CRYSTALLIZATION.
[Illustration: Fig. 86. Crystals of snow.]
It has been already stated that the force of cohesion binds the similar
particles of substances together, whether they be _amorphous_ or
shapeless, _crystalline_ or of a regular symmetrical and mathematical
figure. The term crystal was originally applied by the ancients to
silica in the form of what is usually termed rock crystal, or Brazilian
pebble; and they supposed it to be water which had been solidified by a
remarkable intensity of cold, and could not be thawed by any ordinary or
summer heat. Indeed, this idea of the ancients has been embodied (to a
certain extent) in the shape of artificial ice made by crystallizing
large quantities of sulphate of soda, which was made as flat as
possible, and upon [Page 74] which skaters were invited to describe the
figure of eight, at the usual admittance fee, representing twelve pence.
A crystal is now defined to be an inorganic body, which, by the
operation of affinity, has assumed the form of a regular solid
terminated by a certain number of planes or smooth surfaces.
Thousands of minerals are discovered in the crystallized state--such as
cubes of iron pyrites (sulphuret of iron) and of fluor spar (fluoride of
calcium), whilst numerous saline bodies called salts are sold only in
the form of crystals. Of these salts we have excellent examples in Epsom
salts (sulphate of magnesia), nitre (nitrate of potash), alum (sulphate
of alumina), and potash; the term salt being applied specially to all
substances composed of an acid and a base, as also to other combinations
of elements which may or may not take a crystalline form. Thus, nitre is
composed of nitric acid and potash; the first, even when much diluted,
rapidly changes paper, dipped in tincture of litmus and stained blue, to
a red colour, whilst potash shows its alkaline nature, by changing
paper, stained yellow with tincture of turmeric, to a reddish-brown. The
latter paper is restored to its original yellow by dipping it into the
dilute nitric acid, whilst the litmus paper regains its delicate blue
colour by being passed into the alkaline solution. An acid and an alkali
combine and form a neutral salt, such as nitre, which has no action
whatever on litmus or turmeric; whilst the element iodine, which is not
an acid, unites with the metallic element potassium, and therefore not
an alkali, and forms a salt that crystallizes in cubes called iodide of
potassium. Again, cane sugar, which is composed of charcoal, oxygen, and
hydrogen, crystallizes in hard transparent four-sided and irregular
six-sided prisms, but is not called a salt. Silica or sand is found
crystallized most perfectly in nature in six-sided pyramids, but is not
a salt; it is an acid termed silicic-acid. Sand has no acid taste,
because it is insoluble in water, but when melted in a crucible with an
alkali, such as potash, it forms a salt called silicate of potash.
Magnesia, from being insoluble, or nearly so, in water, is all but
tasteless, and has barely any alkaline reaction, yet it is a very strong
alkaline base; 20.7 parts of it neutralize as much sulphuric acid as 47
of potash. A salt is not always a crystallizable substance, and _vice
versâ_. The progress of our chemical knowledge has therefore demanded a
wider extension and application of the term _salt_, and it is not now
confined merely to a combination of an acid and an alkali, but is
conferred even on compounds consisting only of sulphur and a metal,
which are termed _sulphur salts_.
So also in combinations of chlorine, iodine, bromine, and fluorine, with
metallic bodies, neither of which are acid or alkaline, the term _haloid
salts_ has been applied by Berzelius, from the Greek ([Greek: _als_], sea
salt, and [Greek: _eidos_] form), because they are analogous in
constitution to sea salt; and the mention of sea salt again reminds us
of the wide signification of the term salt, originally confined to this
substance, but now extended into four great orders, as defined by
Turner:--
ORDER I. _The oxy-salts._--This order includes no salt the acid or base
of which is not an oxidised body (ex., nitrate of potash).
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ORDER II. _The hydro-salts._--This order includes no salt the acid or
base of which does not contain hydrogen (ex., chloride of ammonium).
ORDER III. _The sulphur salts._--This order includes no salt the
electro-positive or negative ingredient of which is not a sulphuret
(ex., hydrosulphuret of potassium).
ORDER IV. _The haloid salts._--This order includes no salt the
electro-positive or negative ingredient of which is not haloidal. (Exs.,
iodide of potassium and sea salt). To fix the idea of salt still better
in the youthful mind, it should be remembered that alabaster, of which
works of art are constructed, or marble, or lime-stone, or chalk, are
all salts, because they consist of an acid and a base.
In order to cause a substance to crystallize it is first necessary to
endow the particles with freedom of motion. There are many methods of
doing this chemically or by the application of heat, but we cannot by
any mechanical process of concentration, compression, or division,
persuade a substance to crystallize, unless perhaps we except that
remarkable change in wrought or fibrous iron into crystalline or brittle
iron, by constant vibration, as in the axles of a carriage, or by
attaching a piece of fibrous iron to a tilt hammer.
If we powder some alum crystals they will not again assume their
crystalline form; if brought in contact there is no freedom of motion.
It is like placing some globules of mercury on a plate. They have no
power to create motion; their inertia keeps them separated by certain
distances, and they do not coalesce; but incline the plate, give them
motion, and bring them in contact, they soon unite and form one globule.
The particles of alum are not in close contact, and they have no freedom
of motion unless they are dissolved in water, when they become
invisible; the water by its chemical power destroys the mechanical
aggregation of the solid alum far beyond any operation of levigation.
The solid alum has become liquid, like water; the particles are now free
to move without let or hindrance from friction. A solution, (from the
Latin _solvo_, to loosen) is obtained. The alum must indeed be reduced
to minute particles, as they are alike invisible to the eye whether
assisted by the microscope or not. No repose will cause the alum to
separate; the solvent power of the water opposes gravitation; every part
of the solution is equally impregnated with alum, and the particles are
diffused at equal distances through the water; the heavy alum is
actually drawn up against gravity by the water.
How, then, is the alum to be brought back again to the solid state? The
answer is simple enough. By evaporating away the excess of water, either
by the application of heat or by long exposure to the atmosphere in a
very shallow vessel, the minute atoms of the alum are brought closer
together, and crystallization takes place. The assumption of the solid
state is indicated by the formation of a thin film (called a _pellicle_)
of crystals, and is further and still more satisfactorily proved by
taking out a drop of the solution and placing it on a bit of glass,
which rapidly becomes filled with crystals if the evaporation has been
carried sufficiently far (Fig. 87).
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After evaporating away sufficient water, the dish is placed on one side
and allowed to cool, when crystals of the utmost regularity of form are
produced, and, denoted by a geometrical term, are called octohedral or
eight-sided crystals, when in the utmost state of perfection (Fig. 88).
[Illustration: Fig. 87. R R. Ring-stand. S S. Spirit-lamps. A. Flask
containing boiling solution of alum.--_Solution._ B. Funnel, with a bit
of lamp-cotton stuffed in the bottom.--Filtration. C. Evaporating
dish.--Evaporation. D. Drop on glass.--Crystallization.]
[Illustration: Fig. 88.]
The science of crystallography is too elaborate to be discussed at
length in a work of this kind; the various terms connected with crystals
will therefore only be explained, and experiments given in illustration
of the formation of various crystals.
When the apices--_i.e._, the tips or points of crystals--are cut off,
they are said to be truncated; and the same change occurs on the edges
of numerous crystals.
If some of the salt called chloride of calcium in the dry and amorphous
state is exposed to the air, it soon absorbs water, or what is termed
_deliquesces_: the same thing occurs with the crystals of carbonate of
potash, and if four ounces are weighed out in an evaporating dish, and
then exposed for about half an hour to the air, a very perceptible
increase in weight is observed by the assistance of the scales and grain
weights. _Deliquescence_ is a term from the Latin _deliqueo_, to melt,
and is in fact a gradual melting, caused by the absorption of water from
the atmosphere. The reverse of this is illustrated with various
crystals, such as Glauber's salt (sulphate of soda), or common washing
soda (carbonate of soda); if a fine clear crystal is taken out of the
solution, called the mother liquor, in which it has been crystallized,
wiped dry, and placed under a glass shade, this salt may remain for a
long period [Page 77] without change, but if it receive one scratch
from a pin, the door is opened apparently for the escape of the water
which it contains, chemically united with the salt, and called water of
crystallization; the white crystal gradually swells out, the little
_quasi_ sore from the pin-scratch spreads over the whole, which becomes
opaque, and crumbling down falls into a shapeless mass of white dust;
this change is called _efflorescence_, from _effloresco_, to blow as a
flower--caused by the abstraction from them of chemically-combined water
by the atmosphere. With reference to the preservation of crystals,
Professor Griffiths recommends them to be oiled and wiped, and placed
under a glass shade, if of a deliquescent nature; or if efflorescent,
they are perfectly preserved by placing them under a glass shade with a
little water in a cup to keep the air charged with moisture and prevent
any drying up of the crystal.
Deliquescent crystals may be preserved by placing them, when dry, in
naphtha, or any liquor in which they are perfectly insoluble. Some
salts, like Glauber's salts, contain so much water of crystallization
that when subjected to heat they melt and dissolve in it, and this
liquefaction of the solid crystal is called "watery fusion." Other
salts, such as bay salt, chlorate of potash, &c., when heated, fly to
pieces, with a sharp crackling noise, which is due sometimes, to the
unequal expansion of the crystalline surface, or the sudden conversion
of the water (retained in the crystal by capillary attraction) into
steam; thus nitre behaves in this manner, and frequently retains water
in capillary fissures, although it is an anhydrous salt, or salt
perfectly free from combined water. The crackling sound is called
_decrepitation_, and is well illustrated by throwing a handful of bay
salt on a clear fire; but this property is destroyed by powdering the
crystals.
Many substances when melted and slowly cooled concrete into the most
perfect crystals; in these cases heat alone, the antagonist to cohesion,
is the solvent power. Thus, if bismuth be melted in a crucible, and when
cooling, and just as the pellicle (from _pellis_, a skin or crust) is
forming on the surface, if two small holes are instantly made by a rod
of iron and the liquid metal poured out from the inside (one of the
holes being the entrance for the air, the other the exit for the metal);
on carefully breaking the crucible, the bismuth is found to be
crystallized in the most lovely cubes. Sulphur, again, may be
crystallized in prismatic crystals by pursuing a similar plan; and the
great blocks of spermaceti exhibited by wax chandlers in their windows,
are crystallized in the interior and prepared on the same principle.
There are other modes of conferring the crystalline state upon
substances--viz., by elevating them into a state of vapour by the
process called sublimation (from _sublimis_, high or exalted), the
lifting up and condensation of the vapour in the upper part of a vessel;
a process perfectly distinct from that of _distillation_, which means to
separate drop by drop. Both of these processes are very ancient, and
were invented by the Arabian alchemists long antecedent to the seventh
century. Examples of sublimation are shown by heating iodine, and
especially [Page 78] benzoic acid; with the latter, a very elegant
imitation of snow is produced, by receiving the vapour, on some sprigs
of holly or other evergreen, or imitation paper snowdrops and crocuses,
placed in a tasteful manner under a glass vessel. The benzoic acid
should first be sublimed over the sprigs or artificial flowers in a gas
jar, which may be removed when the whole is cold, and a clear glass
shade substituted for it. (Fig. 89.)
All electro deposits on metals are more or less crystalline; and copper
or silver may be deposited in a crystalline form by placing a scraped
stick of phosphorus in a solution of sulphate of copper or of nitrate of
silver. The phosphorus takes away the oxygen from the metal, or
deoxidizes the solution, and the copper or silver reappears in the
metallic form. The surface of the phosphorus must not be scraped in the
air, but under water, when the operation is perfectly safe.
A singular and almost instantaneous crystallization can be produced by
saturating boiling water with Glauber's salt, of which one ounce and a
half of water will usually dissolve about two ounces; having done this,
pour the solution, whilst boiling hot, into clean oil flasks, or vials
of any kind, previously warmed in the oven, and immediately cork them,
or tie strips of wetted bladder, over the orifices of the flasks or
vials, or pour into the neck a small quantity of olive oil, or close the
neck with a cork through which a thermometer tube has been passed. When
cold, no crystallization occurs until atmospheric air is admitted; and
it was formerly believed that the pressure of the air effected this
object, until some one thought of the oil, and now the theory is
modified, and crystallization is supposed to occur in consequence of the
water dissolving some air which causes the deposit of a minute crystal,
and this being the turning point, the whole becomes solid. However the
fact may be explained, it is certain that when the liquid refuses to
crystallize on the admission of air, the solidification occurs directly
a minute crystal of sulphate of soda, or Glauber's salt, is dropped into
the vessel.
When the crystallization is accomplished, the whole mass is usually so
completely solidified, that on inverting the vessel, not a drop of
liquid falls out.
[Illustration: Fig. 89. A. Gas-jar, with stopper open at first, to be
shut when the lamp is withdrawn. B. Wooden stand, with hole to carry the
cup C, containing the benzoic acid, heated below by the spirit-lamp, S.
F. Flowers or sprigs arranged on pieces of rock or mineral.]
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It may be observed that the same mass of salt will answer any number of
times the same purpose. All that is necessary to be done, is to place
the vial or flask, in a saucepan of warm water, and gradually raise it
to the boiling point till the salt is completely liquefied, when the
vessel must be corked and secured from the air as before. When the
solidification is produced much heat is generated, which is rendered
apparent by means of a thermometer, or by the insertion of a copper wire
into the pasty mass of crystal in the flask, and then touching an
extremely thin shaving or cutting of phosphorus, dried and placed on
cotton wool. Solidification in all cases produces heat. Liquefaction
produces cold.
In Masters's freezing apparatus certain measured quantities of
crystallized sal-ammoniac, nitre, and nitrate of ammonia, are placed in
a metallic cylinder, surrounded with a small quantity of spring water
contained in an outer vessel. Directly the crystals are liquefied by the
addition of water, intense cold is produced, which freezes the water and
forms an exact cast of the inner cylinder in ice, and this may
afterwards be removed, by pouring away the liquefied salts, and filling
the inner cylinder, with water of the same temperature as the air, which
rapidly thaws the surrounding ice, and allows it to slip off into any
convenient vessel ready to receive it. (Fig. 90.)
[Illustration: Fig. 90. A. The inner cylinder which contains the
freezing mixture. B B. The outer one containing spring water. C C. The
ice slipping away from the inner cylinder.]
For an ingenious method of obtaining large and perfect crystals of
almost any size, experimentalists are indebted to Le Blanc. His method
consists in first procuring small and perfect crystals--say, octohedra
of alum--and then placing them in a broad flat-bottomed pan, he pours
over the crystals a quantity of saturated solution of alum, obtained by
evaporating a solution of alum until a drop taken out crystallizes on
cooling. The positions of the crystals are altered at least once a day
with a glass rod, so that all the faces may be alternately exposed to
the action of the solution, for the side on which the crystal rests, or
is in contact with the vessel, never receives any increment. The
crystals will thus gradually grow or increase in size, and when they
have done so for some time, the best and most symmetrical, may be
removed and placed separately, in vessels containing some of the same
saturated [Page 80] solution of alum, and being constantly turned they
may be obtained of almost any size desired.
Unless the crystals are removed to fresh solutions, a reaction takes
place, in consequence of the exhaustion of the alum from the water, and
the crystal is attacked and dissolved. This action is first perceptible
on the edges and angles of the crystal; they become blunted and
gradually lose their shape altogether. By this method crystals may be
made to grow in length or breadth--the former when they are placed upon
their sides, the latter if they be made to stand upon their bases.
On Le Blanc's principle, beautiful crystal baskets are made with alum,
sulphate of copper, and bichromate of potash. The baskets are usually
made of covered copper wire, and when the salts crystallize on them as a
nucleus or centre, they are constantly removed to fresh solutions, so
that the whole is completely covered, and red, white, and blue sparkling
crystal baskets formed. They will retain their brilliancy for any time,
by placing them under a glass shade, with a cup containing a little
water.
The sketch below affords an excellent illustration of some of Nature's
remarkable concretions in the peculiar columnar structure of basalt.
[Illustration: Fig. 91. The Giant's Causeway.]
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