And by what means can the two ga.s.ses, which compose the atmospheric air, be separated?

MRS. B.

There are many ways of a.n.a.lysing the atmosphere: the two ga.s.ses may be separated first by combustion.

EMILY.

You surprise me! how is it possible that combustion should separate them?

MRS. B.

I should previously remind you that oxygen is supposed to be the only simple body naturally combined with negative electricity. In all the other elements the positive electricity prevails, and they have consequently, all of them, an attraction for oxygen.*

[Footnote *: If chlorine or oxymuriatic gas be a simple body, according to Sir H. Davy's view of the subject, it must be considered as an exception to this statement; but this subject cannot be discussed till the properties and nature of chlorine come under examination.]

CAROLINE.

Oxygen the only negatively electrified body! that surprises me extremely; how then are the combinations of the other bodies performed, if, according to your explanation of chemical attraction, bodies are supposed only to combine in virtue of their opposite states of electricity?

MRS. B.

Observe that I said, that oxygen was the only _simple_ body, naturally negative. Compound bodies, in which oxygen prevails over the other component parts, are also negative, but their negative energy is greater or less in proportion as the oxygen predominates. Those compounds into which oxygen enters in less proportion than the other const.i.tuents, are positive, but their positive energy is diminished in proportion to the quant.i.ty of oxygen which enters into their composition.

All bodies, therefore, that are not already combined with oxygen, will attract it, and, under certain circ.u.mstances, will absorb it from the atmosphere, in which case the nitrogen gas will remain alone, and may thus be obtained in its separate state.

CAROLINE.

I do not understand how a gas can be absorbed?

MRS. B.

It is only the oxygen, or basis of the gas, which is absorbed; and the two electricities escaping, that is to say, the negative from the oxygen, the positive from the burning body, unite and produce caloric.

EMILY.

And what becomes of this caloric?

MRS. B.

We shall make this piece of dry wood attract oxygen from the atmosphere, and you will see what becomes of the caloric.

CAROLINE.

You are joking, Mrs. B--; you do not mean to decompose the atmosphere with a piece of dry stick?

MRS. B.

Not the whole body of the atmosphere, certainly; but if we can make this piece of wood attract any quant.i.ty of oxygen from it, a proportional quant.i.ty of atmospherical air will be decomposed.

CAROLINE.

If wood has so strong an attraction for oxygen, why does it not decompose the atmosphere spontaneously?

MRS. B.

It is found by experience, that an elevation of temperature is required for the commencement of the union of the oxygen and the wood.

This elevation of temperature was formerly thought to be necessary, in order to diminish the cohesive attraction of the wood, and enable the oxygen to penetrate and combine with it more readily. But since the introduction of the new theory of chemical combination, another cause has been a.s.signed, and it is now supposed that the high temperature, by exalting the electrical energies of bodies, and consequently their force of attraction, facilitates their combination.

EMILY.

If it is true, that caloric is composed of the two electricities, an elevation of temperature must necessarily augment the electric energies of bodies.

MRS. B.

I doubt whether that would be a necessary consequence; for, admitting this composition of caloric, it is only by its being decomposed that electricity can be produced. Sir H. Davy, however, in his numerous experiments, has found it to be an almost invariable rule that the electrical energies of bodies are increased by elevation of temperature.

What means then shall we employ to raise the temperature of the wood, so as to enable it to attract oxygen from the atmosphere?

CAROLINE.

Holding it near the fire, I should think, would answer the purpose.

MRS. B.

It may, provided you hold it sufficiently close to the fire; for a very considerable elevation of temperature is required.

CAROLINE.

It has actually taken fire, and yet I did not let it touch the coals, but I held it so very close that I suppose it caught fire merely from the intensity of the heat.

MRS. B.

Or you might say, in other words, that the caloric which the wood imbibed, so much elevated its temperature, and exalted its electric energy, as to enable it to attract oxygen very rapidly from the atmosphere.

EMILY.

Does the wood absorb oxygen while it is burning?

MRS. B.

Yes, and the heat and light are produced by the union of the two electricities which are set at liberty, in consequence of the oxygen combining with the wood.

CAROLINE.

You astonish me! the heat of a burning body proceeds then as much from the atmosphere as from the body itself?

MRS. B.