produced by electricity . 427 
mercury or metal within, either in consequence of mechanical 
motion or heat, alter or suspend their polarity. 
III. Imperfect conducting fluids do not give polarity to steel 
when electricity is passed through them ; but electricity passed 
through air produces this effect. Reasoning on this phaeno- 
menon, and on the extreme mobility of the particles of air, 
I concluded, as M. Arago had likewise done from other con- 
siderations, that the voltaic current in air would be affected 
by the magnet. I failed in my first trial, which I have re- 
ferred to in a note to my former paper, and in other trials 
made since by using too weak a magnet ; but I have lately 
had complete success ; and the experiment exhibits a very 
striking phaenomenon. 
Mr. Pepys having had the goodness to charge the great 
battery of the London Institution, consisting of two thousand 
double plates of zinc and copper, with a mixture of 1168 parts 
of water, 108 parts of nitrous acid, and 25 parts of sulphuric 
acid, the poles were connected by charcoal, so as to make 
an arc, or column of electrical light, varying in length from 
one to four inches, according to the state of rarefaction of 
the atmosphere in which it was produced ; and a powerful 
magnet being presented to this arc or column, having its pole 
at a very acute angle to it, the arc, or column, was attracted 
or repelled with a rotatory motion, or made to revolve, by 
placing the poles in different positions, according to the same 
law as the electrified cylinders of platinum described in my 
last paper, being repelled when the negative pole was on the 
right hand by the north pole of the magnet, and attracted by 
the south pole, and vice versa. 
It was proved by several experiments that the motion de- 
31 
MDCCCXXI. 
