Management of Light in Illumination. 201 



But where so much depends on the degree of atten- 

 tion that is paid to the subject, no estimate can be 

 made with any considerable degree of certainty. 



A chemical analysis has shown us that beeswax, 

 tallow, and the fat oils are composed of nearly the 

 same elements, .and consequently contain nearly the 

 same quantities of inflammable matter (carbon and 

 hydrogen] ; and, as I have lately found that they fur- 

 nish nearly the same quantities of heat in their com- 

 bustion,* it might naturally have been supposed that 

 they must likewise furnish equal quantities of light. 



I have no doubt but they would do so, could they be 

 managed in precisely the same manner ; but their dif- 

 ference of form at the ordinary temperature of the 

 atmosphere, the difference of the temperature at which 

 they become fluid and at which they are reduced to 

 vapour, must necessarily produce a sensible difference 

 in the arrangements employed in burning them, which 

 cannot fail to occasion a sensible difference in the quan- 

 tities of light produced in their combustion. 



The intensity of the heat which accompanies the 

 combustion of an inflammable substance is no doubt 

 always the same ; but it does not follow that the quan- 

 tity of light is always the same. 



As the intensity of the light produced by lamps and 

 candles may be ascertained with great certainty by 

 means of the photometer, the cost of the light may 

 in all cases be exactly determined. 



Taking wax candles, tallow candles, and purified oil 

 of colza at the prices these articles are now sold at 



* An account of these experiments was given in a memoir on the Heat 

 Manifested in the Combustion of Inflammable Substances, which was read 

 before the First Class of the French National Institute, the 24th February, 1812. 



