the a. Particles of Radium. 329 



may be placed in the way of the stream, and so rob every 

 particle of some of its energy, but not a single one is brought 

 to rest by collision with the atoms of metal, and the number 

 of particles in the stream remains unchanged. 



In our experiments two different quantities are measured, 

 one the range of the a particles, the other, the ionization 

 which they produce. The range of the a. particle in a given 

 substance is the distance it can penetrate before losing its 

 power of making ions in that substance, which can be 

 collected and measured by electrical means. We assume 

 that when it ceases to have this power, the energy has fallen 

 to an amount which is very small compared to its initial 

 energy (say 1/500), so that even if it varies in different 

 substances, such a variation cannot affect the range appreciably. 

 As a matter, of fact, it is probable that no such variation 

 exists. 



The range of an a. particle in any substance is therefore 

 dependent on the original energy of projection. It is not 

 quite proportional to it, because, as Ave have pointed out 

 already, the a, particle does not spend energy at exactly the 

 same rate at all periods of its flight ; towards the end of its 

 range its rate of expenditure is somewhat increased. If the 

 particle is made to pass through some material other than 

 air, its range may be much altered. For example, if it passes 

 through a thin silver film whose thickness is '001 cm. its 

 range in air, supposing it to be allowed to complete its course 

 in that medium, is diminished by more than 3 cms. Or 

 again, the a. particle which has a range of 7*0 cms. in air has 

 only a range of 3'3 cms. in methyl bromide at the same 

 pressure and temperature. Consequently, if we measure the 

 range of the a. particles in different gases, or after their 

 passage through thin films of matter, we obtain a measure of 

 the comparative rates at which energy is expended by a 

 particle in passing through these gases and materials. 



The second quantity which is measured by us is the 

 ionization produced by the particle at various distances from 

 its source. From this measurement we are able to discover, 

 amongst other things, whether the stream has suffered in any 

 other respect than that of loss of range when it has passed 

 through thin material films, supposing that the rest of the 

 flight is in ordinary air ; and if we compare the quantities as 

 measured in different gases, we are enabled to compare the 

 conductivities which are imparted to these gases by a. particles 

 moving through them under conditions otherwise similar. 



These two quantities are independent of each other. It is 

 true that if the conductivity of a gas were proportional to the 



