Of M. D E L U C. 49 



fphere, the rule is abfolute, or the aflumed principle with regard 

 to the particular modification of the law of heat, is perfectly 

 confirmed, at the fame time that it is generalized, in being ap- 

 plied to every fpecies of vapour and atmofpheric mixture. 



I considered the two examples of breath and fleam as fuf- 

 ficient to confirm the principle with regard to aqueous conden- 

 fation and evaporation, feeing that they comprehended every pof- 

 fible cafe, fo far at leaft as the theory was concerned. But, in 

 thofe examples, the appearance was only that of mi ft, or viiible 

 condenfed vapour, which correfponded to cloud, and not im- 

 mediately to rain. I therefore thought it neceflary, in order to 

 overcome the ordinary prejudices of mankind, to give an ex- 

 ample feemingly more in point, although perhaps fuperfluous, 

 as it follows fo plainly from the principle. There is, however, 

 fomething fo convincing in the appearance, when a mower of 

 rain is formed artificially in a chamber of experiment, that 

 even a philofopher finds himfelf better fatisfied, after feeing 

 the fact, than by many arguments, by which all the fteps of 

 the operation might be explained, and every effect: foretold. I 

 therefore gave two examples of this fort, which had come to 

 my knowledge. It is concerning thefe that we are now to exa- 

 mine what M. de Luc has faid, (parag. 587.) 



" Le feul des phenomenes cites par le Dr Hutton, qui ait 

 " un rapport immediat avec fa theorie, eft la precipitation nei- 

 " geufe des vapeurs repandues dans fair chaud d'une chambre, 

 " lorfque cet air vient a communiquer a un air exterieur tres- 

 " froid. Mais la preuve qui femble en refulter en faveur de 

 " cette theorie, n'efl qu'apparente ; car il n'y a pas lieu de pre- 

 " fumer, que les vapeurs fuffent a leur maximum dans l'air ex- 

 " terieur, ni a Tornea, ni a Peterfburg : circonftance qui nean- 

 " moins feroit neceffaire pour produire une precipitation d'eau 

 " d'apres l'hypothefe ; a moins qu'on ne fupposat encore, que 

 " quoique les vapeurs ne foient pas a leur maximum dans deux 

 " airs qui fe melent, elles peuvent le depaffer fenfiblement dans 



Vol. II. g " le 



