When men penetrate by narrow passages into the interior of the earth,
their respiration, joined to the combustion of candle and gunpowder, are
not long of vitiating the air. The decomposition of wood contributes to
the same effect, as also the mineral bed itself, especially in coal
mines, by the carburetted hydrogen and carbonic acid evolved, and from
the absorption of oxygen by pyrites. In many cases, arsenical and
mercurial vapours are disengaged. Hence the necessity of maintaining in
subterranean cavities a continual circulation of air, which may renew
the atmosphere round the miners. The whole of the means employed to
produce this effect, constitutes what is called the _ventilation of
mines_.
These means are divided into _natural_ and _artificial_. The _natural
means_ are the currents produced by the difference of density between
the air of mines and the external air; the _artificial_ are
air-exhausters or condensers, fires, &c.
The temperature of the air of the subterranean workings surpasses the
mean temperature of the place in which the mine is opened. Hence it is
lighter in winter, but in summer often heavier than the air of the
atmosphere. For this reason, when the mine presents two openings at
different levels, the air naturally flows out by the most elevated in
winter, and by the lowest in summer. We may take advantage of this
circumstance, to lead the air into the bottom of even a very long
gallery, opening into the side of the mountain, by piercing a shaft into
its roof at some distance from the entrance, and dividing the gallery by
a horizontal floor into two parts, which have no mutual communication,
except at the furthest extremity--the upper part communicating with the
shaft, and the under with the mouth of the gallery. If the two
compartments have different dimensions, the air in the smaller sooner
comes into an equilibrium of temperature with the rock; and the
difference of temperature of the two compartments is sufficient to
produce a current. If a streamlet of water flows through this gallery,
it facilitates the flow of the air along the lower compartment. If a
mine has several openings situated on the same level, it rarely happens
but some peculiar circumstance destroys, during the colds of winter and
the heats of summer, the equilibrium of the air. But in spring and
autumn, when the external air is nearly of the same temperature with
that of the mines, the above-named causes are almost always too feeble
to excite an issuing current. This effect is, however, frequently
obtained by raising over one of the shafts a chimney 20 or 30 yards
high, which alone produces the effect of an opening at a different
level. It has been remarked that stormy weather usually deranges every
system of ventilation. See PITCOAL and VENTILATION.
MINIUM. (Eng. and Fr., _Red lead_; _Mennige_, Germ.) This pigment is a
peculiar oxide of lead, consisting of two atoms of the protoxide and one
of the peroxide; but, as found in commerce, it always contains a little
extra protoxide, or yellow massicot. It is prepared by calcining lead
upon a reverberatory hearth with a slow fire, and frequent renewal of
the surface with a rake, till it becomes an oxide, taking care not to
fuse it. The calcined mass is triturated into a fine powder in a paint
mill, where it is elutriated with a stream of water, to carry off the
finely levigated particles, and to deposit them afterwards in tanks. The
powder thus obtained being dried, is called massicot. It is converted
into minium, by being put in quantities of about 50 pounds into iron
trays, 1 foot square, and 4 or 5 inches deep. These are piled up upon
the reverberatory hearth, and exposed during the night, for economy of
fuel, to the residuary heat of the furnace, whereby the massicot absorbs
more oxygen, and becomes partially red lead. This, after being stirred
about, and subjected to a similar low calcining heat once and again,
will be found to form a marketable red lead.
The best minium, however, called _orange mine_, is made by the slow
calcination of good white lead (carbonate) in iron trays. If the lead
contains either iron or copper, it affords a minium which cannot be
employed with advantage in the manufacture of flint-glass, for pottery
glazes, or for house-painting.
Dumas found several samples of red lead which he examined to consist of
the chemical sesquioxide and the protoxide, in proportions varying from
50 of the former and 50 of the latter, to 95·3 of the former and 4·7 of
the latter. The more oxygen gas it gives out when heated, the better it
is, generally speaking. See NAPLES YELLOW.
MINT. (_Monnaie_, Fr.; _Münze_, Germ.) The chief use of gold and silver
is to serve for the medium of exchange in the sale and purchase of
commodities, a function for which they are pre-eminently fitted by their
scarcity, by being unalterable by common agents, and condensing a great
value in a small volume. It would be very inconvenient in general to
barter objects of consumption against each other, because their carriage
would be expensive, and their qualities, in many cases, easily injured
by external agents, &c. Gold is exempt from spontaneous change, and
little costly in conveyance. Mankind at a very early period recognised
how much easier it was to exchange a certain weight of gold or silver
for objects of commerce, than to barter these objects themselves; and
thenceforth all agreed to pay for their purchases in bars or ingots of
these precious metals. But as their intrinsic value depends upon their
purity, it became necessary to stamp on these bars their standard
quality and their weight.
The inconvenience of using ingots in general trade, on account of the
difficulty of defining fractional values, has determined governments to
coin pieces of money, that is, quantities of metal whose weight and
standard were made known and guaranteed by the effigies of the prince.
It is true, indeed, that kings have become frequently coiners of base
money, by altering the weight and purity of the pieces apparently
guaranteed by their impress. By such reductions modern coins represent
less of the precious metal than they did long ago. The _ordonnance_ of
755, for the coining of _sous_ in France, proves that there was then as
much fine silver in a single _sous_, as there is now in a piece of 5
francs. During the last two centuries, indeed, silver coins have been
diminished two thirds in weight.
But since knowledge has become more generally diffused, it has been
shown that these frauds are equally injurious to the prince and to
public faith. A sovereign may, it is true, declare by a decree that a
shilling-piece is to be held worth five; but let us consider the
consequences of this decree. All the individuals who have rents or
capital sums to receive, will be ruined, by getting in metallic value
only one-fifth of what is due to them; for although the _nominal_ value
should be the same as what they are entitled to, the intrinsic value
would be but a fifth of the former; so that when they go to purchase the
necessaries or comforts of life, the dealer who sells them will at once
raise their price five-fold. Each article of merchandise would thus
acquire a nominal price 5 times greater; and he who had received payment
of a debt in that money, could not with it procure more than one-fifth
of the goods he could have previously commanded. That fraudulent law
would, therefore, favour the debtors at the expense of the creditors;
and as the state is commonly a great debtor, especially when it has
recourse to the depreciation of the currency, it is obvious, that
however illicit the gain which it makes, it still does gain; and this is
the reason why princes have so often tampered with the mint. But let us
examine the other consequences of this decree.
If the sovereign is a debtor, he is also a creditor and consumer, and
even the most considerable of any. The taxes which he imposes are paid
him in this deteriorated money, returned to him at its nominal value;
and the purveyors of his armies, his buildings, and his household, sell
him their commodities only at the actual market price. We may infer from
this simple development that the coin with which he pays for any object
has the same intrinsic value as the object; and that the name given to
the coin is of no consequence. The prince may call it a crown, a ducat,
or a rix-dollar at his pleasure; and he may assign any value to it that
his caprice may suggest, yet this will not affect its value; for this is
fixed beyond his control by the general nature of things. The prince
may, indeed, at the outset, have profited by defrauding his creditors,
and by authorizing each debtor to imitate him, but he will soon lose
whatever he may have gained; and he will thus learn to his cost that it
was bad policy to sacrifice his character by giving an example of a
fraud so truly unprofitable in the issue. Moreover, he will lose still
as much in the following years, because his treasury will receive only
one-fifth part of the taxes, unless he has quintupled the imposts. It
may be said, indeed, that he might do the one thing along with the
other. But every one knows that this power is neither generally
permitted to princes, nor if it were, could it be safely exercised.
Serious political crises would combine to endanger the stability of the
government; which besides, as the main consumer in the nation, must lose
always as much as it seems to gain.
It is therefore manifest that the alteration of the standard and weight
of the coinage is at once a crime and a ruinous action for the sovereign
power to commit; and hence such disastrous measures have been long
abandoned in all well-regulated states. A gold sovereign is
intrinsically worth 20 shillings minus the cost of coinage; for were it
worth more, all our sovereign pieces would be exported or melted down,
to obtain the difference of value, however trifling it might be; and
were it worth less, it would be the source of loss similar to what the
state occasions when it depreciates the coin.
To comprehend the true value of a coin, we must regard this piece as an
article of merchandise, whose value depends, as that of every thing
else, on its usefulness, the esteem in which it is held, and the demand
for it in the market. Grain increases in value when there are few
sellers and many buyers; gold and silver are in the same predicament.
The value of these metals is much augmented, indeed, by the universal
currency they obtain when struck into money; a value additional to what
they possess as objects of the arts. This value of the precious metals
changes with time and place, like that of every merchandise; their
abundance, since the discovery of America, has greatly lowered their
value; that is, with the same weight of metal, we cannot at the present
day purchase the same quantity of corn, land, wool, &c. as formerly. In
the countries where silver abounds, this metal has less value, or, in
other terms, commodities are dearer. Hence the metal tends to resume its
equilibrium in flowing into those places where it is rarer; which means,
that the consumer prefers purchasing his commodities there rather than
in another place, if he can easily transport them to where they are
dearer.
It was formerly believed that a country is rich when it has a great deal
of gold and silver; but this popular illusion has passed away. Spain has
never been poorer than since the discovery of America, because its
national industry has been ruined, and the capitals merely passed
through its hands to spread over the rest of Europe, from which it was
obliged to import every thing that its want of home manufactures made it
necessary to procure from abroad. We may add to these, the prodigalities
of the court, which, supposing its wealth inexhaustible, tried to
corrupt all the ministers of the other powers, in furtherance of the
chimera of universal dominion. The richest state is that in which there
is most industry, whereby the inhabitants may procure every thing
indispensable to the conveniences and comforts of life. Gold as a useful
metal, and a medium of exchange, is undoubtedly very precious, and an
adequate quantity for these exchanges must be had; but as it is good for
very little besides, nay, as an excess is even hurtful, it soon begins
to fly of itself towards the places where it is more needed or less
common.
With regard to the relative value of gold and silver, several details
have already been given in our view of the mineral wealth of the globe.
Three centuries ago, an ounce of gold was worth at London or Paris 10
ounces of silver; now it may be exchanged for 15 ounces and a half.
The _par_ of two coins results from the comparison of their weight and
standard fineness. Let us take for an example the conversion of English
gold sovereigns worth 20 shillings or a pound sterling, in relation to
the French louis of 20 francs. The standard of the sovereign gold is
0·917, fine gold being 1000; its weight is 125·256 gr. English, or
7·980855 grammes; by multiplying this weight into its standard, we have
a product of 7·318444035; this is, in grammes, the quantity of pure gold
contained in the sovereign piece. The piece of 20 francs has a legal
standard of 0·9; and multiplying this number by the weight of the louis,
6·45161 grammes, we find that it contains 5·806449 of pure metal. We
then make this proportion:--
As 5·806449 : 20 francs ∷ 7·31844 : 25·2079 francs; or the value of the
English sovereign is nearly 25·21 francs, in French gold coin. A similar
calculation may be made for silver coins. The French rule for finding
the _par_ of a foreign gold coin, or its intrinsic value in francs, is
to multiply its weight by its standard or titre, and that product by
3-4/9. The par of foreign silver money, or its intrinsic value in
francs, is obtained by multiplying its weight in grammes by its standard
in thousand parts, and by 2/9. The French 5-franc piece has its standard
or titre at 0·9, and weighs 25 grammes.
The assaying of gold for coin and trinkets requires very delicate
management. The French take half a gramme at most (about 7-1/2 grains)
of gold, and fuse it with thrice its weight of silver, as already
described under ASSAY. The parting is the next operation. For this
purpose the button of gold and silver alloy is first hammered flat on a
piece of steel, and then made feebly red hot in burning charcoal or over
a lamp flame. After being thus annealed, the metal is passed through the
rolling press, till it be converted into a plate about 1/70 of an inch
thick. After annealing this riband, it is coiled into a spiral form,
introduced immediately into a small matrass of a pear shape, an assay
matrass, and about 500 grains of nitric acid, sp. grav. 1·185, are
poured over it. Heat being now applied to the vessel, the solution of
the silver and copper alloys ensues, and after 22 minutes of constant
ebullition, the liquid is poured off and replaced by an equal quantity
of nitric acid, likewise very pure, but of the density 1·28. This is
made to boil for about 10 minutes, and is then poured off, when the
matrass is filled up with distilled water to the brim. In conclusion, a
small annealing crucible is inverted as a cup over the mouth of the
matrass, which is now turned upside down with a steady hand; the slip of
metal falls into the crucible through the water; which by sustaining a
part of its weight, softens its descent and prevents its tearing. The
matrass is then dexterously removed, without letting its water overflow
the crucible. The water is gently decanted from the crucible, which is
next covered, placed in the middle of burning charcoal, and withdrawn
whenever it becomes red hot. After cooling, the metal slip is weighed
very exactly, whence the weight of fine gold in the alloy is known.
Stronger acid than that prescribed above would be apt to tear the
metallic riband to pieces, and it would be difficult to gather the fine
particles of gold together again. The metallic plate becomes at last
merely a golden sieve, with very little cohesion. When copper is to be
separated from gold by cupellation, a higher temperature is requisite
than in cupelling silver coin.
The coining apparatus of the Royal Mint of London is justly esteemed a
masterpiece of mechanical skill and workmanship. It was erected in 1811,
under the direction of the inventor, Mr. Boulton; and has since been
kept in almost constant employment.
[Illustration: 738 739]
The melting pots (_fig._ 738.) are made of cast iron, and hold
conveniently 400 pounds of metal. They are furnished with a spout or lip
for pouring out the metal, and with two ears, on which the tongs of the
crane lay hold in lifting them out of the furnace. The pot rests on
pedestals on the grate of the furnace, and has a ring cast on its edge
to prevent the fuel falling into it. Whenever it becomes red hot, the
metal properly prepared and mixed, so as to produce an alloy containing
0·915 parts of gold, is put in, and during the melting, which occupies
some hours, it is occasionally stirred. The moulds are meanwhile
prepared by warming them in a stove, and thereafter by rubbing their
inside surfaces with a cloth dipped in oil, by which means the ingots
cast in them get a better surface. _Fig._ 739. represents a side view of
the carriage, charged with its moulds. When the proper number of moulds
is introduced, the screws at the end, represented at _t_ T, are screwed
fast, to fix them all tight.
[Illustration: 740]
The pot of fused metal is lifted out of the furnace by the crane (_fig._
740.), then swung round, and lowered down into the cradle _l_, _m_, _n_,
_o_ of the pouring machine, until the ring on the edge of it rests on
the iron hoop _n_, which, being screwed tight up, holds it secure, and
the crane-tongs are removed. One of the assistants now takes the winch
handle _s_ in one hand, and _y_ in the other. By turning _y_ he moves
the carriage forward, so as to bring the first mould beneath the lip of
the melting pot; and by turning _s_, he inclines the pot, and pours the
metal into the mould. He then fills the other moulds in succession. The
first portion of liquid metal is received in a small iron spoon, and is
reserved for the assay-master; a second sample is taken from the centre
of the pot, and a third from the bottom part. Each of these is examined
as to its quality.
The ingots, which are about 10 inches long, 7 broad, and 6 tenths of an
inch thick, are now carried to the rolling mill.
[Illustration: 741]
_Fig._ 741., where A represents a large spur wheel, fixed on the
extremity of a long horizontal shaft B B, extending beneath the whole
mill. This wheel and shaft are driven by a smaller wheel, fixed on the
main or fly-wheel shaft of a steam engine of 36-horse power. The main
shaft B of the rolling mill has wheels C, D, E fixed upon it, to give
motion to the respective rollers, which are mounted at F and G, in
strong iron frames, bolted to the iron sills _a a_, which extend through
the whole length of the mill, and rest upon the masonry, in which the
wheels are concealed. The two large wheels C and E give motion to the
wheels H, I, which are supported on bearings between two standards _b_,
_b_, bolted down to the ground sills. On the ends of the axes of these
wheels are heads for the reception of coupling boxes _d_, _d_, which
unite them to short connecting shafts K L; and these again, by means of
coupling boxes, convey motion to the upper rollers _e_, _e_, of each
pair, at F and G. The middle wheel D upon the-main shaft B gives motion
to the lower rollers in a similar manner. Thus both the rollers _e_, _f_
of each frame receive their motion from the main shaft with equal
velocity, by means of wheels of large radius, which act with much more
certainty than the small pinions usually employed in rolling mills to
connect the upper and lower rollers, and cause them to move together.
The rolling mill contains four pairs of rollers, each driven by its
train of wheel work; the mill, therefore, consists of two such sets of
wheels and rollers as are represented in our figure. The two shafts are
situated parallel to each other, and receive their motion from the same
steam engine. This admirable rolling mill was erected by John Rennie,
Esq.
The ingots are heated to redness in a furnace before they are rolled.
The two furnaces for this purpose are situated before two pairs of
rollers, which, from being used to consolidate the metal by rolling
whilst hot, are termed breaking-down rollers. Two men are employed in
this operation; one taking the metal from the furnace with a pair of
tongs, introduces it between the rollers; and the other, catching it as
it comes through, lifts it over the top roller, and returns it to his
fellow, who puts it through again, having previously approximated the
rollers a little by their adjusting screws. After having been rolled in
this manner four or five times, they are reduced to nearly two-tenths of
an inch thick, and increased lengthwise to about four times the breadth
of the ingot. These plates, while still warm, are rubbed over with a
dilute acid or _pickle_, to remove the colour produced by the heat, and
are then cut up into narrow slips across the breadth of the plate, by
means of the circular shears _fig._ 742.
[Illustration: 742]
This machine is worked by a spur-wheel at the extremity of the main
shaft B of the rolling mill (_fig._ 741.) It consists of a framing of
iron A A, supporting two shafts B B, which are parallel to each other,
and move together by means of two equal spur-wheels C C, the lower one
of which works with the teeth of the great wheel above mentioned, upon
the main shaft of the rolling mill. At the extremities of the two
shafts, wheels or circular cutters are fixed with their edges
overlapping each other a little way. F represents a shelf on which the
plate is laid, and advanced forward to present it to the cutter; and G
is a ledge or guide, screwed down on it, to conduct the metal and to
regulate the breadth of the piece to be cut off. Hence the screws which
fasten down the ledge are fitted in oblong holes, which admit of
adjustment. The workman holds the plate flat upon the surface F, and
pushing it towards the shears, they will lay hold of it, and draw it
through until they have cut the whole length. The divided parts are also
prevented from curling up into scrolls, as they do when cut by a common
pair of shears; because small shoulders on E and D, behind the cutting
edge, keep them straight. Behind the standard, supporting the back
pivots of the shafts B B of the cutter, is a frame _l_, with a screw _m_
tapped through it. This is used to draw the axis of the upper cutter D
endwise, and keep its edge in close contact with the edge of the other
cutter E. The slips or ribands of plate are now carried to the other two
pairs of rollers in the rolling mill, which are made of case-hardened
iron, and better polished than the breaking-down rollers. The plates are
passed cold between these, to bring them to exactly the same thickness;
whence they are called adjusting or planishing rollers. The workman here
tries every piece by a common gauge, as it comes through. This is a
piece of steel having a notch in it; the inside lines of which are very
straight, and inclined to one another at a very acute angle. They are
divided by fine lines, so that the edge of the plate being pressed into
the notch, will have its thickness truly determined by the depth to
which it enters, the divisions showing the thickness in fractions of an
inch.
In rolling the plate the second time, all the plates are successively
passed through the rollers; then the rollers being adjusted, they are
passed through another time. This is repeated thrice or even four times;
after which they are all tried by the gauge, and thus sorted into as
many parcels as there are different thicknesses. It is a curious
circumstance, that though the rollers are no less than 14 inches in
diameter, and their frame proportionally strong, they will yield in some
degree, so as to reduce a thick plate in a less degree than a thin one;
thus the plates which have all passed through the same rollers, may be
of 3 or 4 different degrees of thickness, which being sorted by the
gauge into as many parcels, are next reduced to the exact dimension, by
adapting the rollers to each parcel. The first of the parcel which now
comes through is tried, by cutting out a circular piece with a small
hand machine, and weighing it. If it proves either too light or too
heavy, the rollers are adjusted accordingly, till by a few such trials
they are found to be correct, when all the parcel is rolled through. The
trial plates which turn out to be too thin, are returned as waste to the
melting-house. By these numerous precautions, the blanks or circular
discs, when cut out by the next machine, will be very nearly of the same
weight; which they would scarcely be, even if the gauge determined all
the plates to the same thickness, because some being more condensed than
others, they would weigh differently under the same volume.
[Illustration: 743 744]
A great improvement has been made on that mode of lamination, by the
late Mr. Barton’s machine for equalizing the thickness of slips of metal
for making coin, which has been for several years introduced into the
British mint. A side elevation is shown in _fig._ 743., and a plan in
_fig._ 744. It operates in the same way as wire-drawing mechanisms;
namely, pulls the slips of metal forcibly through an oblong opening,
left between two surfaces of hardened steel. The box or case which
contains the steel dies, composed of two hardened cylinders, is
represented at C in _fig._ 743. The pincers employed to hold the metal,
and draw it through, are shown at _s r_.
The slips of metal to be operated on by the drawing machine, are first
rendered thinner at one end, that they may be introduced between the
dies, and also between the jaws of the pincers. This thinning of the
ends is effected by another machine, consisting of a small pair of
rollers, mounted in an iron frame, similar to a rolling-mill. The upper
roller is cylindrical, but the lower is formed with 3 flat sides,
leaving merely portions of the cylinder entire, between these flat
sides. The distance between the centres of the rollers is regulated by
screws, furnished with wheels on their upper ends, similar to what is
seen in the drawing dies at C. The two rollers have pinions on their
axes, which make them revolve together; they are set in motion by an
endless strap passing round a drum, upon whose axis is a pinion working
into the teeth of a wheel fixed upon the axis of the lower roller.
The end of a slip of metal is presented between the rollers while they
are in motion, not on that side of the roller which would operate to
draw in the slip between them, as in the rolling-press above described,
but on the contrary side, so that when one of the flat sides of the
under roller fronts horizontally the circumference of the upper roller,
an opening is formed, through which the slip of metal is to be inserted
until it bears against a fixed stop at the back of the rollers. As the
rollers continue to turn round, the cylindrical portions come opposite
to each other, and press the metal between them, forcing it outwards,
and rendering the part which has been introduced between the rollers as
thin as the space between their cylindrical surfaces. Thus the end of
the slip of metal becomes attenuated enough to pass between the dies of
the drawing machine, and to be seized by the pincers.
In using the drawing machine, a boy takes hold of the handle _s_ of the
pincers, their hook of connexion with the endless chain _l_, _l_, not
shown in the present figure, being disengaged, and he moves them upon
their wheels towards the die-box C. In this movement the jaws of the
pincers get opened, and they are pushed up so close to the die-box that
their jaws enter a hollow, which brings them near the dies, enabling
them to seize the end of the slip of metal introduced between them by
the action of the preparatory rollers. The boy now holds the handle _s_
on the top of the pincers fast, and with his other hand draws the handle
_x_ backwards. Thus the jaws are closed, and the metal firmly griped. He
now presses down the handle _x_ till a hook on the under side of the
pincers seizes the endless chain as it moves along, when it carries the
pincers, and their slip of metal, onwards with it. Whenever the whole
length of the metallic riband has passed through between the dies, the
strain on the pincers is suddenly relieved, which causes the weight _r_
to raise their hook out of the chain, and stop their motion. The machine
in the mint has two sets of dies, and two endless chains, as represented
in the plan, _fig._ 744. N N, are toothed wheels in the upper end of the
die-box, furnished with pinions and levers, for turning them round, and
adjusting the distance between the dies. A large spur-wheel G, is fixed
upon the axis F, to give motion to the endless chains; see both figures.
This spur-wheel is turned by a pinion H, fixed upon an axis _m_,
extending across the top of the frame, and working in bearings at each
end. A spur-wheel I, is fixed upon the axis _m_, and works into the
teeth of a pinion K, upon a second axis across the frame, which carries
likewise a drum wheel L, through which motion is communicated to the
whole mechanism by an endless strap.
[Illustration: 745]
The cutting-out machine is exhibited in _fig._ 745. A A is a basement of
stone to support an iron plate B B, on which stand the columns C C, that
bear the upper part D of the frame. The iron frame of the machine E, F,
E, is fixed down upon the iron plate B, B. The punch _d_ is fixed in the
lower part of the inner frame, and is moved up and down by the screw
_a_, which is worked by wipers turned by a steam engine, impelling the
lever H, and turning backwards and forwards the axis G, through a
sufficient space for cutting the thickness of the metallic lamina. A boy
manages this machine. There are twelve of them mounted on the same
basement frame in a circular range contained in an elegant room, lighted
from the roof. The whole are moved by a steam engine of 16-horse power.
The _blanks_ or _planchets_ thus cut out, were formerly adjusted by
filing the edges, to bring them to the exact weight; a step which Mr.
Barton’s ingenious mechanism has rendered in a great measure
unnecessary. The edge is then milled, by a process which Mr. Boulton
desires to keep secret, and which is therefore not shown in our mint.
But the French mint employs a very elegant machine for the purpose of
lettering or milling the edges, called the _cordon des monnaies_,
invented by M. Gengembre, which has entirely superseded the older
milling machine of M. Castaing, described in the Encyclopedias. The
Napoleon coins of France bear on the edge, in sunk letters, the legend,
_Dieu protège la France_; and those of the king, _Domine salvum fac
regem_. This is marked before striking the blank or _flan_. One machine
imprints this legend, and its service is so prompt and easy, that a
single man marks in a day 20,000 pieces of 5 francs, or 100,000 francs.
[Illustration: 746]
Each of the two _arc_ dies E, D, (_fig._ 746.) carries one half of the
legend, engraved in relief on the curved face; these arcs are pieces of
steel tempered very hard, and fixed with two screws, one immoveably at
E, on the sill which bears the apparatus; the other at D, at the
extremity of the lever P, D, which turns round the axis C. The letters
of these demi-legends are exactly parallel, and inscribed in an inverse
order on the dies. An alternating circular motion is communicated to the
handle P. The curvatures of the two dies are arcs of circles described
from the centre C; and the interval which separates them, or the
difference of the radii, is precisely the diameter of the piece to be
milled.
As the centre C sustains the whole strain of the milling, and produces,
of consequence, a hard friction, this axis must possess a considerable
size. It is composed of a squat truncated cone of tempered steel, which
enters into an eye of the moveable piece P, D. This cone is kept on the
plate of the metal N N, which bears the whole machine, by a nut, whose
screw, by being tightened or slackened, gives as much freedom as is
requisite for the movement of rotation, or removes the shake which hard
service gives to the cone in its eye. The middle thickness of the hole
of the moveable piece P, D, and the axis of the lever P, which
terminates it, are exactly on a level with the engraved letters of the
die, so that no strain can derange the movable piece, or disturb the
centre by its oscillations.
At _a_ is a vertical tube, containing a pile of blanks for milling. It
is kept constantly full; the tube being open at both ends, a little
elevated above the circular space _a_, K, _b_, which separates the dies,
and fixed by a tail _m_ with a screw to the motionless piece A, B. The
branch I, _c_, movable with the piece P, D, passes under the tube, and
pushes before it the blank at the bottom of the column, which is
received into a small excavation in the form of a circular step, and
carried forwards. Matters are thus so arranged as to regulate the issue
of the blanks, one by one, on the small step, called the _posoir_ (bed.)
As soon as the blank is pushed forwards into contact with the lower edge
of the engraved grooves, it is seized by them, and carried on by the
strain of milling, without exposing the upper or under surfaces of the
_blank_ to any action which may obstruct the printing on its edge.
The blank is observed to revolve between the two dies according as the
lever P completes its course, and this blank passing from _a_ to K, then
to _b_, meets a circular aperture _b_, through which it falls into a
drawer placed under the sill.
The range of the movable lever P is regulated by four pieces, F, F, F,
F, solidly sunk in the plate N, N, which bears the whole apparatus. A
stud placed on this lever towards D, makes the arm of the _posoir_ I _c_
retire no farther than is necessary for the little blank to issue from
the column; and a spring fixed to the centre _c_, and supported on a
peg, brings back the _posoir_; so that when a screw I comes to strike
against the column, the _posoir_ stops, and the movable die D, which
continues its progress, finds the blank in a fit position for pressing,
seizing, and carrying it on, by reaction of the fixed die E. Thus the
edge of the blank is lettered in half a second. A hundred may easily be
marked in about three minutes.
The coining press is the most beautiful part of the whole mechanism in
the British mint; but the limits of this volume will not allow of its
being figured upon an adequate scale. An engraving of it may be seen in
the Encyclopedia Britannica.
The only attention which this noble machine requires is that of a little
boy, who stands in a sunk place before the press, and always keeps the
tube full of blanks. He has two strings, one of which, when pulled, will
put the press in motion by the concealed mechanism in the apartment
above; and the other string, when snatched, stops the press. This
coining operation goes on at the rate of 60 or 70 strokes per minute;
and with very few interruptions during the whole day. The press-room at
the Royal Mint contains eight machines, all supported on the same stone
base; and the iron beams between the columns serve equally for the
presses on each side. The whole has therefore a magnificent appearance.
The eight presses will strike more than 19,000 coins in an hour, with
only a child to supply each. The grand improvement in these presses,
consists; 1. in the precision with which they operate to strike every
coin with equal force, which could not be ensured by the old press
impelled by manual labour; 2. The rising collar or steel ring in which
they are struck, keeps them all of one size, and makes a fair edge,
which was not the case with the old coins, as they were often rounded
and defaced by the expansion of the metal under the blow; 3. The
twisting motion of the upper die is thought to produce a better surface
on the flat parts of the coin; but this is somewhat doubtful; 4. The
feeding mechanism is very complete, and enables the machine to work much
quicker than the old press did, where the workman, being in constant
danger of having his fingers caught, was obliged to proceed cautiously,
as well as to place the coin true on the die, which was seldom perfectly
done. The feeding mechanism of the above press is a French invention;
but Mr. Boulton is supposed to have improved upon it.
MIRRORS. See COPPER and GLASS.
MISPICKEL, is arsenical pyrites.
MOHAIR, is the hair of a goat which inhabits the mountains in the
vicinity of Angora, in Asia Minor.
MOIRÉE METALLIQUE, called in this country crystallized tin-plate, is a
variegated primrose appearance, produced upon the surface of tin-plate,
by applying to it in a heated state some dilute nitro-muriatic acid for
a few seconds, then washing it with water, drying, and coating it with
lacquer. The figures are more or less beautiful and diversified,
according to the degree of heat, and relative dilution of the acid. This
mode of ornamenting tin-plate is much less in vogue now than it was a
few years ago.
MOLASSE, is a sandstone belonging to the tertiary strata, employed under
that name by the Swiss for building.
MOLASSES, is the brown viscid uncrystallizable liquor, which drains from
cane sugar in the colonies. See SUGAR.
MOLYBDENUM (_Molybdène_, Fr.; _Molybdan_, Germ.); is a rare metal which
occurs in nature sometimes as a sulphuret, sometimes as molybdic acid,
and at others as molybdate of lead. Its reduction from the acid state by
charcoal requires a very high heat, and affords not very satisfactory
results. When reduced by passing hydrogen over the ignited acid, it
appears as an ash-gray powder, susceptible of acquiring metallic lustre
by being rubbed with a steel burnisher; when reduced and fused with
charcoal, it possesses a silver white colour, is very brilliant, hard,
brittle, of specific gravity 8·6; it melts in a powerful air-furnace,
oxidizes with heat and air, burns at an intense heat into molybdic acid,
dissolves in neither dilute sulphuric, muriatic, nor fluoric acids, but
in the concentrated sulphuric and nitric.
The protoxide consists of 85·69 of metal, and 14·31 of oxygen; the
deutoxide consists of 75 of metal, and 25 of oxygen; and the peroxide,
or molybdic acid, of 66·6 of metal, and 33·4 of oxygen. These substances
are too rare at present to be used in any manufacture.
MORDANT, in dyeing and calico-printing, denotes a body which, having a
twofold attraction for organic fibres and colouring particles, serves as
a bond of union between them, and thus gives fixity to dyes; or it
signifies a substance which, by combining with colouring particles in
the pores of textile filaments, renders them insoluble in hot soapy and
weak alkaline solutions. In order properly to appreciate the utility and
the true functions of mordants, we must bear in mind that colouring
matters are peculiar compounds possessed of certain affinities, their
distinctive characters being not to be either acid or alkaline, and yet
to be capable of combining with many bodies, and especially with
salifiable bases, and of receiving from each of them modifications in
their colour, solubility, and alterability. Organic colouring
substances, when pure, have a very energetic attraction for certain
bodies, feeble for others, and none at all for some. Among these
immediate products of animal or vegetable life, some are soluble in pure
water, and others become so only through peculiar agents. We may thus
readily conceive, that whenever a dye-stuff possesses a certain affinity
for the organic fibre, it will be able to become fixed on it, or to dye
it without the intervention of mordants, if it be insoluble by itself in
water, which, in fact, is the case with the colouring matters of
safflower, annotto, and indigo. The first two are soluble in alkalis;
hence, in order to use them, they need only be dissolved in a weak
alkaline lye, be thus applied to the stuffs, and then have their
tinctorial substance precipitated within their pores, by abstracting
their solvent alkali with an acid. The colouring matter, at the instant
of ceasing to be liquid, is in an extremely divided state, and is in
contact with the organic fibres for which it has a certain affinity. It
therefore unites with them, and, being naturally insoluble in water,
that is, having no affinity for this vehicle, the subsequent washings
have no effect upon the dye. The same thing may be said of indigo,
although its solubility in the dye-bath does not depend upon a similar
cause, but is due to a modification of its constituent elements, in
consequence of which it becomes soluble in alkalis. Stuffs plunged into
this indigo bath get impregnated with the solution, so that when again
exposed to the air, the dyeing substance resumes at once its primitive
colour and insolubility, and washing can carry off only the portions in
excess above the intimate combination, or which are merely deposited
upon the surface of the stuff.
Such is the result with insoluble colouring matters; but for those which
are soluble it should be quite the reverse, since they do not possess an
affinity for the organic fibres which can counterbalance their affinity
for water. In such circumstances, the dyer must have recourse to
intermediate bodies, which add their affinity for the colouring matter
to that possessed by the particles of the stuff, and increase by this
twofold action the intimacy and the stability of the combination. These
intermediate bodies are the true _mordants_.
Mordants are in general found among the metallic bases or oxides; whence
they might be supposed to be very numerous, like the metals; but as they
must unite the twofold condition of possessing a strong affinity for
both the colouring matter and the organic fibre, and as the insoluble
bases are almost the only ones fit to form insoluble combinations, we
may thus perceive that their number may be very limited. It is well
known, that although lime and magnesia, for example, have a considerable
affinity for colouring particles, and form insoluble compounds with
them, yet they cannot be employed as mordants, because they possess no
affinity for the textile fibres.
Experience has proved, that of all the bases, those which succeed best
as mordants are alumina, tin, and oxide of iron; the first two of which,
being naturally white, are the only ones which can be employed for
preserving to the colour its original tint, at least without much
variation. But whenever the mordant is itself coloured, it will cause
the dye to take a compound colour quite different from its own. If, as
is usually said, the mordant enters into a real chemical union with the
stuff to be dyed, the application of the mordant should obviously be
made in such circumstances as are known to be most favourable to the
combination taking place; and this is the principle of every day’s
practice in the dyehouse.
In order that a combination may result between two bodies, they must not
only be in contact, but they must be reduced to their ultimate
molecules. The mordants that are to be united with stuffs are, as we
have seen, insoluble of themselves, for which reason their particles
must be divided by solution in an appropriate vehicle. Now this solvent
or menstruum will exert in its own favour an affinity for the mordant,
which will prove to that extent an obstacle to its attraction for the
stuff. Hence we must select such solvents as have a weaker affinity for
the mordants than the mordants have for the stuffs. Of all the acids
which can be employed to dissolve alumina, for example, vinegar is the
one which will retain it with least energy, for which reason the acetate
of alumina is now generally substituted for alum, because the acetic
acid gives up the alumina with such readiness, that mere elevation of
temperature is sufficient to effect the separation of these two
substances. Before this substitution of the acetate, alum alone was
employed; but without knowing the true reason, all the French dyers
preferred the alum of Rome, simply regarding it to be the purest; it is
only within these few years that they have understood the real grounds
of this preference. This alum has not, in fact, the same composition as
the alums of France, England, and Germany, but it consists chiefly of
cubic alum having a larger proportion of base. Now this extra portion of
base is held by the sulphuric acid more feebly than the rest, and hence
is more readily detached in the form of a mordant. Nay, when a solution
of cubic alum is heated, this redundant alumina falls down in the state
of a subsulphate, long before it reaches the boiling point. This
difference had not, however, been recognised, because Roman alum, being
usually soiled with ochre on the surface, gives a turbid solution,
whereby the precipitate of subsulphate of alumina escaped observation.
When the liquid was filtered, and crystallized afresh, common octahedral
alum alone was obtained; whence it was most erroneously concluded, that
the preference given to Roman alum was unjustifiable, and that its only
superiority was in being freer from iron.
Here a remarkable anecdote illustrates the necessity of extreme caution,
before we venture to condemn from theory a practice found to be useful
in the arts, or set about changing it. When the French were masters in
Rome, one of their ablest chemists was sent thither to inspect the
different manufactures, and to place them upon a level with the state of
chemical knowledge. One of the fabrics, which seemed to him furthest
behindhand, was precisely that of alum, and he was particularly hostile
to the construction of the furnaces, in which vast boilers received heat
merely at their bottoms, and could not be made to boil. He strenuously
advised them to be new modelled upon a plan of his own; but,
notwithstanding his advice, which was no doubt very scientific, the old
routine kept its ground, supported by utility and reputation, and very
fortunately, too, for the manufacture; for had the higher heat been
given to the boilers, no more genuine cubical alum would have been made,
since it is decomposed at a temperature of about 120° F., and common
octahedral alum would alone have been produced. The addition of a little
alkali to common alum brings it into the same basic state as the alum of
Rome.
The two principal conditions, namely, extreme tenuity of particles, and
liberty of action, being found in a mordant, its operation is certain.
But as the combination to be effected is merely the result of a play of
affinity between the solvent and the stuff to be dyed, a sort of
partition must take place, proportioned to the mass of the solvent, as
well as to its attractive force. Hence the stuff will retain more of the
mordant when its solution is more concentrated, that is, when the base
diffused through it is not so much protected by a large mass of
menstruum; a fact applied to very valuable uses by the practical man. On
impregnating in calico printing, for example, different spots of the
same web with the same mordant in different degrees of concentration,
there is obtained in the dye-bath a depth of colour upon these spots
intense in proportion to the strength of their various mordants. Thus,
with solution of acetate of alumina in different grades of density, and
with madder, every shade can be produced, from the fullest red to the
lightest pink; and, with acetate of iron and madder, every shade from
black to pale violet.
We hereby perceive that recourse must indispensably be had to mordants
at different stages of concentration; a circumstance readily realized by
varying the proportions of the watery vehicle. See CALICO-PRINTING and
MADDER. When these mordants are to be topically applied, to produce
partial dyes upon cloth, they must be thickened with starch or gum, to
prevent their spreading, and to permit a sufficient body of them to
become attached to the stuff. Starch answers best for the more neutral
mordants, and gum for the acidulous; but so much of them should never
be used, as to impede the attraction of the mordant for the cloth. Nor
should the thickened mordants be of too desiccative a nature, lest they
become hard, and imprison the chemical agent before it has had an
opportunity of combining with the cloth, during the slow evaporation of
its water and acid. Hence the mordanted goods, in such a case, should be
hung up to dry in a gradual manner, and when oxygen is necessary to the
fixation of the base, they should be largely exposed to the atmosphere.
The foreman of the factory ought, therefore, to be thoroughly conversant
with all the minutiæ of chemical reaction. In cold and damp weather he
must raise the temperature of his drying-house, in order to command a
more decided evaporation; and when the atmosphere is unusually dry and
warm, he should add deliquescent correctives to his thickening, as I
have particularized in treating of some styles of calico-printing. But,
supposing the application of the mordant and its desiccation to have
been properly managed, the operation is by no means complete; nay, what
remains to be done is not the least important to success, nor the least
delicate of execution. Let us bear in mind that the mordant is intended
to combine not only with the organic fibre, but afterwards also with the
colouring matter, and that, consequently, it must be laid entirely bare,
or scraped clean, so to speak, that is, completely disengaged from all
foreign substances which might invest it, and obstruct its intimate
contact with the colouring matters. This is the principle and the object
of two operations, to which the names of _dunging_ and _clearing_ have
been given.
If the mordant applied to the surface of the cloth were completely
decomposed, and the whole of its base brought into chemical union with
it, a mere rinsing or scouring in water would suffice for removing the
viscid substances added to it, but this never happens, whatsoever
precautions may be taken; one portion of the mordant remains untouched,
and besides, one part of the base of the portion decomposed does not
enter into combination with the stuff, but continues loose and
superfluous. All these particles, therefore, must be removed without
causing any injury to the dyes. If in this predicament the stuff were
merely immersed in water, the free portion of the mordant would
dissolve, and would combine indiscriminately with all the parts of the
cloth not mordanted, and which should be carefully protected from such
combination, as well as the action of the dye. We must therefore add to
the scouring water some substance that is capable of seizing the mordant
as soon as it is separated from the cloth, and of forming with it an
insoluble compound; by which means we shall withdraw it from the sphere
of action, and prevent its affecting the rest of the stuff, or
interfering with the other dyes. This result is obtained by the addition
of cow-dung to the scouring bath; a substance which contains a
sufficiently great proportion of soluble animal matters, and of
colouring particles, for absorbing the aluminous and ferruginous salts.
The heat given to the dung-bath accelerates this combination, and
determines an insoluble and perfectly inert coagulum.
Thus the dung-bath produces at once the solution of the thickening
paste; a more intimate union between the alumina or iron and the stuff,
in proportion to its elevation of temperature, which promotes that
union; an effectual subtraction of the undecomposed and superfluous part
of the mordant, and perhaps a commencement of mechanical separation of
the particles of alumina, which are merely dispersed among the fibres; a
separation, however, which can be completed only by the proper scouring,
which is done by the dash-wheel with such agitation and pressure (see
BLEACHING and DUNGING) as vastly facilitate the expulsion of foreign
particles. See also BRAN.
Before concluding this article, we may say a word or two about
astringents, and especially gall-nuts, which have been ranked by some
writers among mordants. It is rather difficult to account for the part
which they play. Of course we do not allude to their operation in the
black dye, where they give the well known purple-black colour with salts
of iron; but to the circumstance of their employment for madder dyes,
and especially the Adrianople red. All that seems to be clearly
established is, that the astringent principle or tannin, whose peculiar
nature in this respect is unknown, combines like mordants with the
stuffs and the colouring substance, so as to fix it; but as this tannin
has itself a brown tint, it will not suit for white grounds, though it
answers quite well for pink grounds. When white spots are desired upon a
cloth prepared with oil and galls, they are produced by an oxygenous
discharge, effected either through chlorine or chromic acid.
MORDANT, is also the name sometimes given to the adhesive matter by
which gold-leaf is made to adhere to surfaces of wood and metal in
gilding. Paper, vellum, taffety, &c., are easily gilt by the aid of
different mordants, such as the following: 1. beer in which some honey
and gum arabic have been dissolved; 2. gum arabic, sugar, and water; 3.
the viscid juice of onion or hyacinth, strengthened with a little gum
arabic. When too much gum is employed, the silver or gold leaf is apt to
crack in the drying of the mordant. A little carmine should be mixed
with the above colourless liquids, to mark the places where they are
applied. The foil is applied by means of a dossil of cotton wool, and
when the mordant has become hard, the foil is polished with the same.
The best medium for sticking gold and silver leaf to wood, is the
following, called _mixtion_ by the French artists:--1 pound of amber is
to be fused, with 4 ounces of mastic in tears, and 1 ounce of Jewish
pitch, and the whole dissolved in 1 pound of linseed oil rendered drying
by litharge.
Painters in distemper sometimes increase the effect of their work, by
patches of gold leaf, which they place in favourable positions; they
employ the above mordant. The manufacturers of paper hangings of the
finer kinds attach gold and silver leaf to them by the same varnish.
MOROCCO. See LEATHER.
MORPHIA (_Morphine_, Fr.; _Morphin_, Germ.), is a vegeto-alkali which
exists associated with opian, codeïne, narcotine, meconine, meconic
acid, resin, gum, bassorine, lignine, fat oil, caoutchouc, extractive,
&c., in opium. Morphia is prepared as follows: Opium in powder is to be
repeatedly digested with dilute muriatic acid, slightly heated, and
sea-salt is to be added, to precipitate the opian. The filtered liquid
is to be supersaturated with ammonia, which throws down the morphia,
along with the meconine, resin, and extractive. The precipitate is to be
washed with water, heated, and dissolved in dilute muriatic acid; the
solution is to be filtered, whereby the foreign matters are separated
from the salt of morphia, which concretes upon cooling, while the
meconine remains in the acid liquid. The muriate of morphia having been
squeezed between folds of blotting paper, is to be sprinkled with water,
again squeezed, next dissolved in water, and decomposed by water of
ammonia. The precipitate, when washed, dried, dissolved in alcohol, and
crystallized, is morphia.
These crystals, which contain 6·32 per cent. of combined water, are
transparent, colourless, four-sided prisms, without smell, and nearly
void of taste, fusible at a moderate heat, and then concrete into a
radiated translucent mass, but at a higher temperature they grow
purple-red. Morphia consists of 72·34 of carbon; 6·366 of hydrogen; 5 of
azote; and 16·3 of oxygen. It burns with a red and very smoky flame, is
stained red by nitric acid, is soluble in 30 parts of boiling anhydrous
alcohol, in 500 parts of boiling water, but hardly if at all in cold
water, and is insoluble in ether and oils. The solutions have a strong
bitter taste, and an alkaline reaction upon litmus paper. The saline
compounds have a bitter taste, are mostly crystallizable, are soluble in
water and alcohol (but not in ether), and give a blue colour to the
peroxide salts of iron. It is a very poisonous substance. Acetate of
morphia is sometimes prescribed, instead of opium, in medicine.
MORTAR, HYDRAULIC, called also _Roman Cement_, is the kind of mortar
used for building piers, or walls under or exposed to water, such as
those of harbours, docks, &c. The poorer sorts of limestone are best
adapted for this purpose, such as contain from 8 to 25 per cent. of
foreign matter, in silica, alumina, magnesia, &c. These, though
calcined, do not slake when moistened; but if pulverized they absorb
water without swelling up or heating, like _fat_ lime, and afford a
paste which hardens in a few days under water, but in the air they never
acquire much solidity. Smeaton first discovered these remarkable facts,
and described them in 1759.
The following analyses of different hydraulic limestones, by Berthier,
merit confidence:--
+---------------------------------+------+------+------+------+------+
| |No. 1.|No. 2.|No. 3.|No. 4.|No. 5.|
| +------+------+------+------+------+
| A. _Analyses of limestones._ | | | | | |
|Carbonate of lime | 97·0 | 98·5 | 74·5 | 76·5| 80·0|
|Carbonate of magnesia | 2·0 | -- | 23·0 | 3·0| 1·5|
|Carbonate of protoxide of iron | -- | -- | -- | 3·0| -- |
|Carbonate of manganese | -- | -- | -- | 1·5| -- |
|Silica and alumina | 1·0 | 1·5 | 1·2 |} 15·2|} 18·0|
|Oxide of iron | | | |} |} |
| +------+------+------+------+------+
| |100·0 |100·0 |100·0 | 100·0| 100·0|
+---------------------------------+------+------+------+------+------+
| B. _Analyses of the burnt lime._| | | | | |
|Lime | 96·4 | 97·2 | 78·0 | 68·3| 70·0|
|Magnesia | 1·8 | -- | 20·0 | 2·0| 1·0|
|Alumina | 1·8 | 2·8 | 2·0 | 24·0| 29·0|
|Oxide of iron | -- | -- | -- | 5·7| -- |
| +------+------+------+------+------+
| |100·0 |100·0 |100·0 | 100·0| 100·0|
+---------------------------------+------+------+------+------+------+
No. 1. is from the fresh-water lime formation of Château-Landon, near
Nemours; No. 2. the large-grained limestone of Paris; both of these
afford a fat lime when burnt. Dolomite affords a pretty fat lime, though
it contains 42 per cent. of carbonate of magnesia; No. 3. is a limestone
from the neighbourhood of Paris, which yields a poor lime, possessing no
hydraulic property; No. 4. is the secondary limestone of Metz; No. 5. is
the lime marl of Senonches, near Dreux; both the latter have the
property of hardening under water, particularly the last, which is much
used at Paris on this account.
All good hydraulic mortars must contain alumina and silica; the oxides
of iron and manganese, at one time considered essential, are rather
prejudicial ingredients. By adding silica and alumina, or merely the
former, in certain circumstances, to fat lime, a water-cement may be
artificially formed; as also by adding to lime any of the following
native productions, which contain silicates; puzzolana, trass or tarras,
pumice-stone, basalt-tuff, slate-clay. Puzzolana is a volcanic product,
which forms hills of considerable extent to the south-west of the
Appenines, in the district of Rome, the Pontine marshes, Viterbo,
Bolsena, and in the Neapolitan region of Puzzuoli, whence the name. A
similar volcanic tufa is found in many other parts of the world.
According to Berthier, the Italian puzzolana consists of 44·5 silica;
15·0 alumina; 8·8 lime; 4·7 magnesia; 1·4 potash; 4·1 soda; 12 oxides of
iron and titanium; 9·2 water; in 100 parts.
The _tufa_ stone, which when ground forms _trass_, is composed of 57·0
silica, 16·0 clay, 2·6 lime, 1·0 magnesia, 7·0 potash, 1·0 soda, 5
oxides of iron and titanium, 9·6 water. This tuff is found abundantly
filling up valleys in beds of 10 or 20 feet deep, in the north of
Ireland, among the schistose formations upon the banks of the Rhine, and
at Monheim in Bavaria.
The fatter the lime, the less of it must be added to the ground
puzzolana or trass, to form a hydraulic mortar; the mixture should be
made extemporaneously, and must at any rate be kept dry till about to be
applied. Sometimes a proportion of common sand mortar instead of lime is
mixed with the trass. When the hydraulic cement hardens too soon, as in
12 hours, it is apt to crack; it is better when it takes 8 days to
concrete. Through the agency of the water, silicates of lime, alumina,
(magnesia), and oxide of iron are formed, which assume a stony hardness.
Besides the above two volcanic products, other native earthy compounds
are used in making water cements. To this head belong all limestones
which contain from 20 to 30 per cent. of clay and silica. By gentle
calcination, a portion of the carbonic acid is expelled, and a little
lime is combined with the clay, while a silicate of clay and lime
results, associated with lime in a subcarbonated state. A lime-marl
containing less clay will bear a stronger calcining heat without
prejudice to its qualities as a hydraulic cement; but much also depends
upon the proportion of silica present, and the physical structure of all
the constituents.
The mineral substance most used in England for making such mortar, is
vulgarly called _cement-stone_. It is a reniform limestone, which occurs
distributed in single nodules or rather lenticular cakes, in beds of
clay. They are mostly found in those argillaceous strata which alternate
with the limestone beds of the oolite formation, as also in the clay
strata above the chalk, and sometimes in the London clay. On the coasts
of Kent, in the isles of Sheppey and Thanet, on the coasts of Yorkshire,
Somersetshire, and the Isle of Wight, &c., these nodular concretions are
found in considerable quantities, having been laid bare by the action of
the sea and weather. They were called by the older mineralogists
_Septaria_ and _Ludus Helmontii_ (Van Helmont’s coits). When sawn
across, they show veins of calc-spar traversing the siliceous clay, and
are then sometimes placed in the cabinets of _virtuosi_. They are found
also in several places on the Continent, as at Neustadt-Eberswalde, near
Antwerp, near Altdorf in Bavaria; as also at Boulogne-sur-mer, where
they are called Boulogne-pebbles (_galets_). These nodules vary in size
from that of a fist to a man’s head, they are of a yellow-gray or brown
colour, interspersed with veins of calc-spar, and sometimes contain
cavities bestudded with crystals. Their specific gravity is 2·59.