e steam rise from the
surface, instead of the bottom of the liquid?

MRS. B.

The steam or vapour does ascend from the bottom, though it seems to
arise from the surface of the liquid. We shall boil some water in this
Florence flask, (PLATE IV. Fig. 1.) in order that you may be well
acquainted with the process of ebullition;--you will then see, through
the glass, that the vapour rises in bubbles from the bottom. We shall
make it boil by means of a lamp, which is more convenient for this
purpose than the chimney fire.

[Illustration: Plate IV. Vol. I. p. 84.

Fig. 1. Pneumatic Pump.
Ether evaporated & water frozen in the air pump.
A Phial of Ether.
B Glass vessel containing water.
C.C Thermometers one in the Ether, the other in the water.

Fig. 2. Boiling water in a flask over a Patent lamp.]

EMILY.

I see some small bubbles ascend, and a great many appear all over the
inside of the flask; does the water begin to boil already?

MRS. B.

No; what you now see are bubbles of air, which were either dissolved in
the water, or attached to the inner surface of the flask, and which,
being rarefied by the heat, ascend in the water.

EMILY.

But the heat which rarefies the air inclosed in the water must rarefy
the water at the same time; therefore, if it could remain stationary in
the water when both were cold, I do not understand why it should not
when both are equally heated?

MRS. B.

Air being much less dense than water, is more easily rarefied; the
former, therefore, expands to a great extent, whilst the latter
continues to occupy nearly the same space; for water dilates
comparatively but very little without changing its state and becoming
vapour. Now that the water in the flask begins to boil, observe what
large bubbles rise from the bottom of it.

EMILY.

I see them perfectly; but I wonder that they have sufficient power to
force themselves through the water.

CAROLINE.

They _must_ rise, you know, from their specific levity.

MRS. B.

You are right, Caroline; but vapour has not in all liquids (when brought
to the degree of vaporization) the power of overcoming the pressure of
the less heated surface. Metals, for instance, mercury excepted,
evaporate only from the surface; therefore no vapour will ascend from
them till the degree of heat which is necessary to form it has reached
the surface; that is to say, till the whole of the liquid is brought to
a state of ebullition.

EMILY