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
into the skies, we know that it goes there, for it comes down
again in rain, and so it must go up invisibly. But where does the
heat come from which makes this water invisible? Not from below,
as in the case of the kettle, but from above, pouring down from
the sun. Wherever the sun-waves touch the rivers, ponds, lakes,
seas, or fields of ice and snow upon our earth, they
carry off invisible water-vapour.
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