[ 331 ] 



of 32° a barometer would ftand at 12,7 inches. At fUch heights, 

 and at much inferior, fince evaporation proceeds much more 

 quickly, it is not to be fuppofed that all the vapour fo rapidly 

 produced is diffolved in the ambient air, but part rifes uncombined 

 as it does under an exhaufted or half exhaufted receiver, and in 

 this cafe Mr. De Luc's fyftem is admifTible. This emiffion of pure 

 vapour feems to begin at heights at which the denfity of the air is 25 

 (that is at heights at which the barometer would ftand at twenty- 

 five inches, and thus Ifhall in future exprefs the various denfities 

 of air,) atleaft it is very confiderable where the denfity is twenty,., 

 as already feen, p. 309. This leads me to treat of the properties and 

 ftate of pure invifible vapour namely, its fpecific heat, elafticity, 

 and fpecific gravity. 



The immortal Dbdtor Black, the father of all difcoveries of this', 

 kind, informed me that the vapour of water, boiling at 212*', 

 that is at 180° above the freezing point, and 'poflTefling the fame 

 fenfible heat as the water, contains nine hundred and forty times- 

 more latent heat than an equal weight of water does heated to = 

 212°, or 5,222 times more latent heat than it does of fenfible 

 heat, counting from the freezing point, for 180 x 5?222 =. 940 

 nearly. In this cafe the prefiTure or denfity of the atmofphere is 

 thirty, the barometer ftanding at the height of thirty inches ; and; 

 with Dodor Black's account the experiments of Mr. Schmidt of" 



GielTen ■ 



