138 



ordinary circumstances scarcely rises to within a quarter of an inch 

 of the apex of the wick, now extended to the height of ^th of an inch 

 above the cotton, thus greatly reducing the size of the luminous 

 portion of the flame. 



On returning to England, I repeated the experiments under cir- 

 cumstances which enabled me to ascertain, by photometrical measure- 

 ments, the extent of this loss of illuminating effect in rarefied air. 

 The results prove that a great reduction in the illuminating power of 

 a candle ensues when the candle is transferred from air at the ordi- 

 nary atmospheric pressure to rarefied air. It was, however, found that, 

 owing to the circumstances mentioned above, no satisfactory quan- 

 titative experiments could be made with candles in artificially rarefied 

 air, and recourse was therefore had to coal-gas, which, although also 

 liable to certain disturbing influences, yet yielded results, during an ex- 

 tensive series of experiments, exhibiting sufficient uniformity to render 

 them worthy of confidence. The gas was in all cases passed through a 

 governor to secure uniformity of pressure in the delivery tubes. A 

 single jet of gas was employed as the standard of comparison, and 

 this was fixed at one end of a Bunsen's photometer, whilst the flame 

 to be submitted to various pressures, and which I will call the expe- 

 rimental flame, was placed at the other. The experimental flame 

 was made to burn a uniform amount of gas, viz. 0*65 cubic foot 

 per hour in all the experiments. 



The products of combustion were completely removed, so that 

 the experimental flame, which burnt with perfect steadiness, was 

 always surrounded with pure air, the supply of which was, however, 

 so regulated as to secure a maximum of illuminating effect in each 

 observation. 



In all the following series of experiments, the illuminating power 

 given under each pressure is the average of twenty observations, 

 which accord with each other very closely. In each series:, the 

 maximum illuminating effect, that is the light given by the expe- 

 rimental flame when burning under the full atmospheric pressure, 

 is assumed to be 100. The following is a summary of the results : 



