214 HEAT. 



by the manometer was then observed, and subtracting that due to the 

 residual air, the difference gave that of the water-vapour. 



He found that if the pressure did not exceed '8 of the vapour- 

 pressure for the temperature of the experiment, the relative density 

 of the water- vapour was about '621. 



When near saturation the density appeared much greater, but this 

 must have been due to some error in the method, inasmuch as the 

 density of saturated vapour in air agrees very nearly with the value '621. 

 Probably the vapour condensed on the glass before saturation. 



At Temperatures near the Boiling Point, and at Pressures 



far below 760 mm. A large globe containing some water was kept 

 for a long time in steam until all the air was expelled, its place being 

 taken by the water-vapour. Then the globe was connected with a con- 

 denser outside, some of the vapour being thus withdrawn. All the 

 water in the globe having evaporated, the vapour-pressure was reduced 

 below 760, its value being determined by a manometer. The globe was 

 then closed, allowed to cool, and weighed. It was once more heated in 

 steam and some more of the vapour withdrawn by external condensation, 

 the pressure being still further diminished. The new value was observed 

 and the globe was then closed. After cooling it was weighed again. 

 Assuming that the laws of Boyle and Gay-Lussac could be applied, the 

 loss in weight would be the weight of the vapour at the steam tem- 

 perature filling the globe at a pressure equal to the difference in the 

 pressures before the two weighings. Of course allowance was made for 

 any variations in the steam temperature in the two parts of the 

 experiment. 



As long as the pressure did not approach 760 mm. the results agreed 

 in all cases with a relative density of about -620, but near 760 mm. the 

 density became decidedly greater. Calculations founded on thermo- 

 dynamic principles also show that the relative density of saturated steam 

 is decidedly above *623. 



Using the method of Dumas, Cahours has shown that at the at- 

 mospheric pressure the relative density falls again to about the normal 

 value when the temperature is raised considerably above 100. 



At Ordinary Temperatures in Saturated Air. Air was drawn 

 by an aspirator through a balloon filled with wet sponge into a box in 

 which hung wet linen cloths, so that it was thoroughly saturated at the 

 temperature of the box. It was then drawn on through drying tubes, 

 where all the water-vapour was left behind, into the aspirator, where 

 it became thoroughly saturated once more. Let us suppose the 

 volume of the aspirator to be Y, and for simplicity let its temperature 

 be taken as equal to that of the box, both being t ; let the gain in 

 weight of the drying tubes while the aspirator is once filled with air, 

 be w. Let P be the maximum pressure of water-vapour at t", and let 

 us suppose that the relative density of the water-vapour is '622, then 



. 

 760 1 + at 



The observed values very nearly agreed with this calculated value 

 though in all cases they were slightly lower in general by about 1 per 

 cent. a difference which may be due either to a real difference in 



