330 M. De la Rive's Researches oti the Voltaic Jrc. 



point. Moreover, the curve of the helix is fringed throughout 

 by minute ramifications, precisely similar to the tufts which 

 mark the passage of positive electricity in a Leyden jar. When 

 the plate is negative and the point positive, the marks are 

 totally different, being merely a simple point, or rather a circle 

 of a very small diameter, whence proceeds a line more or less 

 curved, forming a kind of tail to the comet, of which the small 

 circle might be the nucleus: the direction of this tail depends 

 upon the direction in which the luminous arc has been pro- 

 jected. 



When, instead of a plate and a point, two points are used 

 for electrodes, it is evident that no visible trace of this phseno- 

 menon can be obtained ; but both the sharp hissing and the 

 detonations may be produced, which latter are sometimes so 

 loud as to bear a resemblance to distant discharges of mus- 

 ketry. For this the electro-magnet must be very powerful, 

 and the current which produces the arc very intense. I had 

 observed that when 1 took for a positive electrode a point of 

 platinum, and for a negative electrode a point of copper, and 

 placed them between the two poles of the electro-magnet, the 

 production of the voltaic arc between the two poles was ac- 

 companied by a sharp hissing noise ; whereas in the opposite 

 case, the copper being positive, and the platinum negative, 

 the detonations were heard, attended by a frequent breaking 

 of the arc. On examining this phasnomenon more closely, I 

 perceived that the fact I have just meTitioned was due to the 

 platinum becoming heated much more rapidly than the copper 

 when they were employed as electrodes in producing the vol- 

 taic arc ; and I have satisfied myself that in order to obtain 

 the hissing sounds, it is necessary that the positive electrode 

 should be at a sufficiently high temperature to experience a 

 commencement of liquefaction ; for without this condition, 

 only a series of detonations are heard. The hissing would be 

 the result of the easy and continuous transport of matter more 

 or less liquefied from the positive electrode, whilst the deto- 

 nations would be the effect of the resistance opposed by the 

 same matter to the disintegration of its particles when it is not 

 sufficiently heated. Numerous experiments made with metal- 

 lic points^ whether of the same or different natures, as silver, 

 iron, brass, as also platinum and copper, some of which be- 

 come heated sooner than others under the same circumstances, 

 have quite confirmed me in this view of the subject. It is 

 merely necessary to be careful, in order to produce the hissing 

 noise, to maintain as much as possible the continuity of the 

 arc when once the positive electrode becomes incandescent; 

 while, on the other hand, to obtain the detonations, one of the 



