576 



II Y G R O M E T R Y. 



e- ton. The diflfercnre between the corresponding num- 

 bers in the two columns seldom exceeds the 1000th part 

 of an inch, except between 75 and 90, where Mr Dai- 

 Ion's table seems to be a little faulty. 



Force of Vapour, in Inches of Mercury, from to 100 

 FaJtrcnheit. 



Force of va- 

 pour in 

 incho of 

 mercury. 



32. Having thus deduced a general expression for 

 the elasticity or tension of vapour at different tempera- 

 tures, we shall now proceed to investigate its density or 

 absolute weight. The experiments of Gay Lussac, t 

 which were conducted with the utmost attention to weight of 

 accuracy, have established, beyond the possibility of vapour at 

 doubt, thut vapours, when they are subjected to pres- different 

 sure, suffer within certain limits, that is, so long as tel 

 they retain the elastic form, the same reduction of bulk 

 as permanently elastic fluids. Hence if P be the weight 

 of the liquid reduced to vapour in grammes; N the 

 number of divisions of the receiver, which it occupies 

 in the vaporous, state at the temperature of 100 centi- 

 grade ; v the capacity of one of these divisions in litres 

 at the freezing point: then since N v would be the vo- 

 lume of the vapour, if the receiver itself suffered no ex- 

 pansion by the increase of temperature, its real volume 

 will be N v( 1 + 100 ), the quantity k being the cubical 

 dilatation of glass for each degree of the centigrade 

 scale. Also if p be the pressure of the atmosphere at 

 the time of the experiment, and /; the height of the mer- 

 cury in the receiver above its external level, both being 

 expressed in metres, then the volume of vapour in li- 

 tres, produced by the quantity of liquid whose weight 



-n -iiu 

 is P, will be 



gle gramme 



, ,i . r 

 and that of a sin- 



h) 



.76 P 



} 



33. Now in one of his experiments, Gay Lussac Important 

 found, that V^. grammes of water being introduced ^ ( 

 under a receiver, and the temperature raised to 100, sa( / 

 gave a volume of vapour which occupied 220 divisions, 

 each of which was equal in capacity to .00499316 li- 

 tres ; the column of mercury in the inside of the recei- 

 ver was .052 metres above the external level, and the 

 barometer at the time of the experiment stood at .7555 

 metres. As mercury is expanded -ffrs f ' ts bulk 

 from the freezing to the boiling point, or T ^r T f r ea ch 

 degree of the centigrade scale, and the temperature of 

 the mercury in the barometer at the time of the expe- 

 riment was 1 5, the height of the column of mercury in 

 the receiver reduced to what it would be at the freez- 



ing point, is 100 x-052, or .051056 metres, and 



1 TTTFiT 



the height of the mercury in the barometer, corrected 

 in like manner, is , X-7555, or .75341 metres. 



Hence P = .6; N=220; t; = .00499316 ; p=. 75341 ; 

 A=.051056; and since the cubical dilatation of glass is 

 .0000262716 for each degree of the centigrade scale rec- 

 koned from the freezing point, 1 + 100 k= 1.002627 16. 



.76 P 



220 X- 00499316 X 1.00262716 x -75341 .051056 _ 



.76 x- 6 



1.69641 litres. Therefore a volume of vapour equal to 

 1.69641 litres, would contain at the boiling point, un- 

 der a pressure of .76 metres of mercury, a quantity of 

 moisture equal in weight to a gramme ; or a litre of va- 

 pour in the same circumstances would weigh .589481 

 grammes. 



34. If we reduce the result of this experiment to 



. English measures, we shall find that a cubic inch of u 



vapour at the boiling point, when the barometer stands nieasun* 

 at 29.92196 inches, weighs .149176 grains troy; and 

 consequently a cubic foot of vapour in the same circum- 



1 



