196 



KNOBLAUCH ON RADIANT HEAT. 



Table I. (continued). 



It is thus evident that the radiation of heat through diather- 

 manous bodies does not stand in relation to the temperature of 

 the source of heat in any one of the instances which occur here. 



2. To render the experiment as clear as possible, I also ob- 

 served the transition of the heat emitted by radiation from one 

 and the same body at different temperatures. 



(1.) For this purpose, with low degrees of heat, I made use of 

 a Leslie's cubefj the sides of which were 8 centim. in length; 

 in this I heated water to ebullition, and then allowed it to cool 

 gradually. The cooling took place so slowly, that the tempera- 

 ture of the cube, during the short period of the insertion of a 

 diathermanous substance, was considered as constant. 



The following phcEnomenon occurred : — When, by the ap- 

 proximation of the cooling cube before each insertion, a constant 

 deflection of 35° was produced, the needle each time receded to 

 11° when the colourless glass 13 millim. in thickness was in- 

 troduced between the source of heat and the thermal pile, even 

 when the temperature of the former was between 100° and 212° F. 

 Thus the heat was capable of passing through the glass plate 



* It may perhaps appeal- remarkable that J have not produced the same 

 direct deflections of the thermoscope for all diathermanous bodies, for the sake 

 of greater unit'ormity. The reason is, that I was compelled to be as sparing as 

 possible with the hydrogen which formed one of the flames, because each re- 

 production of it interrupted the proper series of experiments for a considerable 

 time, and disturbed the comparison of the results. I therefore always started 

 from that deflection which the radiation of the hydrogen-flame produced without 

 continued regulation, by arranging that of the other sources of heat according 

 to it. I might ce/tainly have reduced the various observations by calculation 

 to a common one; however, I have omitted this tedious process, because not 

 the slightest object would be gained by it except the more elegant form. 



I'he above table i>koirs thai the rock salt which I used did not allow the rays 

 from all sources of heat to pass through it in the same manner as was observed 

 by Melloni with his. 



^ J. Leslie, An Experimental Inquiry into the Nature and Propagation of Heat. 

 Lond. 1804, p. 6. 



