Conversations on Chemistry Part 40

No; in the early part of the enquiry a lamp of this kind was actually proposed; but it was but a rude sketch compared to its present state of improvement. Sir H. Davy, after a succession of trials, by which he brought his lamp nearer and nearer to perfection, at last conceived the happy idea that if the lamp were surrounded with a wire-work or wire-gauze, of a close texture, instead of gla.s.s or horn, the tubular contrivance I have just described would be entirely superseded, since each of the interstices of the gauze would act as a tube in preventing the propagation of explosions; so that this pervious metallic covering would answer the various purposes of transparency, of permeability to air, and of protection against explosion. This idea, Sir Humphry immediately submitted to the test of experiment, and the result has answered his most sanguine expectations, both in his laboratory and in the collieries, where it has already been extensively tried. And he has now the happiness of thinking that his invention will probably be the means of saving every year a number of lives, which would have been lost in digging out of the bowels of the earth one of the most valuable necessaries of life. Here is one of these lamps, every part of which you will at once comprehend. (See PLATE X. fig. 1.)

[Ill.u.s.tration: Plate X.

Fig. 1.

A. the cistern containing the Oil B. the rim or screw by which the gauze cage is fixed to the cistern.

C. apperture for supplying Oil.

E. a wire for tr.i.m.m.i.n.g the wick.

D. F. the wire gauze cylinder.

G. a double top.

Fig. 2.

A. the reservoir of condensed air.

B. the condensing Syringe.

C. the bladder for Oxygen.

D. the moveable jet.]

CAROLINE.

How very simple and ingenious! But I do not yet well see why an explosion taking place within the lamp should not communicate to the external air around it, through the interstices of the wire?

MRS. B.

This has been and is still a subject of wonder, even to philosophers; and the only mode they have of explaining it is, that flame or ignition cannot pa.s.s through a fine wire-work, because the metallic wire cools the flame sufficiently to extinguish it in pa.s.sing through the gauze.

This property of the wire-gauze is quite similar to that of the tubes which I mentioned on introducing the subject; for you may consider each interstice of the gauze as an extremely short tube of a very small diameter.

EMILY.

But I should expect the wire would often become red-hot, by the burning of the gas within the lamp?

MRS. B.

And this is actually the case, for the top of the lamp is very apt to become red-hot. But, fortunately, inflammable gaseous mixtures cannot be exploded by red-hot wire, the intervention of actual flame being required for that purpose; so that the wire does not set fire to the explosive gas around it.

EMILY.

I can understand that; but if the wire be red-hot, how can it cool the flame within, and prevent its pa.s.sing through the gauze?

MRS. B.

The gauze, though red-hot, is not so hot as the flame by which it has been heated; and as metallic wire is a good conductor, the heat does not much acc.u.mulate in it, as it pa.s.ses off quickly to the other parts of the lamp, as well as to any contiguous bodies.

CAROLINE.

This is indeed a most interesting discovery, and one which shows at once the immense utility with which science may be practically applied to some of the most important purposes.

CONVERSATION VIII.

ON SULPHUR AND PHOSPHORUS.

MRS. B.

SULPHUR is the next substance that comes under our consideration. It differs in one essential point from the preceding, as it exists in a solid form at the temperature of the atmosphere.

CAROLINE.

I am glad that we have at last a solid body to examine; one that we can see and touch. Pray, is it not with sulphur that the points of matches are covered, to make them easily kindle?

MRS. B.

Yes, it is; and you therefore already know that sulphur is a very combustible substance. It is seldom discovered in nature in a pure unmixed state; so great is its affinity for other substances, that it is almost constantly found combined with some of them. It is most commonly united with metals, under various forms, and is separated from them by a very simple process. It exists likewise in many mineral waters, and some vegetables yield it in various proportions, especially those of the cruciform tribe. It is also found in animal matter; in short, it may be discovered in greater or less quant.i.ty, in the mineral, vegetable, and animal kingdoms.

EMILY.

I have heard of _flowers of sulphur_, are they the produce of any plant?

MRS. B.

By no means: they consist of nothing more than common sulphur, reduced to a very fine powder by a process called _sublimation_. --You see some of it in this phial; it is exactly the same substance as this lump of sulphur, only its colour is a paler yellow, owing to its state of very minute division.

EMILY.

Pray what is sublimation?

MRS. B.

It is the evaporation, or, more properly speaking, the volatilisation of solid substances, which, in cooling, condense again in a concrete form.

The process, in this instance, must be performed in a closed vessel, both to prevent combustion, which would take place if the access of air were not carefully precluded, and likewise in order to collect the substance after the operation. As it is rather a slow process, we shall not try the experiment now; but you will understand it perfectly if I show you the apparatus used for the purpose. (PLATE XI. fig. 1.) Some lumps of sulphur are put into a receiver of this kind, which is called a _cucurbit_. Its shape, you see, somewhat resembles that of a pear, and is open at the top, so as to adapt itself exactly to a kind of conical receiver of this sort, called the head. The cucurbit, thus covered with its head, is placed over a sand-bath; this is nothing more than a vessel full of sand, which is kept heated by a furnace, such as you see here, so as to preserve the apparatus in a moderate and uniform temperature.

The sulphur then soon begins to melt, and immediately after this, a thick white smoke rises, which is gradually deposited within the head, or upper part of the apparatus, where it condenses against the sides, somewhat in the form of a vegetation, whence it has obtained the name of flowers of sulphur. This apparatus, which is called an _alembic_, is highly useful in all kinds of distillations, as you will see when we come to treat of those operations. Alembics are not commonly made of gla.s.s, like this, which is applicable only to distillations upon a very small scale. Those used in manufactures are generally made of copper, and are, of course, considerably larger. The princ.i.p.al construction, however, is always the same, although their shape admits of some variation.

[Ill.u.s.tration: Plate XI. Vol. I. p. 237.

Fig. 1. Sublimation of Sulphur.

A Alembic.

B Sand-bath.

C Furnace.

Fig. 2. Eudiometer.

Fig. 3. Decomposition of water by Carbon.

A Retort containing water.

B Lamp to heat the water.

C.C Porcelain tube containing Carbone.

D Furnace through which the tube pa.s.ses.

E Receiver for the gas produced.