732 Mr. & Mrs. Soddy and Mr. A, S. Russell on the 



uranium X, it must be unimportant relatively to the hard 

 type. 



The curves on the whole, however, do not bear out the 

 view of the existence of a soft primary radiation. It is 

 possible by choosing the values of X 3 and X 3 near together to 

 get a more or less close approximation of the curves for the 

 lighter substances with the theoretical equation consisting of 

 the sum of three exponential terms, two positive and one 

 negative, but only by using values of B inconsistent with the 

 dotted-line curves. 



Experiments were performed with the uncovered uranium 

 X preparation between the poles of a powerful electromagnet 

 to see if /3-rays had any influence on the results and to explore 

 the curves over smaller values of the equivalent thickness in 

 the hope of getting evidence of the negative exponential term. 

 The electroscope rested on a thick cast-iron plate supported 

 as a table above the magnet, over a circular hole cut in the 

 plate, to the under side of which the absorbing plates were 

 clamped up. Using just sufficient card or lead completely to 

 absorb /3-rays it was found there whs no difference in the 

 value of the leak whether the magnet was excited or not. 

 The same was true when card of equivalent thickness 4*4 was 

 used. Hence the /3-rays do not affect the results, or produce 

 any appreciable secondary 7-radiation. The dotted part on 

 the curve (fig. 1) indicates the reading obtained for card 

 over the region usually masked by the /3-rays. The ordinate 

 of the curve is naturally arbitrary. It must be remembered 

 that /3-rays are never completelv deviated bv a magnet 

 (Soddy, Phil. Mag. 1909, xvi. p. 860), although with the 

 arrangement used they were reduced probably to less than 

 one per cent, of their initial value. The results are therefore 

 not of great value. For the metals sufficiently accurate 

 results to be of service could not be obtained owing to the 

 heat of the magnet disturbing the readings of the electro- 

 scope. 



In light of the results to be given in the succeeding 

 sections it seems that the distribution of the emergent 

 radiation as regards direction in space must be taken into 

 account in the interpretation of such 7-ray absorption curves. 

 Bragg and Madsen (Phil. Mag. 1908, xvi. p. 926) have 

 suggested that the differences between light and heavy 

 metals are due to the soft type of 7-radiation, supposed to be 

 present initially, being more readily absorbed, relatively to 

 the hard type, by dense than by light substances, the 

 absorption causing a proportional production of soft /^-radi- 

 ation. The soft 7-radiation is thus used up rapidly in lead, 



