OF NATURAL PHILOSOPHY. 315 



sun's rays), differs very materially from solar heat 

 in their power of penetrating transparent substances. 

 This singular and important difference was first 

 noticed by Mariotte, and afterwards made the sub- 

 ject of many curious and interesting experiments 

 by Scheele, who found that terrestrial heat, or that 

 radiated from fires or heated bodies, is intercepted 

 and 'detained by glass or other transparent bodies, 

 while solar heat is not ; and that, being so detained, 

 it heats them : which the latter, as it passes freely 

 through them, is incapable of doing. The more 

 recent researches of Delaroche, however, have 

 shown that this detention is complete only when 

 the temperature of the source of heat is low ; but 

 that, as that temperature is higher, a portion of the 

 heat radiated acquires a power of penetrating glass ; 

 and that the quantity which does so bears con- 

 tinually a larger and larger proportion to the whole, 

 as the heat of the radiant body is more intense. 

 This discovery is very important, as it establishes 

 a community of nature between solar and terrestrial 

 heat ; while at the same time it leads us to regard 

 the actual temperature of the sun as far exceeding 

 that of any earthly flame. 



(351.) A variety of theories have been framed to 

 account for these curious phenomena ; but the sub- 

 ject stands rather in need of further elucidation 

 from experiment, and is one which merits, and will 

 probably amply repay, the labours of those who may 

 hereafter devote their attention to it. The theory 

 of the radiation of heat, in general, which seems to 

 agree best with the known phenomena, is that of 

 M. Prevost, who considers all bodies as constantly 



