from the Indications of the Wet-bulb Hygrometer, *251 



under/ 7 : density at t under/' : : 448 + / : 660 ; v density un- 

 der 30 at 212° : density at t' under/' : : 448+7 x 30 : 660/' 



: : 448 + / : 22/'. 



Assuming now the atomic theory of volumes, the number 

 of particles will be the same in a given volume of air at 212° 

 under 30, and of vapour at 212° with same elasticity. Also 

 air at t under/ 7 contains (in given volume) same number of 

 atoms as vapour formed at / with elasticity =/'. Hence the 

 relative densities of vapour at different temperatures will be 

 given by the above proportion, or, taking density of air at 

 212° under 30 as = 1, density of vapour at 212° under 30 

 = '625 (Despretz), therefore density of vapour at t 



' 625 x 22/' 13-75 x/ r / V \ , „ . . 



= 448+^ r 448+/ 1*0 tS lhem e 



tension of vapour at /)*. 



Now, taking latent heat of vapour formed at 212° = 956 

 (Despretz), and considering the sum of the latent and ther- 

 mometric heat to be a constant quantity, we have the latent 

 heat of vapour at any other temperature / = 956 + 212 — / 

 = 1 168 — /. But the quantity of heat requisite for converting 

 into vapour all the moisture that can exist at the temperature 

 / is found by compounding the ratios of the quantities of mois- 

 ture with the ratios of the latent heats at different tempera- 

 tures: thus, 



Latent heat at 212° : latent heat at / : : 956 : 1168—/, and 



quantity of moisture at 212° : quantity at / : : -625 : - — €\ 



therefore, putting Q and Q' for the quantities of heat requisite 

 to convert into vapour all the moisture that can exist (in a 

 given volume) at 212° and jit /, we have Q (at 212) : Q' (at 



t)"056- U68 - tx22f (K\ 



* This formula may be proved otherwise ; thus, putting D (or density 

 of vapour of saturation at t with elasticity =/'; D' for density of same 

 vapour heated to 2\2° and preserving same elasticity; and D" for density 

 of vapour at 212° under 30 pressure, we have 



D : D' : : 660 : 448 + t 

 and D' : D": :/' : 30 



v.D :D"::22/':448 + *and D = ^^ . 



My object in deducing it in the other manner is to show that the fact of 

 the "maximum tension of vapour being wholly independent of the pres- 

 sure, and dependent solely on the temperature," is a necessary conse- 

 quence of the above view of the atomic theory. 



Third Series. Vol. 7. No. 10. Oct. 1835. 2 L 



