438 RIESS ON THE INCANDESCENCE OF 
from wires of platina, palladium, and German silver ; less brilliant 
with silver and brass: they were not at all seen with copper. 
The spreading of the sparks depends upon the brittleness of 
the metal and its affinity for oxygen; it did not appear with 
silver, and with the other metals only in short flashes; but with 
iron very extensively. The formation of the cloud of vapour is 
much more constant than the appearance of the sparks, and did 
not fail in any metal. The facility with which the vapour is 
formed varies with the different metals, but not in a greater de- 
gree than with different wires of the same metal. A certain sort 
of platina wire produced vapour so copiously, that on the first 
discharge, each time, a streak of vapour was formed over the 
whole length of the wire, with other kinds only partial streaks 
appeared. Sometimes the formation of vapour only occurs at 
one discharge and not at the succeeding ones: sometimes, how- 
ever, it is seen in consecutive increasing discharges, but then 
with decreasing force. The formation of vapour is essentially 
promoted by a peculiar state of surface in the wire, and the 
greater or lesser number of furrows left by the wire drawer on 
the wire appears to have considerable influence upon it. I have 
sometimes remarked, that by carefully polishing a wire the pro- 
duction of vapour took place in a less degree than was other- 
wise the case with the same kind of wire. 
Bending of the Wire. 
The phenomena observed on the wire are worthy of notice, 
as they show the powerful effect exerted upon it long before it — 
is brought to a red heat by the electrical discharge; but the 
stage or period of their appearance and their intensity depend, 
as has been remarked, much upon casualties. A better drawn, 
more firmly connected wire, will only show the spark, the vapour, 
and perhaps even the violent vibration with the application of a 
greater quantity of electricity, than a wire less carefully prepared. 
We cannot therefore conclude in general, from the appearance 
of such phenomena, that a certain increase of power of discharge 
will bring a wire to a red heat. It is however different with the 
permanent change in the wire, which must now be noticed, and 
which is intimately connected with the incandescence, and oc- 
eurs immediately before it,—I allude to the angular bend in the 
wire which suddenly takes place at the discharge, as if a sharp- 
edged instrument had been pressed upon it. On the first dis- 
