

Energy of ol particles from Radioactive Substances. 117 



proportional to the quantity of matter present. It is con- 

 sequently quite valid to assume, for example, that the heating 

 effect of radium distributed in minute quantities through the 

 earth's crust is proportional to the amount of radium present. 



General considerations. 



We have seen that the velocity of expulsion of the ol par- 

 ticles from various radioactive substances all lie within a 

 fairly narrow range, viz. between l'5b" x 10° and 2*25 X 10° cms. 

 per second. In a previous paper I have shown that the 

 a particle loses its power of producing ionization, of acting 

 on a photographic plate and of producing phosphorescence 

 when its velocity falls to about *-! V , where V is the velocity 

 of the a particles from radium C The value of V has been 

 shown to be 2'0G x 10 9 cms. per sec, so that this critical 

 velocity of the ol particle is about '82 X 10° cms. per sec. It 

 is consequently seen that the velocity of expulsion of the 

 ol particles in general lies approximately between two and 

 three times this critical velocity. An a. particle escaping 

 below this critical velocity would be difficult to detect and 

 would produce little if any ionization. If the a particles 

 from the active substances had been projected on an average 

 with half their observed velocity, it would have been difficult 

 to detect the presence of the ol particles at all. 



In the absence of any definite knowledge of the causes 

 which lead to the successive disintegrations of the atom, it 

 does not seem possible at the present time to give any 

 adequate explanation of the modes of transformation observed 

 in radioactive matter. At the same time, there can be little 

 doubt that the data so far accumulated of the character and 

 period of the changes and of the nature and velocity of the 

 particles emitted will ultimately prove of very great value in 

 obtaining a clearer idea of atomic constitution. 



A study of radioactive phenomena has emphasized the 

 importance of the ol particle as one of the units of which 

 the heavier atoms are built up, and it is not improbable that 

 the a particle may play an equally important role in the 

 constitution of other atoms besides those of uranium, thorium, 

 radium, and actinium. 



McGill University, Montreal. 

 November 1, 1906. 



