Mr Growther, On the Transmission of ^-rays. 453 



rate of leak, and may cause considerable errors in the results. 

 This point is of special importance in the present case, as it is 

 always difficult to secure really good insulation in the presence of 

 the 7-radiation from any considerable amount of radium. 



(3) The method is found to be very convenient in use, and 

 capable of very considerable sensitiveness and accuracy. 



Experiments have been made so far on aluminium and 

 platinum. The results obtained are given in Table II, and are 

 represented graphically in figs. 5 and 6. The abscissae represent 



Fig. 5. 



in every case the thickness of the absorbing layer; the ordinates 

 represent in the case of the full curves the corresponding values 

 of ///o and in the case of the dotted curves the values of logio ///o . 

 It will be seen that the two substances behave quite differently 

 in their absorption of the /3-rays, and we will consider each 

 separately. 



The curve for aluminium at first descends very gradually and 

 the quantity of rays absorbed increases very slowly with the thick- 

 ness during the initial stages. On adding further thicknesses, 

 however, the curve begins to descend more rapidly, and for part 

 of its course becomes practically a straight line. For greater 



