50 



CORRECTIONS FOR MOISTURE IN GAS. 



Gas, when standing over water, becomes saturated with aqueous vapour, the quan- 

 tity being proportional to the temperature. In these cases a part of the volume observed, 

 and also a part of the weight, is due to the vapour, which therefore must be ascertained 

 before the true weight of the gas under examination can be determined. The following 

 Table exhibits the proportion by volume of aqueous vapour existing in any gas standing 

 over or in contact with water, at the corresponding temperatures and at mean barometric 

 pressure of 30 inches. 



By reference to this Table, which is founded upon the experiments of Dr. 

 Dalton, and includes any temperature at which gases are likely to be weighed, 

 the proportions in bulk of vapour present, and consequently of the dry gas, may 

 easily be ascertained. For this purpose, the observed temperature of the gas 

 should be looked for, and opposite to it will be found the proportion in bulk 

 of aqueous vapour, at a pressure of thirty inches. The volume to which this 

 amounts should be ascertained, and corrected to mean temperature. Then 

 the whole volume is to be corrected to mean temperature and pressure, and 

 the corrected volume of vapour subtracted from it : this will leave the cor- 

 rected volume of dry gas. It has been ascertained in a manner approaching 

 to perfect accuracy, that a cubic inch of permanent aqueous vapour, corrected 

 to the temperature of 60, and a mean pressure of thirty inches, weighs O1929 

 grains ; the weight therefore of the known volume of aqueous vapour is now 

 easily ascertained, and this being subtracted from the weight of the moist gas, 

 will give the weight of the dry gas, the volume of which is also known. 



As an illustration, suppose a gas standing over water had been thus weighed, 

 and that 220 cubic inches, (at the temperature of 50 Fahr., and barometer 

 pressure of 29'4 inches,) had entered into the globe, and caused an increase in 

 weight of 101 '69 grains. By reference to the Table, it will be found that at 

 the temperature of 50 the proportion of aqueous vapour in gas standing over 

 water is '01333, which in the 220 cubic inches will amount to 2 '933 cubic 



