150 DR. FRANK HORTON ON THE DISCHARGE OF 



apparatus. The platinum strip was then covered with a layer of calcium by meaus ot 

 sublimation, and the current between the two electrodes was measured again. It 

 might here be stated that a current between the two electrodes was observed only 

 when the platinum strip was used as the cathode, the positive leak being too small to 

 be detected by the galvanometer used. In what follows, therefore, the platinum strip 

 will be spoken of as the cathode. 



When the observations of the negative leak from the calcium-covered cathode had 

 been made, the calcium was oxidised to lime, and the leak measured again. In this 

 way the negative leak from metallic calcium was compared with the negative leak 

 from the same amount of metal in the form of oxide. The full account of the 

 experiments and results is divided for convenience into the following sections : 



( 1 ) Description of the apparatus, &c. ; 



(2) Investigation of the negative leak from platinum ; 



(3) The negative leak from calcium in helium ; 



(4) The negative leak from lime in helium and in hydrogen ; 



(5) Summary of results, and conclusion. 



(1) Description of the Apparatus, &c. 



The glass apparatus shown in fig. 1 was found to be the most convenient form of 

 discharge tube for these experiments. 



A is a platinum strip which can be heated electrically, the current being supplied 

 by means of the thick platinum leads G. The leak from this to the platinum 

 electrodes B, C, which together form the anode, was measured by means of a delicate 

 d'Arsonval galvanometer, giving a deflection of 1 millim. for a current of 7'19x 10~ 10 

 ampere. The calcium wire D, from which the metal is to be sublimed, is beneath the 

 platinum strip, and about two centimetres from it. It can be heated by an electric 

 current led in through the thick copper leads F, which enter the bulb through 

 sealing-wax joints. The apparatus is connected with the mercury pump McLeod 

 gauge and P 2 O 5 drying bulb by the side tube shown in the figure. The platinum 

 strip was 3'5 centims. long and 2 millims. wide. The calcium wire was about 

 6 centims. long and 1 millim. in diameter. 



Since calcium is readily attacked by all the more common gases, the experiments 

 had to be conducted in an atmosphere of argon or helium. Either of these gases 

 could be admitted into the apparatus through a side tube. For the purpose of 

 purifying the gas a small discharge tube was fitted on to the apparatus. In this the 

 cathode was an alloy of potassium and sodium, made by mixing the metals in atomic 

 proportions. When a discharge from an induction coil is sent through this tube the 

 alloy gradually absorbs any gas that may be present, except argon or helium. The 

 argon or helium in the apparatus could thus be purified by running the discharge 



