Suspension of the Magnetic Needle. 95 
a quantity of harpsichord wire bent in a great number of di- 
rections, will possess in some parts a polarity attractive, and 
in others repulsive, of the south pole of the needle ; and when 
placed in an earthen vessel, with its bottom somewhat above 
the needle, it attracts the south pole, first by those parts which 
have acquired a north polarity by bending ; and this attrac- 
tion is strengthened by position ; and also position gives north 
polarity to other parts, in other respects indifferent : then, 
during: the effervescence, the heat of the mixture dilates the 
w’re, and in some places thereby bends it one way or other, 
which occasions it to acquire the magnetism of position, whence 
there must sometimes result an increased attraction. From 
these and other experiments it appears, that whenever the 
particles of steel (hard enough to be capable of fixed polarity) 
are by any means displaced, they admit their natural mag- 
netism to become rarefied and condensed by the influence of 
the earth’s magnetic atmosphere ; and thus the effect of elec- 
trical shocks on steel wires may be explained : but it does not 
appear probable that the action of acids increases the magnetic 
attraction of iron-, unless accompanied by other circumstances, 
to which an increased attraction may be more reasonably attri- 
buted. 
EXPERIMENT XVI. 
On reading Mr. Cavallo’s experiments on the increased 
attraction of iron by effervescence, Dr. Darwin was led to in- 
quire, whether inflammable air be magnetic. I therefore, 
at his request, caused inflammable air to issue through a 
paper tube held near the north and south pole of the needle 
alternately ; the air was also received in _ a bladder, and 
