February 22nd, ipo^.] PROCEEDINGS. x\r 



they acquire the power of induced radio-activity : the radio- 

 activity of the latter shows that even here the final state was not 

 reached. 



The following highly interesting experiments were shown : — 



I. A radio-graph, taken by M. Becquerel by the uranium 

 rays with a fortnight's exposure, and a negative produced by 

 writing on a photographic plate with a glass tube containing a 

 few milligrams of pure radium bromide, were shown to illustrate 

 the million-fold more intense activity of radium as compared with 

 uranium. 



II. The ionisation of the air by the radium rays was shown 

 by their power of making collapse a silk tassel electrified by^ 

 stroking with a rubber tobacco-pouch. 



III. The a rays were illustrated by means of a preparation, 

 of polonium which gives only this type of radiation and no /3 and y 

 rays. The silk tassel was discharged by the bare preparation as 

 in the case of radium, but a single sheet of paper was shown to 

 completely stop the (a) rays of polonium while not appreciably 

 affecting the (/3 and y) rays of radium. The a rays of radium 

 were shown by means of several of Sir William Crookes' spin- 

 thariscopes. 



IV. The /3 rays of radium were illustrated by means of a 

 screen of platino-cyanide of barium, which fluoresced a deep 

 green colour when the radium was held behind, the rays passing, 

 through the opaque card and through copper foil without 

 appreciable loss. 



V. The y rays of radium were shown passing through f inch 

 of steel, and plainly illuminating barium platino-cyanide on the 

 other side. 



VI. The deviation of the cathode-ray of the Crookes' tube 

 by a magnet was first shown, and afterwards the deviation of the 

 /3 ray of radium by a more powerful electro-magnet, the image 

 on the screen being displaced when the current was applied. 



VII. The heating of radium by the bombardment of its- 

 own (a) radiations was illustrated by an analogous experiment of 



