MATTER FORMED OF SMALL PARTICLES 



103 



density of mercury varies with its temperature. Hence the 

 height must always be corrected to the same temperature for 

 the comparison to be exact. In addition, the scale itself varies 

 with change of temperature. If we know the coefficient of 

 expansion of the brass of which it is made, correction is very 

 easy. It is useful to take the heights of the columns of two 

 barometers at the same time and note that they are alike, in 

 so far as they are correctly constructed, although the tubes 

 may vary in diameter. 



69. The Volume of a given Mass of Air maintained at the 

 same Temperature varies inversely as the Pressure, that is, 

 the Density of Air varies directly as the Pressure A long 

 thick glass tube of even bore, closed at the end A and enlarged 

 at the other end B (fig. 35), has mercury carefully poured in 

 until it stands at the same level in both limbs. B 



This may be readily ascertained by a scale fixed 

 behind, or better, by using the cathetometer. The 

 height of the barometer is now taken, and then 

 mercury added until the difference between the 

 two levels is equal to the observed height of the 

 barometer. It will now be found that the air in 

 the shorter limb occupies half its previous volume, 

 and it is evidently subject to double its original 

 pressure that is, double the pressure of the atmo- 

 sphere. It is ^-necessary to calibrate the shorter 

 limb in order to measure the volume exactly. If, 

 instead of adding a column of mercury equal to 

 that supported in the barometer, we pour in 

 mercury to half this height, the gas will be found 

 to occupy two-thirds its original space ; for the 

 change of pressure is in the ratio of 1 to 1J or 

 2 to 3, and, therefore, the volume changes in 

 the ratio of 3 to 2. It must be remembered, how- 

 ever, that a given change of pressure might not 

 produce the same change of volume at different 

 temperatures. In order to ascertain this, the changes in 

 volume may be observed when the tube is surrounded a 

 completely as is practicable by snow or ice, 



Fig. 35. 



