M. De la Rive's Researches on the Voltaic Arc. 323 



nary calorific and electro-chemical phaenomena. The neces- 

 sity of this condition proves the great resistance presented to 

 the passage of the electric current by the minutely divided 

 matter, whatever it may be, which connects the two poles. 



2. The luminous arc cannot exist, unless contact be pre- 

 viously made between the electrodes, and unless these, or at 

 least one of them, be terminated at the point of contact by 

 points fine enough to produce in them an increase of tempera- 

 ture. When this increased temperature is once produced, we 

 may, by separating the electrodes gradually and with pre- 

 caution from each other, obtain the luminous arc, the length 

 of which will depend on the intensity of the pile. Daniell 

 discovered the important fact, which was confirmed by M. 

 Van Breda in a very recent investigation inserted in the 

 Comptes Rendus de V Academie^^, that without contact having 

 taken place, the luminous arc may be produced between two 

 electrodes placed very near together, by causing the discharge 

 of a Leyden jar to pass between them : this is owing to the 

 discharge being always attended by the transference of highly 

 diffused matter, which closes the circuit during the instant of 

 time necessary for the formation of the arc. 



3. Tiie enormous elevation of temperature which accom- 

 panies the production of the luminous arc, is also manifested 

 in the electrodes, especially in the positive ones, which become 

 much more strongly healed than the negative. 



4. Matter is thus transported from the positive electrode to 

 the negative, a fact which may be verified with electrodes of 

 all kinds, but particularly with those of charcoal. 



5. The various phaenomena presented by the voltaic arc, 

 are modified to a greater or less extent by the nature of the 

 electrodes and by that of the surrounding medium. Thus 

 Mr. Grove adduces facts from which it appears that the pre- 

 sence of oxygen is necessary in most cases to produce a very 

 luminous and brilliant arc. It results also from his experi- 

 ments, as well as those of other philosophers, that when two 

 different substances are made use of for the electrodes, it is 

 not a matter of indifference which of the two is placed at the 

 positive pole. 



I now proceed to my own researches. I commenced by 

 studying the production of a luminous arc between a plate and 

 a point of the same material in air, and in vacuo. By means 

 of a micrometer screw I was able to make the point recede 

 from the plate very gradually, and judge of their mutual di- 

 stance with great precision. The limit of distance beyond 

 which the luminous arc ceases to appear, is constant for the 



[* See also p. 638 of the December Number of this Journal for 1846.J 



Y2 



