i88o.] 
On Water and Air. 
185 
duced by the sun, you liberate not the molecules of the salt, 
but the molecules of the water only, and thus from the salt 
ocean you get sweet water. 
You must, at the same time, have perfectly definite ideas 
of the meaning of vapour. Here is a boiler, b (Fig. 18), 
and beneath it we will now start a flame so as to make the 
boiling of the water go on vigorously ; and there you have, 
issuing from the jet j, — what ? Will you call it steam, or 
will you call it vapour ? If you called that cloud either 
steam or vapour you would be in error. That cloud that 
you see issuing from the boiler is not vapour. It is com- 
posed of fine particles of water. It is, as I have elsewhere 
expressed it, a kind of dust of water, — particles of water 
finely divided and precipitated, so as to produce this appear- 
ance. But if I take a flame and bring it underneath the 
cloud, at a short distance from the jet, the cloud entirely 
disappears. I can cut it off in this way, at the very nozzle 
of the tap, and no cloud whatever is seen. Now that invi- 
sible thing into which I have converted the visible cloud is 
true vapour. The vapour of water is as invisible as the air 
that you breathe. This cloud that you see is the vapour of 
water precipitated so as actually to form water ; and in that 
way our clouds are produced. 
Now I want your patience to accompany me while I make 
a thorough investigation of the manner in which this vapour 
is raised from the ocean — an investigation of the agent 
which is instrumental in producing this vapour. 
Before I do that, however, here is an experiment which 
my friend Prof. Dewar has arranged for me, and which 
enables me to follow up the idea which I threw out before 
you — that heat consists of this intense vibratory motion to 
which I have referred. I mentioned in our last ledture the 
name of Sir William Grove, — Mr. Justice Grove, — and I 
told you how he had operated upon boiling water. He made 
a series of extremely interesting experiments many years ago 
upon the adtion of heat upon water. He placed pieces of 
intensely incandescent platinum, or platinum wire, in water, 
and he found that the atomic vibrations of which the intense 
heat consisted were so violent that they adtually shook 
asunder the molecules of water, and reduced them to their 
constituent atoms of oxygen and nitrogen. Here we have 
an apparatus which will show you this adtion (Fig. 19). At 
the present moment Prof. Dewar is goingto ignite, by means 
of eledtricity, the spiral of platinum wire contained in the 
glass tube s, by attaching a battery to its terminal wires. 
Observe what is going on. The water is boiling vigorously 
