Conversations on Chemistry Part 41

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CAROLINE.

What is the use of that neck, or tube, which bends down from the upper piece of the apparatus?

MRS. B.

It is of no use in sublimations; but in distillations (the general object of which is to evaporate, by heat, in closed vessels, the volatile parts of a compound body, and to condense them again into a liquid,) it serves to carry off the condensed fluid, which otherwise would fall back into the cucurbit. But this is rather foreign to our present subject. Let us return to the sulphur. You now perfectly understand, I suppose, what is meant by sublimation?

EMILY.

I believe I do. Sublimation appears to consist in destroying, by means of heat, the attraction of aggregation of the particles of a solid body, which are thus volatilised; and as soon as they lose the caloric which produced that effect, they are deposited in the form of a fine powder.

CAROLINE.

It seems to me to be somewhat similar to the transformation of water into vapour, which returns to its liquid state when deprived of caloric.

EMILY.

There is this difference, however, that the sulphur does not return to its former state, since, instead of lumps, it changes to a fine powder.

MRS. B.

Chemically speaking, it is exactly the same substance, whether in the form of lump or powder. For if this powder be melted again by heat, it will, in cooling, be restored to the same solid state in which it was before its sublimation.

CAROLINE.

But if there be no real change, produced by the sublimation of the sulphur, what is the use of that operation?

MRS. B.

It divides the sulphur into very minute parts, and thus disposes it to enter more readily into combination with other bodies. It is used also as a means of purification.

CAROLINE.

Sublimation appears to me like the beginning of combustion, for the completion of which one circ.u.mstance only is wanting, the absorption of oxygen.

MRS. B.

But that circ.u.mstance is every thing. No essential alteration is produced in sulphur by sublimation; whilst in combustion it combines with the oxygen, and forms a new compound totally different in every respect from sulphur in its pure state. --We shall now _burn_ some sulphur, and you will see how very different the result will be. For this purpose I put a small quant.i.ty of flowers of sulphur into this cup, and place it in a dish, into which I have poured a little water: I now set fire to the sulphur with the point of this hot wire; for its combustion will not begin unless its temperature be considerably raised.

--You see that it burns with a faint blueish flame; and as I invert over it this receiver, white fumes arise from the sulphur, and fill the vessel. --You will soon perceive that the water is rising within the receiver, a little above its level in the plate. --Well, Emily, can you account for this?

EMILY.

I suppose that the sulphur has absorbed the oxygen from the atmospherical air within the receiver, and that we shall find some oxygenated sulphur in the cup. As for the white smoke, I am quite at a loss to guess what it may be.

MRS. B.

Your first conjecture is very right: but you are mistaken in the last; for nothing will be left in the cup. The white vapour is the oxygenated sulphur, which a.s.sumes the form of an elastic fluid of a pungent and offensive smell, and is a powerful acid. Here you see a chemical combination of oxygen and sulphur, producing a true gas, which would continue such under the pressure and at the temperature of the atmosphere, if it did not unite with the water in the plate, to which it imparts its acid taste, and all its acid properties. --You see, now, with what curious effects the combustion of sulphur is attended.

CAROLINE.

This is something quite new; and I confess that I do not perfectly understand why the sulphur turns acid.

MRS. B.

It is because it unites with oxygen, which is the acidifying principle.

And, indeed, the word _oxygen_ is derived from two Greek words signifying _to produce an acid_.

CAROLINE.

Why, then, is not water, which contains such a quant.i.ty of oxygen, acid?

MRS. B.

Because hydrogen, which is the other const.i.tuent of water, is not susceptible of acidification. --I believe it will be necessary, before we proceed further, to say a few words of the general nature of acids, though it is rather a deviation from our plan of examining the simple bodies separately, before we consider them in a state of combination.

Acids may be considered as a peculiar cla.s.s of _burnt_ bodies, which during their combustion, or combination with oxygen, have acquired very characteristic properties. They are chiefly discernible by their sour taste, and by turning red most of the blue vegetable colours. These two properties are common to the whole cla.s.s of acids; but each of them is distinguished by other peculiar qualities. Every acid consists of some particular substance, (which const.i.tutes its basis, and is different in each,) and of oxygen, which is common to them all.

EMILY.

But I do not clearly see the difference between acids and oxyds.

MRS. B.

Acids were, in fact, oxyds, which, by the addition of a sufficient quant.i.ty of oxygen, have been converted into acids. For acidification, you must observe, always implies previous oxydation, as a body must have combined with the quant.i.ty of oxygen requisite to const.i.tute it an oxyd, before it can combine with the greater quant.i.ty that is necessary to render it an acid.

CAROLINE.

Are all oxyds capable of being converted into acids?

MRS. B.

Very far from it; it is only certain substances which will enter into that peculiar kind of union with oxygen that produces acids, and the number of these is proportionally very small; but all burnt bodies may be considered as belonging either to the cla.s.s of oxyds, or to that of acids. At a future period, we shall enter more at large into this subject. At present, I have but one circ.u.mstance further to point out to your observation respecting acids: it is, that most of them are susceptible of two degrees of acidification, according to the different quant.i.ties of oxygen with which their basis combines.

EMILY.

And how are these two degrees of acidification distinguished?

MRS. B.

By the peculiar properties which result from them. The acid we have just made is the first or weakest degree of acidification, and is called _sulphureous acid_; if it were fully saturated with oxygen, it would be called _sulphuric acid_. You must therefore remember, that in this, as in all acids, the first degree of acidification is expressed by the termination in _ous_; the stronger, by the termination in _ic_.

CAROLINE.

And how is the sulphuric acid made?

MRS. B.