Horcli 20, 1873.] 
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PHAR VIAC3UTICAL JOURNAL AND TRANSACTIONS. 
76u 
bability is, that the progress of science by connecting the 
phenomena of magnetism with the luminiferous ether, 
will prove these “lines of force,” as Faraday loved to 
call the magnetic curves, to represent a condition of this 
mysterious substratum of all radiant action. But it is 
not with the magnetic curves as such that I now wish to 
occupy your attention ; it is their relationship to theo¬ 
retic conceptions that we have now to consider. By the 
action of the bar magnet upon the needle we obtain a 
notion of a polar force ; by the breaking of the strip of 
magnetized steel, we attain the notion that polarity can 
attach itself to the ultimate particles of matter. The ex¬ 
periment with the iron filings introduces a new idea into 
the mind ; the idea, namely, of structural arrangement. 
Every pair of filings possesses four poles, two of which 
are attractive and two repulsive. The attractive poles 
approach, the repulsive poles retreat; the consequence 
being a certain definite arrangement of the particles with 
reference to each other. 
Now, this idea of structure, as produced by polar force, 
opens a way for the intellect into an entirely new region, 
and the reason I ask you now to accompany me into this 
region is, that our next inquiry relates to the action of 
crystals upon light. Befoi'e I speak of this action, I wish 
you to realize the process of crystalline architecture. 
Look then into a granite quarry, and spend a few minutes 
in examining the rock. It is not of perfectly uniform 
texture. It is rather an agglomeration of pieces, which, 
on examination, present curiously defined forms. You 
have there what mineralogists call quartz, you have 
felspar, you have mica. In a mineralogical cabinet, 
where these substances are preserved separately, you 
■will obtain some notion of their forms. You will see 
there, also, specimens of beryl, topaz, emerald, tourma¬ 
line, heavy spar, fluor-spar, Iceland spar — possibly a 
full-formed diamond, as it quitted the hand of nature, 
not yet having got into the hands of the lapidary. 
These crystals, you will observe, are put together ac¬ 
cording to law ; they are not chance productions ; and 
if you care to examine them more minutely, you will 
find them architecture capable of being to some extent 
revealed. They split in certain directions befoi’e a knife- 
edge, exposing smooth and shining surfaces, which are 
called planes of cleavage ; and by following these planes 
you sometimes reach an internal form, disguised beneath 
the external form of the crystal. Ponder these beautiful 
edifices of a hidden builder. You cannot help asking 
yourself how they were built ; and familiar as you now 
are with the notion of a polar force, and the ability of that 
force to produce structural arrangement, your inevitable 
answer will be, that those crystals are built by the play of 
polar forces with which their ultimate molecules are 
endowed. In virtue of these forces, atom lays itself to 
atom in a perfectly definite way, the final visible form of 
the crystal depending upon this play of its molecules. 
Every’,vhere in nature we observe this tendency to run 
into definite forms, and nothing is easier than to give 
scope to this tendency by artificial arrangements. Dis¬ 
solve nitre in water, and allow the water slowly to eva¬ 
porate ; the nitre remains, and the solution soon becomes 
so concentrated that the liquid form can no longer be 
preserved. The nitre molecules approach each other, 
and come at length within the range of their polar forces. 
They arrange themselves in obedience to these forces, 
a minute crystal of nitre being at first produced. On 
this crystal the molecules continue to deposit themselves 
from the surrounding liquid. The crystal grows, and 
finally we have large prisms of nitre, each of a perfectly 
definite shape. Alum crystallizes with the utmost ease 
in this fashion. The resultant crystal is, however, differ¬ 
ent in shape from that of nitre, because the poles of the 
molecules are differently disposed ; and if they be only 
nursed with proper care, the crystals of these substances 
may be caused to grow to an enormous size. The condi¬ 
tion of perfect crystallization is, that the crystallizing 
force should act with deliberation. There should be no 
hurry in its operations ; but every molecule ought to bo 
permitted, without disturbance from its neighbours, to 
exercise its own molecular rights. If the crystallization 
be too sudden, the regularity disappears. 
Water may be saturated with sulphate of soda, dis¬ 
solved when the water is hot, and afterward permitted 
to cool. When cold, the solution is supersaturated; 
that is to say, more solid matter is contained in it than 
corresponds to its temperature. Still, the molecules show 
no sign of building themselves together. This is a very 
remarkable, though a very common fact. The molecules 
in the centre of the liquid are so hampered by the action 
of their neighbours that freedom to follow their own 
tendencies is denied to them. Fix your mind’s eye upon, 
a molecule within the mass. It wishes to unite with its 
neighbour to the right, but it wishes equally to unite 
with its neighbour to the left; the one tendency neutralizes 
the other, and it unites with neither. We have here, in 
fact, translated into molecular action, the well-known 
suspension of animal volition produced by two equally 
inviting bundles of hay. But if a crystal of sulphate of 
soda be dropped into the solution, the molecular indecision 
ceases. On the crystal the adjacent molecules will imme¬ 
diately precipitate themselves ; on these again others wall 
be precipitated, and this act of precipitation will continue 
from the top of the flask to the bottom, until the solution 
has, as far as possible, assumed the solid form. The crystals- 
here formed are small, and confusedly arranged. The 
process has been too hasty to admit of the pure and 
orderly action of the crystallizing force. It typifies the 
state of a nation in which natural and healthy change is 
resisted, until society becomes, as it were, supersaturated 
with the desire for change, the change being then effected 
through confusion and revolution, which a wise foresight 
might have avoided. 
Let me illustrate the action of crystallizing force by 
two examples of it : nitre might be employed, but 
another well-known substance enables me to make the 
experiment in a better form. The substance is common 
sal-ammoniac, or chloride of ammonium, dissolved in 
water. Cleansing perfectly a glass-plate, the solution of 
the chloride is poured over the glass, to which, when the 
plate is set on edge, a thin film of the liquid adheres. 
Warming the glass slightly, evaporation is promoted; 
the plate is then placed in a solar microscope, and an 
image of the film is thrown upon a white screen. The 
warmth of the illuminating beam adds itself to that 
already imparted to the glass-plate, so that after a 
moment or two the film can no longer exist in the liquid 
condition. Molecule then closes with molecule, and you 
have a most impressive display of crystallizing energy 
overspreading the whole screen. You may produce 
something similar if you breathe upon the frost-ferns 
which overspread your window-panes in winter, and then 
observe through a lens the subsequent recongelation of the 
film. 
Here the crystallizing force is hampered by the adhe¬ 
sion of the film to the glass; nevertheless, the play of 
power is strikingly beautiful. Sometimes the crystals 
start from the edge of the film and run through it from 
that edge, for the crystallization being once started, the-, 
molecules throw themselves by preference on the crystals 
already formed. The solid chloride of ammonium pos¬ 
sesses a power of provoking precipitation upon itself 
which is not possessed by the solid glass. Sometimes the 
crystals start from definite nuclei in the centre of the film ; 
every small crystalline particle which rests in the film 
furnishes a starting-point. Throughout the process you. 
notice one feature which is perfectly unalterable, and that 
is, angular magnitude. The spicul<e branch from the 
trunk, and from these branches others shoot; but the 
angles inclosed by the spiculos are unalterable. In like 
manner you may find alum-crystals, quartz-crystals, and. 
all other crystals, distorted in shape. They are thus far- 
at the mercy of the accidents of crystallization; but 
in one particular they assert their superiority over all 
