1819.] Dr. Henry's Experiments on the Gas from Coal. 343 



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 tem- 

 perature about its fusing point. On restoring this temperature 

 by adding fresh metal, the evolution of gas was always sus- 

 pended. I placed also one of Mr. Wedge wood's pyrometer 

 pieces in contact with a retort which was at work at Mr. Lee's 

 manufactory, and which showed only a dull red or blood- 

 coloured heat ; but, after remaining in that situation half an hour, 

 a contraction of barely one degree of the scale had taken place. 

 This temperature, however, \ 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 ascertaining 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 appears that the sudden application of the 

 requisite heat evolves from coal much more gas than the gradual 

 heating of a cool retort up to the point of ignition.* 



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 aeriform products, from the beginning to the end of 

 the operation. But in experiments on the gas obtained at other 

 times from the same substance no appreciable quantity of azotic 

 gas could be discovered till after the sixth hour of the process, 

 when it began to appear, and progressively rose to 20 parts in 

 the hundred. Of this purity of the early products from azote, 

 and appearance of it in the latter ones, Mr. Dalton was an eye 

 witness on one occasion, when he was so good as to co-operate 

 with me ; and 1 had afterwards repeated opportunities of verify- 

 ing the fact. With the view of ascertaining whether the azote 

 found its way from the atmosphere into the distilling vessels, I 

 subjected 100 gr. of cannel coal to heat in a glass retort, the 

 capacity of whose body and neck did not together exceed l£ 

 cubic inch. Besides a portion of gas which was lost, 50 cubic 

 inches were collected, which, on careful analysis, were found 

 to contain five cubic inches of azotic gas. Of these, only one 

 cubic inch can be traced to the common air present in the 

 retort at the outset ; and the other four cubic inches must have 

 been furnished by the coal itself. 



It is reasonable indeed to expect that a substance like ceal y 

 which affords ammonia under some circumstances, should, 

 under others, yield the elements of that alkali iu a detached 

 state ; and the reason why azote is for the most part not to be 

 found in the gas which is first evolved is, that at a low temper- 

 ature that element unites with hydrogen, and composes ammonia 



• Journal of Science, vol. i. p. 75. 



