40 EXPERIMENTS ON THE 



rated and unite, forming ammonia, vapour o'f bitumen (which afterwards con- 

 denses, forming tar, ammoniacal liquor and essential oil), and carbonic oxide. 

 If the retort is too hot, all the dense hydro-carburets are resolved into char- 

 coal and hydrogen ; the product is greater, but the specific gravity little more 

 than that of hydrogen, and the illuminating power of the gas decreased in 

 the same ratio. At the heat of 27 of Wedgewood, or that of melting copper, 

 which has been found the best, the bitumen is decomposed, at the same time 

 that the hydrogen is liberated and unites with its carbon, forming olefiant 

 and carburetted hydrogen gases, often of the specific gravity O470. The 

 quantity of gas of the above specific gravity produced from 3 pounds of the 

 best Newcastle coal, ought to be about 19 cubic feet in one quarter of an hour. 

 The operation must not be carried on too long, for the process in the end 

 would be productive almost exclusively of carbonic oxide and hydrogen. 



The gas, as it is produced in the experimental retort, escapes by the stand- 

 pipe S, (which ought to be at least seven feet long, to allow some portion of 

 the bituminous vapour to fall back and be converted into gas,) and passes 

 through one or two other vertical pipes, to the gasometer, in order that it 

 may be thoroughly condensed : at the bottom bends of these pipes a siphon 

 must be attached, furnished with a stop-cock to draw off the tar and other 

 condensed vapours that will be deposited. The cup, it will be obvious, is 

 absolutely necessary, otherwise the condensed vapours would seal the pipes 

 and stop the flow of the gas. 



The tank of the gasometer should contain as little water as possible, for 

 the reason I shall state hereafter. It is formed of two concentric cylinders, 

 two inches apart, filled with water, between which the rising part works : the 

 whole is fixed in a frame, on which pullies rest, to support the specific-gravity- 

 chain and balance-weights. The chain is best made of stout tape, with pieces 

 of sheet-lead sewn on to it, equal to the difference between the weight of the 

 gasometer when out of the water, and when immersed in it. 



The operation being concluded, it only remains to analyse the gas, which 

 should be done as soon as it is produced, by the following simple processes, 

 which are found sufficiently correct for all practical purposes. 



The intensity of light is ascertained by an instrument called the Photometer, 

 invented by Count Rumford ; it is constructed on the principle that the power 

 of a burning body to illuminate any defined space is directly as the intensity 

 of the light, and inversely as the square of the distance. If two unequal 



