in which the proportions of light carburetted hydrogen 

 and hydrogen were reversed, although the illuminating power 

 of the former would be twice as great. 



It will be seen, from what has already been said respecting 

 the illuminating power of hydrocarbons, that the more dense 

 these bodies are the greater is the amount of light they yield. 

 This important fact was first pointed out by Mr. Leigh, who 

 was also the first to make a near approach towards accurately 

 estimating the illuminating power of gas from its analysis. 

 Mr. Leigh regards the illuminating power of coal gas as being 



