in Gases by the Aid of Ultra- Violet Liyht. 391 



desired amount. The sulphuric acid and phosphorus pent- 

 oxide absorbed the water vapour on the right of the mano- 

 meter, and the air which was also carried through was 

 estimated by the McLeod gauge. The air was pumped out 

 by the Toepler pump when the pressure exceeded about a 

 tenth of a millimetre of mercury. It was found that the 

 air was very rapidly removed from the vessel A by the above 

 process. 



The pressure of the vapour in A was adjusted to any re- 

 quired value by closing S, and allowing the excess of vapour 

 to pass through the oil column in H,. The pressure ex- 

 pressed in millimetres of mercury is . -f //, where h is 



the height of the column of oil in H^ above the column in H 2 , 

 and h! the pressure of the air on the right of the manometer 

 as found by the McLeod gauge. 



It was easy to measure the height h within half a milli- 

 metre, so that the pressure was found to a thirtieth of a 

 millimetre of mercury, which is about the accuracy required 

 when the pressure of the water vapour was not less than one 

 millimetre of mercury. 



The experiments were made with the pressure below the 

 maximum pressure at the temperature of the room, so that 

 the water vapour did not condense and injure the ebonite 

 insulation in the electrical part of the apparatus. 



The condensation on the surface of the glass vessel A and 

 the tube T gave rise to some inconvenience in adjusting the 

 pressure. Thus the pressure in A does not attain its final 

 value immediately after some vapour is admitted from B 7 or 

 removed through M. In the former case vapour would con- 

 dense slowly on the glass, and the pressure would gradually 

 fall a little for some time after the change in pressure had 

 been made. The reverse took place after some vapour had 

 been removed. It was not possible, therefore, to make a 

 large number of experiments at precisely the same pressure. 



The conductivities were measured in the manner described 

 in the previous paper, and the values found are given in the 

 following tables. The pressure P is given in millimetres of 

 mercury, and the experiments are numbered according to the 

 pressure at which they were made. The electric force X is 

 given in volts per centimetre. The corresponding conduc- 

 tivities for the different distances between the plates are 

 given in the columns n h n 2 , n 3 , &c, the suffix denoting the 

 distance between the plates in millimetres. The ratios of the 

 conductivities are given in the column R. 



2D 2 



