lid-cited Activity of Thorium. 041 



to the melting-point of the crystals generally differed by only 

 a few millimetre-, the greatest variation being about 1 cm., 

 which corresponds to about 25 c : so that the temperature to 

 which the wire was heated may be considered to be known 

 within these limits, and a greater accuracy was not aimed at. 

 The object of these experiments was to find the general nature 

 of the effect, rather than to get very exact determinations of 

 temperature. 



When it was desired to investigate the activity driven off 

 from the wire, a sheet of lead-foil rolled into a cylinder was 

 placed inside the glass tube, and a large proportion of the 

 activity lost by the wire was found deposited on this cylinder. 

 After heating, the wire and lead were removed, the inner 

 (active) surface of the cylinder was turned outwards, and its 

 activitv was tested in the same way as that of the wire. 

 Fig. 3 shows the arrangements for heating. 



Fiii'. 



C 

 O 



-O 



A. Active platinum wire. B. Thermocouple. C,C. Lead cylinder. 



Results. 



As a preliminary experiment, an active wire was heated for 

 1 minnte to about 1100°, and at once tested. Its activity 

 was found to have diminished by about -it; per cent., but 

 there was a much more striking change in the rate of decay; 

 for the activity decreased rapidly and at a steady rate, falling 

 to half value in just over an hour (diagram 4, PI. YJIL). _ The 

 experiment was repeated, ami the same result again obtained. 

 Ir was thus at once evident that there had been a very con- 

 siderable separation of the two active substances, thorium A 

 and thorium B. The effect of heat is, however, the reverse 

 of that obtained with cathode rays in that, while in the latter 



- the thorium B i- the more volatile, in the Bonner it is the 

 thorium A which i- driven off in greatest amount. 



To confirm this result, the activity driven off was collected 



