Conversations on Chemistry Part 23

EMILY.

But why does not water, as well as ether, reduce its own temperature by evaporating?

MRS. B.

The fact is that it does, though much less rapidly than ether. Thus, for instance, you may often have observed, in the heat of summer, how much any particular spot may be cooled by watering, though the water used for that purpose be as warm as the air itself. Indeed so much cold may be produced by the mere evaporation of water, that the inhabitants of India, by availing themselves of the most favourable circ.u.mstances for this process which their warm climate can afford, namely, the cool of the night, and situations most exposed to the night breeze, succeed in causing water to freeze, though the temperature of the air be as high as 60 degrees. The water is put into shallow earthen trays, so as to expose an extensive surface to the process of evaporation, and in the morning, the water is found covered with a thin cake of ice, which is collected in sufficient quant.i.ty to be used for purposes of luxury.

CAROLINE.

How delicious it must be to drink liquids so cold in those tropical climates! But, Mrs. B., could we not try that experiment?

MRS. B.

If we were in the country, I have no doubt but that we should be able to freeze water, by the same means, and under similar circ.u.mstances. But we can do it immediately, upon a small scale, in this very room, in which the thermometer stands at 70 degrees. For this purpose we need only place some water in a little cup under the receiver of the air-pump (PLATE V. fig. 1.), and exhaust the air from it. What will be the consequence, Caroline?

[Ill.u.s.tration: Plate V. Vol. I. page 138.

Fig. 1.

The air-pump & receiver for Mr. Leslie's experiment.

C a saucer with sulphuric Acid.

B a gla.s.s or earthen cup containing Water.

D a stand for the cup with its legs made of Gla.s.s.

A a Thermometer.

Fig. 2. Dr. Wollaston's Cryophorus.

Fig. 5. Dr. Marcet's mode of using the Cryophorus.

Fig. 3. & 4. the different parts of Fig. 5. seen separate.]

CAROLINE.

Of course the water will evaporate more quickly, since there will no longer be any atmospheric pressure on its surface: but will this be sufficient to make the water freeze?

MRS. B.

Probably not, because the vapour will not be carried off fast enough; but this will be accomplished without difficulty if we introduce into the receiver (fig. 1.), in a saucer, or other large shallow vessel, some strong sulphuric acid, a substance which has a great attraction for water, whether in the form of vapour, or in the liquid state. This attraction is such that the acid will instantly absorb the moisture as it rises from the water, so as to make room for the formation of fresh vapour; this will of course hasten the process, and the cold produced from the rapid evaporation of the water, will, in a few minutes, be sufficient to freeze its surface.* We shall now exhaust the air from the receiver.

[Footnote *: This experiment was first devised by Mr. Leslie, and has since been modified in a variety of forms.]

EMILY.

Thousands of small bubbles already rise through the water from the internal surface of the cup; what is the reason of this?

MRS. B.

These are bubbles of air which were partly attached to the vessel, and partly diffused in the water itself; and they expand and rise in consequence of the atmospheric pressure being removed.

CAROLINE.

See, Mrs. B.; the thermometer in the cup is sinking fast; it has already descended to 40 degrees!

EMILY.

The water seems now and then violently agitated on the surface, as if it was boiling; and yet the thermometer is descending fast!

MRS. B.

You may call it _boiling_, if you please, for this appearance is, as well as boiling, owing to the rapid formation of vapour; but here, as you have just observed, it takes place from the surface, for it is only when heat is applied to the bottom of the vessel that the vapour is formed there. --Now crystals of ice are actually shooting all over the surface of the water.

CAROLINE.

How beautiful it is! The surface is now entirely frozen--but the thermometer remains at 32 degrees.

MRS. B.

And so it will, conformably with our doctrine of latent heat, until the whole of the water is frozen; but it will then again begin to descend lower and lower, in consequence of the evaporation which goes on from the surface of the ice.

EMILY.

This is a most interesting experiment; but it would be still more striking if no sulphuric acid were required.

MRS. B.

I will show you a freezing instrument, contrived by Dr. Wollaston, upon the same principle as Mr. Leslie's experiment, by which water may be frozen by its own evaporation alone, without the a.s.sistance of sulphuric acid.

This tube, which, as you see (PLATE V. fig. 2.), is terminated at each extremity by a bulb, one of which is half full of water, is internally perfectly exhausted of air; the consequence of this is, that the water in the bulb is always much disposed to evaporate. This evaporation, however, does not proceed sufficiently fast to freeze the water; but if the empty ball be cooled by some artificial means, so as to condense quickly the vapour which rises from the water, the process may be thus so much promoted as to cause the water to freeze in the other ball. Dr.

Wollaston has called this instrument _Cryophorus_.

CAROLINE.

So that cold seems to perform here the same part which the sulphuric acid acted in Mr. Leslie's experiment?

MRS. B.

Exactly so; but let us try the experiment.

EMILY.

How will you cool the instrument? You have neither ice nor snow.

MRS. B.

True: but we have other means of effecting this.* You recollect what an intense cold can be produced by the evaporation of ether in an exhausted receiver. We shall inclose the bulb in this little bag of fine flannel (fig. 3.), then soke it in ether, and introduce it into the receiver of the air-pump. (Fig. 5.) For this purpose we shall find it more convenient to use a cryophorus of this shape (fig. 4.), as its elongated bulb pa.s.ses easily through a bra.s.s plate which closes the top of the receiver. If we now exhaust the receiver quickly, you will see, in less than a minute, the water freeze in the other bulb, out of the receiver.

[Footnote *: This mode of making the experiment was proposed, and the particulars detailed, by Dr. Marcet, in the 34th vol. of Nicholson's Journal, page 119.]

EMILY.