Mr Growther, On the Transmission of ^-rays. 455 



gives the values of x\og{l —I/Io) which is proportional to k, and 

 it will be seen that this product does diminish for the thicker sheets. 

 On the whole the agreement between the experimental results 

 and the theory is even closer than might have been expected, but 

 it is evident that the phenomena of absorption are too complicated 

 to afford any rigorous test of the theory. Experiments are now 

 being made along other lines in order to determine this point. 



It will be seen that the curve differs from those given by 

 Wilson in his paper, mainly in the initial portion. His curves 

 show the final bending round along the a;-axis, though he seems 

 inclined to ascribe it to experimental imperfections. It seems 

 possible that the greater width of his apertures (1'2 cms. as com- 

 pared with 0*5 cm. in these experiments) and the fact that the rays 

 fell upon the absorbing sheets over a fairly considerable angle 

 instead of in a nearly parallel pencil, caused a kind of general 

 levelling out, and thus obscured the initial point of inflection of 

 the curves. 



If now we turn to the absorption curves for platinum, we find 

 an entirely different law of absorption. An examination of the 

 dotted curve in fig. 6, representing the relation between log I/Iq 

 and the thickness of the absorbing layer, will show that, except 

 for a slight initial steepening, the curve may be represented by 

 a straight line to an accuracy well within the probable errors of 

 experiment. On the other hand the lower curve, giving the 

 values of I/Iq, does not show the slightest approximation to a 

 linear law. In fact the absorption of homogeneous /3-rays by 

 platinum follows exactly the same law as the absorption by plati- 

 num of the /S-rays from a single radio-active substance, that is it 

 is exponential. 



The velocity of the /3-rays used in these experiments was 

 2'77 X 10" cms, per second. The mean velocity of the /3-rays from 

 uranium X, according to the measurements of H. W. Schmidt*, 

 is 2"79 X 10" cm./sec. The homogeneous rays used in these 

 experiments had therefore practically the same velocity as the 

 rays used in previous experiments in the absorption of the ^-rays 

 from uranium -j-. The value of X/p for platinum (where \ is the 

 coefficient of absorption assuming the exponential law and p the 

 density) found from the present experiments is 11*9; the value 

 obtained with the uranium rays was 9*4 Considering the very 

 different nature of the two experiments the agreement is fairly 

 close. 



This result must undoubtedly be ascribed to some secondary 

 radiation excited in the platinum by the incident rays. We have 

 already seen that the rays emerging from platinum betray an 



* H. W. Schmidt, Le Radium, Vol. vi. 1909, p. 5. 

 t J. A. Crowther, Phil. Mag. Vol. xii. 1906, p. 379. 



