390 Prof. Townsend on the Conductivity produced 



The experiments with hydrochloric acid gas were con- 

 ducted in precisely the same manner as has been already 

 described for the first three gases. In order to use water 

 vapour it was necessary to alter the arrangement of the ap- 

 paratus, since the pressures cannot be found with a McLeod 

 gauge, and it is undesirable to introduce the vapour into the 

 Toepler pump. 



Plate IX. fig. 1 shows the apparatus which was used 

 for adjusting and measuring the pressure of the water 

 vapour. The parallel plates for measuring the conductivities 

 were set up in the air-tight vessel A, which was connected 

 with one side of the oil manometer M by the tube T. The 

 bulb B containing water was joined to a side tube provided 

 with a stop-cock S, through which the water vapour was 

 admitted to the apparatus. The other side of the manometer 

 was connected through drying-tubes to the McLeod gauge 

 and the Toepler pump. Sulphuric acid was used in the 

 tube D x near the manometer, and phosphorus pentoxide in 

 the second tube D 2 . 



The oil used in the manometer was that which is supplied 

 for lubricating the Geryk vacuum-pump ; it had a specific 

 gravity *87, and a small vapour-pressure which may be 

 neglected in comparison with the pressure of the water 

 vapour in any of the experiments. The oil floated on a 

 mercury column in the tube U, which was connected by 

 flexible tubing with a mercury reservoir. 



In order to remove the air from the apparatus the stop- 

 cock S and the manometer were opened, and the pressure 

 was taken down to a few millimetres of mercury. The stop- 

 cock was then closed and the pressure was further reduced 

 to about a tenth of a millimetre by the Toepler pump. The 

 exact value of the pressure could be found by the McLeod 

 gauge. 



In order to reduce the pressure of the air in A and B to 

 about 1/1000 millimetre, the oil was raised in the manometer 

 so as to stop the connexion between the two sides of the ap- 

 paratus. The stop-cock S was then opened and the vapour 

 from B was allowed to pass into A until the surface of the 

 oil in the tube H 2 was about fifteen centimetres lower than 

 the surface in H^ After closing the stopcock the mercury 

 reservoir was lowered carefully in order to allow the gas on 

 the left of the manometer to bubble through the oil in the 

 column H x until the difference in level on the two sides of 

 the manometer was reduced to about one centimetre. The 

 mercury reservoir was again raised and the process was re- 

 peated, until the pressure of the air in A was reduced to the 



