Of M. D E L U^ C. 45 



covered long ago. But firft to confine our attention to the 

 cooling of the fleam, and then to explain the appearance of 

 condenfation from this cooling alone, is a fpecies of reafoning 

 that one would not have expelled from the author of the Mo- 

 difications of the Atmofphere. 



The queftion is not, if a body of fleam, in the 212th de- 

 gree of heat, mixed with a body of air, in the ordinary tempe- 

 rature of the atmofphere, fhould preferve its degree of heat, 

 that is to fay, fhould be cooled or not ; the queflion is, If the 

 mean heat of this mixed mafs be fufEcient to preferve all the 

 humidity in a tranfparent flate ; or, If there fhall be formed a 

 condenfation of vifible mill, in this cafe as well as in the other, 

 where moifl and warm air was mixed with the atmofphere ? 

 Had no condenfation in this experiment been formed, the prin- 

 ciple of condenfation, confequently of evaporation, could not 

 have been extended to the cafe of fleam, or the rule of evapo- 

 ration would not have been abfolute, as comprehending both 

 the cafe of the atmofphere and that of water by itfelf ; but the 

 condenfation adlually taking place in the experiment, genera- 

 lizes this law of nature with refpefl to every poflible combina- 

 tion of water, air and heat. This condenfation does not hap- 

 pen in confequence of the fleam being expofed to any preffiire 

 which it had not fuflained before, but becaufe the heat of the 

 mixed mafs, which is the medium between the heats of the two 

 maffes, is not fufficient to preferve all the w^ater in the ftate of 

 vapour ; and this is precifdy what, according to the theory, 

 the experiment is meant to prove. 



But M. de Luc, though he has had recourfe to the cooling 

 of the fleam alone, to account for the mifl which inflantly ap- 

 pears upon the mixing of the fteam and air, does not lofe fight 

 of the heat which he knows is not loft ; but he brings it into 

 adlion again, for the evaporation of that mifl which has ap- 

 peared. It is neceffary to give his reafoning in relation to that 

 fubjedl. " Cependant ces vapeurs decompofees ont augmente 



" la 



