COHESION AND CRYSTALLIZATION
Chemical affinity is the force which holds together the different atoms in
a molecule. Cohesion is the force which holds together molecules which are
chemically similar. Although physical science distinguishes three states of
matter, solid, liquid, and gaseous, yet here as elsewhere there are no
sharp dividing lines, but rather an absolute continuity. We have in fact
many intermediate states; between liquids and gases there are the various
conditions of vapour, and between liquids and solids we get viscous,
gelatinous, and paste-like conditions. The only real difference between
solids, liquids, and gases is the intensity of the force of cohesion, which
is considerable in solids, feeble in liquids, and absent in gases.
A living organism is the arena in which are brought into play the opposing
forces of cohesion and disintegration. The study of cohesion is therefore a
vital one for the biologist, and especially cohesion under the conditions
which obtain in living beings, viz. in liquids of heterogeneous
constitution. The forces of cohesion brought into play under these
conditions may be beautifully illustrated by a simple experiment. We take a
plate of glass, well cleaned and absolutely horizontal. On it we pour a
layer of salt water, and in the middle we carefully drop a spot of Indian
ink. The drop at once begins to diffuse, and we obtain a circular figure,
like the monopolar field of diffusion already described, the rays of
diffusion radiating from the centre in all directions.
[Illustration: FIG. 19.--Muriform cohesion figure formed by a drop of
Indian ink in a solution of salt.]
If we keep the plate carefully protected from all disturbing influences,
after some ten to twenty minutes we shall see the coloured particles
returning on their path, and the centre of {79} the drop becoming more and
more black. Each line of force becomes segmented into granules, which
gradually increase in size, and approach nearer to one another and to the
centre of the drop, until it assumes the mulberry appearance shown in the
photograph (Fig. 19).
[Illustration: FIG. 20.--Seven similar drops of Indian ink diffusing in a
salt solution. Two minutes after introducing the drops.]
If we sow a number of drops of Indian ink in regular order on the surface
of a salt solution, we obtain most beautiful patterns formed by the mutual
repulsion of the drops. Figs. 20, 21, and 22 represent the successive
aspects of seven drops of Indian ink thus sown on a layer of salt solution,
and kept undisturbed long enough to allow of their evolution. Fig. 20 shows
the aspect after two minutes, when the diffusion is almost complete. In
Fig. 21, photographed after fifteen {80} minutes, the colouring matter has
almost entirely reunited to form separate granulations; whilst in Fig. 22,
taken after thirty minutes, these granulations are rearranged to form an
agglomeration around the centre of each drop.
[Illustration: FIG. 21.--The same drops 15 minutes later, showing the
granulation appearance.]
The following experiment, which is more difficult, will show the cohesive
attraction of one drop for another. A plate of glass is adjusted absolutely
horizontal, and covered as before with a layer of salt solution. On this we
sow a number of drops of the same salt solution coloured with Indian ink.
The drops must be of exactly the same concentration as the salt medium, so
as to avoid any difference of osmotic pressure between the drops and the
medium, otherwise the drops would not remain intact but would diffuse into
the solution. Since under these conditions the liquid of the medium around
the drops is perfectly symmetrical and homogeneous, it cannot exercise any
influence on the liquid of the drops.
[Illustration: FIG. 22.--The same drops after 30 minutes. The granulations
have agglomerated at the centre of the drops.]
It is otherwise, however, with the colouring matter of the {81} drops. The
particles of Indian ink may be seen passing from one drop to another, the
coloured circles become elongated towards one another, touch, and finally
unite. If, as in Fig. 23, the drops are of different size, the larger one
will have a preponderating attractive action and eat up the smaller drops.
In the figure, six small drops are placed around a large one, and the
smaller drops have begun to be deformed and to move towards the larger
drop. This central drop is also deformed, and has assumed a more or less
hexagonal form, under the influence of the attraction of the six smaller
ones. It may be noticed that the least prominent angle of the hexagon is
opposite the small drop which is farthest away from it, whilst one of the
smaller drops has already begun to be swallowed up by the large one. This
cohesion phenomenon is very slow in its action, but after an hour or two
the central drop will be found to have {82} completely absorbed the six
smaller ones, and only one large drop will remain.
[Illustration: FIG. 23.--Attraction between coloured drops in an isotonic
solution.]
_Incubation._--In the living organism we frequently find conditions similar
to those realized in this experiment, viz. very slow movements of diffusion
in liquids containing particles in suspension. In such cases the
consequences must be the same, viz. granulation and segmentation. Consider
for a moment the incubation of an egg. The heat of incubation determines a
certain amount of evaporation through the shell, with a concentration of
the liquid near the surface. As a consequence of this superficial
concentration we get segmentation of the vitellus, with the production of a
morula.
_Artificial Parthenogenesis._--The experimental parthenogenesis of Loeb and
Delage consists in plunging the egg into a liquid other than sea water, and
returning it again to its original medium. This operation will necessarily
determine slow movements of diffusion in the egg, which will give rise to
segmentation. It may be objected that segmentation is also produced by a
solution which is isotonic with sea water. Such a solution would not indeed
produce an exchange of water with the egg, but it would set up an exchange
of electrolytes, since there would be a difference of their osmotic
pressure in the egg and in the new isotonic medium. The extremely slow
movements of diffusion thus produced would be very favourable to the action
of the cohesive force on the particles in suspension, and hence to the
segmentation of the egg.
[Illustration: FIG. 24.--A circle of eight drops of Indian ink 30 minutes
after they have been sown in a salt solution. The drops have undergone
diffusion and subsequent cohesion, resulting in a reticulate structure.]
Few physical phenomena give us a deeper insight into the phenomena of life
than those which we here contemplate. There is still another experiment
which is even more convincing. On the surface of our horizontal salt
solution we sow a number of drops of a more concentrated salt solution at
equal distances around the circumference of a circle. Movements of
diffusion are thus set up in the interior of the circle, and after a time,
when this diffusion has become so slow as to be almost imperceptible, a
furrow begins to appear in the coloured mass. Then a second and third
appear, and others crossing the former break up the mass {83} into
segments. Finally the segmentation becomes complete, and the preparation
presents a muriform appearance, looking in fact something like a mulberry
(Fig. 24). If the preparation is preserved for several hours longer, we may
see the cells formed by segmentation unite around the circumference so as
to form a hollow bag corresponding to a gastrula, as shown in Fig. 25.
[Illustration: FIG. 25.--The same preparation several hours later, showing
a cellular gastrula-like structure.]
These preparations are extremely sensitive to external influences, which
renders the demonstration of cohesion phenomena difficult. I have
nevertheless on several occasions been able to project the experiment on
the screen during a lecture. The segmentation is influenced by very slight
currents of diffusion, and I have many preparations showing the
segmentation regularly distributed in various ways along radial diffusion
lines. We may in this way produce many varieties of structure lamellar,
vacuolate, or cellular, in fact {84} all the tissue structures which are
met with in living organisms. All these structures are retractile, the
retraction going on very slowly for a long time, as if the force of
cohesion continued to act in the web of the structure even after its
formation was complete. The phenomenon is a purely physical synthetic
reproduction of the phenomenon of coagulation, the cohesion figure being in
fact a retractile clot.
[Illustration: FIG. 26.--Field of crystallization of sodium chloride
(magnified 60 diameters).]
_Crystallization._--When we evaporate a solution of a crystalloid it
becomes more concentrated, slow movements of diffusion are set up, and at a
given moment agglomeration occurs, the agglomerates taking the form of
crystals. Thus crystallization may be regarded as a particular case of
conglomeration by cohesion, differing only in the regularity of the
arrangement of the molecules, which gives the geometrical form of the
crystal. Hence we can easily understand how the presence of a crystalline
fragment may facilitate the process of crystallization. Consider a liquid
in which extremely slow movements of diffusion are taking place. If the
liquid is perfectly homogeneous there will be no centre of attraction to
which the molecules may become attached. {85}
[Illustration: FIG. 27.--Field of crystallization around a crystal of
sodium chloride in process of formation.]
If, however, a crystal or other heterogeneous structure is present, it
forms a centre of cohesion which will attach any molecules that are brought
by diffusion into its sphere of attraction. We have succeeded in
photographing the arrangement of the molecules of a liquid around a crystal
in the act of formation (Fig. 26). For this purpose we add to the solution
traces of some colloidal substance, such as gelatine or gum, so as to delay
the crystallization. It may thus be shown that the molecules of the
surrounding liquid are already arranged in crystalline order for some
distance from the crystal, forming a sort of field of crystallization. The
arrangement of this regular field varies in different cases, and is more or
less complicated according to circumstances. One of the most frequent forms
is that shown in Fig. 27, which is the field around a crystal of sodium
chloride. In the centre {86} of the crystal is a square with well-marked
outline. At each corner of this square there is a straight line at right
angles to the diagonal, which will form the sides of the crystal in process
of formation. From the middle of each side arise yet other perpendiculars,
which in their turn bear other cross lines, each new line being set at
right angles to its predecessor. A later stage of crystallization is shown
in Fig. 27, where the two squares one inside the other at an angle of 45deg
are clearly indicated.
[Illustration: FIG. 28.--Three crystals of sodium chloride in process of
formation, each in the centre of a field of crystallization.]
Every crystallizable substance gives a different characteristic field of
crystallization. In 1903, at the Congress of Angers, I terminated my
address by these words: "The field of crystallization may serve to
determine the character of a substance in solution." I have subsequently
received from Carbonell y Soles of Barcelona an interesting work on this
subject, which he contributed to the International Congress of Medicine at
Madrid in 1903, entitled _Applicacion de la crystalogenia experimental a la
investigacion toxicologica de cas alcaloides_. {87}
Six years ago I received from Australia an exceedingly beautiful photograph
of a thin pellicle found in a rain gauge. My correspondent supposed that
this strange figure might have been produced under the influence of an
electric or magnetic field. I was able to assure him by return of post that
the figure was the result of the crystallization of copper sulphate in a
colloidal medium. In return I received a letter verifying this fact, and
saying that there were copper works in the neighbourhood, and the air was
filled with the dust of copper sulphate.
Living beings are but solutions of colloids and crystalloids, and their
tissues are built up by the aggregation of these solutes. We have already
seen how the forces of crystallization are modified in colloid solutions.
This force of crystallization must play an important role in the
metamorphoses of the living organism, and influence their morphology. It
may therefore be of interest to investigate some of the numberless forms of
crystallization in colloidal solutions.
[Illustration: FIG. 29.--Crystallization of sodium chloride in a colloidal
solution, giving a plant-like form.]
[Illustration: FIG. 30.--Form produced by the crystallization of chloride
of ammonium in a colloidal solution.]
Figs. 29 and 30 represent the forms produced by chloride of sodium and
chloride of ammonium respectively, in solutions of gelatine of different
degrees of concentration. Their resemblance to vegetable growth is so
remarkable that several observers on first seeing them have called them
"Fern-crystals."
I should like here to recall to your notice the work of an English
observer, Dr. E. Montgomery of St. Thomas's {88} Hospital, which was
published as long ago as 1865. This work was recently brought to my notice
by the kindness of Professor Baumler of Freiburg. He says: "Crystals are
not strangers in the organic world. Many organic compounds are able to
assume crystalline forms under certain conditions. Rainey has shown that
many shells consist of globular crystals _i.e._ of mineral substances made
to crystallize by the influence of viscid material." In this connection I
may also mention the interesting work of Otto Lehmann of Karlsruhe on
liquid crystals.
In conclusion, we may recall the words of Schwann himself, the originator
of the cell theory: "The formation of the elementary shapes of an organism
is but a crystallization of substances capable of imbibition. The organism
is but an aggregate of such imbibing crystals."
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