Metals under Influence of Alpha Hays. 411 



was closed with a rather thick stopper of ebonite, in the center 

 of which was a hole through which passed a copper plug of 

 the same dimensions as that on which the polonium had been 

 deposited (4""" in diameter). The lower end of this plug was 

 flush with the bottom of the ebonite stopper, which was about 

 half way between the top and bottom of the cup. By this 

 arrangement, when the cup was charged to +80 volts with 

 respect to the copper plug, most of the lines of force which 

 passed through the emanation fell upon the end of the plug, 

 and the greater part of the active deposit was collected there. 

 After an exposure of 2-1 hours the accumulated deposit was 

 sufficient to give very good readings in the apparatus described 

 above, though somewhat less than had been obtained with the 

 polonium. As there is an appreciable amount of radium with 

 the mesothorium, it is necessary to wait about two or three 

 hours before beginning measurements, in order to allow the 

 radium-active deposit to decaj' to a small value. 



Series of observations were made in the same manner as 

 with the polonium except that the thicker aluminium foil 

 alone was used. On account of the decay of the active deposit 

 it was necessary to complete a series within a reasonable num- 

 ber of hours, and it was desirable to repeat each measurement 

 several times to guard against possible vagaries of the electro- 

 scope. Each reading was corrected in the usual manner for 

 the decay of the active deposit. Curve II, tig. 2, shows the 

 result of the series of March 9, 1911, which is typical of other 

 similar series. The two "knees'' are distinctly shown, and 

 their position is in excellent agreement with the ionization 

 curve in air given by Hahn.* 



It appears from these experiments that (as was anticipated) 

 the secondary S-radiation from aluminium varies with the speed 

 of the a-rays producing it in a manner entirely analogous 

 to the variation in the gaseous ionization under like conditions. 

 But the possibility of an error in this conclusion here suggests 

 itself. When the a-particle is not near the end of its.range it 

 passes through the thin aluminium electrode and the latter 

 receives its positive charge solely on account of the electrons 

 emitted by it. Near the end of the range, however, some of 

 the a-particles (those entering most obliquely) will stop in the 

 foil, and it is possible that the observed increase in the positive 

 charge acquired by the electrode is due to the positive charges 

 on these a-particles and not to any true increase in the S-radia- 

 tion. It is easy to test this by charging the case negatively, 

 and measuring the negative charge received by the electrode 

 from the electrons emitted by the case, for under these circum- 

 stances the stoppage of a-particles by the electrode would 

 *Phys. Zeitsch., vii, p. 415, 1906. 



