Stoney — On the Determination of Gaseous Volumes, etc. 380 



These formulae refer to standard pressure and 21° temperature. 



At other pressures or temperatures a correction has to be 

 applied. 



This for variations of pressure evidently amounts approxi- 

 mately to one per cent, for every 1\ mm. [more exactly 7*6 mm.], 

 or *3 of an inch, that the barometer deviates from its standard 

 height. 



To find the temperature correction we must use Boyle and 

 Mariotte's formula, remembering that 294 is the Absolute 

 Temperature of 21° C. "Whence 



The weight of a litre of the gas at (21 + r)° _ 294 



Its weight at 21° 294 + r 



and 



The volume of a gramme of the gas at (21 + t)° 294 + r 

 Its volume at 21° = 294 ' 



It appears from these formulae that the correction will amount 

 to one per cent, in the first formula when r is such that 



294 = 1 



294 + t 100' 



and, in the second formula, when r is such that 



294 + r _1_ 



294 " + 100' 



i.e. when r = 2°'97 in the first formula, 



and r = 2°-94 in the second. 



In ordinary laboratory determinations it is legitimate to use 

 3° instead of either of these; by doing which we obtain the 

 very simple rule given in the beginning of this Paper. 



APPENDIX. 



As an example, apply the formulae to determine how much 

 zinc is consumed in displacing 7 litres of hydrogen from acidulated 

 water at a temperature of 15° when the barometer reads 31 inches. 



