Everywhere 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.  Dissolve nitre in water, and allow the water slowly to evaporate; the nitre remains and the solution soon becomes so concentrated that the liquid condition 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, different in shape from that of nitre, because the poles of the molecules are differently disposed.  When they are nursed with proper care, crystals of these substances may be caused to grow to a great size.

The condition of perfect crystallization is, that the crystallizing force shall act with deliberation.  There should be no hurry in its operations; but every molecule ought to be permitted, without disturbance from its neighbours, to exercise its own rights.  If the crystallization be too sudden, the regularity disappears.  Water may be saturated with sulphate of soda, dissolved when the water is hot, and afterwards 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.  But, if a crystal of sulphate of soda be dropped into the solution, the molecular indecision ceases.  On the crystal the adjacent molecules will immediately precipitate themselves; on these again others will 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 produced 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.