by Metals under the Influence of Alpha Rays. 915 



polonium curve can be obtained. To obtain a- rays o£ greater 

 range, I used the active deposit from the thorium emanation 

 which emits a-rays of two different speeds ; the rays from 

 thorium B have a range in air of 5'0 cm., and those from 

 thorium C, of S'6 cm. An additional advantage is the slow 

 rate of decay of the thorium active deposit which falls to 

 half value in 10*6 hours. 



A small quantity ("activity of 2*4 mg. RaBr 2 ") of 

 Professor Halm's mesothorium, obtained from Knofler & Co. 

 of Berlin, was available. This was placed in a small cylin- 

 drical cup of platinum with a hemispherical bottom, and was 

 slightly moistened to increase its emanating power. The top 

 of the cup was closed with a rather thick stopper of ebonite, 

 in the centre 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 mm. in diameter). The 

 lower end of this plug was flush with the bottom of the 

 ebonite stopper which was about halfway 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 24 

 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 meso- 

 thorium, it is necessary to wait about two or three hours 

 before beginning measurements, in order to allow the radium 

 active deposit to decay 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 reason- 

 able number of hours, and it was desirable to repeat each 

 measurement several times to guard against possible 

 vagaries of the electroscope. Each reading was corrected 

 in the usual manner for the decay of the active deposit. 

 Curve II. (fig. 2) shows the result of the series of Mar. 9, 1911, 

 which is typical of other similar series. The two " knees " 

 are distinctly shown and their position is in excellent agree- 

 ment with the ionization curve in air given by Halm*. 



It appears from these experiments that (as was anticipated) 

 the secondary 8-radiation from the aluminium varies with 



* Fhys. Zeitsch. vii. p. 415 (190G). 



