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XXTIi, Of the State of Vapour fn^fijVing In the Alnufphere. 

 By Richard Khuvan, Ejq\ LL.D. F.R.S. and 

 P. R. I. A. * 



V APOUR or moifture in the atmofphere may fubfift in 

 tlenfe air, or in air highly rarefied: that it is found in the 

 former is well known, and that it may fubfift in the latter 

 appears by the obfervations of Bougaer ; for he faw clouds 

 three or four hundred toifes above Chiromboracho, and 

 confequently at the height of twenty-two thoufand five hun- 

 dred and twenty-eight Engiilh feet, or 4-3 miles over the 

 level of the fea; a height at'which in the temperature of 33=' 

 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 diUblved in the ambient air, but part rifes' un- 

 combined, as it does under im exhaufted or half-exhaufled re- 

 ceiver, and in this cafe Mr. De Luc's fvflem is admiffible. 

 This em i (lion of pure vapour feems to begin at heights at 

 which thedenfityof the i.lr is 25, (that is, at heights at which 

 fhe barometer would (land at twenty-five inches, and thus I 

 {hall in future exprefs the various dcnfities of air,) at leaft it 

 \i very coufiderable where the denfitv is twenty. Tliis 

 leads me to treat of the properties and flate of pure invi- 

 fible vapour, namely, its fpecific heat, elafticity, and fpecific 

 gravity. 



The immortal Doftor Elack, the father of all difcoveries 

 of this kind, informed mc that the vapour of water, boiling 

 at 212°, that IS, at i.'io" above the freezing point, and polfef- 

 hng the fame lenfible heat as the water, contains nine hun- 

 dred and forty times more latent heat than an equal weight 

 of water does heated to 212°, or5-222 limes more latent heat 

 than It does of ienfibie heat, counting from the freezing 

 point, for 180 X 5222 = 940 nearly. In this cafe the 

 preirure or denfily of the atmofphere is thirty, the barometer 

 Itanding at the height of thirty inches ; and \Vith Doftor 

 Black's account the experiments of Mr. Sclimidt of Gieflen 

 very nearly agree; for according to him the latent heat uf the 

 vapour of water, barometer 29-84 inches, and the heal ai2% 

 '*5'33 I'ln^-'S greater than its fenfible heat above the freezing 

 poml: now i«o X 5-33 = c)S9'Ai. The difference or ex- 

 cels m his experiment proceeds from the picfilire of the at- 

 "* From hit paper iniiiltd, •' Of the Vdriations of the Atmofphere." 

 t Gren's Phyfical Journal, iv. p 315. 



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