M. De la Rive's Researches on the Voltaic Arc, 327 



exert on the phsenomenon of oxidation and on the nature of 

 the oxide formed. A plate and a point of soft iron were used 

 as positive and negative electrodes, both in a vacuum and in 

 the atmosphere ; the same results appeared with a plate and 

 a point of silver, a plate and a point of copper, and a plate 

 and point of argentane*. The blue light was perceived in 

 all the experiments ; coloured circles were likewise seen on all 

 the plates when they had acted as positive electrodes in rare- 

 fied air. The silver and copper plates presented in this case 

 very decided cavities, caused by the passage of the matter 

 from the positive to the negative pole. The points became 

 incandescent throughout when they served as positive elec- 

 trodes ; whereas when negative, they were heated only at their 

 extremities. The copper point when positive became isolating 

 at its extremity, and it was necessary to excite it by friction 

 in order to renew the experiment. This circumstance is pro- 

 bably attributable to the formation of a thin film of oxide. 

 The point and plate of copper gave out a luminous arc of a 

 beautiful green light, which contrasted in a remarkable manner 

 with the blue reflexion visible in this, as in the other experi- 

 ments. Mercury was likewise employed, both as a positive 

 and negative electrode. In a vacuum as well as in atmospheric 

 air, the luminous effect was most brilliant. The mercury was 

 excessively agitated, rising up in the form of a cone when it 

 was positive, and sinking considerably below the positive point 

 when it was negative. The quantity of vapour thrown off by 

 the mercury during this experiment filled the bell so quickly 

 that it was not easy to observe the details. 



I shall terminate this section by stating a fact which appears 

 to me to be important; it is the influence which the nature of 

 the metallic points forming the electrodes exercises on the 

 temperature which they acquire in relation to the production 

 of the voltaic arc. If the two points are of the same metal, 

 both platinum, or both silver, the positive one alone becomes 

 incandescent throughout its whole length. If the silver point 

 be positive and that of the platinum negative, the latter be- 

 comes incandescent, and the silver one is much less heated. 

 Thus, when the voltaic arc is formed, the circuit must be re- 

 garded as completed, and then it is those parts of the circuit 

 which present the greatest resistance to the current which 

 become the hottest ; at first it is that portion forming the arc 

 itself, and then, in the rest of the circuit, the metal which is 

 the worst conductor. But if the conductors be of the same 

 material on both sides of the arc, or if there be only a slight 



* An alloy of copper and nickel : also known by the names oi pachfong 

 and melchior. 



