Of M. D E L U C. 47 



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

 feems to adduce 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 abftracting the heat of the fleam, without 

 mixing with that fleam. It is, therefore, necefTary, that I 

 fliould anfwer that fuppofition with regard to the evaporation 

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

 flances, in order to fee, in a jufl 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, mufl 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 diffolving or evaporating power of heat 

 proceeded uniformly with its diflending power or fenfible heat, 

 the mixed mafs of air and fleam fhould flill remain tranfparent, 

 without producing mift or condenfation. For the fleam lofes 

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

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

 cifely that the condenfation i3 produced. But there would be 

 no condenfation, if water could be retained tranfparent, elaflic 

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

 tact of thofe two bodies. Therefore, the condenfation, which 

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

 of fleam 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 humidU 

 ties which the two united maffes had feparately. 



I might now content myfelf with this obfervation, That it 

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

 propofition is concerned, and not with the difTolution of that 

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



the 



