THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



♦ 



[FOURTH SERIES.] 



APRIL 1873. 



XXX. On Spectral Lines of Low Temperature. 

 By The Marquis of Salisbury, F.R.S.* 



JF one secondary pole of a powerful inductorium be connected 

 with an insulated metal plate, the other pole being left un- 

 connected, and a thermometer be fixed upright upon the plate, a 

 green light will be visible in the vacuum above the mercury. 

 In order to obtain the effect at its best, the battery should be 

 slightly stronger than is necessary to produce the maximum 

 spark between the secondaries of the coil used; and the plate 

 should be completely insulated. By what kind of electric 

 action this light is produced is not quite clear. Plucker and 

 Mr. Gassiot, and others following them, speak of a similar light 

 produced in a closed tube, without wire electrodes, as being 

 caused by induction. The process appears to me more nearly 

 to resemble conduction, the circuit being completed by leakage. 

 At the point where the bulb rests upon the plate a discharge is 

 visible, oxidizing the plate. At the other end of the thermo- 

 meter a brush-discharge may (in the dark) be seen escaping. If 

 a metal conductor be supported vertically parallel to the ther- 

 mometer, with a slight interval between itself and the plate, and 

 insulated at the other end, a similar discharge and similar, 

 though more abundant, escape will be visible. If a piece of wax 

 be inserted between the metal conductor and the plate, the re- 

 semblance will be closer still. As long as the conductor is 

 there, the light in the thermometer will not appear, or will 

 appear only by flashes. When the conductor is removed, the 

 light returns to the thermometer. It appears, therefore, that 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 45. No. 300. April 1873. R 



