Experiments on the Gas from Coal. 125 



great step in the improvetnent of the manufacture of coal-gas, if 

 the whole of the hydrogen could be obtained in combination with 

 that proportion of charcoal which constitutes olefiant gas ; and 

 it is satisfactory to know, that no impediment to this arises out 

 of the proportion of the hydrogen and cliarcoal present in coal. 

 If this object be ever accomplished, it will probably be by the 

 discovery of means of uniformly supporting such a temperature 

 as shall be adequate to the production of olefiant gas, and shall 

 never rise above it ; and some probability of success is perhaps 

 derivable from the fact, that M. Berthollet, by the careful de- 

 composition of oil, which in my experiments afforded a mixture 

 of gases, succeeded in obtaining olefiant gas in a state of pu- 

 rity*. 



With the view of ascertaining how low a degree of heat is ade- 

 quate to the production of gas from coal, I placed a small iron 

 retort, containing cannel, in melted solders of various composi- 

 tion, without obtaining more than the common air of the vessel. 

 The retort charged with fresh materials, was then immersed in 

 melted lead ; but after expelling the common air, no more than 

 a few bubbles of gas came over, and that only when the lead, by 

 being kept over the fire, had acquired a temperature about its 

 fusing point. On restoring this temperature by adding fresh me- 

 tal, the evolution of gas was always suspended. I placed also 

 one of Mr. Wedgwood's pyrometer pieces in contact with a re- 

 tort which was at work at Mr. Lee's manufactory, and which 

 showed onlv a dull red or blood-coloured heat; but, after remain- 

 ing in that situation half an hour, a contraction of barely one 

 degree of the scale had taken place. This temperature, how- 

 ever, I suspect is rather too low, and has a tendency to distil 

 over too much tar, and consequently to produce less gas than 

 might be obtained by a degree of heat somewhat higher. The 

 best adapted temperature will probably be found to vary with 

 different kinds of coal ; and I have been prevented from ascer- 

 taining it with respect to cannel, by the inconveniences that 

 would arise from disturbing the regular arrangements of a large 

 manufactory. From some experiments of Mr. Brande, it ap- 

 pears that the sudden application of the requisite heat evolves 

 from coal much more gas, than the gradual heating of a cool re- 

 tort up to the point of ignition f. 



In the experiments upon gas from Wigan cannel, the results of 

 which are comprised in the first table, azotic gas was found in 

 all the aciriform products, from the beginning to the end of the 



• Mimoircs ilc hi Soc. dArcvcU, ii. H4. 

 f Journal of Science, vol. i. page lb. 



operation. 



