KNOBLAUCH ON RADIANT HEAT. 211 



trate them least, can also be proved by observation in colourless 

 glass, which, as is known, is also less perfectly penetrated by 

 the heat of the metallic cylinder than that of the Argand lamp; 

 for a glass mirror 1*5 millim. in thickness, the rough metallic 

 surface of which was turned towards the thermal pile, when acted 

 upon by the rays of the former, produced a deflection of the gal- 

 vanometer-needle to 12°*25; by those of the lamp to 11°, when 

 direct incidence from both these sources of heat had deflected 

 the needle to 45°. 



It scarcely requires to be again mentioned, that this proof of 

 diathermanous bodies becoming heated by radiation does not 

 aflfect the results detailed in the first section, as it has been ex- 

 perimentally shown (p. 203 and 204), that under the circum- 

 stances in which the experiments on the transmission were in- 

 stituted, it had no perceptible share in the production of the 

 results. 



III. On the Property of radiating heat hi Bodies. 



It is already known that different substances radiate heat at 

 the same temperature in an unequal degree, and that this pro- 

 perty, in one and the same substance, is dependent, — 1st, upon 

 the structure of its surface ; 2nd, upon its thickness. 



1. Although Leslie had previously expressed the view, that 

 the hardness of bodies influenced their radiating power, Melloni 

 first endeavoured to prove that the changes produced in the 

 radiation of one and the same body by scratching its surface, 

 could only be ascribed to the modifications of its hardness pro- 

 duced at the spots concerned. 



He obtained the following deflections of the thermo-multiplier; 

 by radiation irom 



A silver plate, beaten out and pohshed . . . 10°*0 

 A silver plate, beaten out and scratched . . 18°-0 



A silver plate, cast and polished 13°-7 



A silver plate, cast and scratched 11°'3; 



and found that in agate, ivory and marble, the degree of rough- 

 ness did not produce any alteration in the radiation, a remark 

 which had previously been made by Leshe with regard to glass, 

 paper and lamp-black. Hence Melloni drew the conclusion, 

 that more heat is always emitted when the scratching exposes 

 softer parts of the radiating substance ; less when it produces a 



