M. De la Rive's Researches on the Voltaic Arc. 325 



and the plate, by touching another point of the plate, the first 

 point which was touched appearing to have undergone such 

 a modification as to prevent the re-formation of the arc. The 

 same effect is produced when the experiments are made in the 

 air, but it ceases when the power of the battery is increased : 

 this is probably due to an augmentation of cohesion consequent 

 on the increase of temperature in that part of the plate which 

 acts as the positive electrode. Besides, when the experiment 

 is made in air, the voltaic arc is more marked and of greater 

 length than when it is made in vacuo, at least if the battery be 

 weak; for when the battery is powerful, composed, for ex- 

 ample, of fifty pairs of plates freshly charged, it appeared to me 

 that the contrary obtained. 1 did not, however, perceive any 

 great difference ; but the vacuum in which I experimented was 

 far from being perfect; it was that of a pneumatic pump, en- 

 closing therefore highly rarefied air. 



In the latter case, that is to say, with the pile composed of 

 fifty pairs strongly charged, and in highly rarefied air, a bluish 

 spot, perfectly circular and presenting the appearance of a 

 coloured ring of Nobili, was formed on the plate of platinum 

 when it served as the positive electrode. The same spot ap- 

 peared in atmospheric air, but its diameter was one-half less, 

 and its colours much less vivid. In hydrogen, no coloured 

 spot was formed ; its formation is therefore evidently the result 

 of the oxidation of the platinum at a high temperature when 

 acting as a positive electrode in the ordinary atmosphere, and 

 still more so, perhaps, in rarefied air*. When the same plate 

 of platinum was made use of as a negative electrode, the point 

 being positive, it became covered with a white circular spot, 

 formed of a vast number of minute grains of platinum, which, 

 having been raised to a high temperature, remained adhering 

 to the surface. The white spot, like the blue one, was much 

 larger in rarefied air than in a vacuum. If the experiment be 

 prolonged for a minute or two when the plate is negative, the 

 rod of platinum terminating in a point,'which is positive, soon 

 becomes highly incandescent ; its end is fused and falls on the 

 plate in the form of a perfectly spherical globule. When the 

 plate is positive and the point negative, the latter is less heated, 

 and does not become fused ; but the plate, unless it be very 

 thick, is liable to be perforated : besides, as may easily be 



* This effect may possibly have been owing to the action of the oxygen 

 brought by the voltaic current into that particular state which Schonbein 

 first described under the name oi ozone. Indeed, in this state the oxygen 

 may attack those metals which are supposed to be inoxidizable ; and M. 

 Marignac and I have shown that this may be effected by causing a succes- 

 sion of electric discharges to pass through the oxygen, even when very dry, 

 with which the phaenomenou of the voltaic arc has a great resemblance. 



