6 P>'of- Leslie on Heat and Climate. [July, 



away ; and, for the same reason, if heat consisted in intestine 

 motion, it would quickly disappear. But this conclusion is 

 belied by fact. Heat in no case suffers any destruction ; it is 

 only transferred to other bodies, and gradually diffused through 

 the general system; and could any substance be completely 

 detached from other matter, there is the strongest reason to 

 believe that it would for ever retain the same temperature. The 

 communication of heat in rarified air becomes sensibly dimi- 

 nished ; and in the imperfect vacuums which we are able to pro- 

 duce, the difference is very remarkable. It is highly probable, 

 therefore, that if the air in which a body is immersed were 

 completely abstracted, there could be no diffusion whatever of 

 heat. 



Language was formed to express human sentiment and feel- 

 ings ; and all the epithets which it employs refer to our own 

 frame and constitution as the standard of comparison. Such 

 are the terms heat and cold, which, though opposed to each 

 other in common discourse, denote the impressions that may 

 originate from the same cause, only varying in intensity. Had 

 the vital functions been maintained at a much lower temperature, 

 many objects at present denominated cold would have received 

 the appellation of hot. If heat then be material, the term cold 

 may be omitted altogether in philosophical discourse ; and the 

 various temperatures which substances exhibit may be ascribed 

 to the different proportions which they contain of the igneous 

 fluid. But the question occurs, what is this igneous fluid ? Is it of 

 a specific and unalterable nature, incapable of being invested with 

 ■any other form ? Or, since it evidently has some relation to light, 

 is it a modification of this subtle matter ? The former is the 

 opinion generally received, but it is contrary to analogy. Every 

 substance with which we are acquainted is susceptible of various 

 aspects and combinations. 



Heat, then, is manifestly allied to light. Is it a modification 

 of that fluid, or is it not the same matter, only in a state of 

 combination with other bodies ? The latter hypothesis is 

 recommended by its simplicity, which is the great object of 

 philosophical research. Nature herself courts simplicity, and 

 her boundless extent of operations result from the application 

 of only a few general laws. That quality affords likewise a 

 strong presumption of the truth of an hypothesis ; for the 

 difficulty of preserving simplicity increases in proportion to 

 this quality itself of the structure, since the possible chances 

 of combination are thus abridged. The arguments which 

 may be brought to prove that heat is only light in a state of 

 union with bodies are solid and conclusive. If a black body 

 be exposed to the sun's rays, the incident light is no longer 

 recognized by the senses ; but it cannot suffer annihilation ; it, 

 therefore, unites with the body, and heat ensues. The rate too 

 with which the heat accumulates is exactly proportional to the 



