f>46 Prof. J. Joly and Mr. A. L. Fletcher : 



the halos in this Leinster biotite may be abruptly bisected 

 where they abut against the muscovite. This, too. is a 

 frequently observed appearance in granites, syenites, &c. The 

 halo is not continued into quartz or felspar adjacent to the 

 biotite or hornblende, in which it is developed. I£ the 

 appearance is due to a mere storage of helium this should 

 not be. 



On the other hand, the view that the molecular structure 

 is broken down by the passage of the positively charged ray 

 seems in harmony with the photographic and other chemical 

 effects which have been shown to attend the passage of the 

 ray. The nature of the changes produced is as yet unknown, 

 or whether the crystallographic structure influences them. 

 There might be phenomena resembling " reversal " in photo- 

 graphic films. We are really in very complete ignorance of 

 the nature of the wonderfully graphic yet minute phenomena 

 of the halo, as the number of speculative possibilities associated 

 therewith demonstrates. 



It is of interest to note the extremely minute quantity of 

 radioactive material which suffices to make a halo. We have 

 already suggested that the nuclei are in some cases possibly 

 uraninite or some related substance. For they may be black 

 and opaque, and, when their lustre can be observed by reflected 

 light, show the coaly appearance of pitchblende. The assump- 

 tion does not involve an amount of radioactive matter greater 

 th-in the ascertained gross amount per gram of mica might 

 indicate. The diameter of a nucleus sufficing to evolve all the 

 characters of the halo, pupil and corona, is very often less than 

 5 x 10~ 4 cm. Thecorrespondingvolumeisabout65 x 10 -12 cm. 

 If composed of uraninite having the density 8 the mass 

 would be approximately 5 x 10 ~ 10 gram, and the associated 

 radium in each nucleus about 10 ~ 16 gram. As one gram of 

 the mica contained nearly 12 x 10 ~ 12 gram radium there is 

 sufficient to build 100,000 such halos per gram. We may, 

 therefore, without unduly taxing the resources available, sup- 

 pose uraninite to be one of the radioactive substances present. 



The quantity of helium stored in a halo must be small on 

 any probable estimate of the time which has elapsed since its 

 initiation. On Rutherford and Geiger's estimate that 3*4 X 10 l0 

 helium atoms are expelled per second per gram of radium, 

 the quantity 10~ 16 gram radium and the 8 a ray pro- 

 ducing elements in equilibrium with it evolve only about 

 850 helium atoms in a year. If the time was 10 8 years the 

 number of atoms is 85 x 10 9 . Rutherford and Geiger have 

 calculated that there are in one cubic centimetre of a gas at 

 standard pressure and temperature 2*72 xlO 19 molecules. 



