CHANGES INFLUENCED BY RADIUM. — CREIGHTON. 



35 



Five milligrammes of radium bromide of activity of about 

 1,000,000 were employed. The radium was enclosed in a. small 

 glass tube, so that only the a - and yS-rays were used. 



Action of Radium on Lead and Tin. 



After a particularly cold winter, 1867-68, it was found that 

 some blocks of tin that had been stored in the customs house 

 at St. Petersburg, had mysteriously crumbled to a srey powder. 

 It has since been shown that tin exists in two allotropic forms, 

 one of which is this grey powder, the other the ordinary white 

 malleable metal. The transition temperature of these two 

 varieties of tin is about 20° C, the former being stable below, 

 and the latter above this temperature. The reason all ordinary 

 tin, most of which is at a temperature below that of transition, 

 does not change into the grey kind, is due to its being in a state 

 of unstable equilibrium, aind kept there by an unkno^vn agent, 

 to which the name passive resistance has been given. If in 

 some way this passive resistance could be overcome, then the 

 transition of white into grey tin w^ould readily take nlace. 



It was in order to see whether radium would do this that 

 the following experiment was carried out. 



Tw^» pieces of white tin, about two and a half centimetres 

 square and a millimetre thick were prepared from pure mossy 



tin, and their surfaces made 

 smooth and clean. These 

 were placed in a small 

 leaden box, Fig. 1, and 

 separated from each other 

 by means of a leaden 

 partition, which was suffi- 

 ciently thick to keep all but 

 the fastest ^-rays from 

 passing from one compart^ 

 ment to the other. The ends of the box were left open. The 



J^L^ 1 



