



Different Kinds of Coal for the purpose of Illumination. 85 



from Wigan cannel coal had condensation as 7o, but the dura- 

 bility was only 57' ; the Yorkshire cannel-coal gas was, condens- 

 ation 7-66, and durability only 52' 30". Had the durability of 

 the English cannel-coal gas kept pace with the condensation test, 

 it ought to have been at least 87 / instead of 52' and 51*. 



It is evident from this that the durability is affected by other 

 circumstances than the presence of the ingredient, whatever it 

 may be, which causes the condensation by chlorine, in other 

 words, than by the illuminating power. 



Considering this still farther, I observed a remarkable coinci- 

 dence between the durability and the specific gravity, as is shewn 

 in the following table : 



Specific 

 Durability 



gravity, 



55' 



64' 



645 

 66' 



6-51) 

 67' 



704 

 77'o" 



741) 

 91 '7" 



8M6 

 106' 



# 



In the above table it is shewn, that as the specific gravity be- 

 comes greater, the times required for the consumption of equal 

 quantities become longer; but the increase of the one dees not keep 

 pace with that of the other. There is, however, some connec- 

 tion between them, and on farther investigation it occurred to 

 rcie, that perhaps the consumption of gases by combustion is regu- 

 lated by the same law as the diffusion of gases, as pointed out by 

 Professor Graham, viz., that under equal pressures the diffusion 

 ts inversely as the square roots of the specif c gravities. Accor- 

 dingly, in equal times, the consumption should be inversely as the 

 square roots of the specific gravities : and conversely, the times 

 for the consumption of equal volumes, from similar burners, and 

 under the same circumstances will be as the roots of the gravities. 



Again, if this be true, then, under different pressures, the es- 

 cape should be as the square roots of the pressures; and, accor- 

 dingly, the time for equal consumptions should be inversely as the 

 roots of these pressures. 



To put this to the test of experiment, I procured a platinum jet, 

 furnished with a graduated pressure gauge, and adapted it to an 

 experimental metre, by which I could consume the gas, under the 

 same and different pressures, and mark the quantity consumed in 

 a given time ; and consequently the times for the consumption of 

 equal quantities. The gauge had a vernier fitted to it, by which 

 I could easily read off to one-hundredths of an inch. The tem- 

 perature and barometer were also noted for each experiment, and 



the specific gravity, when necessary, was ascertained in the usual 

 way. 



The following are the results of trials made for the 

 putting these opinions to the test. 



purpose 



of Gases under difft 



experimen 



first 



agree 



