Air by Uranium and its Compoimds. 107 



On again connecting both uranium and tinfoil to case the 

 air was found to be positively electrified, the deviation on the 

 electrometer in the first minute indicating a rise of potential 

 of + '65 of a volt. 



It will be seen from these results that, in addition to the 

 electrification deposited in the air and carried to the filter, 

 electricity also passes through the air to the sides of the 

 paraffin cylinder. This electricity, when the uranium and the 

 tinfoil are connected to case, induces the opposite electricity 

 on the uranium, and this is then again given off to the air by 

 the uranium. It will easily be seen from the note appended 

 below why the electrification of the air in the paraffin cylinder 

 at any given potential is greatest when the uranium had not 

 been previously electrified or had been electrified previously 

 with electricity of the opposite kind. 



§ 4. Electrification of Air by Uranium Salts. — Two salts of 

 uranium were used, uranium acetate and uranium nitrate. 

 The nitrate was deposited on a strip of platinum which was 

 then insulated on paraffin in the copper cylinder described 

 above. The same series of experiments was gone through 

 and results of a similar nature to those described in § 3 were 

 obtained. The electrifications of air observed were, how- 

 ever, much smaller in amount. 



The uranium acetate was deposited on tinfoil and placed in 

 a zinc cylinder which was provided with a mica window. By 

 means of this window ultra-violet light from an arc lamp 

 could be caused to shine on the uranium acetate. The 

 electrifications of the air were similar in kind to those already 

 described, and were of about the same amount as those ob- 

 tained with the nitrate of uranium. No change in the 

 electrification of the air from that produced in ordinary 

 daylight was observed when the ultra-violet light was shone 

 on the acetate. 



Physical Laboratory, 

 University of Glasgow, May 1897. 



Note by Lord Kelvin on the sign of the Electrification found in 

 Air drawn from Space surrounding Electrified Uranium. 



In some of our experiments with high voltages we found 

 sparks to pass between uranium and other metals * apparently 

 according to the laws of disruptive discharge subject to but 

 little modification by the special quasi-conductivity produced 

 in air by the " uranium rays." On the other hand, all our 



* Kelvin, Beattie, Smolan, Proc. R. S. E., April 4th, 1897. 



