CRYSTALLOGENIC FORCES.
Crystallisation and Molecular Force distinguished--Experimental
Proof--Polarity of Particles forming a Crystal--Difference
between Organic and Inorganic Forms--Decomposition
of Crystals in Nature--Substitution of Particles in
Crystals--Pseudomorphism--Crystalline Form not dependent
on Chemical Nature--Isomorphism--Dimorphism--Theories of
Crystallogenic Attraction--Influence of Electricity and
Magnetism--Phenomena during Crystallisation--Can a change of
Form take place in Primitive Atoms?--Illustrative Example of
Crystallisation.
“Crystallisation is a peculiar and most admirable work of nature’s
geometry, worthy of being studied by all the power of genius, and the
whole energy of the mind, not on account of the delight which always
attends the knowledge of wonders, but because of its vast importance
in revealing to us the secrets of nature; for here she does, as it
were, betray herself, and, laying aside all disguise, permits us
to behold, not merely the results of her operations, but the very
processes themselves.”--Such is the language of an Italian philosopher,
Gulielmini; and it is the striking peculiarity of beholding the process
of the formation of the regular geometric figures of crystals, the
gradual accretion of particle to particle, which induces us to separate
crystallization from mere molecular aggregation. Without doubt the
formation of a crystal and the production of an amorphous block are
due to powers which bear a close resemblance in many points; but they
present remarkable differences in others.
Let us take some simple case in illustration. In quiet water we
have very finely divided matter suspended, and matter in a state of
solution. The first is slowly precipitated, and in process of time
consolidates into a hard mass at the bottom, presenting no particular
character, unless it has been placed in some peculiar physical
conditions; when, as in nature, we have a regular bedding which is
intersected by lines of lamination or of cleavage, which we are, from
experiment, enabled to refer to the influence of current electricity.
The second--the matter in solution--is also slowly deposited; but it is
accumulated upon nuclei which possess some peculiar disposing powers,
and every particle is united by some particular face, and an angular
figure of the most perfect character results. Many pleasing experiments
would appear to show that electricity has much to do in the process
of crystallization; but it is evident that it must be under some
peculiarly modified conditions that this power is exerted, if, indeed,
it has any direct action.
The same substances always crystallize in the same forms, unless the
conditions of the crystallizing body are altered. It has been supposed
that each particle of a crystalline mass has certain points or poles
which possess definite properties, and that cohesion takes place only
along lines which have some relation to the attracting or repelling
powers of these poles. We shall have, eventually, to consider results
which appear to prove that magnetism is universal in its influence, and
that this polarity of the particles of matter may be referred to it.
Be the cause of crystallisation what it may, it presents to us in
appearance a near approach in inorganic nature to some of the peculiar
conditions of growth in the organised creation. In one, we have the
gradual production of parts and the formation of members due to
peculiar powers of assimilation, each individual preserving all its
distinguishing features; and in the other, we have a regular order of
cohesion occurring under the influence of a power which draws like to
like, and arranges the whole into a form of beauty.
This appears to be the proper place for correcting an error too
prevalent, relative to the formation of crystals, the development
of cells, and the yet more fatal falsehood of referring the great
phenomena of Life to any of the physical forces with which we are
acquainted.
THE CRYSTAL _forms_, by the accretion of particle to particle, along
lines determined by some yet unknown power. There is no change in the
character of any particle--like coheres to like; the first atom and the
last of the series being identical in character.
THE PLANT _grows_, not by the gathering together of similar particles
of matter, but by the absorption of a compound particle--by that one
which must be regarded as the primary nuclear atom or cell. After
this absorption--in virtue of a power which we call LIFE, excited
into action by LIGHT--the compound particle is decomposed, and one
constituent is retained to effect the formation of a new cell, whilst
the other is liberated as an invisible air. Here we have a change of
chemical constitution effected; and this takes place through the whole
period of vegetable growth, from the development of the plumule up to
the formation of the latest leaf upon the topmost branch of the most
lordly tree.
Life has been referred to electricity and to chemical power--as the
effect of a known cause. Without doubt, during the operations of life
the whole of the physical powers are necessary to the production of
all the phenomena of growth in the vegetable and the animal world. But
these powers are ever subsidiary to vital force, and are like attendant
spirits chained to do an enchanter’s bidding.
Life is a force beyond the reach of human search, and he who fancies he
has a hold upon the principle which produced biological phenomena, has
committed himself to as wild a pursuit as he who rashly endeavours to
catch a morass-meteor.
Subtile as are the forces of light, heat, and electricity--that of
life, _vitality_, is infinitely more refined, and it must for ever
elude the search of the philosopher.
Man is permitted to test and try all things which are created, and
to apply to useful ends the discoveries which he may make. But man
can never become a creator; and he who would attempt to give sense to
an inert mass of matter, by electricity, heat, or light, will prove
himself as ignorant of nature’s truth as is the senseless mass upon
which he works.
“So far shalt thou go, and no further,” was said equally to the
great tide-wave of human intellect, as to the mighty surge of the
earth-girdling ocean.
It must not be forgotten that a striking difference exists between
the productions of the mineral and the other kingdoms of nature.
Animals and vegetables arrive at maturity by successive developments,
and increase by the assimilation of substances, having the power of
producing the most important chemical changes upon such matter as comes
within the range of their influence; but minerals are equally perfect
in the earliest stages of their formation, and increase only, as
previously said, by the accretion of particles without their undergoing
any change.
The animal and vegetable tribes cease to continue the functions of
life: death ensues, and a complete disorganisation takes place; but
this is not the case in the mineral world: the crystal being the result
of a constantly acting force is not necessarily liable to decomposition.
Nevertheless, we sometimes find in nature that crystals, after arriving
at what may be regarded as, in some sort, their maturity, are, owing
to a change of the conditions under which they were formed, gradually
decomposed. In our mines we discover skeletons of crystals, and
within the hollow shell thus formed, other crystals of a different
constitution and figure find nuclei, and the conditions required for
their development. Again, to give a striking instance, the felspar
crystals of the granitic formations are liable to decomposition in a
somewhat peculiar manner. In decomposing, these crystals leave moulds
of their own peculiar forms, and it not unfrequently happens, in the
stanniferous districts of Cornwall, that oxide of tin gradually fills
these moulds, and we procure this metallic mineral in the form of the
earthy one. Then we have the curious instances of bodies crystallising
in a false form under change of circumstances. We find, for example,
Pseudomorphism, (or _false-form_), as this class of phenomena is named,
occurring by the removal of the constituent atoms of one crystal, while
another set--which naturally assumes a different form--takes their
place, yet still preserving the original shape. It often happens that
copper pyrites will, in this manner, exhibit the angles of an ordinary
variety of crystallised carbonate of iron. These curious changes may
be familiarised by supposing a beautiful statue of gold, from which
some skilful mechanic removes particle by particle, and so skilfully
substitutes a grain of brass for every one of gold removed, that the
loss of the precious metal cannot be detected by any mere examination
of its form.
Crystalline form is not strictly dependent upon the chemical nature
of the parts forming the crystal. The same number of atoms, arranged
in the same way, produce the same form. Substances much unlike each
other will assume the same crystalline arrangement. Magnesia, lime,
oxide of cadmium, the protoxides of iron, nickel, and cobalt, combined
with the same acid, present similarly formed bodies. These Isomorphic
(_like-form_)[37] peculiarities are exceedingly common, and the
discoverer of the phenomena, Mitscherlich, announced the above law.
It cannot, however, be regarded as a philosophical expression of the
fact, and requires reconsideration--chemical elements of a dissimilar
character may have the same law of aggregation, and thus produce the
same form, without having any relation to the number of atoms.
We also find compounds which have two distinct systems of
crystallisation. This property, Dimorphism, is very strikingly shown
in carbonate of lime, which occurs in rhombohedrons, in calc spar, and
in rhombic prisms in arragonite. The molecular arrangements here are
not, however, of equal stability, and one form is evidently forced upon
the other, and is abandoned by it on the slightest disturbance. When a
prism of arragonite is heated it breaks up into the rhombs of common
calc spar, at a temperature far below that at which the carbonate of
lime is decomposed; but no alteration of temperature can convert calc
spar into arragonite.
Crystals are found in the most microscopic character, and of an
exceedingly large size. A crystal of quartz at Milan is three feet and
a quarter long, and five feet and a half in circumference, and its
weight is 870 pounds. Beryls have been found in New Hampshire measuring
four feet in length.[38]
In the dark recesses of the earth, where the influences which produce
organisation and life cease to act, a creative spirit still pursues its
never-ending task of giving form to matter.
The science of crystallogeny,[39] embracing the theoretical and
practical question of the causes producing these geometric forms,
has in various ways attempted to explain the laws according to which
molecules arrange themselves on molecules in perfect order, giving rise
to a rigidly correct system of architecture. But it cannot be said that
any theory yet propounded is sufficiently exact to embrace the whole
of the known phenomena, and the questions,--What is crystallogenic
attraction, and what is the physical nature of the ultimate particles
of matter,--are still open for the inquiries of that genius which
delights in wrestling with the secrets of nature.
The great Epicurus speculated on the “plastic nature” of atoms, and
attributed to this _nature_ the power they possess of arranging
themselves into symmetric forms. Modern philosophers satisfy themselves
with attraction, and, reasoning from analogy, imagine that each atom
has a polar system.
Electricity, and light, and heat, exert remarkable powers, and
accelerate or retard crystallisation according to the conditions under
which these forces are brought to bear on the crystallising mass. We
have recently obtained evidence which appears to prove that some form
of magnetism has an active influence in determining the natural forms
of crystals, and we discover that magnetism exerts a peculiar influence
in relation to the optic axes of crystals, which is not exerted in
lines at right angles to these. Electricity appears to quicken the
process of crystalline aggregation--to collect more readily together
those atoms which seek to combine--to bring them all within the limits
of that influence by which their symmetrical forms are determined; and
strong evidence is now afforded, in support of the theory of magnetic
polarity, by the refined investigations of Faraday and Plücker, which
prove that magnetism has a _directing_ influence upon crystalline
bodies.[40]
It has been found that crystals of sulphate of iron, slowly forming
from a solution which has been placed within the range of sufficiently
powerful magnetic force, dispose themselves along certain magnetic
curves, such as are formed around a magnet by steel filings; whereas
the crystals of the Arbor Dianæ, or silver tree, forming under the
same circumstances, take a position nearly at right angles to these
curves. Certain groups of crystals have been found in nature, which
appear to show, by their positions, that terrestrial magnetism has been
active in producing the phenomena they exhibit; indeed, nearly all our
mineral formations indicate the influences of this, or some similarly
acting power.[41]
During rapid crystallisation, some salts--as the sulphate of
soda, boracic acid, and arsenious acid crystallising in muriatic
acid--exhibit decided indications of electrical excitement; light is
given out in flashes. We have evidence that crystals exhibit a tendency
to move towards the light, and that crystallisation takes place more
readily, and progresses with greater activity in the sunshine than in
the shade. Professor Plücker has recently ascertained that certain
crystals--in particular the cyanite--“point very well to the north,
by the magnetic power of the earth only. It is a true compass needle;
and, more than that, you may obtain its declination.” We must remember
that this crystal, the cyanite, is a compound of silica and alumina
only. This is the amount of experimental evidence which science has
afforded in explanation of the conditions under which nature pursues
her wondrous work of crystal formation. We see just sufficient of
the operation to be convinced that the luminous star which shines in
the brightness of Heaven, and the cavern-secreted gem, are equally
the result of forces which are known to us in only a few of their
modifications.
Every substance, when placed under circumstances which allow of the
free movement of its molecules, has a tendency to crystallise. All the
metals may, by slowly cooling from the melting state, be exhibited
with a crystalline structure. Of the metallic and earthy minerals,
nature furnishes us with an almost infinite variety of crystals, and,
by a reduction of temperature, yet more simple bodies assume the
most symmetric forms. Water, in the conditions of ice and snow, is a
familiar and beautiful example; and, by such extreme degrees of cold as
are artificially produced, many of the gases exhibit a tendency to a
crystalline condition.
May not the solid elementary atoms be susceptible of change of form
under different influences? May not the different states under which
the same bodies are found--as, for example, silica, carbon, and
iron--be due entirely to a change in the form of the primitive atom?
Admitting the probability of this, we then easily see that the central
molecule, formed of an aggregation of such atoms, uniting by particular
faces, would present a determinate form; and that the resulting
crystal, a mass of such molecules, cohering according to a given law,
at certain angles, would present such geometric figures as we find in
nature, or produce in our laboratories, when we avail ourselves of
processes which nature has taught us.
If we take a particle of marble, and place it in a large quantity
of water acidulated with sulphuric acid, it dissolves, and a new
compound results. The marble disappears--the eye cannot detect it by
form or colour: the acid also has been disguised--the taste discovers
nothing sour in the fluid. We have, in combination with the water,
the lime and sulphuric acid; but that combination appears to the eye
in no respect different from the water itself. It is colourless and
perfectly transparent, although it holds a mass of solid matter which
previously would not allow of the permeation of a ray of light. Let
us expose this fluid to such circumstances that the water will slowly
evaporate, and we shall find forming in it, after a time, microscopic
particles of solid, light-refracting matter. These particles gradually
increase in size, and we may watch their growth until eventually we
have a symmetric figure, beautifully shaped, the primary form of
which is a right rhomboidal prism. Thus in nature, by the action,
in all probability, of vegetable matter on the sulphates held in
solution by the water of the great rivers and the ocean--aided by our
oxidizing atmosphere--sulphuric acid is produced to do its work upon
the limestone formations, and from this combination would result the
well-known gypsum, or plaster of Paris, which ordinarily exists as an
amorphous mass, but is often found in a crystalline form.[42]
This is a very perfect illustration of the wonderful process we have
been considering, and in which, simple though it appears to be, we
have set to work a large proportion of the known physical elements
of the universe. By studying aright the result which we have it in
our power to obtain in a watch-glass, we may advance our knowledge of
gigantic phenomena, which are now progressing at the bottom of the
ocean, or of the wondrous agencies which are in operation, producing
light-refracting gems within the secret recesses of the rocky crust of
our globe.
The force of crystallisation is a subject worthy of much consideration.
If we examine our slate rocks, through which little veins filled with
quartz crystals are spread, we shall see that the mechanical force
exerted during the production of these crystals has been capable of
rending those rocks in every direction. Those fissures formed by the
first system of crystalline veins, in order of time, are filled in by
another set of crystalline bodies, which equally exert their mechanical
power, and thus produce those curious intersections and dislocations
which were long a puzzle to the geologist. The simplest power, slowly
and constantly acting through a long period of time, may become
sufficient, eventually, to rend the Andes from base to summit, or to
lift a new continent above the waters of the ocean.
FOOTNOTES:
[37] “Gay Lussac first made the remark, that a crystal of potash alum,
transferred to a solution of ammonia alum, continued to increase
without its form being modified, and might thus be covered with
alternate layers of the two alums, preserving its regularity and proper
crystalline figure. M. Beudant afterwards observed that other bodies,
such as the sulphates of iron and copper, might present themselves
in crystals of the same form and angles, although the form was not a
simple one, like that of alum. But M. Mitscherlich first recognised
this correspondence in a sufficient number of cases to prove that it
was a general consequence of similarity of composition in different
bodies.”--Graham’s _Elements of Chemistry_ (1842), p. 136.
The following remarks are from a paper by Dr. Hermann Kopp, _On the
Atomic Volume and Crystalline Condition of Bodies, &c._, published in
the Philosophical Magazine for 1841:--“The doctrine of isomorphism
shows us that there are many bodies which possess an analogous
constitution, and the same crystalline form. Our idea of the volume
(or, in other words, of the crystalline form) of these bodies must
therefore be the same. From this it follows that their specific
weight is connected with mass contained in the same volume. From
these considerations the following law may be deduced: _The specific
weight of isomorphous bodies is proportional to their atomic weight,
or isomorphous bodies possess the same atomic volume_.”--page 255.
A translation appears in the Cavendish Society, from Dr. Otto’s
Chemistry, _On Isomorphism_, which may be advantageously consulted. See
also a paper by M. Rose, translated from the _Proceedings of the Royal
Berlin Academy_ for the _Chemical Gazette_, Oct. 1848, entitled, _On
the Isomeric Conditions of the Peroxide of Tin_.
[38] _A System of Mineralogy, comprising the most recent discoveries_,
by James D. Dana, A.M., New York, 1844.
[39] Crystallogeny, or the formation of crystals, is the term employed
by Dana, in his admirable work quoted above: whose remarks on
_Theoretical Crystallogeny_, p. 71, are well worthy of all attention.
[40] _On the Magnetic Relations of the Positive and Negative Optic Axes
of Crystals_, by Professor Plücker, of Bonn.--Philosophical Magazine,
No. 231 (3rd Series), p. 450. _Experimental Researches on Electricity;
On the Crystalline Polarity of Bismuth and other bodies, and on its
Relation to the Magnetic form of Force_: by Michael Faraday, Esq.,
F.R.S.--Transactions of the Royal Society for 1848.
[41] In the _Memoirs of the Geological Survey of the United Kingdom,
and of the Museum of Economic Geology_, vol. i. 1846, will be found a
paper, by the author of this volume, _On the Influences of Magnetism on
Crystallisation, and other Conditions of Matter_, in which the subject
is examined with much care. See also _Magnétisme polaire d’une montagne
de Chlorite schisteuse et de Serpentine_: Annales de Chimie, vol. xxv.
p. 327; _Influence du Magnétisme sur les actions chimiques_, by l’Abbé
Rendus; and also a notice of the experiments of Ritter and Hansteen,
“Analysées par M. Œrsted;” also _Effets du Magnétisme terrestre sur
la précipitation de l’Argent, observés par M. Muschman_: Annales de
Chimie, vol. xxxviii. p. 196-201.
[42] The transparent varieties of sulphate of lime are distinguished
by the name _Selenite_; and the fine massive varieties are called
_Alabaster_. Gypsum forms very extensive beds in secondary countries,
and is found in tertiary deposits; occasionally, in primitive rocks; it
is also a product of volcanoes. The finest foreign specimens are found
in the salt mines of Bex, in Switzerland; at Hall, in the Tyrol; in the
sulphur-mines of Sicily; and in the gypsum formation near Ocana, in
Spain. In England, the clay of Shotover Hill, near Oxford, yields the
largest crystals.--See Dana’s _Mineralogy_, second edition, p. 241.