Conductivity of Air enclosed in Metallic Receivers. 303 

 Table V. 



March 6tb, 



2o= Number of ions per c.c. per sec. reduced to Zero centigrade. 



Lead 

 Cylinder. 



By 



Electroscope. 



By 



Electrometer. 



Values obtained 



by 



Prof. McLennan. 



No. 1 



1514 

 135-8 



28-55 



" So" 

 15-42 

 139-6 

 29-21 



" q." 



23 

 160 

 37 



No. 2 



No. 3 





This result seemed somewhat surprising in view of the 

 fact that Prof. McLennan had obtained the same numbers 

 repeatedly during a period of over six months. As the ex- 

 periments made by the writer with the three lead cylinders 

 were carried out in a room in the new physical laboratory, 

 and those by him in the older building, it was thought well to 

 make a redetermination in the latter. On doing this, however, 

 it was found that a slightly lower value even was obtained for 

 the ionization in the measurements taken in this room, than 

 in those in the new laboratory. 



Although the experiments were made in a room supposed 

 to be free or far removed from any active substances, it seems 

 probable that there was in the old department, or in the 

 rooms adjacent, some unobserved source of radiation present 

 during Prof. McLennan's investigation which was absent 

 during the measurements made by the writer, and this 

 impurity was very probably removed when the old laboratory 

 was vacated by the department of Physics and adapted to 

 other purposes. 



It might be pointed out that the lowest value for " q " 

 hitherto recorded for the ionization in a lead cylinder, even 

 with this possible additional influence, is that of 23 ions 

 per c.c. per sec. given by Professor McLennan. The present 

 value of 15 obtained with this cylinder is still lower, and 

 would seem to indicate that we possessed in this lead receiver, 

 one which contained little if any active impurity. It was 

 therefore especially suitable, if proper screens could be found, 

 for the investigation of any intrinsic activity associated with 

 the metal lead itself. 



