SPECIFIC GRAVITY OF A MIXTURE. 73 



33. Specific Gravity of Air. Take a large flask 

 which can be completely closed by a stop-cock, and weigh it 

 when filled with air ; withdraw the air by means of an air- 

 pump and weigh the flask again ; finally, fill the flask with 

 water and weigh again. This last weight minus the second 

 will give the weight of the water that filled the flask, and 

 the first weight minus the second will give the weight of an 

 equal volume of air; divide the weight of the air by that of 

 the water ; the result will be the specific gravity of air as 

 compared with that of water. 



Let to = the weight of the exhausted flask ; w', w" its 

 weights when filled with air and water ; then 



w' iv = weight of the air contained by the flask, 

 w" w = weight of the water contained by the flask ; 



w' w 



therefore, $ = r , (1) 



w w 



SCH. In the same manner the specific gravity of any gas 

 can be obtained. The specific gravity of water at 20.5 is 

 about 768 times that of air at under the pressure of 29. 9 

 inches of mercury at 0. 



The atmosphere in which these operations must be per- 

 formed varies at different times, even during the same day, 

 in respect to temperature, the weight of its column which 

 presses upon the earth, and the quantity of moisture it con- 

 tains. On these accounts, corrections must be made before 

 the specific gravity of air, or that of any gas exposed to its 

 pressure, can be accurately determined. The discussion of 

 the principles according to which these corrections are 

 made, is given in Chap. III. 



34. Specific Gravity of a Mixture. (1) When the 

 volumes and specific gravities of the components are 

 given. 



Let v, v' y v", etc., be the volumes of the bodies of which 



