216 Mr. W. Wilson on the Effect of Pressure on 



necessary instruments are available, photometrical measure- 

 ments to test this can be made. 



The experiments and observations recorded in this note 

 were made at the Physical Laboratory of the Indian 

 Association for the Cultivation of Science, Calcutta, and 

 formed the subject of a demonstration held at a Special 

 Meeting of that body on the 18th of January last. 



P.S. dated the 26th of Nov. — Photometrical measure- 

 ments in verification of the above have been carried out. 

 These will be dealt with in a future communication. 



XIV. The Effect of Pressure on the Natural Ionization in a 

 Closed Vessel, and on the Ionization produced by the <y Rays. 

 By W. Wilson, M.Sc, Graduate and Hatfield Scholar of 

 the University of Manchester *. 



ri^HE natural ionization in gases has been very largely 

 JL studied since the experiments of Geitel t and C. T. R. 

 Wilson J in 1900. Experiments have been generally made 

 with the object of determining whether ordinary matter is 

 radioactive or not. Wilson observed the effect of diminishing 

 the pressure of the gas inside the electroscope, and found 

 that for pressures of 43 to 743 mm. of mercury, the ionization 

 was proportional to the pressure. Patterson § also studied 

 the effect of varying the pressure and found that with large 

 vessels (20 cms. long, 30 cms. diameter) the ionization 

 increased with the pressure at first, but that at one atmosphere 

 it had become sensibly constant. Strutt || observed that the 

 ionization varied for vessels of different materials ; and 

 McLennan H and Burton found that it was proportional to 

 the pressure up to seven atmospheres. 



Rutherford and Cooke ** observed that the rate of leak 

 was diminished on surrounding the electroscope with lead, 

 thereby proving the existence of a penetrating radiation. 

 Woodft showed that the decrease in ionization depends 

 both on the material of the screen and the material of the 

 ionization vessel. He concludes that the natural ionization 

 is due to an external radiation and its attendant secondary 

 radiation, together with intrinsic rays from the sides of the 



* Communicated by Prof. E. Rutherford, F.R.S. 

 t Geitel, Phys. Zeit. ii. p. 116 (1900). 



% C. T. R. Wilson, Proc. Camb. Phil. Soc. xi. p. 32 (1900) ; Proc. 

 Roy. Soc. lxviii. p. 151 (1901). § Patterson, Phil. Mag. Aug. 1903 



|| Strutt, Phil. Mag. June 1903. 

 •f| McLennan & Burton, Phys. Review, No. 4, 1903. 

 ** Rutherford & Cooke, Am. Phys Soc. Dec. 1902. 

 tt Wood, Phil. Mag. April 1905. 



