42 EXPERIMENTS ON THE 



able) ; C is the substance intercepting the light of the gas-lamp D, and throw- 

 ing a shadow on the paper. C is formed of a strip of thin brass, about a 

 quarter of an inch broad, moveable round its axis on a pin at the top and 

 bottom, so that its shadow may be adjusted to correspond in breadth to 

 that cast by the lamp ; for the candle being nearer, if its intercepting wire 

 were of the same diameter as that of the lamp, the shadow would of course 

 be much broader, and tend to deceive the operator. D is the lamp placed 

 5 feet from the wire C ; E is the candle-socket sliding upon a rod, which 

 is marked according to the number of candles the gas-lamp is equal to. 



If, when the candle is placed at 1 , the two shadows are equal, the lamp 

 only gives a light equal to one candle ; if at 2, the lamp is equal to 4 candles ; 

 and if at 4, the lamp is equal to 16 candles. 



A simple rule-of-three statement will give the comparative quantities of 

 light, the candle being at any distance. The burner remaining 5 feet from 

 the interposed wire, supposing the candle to be l^ths of a foot from its 

 wire, 



The square of 1'5 = 2'25. 

 The square of 5'0 = 25'00. 

 Then as 2'25, the square of the candle's distance, is to 1', 



so is 25'00, the square of the lamp's distance, to 11*11, the number 

 of candles the gas-lamp is equal to. 



The candle should be of the same size for all the experiments, and of wax : 

 the lamp should consume about 4^ cubic feet of gas each hour. 



After having determined the intensity of light, it is requisite also to find the 

 specific gravity of the gas, which is obtained by weighing a given volume, and 

 comparing it with the weight of the same volume of atmospheric air, which is 

 the standard of comparison. 



One hundred cubic inches of common air weigh, at the temperature of 60 

 Fahr., and when the barometer stands at 30 inches, exactly 3O50 grains. 

 Supposing, under the same circumstances, 100 cubic inches of coal-gas to 

 weigh 16-520 grains, then the specific gravity is deduced as follows : 



As 30'5, the weight of air, is to 16'52, the weight of the gas, 



so is 1 -0, the specific gravity of air, to 0'54l , the specific gravity of the gas. 



In weighing the gas due regard must be paid to the moisture, which is 

 always present, and which must either be allowed for by calculation, or re- 

 moved by the methods described in the next section. 



