HEAT. 



31 



paper is interposed, the effect produced 

 upon the thermometer is not produced 

 by portions of radiant heat passing 

 through these substances, but by the 

 undulations before mentioned, which 

 convey a portion of ^the heat given out 

 from the tin vessel ; and the screen being 

 heated by this process, it also gives out 

 heat, occasioning new undulations on 

 the side nearest to the reflector, by which 

 heat is conveyed to it, and finally to the 

 bulb of the thermometer in its focus. 



This view of the subject is supported 

 by other experiments. Mr. Leslie found 

 that when a thin sheet of ice, which can- 

 not have its temperature raised, was in- 

 terposed, no heat was imparted to the 

 thermometer. 



Two panes of glass were coated on 

 one side with tinfoil. When they were 

 both placed in the screen, with the tin- 

 foil inwards, so that one of the unco- 

 vered surfaces of the glass was next the 

 source of heat and the other towards 

 the reflector, the thermometer was 

 elevated in temperature 18 ; but when 

 the plates of glass were so placed in the 

 screen as to have their uncovered sur- 

 faces inwards, and their metallic surfaces 

 presented to the cubical vessel and the 

 reflector, no effect whatever was pro- 

 duced upon the thermometer. 



Mr. Leslie contends, that since the 

 resistance opposed to the passage of 

 radiant heat in both these experiments 

 was precisely the same, the effect must 

 be accounted for in some other way 

 than by supposing a portion of heat 

 capable of passing through. In the 

 first experiment with the compound 

 screen, surfaces were presented which 

 had the power of receiving and emitting 

 heat ; the screen was heated, therefore, 

 by the undulatory process, and the heat 

 which was emitted was conveyed to the 

 reflector in the same way. But when 

 the metallic surfaces of the compound 

 screen were exposed, heat could not be 

 received readily, and if it had been re- 

 ceived, it could not readily have been 

 imparted : the effects of the undulations 

 could not, therefore, extend beyond the 

 screen. 



It is not necessary, however, to 

 adopt Mr. Leslie's theory in explaining 

 these experiments, as they admit of an 

 equally plausible explanation accord- 

 ing to "the usual theory of radiant heat. 

 Admitting the possibility of the undu- 

 lations supposed by Mr. Leslie, it is 

 difficult to imagine by what agency ca- 

 loric is transferred so rapidly from par- 



ticle to particle ; especially as the pro- 

 pagation of heat through elastic fluids 

 is usually so slow. This theory is at 

 variance with the results obtained by 

 the experiments of Herschel and Be- 

 rard. It is an essential part of the 

 hypothesis alluded to, that these pulsa- 

 tory undulations are incapable of exist- 

 ing in a vacuum, and consequently that 

 heat cannot pass through a vacuum, as 

 its propagation depends upon the agency 

 of aii\ 



The experiments of MM. Dulong 

 and Petit, are hostile to this part of the 

 theory, the overthrow of which must 

 destroy the whole ; and an experiment 

 of Sir H. Davy, now to be described, 

 seems quite conclusive against it. A 

 piece of platinum-wire w r as heated by 

 voltaic electricity within a receiver 

 (fig. 12,) containing concave reflectors 



fig- 12. 



with a thermometer in the focus of one 

 of them, the heated wire being in the 

 focus of the other. The effect was first 

 tried in air of the natural density, 

 and then repeated when the receiver 

 was exhausted to T ijjth part of what it 

 contained before ; the temperature of 

 the thermometer was three times as 

 much raised when the receiver was thus 

 exhausted, as when it contained ah- in 

 its natural state. A similar result was 

 obtained by the ignition of charcoal 

 under the same circumstances. 



Mr. Leslie, Count Rumford, ^nd 

 others, have made numerous experi- 

 raents_on the cooling of ..bodies under 



