Conversations on Chemistry Part 51

MRS. B.

Whilst it is red-hot, I shall drop some iron filings on it, and supply them with a current of oxygen gas, by means of this apparatus, (PLATE XII. fig 2.) which consists simply of a closed tin cylindrical vessel, full of oxygen gas, with two apertures and stop-c.o.c.ks, by one of which a stream of water is thrown into the vessel through a long funnel, whilst by the other the gas is forced out through a blow-pipe adapted to it, as the water gains admittance. --Now that I pour water into the funnel, you may hear the gas issuing from the blow-pipe--I bring the charcoal close to the current, and drop the filings upon it--

CAROLINE.

They emit much the same vivid light as the combustion of the iron wire in oxygen gas.

MRS. B.

The process is, in fact, the same; there is only some difference in the mode of conducting it. Let us burn some tin in the same manner--you see that it is equally combustible. --Let us now try some copper--

CAROLINE.

This burns with a greenish flame; it is, I suppose, owing to the colour of the oxyd?

EMILY.

Pray, shall we not also burn some gold?

MRS. B.

That is not in our power, at least in this way. Gold, silver, and platina, are incapable of being oxydated by the greatest heat that we can produce by the common method. It is from this circ.u.mstance, that they have been called perfect metals. Even these, however, have an affinity for oxygen; but their oxydation or combustion can be performed only by means of acids or by electricity. The spark given out by the Voltaic battery produces at the point of contact a greater degree of heat than any other process; and it is at this very high temperature only that the affinity of these metals for oxygen will enable them to act on each other.

I am sorry that I cannot show you the combustion of the perfect metals by this process, but it requires a considerable Voltaic battery. You will see these experiments performed in the most perfect manner, when you attend the chemical lectures of the Royal Inst.i.tution. But in the mean time I can, without difficulty, show you an ingenious apparatus lately contrived for the purpose of producing intense heats, the power of which nearly equals that of the largest Voltaic batteries. It simply consists, you see, in a strong box, made of iron or copper, (PLATE X.

fig. 2.) to which may be adapted this air-syringe or condensing-pump, and a stop-c.o.c.k terminating in a small orifice similar to that of a blow-pipe. By working the condensing syringe, up and down in this manner, a quant.i.ty of air is acc.u.mulated in the vessel, which may be increased to almost any extent; so that if we now turn the stop-c.o.c.k, the condensed air will rush out, forming a jet of considerable force; and if we place the flame of a lamp in the current, you will see how violently the flame is driven in that direction.

CAROLINE.

It seems to be exactly the same effect as that of a blow-pipe worked by the mouth, only much stronger.

EMILY.

Yes; and this new instrument has this additional advantage, that it does not fatigue the mouth and lungs like the common blow-pipe, and requires no art in blowing.

MRS. B.

Unquestionably; but yet this blow-pipe would be of very limited utility, if its energy and power could not be greatly increased by some other contrivance. Can you imagine any mode of producing such an effect?

EMILY.

Could not the reservoir be charged with pure oxygen, instead of common air, as in the case of the gas-holder?

MRS. B.

Undoubtedly; and this is precisely the contrivance I allude to. The vessel need only be supplied with air from a bladder full of oxygen, instead of the air of the room, and this, you see, may be easily done by s.c.r.e.w.i.n.g the bladder on the upper part of the syringe, so that in working the syringe the oxygen gas is forced from the bladder into the condensing vessel.

CAROLINE.

With the aid of this small apparatus, therefore, we could obtain the same effects as those we have just produced with the gas-holder, by means of a column of water forcing the gas out of it?

MRS. B.

Yes; and much more conveniently so. But there is a mode of using this apparatus by which more powerful effects still may be obtained. It consists in condensing in the reservoir, not oxygen alone, but a mixture of oxygen and hydrogen in the exact proportion in which they unite to produce water; and then kindling the jet formed by the mixed gases. The heat disengaged by this combustion, without the help of any lamp, is probably the most intense known; and various effects are said to have been obtained from it which exceed all expectation.

CAROLINE.

But why should we not try this experiment?

MRS. B.

Because it is not exempt from danger; the combustion (notwithstanding various contrivances which have been resorted to with a view to prevent accident) being apt to penetrate into the inside of the vessel, and to produce a dangerous and violent explosion. --We shall, therefore, now proceed in our subject.

CAROLINE.

I think you said the oxyds of metals could be restored to their metallic state?

MRS. B.

Yes; this is called _reviving_ a metal. Metals are in general capable of being revived by charcoal, when heated red hot, charcoal having a greater attraction for oxygen than the metals. You need only, therefore, decompose, or unburn the oxyd, by depriving it of its oxygen, and the metal will be restored to its pure state.

EMILY.

But will the carbon, by this operation, be burnt, and be converted into carbonic acid?

MRS. B.

Certainly. There are other combustible substances to which metals at a high temperature will part with their oxygen. They will also yield it to each other, according to their several degrees of attraction for it; and if the oxygen goes into a more dense state in the metal which it enters, than it existed in that which it quits, a proportional disengagement of caloric will take place.

CAROLINE.

And cannot the oxyds of gold, silver, and platina, which are formed by means of acids or of the electric fluid, be restored to their metallic state?

MRS. B.

Yes, they may; and the intervention of a combustible body is not required; heat alone will take the oxygen from them, convert it into a gas, and revive the metal.

EMILY.

You said that rust was an oxyd of iron; how is it, then, that water, or merely dampness, produces it, which, you know, it very frequently does on steel grates, or any iron instruments?

MRS. B.

In that case the metal decomposes the water, or dampness (which is nothing but water in a state of vapour), and obtains the oxygen from it.

CAROLINE.

I thought that it was necessary to bring metals to a very high temperature to enable them to decompose water.