704 Messrs. McLennan and Burton on the 



which had been cut off from a second containing thorium 

 oxide, alter the two had been in communication for some time,, 

 while the second is similar to that given by him for the 

 increase in the conductivity of the air in one chamber when 

 jylaced in communication with another containing thorium 

 oxide. 



It will thus be seen that the first portion of the curves in 

 figs. 2 and 3 can be explained upon the supposition that a 

 radioactive emanation, probably having its origin in the 

 earth's surface, was introduced into the cylinder with the 

 air, the decay of this emanation being the cause of the decrease 

 in the conductivity, and the second portion upon the sup- 

 position that a radioactive emanation is given off by the walls 

 of the containing vessel. On this view the limiting value to 

 which the conductivity curves tend would represent a con- 

 dition of equilibrium, where the rate of decay of the emanation 

 was equal to the rate at which it was produced. 



As both the low and the high pressure cylinders were made 

 of the same material and were of the same size, one would 

 expect the same amount of the emanation to be present in 

 both when the steady state was reached. With an easily 

 absorbed radiation from this emanation, we should obtain a 

 limiting conductivity independent of the pressure. But, 

 since a very great difference was found in the limiting con- 

 ductivities at the tw T o pressures, it would appear that the 

 radiation possesses considerable power of penetration and is 

 not easily absorbed. 



The difference in the initial conductivities given in columns 

 2 and 3, Table L, may also be readily explained by the differ- 

 ence in the air-pressures. The time required to fill the high- 

 pressure cylinder, and the decay taking place during that 

 time in the emanation introduced with the air, preclude a 

 comparison of the amounts of active emanation present in 

 each cylinder when the first observations upon their con- 

 ductivities were made ; but, if the amount in the high-pressure 

 cylinder were equal to or greater than that in the low- 

 pressure cylinder, the difference in the initial conductivities 

 is explained, while, if it were less, the greater density of the 

 air in the high-pressure cylinder, and the consequent greater 

 absorption, would still account for the higher conductivity. 



4. Effect of Different Metals. 



To ascertain whether the conductivity of the inclosed air 

 w^as affected by a change in the metal composing the walls of 

 the receiver, linings of tin and lead were in turn fitted into 

 the zinc cylinder used in the first experiment. 



