Different kinds of Coal for the purpose of Illumination. 279 



A screen fitted up in this way, stretched on a frame, with 

 a holder for the light at the back, and with another holder in 

 front for the other light, is all that is required : of course, if 

 gases are to tried, the holder in front must be furnished with 

 a jet of the diameter previously fixed on, and with a flexible 

 tube to be connected with the gas-pipe. This holder can be 

 made to move backwards and forwards on a scale, graduated 

 to inches, with their corresponding squares ; or to lights of 

 candles, as may be thought proper. To make the apparatus 

 more complete, the gas-jet can be furnished with the pres- 

 sure-gauge, which I have recommended for ascertaining the 

 durability ; and thus, by the two together, the value of gases 

 may be quickly ascertained. 



This mode of trying the illuminating power will be found 

 very useful, not that I prefer it to the chlorine test, because, 

 by the latter, all gases can be compared with a standard of 

 unity, the illuminating power being just as the condensa- 

 tion ; but because the former is easily managed, especially by 

 those not pi'actised to work with water-troughs. Could a 

 truly uniform source of light be procured, and, by general 

 consent, be fixed on, as that for the back of the screen, then 

 the light from gases, or indeed from other sources, could be 

 compared with it ; but as yet, we are not, so far as I am 

 aware, in possession of such a light. 



With the use of the screen now described, I was anxious 

 to repeat the trials made, with the view of ascertaining the 

 light for equal consumpts of gases by different burners ; an 

 account of which was published in the Transactions of the 

 Society for 1842, because 1 have again and again heard the 

 accuracy of these results called in question. In conducting 

 the trials, I had, as before, recourse to an experimental metre, 

 to ascertain the consumpt, and made use of a gas-jet flame, 

 always of the same height, and always at the same distance 

 from the back of the screen. Each series of trials being con- 

 ducted in one day, the gas was, of course, of the same quality 

 for each day, by which a uniform transmission of liglit from 

 behind was obtained. 



