Of M. D E LU C. 51 



that which he had already given. It is this, " Car d'ailleurs, 

 " les phenomenes dont il s'agit peuvent sexpliquer fans avoir 

 " recours a cette hypothefe." Here, indeed, is a very good 

 reafon for objeding to the appUcation of thofe experimental 

 cafes ; and now we are to examine this explanation which M. 

 DE Luc is to give of the phenomenon. 



He fays, " La mafTe (comparativement fort petite) de I'air de 

 " la chambre, perdoit tres-promptement une quantite fenfible 

 ** de fa chaleur par I'ouverture qu'on y faifoit, fans que I'air 

 " exterieure fe rechaufFat fenfiblement a cette ouverture, aupres 

 ** de laquelle fair qui commeni^oit a s'echaufFer, faifoit bientot 

 " place a de fair froid, en s'elevant. Les vapeurs chaudes de- 

 " voient done fe precipiter en brouillard dans la chambre ; par- 

 " ceque I'air exterieur n'en recevoit prefque point." 



The explanation which M. de Luc has here attempted, 

 either is not conceived with that diftindnefs of idea which is 

 required for inveftigating the laws of nature, or is not expref- 

 fed in fuch precife terms as might make it eafy to bring his 

 propofition to a fcientific iflue, in applying principles. Let us, 

 however, endeavour to follow the argument of our author 

 through the obfcurity in which it is involved. 



M. DE Luc fays, that the air of the chamber would quickly 

 lofe a fenfible quantity of its heat, without the external air being 

 fenfibly heated at this opening. Does he mean, that the air of 

 the chamber would fufFer any lofs of its heat vipon this occa- 

 fion, befides what happened by the interchanging of the ex- 

 ternal and internal air ? Such a fuppofition as that might truly 

 form the foundation of an argument ; but this, it is found, 

 will not conform to the laws of hydroftatics. He, therefore, 

 muft be obliged to fuffer fome of the heated air to efcape, and 

 its place to be fupplied with the cold air which comes in. 

 Things being in this (late, our author fays, that the warm va- 

 pours ought to precipitate, in forming mift in the chamber, be- 

 caufe the external air receives fcarce any of them. This evi- 



g 2 dently 



