464 M, A. De la Rive on the Rotation of the 



korff's induction apparatus through Geisslar's tubes*, and the 

 influence which magnetism exerts over them, I recall to your 

 mind that I vA'as the first to point out this influence under a 

 form difi'ereut, it is true, but more remarkable, in my opinion, 

 than that which M. Pliicker has made known. The note which 

 the learned philosopher of Bonn has published respecting this 

 subject induced me to repeat my experiment, and has made me 

 feel the necessity of adding to its description some details neces- 

 sary for the reproduction of the results with certainty. I have 

 just had the opportunity, during a recent residence at Berlin, of 

 making the experiment in the laboratory of M. IMagnus, in the 

 presence of that illustrious philosopher, and of MM. Riess, Dove, 

 Dubois-Reymond, and Wiedemann, all of whom were witnesses 

 of the facility with which it succeeded. 



I have already described, in a letter addressed to M. Regnault, 

 and inserted in the Comptes Rendus of the Academy of Sciences 

 of Paris of 1849, vol. xxix., and also in my 'Treatise on Elec- 

 tricity,'' vol. ii. p. 308, the apparatus intended to show the action 

 in question. It consists of a glass balloon, associated with which 

 is a rod of soft iron, surrounded, except at its two extremities, 

 by a very thick insulating layer; one end of the rod is nearly in 

 the centre of the balloon : the other issuing from the balloon 

 through a tubulature, is external and capable of being placed 

 upon the pole of an electro-magnet. The requisite electrical 

 discharges are set up, when the air is sufficiently exhausted in 

 the balloon, between the interior extremity of the iron rod and a 

 metal ring which surrounds this rod, and outside the insulating 

 layer, at the portion nearest to the tubulature. 



The first time that I made the experiment, I employed an 

 Armstrong's hydro-electric machine to produce the discharges ; 

 subsequently I made use of an ordinary electrical machine, but 

 then the phsenomenon was less distinct, which is due to the in- 

 ferior power of the source of electricity, and especially to the less 

 degree of continuity of the discharges. But as Ai-mstrong's 

 machine is not convenient to manage, I found that it might be 

 replaced with advantage by Ruhmkorfi'^s induction apparatus, by 

 establishing a communication between one end of the secondary 

 coil and the rod of soft iron, and between the other and the inte- 

 rior ring of copper, by means of a wire soldered to this ring, and 

 traversing the tubulature, but still insulated from the soft iron. I 

 should add, that it is necessary for the success of the experiment 

 that the balloon should contain a certain quantity of vapour, of 

 which the tension may be equal to 4 or 6 millims. of mercury ; 

 the aqueous vapour which usually remains in the balloon when 



* [Messrs. Gassiot and Faiaday liave worked together only on the peculiar 

 tubes constructed under Mr. Gassiot's direction. — Ed.] 



