1903.] The Evaporation of Water in a Current of Air. 77 



read to '01 was placed in the wash-bottle through a cork in the 

 neck, and air was drawn through at varying rates. In most of the 

 experiments the air passed first of all through another wash-bottle 

 containing water, and then through the flask containing the thermo- 

 meter. The results are here tabulated : 



It will be noted that at 40, with only two flasks, no perceptible fall 

 of temperature is produced by such a rapid current of air as 3 litres 

 per minute, while at 76, with the same number of flasks, a current of 

 0-14 litre per minute has no perceptible effect. Since the rates used 

 in the actual experiments for the determination of vapour pressure 

 were much less than 0'14 litre per minute, and four flasks (in some 

 cases five) were employed, there can be no doubt as to the saturation 

 of the air in these experiments. 



This was confirmed also by varying the rate during the experiments, 

 when the variation was not found to have any effect on the amount of 

 water driven off. 



Method of Calculation. 



An experiment carried out in the manner described furnishes the 

 following data : 



W = Weight of water drawn off. 

 P = Pressure in the last aspiration flask. 

 T = Absolute temperature of air in aspirator. 

 V = Volume of air drawn through the water, measured at tem- 

 perature and pressure of aspirator. 

 p = Pressure of air in aspirator. 



If we assume that the vapour pressure of water is the same in the 

 presence of air as in the absence of air or any other gas, and that 

 the partial pressure of each constituent of the gaseous mixture is the 

 same as if each occupied the whole space (Dalton's laws), and also 

 that the density of saturated water vapour is normal, then the vapour- 

 pressure can be calculated from the relation 



