METALLIC WIRES BY ELECTRICITY. 455 
coating of the battery) and progresses to the negative end, and 
he sees in this an ocular demonstration of the theory that elec- 
tricity is a material substance. My attention was attracted to 
the fact before I had seen that statement ; I observed however, 
with only one exception, exactly the reverse to take place, 
namely, that the incandescence proceeded from the negatively 
charged (outer) coating of the battery towards the positive (inner) 
coating. In order to remove all doubt that an ocular deception 
was effected by the position of the apparatus, I charged a bat- 
tery with positive electricity, and observed clearly the progress- 
ive incandescence of the wire from the outer towards the inner 
side of the wire, and exactly the same when I charged the bat- 
tery with negative electricity. 
Van Marum * pretends to have found, that whenever a wire is 
partially destroyed (he ascribes it in all cases to fusion) always 
that part is affected which is nearest to the positive coating of 
the battery, the remaining whole portion being in connexion 
with the negative coating; and he also explains this by the as- 
sumption of an electrical materia. I found however, with a 
battery charged positively, that sometimes the wires were broken 
off at the outer and sometimes at the inner side, so that it ap- 
peared indifferent whether the positive or negative clamp retained 
the remaining portion of wire. 
From the experiments related at page 454, it follows that the 
quantity of electricity necessary to break asunder a platina wire 
is considerably greater than that which suffices to produce the 
first appearance of incandescence. This quantity of electricity 
must also be increased if the wire is to be broken on the first 
discharge ; for a platina wire which has already passed through 
several grades of incandescence is broken by a discharge which 
would only produce an intense white heat in a new wire. In 
addition to this, all wires used for experiments on incandescence 
must lie slack in the connecting circuit, as wires tightly stretched 
will often be broken by discharges of small quantities of electri- 
city before incandescence occurs, as was mentioned above at 
page 439. 
Wires also of other metals, which are made incandescent by 
the discharge of a certain quantity of electricity through them, 
are broken away from their fastenings by a larger quantity: but 
the exact quantity of electricity to be added to that which already 
produces incandescence, in order that this effect may take place, 
* Beschreibung einer grossen Electrisirmaschine, Erste Fortsetzung, p. 11. 
