Mr. F. S. Spiers on Contact Electricity. 81 



place when the tube was heated under exactly the same con- 

 ditions but in this case full of air, for on doing so the P.D. 

 fell to 



0*30 volt 



when cold, and rose again to 



1-10 volts 



on thoroughly cleaning the aluminium. 



§ 11. The experiments described in § 10 satisfied me that it 

 was useless to attempt to drive off the last remnants of air from 

 the metal plates by merely heating them in a vacuum, so I 

 decided first to thoroughly wash out the apparatus three or 

 four times with fairly pure dry hydrogen, and then strongly 

 heat it in a high vacuum of that gas. I expected that such 

 a molecularly active gas as hydrogen would displace even the 

 last traces of air from the pores of the metal plates. 



The hydrogen was prepared from fairly pure zinc (such as 

 is used in Marsh's test for arsenic) and pure hydrochloric 

 acid. It was washed by passing through distilled water and 

 carefully dried by means of tubes of pumice and strong 

 sulphuric acid and of phosphorus pentoxide. Such hydrogen 

 should contain but the barest traces of foreign gases, and I 

 considered it sufficiently pure for my purpose. (1 had tried 

 other methods of preparing the hydrogen, but found the 

 above the most satisfactory on the whole. The chief thing- 

 was to avoid having such impurities in the gas as oxygen, 

 ozone, water-vapour, or other oxygen- containing electrolytes.) 

 There was probably a far greater quantity of strange gases in 

 the vacuum-tube even after exhaustion than was in the 

 hydrogen. 



The hydrogen generating apparatus and drying-tubes were 

 joined directly to the apparatus through a mercury-sealed tap. 

 The generating apparatus, when not producing gas, was 

 always kept full of hydrogen at a pressure greater than that 

 of the atmosphere, so that if any leakage took place through 

 the various joints it would be hydrogen leaking out into the 

 atmosphere and not air leaking in. 



§ 12. At the beginning of this new experiment the P.D. 

 between the plates was 



0-98 volt. 



I exhausted to a vacuum of 30V0 mm «> an d let in hydrogen 

 until the pressure was 1 atmosphere. The P.D. fell to 



0-91 volt. 

 Phil Mag. S. 5. Vol. 49. No. 296. Jan. 1900. Q 



