IRON. 
of this metal, that all substances which ex- 
hibit magnetism do contain iron ; but it 
must be confessed, that there remain many 
experiments to be made among the earths 
and powders which exhibit magnetical pro- 
perties, before this negative proposition, 
which coniines magnetism to iron, can be 
admitted as proved. 
When iron is exposed to the action of 
pure water, it acquires weight by gradual 
oxydation, and hydrogen gas escapes : this 
is a very slow operation. But if the steam 
of water be made to pass through a red hot 
gun-barrel, or through an ignited copper or 
glass tube, containing iron wire, the iron 
becomes converted into an oxide, while hy- 
drogen gas passes out at the other end of 
the barrel. The action of- air, assisted by 
heat, converts iron into a black oxide, con- 
taining twenty-five of oxygen. By the ac- 
tion of stronger heat this becomes a red- 
dish brown oxide, containing forty-eight of 
oxygen. The yellow rust, formed when 
iron is long exposed to damp air, is not a 
simple oxide, as it contains a portion of 
carbonic acid. According to M. Chene- 
vix, there are four stages of oxydation of 
iron : the first, or minimum, white ; the se- 
cond, green; the third, black; the fourth, 
or maximum, red. Thenard admits only 
three, the white, green, and red. 
The concentrated sulphuric acid scarcely 
acts on iron, unless it is boiling. If the 
sulphuric acid be diluted with two or three 
parts of water, it dissolves iron readily, 
without the assistance of any other heat than 
what is produced by the act of combination, 
During this solution, hydrogen gas escapes 
in large quantities. 
Sulphate of iron is not made in the direct 
way, because it can be obtained at less 
charge from the decomposition of martial 
pyrites. It exists in two states, one 
containing oxide of iron, with .27 of 
oxygen, which is of a pale green, not al- 
tered by gallic acid, and giving a white 
precipitate with prussiate of potash. The 
other, in which the iron is combined with 
.48 of oxygen, is red, not crystallizable, 
and gives a black precipitate with gallic 
acid, and a blue with prussiate of potash. 
In the common sulphate these two are mix- 
ed in various proportions. 
Distillation separates the acid from sul- 
phate of iron, and leaves the brown oxide 
of iron, called colcothar. 
Vegetable astringent matters, such as 
nut-galls, the husks of nuts, logwood, tea, 
&c. which contain the gallic acid, precipi- 
tate a fine black fecula from sulphate of 
iron, which remains suspended for a con- 
siderable time in the fluid, by the addition 
of gum arabic. This fluid is well known 
by the name of ink. See Ink. - 
The beautiful pigment, well known in 
the arts by the name of Prussian blue, is 
likewise a precipitate afforded by sulphate 
of iron. 
If two parts of alum, and one of sulphate 
of iron, be dissolved in eight or ten parts 
of boiling water, and a solution of prussiate 
of potash be added as long as any efferves- 
cence and precipitation are produced, the 
precipitate, thoroughly washed by affusion 
of boiling water, will have a green colour. 
This is owing to the yellow oxide of iron 
thrown down with the prussiate, which must 
be dissolved by adding muriatic acid. The 
deep blue powder, insoluble in this acid, is 
then to be washed and dried for use. Ac- 
cording to Professor Proust, the iron in 
Prussian blue contains .48 of oxygen, 
and is obtained only from a super-oxy- 
genated sulphate; the precipitate from 
a pure alkaline prussiate and sulphate of 
iron with a minimum of oxygen being 
white, and containing only .27* of oxy- 
gen. This may explain a fact observed by 
a French colourman, who, having mixed 
some Prussian blue and white lead with nut 
oil, and set it by for some time covered 
with water, found the surface only blue, 
and all the rest white. On pouring it out 
on his stone, and beginning to grind it 
afresh, with intention to add more Prussian 
blue, he found the colour gradually returning 
of itself. Here it might be supposed that the 
oxide of the prussiate had parted with oxy- 
gen to the oil, or the oxide of lead, or both, 
thus becoming white ; except that on the 
surface, which was supplied with oxygen 
from the superincumbent water ; and that 
it recovered its colour by attracting oxygen 
from the air. But on this supposition it 
would seem, that light must contain oxygen, 
since the colour of this paint, spread on 
wood or paper, returned by exposure to 
light in vacuo as well as in the open air. 
The colour of Prussian blue is affected by 
the contact of iron. Mr. Gill, finding a 
knife with which he was mixing some Chi- 
nese blue acquire a green tinge, spread a 
little of it, and afterward a little Prussian 
blue, sufficiently diluted on the blade of a 
knife, and with a camel hair pencil took off 
enough to form a tint on paper, and thus 
continued, till lie had taken off in the first 
instance thirty-six, and in tire second eighty- 
