Variable Illuminating- Power of Coal Gas. 139 



gas the adjustment became simply a gradual withdrawal of the 

 candle. 



The capped receiver from which the gas was passed floated 

 freely in a large glass jar, supported in an erect position by the 

 perpendicular sides of the jar, its own weight, with all attach- 

 ments, making a difference of level between the water around it 

 and that within equal to 3£ inches, a little exceeding the ordinary 

 evening pressure in the gas pipes. This difference of level, and 

 consequently the pressure on the escaping gas, was kept uniform 

 by the spontaneous sinking of the receiver as the gas was con- 

 sumed, a flexible tube communicating between the stop of the 

 receiver and the gas burner. This arrangement gave me a steady, 

 equable flame, which continued perfectly uniform long enough 

 to enable me, after a few trials, to note, very exactly, its true 

 value. The results as first obtaiued were too startling to be at 

 once believed, but subsequent repeated trials satisfied me that 

 they were very close approximation to the truth. The first 

 trial was with the gas from the street main, which I found equal 

 to 10*71 candles. The same gas, transferred from the pipe to 

 the capped receiver, and burned immediately, gave exactly the 

 same power, 10*71 candles. Gas No. 1 was next used, and found 

 equal to only 3-50 candles; Gas No. 2. after standing two days, 

 gave the light of 3*20 candles; Gas No. 3, three days old, was 

 equal to 1*90 candles; and Gas No. 4, four days old, gave the 

 light of l'Yo candles — these quantities representing the average 

 of repeated trials. 



It thus appears that the illuminating material of our coal gas 

 is so rapidly abstracted by suffering it to remain in contact with 

 water, that the same volume *of gas which to-day will give me the 

 light of nearly 11 candles, by standing until to-morrow will give 

 the light of only 3^ candles, and if left standing four days will 

 give the light of only 1| candles, while the only means left to the 

 consumer to get the light he requires from this deteriorated gas 

 is to burn more of it, as we have all been doing through the past 

 winter. If we now take into account the well known fact that 

 gas of less illuminating power has less density, and that gas of less 

 density passes more rapidly through a given aperture than gas of 

 greater density we have another cause operating to increase the 

 consumption. In Hedley's experiments the Argand burner which 

 gave the light of 25 candles when supplied with 3 cubic feet per 

 hour of gas from Welsh cannel coal, with a specific gravity of 



