feel, was once in a sunbeam, and has travelled from it through
the food you have eaten, and has now been at work keeping up the
heat of your body.

But there is still another way in which these plants may give out
the heat-waves they have imprisoned. You will remember how we
learnt in the first lecture that coal is made of plants, and that
the heat they give out is the heat these plants once took in.
Think how much work is done by burning coals. Not only are our
houses warmed by coal fires and lighted by coal gas, but our
steam-engines and machinery work entirely by water which has been
turned into steam by the heat of coal and coke fire; and our
steamboats travel all over the world by means of the same power.
In the same way the oil of our lamps comes either from olives,
which grow on trees; or from coal and the remains of plants and
animals in the earth. Even our tallow candles are made of mutton
fat, and sheep eat grass; as so, turn which way we will, we find
that the light and heat on our earth, whether it comes from
fires, or candles, or lamps, or gas, and whether it moves
machinery, or drives a train, or propels a ship, is equally the
work of the invisible waves of ether coming from the sun, which
make what we call a sunbeam.

Lastly, there are still some hidden waves which we have not yet
mentioned, which are not useful to us either as light or heat,
and yet they are not idle.

Before I began this lecture, I put a piece of paper, which had
been dipped in nitrate of silver, under a piece of glass; and
between it and the glass I put a piece of lace. Look what the