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



same plate and the same point: when, however, the plate 

 communicates with the positive pole, it is in general double 

 that which it is when the point communicates with the same 

 pole. But in proportion as the strength of the pile is greater, 

 the difference is so much the smaller. 



With respect to the absolute amount of this distance, it is 

 very variable, depending on the strength of the pile, on the 

 nature and molecular state of the electrodes, and on the time 

 occupied in the experiment. Thus, with a Grove battery 

 composed of fifty pairs of plates sixteen square inches in sur- 

 face, it is two or three times greater than with a pile of seventy 

 elements of two or three square inches. With metals easily 

 fused or oxidized, as zinc and iron, it is much greater than 

 with platinum or silver. The duration of the phtenomenon 

 influences the result, inasmuch as the high temperature of the 

 electrodes allows them to be drawn asunder to a greater di- 

 stance without breaking the arc. The same effect may be 

 produced by heating them artificially, by means of a spirit- 

 lamp. It is evident from what I have said that the length of 

 the luminous arc has a relation to the greater or less facility 

 which the substances composing the electrodes possess of being 

 segregated, a facility which may depend upon their tempera- 

 ture diminishing their cohesion, upon their tendency to oxi- 

 dize (which produces the same effect), upon their molecular 

 state, and lastly upon their peculiar nature. Carbon derives 

 from its molecular constitution, which renders it so friable, 

 the property of being one of the substances which produces 

 the longest luminous arc. 



The deposits of the transported matter, form upon the plate, 

 when it is negative and the point positive, a species of very 

 regular ring, the centre of which is the projection of the point 

 upon the plate. This takes place equally, whether the plate be 

 vertical or horizontal, plainly indicating a determinate direc- 

 tion in the transfer of the substance from die positive to the 

 negative electrode; in the air and with metallic electrodes, 

 the deposits always consist of the oxidized dust of the meial, 

 of which the positive electrode is composed. 



I shall here enter into some details. A plate and a point 

 of platinum have been used as electrodes in a vacuum, in air 

 and in hydrogen. In a vacuum with a Grove battery of fifty 

 pairs of plates, which had previously been used, I had only a 

 very feeble effect, and particularly when the plate served as 

 the positive electrode. The point was hardly removed a mil- 

 limetre* from the plate when the arc broke ; to re-establish it, 

 it became necessary to renew the contact between the point 

 * 1 millimetre = 0-03937 inch.— Trans, 



