PRODUCTS FROM COAL. 43 



The vessel in which gas is usually weighed in the laboratory is a thin glass 

 globe, of known capacity, furnished at one end with a brass stop-cock, by 

 which, after being exhausted by an air-pump, it is attached to the stop-cock 

 of a transfer jar standing upon the shelf of a pneumatic trough, or on to that 

 of the gasometer containing the gas. 



As an air-pump, however, is not always at hand, the weighing-vessel may 

 have an aperture at the opposite end from the stop-cock ; which, after the gas 

 has been allowed to blow through and expel the atmospheric air, can be 

 closed with a tight cork. Care must be taken to ascertain exactly the cubical 

 contents of the weighing- vessel. 



It will be found that the illuminating power of the gas is almost directly as 

 the specific gravity ; the heavier the gas, the greater the light given. If gas 

 of sp. gr. 0'300 gives the light of six candles, that of sp. gr. 0'500 will give 

 the light of ten candles, or as 3 to 5. This has been doubted ; but Mr. Clegg 

 ascertained the fact in 1817, from many experiments, and I have myself fre- 

 quently proved it beyond all question. 



The presence of olefiant gas may be detected, and the quantity ascertained, 

 by passing into a bottle, inverted over water, two measures of coal-gas and 

 one of chlorine ; a diminution of volume will take place, the water will rise in 

 the jar, and an oily liquid be formed by the olefiant gas uniting with chlorine, 

 which ought to be in excess. Remove the remaining chlorine by a solution 

 of pure potash, and the diminution in volume which the coal-gas has sustained 

 will give the quantity of the olefiant gas sought. 



The most important impurity is sulphuretted hydrogen ; the method of 

 ascertaining the amount is simply by inverting a well-stopped bottle contain- 

 ing a solution of the acetate of lead over the shelf of a pneumatic trough, and 

 transferring into it, from a bladder or other vessel, a portion of coal-gas, say 

 about one-fourth the contents of the bottle ; re-place the stopper (which has 

 been removed for the admission of the gas) , agitate the solution and gas well 

 together, suffer it to stand an hour or so, and a black precipitate of the sul- 

 phuret of lead will have fallen : decant the clear liquid, and evaporate the 

 precipitate to dryness ; the weight will give the proportion of sulphuretted 

 hydrogen. 



The quantity of tar, ammoniacal liquor and oil produced can be measured, 

 when drawn off from the siphons attached to the lower bends of the vertical 

 condensing pipes. 



G 2 



