126 Experiments on the Gas from Coal. 



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 hun- 

 dred. Of this purity of the early products from azote, and ap- 

 pearance 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 I had afterwards repeated opportunities of verifying the fact. 

 With the view of ascertaining whether the azote found its way 

 from the atmosphere into the distilling vessels, I subjected 100 

 grains of cannel coal to heat in a glass retort, the capacity of 

 whose body and neck did not together exceed 1;; cubic inch. Be- 

 sides a portion of gas which was lost, 50 cubic inches were col- 

 lected, which, on careful analysis, were found to contain 5 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 4 cubic inches must have been furnished by the coal itself. 



It is resonabie indeed to expect, that a substance like coal, 

 which affords ammonia under some circumstances, should, under 

 others, yield the elements of that alkali in a detached state ; and 

 the reason, whv azote is for the most part not to be found in the 

 gas which is first evolved, is, that at a low temperature, that ele- 

 ment unites with hydrogen, and composes ammonia. But when 

 the contents of the retort, which, for some time, have been kept 

 comparatively cool by the escape of condensable fluids, become 

 more intensely heated, anmionia is either not formed, or, if 

 formed, is decomposed again into azotic and hydrogen gases, 

 both of which may be traced in the aeriform products of the ad- 

 vanced stages of distillation. As a matter of practice, it is cer- 

 tainly desirable that the azote existing in coal should enter into 

 the composition of a condensable fluid, rather than that it should 

 escape in a gaseous state ; for it is an impurity which, when once 

 mingled with the combustible gas, cannot be removed by any 

 known method, and must materially impair its illuminating power. 

 That such an effect must result from its presence, may be in- 

 ferred from the experiments of Sir H. Davy, who found that an 

 explosive mixture of carburetted hydrogen and common air was 

 deprived of its combustibility by being mixed with one sixth of 

 its bulk of azotic gas*. 



[To be continued.] 



• On the Safety-lamp, page 30. 



XXII. Free 



