﻿792 Dr. Norman Campbell on 



determined by actions quite different from that which results- 

 in the ejection of an electron from an atom into which the 

 ionizing particle penetrates. 



Now the experiments which have been described do not 

 confirm completely Ramsauer's suggestion. Garratt * has 

 shown that of the positive particles emitted from a heated 

 phosphate some 10 per cent, are hydrogen atoms singly 

 charged. According to Ramsauer, such particles ought to 

 begin to display the high penetration and high ionizing 

 power of a-rays when their Telocity is that of 11-volt 

 electrons ; such a velocity they would attain by falling 

 through about 20,000 volts- Assuming, then, that Grarratt's 

 conclusion applies to the conditions of these experiment s y 

 there ought to have been found, according to Ramsauer, a 

 rapid increase in the value of P for values of V greater than 

 20,000. No such increase was found, and so far the more 

 advanced developments of Ramsauer's theory are rendered 

 doubtful. 



On the other hand we may, perhaps, express the funda- 

 mental idea of Ramsauer's theory in the following way : — 

 The great difference in penetrating power and in ionizing 

 power between a-rays and canal-rays is physically similar to 

 the great difference in the same respects between electrons 

 of which the energy is and of which the energy is not greater 

 than 11 volts. In both cnses he supposes the difference to 

 arise from a difference in the power to penetrate the atom. 

 Now in my experiments it is clear that the values of V (if 

 there are such) for which the positive particles investigated 

 become a-rays have not been reached ; accordingly it is to 

 be expected that the relation between P and V should 

 resemble the relation between these quantities for electrons 

 of which the energy is less than 11 volts rather than that of 

 electrons of which the energy is greater than 11 volts. 



This expectation is fulfilled. From v. Baeyer's work f 

 we know that as the energy of cathode rays falling on a 

 metal plate is increased from 0, the number of electrons 

 leaving the plate increases to a maximum at 5 volts and 

 then falls to a sharp minimum just before 11 volts, where 

 the rapid rise begins. 



It is generally believed (for reasons which seem perfectly 

 adequate) that these electrons leaving the plate when the 

 energy of the incident cathode rays is less than 11 volts 

 represent, not an ionization of the plate, but a reflexion of 

 the incident rays. Nevertheless it is possible that the 



* A. E. Garratt, Phil. Mag. xx. p. 573 (1910). 

 t O. v. Baeyer, Pfojs. Zeit. x. p. 176 (1909). 



