SUNBEAMS AND THEIR WORK. 47 
Breathe upon your hand, and feel how hot your breath 
is; well, that heat which you feel, was once in a sun- 
beam, 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 coal. 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 fires ; and our steamboats 
travel all over the world by means of the same power. 
In the same way the oil of our lamps comes -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; and 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 pro- 
pels 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 
