Professor Tyndall, in a lecture delivered by him at the Royal
Institution, London, on the properties of Ice, gave the following
interesting illustration of crystalline force. By perfectly cleaning a
piece of glass, and placing on it a film of a solution of chloride of
ammonium or sal ammoniac, the action of crystallisation was shown to
the whole audience. The glass slide was placed in a microscope, and the
electric light passing through it was concentrated on a white disc. The
image of the crystals, as they started into existence, and shot across
the disc in exquisite arborescent and symmetrical forms, excited the
admiration of every one. The lecturer explained that the heat, causing
the film of moisture to evaporate, brought the particles of salt
sufficiently near to exercise the crystalline force, the result being
the beautiful structure built up with such marvellous rapidity.