Assays ought to occupy an important place in metallurgic instructions,
and there is reason to believe that the knowledge of assaying is not
sufficiently diffused, since its practice is so often neglected in
smelting houses. Not only ought the assays of the ores under treatment,
to be frequently repeated, because their nature is subject to vary; but
the different products of the furnaces should be subjected to reiterated
assays, at the several periods of the operations. When silver or gold
ores are in question, the docimastic operations, then indispensable,
exercise a salutary controul over the metallurgic processes, and afford
a clear indication of the quantities of precious metal which they ought
to produce.
By the title _Assays_, in a metallurgic point of view, is meant the
method of ascertaining for any substance whatever, not only the presence
and the nature of a metal, but its proportional quantity. Hence the
operations which do not lead to a precise determination of the metal in
question, are not to be arranged among the assays now under
consideration. Experiments made with the blow-pipe, although capable of
yielding most useful indications, are like the touchstone in regard to
gold, and do not constitute genuine assays.
Three kinds of assays may be practised in different circumstances, and
with more or less advantage upon different ores. 1. The mechanical
assay; 2. the assay by the dry way; 3. the assay by the humid way.
1. _Of mechanical assays._--These kinds of assays consist in the
separation of the substances mechanically mixed in the ores, and are
performed by a hand-washing, in a small trough of an oblong shape,
called a _sebilla_. After pulverizing with more or less pains the
matters to be assayed by this process, a determinate weight of them is
put into this wooden bowl with a little water; and by means of certain
movements and some precautions, to be learned only by practice, the
lightest substances may be pretty exactly separated, namely, the earthy
gangues from the denser matter or metallic particles, without losing any
sensible portion of them. Thus a _schlich_ of greater or less purity
will be obtained, which may afford the means of judging by its quality
of the richness of the assayed ores, and which may thereafter be
subjected to assays of another kind, whereby the whole metal may be
insulated.
Washing, as an assay, is practised on auriferous sands; on all ores from
the _stamps_, and even on _schlichs_ already washed upon the great
scale, to appreciate more nicely the degree of purity they have
acquired. The ores of tin in which the oxide is often disseminated in
much earthy gangue, are well adapted to this species of assay, because
the tin oxide is very dense. The mechanical assay may also be employed
in reference to the ores whose metallic portion presents an uniform
composition, provided it also possesses considerable specific gravity.
Thus the ores of sulphuret of lead (galena) being susceptible of
becoming almost pure sulphurets (within 1 or 2 _per cent._) by mere
washing skilfully conducted, the richness of that ore in pure galena,
and consequently in lead, may be at once concluded; since 120 of galena
contain 104 of lead, and 16 of sulphur. The sulphuret of antimony
mingled with its gangue may be subjected to the same mode of assay, and
the result will be still more direct, since the crude antimony is
brought into the market after being freed from its gangue by a simple
fusion.
The assay by washing is also had recourse to for ascertaining if the
_scoriæ_ or other products of the furnaces contain some metallic grains
which might be extracted from them by stamping and washing on the great
scale; a process employed considerably with the _scoriæ_ of tin and
copper works.
_Of assays by the dry way._--The assay by the dry way has for its
object, to show the nature and proportion of the metals contained in a
mineral substance. To make a good assay, however, it is indispensably
necessary to know what is the metal associated with it, and even within
certain limits, the quantity of the foreign bodies. Only one metal is
commonly looked after; unless in the case of certain argentiferous ores.
The mineralogical examination of the substances under treatment, is most
commonly sufficient to afford data in these respects; but the assays may
always be varied with different views, before stopping at a definite
result; and in every instance, only such assays can be confided in, as
have been verified by a double operation.
This mode of assaying requires only a little experience, with a simple
apparatus; and is of such a nature as to be practised currently in the
smelting works. The air furnace and crucibles employed are described in
all good elementary chemical books. These assays are usually performed
with the addition of a flux to the ore, or some agent for separating the
earthy from the metallic substances; and they possess a peculiar
advantage relative to the smelting operations, because they offer many
analogies between results on the great scale and experiments on the
small. This may even enable us often to deduce, from the manner in which
the assay has succeeded with a certain flux, and at a certain degree of
heat, valuable indications as to the treatment of the ore in the great
way. See FURNACE.
In the smelting houses which purchase the ore, as in Germany, it is
necessary to bestow much attention upon the assays, because they serve
to regulate the quality and the price of the schlichs to be delivered.
These assays are not by any means free from difficulties, especially
when ores containing several useful metals are treated, and which are to
be dosed or proportioned; ores, for example, including a notable
quantity of lead, copper, and silver, mixed together.
In the central works of the Hartz, as well as in those of Saxony, the
_schlichs_ as delivered are subjected to docimastic assays, which are
verified three times, and by three different persons, one of whom is
engaged for the interests of the mining partners, another for that of
the smelting house, and a third as arbiter in case of a difference. If
the first two results of assaying differ by 1/2 _loth_ (or 1/4 ounce) of
silver per cwt. of _schlich_, the operations must be resumed; but this
rarely happens. When out of the three assays, the one differs from the
two others by no more than 1/4 loth of silver per cwt., but by more in
one, and by less in another, the mean result is adopted. As to the
contents of the _schlich_ in lead, the mean results of the assays must
be taken. The differences allowed, are three pounds for the _schlich_,
when it contains from 12 to 30 per cent. of lead, increasing to six
pounds for _schlich_, when it contains less than 55 per cent. of that
metal.
Assaying forms in great establishments, an important object in reference
to time and expense. Thus, in the single work of Franckenscharn, in the
Hartz, no less than 300 assays have to be made in a threefold way, every
Monday, without taking into account the several assays of the smelting
products which take place every Thursday. Formerly fluxes more or less
compound were employed for these purposes, and every assay cost about
fifteen pence. At present all these assays are made more simply, by much
cheaper methods, and cost a penny farthing each upon an average.
_Of the assays by the humid way._--The assays by the humid way, not
reducible to very simple processes, are true chemical analyses, which
may in fact be applied with much advantage, either to ores, or to the
products of the furnace; but which cannot be expected to be practised in
smelting-houses, on account of the complication of apparatus and
reagents they require. Moreover, an expert chemist is necessary to
obtain results that can be depended on. The directors of
smelting-houses, however, should never neglect any opportunities that
may occur of submitting the materials operated upon, as well as their
products, to a more thorough examination than the dry way alone can
effect. One of the great advantages of similar researches is, to
discover and appreciate the minute quantities of injurious substances
which impair the malleability of the metals, which give them several bad
qualities, about whose nature and cause, more or less error and
uncertainty prevail. Chemical analysis, rightly applied to metallurgy,
cannot fail to introduce remarkable improvements into the
processes.--See the different metals, in their alphabetical places.
For assays in the dry way, both of stony and metallic minerals, the
process of Dr. Abich deserves recommendation. In consists in mixing the
pulverized mineral with 4 or 6 times its weight of carbonate of baryta
in powder, fusing the mixture at a white heat, and then dissolving it
after it cools, in dilute muriatic acid. The most refractory minerals,
even corundum, cyanite, staurolite, zircon, and felspar, yield readily
to this treatment. This process may be employed with advantage upon poor
refractory ores. The platinum crucible, into which the mixed materials
are put for fusion, should be placed in a Hessian crucible, and
surrounded with good coak.
* * * * *
The following tabular view of the metallic produce of the British mines,
is given by two very skilful observers, in a work published in 1827,
entitled _Voyage Metallurgique en Angleterre, par MM. Dufrénoy et Elie
de Beaumont_:--
Tons. Tons.
Tin Cornwall alone 3,160
{ Cornwall 9,331 }
{ Devonshire 537 }
{ Staffordshire 38 }
Copper { Anglesey 738 }
{ Wales 55 } 11,469
{ Cumberland 21 }
{ Ireland 738 }
{ Scotland 11 }
{ Wales 7,500 }
{ Scotland 2,800 }
Lead { Cornwall and Devonshire 800 } 31,900
{ Shropshire 800 }
{ Derbyshire 1,000 }
{ Cumberland 19,000 }
Cast Iron about 600,000[32]
[32] I have converted the weights of lead and cast iron, given in
kilogrammes, into tons, at the rate of 1000 kilogrammes per ton; which
is sufficiently near.
The manganese raised in England exceeds 2000 tons.
M. Heron de Villefosse inserted in the last number of the _Annales des
Mines_ for 1827, the following statistical view of the metallic products
of France:--
Tons.
Lead in pigs (_saumons_) 103
Litharge 513
Sulphuret of lead, ground galena (_alquifoux_) 112
Black copper 164
Antimony 91
Manganese 765
Crude cast iron 25,606
Bar iron 127,643
Steel 3,500
Silver in ingots 1-1/6
The total value of which is estimated at 80 millions of francs; or about
3,400,000 pounds sterling.
METALS; (_Metaux_, Fr.; _Metalle_, Germ.) are by far the most numerous
class of undecompounded bodies in chemical arrangements. They amount to
41; of which 7 form, with oxygen, bodies possessed of alkaline
properties; these are, 1. potassium; 2. sodium; 3. lithium; 4. barytium,
or barium; 5. strontium; 6. calcium; 7. magnesium; for even magnesia,
the last and feeblest base, tinges turmeric brown, and red cabbage
green. The next 5 metals form, with oxygen, the earths proper; they are,
8. yttrium; 9. glucinum; 10. alumium; 11. zirconium; 12. thorinum. The
remaining 29 may be enumerated in alphabetical order, as they hardly
admit of being grouped into subdivisions with any advantage. They are as
follows: 13. antimony; 14. arsenic; 15. bismuth; 16. cadmium; 17.
cerium; 18. chromium; 19. cobalt; 20. copper; 21. gold; 22. iridium; 23.
iron; 24. lead; 25. manganese; 26. mercury; 27. molybdenum; 28. nickel;
29. osmium; 30. palladium; 31. platinum; 32. rhodium; 33. silver; 34.
tantalum; 35. tellurium; 36. tin; 37. titanium; 38. tungstenium; 39.
vanadium; 40. uranium; 41. zinc.
1. They are all, more or less, remarkable for a peculiar lustre, called
the metallic. This property of strongly reflecting light, is connected
with a certain state of aggregation of their particles, but is
possessed, superficially at least, by mica, animal charcoal, selenium,
polished indigo;--bodies not at all metallic.
2. The metals are excellent conductors of caloric, and most of them also
of electricity, though probably not all. According to Despretz, they
possess the power of conducting heat according to the following
numbers:--Gold, 1000; platinum, 981; silver, 973; copper, 898; iron,
374; zinc, 363; tin, 304; lead, 179·6.
Becquerel gives the following table of metals, as to electrical
conduction:--
Copper, 100; gold, 93·6; silver, 73·6; zinc, 28·5; platina, 16·4; iron,
15·8; tin, 15·5; lead, 8·3; mercury, 3·5; potassium, 1·33.
The metals which hardly, if at all, conduct electricity, are, zirconium;
alumium; tantalum, in powder; and tellurium.
3. Metals are probably opaque; yet gold leaf, as observed by Newton,
seems to transmit the green rays, for objects placed behind it in the
sunbeam appear green. This phenomena has, however, been ascribed to the
rays of light passing through an infinite number of minute fissures in
the thinly hammered gold.
4. All metals are capable of combining with oxygen, but with affinities
and in quantities extremely different. Potassium and sodium have the
strongest affinity for it; arsenic and chromium, the feeblest. Many
metals become acids by a sufficient dose of oxygen, while, with a
smaller dose, they constitute salifiable bases.
5. Metals combine with each other, forming a class of bodies called
alloys, except when one of them is mercury, in which case the compound
is styled an amalgam.
6. They combine with hydrogen into _hydrurets_; with carbon, into
_carburets_; with sulphur, into _sulphurets_; with phosphorus, into
_phosphurets_; with selenium, into _seleniurets_; with boron, into
_borurets_ (_borides?_); with chlorine, into _chlorides_; with iodine,
into _iodides_; with cyanogen, into _cyanides_; with silicon, into
_silicides_; and with fluorine, into _fluorides_.
7. Metallic salts are definite compounds, mostly crystalline, of the
metallic oxides with the acids. See HALOID.
METEORITES, (_Aerolithes_, Fr.), are stones of a peculiar aspect and
composition, which have fallen from the air.
METHYLÈNE, a peculiar liquid compound of carbon and hydrogen, extracted
from pyroxilic spirit, which is reckoned to be a bi-hydrate of
_methylène_.
MICA, is a finely foliated mineral, of a pearly metallic lustre. It is
harder than gypsum, but not so hard as calc-spar; flexible and elastic;
spec. grav. 2·65. It is an ingredient of granite and gneiss. The large
sheets of mica exposed for sale in London, are mostly brought from
Siberia. They are used, instead of glass, to enclose the fire, without
concealing the flame, in certain stoves.
The mica of Fahlun, analyzed by Rose, afforded, silica, 46·22; alumina,
34·52; peroxide? of iron, 6·04; potash, 8·22; magnesia, with oxide of
manganese, 2·11; fluoric acid, 1·09; water, 0·98.
MICROCOSMIC SALT; a term given to a salt extracted from human urine,
because man was regarded by the alchemists as a miniature of the world,
or the microcosm. It is a phosphate of soda and ammonia; and is now
prepared by mixing, equivalent proportions of phosphate of soda and
phosphate of ammonia, each in solution, evaporating and crystallizing
the mixture. A small excess of ammonia aids the crystallization.
MILK; (_Lait_, Fr.; _Milche_, Germ.) owes its whiteness and opacity to
an emulsion composed of the caseous matter and butter, with sugar of
milk, extractive matters, salts, and free lactic acid; the latter of
which causes fresh milk to redden litmus paper. Milk, in general,
contains from 10 to 12 per cent. of solid matter, on being evaporated to
dryness by a steam heat. The mean specific gravity of cows’ milk is
1·030, but it is less if the milk be rich in cream. The specific gravity
of the skimmed milk is 1·035; and of the cream is 1·0244. 100 parts of
creamed milk, contain--
Caseous matter, containing some butter, 2·600
Sugar of milk 3·500
Alcoholic extract, lactic acid, and lactates 0·600
Salts; muriate and phosphate of potash, and phosphate of lime 0·420
Water 92·875
Cream consists of,--Butter separated by churning 4·5
Caseous matter precipitated by the coagulation of the milk of
the butter 3·5
Butter-milk 92·0
-------
100·0
When milk contained in wire-corked bottles, is heated to the boiling
point in a water bath, the oxygen of the included small portion of air
under the cork seems to be carbonated, and the milk will afterwards keep
fresh, it is said, for a year or two; as green gooseberries and peas do
by the same treatment.
MILL-STONE, or BUHR-STONE. This interesting form of silica, which occurs
in great masses, has a texture essentially cellular, the cells being
irregular in number, shape, and size, and are often crossed by thin
plates, or coarse fibres of silex. The Buhr-stone has a straight
fracture, but it is not so brittle as flint, though its hardness is
nearly the same. It is feebly translucent; its colours are pale and
dead, of a whitish, grayish, or yellowish cast, sometimes with a tinge
of blue.
The Buhr-stones usually occur in beds, which are sometimes continuous,
and at others interrupted. These beds are placed amid deposits of sand,
or argillaceous and ferruginous marls, which penetrate between them,
filling their fissures and honeycomb cavities. Buhr-stones constitute a
very rare geological formation, being found in abundance only in the
mineral basin of Paris, and a few adjoining districts. Its place of
superposition is well ascertained: it forms a part of the lacustrine, or
fresh-water formation, which, in the locality alluded to, lies above the
fossil-bone gypsum, and the stratum of sand and marine sandstone which
cover it. Buhr-stone constitutes, therefore, the uppermost solid stratum
of the crust of the globe; for above it there is nothing but alluvial
soil, or diluvial gravel, sand, and loam.
Buhr-stones sometimes contain no organic forms, at others they seem as
if stuffed full of fresh-water shells, or land shells and vegetables of
inland growth. There is no exception known to this arrangement; but the
shells have assumed a siliceous nature, and their cavities are often
bedecked with crystals of quartz. The best Buhr-stones for grinding
corn, have about an equal proportion of solid matter, and of vacant
space. The finest quarry of them is upon the high ground, near _La
Ferté-sous-Jouarre_. The stones are quarried in the open air, and are
cut out in cylinders, from one to two yards in diameter, by a series of
iron and wooden wedges, gradually but equally inserted. The pieces of
buhr-stones are afterwards cut into parallelopipeds, called _panes_,
which are bound with iron hoops into large millstones. These pieces are
exported chiefly to England and America. Good millstones of a bluish
white colour, with a regular proportion of cells, when six feet and a
half in diameter, fetch 1200 francs a-piece, or 48_l._ sterling. A
coarse conglomerate sandstone or breccia is, in some cases, used as a
substitute for buhr-stones; but it is a poor one.
MINERAL WATERS. See SODA WATER, and WATERS, MINERAL.
MINES; (_Bergwerke_, Germ.) Amidst the variety of bodies apparently
infinite, which compose the crust of the globe, geologists have
demonstrated the prevalence of a few general systems of rocks, to which
they have given the name of _formations_ or _deposits_. A large
proportion of these mineral systems consists of parallel planes, whose
length and breadth greatly exceed their thickness; on which account they
are called stratified rocks; others occur in very thick blocks, without
any parallel stratification, or horizontal seams of considerable extent.
The stratiform deposits are subdivided into two great classes; the
primary and the secondary. The former seem to have been called into
existence before the creation of organic matter, because they contain no
exuviæ of vegetable or animal beings; while the latter are more or less
interspersed, and sometimes replete with organic remains. The primary
strata are characterized, moreover, by the nearly vertical or highly
inclined position of their planes; the secondary lie for the most part
in a nearly horizontal position.
Where the primitive mountains graduate down into the plains, rocks of an
intermediate character appear, which, though possessing a nearly
vertical position, contain a few vestiges of animal beings, especially
shells. These have been called _transition_, to indicate their being the
passing links between the first and second systems of ancient deposits;
they are distinguished by the fractured and cemented texture of their
planes, for which reason they are sometimes called conglomerate.
Between these and the truly secondary rocks, another very valuable
series is interposed in certain districts of the globe; namely, the
coal-measures, the paramount formation of Great Britain. The coal strata
are disposed in a basin-form, and alternate with parallel beds of
sandstone, slate-clay, iron-stone, and occasionally limestone. Some
geologists have called the coal-measures the medial formation.
In every mineral plane, the inclination and direction are to be noted;
the former being the angle which it forms with the horizon, the latter
the point of the azimuth or horizon, towards which it dips, as west,
north-east, south, &c. The direction of the bed is that of a horizontal
line drawn in its plane; and which is also denoted by the point of the
compass. Since the lines of direction and inclination are at right
angles to each other, the first may always be inferred from the second;
for when a stratum is said to dip to the east or west, this implies that
its direction is north and south.
The smaller sinuosities of the bed are not taken into account, just as
the windings of a river are neglected in stating the line of its course.
_Masses_ are mineral deposits, not extensively spread in parallel
planes, but irregular heaps, rounded or oval, enveloped in whole or in a
great measure by rocks of a different kind. Lenticular masses being
frequently placed between two horizontal or inclined strata, have been
sometimes supposed to be stratiform themselves, and have been
accordingly denominated by the Germans _liegende stocke_, _lying heaps_
or _blocks_.
The orbicular masses often occur in the interior of unstratified
mountains, or in the bosom of one bed.
_Nests_, _concretions_, _nodules_, are small masses found in the middle
of strata; the first being commonly in a friable state; the second often
kidney-shaped, or tuberous; the third nearly round, and encrusted, like
the kernel of an almond.
_Lodes_, or large veins, are flattened masses, with their opposite
surfaces not parallel, which consequently terminate like a wedge, at a
greater or less distance, and do not run parallel with the rocky strata
in which they lie, but cross them in a direction not far from the
perpendicular; often traversing several different mineral planes. The
_lodes_ are sometimes deranged in their course, so as to pursue for a
little way the space between two contiguous strata; at other times they
divide into several branches. The matter which fills the lodes is for
the most part entirely different from the rocks they pass through, or at
least it possesses peculiar features.
This mode of existence, exhibited by several mineral substances, but
which has been long known with regard to metallic ores, suggests the
idea of clefts or rents having been made in the stratum posterior to its
consolidation, and of the vacuities having been filled with foreign
matter, either immediately or after a certain interval. There can be no
doubt as to the justness of the first part of the proposition, for there
may be observed round many lodes undeniable proofs of the movement or
dislocation of the rock; for example, upon each side of the rent, the
same strata are no longer situated in the same plane as before, but make
greater or smaller angles with it; or the stratum upon one side of the
lode is raised considerably above, or depressed considerably below, its
counterpart upon the other side. With regard to the manner in which the
rent has been filled, different opinions may be entertained. In the
lodes which are widest near the surface of the ground, and graduate into
a thin wedge below, the foreign matter would seem to have been
introduced as into a funnel at the top, and to have carried along with
it in its fluid state portions of rounded gravel and organic remains. In
other cases, other conceptions seem to be more probable; since many
lodes are largest at their under part, and become progressively narrower
as they approach the surface; from which circumstance, it has been
inferred that the rent has been caused by an expansive force acting
from within the earth, and that the foreign matter, having been injected
in a fluid state, has afterwards slowly crystallized. This hypothesis
accounts much better than the other for most of the phenomena observable
in mineral veins, for the alterations of the rock at their sides, for
the crystallization of the different substances interspersed in them,
for the cavities bestudded with little crystals, and for many minute
peculiarities. Thus, the large crystals of certain substances which line
the walls of hollow veins, have sometimes their under surfaces
besprinkled with small crystals of sulphurets, arseniurets, &c., while
their upper surfaces are quite smooth; suggesting the idea of a slow
sublimation of these volatile matters from below, by the residual heat,
and their condensation upon the under faces of the crystalline bodies,
already cooled. This phenomenon affords a strong indication of the
igneous origin of metalliferous veins.
In the lodes, the principal matters which fill them are to be
distinguished from the accessory substances; the latter being
distributed irregularly, amidst the mass of the first, in crystals,
nodules, grains, seams, &c. The non-metalliferous exterior portion,
which is often the largest, is called _gangue_, from the German _gang_,
_vein_. The position of a vein is denoted, like that of the strata, by
the angle of inclination, and the point of the horizon towards which
they dip, whence the direction is deduced.
_Veins_, are merely small lodes, which sometimes traverse the great
ones, ramifying in various directions, and in different degrees of
tenuity.
A metalliferous substance is said to be _disseminated_, when it is
dispersed in crystals, spangles, scales, globules, &c., through a large
mineral mass.
Certain ores which contain the metals most indispensable to human
necessities, have been treasured up by the Creator in very bountiful
deposits; constituting either great masses in rocks of different kinds,
or distributed in lodes, veins, nests, concretions, or beds with stony
and earthy admixtures; the whole of which become the objects of mineral
exploration. These precious stores occur in different stages of the
geological formations; but their main portion, after having existed
abundantly in the several orders of the primary strata, suddenly cease
to be found towards the middle of the secondary. Iron ores are the only
ones which continue among the more modern deposits, even so high as the
beds immediately beneath the chalk, when they also disappear, or exist
merely as colouring matters of the tertiary earthy beds.
The strata of gneiss and mica-slate constitute in Europe the grand
metallic domain. There is hardly any kind of ore which does not occur
there in sufficient abundance to become the object of mining operations,
and many are found no where else. The transition rocks and the lower
part of the secondary ones, are not so rich, neither do they contain the
same variety of ores. But this order of things, which is presented by
Great Britain, Germany, France, Sweden, and Norway, is far from forming
a general law; since in equinoxial America the gneiss is but little
metalliferous; while the superior strata, such as the clay-schists, the
sienitic porphyries, the limestones, which complete the transition
series, as also several secondary deposits, include the greater portion
of the immense mineral wealth of that region of the globe.
All the substances of which the ordinary metals form the basis, are not
equally abundant in nature; a great proportion of the numerous mineral
species which figure in our classifications, are mere varieties
scattered up and down in the cavities of the great masses or lodes. The
workable ores are few in number, being mostly sulphurets, some oxides,
and carbonates. These occasionally form of themselves very large masses,
but more frequently they are blended with lumps of quartz felspar, and
carbonate of lime, which form the main body of the deposit; as happens
always in proper lodes. The ores in that case are arranged in small
layers parallel to the strata of the formation, or in small veins which
traverse the rock in all directions, or in nests or concretions
stationed irregularly, or finally disseminated in hardly visible
particles. These deposits sometimes contain apparently only one species
of ore, sometimes several, which must be mined together, as they seem to
be of contemporaneous formation; whilst, in other cases, they are
separable, having been probably formed at different epochs. In treating
of the several metals in their alphabetical order, I have taken care to
describe their peculiar geological positions, and the rocks which
accompany or mineralize them.
In mining, as in architecture, the best method of imparting instruction
is to display the master-pieces of the respective arts, which speak
clearly to the mind through the medium of the eye. It is not so easy,
however, to represent at once the general effect of a mine, as it is of
an edifice; because there is no point of sight from which the former can
be sketched at once, like the latter. The subterraneous structures
certainly afford some of the finest examples of the useful labours of
man, continued for ages, under the guidance of science and ingenuity;
but, however curious, beautiful, and grand in themselves, they cannot
become objects of a panoramic view. It is only by the lights of geometry
and geology that mines can be contemplated and surveyed, either as a
whole or in their details; and, therefore, these marvellous subterranean
regions, in which roads are cut many hundred miles long, are altogether
unknown or disregarded by men of the world. Should any of them,
perchance, from curiosity or interest, descend into these dark recesses
of the earth, they are prepared to discover only a few insulated
objects, which they may think strange or possibly hideous; but they
cannot recognize either the symmetrical disposition of mineral bodies,
or the laws which govern geological phenomena, and serve as sure guides
to the skilful miner in his adventurous search. It is by exact plans and
sections of subterraneous workings, that a knowledge of the nature,
extent, and distribution of mineral wealth, can be acquired.
[Illustration: 698. _A general view of mining operations._]
As there is no country in the world so truly rich and powerful, by
virtue of its mineral stores, as Great Britain, so there are no people
who ought to take a deeper interest in their scientific illustration. I
have endeavoured in the present article to collect from the most
authentic sources the most interesting and instructive examples of
mining operations.
To the magnificent work of Ville-Fosse, _Sur la Richesse Minerale_, no
longer on sale, I have to acknowledge weighty obligations; many of the
figures being copied from his great Atlas.
Lodes or mineral veins are usually distinguished by English miners into
at least four species. 1. The rake vein. 2. The pipe vein. 3. The flat
or dilated vein; and 4. The interlaced mass (_stock-werke_), indicating
the union of a multitude of small veins mixed in every possible
direction with each other, and with the rock.
1. The _rake_ vein is a perpendicular mineral fissure; and is the form
best known among practical miners. It commonly runs in a straight line,
beginning at the superficies of the strata, and cutting them downwards,
generally further than can be reached. This vein sometimes stands quite
perpendicular; but it more usually inclines or hangs over at a greater
or smaller angle, or slope, which is called by the miners the _hade_ or
_hading_ of the vein. The line of direction in which the fissure runs,
is called the _bearing of the vein_.
2. The _pipe_ vein resembles in many respects a huge irregular cavern,
pushing forward into the body of the earth in a sloping direction, under
various inclinations, from an angle of a few degrees to the horizon, to
a dip of 45°, or more. The pipe does not in general cut the strata
across like the rake vein, but insinuates itself between them; so that
if the plane of the strata be nearly horizontal, the bearing of the pipe
vein will be conformable; but if the strata stand up at a high angle,
the pipe shoots down nearly headlong like a shaft. Some pipes are very
wide and high, others are very low and narrow, sometimes not larger than
a common mine or drift.
3. The _flat_ or _dilated_ vein, is a space or opening between two
strata or beds of stone, the one of which lies above, and the other
below this vein, like a stratum of coal between its roof and pavement;
so that the vein and the strata are placed in the same plane of
inclination. These veins are subject, like coal, to be interrupted,
broken, and thrown up or down by slips, dykes, or other interruptions of
the regular strata. In the case of a metallic vein, a slip often
increases the chance of finding more treasure. Such veins do not
preserve the parallelism of their beds, characteristic of coal seams;
but vary excessively in thickness within a moderate space. Flat veins
occur frequently in limestone, either in a horizontal or declining
direction. The flat or strata veins open and close, as the rake veins
also do.