342 Dr. Henry's Experiments on the Gas from Coal. [Nov. 



In the small experiments made several years ago, I never 

 found, in the early products of gas from cannel coal, a propor- 

 tion of olefiant gas at all approaching that which is noted in 

 Table I, and its quantity in small distillations rapidly decreased, 

 until ia the latter products it could be no longer traced at all. 

 The method of analysis, which I formerly employed, led me, 

 however, as I have lately discovered, to under-rate the propor- 

 tion of olefiant gas, and to over-estimate that of sulphuretted 

 hydrogen. But making due allowance for this error, the supe- 

 riority of the products of large operations, so far as respects 

 olefiant gas, still exists, and is confirmed by comparative experi- 

 ments on a small scale which I have lately made. Thus it 

 appears from Table I, that even after 12 hours continuance of 

 the process, olefiant gas still constitutes four per cent, of the 

 gases evolved from cannel. The other inflammable gases also, 

 when obtained in large quantity, are more uniform in quality, 

 and possess, towards the. close of the process, much greater 

 combustibility and illuminating power than when procured in 

 small experiments. This superiority is obviously dependent on 

 the greater facility of preserving an uniform temperature in all 

 chemical processes which are carried en upon a scale of magni T 

 tude. 



The temperature to which the coal is subjected must necessa- 

 rily be a point of the greatest importance to the quantity and 

 quality of the aeriform products ; for while too low a heat distils 

 over in the form of a condensible fluid, the bituminous part of 

 the coal which ought to afford gas, too high a temperature, on 

 the contrary, occasions the production of a large relative propor- 

 tion of the lighter and less combustible gases. It would be a 

 o-reat step in the improvement 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 charcoal present in 

 coal. If this object be ever accomplished, it will probably be by 

 the discovery of means of uniformly supporting such a tempera- 

 ture 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 care- 

 ful decomposition of oil, which in my experiments afforded a 

 mixture of gases, succeeded in obtaining olefiant gas in a state 

 of purity.* 



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

 adequate to the production of gas from ccal, I placed a small 

 iron retort containing cannel in melted solders of various com- 

 position 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, 



* Memoires dela Soc. d'Arcueil, ii. 84. . . • 



