' on the terrestrial Rays that occasion Heat. 459 



mitted 259 rays of heat, and 203 of light, the alteration pro- 

 duced in the texture of its surface acts very differently upon 

 these two principles ; occasioning an additional stoppage of 

 only 205 rays of heat, but of 651 rays of light. 



51st Experiment. 

 One of the pieces of coach glass, mentioned in the 27th 

 experiment, was prepared in the same manner. 



Sun. Coach glass ; one side rubbed on emery, 



the rough side exposed. 



o' 66± 66j- 



5 73i 69t ■ • • 7 = 3 = >4 2 9 



It stops 571 scattered rays of heat, and 885 of light; so that 



the fine scratches on its surface, made by the operation of 



emery, have again acted very differently upon the rays of heat, 



and of light, occasioning an additional stoppage of 375 of the 



former, but of no less than 717 of the latter. 



5<id Experiment. 

 I took another of the pieces of crown glass, mentioned in the 

 26th experiment, and rubbed both sides on emery. 



Sun. Crown glass ; both sides rubbed on emery. 



o' 6 9 ± 6 9 ± 



5 pj 7H-- 6:< * = >333 



The glass thus prepared, stops 667 scattered rays of heat, 



and 932 of light. 



5gd Experiment. 

 Another piece of coach glass, one of those that were men- 

 tioned in the 27th experiment, was prepared in the same 

 manner, 



