37 2 Of the Propagation of Heat 



The difference in the times necessary to produce simi- 

 lar effects in these two cases is no proof that they are not 

 produced in the same manner; for if they are effected 

 merely by the agency of Heat (which I suppose), then 

 the effects produced in any given time will not be as the 

 density of the light or as the number of rays, but as 

 that part of the Heat generated which, not being im- 

 mediately dispersed or carried off by the air, has time to 

 produce the action proper td it in the wood ; and conse- 

 quently must be incomparably greater, in proportion, 

 when the rays are concentrated, than when they are not. 



Luna cornea exposed to the action of light changes 

 colour ; but why should we not attribute this change to 

 the expulsion of the oxygen united with the metal, by 

 the agency of the Heat generated by the light ? To re- 

 move every possible objection to this explanation of the 

 phenomenon nothing more appears to be necessary than 

 to admit what is well known, that this metallic oxyd 

 may be reduced, without addition, with some degree of 

 Heat, and that this substance is a bad conductor of 

 Heat. 



Will not the admission of our hypothesis respecting 

 the intensity of the Heat which is supposed to be gen- 

 erated where light is stopped, and of that respecting the 

 non-conducting power of Fluids with regard to Heat, 

 enable us to account, in a manner more satisfactory than 

 has hitherto been done, for the effects of the sun's light 

 in bleaching linen, when it is exposed wet to the action 

 of his direct rays ? as also for the reduction of those 

 metallic oxyds which have been found to be revived by 

 exposure to light ? And will it not also assist us in ac- 

 counting for the production of pure air in the beautiful 

 experiment of Dr. Ingen-Housz, in which the green leaves 



