652 Mr. Gr. Shearer on the 



times to ensure that the air had been completely removed. 

 In order to raise the pressure, mercury was introduced into 

 this second chamber from a reservoir, thus displacing the 

 gas into the ionization chamber and increasing the pressure, 

 the value of" which was measured by the pressure gauge. 



Later on in the course of the experiments, when the 

 hydrogen was being prepared electrolytically, another and 

 more convenient method of increasing the pressure was used. 

 The voltameter used consisted of three glass tubes with 

 platinum electrodes, the two outer ones being anodes while 

 the central one was the cathode at which the hydrogen was 

 produced. These were provided with stopcocks, and the 

 central one was connected directly to the ionization chamber. 

 When it was desired to raise the pressure, the two external 

 tubes were closed and the current turned on, the hydrogen 

 produced passed into the ionization chamber and the pressure 

 increased. It was of course necessary from time to time to 

 allow some of the oxygen which collected in the other tubes 

 to escape. This method of varying the pressure was found 

 to work very well, and it had the advantage that there was 

 no possibility of any air entering along with the hydrogen, 

 whereas in the other method there was always the danger of 

 bubbles being carried in with the mercury. 



Preparation of Hydrogen. 



In the first series the hydrogen was prepared in a Kipp 

 generator by the use of zinc and sulphuric acid. The 

 chemicals used were the purest obtainable and were supplied 

 by various firms, those used first being Merck's or Kahlbaum's, 

 while later some products of English firms were used. The 

 water used was special " conductivity " water, and was kindly 

 supplied to me by the Chemistry Department. In order to 

 get the action to start, a little pure copper sulphate was 

 introduced into the Kipp. The gas produced was passed 

 through soda-lime and then through acidified potassium 

 permanganate, the latter being inserted to remove any arsenic. 

 Subsequently it passed through a long tube with cotton-wool, 

 up a calcium-chloride drying-tower, finally entering the 

 chamber through a tube containing phosphorus pentoxide. 



In the later experiments the hydrogen was prepared by 

 the electrolysis of dilute pure sulphuric acid, the water 

 employed being again "conductivity" water. The gas 

 entered the chamber through calcium chloride and phos- 

 phorus pentoxide drying-tubes. 



The hydrogen was subjected on two occasions to Marsh's 



