M A G N 
\o the brafs by the tools employed, 6r naturally mixed 
with the brafs. jdly. Thofe pieces of brafs which have 
that property, retain it without any diminution after a 
great number of repeated trials, viz. after having been re¬ 
peatedly hardened and foftened. But I have not found 
any means of giving that property to fuch brafs as had it 
not naturally. 4thly. A large piece of brafs has generally 
fl magnetic power fomewhat (ironger thanafmaller piece j 
and the flat furface of the piece draws the needle more 
forcibly than the edge or corner of it. Sthly. If only one 
end of a large piece of brafs be hammered, then that end 
•lone will dirturb the magnetic needle, and not the reft. 
6 -thly. The magnetic power which brafs acquires by ham¬ 
mering has a certain limit, beyond which it cannot be in- 
creafed by farther hammering. This limit is various in 
pieces of brafs of different thicknefs, and likewife of dif¬ 
ferent quality. 7thly. Though there are fome pieces of 
brafs which have not the property of being rendered mag¬ 
netic by hammering, yet all the pieces of magnetic brafs, 
that I have tried, lofe their magnetifm, fo as no longer to 
aft'eCt the needle, by being made red-hot; excepting in¬ 
deed when fome pieces of iron are concealed in them, 
which fometimes occurs; but in this cafe, the pieces of 
hrafs, after having been made red-hot and cooled, will at¬ 
tract the needle more forcibly with one part of its furface 
than with the reft of it; and hence, by turning the piece 
of brafs about, and prefenting every part of it fuccef- 
fively to the fufpended magnetic needle, one may eafily 
difcover in what part of it the iron is lodged. Sthly. In 
the courfe of my experiments on the magnetifm of brafs, 
f have twice obferved the following remarkable circum- 
ftance : A piece of brafs, which had the property of be¬ 
coming magnetic by hammering, and of lofing the mag- 
cetifm by foftening, having been left in the fire till it was 
partially melted, I found, upon trial, that it had loft the 
property of becoming magnetic by hammering ; but, hav¬ 
ing been afterwards failly fufed in a crucible, it thereby 
acquired the property it had originally, viz. that of be¬ 
coming magnetic by hammering. 9thly. I have likewife 
often obferved, that a long continuance in a fire fo ftrong 
as to be little fhort of melting hot, generally diminifhe*, 
and fometimes quite deftroys, the property of becoming 
magnetic in brafs. At the fame time, the texture of the 
metal is confiderably altered, becoming what fome work¬ 
men call rotten. From this it appears, that the property 
of becoming magnetic in brafs by hammering, is rather 
owing to feme configuration of its parts than to the 
admixture of any iron; which is confirmed ftill farther by 
obferving that Dutch plate-brafs, which is made not by 
melting the copper, but by keeping it in a ftrong degree of 
heat whilft furrounded with lapis calaminarin, alfo poflefles 
that property ; at lead, all the pieces of it, which I have 
tried, have that property. From thefe obfervations it fol¬ 
lows, that when brafs is to be ufed for the conftruCtion 
of inftruments wherein a magnetic needle is concerned, as 
dipping needles, variation compaflts, &c. the brafs fliould 
be either left quite foft, or it fliould be chofen of fuch a 
fort as will not be made magnetic by hammering, which 
however does not occur very frequently,” 
Mr. Cavallo alfo examined other metallic fubftances, 
viz. copper, zinc, and platina. The two former manifefted 
no figns of being magnetic ; and of various pieces of pla¬ 
tina, fome did not acquire any magnetifm by hammering, 
and others were rendered evidently attractable by the mag¬ 
net by three orfour ftrokes, and about ten Itrokes gave them 
the fullpower ofwhich they werefufceptible. But,when the 
grains of platina that were made capable of being attracted 
by the magnet under the operation of hammering were 
put upon a charcoal fire, and made red-hot by means of a 
blowpipe, and were afterwards prefented firft to the mag¬ 
net and alfo to the fufpended needle, they ftiowed not the 
leaft fign of attraction. Heat, therefore, deprives them, 
as well as brafs, of the property acquired by hammering. 
Mr. Cavallo concludes upon the whple, from experiments 
Vox.. XIV. Sio. 961. 
E T I S M. 117 
of this kind, that the power of being attracted by the mag¬ 
net may exift, or may belong to other fubftances, indepen¬ 
dent of iron ; and therefore that the attraction of a few par¬ 
ticles of any unknown fubftance by the magnet is not a fure 
fign of the prefence of iron. Although it be true, that iron 
is always attractable by the magnet, yet it does not hence 
follow, that whatever is attracted by the magnet mull be 
iron. Thus amber, and other combuftible minerals, are 
generally attracted by the magnet, Specially after burn¬ 
ing. Of the precious (tones, thofe that are pellucid, aa 
the diamonds find cryftals, are not attracted. The ame- 
thyft, topaz, chalcedony, and generally thofe which are 
deprived of their colour by fire, are not attracted. The 
other precious (tones are all attracted, viz. the ruby, ef- 
pecially the oriental, the chryfolite, and the tourmalins. 
The emerald, and particularly the garnet, are not only 
attracted, but frequently acquire an evident polarity from 
the influence of a ftrong magnet, fo that afterwards they 
are attracted from one fide and repelled from the other. 
The opal is but weakly attracted. Almoft every part of 
animal or vegetable bodies, after combuftion, is in great 
meafure attracted by the magnet. The fle(h, and elpeci- 
ally the blood, after burning, are attracted mod; but the- 
bones are attracted lefs powerfully. Vegetables, after"' 
burning, are almoft all, though not with equal force, at¬ 
tracted by the magnet. But unburned animal or vegeta¬ 
ble fubftances very feldorn, if ever, (how any perceptible 
attraction towards the magnet. Even foot, and the dull 
which ufually falls upon whatever is left expofed to the 
atmofphere, are fenfibly attracted by the magnet. Hence 
it appears that iron, though divided into exceedingly fmall 
particles, is in fome ftate or other mixed with every fub¬ 
ftance; that it is to be found in animals, in vegetables* 
in minerals, and even in the air; that in every ftate of ex- 
iftence it always (bows fome attraction towards the mag¬ 
net; and that its exigence in feveral fubftances can be dtf- 
covered by no other known method befides the magnet. 
But there is reafon for prefuming, that lome bodies inde¬ 
pendent of iron are attracted by the magnet; for the experi¬ 
ments which Coulomb made and repeated before the Na¬ 
tional Inltiture, lead us to conclude, that theaCtion of mag- 
nerifm extends through all nature; for none of the bodies 
he tried was found to refift this power. But, however 
real this action may be, it is not alike in all bodies ; and 
in molt of them it mult be neceflarily very fmall, to have 
efcaped the attention of philofophers to this time. In 
order, therefore, to exhibit, and to meafure, thefe refults; 
we mult begin by placing the bodies in a fituation which 
(hall allow them to yield to the weakelt action. For 
this purpofe, Coulomb fafliioned his fubjects into the 
form of a cylinder or fmall bar; and in this ftate he 
fufpended them to a filken thread, fuch as is drawn from 
the (ilk-worm’s cone, and in this ftate he placed them be- 
tween the oppofite poles of two magnetic bars of lteel. 
The (ingle thread of filk could hardly bear the weight of 2 
quarter of an ounce without breaking, confequently it be¬ 
came neceflary to form fmall bars very light and thin. 
Citizen Coulomb made them about feven or eight milli¬ 
metres in length (or lefs than half an inch), with three- 
fourths of a millimetre (or about an hundredth part of an 
inch) in thicknefs, and he gave the metals about one-third 
of this thicknefs. In his experiments lie placed the fteel 
bars in the fame right line, their oppofite poles being five 
or fix millimetres farther afunder than the length of the 
needle intended to ofcillate between them. The refultof 
the experiment ftiowed, that, whatever might be the fub¬ 
ftance of the needle, it always difpofed it(elf according to 
the direftion of the two bars ; and that, if they were turned 
from this direction, they always recovered it, after ofcil- 
lations of which the number was often more than thirty 
per minute. It was therefore eafy in every cafe to de¬ 
termine, from the weight and figure of the needle, the 
force which had produced the ofcillation. Thefe experi¬ 
ments were fucceisfully made with fmall needles, of sold, 
H b (liver* 
