388 Scientific Proceedings, Royal Dublin Society. 



per cent, for every 7^- mm., or three-tenths of an inch, that the baro- 

 meter deviates from its standard height. 



Another very convenient formula — formula (3) — is given in the 

 Appendix. (See page 390.) 



These approximate formulae give results within a few 

 thousandths of the truth under atmospheric changes of the 

 barometer and at atmospheric temperatures, say between - 5° 

 and + 30°. They are therefore quite sufficient for most 

 laboratory purposes. 



They are easily proved. The weight of a litre of dry air 

 at freezing temperature, and under the pressure of 760 mm. of 

 mercury at Paris, was determined by Regnault to be, 



1-2932 grammes. 



Using this determination, and Boyle and Mariotte's formula, viz. : — 



z T 



we find that the weight of a litre of air under the above standard 

 pressure becomes exactly, 



1*2 grammes. 



when its temperature is brought to 21°*2 C. This temperature may 

 be taken as 21° in most laboratory work. Next, taking the 

 relative density of air to hydrogen as 14*4, we deduce from the 

 above that at the same pressure and temperature, 



the weight of a litre of hydrogen = — gramme, 



-L<0 



and the volume of a gramme of hydrogen = 12 litres; 

 whence, for any other gas, 



W (the weight of a litre of the gas) = — grammes, 



12 

 V (the volume of a gramme of the gas) = — litres, 



where D is the relative density of the gas compared with that of 

 hydrogen as unit. 



