MAG 
MAG 
M A G 
93 
the while holding his hand to his breast, and 
at last solemnly swearing faithfully and in- 
violably to observe all the things therein 
contained, &c. ; then the bishops extinguish- 
ing the candles, and throwing them on the 
ground, cried out, “ Thus let him be extin- 
tinguished, and stink in hell, who violates 
this charter.” It is observed, that notwith- 
standing the solemnity of this confirmation, 
king Henry, the very- next year, again in- 
vaded the rights of his people, till the barons 
entered into a war against him ; when, after 
various success, he confirmed this charter, 
and the charter of the forest, in the fifty- 
second year of his reign. This excellent 
charter, so equitable and beneficial to the 
subject, is the most antient written law in the 
kingdom : by the 25 Edw. I. it is ordained, 
that it shall be taken as the common law ; 
and by the 43 Edw. III. all statutes made 
against it are declared to be void. 
MAGNESIA. About the begi lining of 
the eighteenth century, a Roman canon ex- 
posed a white powder to sale at Rome as 
a cure for all diseases. This powder he 
called magnesia alba. He kept the manner 
of preparing it a profound secret ; but in 1707 
Valentini informed the public that it might 
be obtained by calcining the lixivium which 
remains after the preparation of nitre ; and 
two years after, Slevogt discovered that it 
might be precipitated by potass from the 
mother-ley of nitre. This powder was gene- 
rally supposed to be lime, till Frederic Hoff- 
man observed that it formed very different 
combinations with other bodies. But little was 
known concerning its nature, and it was even 
confounded with lime by most chemists, till 
Dr. Black made his celebrated esperimen's 
on it in 1755. Margraff published a disser 
tation on it in 1759* and Bergman another in 
1775, in which he collected the observations 
of these two philosophers, and which he en- 
riched also with many additions of his own. 
Butini of Geneva likewise published a valu- 
able dissertation on it in 177p. 
As magnesia has never yet been found 
native in a state of purity, it may be pre- 
pared in the following manner: sulphat of 
magnesia, a salt composed of this earth and 
sulphuric acid, exists in sea-water, and in 
many springs, particularly in some about 
Epsom; from which circumstance it was for- 
merly called Epsom salt. This salt is to be 
dissolved in water, and half its weight of pot- 
ass added. The magnesia is immediately 
precipitated, because potass has a stronger 
affinity for sulphuric acid. It is then to be 
washed with a sufficient quantity of water, 
and dried. 
Magnesia thus obtained is a very soft white 
powder, which lias very little taste, and is 
totally destitute of smell. Its specific gravity 
fs about 2.3. It converts delicate vegetable 
blues (paper for instance, stained with the 
petals of the mallow) to green. 
It is not melted by the strongest heat which 
it has been possible to apply ; but M. Darcet 
observed that, in a very high temperature, 
it became somewhat agglutinated. When 
formed into a cake with water, and then ex- 
posed to a violent heat, the water is gradually 
driven off, and the magnesia contracts in its 
dimension ; at the same time, it acquires the 
property of shining in the dark when rubbed 
upon allot iron plate. 
It is almost insoluble in water ; for, ac- 
cording to Mr. Kirwan, it requires 7900 
times its weight of water at the temperature 
of 60° to dissolve it. It is capable, however, 
of combining with water in a solid state ; for 
100 parts of magnesia, thrown into water, 
v.nd then dried, are increased in weight to 
118 parts. Even when combined with car- 
bonic acid (for which it has a strong affinity) 
it is capable of absorbing and retaining l| 
times its own weight ot water without letting 
go a drop; but on exposure to the air, this 
water evaporates, though more slowly than 
it would from lime. 
Magnesia has never yet been obtained in 
a crystallized form. 
When exposed to the air, it attracts car- 
bonic acid gas and water ; but exceeding- 
ly slowly. Butini left a quantity of it for two 
years in a porcelain cup merely covered with 
paper ; its weight was only increased — im- 
part. 
Magnesia does not combine with oxygen ; 
nor is it altered by any of the compounds 
into which oxygen enters. The only one of 
the simple combustibles with which it can 
be united is sulphur. No person has hitherto 
succeeded in forming a phosphuret of mag- 
nesia. The sulphuret of magnesia may be 
formed by exposing a mixture of two parts 
of magnesia and one part of sulphur, to a 
gentle heat in a crucible, The result is a 
yellow powder, slightly agglutinated, which 
emits very little sulphureted hydrogen gas, 
when thrown into water. A moderate heat 
is sufficient to drive off the sulphur. 
Magnesia does not combine with azote, 
but it unites with muriatic acid, and forms a 
compound called muriat of magnesia. It 
has no action upon the me als : nor does it 
combine, as far as is known at present, with 
the metallic oxides, unless some intermedi/te 
substance is present. It does not combine 
with tiie fixed alkalies, neither are its proper- 
ties altered by these bodies; but it. has a 
strong propensity to enter into triple com- 
pounds with ammonia. 
There seems to be little affinity between 
magnesia and barytes; at least no mixture 
of the two earths is fusible in the strongest 
heat which it has been possible to apply. 
Mr. Kirwan has shown that there is but 
little affinity between strontian and magnesia. 
They do not melt when exposed to a strong 
heat, at least when the strontian exceeds 
or equals the magnesia. 
Equal parts of lime and magnesia, mixed 
together, and exposed by Lavoisier to a 
very violent beat, did not melt ; neither did 
they melt when Mr. Kirwan placed them in 
the temperature of 150° Wedgewood. 
The affinities of magnesia, according to 
Bergman, are as follows : 
Oxalic acid, 
Phosphoric, 
Sulphuric, 
Fluoric, 
Tartaric, 
Citric, 
Lactic, 
Benzoic, 
Arsenic, 
Saclactic, 
Succinic, 
Nitric, 
Muriatic, 
Acetic, 
Boracic, 
Sulphurous, 
Carbonic, 
Prussic. 
Magnesia is used in medicine, to remove 
acidities. 
MAGNETISM. The natur 1 magnet, or 
loadstone, is a hard mineral bo ;iy of a dark 
biown, or almost black colour, and when 
examined, Is found to be an ore of iron. 
It is met with in various countries, generally 
in iron mines, and of all sizes and forms. 
This singular substance was known to the 
antients ; and they had remarked its peculiar 
property of attracting iron, though it does 
not appear that they were acquainted with 
the wonderful property which it also has, of 
turning to the pole When suspended, and left 
at liberty to move freely. 
Upon this remarkable circumstance tiny 
mariner’s compass depends, an instrument 
which gives us such infinite advantages over 
the antients. it is this which enables the 
mariners to conduct their vessels through 
vast oceans out of the sight of land, in any 
given direction; and this directive property 
also guides the miners in their subterranean 
excavations, and the traveller through de- 
serts otherwise impassable. 
It is not precisely known when and by 
whom this directive properly of the magnet 
was discovered. The most probable ac- 
counts seem to prove, that it was known 
early in the 13th century; and that the per- 
son who first made mariner’s compasses, at 
least in Europe, was a Neapolitan of file- 
name of Flavin, or Jolmde Gioga, or Giova,. 
or Gira. 
Tire natural loadstone lias also the qua- 
lity of communicating its properties to iron, 
and steel ; and when pieces of steel properly- 
prepared are touched, as it is called, by the- 
loadstone, they are denominated artificial, 
magnets. 
These artificial magnets are even capable 
of being made more powerful than the na- 
tural ones; and as they cap be made of aav 
form, and are more convenient, they are'’ 
now universally used, so that the loadstoner 
or natural magnet is only kept as a curiosity.. 
All magnets, whether natural or artificial,, 
are distinguished from other bodies by the 
following characteristics, which appear to be* 
inseparable from their nature ; so that no. 
bo.iy can be called a magnet, unless it is. 
possessed of all these properties : 
1. A magnet attracts iron. 
2. When a magnet, is placed so as to be 
at liberty to move freely in every direction, 
its ends point towards the poles of the earth, 
or very nearly so ; and each end always 
points to the same pole. This is called the 
polarity of the magnet ; the ends of the mag- 
net are called poles ; and they are called 
north and south poles oi the magnet, accord- 
ing as they point to the north or south pole 
of the earth. When a magm t places itself.*' 
in this direction, it is said to traverse. 
3. When the north pole of one magnet hr* 
presented to the south of another magnet,, 
these ends attract each other; but if the 
south pole of one -magnet is presented to the 
south pole of another,, or the north pole oft 
one to the north pole of another, these ends 
will repel each other. 
From these criteria, it is easy to ■■determine* 
the names of the poles of a magnetical bar,, 
by applying it near a suspended magnet whose 
poles are known. 
4. When a magnet is situated so a; to he 
at liberty to move itself with sufficient free- 
dom, its two poles- do not lie in a horizontal 
direction, but it generally inclines' one of 
them towards the horizon, and of* coarse it. 
elevates the other pole above it. Tins is 
