THE MEASUREMENT OF GASES. MOLECULAR HYPOTHESIS 45 



at the top, until the pressure of the mercury column inside bal- 

 ances the pressure of the air outside. The atmospheric pressure 

 is therefore expressed by the barometer as equivalent to the 

 pressure exerted by so many millimeters of mercury. For accurate 

 work, all readings obtained must be reduced to a standard tem- 

 perature (0 C.), by correcting for the expansion of mercury above 

 that temperature. 



Correction of the Volume to 760 mm. Pressure. Since 

 the barometer varies in height, and the volume of a sample of gas 

 therefore varies also, it is convenient to " correct " the volume 

 further by " reducing " it to that which the gas would occupy at 

 " standard " pressure, namely 760 mm. Now, the volume of a 

 sample of gas varies inversely with the pressure (Boyle's law). 

 If, for example, the volume is 23 c.c. and the observed pressure 

 of the barometer is 745 mm., Boyle's law enables us to calculate 

 the volume the same sample of gas would occupy at 760 mm. The 

 volume changes in the ratio of these two pressures. If the pres- 

 sure of the gas were actually changed to 760 mm., a greater 

 pressure the gas would assume a smaller volume. Hence, 



745 

 the new volume = 23 X =7^ = 22.5 c.c. That is to say, if the 



new volume is to be kss, we place the smaller pressure in the 

 numerator. 



If a sample of gas occupies 15 c.c. at 850 mm., what volume will 

 it occupy at 500 mm.? Here the new pressure is smaller, and the 



850 



new volume therefore greater. New volume = 15 X -^^ = 25.5 c.c. 



500 



Correction of the Volume to C. All gases at are 

 found to gain 1/273 of their volume when heated 1 degree, 2/273 

 for 2 degrees and 273/273 for 273 degrees. Thus at 273 the 

 volume is doubled. When cooled below 0, every gas similarly 

 loses 1/273 of its volume for each degree. At 273, if the 



