Mr . cavallo’s Magnetlcal Experiments , &c . 63 
needle. The following experiment will ftiew, that an exceed- 
ingly fmall quantity of iron will render a body fenfibly mag- 
netic. 
Having chofen a piece of Turkey-ftone, which weighed 
about an ounce, I examined it by a very fenfible magnetic 
needle, and found that it had not the leaft degree of mag- 
netifm, the needle not being moved from its ufual direction by 
the vicinity of any part of the furface of the ftone ; I then 
weighed a piece of fteel with a pair of fcales that turned with 
the twentieth part of a grain, and afterwards drew one end of 
it over the furface of the ftone in various directions. This 
done, the piece of fteel was weighed again, and was found to 
have loft fo fmall a part of its weight as not to be difcernible 
by that pair of fcales; yet the Turkey- ftone, which had ac- 
quired only that fmall quantity of fteel, affected the magnetic 
needle very fenfibly. Chemiftry feems not to afford any means 
by which fo fmall a quantity of iron may be decifively detected 
in a body that weighs one ounce. Hence it follows, that 
though no iron is to be difcovered in a body by chemical me- 
thods, yet it fhould not be concluded, that the faid body, if it 
affedt the magnetic needle, does not own its magnetifm to fome 
fmall quantity of iron concealed in its fubftance. 
Nickel is a metallic fubftance which has been fufpe&ed to be 
capable of acquiring fome degree of magnetifm independent of 
iron; and this fufpicion has been founded upon obferving, that 
nickel retained its magnetifm after having been repeatedly puri- 
fied*. There are, however, perfons who have denied the 
magnetifm of purified nickel ; and 1 have feen fome pieces of 
it which did not in the leaft affedt the magnetic needle. It is 
probable, that thofe pieces were not pure nickel, and perhaps 
* See Kirwan’s Mineralogy, p, 3^2. and 367. 
fome 
