Let me illustrate the action of the 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, but by evaporation the water only is removed.  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 dissolved salt 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 pocket lens the subsequent recongelation of the film.

In this case the crystallizing force is hampered by the adhesion 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.  Sometimes the crystals start from definite nuclei in the centre of the film, every small crystalline particle which rests in the film furnishing a starting-point.  Throughout the process you notice one feature which is perfectly unalterable, and that is, angular magnitude.  The spiculae branch from the trunk, and from these branches others shoot; but the angles enclosed by the spiculae 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 such accidents—­angular magnitude is always rigidly preserved.

My second example of the action of crystallizing force is this:  By sending a voltaic current through a liquid, you know that we decompose the liquid, and if it contains a metal, we liberate this metal by electrolysis.  This small cell contains a solution of acetate of lead, which is chosen for our present purpose, because lead lends itself freely to this crystallizing power.  Into the cell are dipped two very thin platinum wires, and these are connected by other wires with a small voltaic battery.  On sending the voltaic current through the solution, the lead will be slowly severed from the atoms with which it is now combined; it will be liberated upon one of the wires, and at the moment of its liberation it will obey the polar forces of its atoms, and produce