6 Prof. E. Edlund on the Electrical 



Geissler tubes furnished with platinum electrodes which had 

 been soldered to them, tubes from which the air had been ex- 

 pelled as completely as possible, the current of a Ruhmkorff 

 induction-apparatus passed at first with a whitish light. In the 

 continuation of the experiment, however, the discharge soon be- 

 came interm ittent, to cease totally at the end of a few minutes. 

 Pliicker attributes the cause of this phenomenon to the cir- 

 cumstance that the oxygen of the insignificant quantity of air 

 which was still present in the tube was absorbed by the pla- 

 tinum electrodes, after which the nitrogen remaining was un- 

 able to transmit the discharge. If now strips of tinfoil were 

 placed round the tube near its two extremities, but insulated 

 from the platinum electrodes, and the poles of the induction- 

 apparatus put in contact with the tinfoils, the tube recom- 

 menced shining with electric light, indicating that induced 

 currents were thus*produced in the vacuum by the charge and 

 discharge of the tinfoils. If after some time the electrodes 

 were connected with the poles of the induction-apparatus, the 

 discharges again passed at first with a whitish light, soon 

 became intermittent, and ended by totally ceasing. After- 

 wards, on the poles of the apparatus being connected with 

 the tinfoils, the tube was again seen to shine with electric 

 light ; and these alternations could be continued ad libitum. 

 When the electric light was explored with a magnet, it ap- 

 peared that the induced current producing it circulated with 

 a to-and-fro movement, as there was every reason to expect. 

 Pliicker's experiments consecmently show that, if the air of 

 the tube is so rarefied that the current of the induction-appa- 

 ratus cannot traverse it, it is nevertheless possible to produce, 

 with the same apparatus, induced currents in the rarefied air 

 — that is to say, to set in motion the electric material present 

 there. These experiments therefore entirely confirm the 

 above-mentioned experiments of Gassiot*. 



Lastly, Hittorf f succeeded in producing, by the simulta- 

 neous employment of the air-pump and intense heating of the 

 tube, a still more perfect vacuum than that obtained by Gas- 

 siot with the aid of pure carbonic acid. Even when the dis- 

 tance between the electrodes amounted to only 2 or 3 millim., 



* von Waltenhofen deduces frcin his researches the result that, as is 

 natural, the form of the electrodes influences the limit of rarefaction at 

 which the cm-rent ceases to traverse the rarefied gas. He remarks, in 

 consequence, that if it were possible to introduce the current into the rare- 

 fied gas without the aid of electrodes, the discharge woidd take place even 

 when the rarefaction was carried so far that a discharge could not be pro- 

 duced with the aid of the electrodes (Pogg. Ann. cxxvi. p. 537). 



t Pogg. Ann. cxxxvi. p. 201. 



