124 Experiments on the Gas from Coal. 



wards the latter end. Even in the advanced stages of large di- 

 stillations, the presence of sulphuretted hydrogen in coal-gas may 

 be traced by the proper test, though not in a quantity that ad- 

 mits of being easily measured. The test which I used for some 

 time was the white oxide of bismuth, for which 1 afterwards sub- 

 stituted white lead, ground with a little water to the proper con- 

 sistence, and spread by a camel's hair pencil on a slip of card. 

 This was secured by a small pair of forceps fixed in a cork, by 

 means of which the slip of card could be placed in a jar or bot- 

 tle of the gas, and kept there for some time. By experiments on 

 artificial mixtures, I found that a cubic inch of sulphuretted hy- 

 drogen, diffused through twenty thousand cubic inches of com- 

 mon air, distinctly afi'ected the test, which it changed to a light 

 yellowish or straw colour. By mixing sulphuretted hydrogen 

 with various proportions of common air, I prepared coloured 

 cards of a variety of shades, which served as standards of com- 

 parison for judging of proportions of sulphuretted hydrogen in 

 coal-gas, which were too minute to be accurately measured. 



In the small experiments made several years ago, I never found, 

 in the early products of gas from cannel coal, a proportion of ole- 

 fiant gas at all approaching that which is noted in Table I. and 

 its quantity in small distillations ra|)idly decreased, until in 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 proportion of defiant gas, 

 aud to over-estimate that of sulphuretted hydrogen. But making 

 due allowance for this error, the superiority of the products of 

 large operations, so far as respects olefiant gas, still exists, and 

 is confirmed by comparative experiments on a small scale which 

 I have lately made. Thus it appears from Table I. that even 

 after twelve hours continuance of the process, olefiant gas still 

 constitutes 4 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 comlnistibility and illuminating power, than 

 when procured in small experiments. This superiority is ob- 

 viously dependent on tiie greater facility of preserving an uni- 

 form temperature, in all chemical processes which are carried on 

 upon a scale of magnitude. 



The temperature to which the coal is subjected, must neces- 

 sarily 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 condensihle fluid, the bituminous part of 

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

 the contrary, occasions the ])roduction of a large relative propor- 

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



great 



