24 DoSh-iite tfHiai, with refpeSi to denfe and clajlic Fluids. 



create fufpicion In the minds of many who have confidered them attentively, that the doc- 

 trine which affirms this procefs to confift merely in a direft combination of caloric is pot to 

 be admitted without limitations. They have regarded thefe limitations, hovever, as only 

 partial exceptions to a general law : and my prefent purpofe U to fliow that, even in its 

 moft general acceptation, the doclrine is open to very weigluv obje£lions. 



I fhall endeavour to explain myfelf in examining what grounds we have for the fuppofed 

 analogy between vaporization and liquefa£lion ; what indications we have as to the real 

 aftion of caloric in vaporization j and whether the probability both of analogy and of fa£t» 

 fliould not incline us rather to refer the vaporous modification of bodies to the clafs of 

 chemical decompofitions. 



The moft important and charafteriftic circumftance attending the phenomenon of lique- 

 faftion is the uniformity of temperature at which it takes place. And it is here that a moft 

 obvious failure in the fuppofed analogy of vaporization prefents itfelf. It is not fufficient 

 for this analogy that in fimilar circumftances water, like ice, cannot preferve its form 

 beyond a certain temperature. It would require alfo that water, like ice, (hould not be 

 capable of changing its Ibrm below that temperature. So little do we find this the cafe, 

 that many circumftances connefted with evaporation give us reafon to conclude that, 

 when not impeded by mechanical refiftance, there is fcarcely any temperature at which it 

 may not take place; and that, without any variation but as to the rapidity with which it 

 is performed. I fhall enter into no examination here of the attempt to refcue the analogy 

 in queftion from this objeflion by referring what is called the fpontaneous evaporation of 

 liquids to the folvent aftion of air, becaufe in as much as refpefts the principle of the 

 objeftion, the phenomenon is equally evident whether air be prefent or not. 



But to come more diredlly to the point, let us attend to the circumftance by which fup- 

 pofed analogy between liquefadtion and evaporation is deemed peculiarly exprefled. 

 This is the appearance in each cafe of a quantity of combined caloric. It will be feen in 

 the fequel that I am far from being difpofed to deny this combination in the phenomena of 

 evaporation. All that I have to objeft to is the idea of its indicating a correfpondence of 

 principle between thefe phenomena and thofe of liquefaftion. The appearance from 

 which the combination of caloric in the former cafes is inferred, are thofe of its becoming 

 latent in the production, and again fenfible in the condenfation of vapour. It muft be 

 obfcrved, however, in the firft place, that the caloric concerned in thefe two phenomena 

 is by no means the fame. Mr. Watt found that vapour produced in vacuo at a tem- 

 perature 70°, gave out, on its condenfation, a quantity of caloric equal to 1200° or 1300°. 

 And, as it muft be needlefs to (how that fuch a quantity could not have exifted in the 

 vapour on its original production, it is evident that this at kaft cannot be regarded as 

 having any eflential relation to its conftitution. 



Let us fee how far the caloric which is aftually prefent, and becomes latent in the 

 formation of vapour, can be underftood to denote a conformity between this modification 

 and that of liquefaction. That this is not implied by che mere circumftance itfelf, is 



evident 



