38 Dr Andrew Fyfe on the Comparative Value of 



now that the trials have been greatly extended, I place the 

 utmost confidence. 



For ascertaining the latter, I have followed the usual 

 method, an accurately-adjusted experimental metre; by which 

 the times required for the consumpt of equal volumes, burn- 

 ing under similar circumstances, and also the quantity con- 

 sumed in equal times, were easily determined. The jet burner 

 was the same in all the trials. 



I consider both of these circumstances absolutely necessary, 

 for, though some have insisted only on the one, and others 

 on the other only, yet. unless both be taken into account, we 

 do not arrive at the true value oiUxQ gases, and, consequently, 

 cannot compare one with another for the purpose of illumi- 

 nation. Thus, if two gases afford, by their combustion, from 

 the same or similar burners, with the same height of flame, 

 the same light : but if a foot of the one lasts an hour, and a 

 foot of the other an hour and a-half, then the latter is one- 

 half more in value than the former for yielding light, because 

 it is giving the same light for one-half more time ; or, which 

 is the same thing, one-half more of the former must be used 

 so as to complete the time which the latter will burn. This. 

 T regret to say, has been too often overlooked by many in 

 estimating the value of coal-gas. 



As the chief object I had in view was, not the comparison 

 of the light afforded by coal-gas, or its expense, as compared 

 with other sources of light ; but merely the value of the gases 

 as compared with one another, when obtained from different 

 coals, I shall commence with that got from English caking 

 coal, and take it as the unit for comparison. 



English Caking-Coal Gas. — The gas from this kind of coal, 

 on which my experiments were made, was that at New- 

 castle ; others were also pi'ocured by means of an experimen- 

 tal apparatus, fitted up expressly for the purpose. The con- 

 densation by chlorine in the former, was, on an average of 

 several trials, 4-33 per cent. 



The specific gravity, at Th. 60, B. 30, was, 420. 



The durability, with a four-inch flame, from a platinum 

 jet, g^gd of an inch in diameter, was, 1 cubic foot in 50 mi 



