MAGNESIA. 
gas. Magnesia had, before his time, been 
frequently confounded with lime ; he, how- 
ever, by the most accurate experiments 
shewed that it possessed properties differ- 
ent from all the other earths. Although 
magnesia exists in great abundance in com- 
bination with other substances, it has ne- 
ver been found perfectly pure in nature. 
It is an ingredient in many fossils ; and 
several of the salts, which it forms by com- 
bination with the acids, are found in mine- 
ral springs, and in the water of the ocean. 
From these combinations magnesia is ob- 
tained by different artificial processes. Mr. 
Murray mentions the sulphate of magnesia, 
or Epsom salt, as well adapted to this pur- 
pose. One part of this salt is to be dis- 
solved in twenty of water, and the solution 
filtered ; to this is added, while hot, a solu- 
tion of pure potash or soda, as long as pre- 
cipitation is produced. The alkali com- 
bines with the sulphuric acid, and the mag- 
nesia is separated : being insoluble in water, 
it falls down in white powder: it is then 
washed in water till the fluid comes off 
tasteless. This earth exists under the 
form of a white spongy pow'der, soft to the 
touch, without smell, and having a slightly 
bitter taste. Its specific gravity is 2.3. It 
slightly changes vegetable colours to a green. 
Magnesia, when quite pure, is mfusiblCj 
though exposed to the most intense heat : 
even in the focus of the very powerful 
burning mirror, or in the heat excited by 
oxygen gas, it cannot be melted. When 
made into a paste with water it contracts 
like alumina, if exposed to a sudden heat. 
It is almost insoluble in water. There is 
no action between magnesia and hydrogen, 
or carbon, and very little between it and 
phosphorus. It combines readily with the 
acids, and with them forms neutral salts. 
Of these the greater number are soluble or 
crystallizible, and have a bitter taste. It 
does not enter into combination with the 
fixed alkalies, but in combination with 
some of the other earths, it is fusible by 
means of a very strong heat. With lime, 
in certain proportions, it forms a greenish 
yellow glass. It is much used in medicine 
as a gentle laxative, and as an absorbent to 
destroy acidity in the stomach. It is also 
employed to aid the solution of resinous 
and gummy substances, as camphor and 
opium in water. We shall notice only a 
few of its combinations. 
Magnesia combines with sulphur either 
in the dry or humid way, forming thereby a 
sulphuret of magnesia. The solid sulplm- 
ret of magnesia decomposes rapidly when 
exposed to the air. 
Sulphate of magnesia is a compound of 
sulphuric acid and magnesia, and is found 
in sea water, and in many mineral springs. 
Those at Epsom once afforded a large part 
of what was used in commerce, hence the 
name of Epsom salt. Now indeed it is 
commonly obtained from sea-water. The 
bittern water, or, as it is usually called the 
mother water of common salt, tliat is, the 
water which remains after the crystalliza- 
tion, consists chiefly of sulphate of magne- 
sia. The constituent parts are, according 
to Bergman, 
Sulphuric acid 33 
Magnesia 19 
Water 48 
100 
But Mr. Kirwan gives a different result. 
In crystals. 
Dry. 
.Sulphuric acid .. 
63.32 
Magnesia 
36.68 
Water 
100 
100 
Sulphate of magnesia is formed by pass- 
ing sulphni'ous acid through water, in which 
magnesia is diffused. At first it is in a 
state of powder, which is gradually dis- 
solved, and by exposure to the air, it de- 
posits crystals, and passes into sulphate of 
magnesia. It consists of i 
.Sulphurous acid 39 
Magnesia 16 
Water 45 
100 
Carbonate of magnesia, or the magnesia 
alba, of the physicians, is a very important 
compound. The manufacture of this on 
the large scale is thus conducted. Instead 
of the pure sulphate of magnesia, the bit- 
tern, or liquor remaining after the crystalli- 
zation of sea salt is used, and the magnesia 
is precipitated by carbonate of potash. 
When properly prepared it is perfectly 
white, nearly or wholly tasteless, and very 
sparingly soluble in water. The magnesia 
of commerce is composed of 
Fourcroy, 
Kirwan, 
Carbonic acid .... 
48 
34 
Magnesia 
45 
W'alei' 
21 
100 
100 
