io8 HIS'tORT of the SOCIETr. 



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

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

 " ducing thefe efFedls, it is latent in refpedl to the thermometer, 

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

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

 '' be confidered in nature as the great principle of chemical 

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

 " 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 eledlric matter * ; and though agreeing with 

 " thefe in fome of its efFeds, in its nature poflibly different from 

 ** either." But fiich 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 i-eading in chemiftry does not enable me to fay, how far 

 tlie dodi'ine of latent heat is, in thefe precife terms at leaft, 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 decompofition of igtiifyhig or vital air, manifefting a 

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

 diredl proof of latent heat in the atmofphere, or permanently 

 elaftic fluids, and it is from analogy only that we aflTume 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 j and the heat which we find diC- 



appear 



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

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

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

 philofopby. 



