SUNBEAMS AND THEIR WORK. 47 
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 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 ; and so, turn which way we will, we find that 
the light and heat on our earth, whether they come 
from fires, or candles, or lamps, or gas, and whether 
they move machinery, or drive a train, or propel a 
ship, are 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 sun has been doing 
while I have been speaking. It has been breaking up 
the nitrate of silver on the .paper and turning it into 
a deep brown substance ; only where the threads of 
the lace were, and the sun could not touch the nitrate 
of silver, there the paper has remained light-coloured, 
and by this means I have a beautiful impression of the 
lace on the paper. I will now dip the impression into 
water in which some hyposulphite of soda is dissolved, 
and this -will " fix " the picture, that is, prevent the 
sun acting upon it any more; then the picture will 
remain distinct, and I can pass it round to you ail 
