Of the Light manifested in Combustion. 2 1 3 



the middle of the field of the instrument, when the 

 apparatus is prepared for making an experiment. 



The other divisions of this scale of light are de- 

 termined in such a manner that the numbers which 

 they bear, which I call degrees, are everywhere as the 

 squares of their distances from the middle of the field 

 of the photometer, where the two shadows are in con- 

 tact whose densities are to be compared and equalized. 



To fill the important station of a standard light, with 

 which all others are compared, I have chosen a wax 

 candle of the first quality, just eight tenths of an Eng- 

 lish inch in diameter, and which burning with a clear 

 and steady flame has been found to consume very reg- 

 ularly 1 08 grains Troy of wax per hour. 



To this standard light I have assigned the value of 

 100 degrees; and it is always placed exactly opposite 

 to that division of the scale of the photometer which is 

 marked 100. This division is, of course, at the dis- 

 tance of 31.62 inches from the middle of the vertical 

 field of the instrument, that marked 10 being at the 

 distance of 10 inches. 



In order to express in a commodious manner the 

 quantities of wax, tallow, oil, or other inflammable sub- 

 stance consumed in the experiments, I have supposed 

 the 1 08 grains Troy of wax consumed by the standard 

 light per hour to be divided into 100 equal parts 

 (= i. 08 grains) to serve as a standard weight in all 

 cases. The usefulness of this arrangement will be seen 

 hereafter. 



I have now to request the attention and the indul- 

 gence of the Royal Society while I use my best en- 

 deavours to give them a clear and distinct account of 

 my experiments and their results. 



