310 Bwmstead and McGougan — Emission of Electrons by 



between the source of o-rays and the metal, one can plot a 

 curve entirely similar to the ionization curves first obtained by 

 Bragg. The number of electrons emitted by the metal 

 increases with the number of foils interposed until their com- 

 bined thickness is nearly equal to the range of the o-rays in 

 aluminium, after which the emission of electrons falls off 

 rapidly.* The increase in the number of electrons with dimin- 

 ishing speed of the a-rays, however, is not so great as the 

 increase in the number of ions in a gas. The ionization curve 

 of the metal lies within, or to the left of, the corresponding 

 curve for gaseous ionization and has a less pronounced maxi- 

 mum or "knee" just before the end of the range is reached. 

 This result was anticipated before the experiments were under- 

 taken, for reasons given in the previous paper. The same 

 arguments which led to this conclusion also gave reason to 

 believe that the curve for a metal of high atomic weight, such 

 as gold, would lie within that of a metal of lower atomic 

 weight, such as aluminium, just as the latter would lie within 

 the curve of gaseous ionization. This expectation, however, 

 was not fulfilled by the results of the experiments. The curves 

 for gold and for aluminium coincided within the limits of 

 accuracy of the somewhat rough method of determining them 

 which was used. 



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, 

 nevertheless, the same ionization curves. For the ionization 

 curves of gases and vapors differ considerably among them- 

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

 in their shape. f The close similarity between the curves for 

 aluminium and gold gave rise to the suspicion that the elec- 

 trons 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 possi- 

 bility 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 

 2A XlO" 4 "" 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 evacu- 

 ated chamber in which the S-ray effects took place ; the rods 

 were insulated from the plate by ebonite, an earthed guard ' 



* The increase in the emission as the speed of the a-particles decreases has 

 also heen observed by Campbell, Phil. Mag., xxi, 276, 1911. 

 f Taylor, Phil. Mag., xxi, pp. 573, 575, 1911. 



