M. Matteucci on the Voltaic Arc. 291 



excrescences. By compelling the two iron points to remain in 

 contact, it is very beautiful to observe the formation in the 

 centre of a double cone of incandescent lava of a very brilliant 

 light ; and which, resting by its bases upon the two metallic 

 points, avidently flows from the positive to the negative pole. 



2. A diffuse light is perceived similar to a flame or lu- 

 minous cloud, but transparent, enveloping the two points; 

 this light varies in colour with the nature of the metals, and 

 resembles every other electric light produced between differ- 

 ent metallic points. Thus it is green with copper, of a dirty 

 yellow with zinc, and violet with platina and silver. 



3. This light or flame is constantly traversed by sparkling 

 points, similar to those which are produced by hammering 

 hot iron : these sparks are principally produced with the iron 

 points, and are always seen to glow outside of the electric 

 flame. 



4. Lastly, some very brilliant and mobile luminous par- 

 ticles, and which seem always to congregate towards the ex- 

 tremity of the point, appear constantly and only at the nega- 

 tive pole. The observation of Dr. Neef is, as regards this, 

 perfectly accurate : it is requisite only to change the direction 

 of the current to observe this light immediately spring from 

 one pole to the other. If there is a drop of oil between the 

 points, the light of the negative pole is concentrated at the 

 extremity only, just as with an exceedingly weak current. If 

 a somewhat powerful current be employed, the phaenomena 

 described are no longer seen distinctly, and the two poles then 

 appear to be equally luminous. 



I have likewise studied the transference of matter by the 

 spark between the two metallic extremities, and employed for 

 this purpose a plate and a point of similar or dissimilar metals, 

 making the plate sometimes positive and sometimes negative. 

 I examined only the plate with the microscope after the 

 experiment ; in every case there was a transference of the 

 positive on to the negative metal, and vice versa. The circular 

 stain which is forlned upon the plate is composed of the cen- 

 tral part, where signs of fusion are apparent, and where the 

 metal transferred from the other pole is deposited ; around 

 this central portion there is a radiating circle of a more or less 

 dark colour, which varies with the nature of the inetals. 

 When the plate is positive, the marks of fusion are greater, 

 and the stains of the metal transferred from the negative pole 

 are scarcely perceptible, whilst the border, of a dark colour, is 

 very lai*ge. The reverse occurs when the plate is negative. 

 When a drop of gum-water or of turpentine is interposed 

 between the point and the plate, it soon becomes charged with 



U2 



