A DROP OF WATER. 75 
of water. I have here a kettle (Fig. 18, p. 76) boiling 
over a spirit-lamp, and I want you to follow minutely 
what is going on in it. First, in the flame of the lamp, 
atoms of the spirit drawn up from below are clashing 
with the oxygen-atoms in the air. This, as you know, 
causes heat-waves and light-waves to move rapidly 
all round the lamp. The light-waves cannot pass 
through the kettle, but the heat-waves can, and. as 
they enter the water inside they Agitate it violently. 
Quickly, and still more quickly, the particles of water 
near the bottom of the kettle move to and fro and are 
shaken apart ; and as they become light they rise 
through the 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 com- 
posed 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 
