828 



Mr. E. Marsden on the 



with foils o£ different thicknesses over the remaining three 

 openings. The rates at which the scintillations appeared 

 with the different foils weie expressed as percentages of the 

 number with no foil, and a curve plotted with these per- 

 centages as ordinates and thicknesses of foils as abscissae. 

 Carves obtained for aluminium, copper, tin, and platinum 

 under the same circumstances are shown in fig. 3. 



too 



60 



^ 40 



IV. 











¥ 











\ x 



V\ \ u 











\^N 



LV. ^B/ 



°N^t 







TV 



^ 



N. 



"-»^_ 



•02 -03 



Mass per sq. cm. 



•04 



•05 



The H particles striking the screen will under the experi- 

 mental conditions have widely differing velocities^ and the 

 foils will act by cutting out the ones with the smallest 

 velocities. It will be noticed, however, that for the same 

 mass per unit area, aluminium shows the greatest percentage 

 absorption of the H particles. This is similar to the law of 

 absorption of a particles, and further, as with a. particles, the 

 amounts of the various materials required to produce the 

 same absorption are roughly proportional to the square roots 

 of the atomic weights. This is shown by the figures in the 

 following table. The numbers in Col. IV. are obtained from 

 the curves in fig. 3. 



The determination of the exact ranges of the swiftest H 

 particles will need more observations than are shown in the 

 figure, but the points where the curves cut the axis of abscissae 

 give approximate estimates. The most penetrating H 

 particles will be those which are produced by an end-on 

 collision of an a. particle of radium C just after emergence 

 from the mica window, for such an « particle will be the 

 swiftest entering the hydrogen. The mica had an equivalent 



