therefore, that we owe our rains and snows. They are absorbed close to
the surface of the ocean, and warm the superficial water, while the
luminous rays plunge to great depths without producing any sensible
effect. But we can proceed further than this. Here is a large flask
containing a freezing mixture, which has so chilled the flask, that
the aqueous vapour of the air of this room has been condensed and
frozen upon it to a white fur. Introducing the alum-cell, and placing
the coating of hoar-frost at the intensely luminous focus of the
electric lamp, not a spicula of the dazzling frost is melted.
Introducing the iodine-cell, and removing the alum, a broad space of
the frozen coating is instantly melted away. Hence we infer that the
snow and ice, which feed the Rhone, the Rhine, and other rivers with
glaciers for their sources, are released from their imprisonment upon
the mountains by the invisible ultra-red rays of the sun.

Sec. 6. _Identity of Light and Radiant Heat. Reflection from Plane and
Curved Surfaces. Total Reflection of Heat_.

The growth of science is organic. That which today is an _end_ becomes
to-morrow a _means_ to a remoter end. Every new discovery in science
is immediately made the basis of other discoveries, or of new methods
of investigation. Thus about fifty years ago OErsted, of Copenhagen,
discovered the deflection of a magnetic needle by an electric current;
and about the same time Thomas Seebeck, of Berlin, discovered
thermoelectricity. These great discoveries were soon afterwards turned
to account, by Nobili and Melloni, in the construction of an
instrument which has vastly augmented our knowledge of radiant heat.
This instrument, which is called a _thermo-electric pile_, or more
briefly a thermo-pile, consists of thin bars of bismuth and antimony,
soldered alternately together at their ends, but separated from each
other elsewhere. From the ends of this 'thermo-pile' wires pass to a
galvanometer, which consists of a coil of covered wire, within and
above which are suspended two magnetic needles, joined to a rigid
system, and carefully defended from currents of air.

The action of the arrangement is this: the heat, falling on the pile,
produces an electric current; the current, passing through the coil,
deflects the needles, and the magnitude of the deflection may be made
a measure of the heat. The upper needle moves over a graduated dial
far too small to be directly seen. It is now, h