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



imagined, the phsenomenon lasts much longer in the latter 

 Ciise. The light is less brilliant, but it is accompanied by a 

 reflexion of a superb blue, which may be seen when the ex- 

 periment is made in the interior of a bell, whether the air 

 be rarefied or not. This blue reflexion is observed on the side 

 of the bell, and is to be seen whatever may be the nature of 

 the electrodes, or the colour of the light to which these give 

 rise in the centre of the bell ; only when this central light is 

 very brilliant, it becomes slightly paler by the effect of contrast. 



I substituted for the platinum point a point of coke, but 

 the plate of platinum remained ; this being positive and the 

 point negative, I obtained a luminous arc more than double 

 the length of the arc produced by the point of platinum. 

 With respect to the arc, instead of its being a cone of light, 

 having its base on the plate and its apex at the point, as was 

 the case when the latter was platinum, it was composed of a 

 multitude of luminous jets diverging from different points of 

 the plate, and tending to various parts of the point of coke. 

 This fact shows clearly the influence that may be exercised by 

 the negative electrode, the function of which is very far from 

 being a merely passive one. Let me add, that although the 

 strength of the pile was precisely ihe same as when the point 

 was of platinum, not only was the luminous arc much longer 

 with the point of coke, but the heat developed in the plate of 

 platinum was so much greater that it was soon melted and 

 perforated. The coke being positive and the plate negative, 

 the length of the arc was less than in the preceding case, and 

 particularly so in air, where it was sensibly less than in a 

 vacuum. The heat generated was however still very great, 

 the point of coke becoming quickly incandescent throughout. 

 I ought to add, that with the point of coke, the luminous arc 

 was so brilliant that the blue light which I have mentioned 

 almost entirely disappeared, which was not the case with any 

 other kind of point. 



Leaving the plate of platinum, I adjusted a zinc point. The 

 effects were most brilliant, but of short duration, the point 

 speedily melting. In common air, a deposit of white oxide 

 was precipitated upon the platinum plate; in highly rarefied 

 air (the vacuum of an air-pump), a black deposit was formed : 

 in both cases it communicated with the positive pole. An 

 iron point being substituted for that of zinc, equally produced 

 in common air a brownish-red deposit of oxide of iron, and in 

 rarefied air a deposit of black oxide. 



I call the attention of chemists to these two facts, as well as 

 that of the oxidation of the platinum at a high temperature in 

 rarefied air. They appear to prove the influence which the 

 state of greater pr iess density of the surrounding oxygen may 



