44 ANSWERS to the OBJECTIONS 



of rain, which fvirely does not happen in the mofl ordinary 

 ftate of the atmofphere, at leaft not in nioft countries, thofe 

 particularly in which M. de Luc has made his meteorological 

 obfervations. 



Having thus difcufled the cafe of humid air or natural va- 

 pour, M. DE Luc next proceeds to confider the cafe of fteam, 

 or pure vapour, as he calls it. Here he fays, that the mifl 

 formed above water boiling in the open air, may be explained 

 upon another principle than that of the hypothefis from whence 

 I had concluded that it fhould be fo. It will be proper to give 

 his reafoning upon the fubjedl : 



" La vapeur de I'eau bouillante [Jieam) eft pure, parce 

 " qu'au degre de chaleur de cette eau, les vapeurs font toujours 

 " capable de fupporter feules la preflion de I'atmofphere. Des 

 " vapeurs prefque pures, forment les bulks qui traverfent fans 

 " cefle I'eau bouillante ; et ces bouffees de fluide elaftique tranf- 

 ""^parent, deplacent Pair en fe degageant de I'eau. Si ces va- 

 " peurs fe repandent dans un efpace qui n'ait qu'une petite 

 " iifue a roppofite de leur entree, en amenant cet efpace a leur 

 " temperature, elles en chaffent tout fair, et y demeurent tranf- 

 " parentes ; mais des qu'elles I'ont depafle, et qu'elles fe repan- 

 " dent dans I'air exterieur, leur courant s'y decompofe bientot : 

 " car des la premiere perte fenfible qu'elles eprouvent dans le 

 " degre de chaleur auquel eft attachee leur exiftence, ne pou- 

 " vant plus fupporter la prefFion de I'atmofphere, elles fe tranf- 

 " forment en un brouillard, qui fe mele a I'air environnant." 



Here M. de Luc confiders the tranfparent fteam, when co- 

 ming in contadl with the colder atmofphere, as cooled by the 

 air, without noticing, that it proportionably heats that air by 

 which it is cooled. This overfight in another perfon but M. de 

 Luc, might have been natural ; it might even in M. de Luc 

 himfelf have been more excufable, had he been lefs converfant 

 with the important theory of latent heat which Dr Black dif- 



covered 



