AND MECHANICAL CONDITION ON EADIANT HEAT. 



89 



Were the radiative power of these substances determined by the state of division, I 

 think it must make itself sensible even in a case where the division is eiFected by the 

 pestle and mortar ; but I do not find this to be the case. A plate of glass was fixed 

 against the polished surface of a Leslie's cube, and on the plate the powder of glass, 

 rendered as fine as the pestle and mortar could make it, was strewn. It was caused to 

 adhere without cement of any kind. The cube was filled with boiling water and pre- 

 sented to the thermo-electric pile until a permanent defiection was obtained. Permit- 

 ting the cube to remain in its position, the powder was removed with a camel's-hair 

 brush. An inconsiderable augmentation of the radiation was the result, the increase 

 being such as might be expected to follow from the slight difference of temperature 

 between the surface of the glass plate and the powder which had been strewn upon it. 

 Similar experiments were made with a plate of rock-salt, on which finely divided powder 

 of rock-salt was shaken. The result was precisely similar to that obtained with the 

 glass powder. 



One side of a Leslie's cube was covered by a sheet of bright platinum foil, and a 

 second face by a similar sheet on which black platinum had been deposited by electrolysis. 

 As radiators these two sheets of foil behaved in the following manner : — 



Radiation. 



Bright Platinum foil 6-0 



Platinized Platinum 45"2 



Here the radiation of the black platinum is nearly eight times that of the bright sub- 

 stance. 



Having thus shown, I trust conclusively, that the influence of chemical constitution 

 makes itself felt in all states of aggregation, for the sake of reference, I will here tabu- 

 late the results obtained with a considerable number of powders when subjected to the 

 same conditions of experiment. 



