320 Bumstead and McGougan — Emission of Electrons by 



The points for the different metals lie so closely together 

 that only one curve lias been drawn. , Anyone who has had 

 ex peri (Mice with sucli measurements will recognize that the dif- 

 ferences are too small to have any significance. Even in the 

 case of aluminium, which appears to differ somewhat from the 

 others, the differences are not at least more than 2, or 3 per 

 cent; and differences of this order, which are obviously acci- 

 dental, occur in all the curves. We are forced to conclude, 

 therefore, that the ionization curves as observed, for all metals, 

 have the same form. This is in agreement with the results of 

 the less accurate experiments upon gold and aluminium 

 described in the previous paper. Whether or not the curves 

 so obtained really represent the ionization of the metals is by 

 no means certain. As has been said, the fact that there is no 

 change in the form of the curve, when its magnitude is con- 

 siderably decreased by the removal of a surface film of gas 

 from the metal, makes it not improbable that the whole effect 

 may be due to such a film. The probability of this explana- 

 tion is increased by the fact that the ionization curves of gases 

 and vapors do vary considerably ; and it seems, therefore, 

 unlikely that metals, so different in all their properties as those 

 used above, should show such complete similarity in this 

 respect. 



§ 3. Investigation of an Absorbable Radiation accompanying 



Alpha Hays. 



When the last thin aluminium foil is removed from the path 

 of the a-rays the number of electrons leaving the metal plate 

 is greatly increased ; this is shown by the first line of Table 

 III, § 2. There appears to be, therefore, a very absorbable or 

 " soft " radiation emitted by the polonium, which is completely 

 stopped by 0-64 X 10" 4cm of aluminium. To obtain an idea of 

 the nature of this radiation some experiments were made, in 

 which we used the apparatus described in the previous paper.* 

 It is substantially the same as that shown in fig. 1 of the last 

 section, except for slight differences in dimensions, and the fact 

 that there is only one disc to carry the aluminium foils instead 

 of two. A much stronger preparation of polonium, which we 

 owe to the kindness of Professor Boltwood, permitted the use 

 of an electrometer instead of an electroscope. A brass plate 

 was used as the source of 8-rays, and the negative current from 

 it was measured, with one thin foil interposed and with none, 

 when various positive potentials were applied to the case rang- 

 ing from 40 to 1000 volts. The results of these measurements 

 are shown in fig. 4, where the abscissae are the potentials ap- 



*Phil. Mag., xxii, 917, 1911 ; this Journal, xxxii, 413, 1911. 



