1819.1 Dr. Henru's Experiments en the Gas from Coal. 341 



has, in an equal volume, an illuminating power about one-third 

 greater than that from coal of medium quality. The quantity 

 also from the former substance exceeded by about one-seventh 

 that obtained from coal distilled under precisely similar circum- 

 stances ; 3500 cubic feet of gas having been collected from 1120 

 pounds of cannel, and only 3000 cubic feet from the same quan- 

 tity of coal. The whole product of one distilktion of cannel 

 mixed together in a gasometer was of such quality that 100 

 measures required for combustion 155 measures of oxygen gas, 

 and gave 88 measures of carbonic acid. But as the gas was 

 contaminated with 15 measures of azote in every 100, the oxygen 

 required for saturating 100 measures of the really combustible 

 part of it may be stated at 195 ; and the carbonic acid produced 

 at 110. It may be necessary to observe, that in comparing the 

 value of gases produced from different kinds of coal, or from the 

 same kind of coal differently treated,it is not enough to determine the 

 quantity of aeriform products ; and no satisfactory conclusion can 

 be drawn respecting the relative fitness of any variety of coal for 

 affordino- gas, or the advantages of different modes of distillation, 

 unless the degrees of combustibility of the gases compared be 

 determined, by finding experimentally the proportion of oxygen 

 gas required for their saturation. 



The results expressed in the first table, when contrasted with 

 those which I formerly obtained by the destructive distillation of 

 small quantities of coal, present several circumstances of disa- 

 greement, as to the quality of the products at different stages of 

 the operation. In small experiments, the sulphuretted hydrogen 

 and carbonic acid gases were evolved only at the early stages of 

 the process ; and sulphuretted hydrogen especially could not by 

 the nicest tests be discovered in the fast products of gas. On 

 the large scale both these gases continue to be evolved through- 

 out the whole operation, though in greatly diminished proportion 

 towards the latter end. Even in the advanced stages of large 

 distillations, the presence of sulphuretted hydrogen in coal gas 

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

 admits of being easily measured. The test, which I used for 

 some time, was the white oxide of bismuth, for which I after- 

 wards substituted white lead, ground with a little water to the 

 proper consistence, 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 ajar 

 or bottle of the gas, and kept there for some time. By experi- 

 ments on artificial mixtures, I found that a cubic inch of sulphu- 

 retted hydrogen diffused through 20,000 cubic inches of common 

 air distinctly affected the test, which it changed to a light-yel- 

 lowish or straw colour. By mixing sulphuretted hydrogen with 

 various proportions of common air, I prepared coloured cards of 

 a variety ot shades, which served as standards of comparison for 

 judging of proportions of sulphuretted hydrogen in coal gas, 

 which were too minute to be accurately measured. 



