of Ordinary Materials. 



681 



that, when a large vessel is used, the amount of ionization is 

 not proportional to the pressure, but tends towards a limit, 

 when further increase of pressure no longer affects it. This 

 is exactly the behaviour that might be expected if the effect 

 was due to a feeble radioactivity of the walls of the vessel, 

 the radiation being easily absorbed by the air. I have re- 

 cently carried out a series of experiments, with a view to 

 decide whether the nature of the walls of the vessel had any 

 influence on the rate of discharge of a charged body inside it. 



The figure represents the 

 experimental arrangement 

 adopted, a is a charged wire 

 in the axis of the cylindrical 

 vessel b. The walls of b 

 could be lined with any de- 

 sired material by inserting a 

 cylinder cc composed of it. 

 This could be done by re- 

 moving the glass plate d at 

 the end, which was cemented 

 on. The vessel could be 

 exhausted through the stop- 

 cock / if desired. e was 

 a drying - bulb containing 

 phosphoric anhydride. The 

 wire a passed air-tight 

 through the brass cap g, 

 cemented on to the neck h 

 of b ; h was made of lead- 

 glass, on account of the 

 superior insulating qualities 

 of this kind of glass. The 

 cap g carried a brass strip 

 k carrying a gold leaf I. 

 The whole was surrounded 

 by a vessel m as shown, n 

 was an iron wire attached to 

 a platinum wire o. This iron 

 wire could be brought into 

 contact with h by means of 

 an external magnet in order 

 to charge the system. m 

 could be exhausted through 

 the stopcock p, and dried 

 by means of the phosphoric 

 anhydride contained in <). 

 The position of the gold leaf was read by a microscope with 

 micrometer eyepiece focussed upon it. 



