on the terrestrial Rays that occasion Heat. 523 



The surmise of a difference in the law of the transmission of 

 heat and of light, is considerably supported by an argument drawn 

 from circumstances of a very different nature. In the scattered 

 transmissions arising from rough surfaces, we find, that when 

 crown glass, for instance, has one of its sides rubbed on emery, 

 it will stop 205 rays of heat more than while that surface re- 

 mained polished ; but the effect of the roughness produced by 

 emery scratches, is far more considerable on the rays of light; 

 the additional stoppage of them amounting to no less than 651. 



A confirmation of the same effect we have in coach glass; 

 which, having also one side rubbed on emery, stops only 357 

 rays of heat more than it did before, while there is an additional 

 stoppage of rays of light, amounting to no less than 717. Now, 

 since the interior construction of these glasses, before and after 

 having been rubbed on emery, remains the same, these remark- 

 able effects can only be ascribed to the roughness of their sur- 

 faces. Hence, we may conclude, that as the same cause, when 

 it acts upon the rays of heat and light, produces effects so very 

 different, it can only be accounted for by admitting the rays 

 themselves to be of a different nature, and therefore subject to 

 a different law in being scattered. It has already been shewn, 

 that the rays of heat are, upon an average, less refrangible than 

 those of light ; and now it appears that they are also, if I may 

 introduce a convenient term, less scatter able. 



We ought now also to take a short review of the phaenomena 

 attending the transmission of terrestrial heat. The results of the 

 experiments which have been given, are drawn into one view in 

 the following table. 



