100 ELECTRONS AND IONS [CH. ix. 



we have reckoned the latter fraction, in the inside 

 of an atom of mercury, as 



=10 

 " 



10 



100,000 x |7r(lO- 13 ) 3 10 5 x 10' 3 



Hence the mean free path of an electron inside 

 an atom of mercury will be comparable to 10 9 times 

 the size of an electron, i.e., it will be 10~ 4 centi- 

 metre ; that is, it may get through, on the average, 

 the substance of some 10,000 mercury atoms in a 

 row, before collision with anything. 



In any other less dense substance it will go 

 further. In ordinary air, on an average free journey, 

 it would escape collision with a hundred million 

 molecules in a room, which would be equivalent to 

 a distance of about four inches. In the case of 

 corpuscles plunging into a dense metal, the actual 

 distance achieved by them is very small, only the 

 thousandth part of a millimetre on the average, and 

 it need by no means necessarily be a straight line; 

 so a target of platinum succeeds in stopping them 

 very near its surface, and enables the X-rays 

 generated by the shock fairly to emerge. Some 

 corpuscles will be stopped more suddenly than this, 

 and some will travel further, but 10~ 4 centimetre, 

 or the thousandth of a millimetre, should be com- 

 parable with the average distance travelled in a 

 solid as dense as platinum. 



Effects of an Encounter. 



This distance, however, gives no notion of the 

 value of the negative acceleration during a collision, 

 because the greater part of that thousandth of a 

 millimetre is free flight; the stoppage occurs only 

 as the last episode of that flight, viz. at the instant 





