HYGROMETRY. 



579 



try. 



the mercury to flow out at b, until its level in AB and 

 XT' is the same. It' N be the number of divisions oc- 

 cupied originally by the gas, N' the number of divi- 

 sions occupied by the mixture, and p the pressure of 

 the atmosphere, which we shall suppose remains the 

 same (luring the experiment ; the elastic force of the 

 gas, in consequence of its increase of volume, will now 



be (--\ p, nd iff be the elastic force of the vapour, 



tiplied, in order to reduce it to its real volume, if it 

 were deprived entirely of water. Its value in pneuma- 

 tic researches will be duly estimated by the philosophi- 

 cal chemist. 



the joint elasticity of both will be/ + " N f. 



But this 



being exactly equal to the atmospherical pressure, we 

 have 



N/, 



Np 

 Hence /= p -- or p 



= 



PJ' 



Tbt eJati 43. It appears, by the experiments of Gay Lussac, 

 that when the values of N', N, and p are substituted for 

 Ml'npaur' ***** quantities, the value off deduced from the I'onnu- 

 snaedVtth !> > exactly the same as would be obtained for the elas- 

 unwpherw tic force of vapour at the same temperature, and under 

 air. the the same pressure, by the formula laid down in $ 31. 

 *""* Hence it Das been justly inferred that the vapour, in 



union or mixture with the gas, retains its own peculiar 

 elasticity, and exerts the same tension as if no gas were 

 present. Tbii result furnishes by far the strongest argu- 

 ment which has yet been adduced, that vapour exists 

 in air, not in > state of chemical union, but merely of 

 mechanical mixture. In a liygroroetrical point of view, 

 this is a (act of the utmost importance, as it enables us 

 to determine the precise volume which a mixture of va- 

 pour and dry air would occupy at a given temperature, 

 and under a given pressure. All that U necessary is to 

 determine /oy the formula for calculating the elas- 

 tic force of vapour at the given temperature, and then 

 to substitute it for that quantity in the expression 



/ = p ( 7)- W ahould thus obtain 



Kipsarcn To illustrate this formula by example, let it be re- 



Tabby 



th.tcetaaw 

 f vapoac. 



quired to determine the enlargement of volume which 

 dry air receive* when saturated with vapour, at the 



! of 66 of Fahrenheit, and under pres- 

 sure" of 3O inches of mercury. Since we wish to deter- 

 merely the relative increase of volume, N may be 



u unity; and / = .63795. 



Therefore N'= -= 



. = 1. 



To illustrate the use of the above Tablet by example, miration 

 let it be required to determine the enlargement of vo- of the Table 

 lume which a cubic foot of atmospheric air acquires by by example, 

 standing over water, when the temperature of the air 

 is 70, the barometrical pressure 29.23 inches, and the 

 level of the water in the inside of the receiver 9.5 inches 

 above its level on the outside. 



The increased volume for the temperature 70 being 

 1.02*8, the enlarged bulk of the air, under a pressure 

 of 30 inches of mercury, is 1728 x 1.0248 or 1770.85 

 cubic inches. This accordingly would lie the increased 

 volume under that pressure ; but the pressure being 

 different, a corresponding correction must be applied 

 to the result: in toe first place, a column of watrr <>i' 

 the height of 9.5 inches is equivalent to a column of 

 mercury of the height of .098 inches, the height* being 



i found by hit eiperinwnt*. that dry air at the temperature of ISP.lt Reaumur, or 6C.1I Fahrenheit, wa expanded 

 l-Mtb. wbkfc to wonderfully near the truth, considering the manner in which it waa determined. 



t * the Taa* ea oaily to applied with perfect accuracy, when the leel of tbt water, in the outiidt and inside of the receifcr, 

 I* exactly the tame, ibt results sfiarM by the example* muit be considered merely u near approximation*. 



. 



~pf~ 30 .63795" 29-36205 



Hence dry air at the temperature of 66 is, when satu- 

 rated with uaoiatisiw, expanded ^ of its original bulk.* 

 43. The following Table constructed from the above 

 formula, exhibits the dilatation of air by moisture, from 

 33* to 10O, under a pressure of SO inches of mercury ; 

 and also the numbers by which the bulk of air, in a 

 state of complete saturation with vapour, must be mul- 



