Of M. D E L U C. 47 



But though not in confeqvience of his theory, M. de Luc 

 feems to addtice that explanation in confirmation of it. Now, 

 if this explanation fliould be admitted, it might tend to confirm 

 his fuppofition, that the fteam had been condenfed, not by the 

 medium temperature of the mixed air and vapour, as I con- 

 tend, but by the air abflraifling the heat of the fteam, without 

 mixing with that fteam. It is, therefore, neceffary, that I 

 fhould anfwer that fuppofition with regard to the evaporation 

 of the mift. But it requires ftrirt attention to many circum- 

 ftances, in order to fee, in a juft light, that atmofpheric opera- 

 tion, which had led a natural philofopher to make a fuppofition 

 of that kind. 



Steam, before it can be condenfed into water, muft communi- 

 cate or transfer its latent heat (equal to 900 ° more than the heat of 

 boiling water) to the body by which it is cooled or condenfed ; 

 confequently, if the difl!blving or evaporating power of heat 

 proceeded uniformly with its diftending power or fenfible heat, 

 the mixed mafs of air and fteam fhould ftill remain tranfparent, 

 without producing mift or condenfation. For the fteam lofes 

 no heat but what the air gains ; it is in the contadl of thofe 

 two fluids that this cooling happens ; and it is in this place pre- 

 cifely that the condenfation is produced. But there would be 

 no condenfation, if water covild be retained tranfparent, elaftic 

 or diflblved, in the medium heat which is produced at the con- 

 tad: of thofe two bodies. Therefore, the condenfation, which 

 adlually happens, proves this phyfical truth, that when a mafs 

 of fteam is mixed with a particular mafs of the atmofphere, or 

 with a certain portion of the atmofpheric fluid, the humidity 

 of the new mafs is greater than the mean between the humidi- 

 ties which the two united malfes had feparately. 



I MIGHT now content myfelf with this obfervation. That it 

 is only with the produdtion of mift or vifible vapour that my 

 propofition is concerned, and not with the diflblution of that 

 mift again, when it comes to be mixed with another portion of 



the 



