404 Mr. H. Lester Cooke on Penetrating 



(2) an external radiation capable of penetrating the walls 

 of the inclosing vessel : 



(3) a spontaneous ionization of the gas itself ; or 



(4) a combination of any two, or of all three, of the fore- 

 going. 



In order to explain the facts by (3) it must be assumed 

 that the spontaneous ionization is due to the action of the 

 particles of gas on themselves, and not due to any form of 

 radiation or collision between adjacent particles. If this 

 latter were the case the amount of ionization, instead of 

 being simply proportional to the number of particles present, 

 would vary as the square of this number. It would there- 

 fore seem advisable, before accepting this " Spontaneous ^' 

 theory, to examine the other possible explanations of the 

 phenomena. It was on this account that the following 

 research was undertaken. 



It was decided to employ an electroscope in these experi- 

 ments. The observations could be made either with an 

 electrometer and large testing-vessel (or small vessel in which 

 the gas is under high prossure), or by means of an electro- 

 scope, which could be made small and iDortable. As the 

 effects are somewhat small to come within the range of an 

 ordinary electrometer, and as the method of experimentation 

 required the use of heavy metal screens, and necessitated 

 observations being taken at different places and under vary- 

 ing conditions, it was decided that the electroscope most 

 nearly fulfilled the experimental requirements. 



The form of instrument used, which is very similar to that 

 employed by Wilson, is shown diagrammatically in the ac- 

 companying figure (fig. 1). The case of this electroscope is 

 of brass, about 2 millimetres thick, and the volume of gas 

 inclosed measures 1100 c.c. A metal rod, A, passes through 

 an ebonite cork in the centre of the top plate, and is 

 terminated by a sulphur bead B. In the lower end of this 

 bead a strip of rolled copper wire is secured, and to this the 

 gold leaf, which acts as a potential indicator, is fastened. 

 Another wire, C, bent as shown in the diagram, passes 

 through the ebonite cork, and is capable of being turned so 

 as to be brought into electrical contact with the gold leaf 

 system, and thus serves as a charging device. In the bottom 

 of the vessel is placed a small glass dish, covered with per- 

 forated tinfoil, and containing calcium chloride ; this serves 

 the purpose of removing the moisture from the inclosed gas 

 without causing any electrostatic disturbance. The diver- 

 gence of the leaves was read with a microscope having a 100 

 division scale in the eyepiece. 



