EXAMPLES. 97 



RBM. 1. It has been shown by a series of experiments 

 that this law connecting the elastic force and volume of a 

 gas under a constant temperature is sensibly true for air and 

 most gases as far as a pressure of 100 atmospheres.* It is 

 only when the pressures are very great that variations from 

 the law are observed, and even then the departure from the 

 law is but small, especially with those gases which we are 

 not able to condense into liquids. With gases which undergo 

 liquefaction at moderate pressures, the departure from the 

 law is greater, and increases as the state of liquefaction is 

 approached, f 



REAL 2. In conducting this experiment, care must be 

 taken to have the temperatures the same at the beginning 

 and at the conclusion, as the elastic force of a gas under a 

 given volume is influenced by changes of temperature. For 

 this reason, it is necessary to pour in the mercury gradually, 

 and to allow some time to elapse before the difference of 

 levels is observed, since, whenever a gas is compressed, an 

 elevation of temperature is produced. Therefore, whatever 

 heat is developed by increase of pressure must be allowed to 

 pass off before the volume of gas is observed. 



EXAMPLES. 



1. Let DE (Fig. 33), be 10 inches ; if mercury be poured 

 in until the level in the closed branch stands 3 inches above 

 EF, and in the open branch 15.64 inches, find the elastic 

 force of the air in the closed branch, the barometer standing 

 at 29.5 inches. 



Since the levels of the mercury in the two branches stand 

 at 15.64 and 3 inches, the level in the longer branch is 

 12.64 inches above that in the closed branch; the elastic 

 force of the compressed air, therefore, sustains a column of 



* Galbraith's Hydrostatics, p. 35. 



t Weisbach'B Mechs., p. 782 ; also Twiaden's Mechs., p. 289. 



