Energy of Rontgen and Becguerel Rays, &c. 249 



Energy required to produce an Ion. 



The current produced when a given volume of the gas was ionized 

 by X rays was determined by means of an electrometer. In order to 

 get rid of the secondary radiations set up when X rays strike on a con- 

 ductor, the rays passed between two charged parallel plates without 

 striking them. A guard-ring method was employed to ensure 

 uniformity of the electric field. 



The value of the ionic energy was deduced from the determination 

 of the current, heating effect, and absorption of the rays. The mean 

 value of the energy required to produce an ion in air at atmospheric 

 pressure and temperature was found to be 



l-90xlO- 10 erg. 



This value is much greater than the energy required to produce 

 hydrogen and oxygen ions in the decomposition of water. 



The ionic energy of air was found to be approximately the same 

 from pressures of one-half to three atmospheres. 



The method of determining the ionic energy for other gases is 

 described, and the evidence that the " ionic energy " is the same for all 

 gases is discussed. 



Distance between the CJuirges of the Ions in a Molecule. 



On the assumption that the energy absorbed in producing an ion is 

 due to the work done in separating the ions against the forces of their 

 electrical attraction, it can be shown that the mean distance between 

 the charges of the ions in the molecule is 



1-1 xlO-cm. 



This is only $ of the probable diameter of the atom. This result is 

 in accordance with the view recently advanced by J. J. Thomson, that 

 ionization is produced by the removal of a negative ion from the mole- 

 cule, and that the negative ion is only a small fraction of the mass of 

 an atom. 



Minimum Potential required to produce a Spark. 



If the production of ions is necessary before a spark can pass, it can 

 readily be deduced from the value of ionic energy that a spark cannot 

 pass for a potential difference less than 175 volts. Experiments have 

 shown that the minimum value is over 300 volts. The theoretical 

 value is of the same order, but from the complexity of the phenomena 

 a very close agreement could not be expected. 



