1903. J on Some Becent Investigations on Electrical Conduction. 297 



the brass strip h, so that the leaf I diverged. It was found that the 

 leaf moved over less than 3^ of a scale division in the course of three 

 hours. 



As soon as any air was admitted into b, a leakage of electricity 

 from the wire a was observed. 



The following table gives the results, with various materials : — 



Material of Cylinder. 



Zinc .... 

 Phosphoric acid on glass . 

 Aluminium 



Silver, chemically deposited 

 Copper oxide . 

 Copper .... 

 Tinfoil, 1st sample . 

 „ 2nd sample . 

 Platinum, 1st sample 



„ 2nd sample 



„ 3rd sample 

 Uranium nitrate 

 Strongest radium preparations 



Current, in scale 



divisions per hoar. 



1-2 



1-3 



1-4 



1-6 



1-7 



2-3 



2-3 



3-3 



20 



2-9 



3'9 



12,000 



1,200,000,000 



The number for uranium nitrate was obtained by cementing a 

 small piece of the substance on to the inner wall of the cylinder, and 

 determining the rate of leak. The number thus obtained was cor- 

 rected to the value which would correspond to covering the whole 

 cylinder with uranium nitrate. The value for radium is merely 

 computed from the comparisons which have been made between 

 radium and uranium. 



We conclude, then, that the leakage always found in the air is 

 not an essential property of the air itself, but is due to feeble radio- 

 activity in the surrounding solid objects. 



In the experiment I showed you, with the pair of gold leaves 

 hung from a long electrified wire, the leakage was due to Becquerel 

 rays emitted by the walls and floor of the room, and possibly even 

 by the persons of the audience. 



So far we have been speaking of gases of the ordinary kind, that 

 is, the vapours of volatile inorganic substances. 



There are, however, special reasons for thinking that a metallic 

 vapour should behave diffeiently in its electrical relations. 



We will take mercury vapour as an example. 



Let us suppose some mercury placed in a hermetically closed 

 vessel, and let the vessel be heated. The mercury will partially 

 evaporate, and, at any given temperature, there will be a definite 

 density of vapour which is in equilibrium with the liquid, so that 

 no further evaporation takes place. When the temperature is raised, 

 the equilibrium vapour density increases, while the density of the 

 liquid diminishes, so that, if the temperature be increased sufficiently, 

 the liquid and vapour will have the same density, and will be indis- 

 tinguishable from one another. The temperature at which this 



X 2 



