from a Plate exposed to Rays from Radium. 179 



produce ; (2) the secondary rays sent out from the air, due 

 to the passage through it of the primary rays ; (3) the 

 secondary rays from the lead ; (Ij the air-rays produced 

 by (3) ; and (5) various tertiary rays, &c. The first of these 

 is always present, and with the so-called spontaneous 

 ionization constitutes an invariable amount of leak, which is 

 always added to that from other sources. The second group 

 was first studied, that is, the rays from air due to the passage 

 through it of j3 and y rays. A few numbers are given in 

 Table I. to show the power of the primary rays of different 

 penetrability to produce these air-rays. The numbers denote 

 the rate of leak as observed by the movement of the oold- 

 leaf of the electroscope. In order to preserve similarity of 

 conditions and ensure greater accuracy, the numbers actually 

 observed were for the time in seconds required for the leaf 

 to move over 20 scale-divisions, and these numbers divided 

 by 20 are added in brackets to give a general idea of the 

 actual magnitude of the times involved. The leak as given 

 is 100 times the reciprocal of the above time, or the number 

 of scale-divisions passed over by the leaf in 100 seconds. 

 The leak is given for the full unobstructed beam, and also 

 for the rays left when various plates of lead A (and C) are 

 interposed in the path of the beam, over a large range of 

 thicknesses from '02 to 15*4 mm. The method was susceptible 

 of an accuracy of J of 1 per cent., and where necessary to 

 interpretation the numbers can be relied on to that decree ; 

 in general an accuracy of 1 per cent, may be claimed for 

 them. 



Table I. 



Thickness of | 

 lead plate A L 

 in mm. ... J 



o-oo 



(800) 

 12-5 



0-020 0095 



(18-9) (37-2) 

 5-29 2-69 



0-225 



(44-4) 

 2-25 



100 



(49-6) 

 2-02 



1-80 



(50-6) 

 1-98 



3-60 



(51-5) 

 1-94 



5-60 

 (52-0) 



15-4 



(53-0) 



Time in sec. "1 

 for 1 scale- I 

 div ) 



Leak — no. of | 

 scale- div. in j- 

 100 sec. ...j 



1-92 



1-89 



The leak 1'89 for an absorbing layer A of 15"4 mm. is the 

 smallest noted during the course of the experiments, and is 

 useful as an index when asking whether the leak is as small in 

 any given circumstances as there is a possibility of its being. 

 From the table it is seen that, although the easily absorbable 



N2 



