Suspension of the Magnetic Needle. 97 
south polarity ; the other end, being thicker, did not seem to be 
altered. 
Another nail was hammered with its point towards the north, 
which gave it a fixed north polarity. The polarity of these 
hammered nails might be instantly changed, by bending the 
point, whilst held in a contrary position to that in which they 
were hammered. Several oblong pieces of magnetic brass 
were hammered in the same manner, and thereby made to 
possess a north or south polarity, according to their position 
whilst hammered : hence it appears, that the general effect of 
hammering is to harden the metal, whereby it becomes in some 
degree a non-conductor of magnetism, and retains that rare- 
fied and condensed, and therefore more sensible, state of the 
fluid, which is produced by the influence of the earth's mag- 
netic atmosphere. 
EXPERIMENT XVIII. 
In a small crucible I placed six thin plates of copper, and 
betwixt each of them a plate of zinc ; these being melted, and 
cast in a proper mould, produced an oblong piece of brass, 
which was not sensibly magnetic, nor could I produce any 
magnetism in it by hammering. The same quantity of copper 
and zinc were melted, with the addition of some small bits of 
iron. This brass was very sensibly magnetic, and, when ham- 
mered, acquired polarity, by which it more sensibly attracted 
or repelled the needle. Lastly, a piece of copper was melted, 
with the addition of some iron, which was also sensibly mag- 
netic. From these experiments I conclude, that brass owes 
its magnetism to iron ; but that it may sometimes contain so 
small a quantity as not to be sensible till it be hammered. 
O 
MDCCXCI I . 
