crystalline forms of exquisite beauty.  They are now before you, sprouting like ferns from the wire, appearing indeed like vegetable growths rendered so rapid as to be plainly visible to the naked eye.  On reversing the current, these wonderful lead-fronds will dissolve, while from the other wire filaments of lead dart through the liquid.  In a moment or two the growth of the lead-trees recommences, but they now cover the other wire.

In the process of crystallization, Nature first reveals herself as a builder.  Where do her operations stop?  Does she continue by the play of the same forces to form the vegetable, and afterwards the animal?  Whatever the answer to these questions may be, trust me that the notions of the coming generations regarding this mysterious thing, which some have called ‘brute matter,’ will be very different from those of the generations past.

There is hardly a more beautiful and instructive example of this play of molecular force than that furnished by water.  You have seen the exquisite fern-like forms produced by the crystallization of a film of water on a cold window-pane.[15] You have also probably noticed the beautiful rosettes tied together by the crystallizing force during the descent of a snow-shower on a very calm day.  The slopes and summits of the Alps are loaded in winter with these blossoms of the frost.  They vary infinitely in detail of beauty, but the same angular magnitude is preserved throughout:  an inflexible power binding spears and spiculae to the angle of 60 degrees.

The common ice of our lakes is also ruled in its formation by the same angle.  You may sometimes see in freezing water small crystals of stellar shapes, each star consisting of six rays, with this angle of 60 deg. between every two of them.  This structure may be revealed in ordinary ice.  In a sunbeam, or, failing that, in our electric beam, we have an instrument delicate enough to unlock the frozen molecules, without disturbing the order of their architecture.  Cutting from clear, sound, regularly frozen ice, a slab parallel to the planes of freezing, and sending a sunbeam through such a slab, it liquefies internally at special points, round each point a six-petalled liquid flower of exquisite beauty being formed.  Crowds of such flowers are thus produced.  From an ice-house we sometimes take blocks of ice presenting misty spaces in the otherwise continuous mass; and when we inquire into the cause of this mistiness, we find it to be due to myriads of small six-petalled flowers, into which the ice has been resolved by the mere heat of conduction.

A moment’s further devotion to the crystallization of water will be well repaid; for the sum of qualities which renders this substance fitted to play its part in Nature may well excite wonder and stimulate thought.  Like almost all other substances, water is expanded by heat and contracted by cold.  Let this expansion and contraction be first illustrated:—­