[Illustration: FIG. 4.--Section of a Convex lens.]
It is pleasant on a dry, still day in winter, when the ground is covered
with crisp snow or glistens with hard frost, to feel the warmth of the
sun's rays, and it is becoming quite a fashion for people of leisure to
spend the winter months at the pleasure resorts amid the snow-laden
mountains of Switzerland. It is a matter of some interest to inquire how
it happens that the sun's rays are warm when the thermometer tells us
that the temperature of the air is below freezing-point. There is an old
and pretty experiment in which a burning glass is made of ice; it is not
a difficult thing to do. If the scale-pan of an ordinary balance be made
hot and be pressed against a slice of ice (the concave side of the
scale-pan towards the ice), first on one side of the slice and then on
the other, the ice can be formed into a convex lens (Fig. 4). If now
this lens be placed in the path of a sunbeam and the light be brought to
a focus, that is, to a bright spot on a piece of paper, the paper will
be heated and will take fire while the lens through which the heat
passes remains ice. From this we may surmise that the heat of the sun
does not affect the medium through which it passes.
Clerk Maxwell suggested yet another experiment in illustration of this
law. By means of an ice lens he collected the sunlight to a focus in the
middle of a basin of clear water, and observed that no effect was
discernible in the water. He then directed the focus (the spot of light)
on to a mote in the water. The mote became hot, the water was agitated,
convection currents were formed, and the mote was carried up in them.
This showed that rays of light from the sun do not affect the substances
through which they can pass, and that they heat bodies through which
they do not pass. It has been demonstrated by laboratory experiments
that all hot bodies emit rays of heat, whether we see the rays or not.
When we see the rays the bodies are said to be red or white-hot. The
process by which heat passes from one body to another without warming
the intervening medium is called radiation. Radiation takes place only
through transparent bodies. Rays of heat, like rays of light, pass
through transparent bodies; whereas they are absorbed by, that is they
make hot, opaque bodies. Heat rays travel in straight lines and are
reflected from polished surfaces; their intensity varies inversely as
the square of the distance of the object on which they fall from their
source. The heat of an ordinary fire is radiant heat; when we sit round
the fire we act as opaque bodies and absorb the heat, and are what we
call scorched if the fire is very bright. If we move away from the fire,
still letting the same firelight shine on us, we are not scorched; this
is because the heating power of the rays varies inversely as the
distance from their source, therefore if we move away double the
distance we receive one quarter of the heat that we received before we
moved. If we draw our chairs to one side we are not scorched, because
the rays of heat do not travel round a corner.