A DROP OF WATER. 79 
I will do so by an experiment suggested by Dr. Tyn- 
dall. 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 
FIG. 19. 
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 in- 
visibly 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 
