Value of (liferent kinds of Coal for Illumination. 37 



was present, although the specimens could hardly have 

 yielded a larger quantity of pure and unmixed mineral to 

 make an analysis on a larger scale. 



'■'e Value of different Kinds of Coal for the 

 ination ; and on Methods not hitherto prac- 

 lining the Value of the Gases they afford. 

 FE, M.D., F.R.S.E., F.R.S.S.A., Professor 

 Cing's College University, Aberdeen, &c. 

 by the Royal Scottish Society of Arts.* 



lished in the Transactions of the Society for 

 iccount of numerous trials made with the 

 ing the value of different kinds of coal for 

 illumination. Having been again engaged 

 )eriments for the same purpose, I have beeo 

 some of the results public, because I con- 

 •e interesting, and lead to valuable practical 



hich I have had in view were to ascertain, 

 ubi owij the <,!. , jaratwe amount of light afforded by the gases 

 which the coals yield, but also the durability of these gases, 

 so as to enable me to fix, as far as possible, their comparative 

 value, and, consequently, their comparative expense, for the 

 purpose of illumination. Besides these, however, my atten- 

 tion was drawn to other circumstances connected with the 

 consumption of gas, which, strictly bearing on the other part 

 of the inquiry, are of importance. 



1 . Quality of the Gases. 



In estimating the quality of coal-gases, and, consequently, 

 fixing their comparative value for the purposes of illumina- 

 tion, we must take into account both the light afforded, and 

 the time required for the consumpt of equal volumes. 



In trying the former, I have, in the following experiments, 

 invariably had recourse to the method mentioned in my for- 

 mer papers, viz., the condensation by chlox'ine, and in which, 



* KcaU befwc the Society 2itli April lSi8. 



