So far we have spoken only of light; but hold your hand in the
sun and feel the heat of the sunbeams, and then consider if the
waves of heat do not do work also. There are many waves in a
sunbeam which move too slowly to make us see light when they hit
our eye, but we can feel them as heat, though we cannot see them
as light. The simplest way of feeling heat-waves is to hold a
warm iron near your face. You know that no light comes from it,
yet you can feel the heat-waves beating violently against your
face and scorching it. Now there are many of these dark heat-
rays in a sunbeam, and it is they which do most of the work in
the world.

In the first place, as they come quivering to the earth, it is
they which shake the water-drops apart, so that these are carried
up in the air, as we shall see in the next lecture. And then
remember, it is these drops, falling again as rain, which make
the rivers and all the moving water on the earth. So also it is
the heat-waves which make the air hot and light, and so cause it
to rise and make winds and air-currents, and these again give
rise to ocean-currents. It is these dark rays, again, which
strike upon the land and give it the warmth which enables plants
to grow. It is they also which keep up the warmth in our own
bodies, both by coming to us directly from the sun, and also in a
very roundabout way through plants. You will remember that
plants use up rays of light and heat in growing; then either we
eat the plants, or animals eat the plants and we eat the animals;
and when we digest the food, that heat comes back in our bodies,
which the plants first took from the sunbeam. Breathe upon your
hand, and feel how hot your breath is; well, that heat which you