WATER IN THE ATMOSPHERE. 



213 



or 



w 

 "622 



II 



w 

 : 622 



001293 x -622 

 l + at' 



This method has the disadvantage that it takes a long time to 

 execute. 



Regnault's Researches on the Density of Water- Vapour. 



Regnault investigated the density of water-vapour at pressures below 

 the maximum, both at ordinary temperatures, and near the normal 

 boiling-point. He also determined the density of the vapour present in 

 saturated air at ordinary temperatures, expressing the result in all cases 

 as the ratio of density of water-vapour to that of dry air at the same 



FIG. 125. Chemical Method for Determining Dew-Point. 



temperature and pressure, in order to test the applicability of the laws 

 of Boyle and Gay-Lussac. 



If we calculate the density of water-vapour from the densities of its 

 constituents, assuming that two volumes of hydrogen combine with one 

 of oxygen to form two of water-vapour at the same temperature and 

 pressure, the density compared with that of air is 



0896x2 + 1-43 



2x1-293 



= -622 



Regnault's results, as will be seen below, show that under the various 

 circumstances of the experiments, the density is very nearly this, or we 

 may, to a close approximation, apply the ordinary rules connecting 

 temperature, pressure and volume. 



At Ordinary Temperatures. Regnault introduced a small glass 

 bulb containing a known weight of water into a large glass globe, which 

 was then exhausted, the small quantity of residual air being carefully 

 dried. A manometer attached to the globe gave the pressure within it. 

 The bulb was then broken and the temperature of the globe was raised 

 to a point above that at which all the water evaporated. The vapour- 

 pressure was, therefore, below its maximum. The pressure indicated 



