216 



KNOBLAUCH ON RADIANT HEAT. 



Although it is shown that the density and hardness exert an 

 influence upon the radiation of heat under the circumstances 

 pointed out, of course it must not be understood that it is caused 

 by them alone. In different bodies, in which various other rela- 

 tions are simultaneously called into action, the property of emit- 

 ting heat cannot, as Leslie has attempted, be referred to the 

 hardness alone. 



2. With reference to the increase of the radiation in propor- 

 tion to the thickness of the bodies laid upon a heated cube, I 

 shall communicate two series of experiments, merel}^ because 

 they were made upon those substances which had yielded a 

 greater heat with an increase in the thickness (see p. 209). They 

 were colourless, transparent varnish, and black, opake diatherma- 

 nous asphalt-lac. After painting them in layers in various num- 

 bers or of unequal thickness upon a Leslie's cube, which during 

 the experiment had been retained at a temperature of 212°, their 

 radiation upon the thermal pile produced the deflections of the 

 multiplier contained in the following table : — 



Table X. 



These differences are too considerable to render it necessary 

 to bring forward other examples. This phaenomenon has already 

 been correctly explained by Count Rumford, by assuming that 

 the heat radiates from a certain depth beneath the surface ; and 

 Melloni, on the same principle, has given a satisfactory account 

 of the whole process. 



If this increase in the radiation of bodies be compared with 

 the increase in neating power under increasing thickness of the 

 layers in action, which has been pointed out in the previous sec- 

 tion (pp. 207 to 209), we find in it a new cause for the agree- 

 ment of the radiation and absorption of heat. 



On comparing these functions, however, it must not be over- 



