Of M. D E L U C. 49 



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

 to the p^articular modification of the law of heat, is perfecflly 

 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 leafl as the theory was concerned. But, in 

 thofe examples, the appearance was only that of mift, or vifible 

 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 fhower of 

 rain is formed artificially in a chamber of experiment, that 

 even a philofopher finds himfelf better fatisfied, after feeing 

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

 the operation might be explained, and every effedt 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 I'air 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'eft qu'apparente ; car il n'y a pas lieu de pre- 

 " fumer, que les vapeurs fuffent a leur maximum dans Pair ex- 

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

 " moins feroit necefTaire pour produire une precipitation d'eau 

 " d'apres I'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 depafTer fenfiblement dans 



Vol. II. g " le 



