454 Prof. 0. Timiiia/eff. [Apr. 3 



FIG. 11. 

 Variation de la dissociation avec t et P. 



1000 



2OOO 3OOO 



Temperatures. 



-1000 



I am not mistaken, it was Count Rumford who showed for the first 

 time that what he called the chemical properties of light may be 

 attributed to its heating power. In his paper " An Enquiry concerning 

 the Chemical Properties that have been attributed to Light " we find 

 the following passage : " The heat generated by the absorption of the 

 rays of light must necessarily at the moimnt of its generation at least, 

 exist in almost infinitely small spaces ; and consequently it is in bodies 

 that are inconceivably small that it can produce durable effects in any 

 degree indicative of its extreme intensity." At a still earlier date, in 

 his paper " On the Propagation of Heat in Fluids " Count Rumford 

 even applies this principle of small spaces, of the small sphere to which 

 photo-chemical action is restricted, to the case we are now considering. 

 Here are his words : " Will not the admission of our hypothesis 

 respecting the intensity of the heat which is supposed to be generated 

 where light is stopped . . . assist us in accounting for the production 

 of pure air in the beautiful experiments of Dr. Ingenhousz, in which 

 the green leaves of living vegetables are exposed immersed in water 

 to the sun's rays 1" 



By small spaces Count Rumford probably meant molecular spaces, but 

 it seemed to me that it would be highly interesting to test his 

 hypothesis in a rougher way. Now that we are perfectly sure that the 

 phenomenon takes place in the chloroplast, and is due to the rays 

 absorbed, we may try to make a rough estimate of the sphere of action 

 of the sun's rays. The necessary elements of these calculations are the 

 following: the total amount of the radiation as measured by the 

 pyrheliometer ; the total area of Langley's energy curve ; the fraction 



