LIFE of Dr HUtt ON. 65 



which cold acts in cauftng a precipitation of humidity. It met, 

 however, from M. de Luc with a very vigorous and determi- 

 ned oppofition ; Dr Hutton defended it with fome warmth, 

 and the controverfy was carried on with more iharpnefs, on both 

 fides, than a theory in meteorology might have been expected to 

 call forth. For this Dr Hutton had lead apology, if greateft in- 

 dulgence, on the fcore of temper, is due to the combatant who 

 has the word argument. The merits of the queftion cannot be 

 confidered here : It is fufficient to remark, that they came ulti- 

 mately to reft on a (ingle point, Whether the refrigeration of 

 air is carried on by the mixture of the cold and the hot air, or by 

 the paflage of the heat itfelf, without fuch mixture, from one 

 portion of air to another. If the former holds, Dr Hutton's 

 theory is cftablifhed ; if the latter be true, M. de Luc's objec- 

 tions may at lead merit examination. 



Now, it is certain, that if not the only, yet almoft the only, 

 communication of heat through fluids, is produced by the mix- 

 ture of one part of the fluid with another. The ftatical princi- 

 ple by which heat is thus propagated, was firft, I believe, accu- 

 rately explained by Dr Black, and fince his time has been far- 

 ther illuftrated by the experiments of Count Rum ford. Thefe 

 laft have led their ingenious author to conclude that heat has no 

 tendency to pafs through fluids, otherwife than by the mixture 

 of the parts of different temperature. The accuracy of this con- 

 clufion, in its full extent, may reafonably be queftioned; but this 

 much of it is undoubtedly true, that when the particles of a bo- 

 dy are at liberty to move freely among themfelves, the direct 

 communication of heat, compared with the ftatical, is evanef- 

 cent, and may be regarded as a mere infinitefimal. M. de Luc's 

 objections are therefore of no weight. 



The "Theory of Rain was republished by Dr Hutton in his 

 Phyfwal Differtations feveral years afterwards, together with his 

 anfwers to M. de Luc, and feveral pther meteorological tracts, 



1 2 which 



