and Mechanical Condition on Radiant Heat. 



299 



difference of temperature between the surface of the glass plate 

 and the powder which had been strewn upon it. Similar experi- 

 ments 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 with a sheet of bright 

 platinum -foil, and a second face by a similar sheet on which 

 black platinum had been deposited by electrolysis. As radia- 

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



Radiation. 



Bright platinum-foil 6 



Platinized platinum 45*2 



Here the radiation of the black platinum is nearly eight times 

 that of the bright substance. 



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 tabulate the results obtained 

 with a considerable number of powders when subjected to the 

 same conditions of experiment. 



Table I. — Radiation from Powders imbedded in Sulphur 

 Cement. 



Substance. Radiation. 



Eock-salt 35-3 



Biniodide of mercury . 39*7 



Milk of sulphur . . . 40"6 

 Common salt . . . .41*3 



Yellow iodide of mercury. 46*6 



Sulphide of mercury . . 46*6 



Iodide of lead .... 47*3 



Chloride of lead . . . 55'4 



Chloride of cadmium . 56"5 



Chloride of barium . . 58-2 



Chloride of silver (dark). 58*6 



Pluor-spar 68 '4 



Tersulphide of antimony. 69*4 



Carbonate of lime . . 70*2 



Oxysulphide of antimony. 70*5 



Sulphide of calcium. . . 71-0 



Substance. Radiation. 

 Sulphide of molybdenum. 71*3 

 Sulphate of baryta . . 71*6 

 Chromate of lead . . . 74*1 

 Red oxide of lead . . . 74*2 

 Sulphide of cadmium . 76*3 

 Subchloride of copper . 76-5 

 Oxide of cobalt . . . 767 

 Sulphate of lime . . . 77*7 

 Carbonate of zinc . . . 77'7 

 Red oxide of iron . . . 78'4 

 Sulphide of copper . . 790 

 Hydrated oxide of zinc . 80*4 

 Black oxide of iron . . 81*3 

 Sulphate of iron . . . 81*7 

 Iodide of copper . . . 82*0 

 Lampblack 84*0 



I subsequently endeavoured to get rid of the sulphur cement 

 and to make the powders adhere by wetting them with pure bi- 

 sulphide of carbon, applying them to the cubes while wet. Some 

 of the powders clung, others did not. My ingenious friend Mr. 

 Duppa suggested to me that the powders might be held on by 



