622 Mr. R. D. Kleeman on the Secondary Cathode 



cathode radiation), than when the plate is exposed to both 

 the ft and 7 rays; and there would, therefore, be a greater 

 comparative increase in the radiation beyond a certain thick- 

 ness of substance with the 7 rays, than with both the ft and 

 7 rays. It may be mentioned that according to the experi- 

 ments o£ Eve the 7 rays of radium produce about one-fifth 

 of the total radiation from a plate exposed to both the ft and 

 7 rays. It follows, therefore, that the ratio of the total 

 radiation to the 7 radiation, when a substance is exposed to the 

 7 rays, is about five times greater than when the substance is 

 exposed to both the ft and 7 rays. 



An unexpected difficulty encountered in these experiments 

 will now be described. It was found that a number of leaks 

 taken in succession with a single substance were subject to 

 gradual fluctuations in magnitude: in the course of an hour 

 a change of 5 or 10 per cent, in the value of the leaks might 

 take place. When one of the chambers was disconnected 

 from the electrometer, and the radium placed at such a 

 distance from the other chamber that a convenient leak was 

 obtained, these fluctuations were very small but still de- 

 tectable. It thus appears that the leak in a single chamber was 

 liable to fluctuations which were small in comparison with 

 the whole leak, and therefoie when the difference between 

 two leaks was taken, both of which were large, the fluctua- 

 tion in the observed leak necessarily became much larger in 

 comparison with the whole leak. The fluctuations might 

 have been due to changes in the centre of the 7 radiation, 

 caused by some emanation coming off in gusts from the 

 radium, and the radium A thereby getting deposited on the 

 inside of the walls of the glass tube instead of in the radium 

 itself. By taking a large number of readings, alternately 

 with the two standard substances, and one of the standard 

 substances and the substance under investigation, and taking 

 in each case the mean of the differences obtained, the dis- 

 turbing effect of these fluctuations was rendered very small. 

 About sixty -four observations were, as a rule, taken in all 

 during the investigation of a single substance. It required 

 about one hour and a half to make such a set of observations. 



The differences between the amounts of secondary cathode 

 radiation from a number of substances and the radiation 

 from aluminium, corresponding to a difference of 800 of the 

 standard pair of metals lead and aluminium, are given in 

 Table I. They were not determined in any definite order, 

 but as they came to hand; some of them are the mean of two 

 or three separate determinations made at different times. In 

 the case of those substances which were obtainable in the 



