324 Mr Ritchie on Leslie's Photometer. 



fn the late discussions with regard to the relative values of 

 coal and oil-gas, those who have employed Mr Leslie's pho- 

 tometer, seem to have overlooked the distinction between the 

 terms Quantity of Light and Illuminating Power. By the 

 former term, I would he understood to mean the numher of 

 atoms of light shooting out simultaneously from two luminous 

 sources; hy the latter, the power which these atoms possess 

 of rendering external objects visible. The ratio of the former 

 may be determined by a photometer founded on the expan- 

 sion of air by its combination with light. The ratio of the 

 latter cannot be determined by any method which does not 

 employ the indications of the extremely delicate photoscope, 

 the eye, as one of the elements in the calculation. 



The method which Mr Leslie has proposed, evidently de- 

 pends on the assumed principle, that the illuminating power 

 is proportional to the quantity of light. This principle, from 

 a very simple method, which I have lately employed for de- 

 termining the illuminating powers of different flames, I find 

 to be quite unfounded. When the colours of the flames are 

 nearly the same, the illuminating powers will then be nearly 

 proportional to the quantities of light ; but when the colours 

 are different, the illuminating power of the most brilliant light, 

 increases in a much higher ratio than the mere quantity of 

 light. Mr Leslie's method must, therefore, give results very 

 unfavourable to the illuminating power of oil-gas, when com- 

 pared with that of ordinary coal-gas. 



But, as mere reasoning can never determine a question in 

 physical science, unless that reasoning be founded on accurate 

 experiment, I had recourse to the following experiment, which 

 seems to put the matter beyond the possibility of doubt. I 

 made a quantity of oil-gas of the very best quality, and an 

 equal quantity of coal-gas, of an inferior quality, and not well 

 purified. The one burned with a bluish flame, surrounded 

 by a red fringe. The other threw out a torrent of white bril- 

 liant light. By Mr Leslie's method, the ratio turned out as one 

 to Jive, though the one did not possess one-twentieth part of the 

 illuminating power of the other. I have thus taken the qua- 

 lities of the two gases very different, in order to shew more 

 clearly the fallacy of the method in extreme cases, as it is more 



