706 



Messrs. McLennan and Burton on the 



The curves for the different metals, it will be seen, have 

 the same characteristics. In each there is a rapid drop to a 

 minimum and a gradual rise towards an ultimate limiting 

 value : It is interesting to note that a considerable difference 

 was found in the minimum conductivities for the three metals 

 and that the final limiting values also varied. 



The decay of an emanation introduced into the cylinder with 

 the air would again account for the first portion of the curves, 

 a radioactive emanation from the metallic walls would explain 

 the existence of the second portion, while the differences in 

 the minimum and limiting values may be considered to have 

 their origin in variations in the rate at which the emanation 

 is given off by the different metals. 



In this connexion it will be noted that the limiting values 

 of the conductivities range according to the atomic weights 

 of the metals, lead having the highest, tin the next, and zinc 

 the lowest. 



5. Effect of Variations in Pressure. 



In order to investigate the relation between the con- 

 ductivity of air and the pressure at which it was confined, 

 the heavy cylinder was filled to a pressure of about seven 

 atmospheres, and allowed to stand for some days until its con- 

 ductivity assumed a constant value. 



Table III. 



Pressure 



Current : 



Pressure 



Current : 



in cms. 



Arbitrary Scale. 



in cms. 



Arbitrary Scale. 



501-0 



235 



69-3 



5-8 



481-0 



22-7 



620 



5-4 



420-0 



21-2 



530 



4-7 



384-0 



20-3 



44-2 



4-2 



2720 



15-8 



35-0 



3-5 



2270 



14-1 



22-4 



2-7 



1760 



11-7 



18-2 



2-2 



125-0 



9-3 



140 



1-8 



74-2 



6-5 



4-4 



11 



The air was then allowed to escape gradually, and the 

 pressure reduced from 501*0 cms. to 4*4 cms of mercury, the 

 conductivit} r being measured at these and at various inter- 

 mediate pressures. The results are given in Table III., and 

 the conductivity curve in fig. 5. 



The ionization curve so nearly approaches a straight line 

 that we may almost conclude, in view of the wide range of 



