464 Prof. Bumstead and Mr. McGongan on Emission of 



coincided within the limits of accuracy of the somewhat 

 rough method which was used for determining them. 



Quite apart from the theoretical reasons discussed in the 

 former paper, it was somewhat surprising to find that two metals 

 which differ so much as aluminium and gold gave, neverthe- 

 less, the same ionization curves. For the ionization curves 

 of gases and vapours differ considerably among themselves, 

 not only in the area enclosed (total ionization) but also in 

 their shape *. The close similarity between the curves for 

 aluminium and gold gave rise to the suspicion that the 

 electrons which had been producing the effects observed 

 came not from the metals, but perhaps from a layer of 

 adsorbed gas which was the same in both cases. In order to 

 test this possibility the following experiment was made. 



§ 1. Attempt to remove Adsorbed Gases by Heating. 



A strip of thin platinum foil, 6 cm. long, 3'7 cm. wide, and 

 2*4 X 10 -4 cm. thick, was stretched horizontally between two 

 heavy brass clamps ; the clamps were carried each on a 

 vertical copper rod which passed through the cover-plate of 

 the evacuated chamber in which the S-ray effects took place ; 

 the rods were insulated from the plate by ebonite, an earthed 

 guard-tube, and amber, and the joints made tight with 

 sealing-wax. The platinum foil was arranged so that it 

 could be exposed to a pencil of a-rays from polonium 

 deposited on the end of a copper plug 4 mm. in diameter. 

 Between the polonium and the platinum, one could interpose 

 aluminium foils, without interfering with the vacuum, in the 

 manner described in the former paper. Two, three, four, 

 five, or six layers of foil could be interposed, each 

 3*2 x 10~ 4 cm. thick. The case was exhausted to about 

 *0001 mm. with the help of charcoal and liquid air ; one of 

 the insulated copper rods which carried the platinum foil was 

 connected with a quadrant electrometer, the case surrounding 

 the exhausted chamber was charged positively, and measure- 

 ments of the negative current leaving the platinum were 

 taken in the usual manner. After an "ionization curve " 

 had been determined in this way, the platinum strip could be 

 heated by a current sent through it and the two copper rods, 

 and the curve could be again determined when the liberated 

 gas had been removed. 



Even before the platinum strip was heated its behaviour 

 gave evidence that the occluded gases had some effect upon 

 the phenomena, if only a temporary one. With the metals 



* Taylor, Phil. Mag. xxi. pp. 573, 575 (1911). 



