﻿Hydrogen throng k a Palladium Septum. 209 



the clip on the india-rubber tube was opened. When the 

 temperature had become constant, the reservoir of mercury 

 E was brought approximately to a level with a mark on the 

 capillary tube D, and the capillary tube at C was broken. 

 Gas then escaped through C until the pressure in the tube A 

 became equal to that of the atmosphere. The capillary point 

 was again scaled. An accurate reading was then taken, the 

 mercury reservoir being placed so that an exact volume 

 of gas was contained in the palladium cell under a known 

 pressure. 



Hydrogen carefully purified by washing with potassium 

 permanganate, silver nitrate, and caustic potash, and dried by 

 passage through sulphuric acid and over phosphorus pent- 

 oxide, was then passed in through the tube F, escaping at G. 

 Pressure rapidly rose in the interior of the apparatus, and 

 was measured by raising the mercury reservoir. When it 

 had attained its maximum a reading was taken, the position 

 of the mercury in the capillary stem being so adjusted that 

 the capacity of the cell was accurately the same as at the 

 commencement of the experiments. The difference between 

 the initial and final pressure is due to entry of hydrogen. 



The success of such experiments depends on the condition 



of the palladium. After having been used once or twice the 



interior of the palladium tube became coated with mercury, 



even although the level of mercury (which was cold) lay far 



below the palladium top. It appears that palladium absorbs 



mercury- vapour with avidity, thus rendering the palladium 



cell a partial vacuum to mercury-vapour. Vapour rises from 



the cold mercury to restore pressure, and is again absorbed. 



It was therefore necessary to heat the palladium tube after 



each experiment in order to expel mercury. This had the 



effect of oxidizing the palladium and of covering it with a 



brownish-black film. To remove the oxide, the tube was made 



the negative pole of a battery, and dilute sulphuric acid was 



electrolysed, both on the inside and outside of the palladium 



tube. It was then dried at a low temperature ; the external 



surface was polished with a little dry emery ; the tube was 



cemented into its place, and was ready for a fresh experiment. 



Without such precautions the passage of hydrogen is very 



| slow and incomplete. It should be mentioned that before 



, commencing an experiment it was necessary to jacket the 



: palladium tube at 220° and to pass a current of air over it for 



some time. In this way the combined hydrogen was removed; 



I hydrogen escaped from the exterior of the tube and was 



| replaced by air or some other gas in the interior. 



Phil. Mag. S. 5. Vol. 38, No. 231. Aug. 1891. P 



