The invisible, and not the visible, rays of the sun have the greatest
heating power, and do the principal work in the melting of the mountain
snows, and in vaporising the waters of the seas and rivers. Water
absorbs or filters away the dark rays from the light ones; so that, if
you pass the rays from an electric lamp through a narrow glass trough
filled with water, and then bring them to an intensely sharp and
brilliant focus, although the rays thus filtered contain sufficient heat
to set fire to brown paper, hoar-frost is not touched by them, because,
being transparent to these rays, it lets them pass through without
absorption. Proof:—You can and do set fire to paper in the focus, and
then place a flask, covered outside with hoar-frost, in the same focus,
yet the snow remains unmelted. Then you can stop all the visible rays by
the interposition of a glass trough filled with solution of iodine in
bisulphide of carbon—which is, however, transparent to the invisible
rays—and on removing the trough of water, this allows invisible rays to
pass on to the focus, whilst the visible rays are cut off by the
solution of iodine. When the flask coated with hoar-frost is placed in
the dark focus, the heat at once melts it from the surface, wherever the
glass is brought into the centre of action.