= V X -. The ratio, therefore, of the dry 



S86 Dr. Apjohn on the Specific Gravity df Gases. [May, 



and Gay-Lussac, that -^-^ will represent the volume which any 



^ven bulk of dry gas, as ar, assumes, when saturated with steam 

 whose elasticity is J] p representing the atmospheric pressure at 

 the time. Let the increased volume equal v, and we shall have 



^ z=z V X ^-^, and the expansion produced by the steam 



P - f 



= V — V X - — - 



p p 



gas to the expansion produced in it by the moisture is that of 



"Z : 1. Dr. Thomson, through some oversight, considers this 



as the ratio in volume of the dry gas to the steam with which it 



is saturated, whereas the true ratio of these is that of : 1 , 



' p 



inasmuch as a given volume of gas at any temperature saturated 



with moisture contains as much steam as could exist at that 



temperature in a vacuum of equal capacity. The following, 



therefore, is the true expression for the specific gravity of a gas 



saturated with moisture. Calhng li y, y = ^-^ x a + b, a 



and b denoting the respective specific gravities of the dry gas, 

 and of steam whose elasticity is f. The following table, con- 

 structed from this formula, exhibits the specific gravities of some 

 of the principal gases when saturated with moisture at the tem- 

 perature of 60°. In calculating /;, the specific gravity of steam 

 at 60°, Ihave used Mr. Dalton's table of elasticities. 



Gas. Sp. gr. of dry. Sp. gr. of moist. 



Air 1-0000 0-9907 



Oxygen Mill 1-0998 



Nitrogen 0-9722 0-9634 



Chlorme , 2-5000 2-4644 



• . Hydrogen 0*0694 0-0761 



The expression given above may easily be made to assume the 



form of y = a — — + b. Then, by substituting for b its 



value, which is '472 X -, we shall have ?/ = a — — + -472 x 



-. 'From which it appears that the specific gravity of the moist 



gas is equal to, less,.or greater than that of the dry, according as 

 the specific gravity of the dry gas is equal to, greater, or less than 

 •472, the specific gravity of steam at 212°. This result is worth 

 recollecting, as by it the subject of moisture, as modifying 

 :gaseous specific gravity, is divested of all perplexity. Thus by 

 it we learn, that hydrogen is the only one of the permanent gases 



