466 Mr. J. P. Gassiot on a Ruhrakorff's Induction Apparatm, 



favour of this analogy, the observation which has just been made 

 by Dr. Robinson (Phil. Mag. April 1858), that the light of the 

 aurora borealis, like the electrical light, possesses the property of 

 rendering those substances fluorescent which are capable of being 

 so, a property which is due to the presence in both lights of the 

 most refrangible rays. 



Believe me, &c., 



A. De la Rive. 



P.S. 19M May.— In my letter of the 10th of May, which 

 contained some details relative to my experiments on the rotation 

 of the electrical light round the pole of an electro-magnet, T 

 have omitted a very important point which I now point out. Of 

 the vapours which I have successively introduced into the balloon,, 

 that which has given me the most brilliant, and at the same time 

 the most constant results, is undoubtedly the vapour of sulphuric 

 sether, at the tension of 10 to 12 millims. of mercury. Not 

 only is the rotation very distinct in one direction or the other, 

 according to the direction of the discharges and the nature of 

 the magnetic pole, but the experiment may be continued for a 

 very long time without the ph?enomenon being changed in ap- 

 pearance or rendered more difficult of perception, at the end of 

 a certain time, by the expansion of the jets round the cylinder, 

 as is the case with the other vapours. The streams are also very 

 apparent and more durable. This difference is probably due to 

 the vapour of sether in the balloon being in a state of greater 

 tension and density than the others, and to its being less rapidly 

 decomposed by the passage of the electrical discharges. 



[It may be as well to point out that Davy first observed the effect 

 of the magnet on the electric discharge in air either dense or rarefied 

 (Phil. Trans. 1821, p. 427). Ritchie perhaps first observed it in 

 water, and at a later date. All these motions and rotations obey the 

 law enunciated by Faraday in his account of electro-magnetic rota- 

 tion, Experimental Researches, 8vo, vol. ii. pp. 130, 131. — Ed.] 



LIX. Description of a Ruhmkorff's Induction Apparatus, con- 

 structed for John P. Gassiot, V.P.R.S., by Mr. Ritchie, 



Philosophical Instrument Maker, Boston, U.S. 



To the Editors of the Philosophical Magazine and Journal. 



Clapham Common, 

 Gentlemen, May 24, 1858. 



THE following description of a Ruhmkorflf's induction appa- 

 ratus, which has been recently constructed for me by Mr. 

 Ritchie of Boston, United States, may probably not be devoid of 

 interest to the readers of the Philosophical Magazine. 



