<N AVATKK AND ITS COMPOUNDS 41 



,-iiul snow, soaks into the soil and collects together into springs, lakes, 

 livers, seas, and oceans. It is absorbed from the soil by the roots of 



volume . ; consequently the volume occupied by the aqueous vapour under the 



pre-sure // +/ equals v -_ , or v * . Thus the volumes of the dry gas and of the 



h +f k +/ 



moisture which occurs in it, at a pressure /*+/, are in the ratio /: h. And, therefore, if 

 the aqueous vapour saturates a space at a pressure n, the volumes of the dry air and of 

 the moisture which is contained in it are in the ratio nf:f, where / is the pressure of 

 the vapour according to the tables of vapour tension. Thus, if a volume N of a gas 

 saturated with moisture be measured at a pressure H, then the volume of the gas, when 



TT _ f 



dry, will be equal to N , because the volume N requires to be divided into parts 

 H 



which are in the ratio H /:/. In fact, the entire volume N must be to the volume of 



dry gas x as H is to H-/; therefore, N : x = H : H-/, from which a; = N H ~-^. Under 



H 



TT TT / 



any other pressure for instance, 760 mm. the volume of dry gas will be -2:, or ~^ 



and thus we obtain the following practical rule : If a volume of a gas saturated with 

 aqueous vapour be measured at a pressure H mm., then the volume of dry gas contained 

 in it will be obtained by finding the volume corresponding with the pressure H, less the 

 pressure due to the aqueous vapour at the temperature of observation. For example, 

 37-5 cubic centimetres of air saturated with aqueous vapour was measured at a tempera- 

 ture of 15'3, and under a pressure of 747'3 mm. of mercury (at 0). What will be the 

 volume of dry gas at and 760 mm. ? The pressure of aqueous vapour corresponding 

 witli 15"3 C is equal to 12*9 mm., and therefore the volume of dry gas at 15'3 and 



747-3 mm. is equal to 37'5 x 747 ' 8 ~ 12>y ; at 760 mm. it will be equal to 87'5x Z!i- 

 747-3 TOO ' 



and at the volume of dry gas will be 37'5 x x - = 34'31 c.c. 



760 273-15-3 



From this rule may also be calculated what fraction of a volume of gas is occupied by 

 moisture under the ordinary pressure at different temperatures ; for instance, at 30 C 

 /=31'5, consequently 100 volumes of a moist gas or air, at 760 mm., contain a volume of 



aqueous vapour 100 x >:>1 ;> , or 4'110; also it is found that at there is contained 



0'61 p.c. by volume, at 10 1-21 p.c., at 20 2'29 p.c.,and at 50 up to 12'11 p.c. From this 

 it may be judged how great an error might be made in the volumetric determination 

 of gases were the moisture not taken into consideration. From this it is also evident 

 how great are the variations in volume of the atmosphere when it loses or gains aqueous 

 vapour, which again explains a number of atmospheric phenomena (winds, variation of 

 pressure, precipitations, storms, <fec.). 



If aqueous vapour does not saturate a gas, then it is indispensable that the degree of 

 moisture should be known in order to determine the volume of dry gas from the volume 

 of moist gas. The preceding ratio gives the maximum quantity of water which can 

 be held in a gas, and the degree of moisture shows what fraction of this maximum 

 quantity occurs in a given ease, when the vapour does not saturate the space occupied 

 by the gas. Consequently, if the degree of moisture equals 50 p.c. that is, half the 

 maximum then the volume of dry gas at 760 mm. is equal to the volume of dry gas 



at Till) mm. multiplied by _ -/, or, in general, by ~ t ' ?, where r is the degree of mois- 



ture. It, therefore, it is required to measure the volume of a moist gas, it must either be 

 entirely dried or quite saturated with moisture, or else the degree of moisture deter- 

 mined. The first and last methods are inconvenient, and therefore recourse is usually 

 had to the second. For this purpose water is introduced into the cylinder holding the 

 gas to be measured ; it is left for a certain time so that the gas may become saturated, 



