It was supposed that the caloric, given out during combustion, proceeded entirely, or nearly so, from the decomposition of the oxygen gas; but, according to Sir H. Davy's new view of the subject, both the oxygen gas, and the combustible body, concur in supplying the heat and light, by the union of their opposite electricities.

EMILY.

I have not yet met with any thing in chemistry that has surprised or delighted me so much as this explanation of combustion. I was at first wondering what connection there could be between the affinity of a body for oxygen and its combustibility; but I think I understand it now perfectly.

MRS. B.

Combustion then, you see, is nothing more than the rapid combination of a body with oxygen, attended by the disengagement of light and heat.

EMILY.

But are there no combustible bodies whose attraction for oxygen is so strong, that they will combine with it, without the application of heat?

CAROLINE.

That cannot be; otherwise we should see bodies burning spontaneously.

MRS. B.

But there are some instances of this kind, such as phosphorus, pota.s.sium, and some compound bodies, which I shall hereafter make you acquainted with. These bodies, however, are prepared by art, for in general, all the combustions that could occur spontaneously, at the temperature of the atmosphere, have already taken place; therefore new combustions cannot happen without the temperature of the body being raised. Some bodies, however, will burn at a much lower temperature than others.

CAROLINE.

But the common way of burning a body is not merely to approach it to one already on fire, but rather to put the one in actual contact with the other, as when I burn this piece of paper by holding it in the flame of the fire.

MRS. B.

The closer it is in contact with the source of caloric, the sooner will its temperature be raised to the degree necessary for it to burn. If you hold it near the fire, the same effect will be produced; but more time will be required, as you found to be the case with the piece of stick.

EMILY.

But why is it not necessary to continue applying caloric throughout the process of combustion, in order to keep up the electric energy of the wood, which is required to enable it to combine with the oxygen?

MRS. B.

The caloric which is gradually produced by the two electricities during combustion, keeps up the temperature of the burning body; so that when once combustion has begun, no further application of caloric is required.

CAROLINE.

Since I have learnt this wonderful theory of combustion, I cannot take my eyes from the fire; and I can scarcely conceive that the heat and light, which I always supposed to proceed entirely from the coals, are really produced as much by the atmosphere.

EMILY.

When you blow the fire, you increase the combustion, I suppose, by supplying the coals with a greater quant.i.ty of oxygen gas?

MRS. B.

Certainly; but of course no blowing will produce combustion, unless the temperature of the coals be first raised. A single spark, however, is sometimes sufficient to produce that effect; for, as I said before, when once combustion has commenced, the caloric disengaged is sufficient to elevate the temperature of the rest of the body, provided that there be a free access of oxygen. It however sometimes happens that if a fire be ill made, it will be extinguished before all the fuel is consumed, from the very circ.u.mstance of the combustion being so slow that the caloric disengaged is insufficient to keep up the temperature of the fuel. You must recollect that there are three things required in order to produce combustion; a combustible body, oxygen, and a temperature at which the one will combine with the other.

EMILY.

You said that combustion was one method of decomposing the atmosphere, and obtaining the nitrogen gas in its simple state; but how do you secure this gas, and prevent it from mixing with the rest of the atmosphere?

MRS. B.

It is necessary for this purpose to burn the body within a close vessel, which is easily done. --We shall introduce a small lighted taper (PLATE VII. Fig. 1.) under this gla.s.s receiver, which stands in a bason over water, to prevent all communication with the external air.

[Ill.u.s.tration: Plate VII. Vol. I. p. 181.

Fig. 1. Combustion of a taper under a receiver.

Fig. 2. A Retort on a stand.

Fig. 3. Preparation of oxygen gas.

A Furnace.

B Earthen Retort in the furnace.

C Water bath.

D Receiver.

E.E Tube conveying the gas from the Retort through the water into the Receiver.

F.F.F Shelf perforated on which the Receiver stands.

Fig. 4. Combustion of iron wire in oxygen gas.]

CAROLINE.

How dim the light burns already! --It is now extinguished.

MRS. B.

Can you tell us why it is extinguished?

CAROLINE.

Let me consider. --The receiver was full of atmospherical air; the taper, in burning within it, must have combined with the oxygen contained in that air, and the caloric that was disengaged produced the light of the taper. But when the whole of the oxygen was absorbed, the whole of its electricity was disengaged; consequently no more caloric could be produced, the taper ceased to burn, and the flame was extinguished.

MRS. B.

Your explanation is perfectly correct.

EMILY.

The two const.i.tuents of the oxygen gas being thus disposed of, what remains under the receiver must be pure nitrogen gas?

MRS. B.

There are some circ.u.mstances which prevent the nitrogen gas, thus obtained, from being perfectly pure; but we may easily try whether the oxygen has disappeared, by putting another lighted taper under it. --You see how instantaneously the flame is extinguished, for want of oxygen to supply the negative electricity required for the formation of caloric; and were you to put an animal under the receiver, it would immediately be suffocated. But that is an experiment which I do not think your curiosity will tempt you to try.

EMILY.