[mclennan-kets] electrical CONDUCTIVITY 59 



creased when it is remembered that the experiments were made with 

 a layer of polonium, which had been deposited on the copper plate 

 PQ about nine months previously. This interval should have been 

 sufficient for any beta ray product to have died out, which with any 

 likelihood might have originally contaminated the polonium deposit. 



VIII. Summary of Results. 



(1) In a number of experiments it has been shewn in agreement 



with a number of other experimenters that liquid air 

 when freshly filtered is an extremely good insulator, and 

 that its conductivity in the absence of any ionising radi- 

 ation other than that from the earth is much the same as 

 that of ordinary clean air at atmospheric pressure. 



(2) The dielectric constant of liquid air was found to be 1'43. 



(3) The saturation current obtained in air at ordinary pressure 



by the complete absorption of the alpha radiation emitted 

 by a plate coated with polonium was found to be about 

 16 times the maximum current obtained with the highest 

 fields used when the radiation was absorbed in air at 101 

 atmospheres and about 576 times the maximum current 

 obtained in liquid air when the ionization was produced 

 by the same radiation. 



(4) The mobility of the positive ion produced in air at 116 



atmospheres by alpha rays was found to be 0*0005294 cm. 

 sec. per volt per cm., and that the negative 0-0006217 

 cm. sec. per volt per cm. the latter being about 1 • 18 

 times the former. 



(5) Evidence has been found in the couise of the investigation 



of the existence of a penetrating radiation emitted by 

 the layer of polonium which furnished the alpha rays. 



In conclusion the authors wish to acknowledge the kindness of 

 Professor H. A. Dawes and Mi. P. Blackman, who assisted in taking 

 readings during the course of the investigation. 



