362 O. N. Rood on the Electric Spark. 
instant of time, metallic particles are torn off from each electrode, 
as shown by Feddersen, and, in an ignited state, projected in the 
general direction of the other electrode. The source of the light, 
then, is twofold: 1st, from the heated gas, and 2d, from the 
finely divided heated particles of metal. It therefore becomes 
of interest to inquire by what particular portion of the luminous 
matter these photographic figures are traced. 
Making the sensitive plate the negative electrode, for example, 
it might be supposed that the figure formed on that electrode 
was produced, Ist, by the ignited particles charged with negative 
electricity in their act of departure toward the positive electrode, 
and 2d, that it was partly due to the arrival of luminous metallic 
particles from the positive electrode, and 3d, that a portion of it 
was due to the line of heated gas which extends from one elec- - 
trode to the other. That the figure is produced only by the Ist 
and 3d of these causes the following considerations will render 
probable. Feddersen found during the discharge of a Leyden Jar 
that when a resistance was introduced into the circuit sufficient 
to reduce the discharge to a single one, viz: so that in one act 
the positive electricity flowed toward the negative electrode and 
the negative electricity toward the positive electrode, that me- 
tallic particles were torn off from each electrode, and projected 
merely in the general direction of the opposite electrode, being 
often thrown sidewise wide of the mark, or im other words, that 
very often the luminous finely divided metallic particles did not 
seem to bridge over the chasm between the electrodes, which 
were joined sometimes only by a fine line. 
ig. 1 is from two photographs obtained by him under such 
circumstances, enlarged two diame- 
ters. 
ave made some experiments on 
the spark from an electric machine, 
operating with very different appa- 
ratus, but still have obtained corres. 
ponding results. A glass plate, 4 
- 
then firmly pressed together, the tin foil being next to the sé 
tive surface, and a spark from an electric machine dscns e se 
e arrangement: the tin foil plate was then shifted 
