102 Dr. J. C. Beattie on the Electrification of 



that such a dynamical analogue may yet be found and suc- 

 cessfully treated. I am, Gentlemen, 



Faithfully yours, 

 8 Upper Hornsey Rise, N., G. JOHNSTONE STONEY. 



June 11, 1897. 



[Note added June 22. — Since the above was written I 

 learn from Dr. Larmor that MacOullagh himself refers to his 

 method of proof in a footnote to one of his papers. I had 

 supposed that our knowledge of it depended solely on 

 tradition. — G. J. S.] 



P.S. — The reader is requested to make the following cor- 

 rections : — 



In the footnote on p. 279 of the April number of the 

 Magazine, third line from the bottom of the page — 



Change " deducted " into "allowed for." The allowance to 

 be made is not a simple deduction. 



In the first footnote on p. 372 of the May number of the 

 Magazine, transfer the words " so as to receive all the light 

 which emerges from it " from the end of the first sentence of 

 the footnote, to the end of the second sentence. The words 

 were inadvertently written in at the wrong place. 



XIII. On the Electrification of Air by Uranium and its 

 Compounds. Bi/ J. Carruthers Beattie, D.Sc, F.R.S. E. 

 With a Note by Loud Kelvin, G.C.V.O., F.R.S.L. <y E., 

 eye, $c* 



§ 1. TT is proposed in the following paper to describe 

 _L experiments made to test the electric state of the 

 air in the neighbourhood of metallic uranium, or of other 

 metals on which a salt of uranium had been deposited from a 

 solution when these metals were charged to a positive or 

 negative potential. 



§ 2. Method employed. — To test the electric state of the air, 

 the electric-filter method due to Kelvin, Maclean, and Gait f 

 was employed. The special filter used in the experiments to 

 be described was a block-tin tube 10 cm. long and 1 cm. 

 diameter, filled with brass filings. This was insulated on two 

 tunnelled pieces of paraffin, and put in metallic connexion 

 with the insulated pair of quadrants of a quadrant electro- 

 meter, whose capacity was j^— of a microfarad. From one 

 of the tunnelled pieces of paraffin a metal tube led to an air- 

 pump ; from the other a piece of indiarubber tubing led to 

 the place where the air to be tested was. This air was then 



* Read before the Royal Society of Edinburgh, June 7th, 1897. 

 Comni unicated by Kord Kelvin. 



t Kelvin, Maclean, Gait, Proc. Roy. Soc. London, March 14th, 1895. 



