474 Prof. Bumstead and Mr. McGougan on Emission of 



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 iu the form of the 

 curve when its magnitude is considerably 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 explanation is increased by the fact 

 that the ionization curves of gases and vapours do vary con- 

 siderably ; 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 Rays. 



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 ab- 

 sorbable or " soft '' radiation emitted by the polonium, which 

 is completely stopped by 0*64 X 10 -4 cm. of aluminium. To 

 obtain an idea of the nature of this radiation, some experi- 

 ments were made, in which we used the apparatus described 

 in the previous paper *. It is substantially the same as that 

 shown in figure 1 of the last section, except for slight differ- 

 ences in dimensions and the fact that there is only one disk 

 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 S-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 

 ranging from 40 to 1000 volts. The results of these 

 measurements are shown in fig. 4, where the abscissas are the 

 potentials applied to the case and the ordinates of Curves I. 

 and III. are, respectively, the currents observed with one foil, 

 and with no foils, interposed. The difference between the 

 ordinates of the two curves shows the effect of the assumed 

 soft radiation. This effect is greatly diminished as higher 

 positive potentials are applied to the case, so that at 1000 

 volts it is reduced to about one-fourth of its value at 40 volts. 

 This indicates that the radiation consists of electrons with 

 much higher velocities than those attributed to the S-rays, 

 and with a wide range of velocities. As the positive potential 



* Phil. Mag. xxii. p. 917 (1911); Am. Jour. Sci. xxxii. p. 413 (1911). 



