130 PHYSICAL SCIENCE 



electric field be kept acting in one direction, all 

 the positive ions produced by the Rontgen rays 

 will go to one electrode, and all the negative ions 

 to the other. But if the electric force be reversed 

 before all the ions get across, the charge received 

 by an electrode would be less than before. Thus, 

 measurement of the charges received by the elec- 

 trodes with different speeds of reversal will give a 

 means of calculating the velocities of the ions. At 

 atmospheric pressure, under a potential gradient 

 of I volt per centimetre, the velocities of different 

 ions vary from about three-quarters of a centi- 

 metre per second in the case of carbon dioxide, to 

 about 7 centimetres per second in the case of 

 hydrogen. The velocity of the negative ion is, 

 in general, appreciably greater than that of the 

 positive ion, the ratio, unity for carbon dioxide, 

 rising to 1.24 for air and oxygen. 



We should expect the velocity of an ion to be 

 inversely proportional to the pressure of the gas, 

 and this has been found to be the case with the 

 positive ions. The mobility of the negative ions, 

 on the other hand, increases with decreasing pres- 

 sure much faster than this expectation justifies, 

 and at low pressures, 100 millimetres of mercury 

 and less, the change is very marked. This result 

 indicates an alteration in the nature of the ions 

 themselves, and justifies the belief that they must 

 possess more complex structures at high than at 

 low pressures. 



We shall see later that, at the very low 

 pressures which exist in good vacuum tubes, it 

 is possible to estimate the absolute mass of the 

 ions, with the remarkable result that, whereas 

 the mass of the positive ion appears to be much 



