io8 HIStORT of the SOCIETT. 



ture or chemical phenomena. " Heat," he obferved, " is in na- 

 " ture the principle of fluidity and evaporation, though, in pro- 

 •' ducing thefe effects, it is latent in refpect to the thermometer, 

 " or any fenfation of ours ; and as matter, otherwife quiefcent, 

 " becomes voluble and volatile in liquid and in vapour, heat may 

 <c be confidered in nature as the great principle of chemical 

 " movement and of life. If it pafs through, vacuity as well as 

 V through body, as it certainly does in its communication from 

 " the fun to the planets, we mufl confider it not as an accident 

 " in bodies, but as a feparate and fpecific exiftence, not lefs fo 

 " than light or electric matter * ; and though agreeing with 

 C{ thefe in fome of its effects, in its nature poffibly different from 

 " either." But fuch was Black's caution not to outrun the 

 courfe of actual evidence, that he declined any difcuflion of the 

 queftion, relating to the abfolute nature of this magnificent 

 power in the fyftem of nature. 



My reading in chemiftry does not enable me to fay, how far 

 the doctrine of latent heat is, in thefe precife terms at lead, ad- 

 mitted as a principle in the received theories of combuftion and. 

 animal heat ; but, to my limited apprehenfion, it appears to be 

 the only folid foundation of any theory that proceeds upon the 

 fuppofed decomposition of ignifying or vital air, manifefting a 

 light and heat previoufly latent in fuch air. We can have no 

 direct proof of latent heat in the atmofphere, or permanently 

 elaftic fluids, and it is from analogy only that we affume it to exift 

 in fuch fluids. The maxim of Newton, indeed, may be applied 

 here, that what is uniformly obferved in any department of na- 

 ture, as far as our experience reaches, may be fafely deemed ge- 

 neral within fuch department ', and the heat which we find dis- 

 appear 



* It is no doubt a mighty increment in fcience, to have found fuch powerful 

 fubftances operating, as the writer of thefe minutes apprehends, without gravita- 

 tion, inertia, or impenetrability, the great bafes and columns of the mechanical 

 philofophy. 



