colder water letting another layer come down to be heated in its
turn. The motion grows more and more violent, making the water
hotter and hotter, till at last the particles of which it is
composed fly asunder, and escape as invisible vapour. If this
kettle were transparent you would not see any steam above the
water, because it is in the form of an invisible gas. But as the
steam comes out of the mouth of the kettle you see a cloud. Why
is this? Because the vapour is chilled by coming out into the
cold air, and its particles are drawn together again into tiny,
tiny drops of water, to which Dr. Tyndall has given the
suggestive name of water-dust. If you hold a plate over the steam
you can catch these tiny drops, though they will run into one
another almost as you are catching them.

The clouds you see floating in the sky are made of exactly the
same kind of water-dust as the cloud from the kettle, and I wish
to show you that this is also really the same as the invisible
steam within the kettle. I will do so by an experiment
suggested by Dr. Tyndall. Here is another spirit-lamp, which I
will hold under the cloud of steam - see! the cloud disappears!
As soon as the water-dust is heated the heat-waves scatter it
again into invisible particles, which float away into the room.
Even without the spirit-lamp, you can convince yourself that
water-vapour may be invisible; for close to the mouth of the
kettle you will see a short blank space before the cloud begins.
In this space there must be steam, but it is still so hot that
you cannot see it; and this proves that heat-waves can so shake
water apart as to carry it away invisibly right before your eyes.

Now, although we never see any water travelling from our earth up