742 Mr. & Mrs. Soddy and Mr. A. S. Russell on the 



it, and the lower E and F to the reflected radiation only. 

 There is practically very little difference between the curves 

 C and D, though one refers to a comparatively narrow cone 

 and the other to a cone of angle 180°. Fig. 9 shows similar 

 results for aluminium, the lettering of the six curves referring 

 to the same disposition as in the last figure. Here the 

 hemisphere curves A and C are markedly more straight than 

 the cylinder curves B and D, which, as is well known, are 

 strongly concave to the axis (Levin, Phys. Zeit. viii. 1907, 

 p. 585), the absorption coefficient, indeed, increasing 2*5 times 

 before all the rays are absorbed. So the general result of 

 these experiments is to show that, on the whole, the simple 

 exponential law is wore nearly followed with /3-rays when a 

 hemispherical ionization chamber and cone of rays of angle 

 180° are used than in the common disposition. This suffi- 

 ciently illustrates the great difference between the /3- and 

 7-rays. 



In view of the controversy that is in progress as to the 

 nature of the law of absorption of /3-rays, these results are of 

 interest as showing how artificial the mathematical treatment 

 of the question is which assumes a rectilinear propagation of 

 the ra}^s. The passage of /3-rays through matter probably 

 resembles more a diffusion than a radiation, and before much 

 real advance can be expected to be made it would seem 

 necessary to obtain some information of the free-path of the 

 /3-particle in various metals. 



With the apparatus shown in fig. 3 several other measure- 

 ments of the 7-rays both of uranium X and radium, for zinc 

 hemispheres as well as lead, have been taken. Figs. 5 and 10, 

 PL XII., show the result for radium with a zinc hemisphere 

 9 cm. in radius. For the latter two thirds of the hemisphere 

 the curve agrees remarkably closely with the theoretical curve 

 (X = 0*28). This again is identical with the value (0'278) 

 given in Table II. of the last paper. When X was put equal 

 to 0*30 the agreement was far less perfect. But for the first 

 part of the curve the observed readings fall consistently 

 below the theoretical. The lowest curve in fig. 10 is the 

 difference curve between the observed and theoretical values 

 of It, and it will be seen to be very approximately exponen- 

 tial up to the point at which /3-rays interfere. The value of 

 the coefficient X' of these supposed secondary rays is 1*25, or 

 4*5 times that of the primary (compare Madsen, Phil. Mag. 

 1909, xvii. p. 447). This result thus suggests that for zinc 

 a secondary radiation is generated which does not come into 

 equilibrium with the primar}^ until about 2 cm. of zinc have 

 been penetrated. Again there is no evidence of a soft 

 primary 7- radiation. The remaining two curves (fig. 11, 



