64:6 Mr. G. Shearer on the 



In this work Crowther measured the ionization produced 

 first in hydrogen and then in air at the same pressure, and 

 calculated the ratio directly. He took precautions to prevent 

 the rays from striking the electrodes so that no electrons 

 would be emitted by them, but there must have been a 

 certain gain of electrons at the ends of the chamber. The 

 total effect of these electrons would increase with an increase 

 impenetrating power of the exciting beam, and we should 

 thiis be led to expect that with an increase in penetrating 

 power there would be an increase in ionization under the 

 conditions in Crowther's work. This would not, however, 

 account for the sudden rise in the ionization between rays 

 corresponding to spark-gaps of 16 and 20 mm. The form 

 of the curve which represents the variation of ionization 

 with hardness of the rays, suggests that there was present in 

 the gas some impurity whose characteristic radiation was 

 excited by rays corresponding to a spark-gap greater than 

 16 mm. 



The latest, and probably the most reliable, work on this 

 subject is that o£ Beatty*, who measured the relative ioniza- 

 tion of hydrogen for various types of X-rays. In this work 

 he employed homogeneous X-rays characteristic of Fe, Ou, 

 Zn, As, and Sn, and used a method which entirely eliminated 

 the effect of the electrons emitted from the walls or the 

 electrodes. The method consisted in obtaining pressure- 

 ionization curves for air and hydrogen. If there were 

 neither a gain nor a loss of electrons at the ends of the 

 chamber the curves would be straight lines, and the ratio of 

 their gradients would give the relative ionization by X-rays f. 

 The effect of the loss or gain of electrons at the ends of the 

 chamber, although important at lower pressures, does not 

 vary with the pressure after that point at which there is 

 enough gas present to absorb completely the energy of the 

 electrons. The admission of more gas is not accompanied 

 by any extra ionization due to these electrons, and any addi- 

 tional ionization observed must in the absence of penetrating- 

 radiations from the ends be due to processes taking place 



* Phil. Mag. [6] xx. p. 320 (1910). 



t By " ionization by X-rays " is here meant the ionization produced 

 directly or by easily absorbed secondary radiations from the gas itself. It 

 is in great part due to the high-speed electrons emitted from the gas 

 under the action of the X-rays ; some ionization may he the result of the 

 action on the gas of soft characteristic X-rays excited in the gas by the 

 original beam of X-rays, these, of course, producing their effect through 

 the agency of the slowly moving electrons which they eject. 



