i88o.J 
On Water and Air. 
183 
itself.” You see the image before his mind; and, in teach- 
ing the young, I always like to keep before them certain 
definite, clear images that will answer to the terms which 
we employ. Hooke’s image of the vibrating sand particles 
may, perhaps, help you to conceive of the vibration of the 
particles of water which takes place when ice is converted 
into a liquid. Well, Hooke’s image of fluidity answers very 
well for our conception of the mere liquid state. But now 
suppose that you have a liquid such as I have here, with a 
free surface — with those molecules and atoms vibrating in 
this way. I have now to appeal to the eye of the mind. I 
can not show these things to the eyes of your body, but I am 
sure that every boy here present who will give me his atten- 
tion will be able to see mentally what I mean. Just as in 
the case of the Whirlpool Rapids which I showed you in 
the last ledture, where two waves coalesced and tossed the 
crest of the wave high into the air, so you can imagine the 
vibrations sometimes so coalescing at the free surface of the 
liquid that they shall toss the molecules of the water into 
the air. If you have seized this image, you have now what 
we call evaporation. The particles of liquid are loosened by 
the vibrations produced by heat, and they are incessantly 
jerked into the atmosphere above the liquid ; and therein 
consists all evaporation which will reduce the whole of the 
liquid to vapour if you only allow it sufficient time. This, 
then, is our conception of evaporation. 
Now I approach the subjedi with which we started— the 
conversion of the waters of the ocean into vapour, that 
vapour from which all our rains and rivers are derived. You 
may say to me, “ The water of the ocean is salt, and the 
water coming from the clouds is perfedtly sweet and without 
salt. How can you get the sweet water from the salt 
water ?” Quite easily, I answer. There, for instance, is a 
flask (f, Fig. 17) containing a very strong brine — so strong 
that after boiling it for a time, by means of a gas-flame, a 
large quantity of the salt is rendered solid in the bottom of 
the vessel. We kept that boiling yesterday for a consider- 
able time. We led the steam from that flask through the 
worm-pipe, placed in the glass vessel c, that you see there. 
We have surrounded the worm with cold water, kept cold by 
ice. The steam passes over from that flask of salt water, 
and, after passing through the cool worm-pipe, it is received 
in the form of water in this flask, f'. There is the fresh 
water extradted from the brine. This water is perfedtly 
sweet — sweet as any spring-water. This shows you that in 
the process of distillation, whether it is artificial or is pro- 
