this is true to such an extent that there is scarcely an invention
claiming the reader’s attention in this book but what involves the
indispensable use of these materials. Again, in their production on the
large scale it will be seen that there is a mutual dependence, and that
this is made possible only by means of the invention we have begun with;
for without the steam engine the deep coal mines could not have the
water pumped out of them,—it was indeed for this very purpose that the
steam engine was originally contrived,—nor could the coal be efficiently
raised without steam power. Before the steam engine came into use iron
could not be produced or worked to anything like the extent attained
even in the middle of the nineteenth century, for only by steam power
could the blast be made effective and the rolling mill do its work. On
the other hand, the steam engine required iron for its own construction,
and this at once caused a notable increase in the demand for the metal.
Once more, the engine itself supplies no force; for without the fuel
which raises steam from the water in the boiler it is motionless and
powerless, and that fuel is practically _coal_. In consequence of thus
providing power, and also of supplying a requisite for the production of
iron, coal has acquired supreme industrial importance, so that all our
great trades and places of densest population are situated in or near
coal-fields. But what we have further to say about coal may be
conveniently deferred to a subsequent article, while we proceed to treat
of iron, and of the contrivances in which it plays an essential part.
Iron has also been called “the mainspring of civilization,” and the
significance of the phrase is obvious enough when we consider the
enormous number and infinite variety of the things that are made of it:
the sword and the ploughshare; all our weapons of war and all our
implements of peace; the slender needle and the girders that span wide
rivers; the delicate hair-spring of the tiny watch and the most
tenacious of cables; the common utensils of domestic life and the huge
battle-ships of our fleets; the smoothest roads, the loftiest towers,
the most spacious pleasure palaces. Such extensive applications of iron
for purposes so diverse have been rendered possible only by the greater
facility and cheapness of production, together with the better knowledge
of the properties of the substance and increased skill in its treatment,
that have particularly distinguished our century. Apart again from the
constructive uses of iron, it enters essentially into another class of
inventions of which the age is justly proud, namely, those which utilize
electricity in the production of light, mechanical power, and chemical
action; for it is on a quality possessed by iron, and by _iron alone_,
that the generation of current by the electric dynamo ultimately
depends. This peculiar property of iron, which was first announced by
Arago in 1820, and has since proved so fertile in practical
applications, is that a bar of the metal can, under suitable conditions,
be instantly converted into the most powerful of magnets, and as quickly
demagnetized. What these conditions are will be explained when we come
to treat of electricity.
[Illustration:
FIG. 16.—_Aerolite in the British Museum._
]
Besides the unique property of iron just referred to, and its
superlative utility in arts and industries, there are other
circumstances that give a peculiar interest to this metal. It is the
chief constituent of many minerals, and traces or small quantities are
found in most of the materials that make up the crust of the earth; it
is present also in the organic kingdoms, being especially notable in the
blood of vertebrate (_back-boned_) animals, of which it is an essential
component. Notwithstanding its wide diffusion, iron is not found
_native_, that is, as _metal_, but has to be extracted from its _ores_,
which are usually dull stony-looking substances, as unlike the metal as
can be conceived. In this respect it differs from gold, which is not met
in any other than the metallic state, in the form of _nuggets_, minute
crystals or branching filaments, and from metals such as silver, copper,
and a few others which also are occasionally found native. It is true
that rarely small quantities of metallic iron have been met with in the
form of minute grains disseminated in volcanic rocks; but in contrast
with the practical absence of metallic iron from terrestrial accessible
materials is the fact that masses of iron, sometimes of nearly pure
metal, occasionally descend upon the earth from interplanetary space.
These are _aerolites_, of which there are several varieties, some
consisting only of crystalline minerals without any metallic iron,
others of a mixture of minerals and metals, but the most common are of
iron, always alloyed with a small quantity of nickel, and usually
containing also traces more or less of a few other metals and known
chemical elements. The iron in some specimens has been found to amount
to 93 per cent. of the whole. These _aerolites_, or _meteorites_, as
they are also called, are of irregular shape and vary greatly in size,
which however is sometimes very considerable: one found in South America
was calculated to weigh 14 tons, another discovered in Mexico, 20 tons.
There is in the British Museum a good specimen of an iron meteorite,
which is represented in Fig. 16, where it will be observed that a
portion has been cut off to form a plane surface, which when polished
and etched by an acid, reveals a crystalline structure quite peculiar
and distinctive, so that such meteorites can be recognized with
certainty, even if they did not possess surface characters which are
easily observed and identified when once a specimen has been examined.
The fall of meteorites to the surface of the earth is comparatively
rare, but it has been witnessed by even scientific observers; as when
Gassendi, the French astronomer, saw in Provence the fall of a meteorite
weighing 59 lbs. In the _Transactions of the Royal Society_ for 1802 may
be found a detailed account of an instance in England of the fall which
took place in Yorkshire, on the 13th December 1795, of a stone 56 lbs.
in weight. Aerolites become ignited or incandescent by reason of the
great velocity with which they pass through the atmosphere, whereby the
air in front of them is condensed and heated, the heat often being
sufficient to liquefy or even vaporize the solid matter. The so-called
shooting stars are with good reason believed to be nothing but such
incandescent aerolites, and the aerolites themselves are regarded as
small asteroids, or scattered planetary dust, portions of which
occasionally coming within the sphere of the earth’s attraction are
drawn to its surface. Meteoric iron is too rare to be of any value as a
source of iron, but certain specimens have been found in which the metal
was malleable and of excellent quality. From such meteorites the natives
of India and other places have, it is said, sometimes forged weapons of
wonderful temper and keenness, and we may well imagine that when such
weapons have been made from iron that had actually been observed to fall
from the sky, they would be regarded as endowed with magical powers, so
that we may perhaps ascribe to such circumstances the origin of some of
the legends about enchanted swords, etc. It is significant also that in
some Egyptian inscriptions of the very highest antiquity, the word
indicating iron has for its literal meaning _stone of the sky_.
But as nature has hardly provided man with the _metal_ iron, he has been
obliged to find the art of extracting it from substances which are
utterly unlike the metal itself. In this case, as in many others, the
art has been discovered and practised ages before any scientific
knowledge of the nature of the processes employed had been acquired. The
idea prevails that there are such difficulties in extracting this metal;
that elaborate and complex appliances, not unlike those in use in modern
times, were requisite for the purpose; and therefore that the use of
iron is compatible only with a somewhat late period in man’s history,
and implies a comparatively advanced stage of civilization. Now there
undoubtedly are facts which tend to confirm this view; for instance, the
Spaniards who first colonized North America found the natives perfectly
familiar with the use of copper, but without any acquaintance with iron,
although the region abounded with the finest ferruginous minerals; and,
again, the archæologists who have examined the relics of ancient
civilizations and of pre-historic peoples about the shores of the
Mediterranean, find in the earliest of these relics weapons and
implements of rudely chipped stones, followed later by the use of
better-shaped and polished stones; hence the periods represented by
these, they have respectively designated by the terms _palæolithic_ and
_neolithic_—the old and the new stone ages. At some later time the stone
of these implements was gradually replaced by _bronze_, which is a
mixture of copper and tin, while as yet iron does not occur in any form
among the remains. In the latest layers, however, articles of iron are
found, and it is inferred that this metal came into use only after
bronze had been known for an indefinite period; hence these later
pre-historic periods have come to be respectively called _the bronze
age_ and _the iron age_. No doubt this succession really occurred in the
localities where the observations were made, but it would not be
justifiable to assume that the same was the case in every part of the
world, for much would depend on such circumstances as the presence or
absence of the essential minerals. We may also set against the supposed
difficulty of obtaining iron from the ores, the still greater complexity
of the methods required for the production of copper and of tin. Besides
this is the fact that the ores of tin are found but in very few places
in the world, and of these only the Cornwall mines, so well known in
ancient times, would be likely to furnish a supply to the places where
pre-historic bronzes are found; this implies that navigation and
commerce must have already made considerable progress. On the other
hand, iron has been produced and worked for untold ages by the negro
races all over Central Africa, and the method of treating the ore has no
doubt been that which is there still practised by certain scarcely
civilized tribes, and it is as simple as any metallurgical operation can
possibly be, requiring merely a hole dug in a clay bank, wherein the
fuel and minerals are piled up, and the mere wind supplies sufficient
blast to urge the fire to the needful temperature, or air is blown in
from rude bellows made of a pair of skins alternately raised and
compressed. These very primitive furnaces have in some places developed
into permanent clay structures, seven or eight feet in height. The
natives of Central Africa have therefore long known the method of
extracting iron, as well as of forging and casting it.
The nature and value of what has been done during the century in the
treatment of iron would not be intelligible without some description of
the ordinary processes of extracting the metal from the ores; and a
scientific understanding of these implies some acquaintance with
chemistry. Not because metallurgy has been developed from chemistry, for
the fact is rather the reverse; indeed, as we have seen, the art of
extracting iron from its ores was practised ages before chemistry as a
science was dreamt of. Although we may assume that many of our readers
have sufficient knowledge of chemistry to attach distinct ideas to such
few chemical terms as we shall have occasion to use, yet it may be of
advantage to others to have some preliminary notes of the character of
the chemical actions, and of some properties of the substances that will
have to be referred to. It is certainly the case that people in general,
and even people very well informed in other subjects, have but the
vaguest notions of the nature of chemical actions, and of the meaning of
the terms belonging to that science. For example, one of our most
popular and justly esteemed writers, treating of the very subject of
iron extraction, calls the ore a _matrix_, thereby implying that the
iron as metal is disseminated in detached fragments throughout the mass,
which is a conception inconsistent with the facts. The reader will be in
a more advantageous position for understanding the relation of the ores
of iron to the metal, if he will follow in imagination, or still better
in reality, a few observations and experiments like the following—of
which, however, he is recommended not to attempt the chemical part
unless he is himself practically familiar with the performance of
chemical operations, or can obtain the personal assistance of someone
who is. Taking, say, a few common iron nails, let him note some obvious
properties they possess: they have weight—are hard and tough so that
they cannot be crushed in a mortar—are opaque to light—if a smooth
surface be produced on any part, it will show that peculiar shiny
appearance which is called metallic lustre, in this case without any
decided colour—they are not dissolved by water as sugar or salt is—and
are attracted by a magnet. If several of the nails be heated to bright
redness they may be hammered on an anvil into one mass, and this may be
flattened out into a thin plate, or it may be shaped into a slender rod
and then drawn out into wire; or otherwise the nails may be converted
into the small fragments called iron filings. In these several forms the
nails, as nails, will have ceased to exist; but the material of which
they were formed will remain unchanged, and each and every part of it
however large or small will continue to exhibit all the properties noted
above as belonging to the substance of the nails, which in the cases
supposed has undergone merely _physical_ change of shape. Treating our
nails in yet another way, we may proceed to subject them to a _chemical_
change, by an experiment very simple in itself, but involving certain
precautions, by neglect of which the tyro in chemical operations would
incur some personal risks; these might however be obviated by using only
very small quantities of the materials (a mere pinch of iron filings and
a few drops of sulphuric acid), when the results would still be
sufficiently observable. A few of the iron nails having been placed in a
flask of thin glass, we pour upon them a mixture of oil of vitriol
(sulphuric acid) and water, which has previously been prepared by
gradually adding 1 measure of the acid to 5 measures of water. The
action that takes place is greatly accelerated by heat, and indeed the
contents should be heated to boiling by standing the flask on a layer of
fine sand spread on an iron plate and gently heated from below. The
nails will soon disappear, being completely dissolved by the acid
liquid, and the turbid solution should be filtered through filtering
paper as rapidly as possible and while still hot. This turbid and dirty
looking condition is due to foreign matters in the nails, for these
never consist of _pure_ iron. The filtered liquid is set aside to cool
in a closed vessel, in which after a time will be found a deposit of
crystals of a pale bluish-green colour. The liquor above these having
been poured off, the crystals are to be rinsed with a very small
quantity of _cold_ water, and then dried between folds of
blotting-paper, after which they are ready for examination. The quantity
of the diluted acid put into the flask should have a certain proportion
to the weight of the nails; about 5 _fluid ounces_ to 1 ounce of iron
will be found convenient, for if less is used the nails will not be
entirely dissolved, and an excess will tend to keep the crystals in
solution instead of depositing them when cold. The nails—as such—will
now have passed out of existence: can we say that the iron that formed
them exists in the crystals? Certainly not as the _metal_ iron, for
every property of the metal will have disappeared. The crystals are
brittle, can be crushed in a mortar—they are translucent—they show no
metallic lustre, but only glassy surfaces—they are readily dissolved by
water—they are not attracted by a magnet. The most powerful lens will
fail to show the least particle of iron in them; they have in their
properties no assignable relation to the metal of the nails, but are
matter of quite another sort; and be it noted that this entire
_otherness_ is the special and characteristic sign of _chemical change_.
So complete is the transformation in the case we have been considering
that it would never have been said that iron was contained in these
crystals, but rather that the metal had for ever passed out of
existence, but for one circumstance; and that is, that by subjecting the
crystals to certain processes of _chemical analysis_ we can again obtain
from them the iron in metallic state. Nay more, we should find the
weight of metal so obtained to be exactly equal to that of the pure iron
dissolved from the original nails, supposing of course that we operated
upon the whole of the crystalline matter so produced. The inference
therefore is that although every property of the iron appeared to be
absent from the crystals, the iron entering in them retained there its
original weight, and the correct statement of the change would be, that
in the crystals the iron had lost all its original properties SAVE ONE,
namely, its weight, or gravitating force, if we choose to call it so, a
property belonging to it in common with every material substance.
Chemical analysis can also separate from the crystals their other
constituents and weigh them apart—so much water and so much sulphuric
acid—and when to these weights that of the iron is added, the sum
exactly makes up the weight of the crystals.
A still simpler experiment, which may be performed by anyone with the
greatest ease, may serve as a further illustration of the profound
nature of the change in the properties of bodies brought about by
chemical combination, and it will also serve as the occasion of
directing attention to a remarkable circumstance that invariably
characterizes such changes, and one that should always be present in our
minds when we are considering them. A yard of flat _magnesium_ wire can
be bought for a few pence, and after its metallic character has been
observed in the silvery lustre disclosed by scraping the dull white
surface, a few inches is to be held vertically by a pair of tongs, or by
inserting one extremity in a cleft at the end of a stick, then the lower
part is brought into contact with a candle or gas flame. The metal will
instantly burn with a dazzlingly brilliant light, and some white smoke
(really fine white solid particles) will float into the air; but if a
plate be held under the burning metal, some of the smoke will settle
upon it, together with white fragments that have preserved some shape of
the metallic ribbon, but which a touch will reduce into a fine white
powder, identical with the well-known domestic medicine called “calcined
magnesia”—a substance totally different from the _metal magnesium_. The
reader will scarcely require to be told that in this burning the metal
is entering into combination with the oxygen of the air—by which that
invisible gas somehow becomes fixed in these solid white particles, so
entirely unlike itself. But this experiment might be so arranged that
the quantities of magnesium and oxygen entering into the magnesia could
be weighed. For this purpose special appliances would be required in
order to ensure complete combustion of the metal, for in the experiment
as just described some small particles are liable to be shielded from
the oxygen by a covering of magnesia, and the arrangement would have to
be such that the _whole_ of the white powder could be gathered up and
weighed. In the absence of such appliances, and of a delicate balance,
together with the skill requisite for their use, the reader must for the
time be contented to take our word for what would be the result. In
every experiment the magnesia would be found heavier than the metal
burned in the proportion of 5 to 3; in other words, magnesia always
contains (so the phrase runs) 3 parts of magnesium combined with 2 of
oxygen: never more nor less. A definite proportion between the weights
of the constituent substances characterizes every chemical combination,
and when this is once determined in a single sample of the compound, it
is determined for every portion of the same, wherever found or however
produced. But each compound has its own particular proportion, that is,
the quantitative relations are different for each. For example, the two
constituents of water, hydrogen and oxygen, are combined in the ratio of
1 to 8, etc.; and oxygen combines with metals in a ratio different in
each case. Then occasionally the same ratio of constituents occurs in
compounds of different composition. The elementary student is apt to
suppose that this is _because_ of the _law_ which he finds stated,
probably in almost the first page of his text-book: “Every compound
contains its elements in definite and invariable proportions”; and even
well-educated people entertain the idea of the fact being “governed by”
or “obeying” the law just quoted,—a misconception arising from the other
use of the word “law,” as signifying an enactment. The real case however
is the converse; namely, that a multitude of facts like that above
stated have governed the law, and caused it to be what it is—the general
statement of many observed facts.
We have assumed that the reader’s chemical knowledge had already made
him aware that in every case of ordinary combustion the oxygen of the
atmosphere is in the act of entering into combination with the burning
body: as with the magnesium, so with a coal fire, a gas flame, or a
burning candle; only in these last cases the products of the combustion
pass away invisibly. The candle by burning disappears from sight, but
its matter is not lost, and as in the case of magnesium, the compounds
it forms weigh more than the unburnt candle. The experiment is commonly
shown in courses of elementary lectures on chemistry, of so burning a
candle that the invisible products are retained in the apparatus,
instead of being dissipated in the atmosphere, and the increase of
weight of the burnt candle over the original one is demonstrated by the
balance. Important as is the part played by oxygen in all chemical
actions on the earth, the composition of the atmosphere was not
understood until the end of the eighteenth century, and it was well on
into the nineteenth before the quantities of its constituents were
accurately determined. Now everyone knows that air is mainly made up of
a _mixture_ of the two gases _oxygen_ and _nitrogen_. A _mixture_ of two
or more things is very different from a chemical _combination_ of them;
for in the former each ingredient retains its own properties. (See _Air_
in Index.) Nitrogen being an inert gas that takes no part in combustion,
or in the ordinary chemical actions of the air, acts therein simply as a
diluent of the oxygen. It is necessary in relation to our present
subject to bear this in mind, as well as the relative quantities of the
two gases in air. For our immediate purpose we may neglect the minor
constituents of air—such as watery vapour, carbonic acid, etc., of which
the total weight does not exceed one hundredth part of the whole—and
consider air as a mixture of 23 parts by weight of oxygen with 77 of
nitrogen, or calculated in volumes, 21 measures of oxygen with 79 of
nitrogen. Compounds of oxygen with nearly every one of the other seventy
or more chemical elements are known, and these compounds, which are
called _oxides_, are arranged by chemists under five or six classes,
forming as they do _basic radicles_, _acid radicles_, _saline oxides_,
etc. With some of these compounds belonging to different classes, we
must make acquaintance after noticing the elementary substance with
which the oxygen is united.