1897.] on the Condensation of Water Vapour. 335 



the upper part of the test-tube can suddenly be put in communi- 

 cation with the exhausted vessel F. This causes P to fly till 

 it strikes the indiarubber, thus causing a sudden expansion of 

 the air in A. It remains tightly pressed against the surface of 

 the indiarubber as long as the vacuum is maintained below it, 

 thus preventing the lubricating water which surrounds it from 

 escaping. 



While the piston is in this position, the extent of the next 

 expansion is arranged for, by opening the screw-clip T 2 , and 

 lowering the mercury reservoir R, till the pressure in A, as 

 indicated by the gauge, is the desired amount below the atmo- 

 spheric pressure. The clip is then closed and air readmitted to 

 the interior of the piston P by opening the tap T x . The piston 

 then rises till the pressure in A only differs from the atmospheric 

 pressure by that required to support the weight of the piston, — a 

 small fraction of a millimetre of mercury and negligible in these 

 experiments. 



If B be the barometric pressure, then the pressure of the air 

 before expansion is 



P,=B-7T, 



where it is the maximum vapour pressure at the temperature of 

 experiment. The pressure of the air after expansion, when the 

 temperature has risen to its former value, is 



P, = P 1 -p, 



where p is the reading of the gauge, taken before the previous 

 contraction, as described above. 



Then the ratio of the final to the initial volume is 



V 2 P 1 B-7T 



V x P 2 B-TT-p' 



Any condensation, in the form of drops, resulting from the 

 expansion was made visible by the light of a luminous gas-flame, 

 brought to a focus inside A by means of a condensing lens. 



The Uranium salt (the double sulphate of Uranium and 

 potassium) was contained in a thin- walled glass bulb, forming a 

 layer about 1 centim. in depth. 



The dust-particles originally present having been removed by 

 repeated expansions, observations were made of the result of a 

 given expansion, with the bulb containing the Uranium salt 

 alternately 2 or 3 millimetres above A (as in the figure) and 

 removed to a distance. 



I. v 2 Jv 1 = 1245. 



Result. No drops seen either with or without the rays. 



