632 Prof. Townsend and Mr. K irkb y on Conductivity produced 



seems to be no means of determining how much of the energy 

 of Bontgen rays absorbed by the gas goes to produce ions, 

 and how much is spent in heating the gas. In the case of 

 ordinary light traversing a gas, the energy which is absorbed 

 is converted into heat without the genesis of ions. Prof. 

 Butherford and Mr. McClung's results cannot therefore be 

 held to invalidate the collision theory. 



2. We have found that the phenomena which characterize 

 the conductivity of air at low pressures are also to be met 

 with in hydrogen and carbonic acid gas. The experiments 

 with these gases were made in the same manner as those 

 which were previously made with air. Particular care was 

 taken to make the apparatus air-tight. The pressure of the 

 gas as shown by a McLeod gauge did not alter perceptibly 

 during a week. Some of the experiments were repeated with 

 fresh gas, and the same conductivities were obtained. The 

 method of conducting the experiments has already been 

 described in the previous papers. 



When the electric force was reversed a small difference in 

 the conductivity was obtained, which, as has already been 

 explained, arises from the secondary rays emanating from one 

 of the electrodes. The currents were obtained for various 

 electric forces, while the pressure of the gas and the strength 

 of the radiation were kept constant. Several sets of obser- 

 vations were thus made with different pressures. The fol- 

 lowing tables give the mean currents obtained by reversing 

 the force. The pressure of the gas in millimetres of mercury 

 is given at the head of each column. The difference of 

 potential between the electrodes is given in the first column. 

 The sets of observations given in the different columns are 

 numbered for future reference. 



Experiments were made with different strengths of radia- 

 tion, and it was found that when the pressure was constant 

 the ratios of the currents corresponding to the various forces 

 did not depend upon the intensity of the Bontgen rays. No 

 precautions were therefore taken to have the different sets of 

 observations made with the same intensity of radiation. 



In these experiments the current was practically constant 

 for forces in the neighbourhood of 72 volts per centimetre. 

 Experiments 3 and 4 at pressures of 14*5 and 9*5 mms. were 

 not continued for the higher forces in order to avoid taking 

 observations near the sparking potential. The rays used in 

 the experiments at 9*5 mm. pressure were stronger than 

 those used in the other three sets. If we had maintained the 

 same strength of rays throughout, the first numbers in the 

 fourth column would have been very small and difficult to 



