weight of the atom. If w be the atomic weight, the stop- 

 ping power of an atom should, therefore, be capable of ex- 

 pression by the formula: — 



where the former term is usually by far the most important. 

 The close agreement of the figures in the second and fifth 

 columns of Table A shows that this is very nearly the case. 



The fifth column in liable A show.s the application of the formula 

 a^iv+lii'K Its clo.se agreement with the second column is remarkable, 

 considering that only tM'o constants are emploj'ed. The formula does not 

 seem to apply to the metals, which rather follow a simple square root law. 

 This is certainly a difficulty. 



