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XYI. Action of the a Rays on Glass. By Professor E. 

 Rutherford, F.R.S., University of Manchester*, 



I 1ST 1907 Professor Jolyf showed that the pleocliroic halos 

 observed in brown mica were in all probability to be 

 ascribed to the action of the a rays from radium. These 

 coloured areas, the origin of which had long puzzled geolo- 

 gists, are always found to contain at their centre a minute 

 crystal of zirconite, or more rarely of apatite. Strutt has 

 shown that both of these minerals are rich in radium. Ac- 

 cording to the explanation of Joly, the u rays emitted for 

 long periods of time from the crystal have gradually 

 darkened the mica and have given rise to the halo observed. 

 The evidence indicated that the halos are spherical in shape 

 with an average radius of about *04 millimetre. It is well 

 known that the a. rays from each radioactive product have a 

 definite range of action depending upon the density of the 

 material. The most penetrating a rays from the uranium- 

 radium series arise from radium 0, and have a range in air 

 of about 7'06 cm. In a material like mica the corresponding 

 range is about *04 millimetre. The range of the a particles 

 in mica thus agreed closely with the observed radius of the 

 halo. The evidence seems conclusive that these halos are 

 of radioactive origin. 



Recently, in the course of some experiments with the 

 radium emanation, I had occasion to allow a large quantity 

 of purified emanation to decay in a fine capillary tube of 

 soda glass. The emanation was introduced into the capillary 

 under pressure, and the tube then sealed off. The greater 

 part of the emanation was then condensed near the end of 

 the capillary and left surrounded by liquid air for four days. 

 The tube was then removed from the liquid air and allowed 

 to stand for a month, in which time the greater part of the 

 emanation was transformed. The base of the capillary was 

 not uniform, but gradually tapered over a distance of 

 3 centimetres to nearly a point. The external diameter of 

 the glass tube near the end of the capillary was about '6 mm. 



Notwithstanding that a large quantity of emanation, cor- 

 responding to the equilibrium amount from 150 milligrams 

 of radium, had been kept in the tube, the thin- walled part of 

 the capillary was only slightly coloured when viewed by the 

 eye. The capillary was placed under a microscope to examine 

 whether any indication of the effect of the a. rays from the 

 emanation and its products in the glass could be observed. 



* Communicated by the Author. 

 t Phil. Mag. March 1907. 



