34 TRANSMISSION OF HEAT. 



Of 100 incident rays, there were transmitted : 



By Rock-salt " 92 rays. 



Mirror glass . . . . . 62 



Rock-crystal 62 



Iceland spar 6*2 



Rock-crystal, smoky and brown .57 



Carbonate of lead . . . . 52 



Sulphate of barytes .... 33 



Emerald 29 



Gypsum 20 



Fluor spar 15 



Citric acid 15 



Rochelle salt . . . . . 12 



Alum . . . . . . 12 



Sulphate of copper . . . . 



A piece of smoky rock-crystal, so brown that the traces of letters 

 on a printed page covered by it, could not be seen, and which was 

 fifty-eight times thicker than a transparent plate of alum, trans- 

 mitted 19 rays, while the alum transmitted only 6. One sub- 

 stance which is perfectly opaque, a kind of glass used for the 

 polarization of light, was found by Melloni to allow a conside- 

 rable quantity of rays of heat to pass through it. He applies 

 the term diathermanous to bodies which transmit heat, as dia- 

 phanous is applied to bodies which transmit light. Of all 

 diaphanous or transparent bodies, water is in the least degree 

 diathermanous. With the exception of the opaque glass referred 

 to above, all diathermanous bodies belong also to the class of 

 diaphanous bodies ; for those kinds of metal, wood and marble 

 which totally obstruct the passage of light, obstruct that of heat 

 also. 



The proportion of heat from various sources which radiates 

 through a plate of glass, l-50th of an inch in thickness, was 

 observed by Melloni to be as follows : 



Of 100 rays from the flame 



of an oil-lamp there were . 54 transmitted, 46 absorbed. 

 red hot platinum, 37 63 



blackened copper, 



heated to 732 F. 12 88 



blackened copper 



heated to 2 12 ,0 100 



