PHILOSOPHICAL TRANSACTIONS. 



781 



"VOL. XC.] 



them. Now this does not happen in the transmission of light, which, as far as 

 we know, is instantaneous ; at least a failure in the brightness of an object, when 

 first we look at it through a glass, amounting to 1,2, or even 3 minutes, could 

 not possibly have escaped our observation. This seems to suggest to us, that the 

 law by which heat is transmitted, is different from that which directs the passage of 

 light : and in that case it must become an irrefragable argument of the difference 

 of the rays which occasion them. 



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 dif- 

 ferent 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 remained polished ; but the ef- 

 fect 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 remarkable effects can only be 

 ascribed to the roughness of their surfaces. Hence we may conclude, that as the 

 same cause, when it acts on the rays of heat and light, produces effects so very dif- 

 ferent, 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 shown, that the rays of heat are, on an average, less refrangible than 

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

 venient term, less scatterable. 



We ought now also to take a short review of the phenomena attending the trans- 

 mission of terrestrial heat. The results of the experiments which have been given, 

 are drawn into one view in the following table. 



Bluish- white glass stops .... 625 750. 



Flint glass 591 750. 



Crown glass 636". . . . 722. 



Coach glass 458. . ... 714. 



Iceland crystal 516. . . . 7^6. 



Talc 375.... 713. 



Very dark red glass 636 6 13 



Dark-red 526. . . . 573. 



Orange 560 643. 



Yellow 583 685. 



Pale-green 500 688. 



Dark-green 739 745. 



Bluish-green 652. . . . 696. 



Pale-blue 609. . . . 676. 



Dark-blue 6*19 704: 



Jndigo t 679 721. 



Table 6. 



Invisible Ha>* of 



heat. flame. Fire. 



. 700 Pale-indigo .571 655. 



. 533 Purple 520 679. 



. 783 Violet 500 6l5. 



. 625 Crown glass, one side rough. . 741 .... 723. 



. 726 Coach glass, ditto 667. . . . 758. , 



.615 Crown glass, both sides rough 6l 5. ... 791. 



Coach glass, both 680 854 



. 630 The two last but two, together 20. . . . 849 



. 524 The two last together 667 897 



.531 The four last together 87 902 



. 632 Olive-colour, burnt in glass . 792. . . . 849. 



. 700 White paper 792 912. 



. 556 White linen 690. . . . 910. 



. 548 White persian 593 82Q 



. 632 Black muslin 565 706 



.659 



Inriiiblt 

 heat. 



..700 

 .730 

 .684 

 .775 

 .741 

 .833 

 .769 



.636 

 .535 

 .457 



