208 



Latent Heat of 

 ft earn. 



— is lefs the 

 greater its 

 temperature* 



STATE OF VAPOUR SUBSISTING IN THE ATMOSPHERE. 



at which in the temperature of 32 Q a barometer would Hand 

 at 12,7 inches. At fuch heights, and at much inferior, fince 

 evaporation proceeds much more quickly, it is not to be fup- 

 pofed that all the vapour fo rapidly produced is diflblved in the 

 ambient air, but part rifes uncombined as it does under an ex- 

 haufted or half exhaufted receiver, and in this' cafe Mr. De 

 Luc's fyftem is admiffible. This emiflion 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 I fhall in future exprefs the various denfities 

 of air,) at lead 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 Doclor 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 pofleffing 

 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 preflure or den- 

 fity of the atmofphere is thirty, the barometer Handing at the 

 height of thirty inches; and with Doclor Black's account the 

 experiments of Mr. Schmidt of Gieflen very nearly agree, 

 for according to him the latent heat of the vapour of water, 

 barometer 29,84 inches, and the heat 212°, is 5,33 times 

 greater than its fenfible heat above the freezing point, now 

 180 x 5,33 = 959*4*. The difference or excefs in his 

 experiment proceeds from the preflure of the atmofphere being 

 fomewhat lower, as Mr. Watt's experiments prove. 



Mr. Watt difcovered that the latent heat of fieam diminifh- 

 ed in proportion as its fenfible heat increafed, Phil. Tranf. 

 1784, p. 335. Now the fenfible heat of fleam exceeds 180 Q 

 above the freezing point when the barometer fiands above 

 thirty inches, and is lefs than 180° when the barometer ftands 

 lower than thirty inches, as Mr. De Luc firft. difcovered, and 

 may be feen in Sir George Shuckburgh's, and*Mr. De Luc's 

 tables, Phil. Tranf. 1779. p. 375. From thefe I have deduced 

 the following table: 



* 4 Gren's Phys. Journal, p. 315. 



Heat 



