Conversations on Chemistry Part 61

EMILY.

The blue solution is already turning red all round the wire.

CAROLINE.

And the violet solution is beginning to turn green. This is indeed very singular!

MRS. B.

You will be still more astonished when we vary the experiment in this manner:-- These three gla.s.ses (fig. 3. f, g, h,) are, as in the former instance, connected together by wetted cotton, but the middle one alone contains a saline solution, the two others containing only distilled water, coloured as before by vegetable infusions. Yet, on making the connection with the battery, the alkali will appear in the negative gla.s.s (h), and the acid in the positive gla.s.s (f), though neither of them contained any saline matter.

EMILY.

So that the acid and alkali must be conveyed right and left from the central gla.s.s, into the other gla.s.ses, by means of the connecting moistened cotton?

MRS. B.

Exactly so; and you may render the experiment still more striking, by putting into the central gla.s.s (k, fig. 3.) an alkaline solution, the glauber salt being placed into the negative gla.s.s (l), and the positive gla.s.s (i) containing only water. The acid will be attracted by the positive wire (m), and will actually appear in the vessel (i), after pa.s.sing through the alkaline solution (k), without combining with it, although, you know, acids and alkalies are so much disposed to combine.

--But this conversation has already much exceeded our usual limits, and we cannot enlarge more upon this interesting subject at present.

CONVERSATION XIV.

ON ALKALIES.

MRS. B.

Having now given you some idea of the laws by which chemical attractions are governed, we may proceed to the examination of bodies which are formed in consequence of these attractions.

The first cla.s.s of compounds that present themselves to our notice, in our gradual ascent to the most complicated combinations, are bodies composed of only two principles. The sulphurets, phosphurets, carburets, &c. are of this description; but the most numerous and important of these compounds are the combinations of oxygen with the various simple substances with which it has a tendency to unite. Of these you have already acquired some knowledge, but it will be necessary to enter into further particulars respecting the nature and properties of those most deserving our notice. Of this cla.s.s are the ALKALIES and the EARTHS, which we shall successively examine.

We shall first take a view of the alkalies, of which there are three, viz. POTASH, SODA, and AMMONIA. The two first are called _fixed alkalies_, because they exist in a solid form at the temperature of the atmosphere, and require a great heat to be volatilised. They consist, as you already know, of metallic bases combined with oxygen. In potash, the proportions are about eighty-six parts of pota.s.sium to fourteen of oxygen; and in soda, seventy-seven parts of sodium to twenty-three of oxygen. The third alkali, ammonia, has been distinguished by the name of _volatile alkali_, because its natural form is that of gas. Its composition is of a more complicated nature, of which we shall speak hereafter.

Some of the earths bear so strong a resemblance in their properties to the alkalies, that it is difficult to know under which head to place them. The celebrated French chemist, Fourcroy, has cla.s.sed two of them (barytes and stront.i.tes) with the alkalies; but as lime and magnesia have almost an equal t.i.tle to that rank, I think it better not to separate them, and therefore have adopted the common method of cla.s.sing them with the earths, and of distinguishing them by the name of _alkaline earths_.

The general properties of alkalies are, an acrid burning taste, a pungent smell, and a caustic action on the skin and flesh.

CAROLINE.

I wonder they should be caustic, Mrs. B., since they contain so little oxygen.

MRS. B.

Whatever substance has an affinity for any one of the const.i.tuents of animal matter, sufficiently powerful to decompose it, is ent.i.tled to the appellation of caustic. The alkalies, in their pure state, have a very strong attraction for water, for hydrogen, and for carbon, which, you know, are the const.i.tuent principles of oil, and it is chiefly by absorbing these substances from animal matter that they effect its decomposition; for, when diluted with a sufficient quant.i.ty of water, or combined with any oily substance, they lose their causticity.

But, to return to the general properties of alkalies--they change, as we have already seen, the colour of syrup of violets, and other blue vegetable infusions, to green; and have, in general, a very great tendency to unite with acids, although the respective qualities of these two cla.s.ses of bodies form a remarkable contrast.

We shall examine the result of the combination of acids and alkalies more particularly hereafter. It will be sufficient at present to inform you, that whenever acids are brought in contact with alkalies, or alkaline earths, they unite with a remarkable eagerness, and form compounds perfectly different from either of their const.i.tuents; these bodies are called _neutral_ or _compound salts_.

The dry white powder which you see in this phial is pure caustic POTASH; it is very difficult to preserve it in this state, as it attracts, with extreme avidity, the moisture from the atmosphere, and if the air were not perfectly excluded, it would, in a very short time, be actually melted.

EMILY.

It is then, I suppose, always found in a liquid state?

MRS. B.

No; it exists in nature in a great variety of forms and combinations, but is never found in its pure separate state; it is combined with carbonic acid, with which it exists in every part of the vegetable kingdom, and is most commonly obtained from the ashes of vegetables, which are the residue that remains after all the other parts have been volatilised by combustion.

CAROLINE.

But you once said, that after all the volatile parts of a vegetable were evaporated, the substance that remained was charcoal?

MRS. B.

I am surprised that you should still confound the processes of volatilisation and combustion. In order to procure charcoal, we evaporate such parts as can be reduced to vapour by the operation of heat alone; but when we _burn_ the vegetable, we burn the carbon also, and convert it into carbonic acid gas.

CAROLINE.

That is true; I hope I shall make no more mistakes in my favourite theory of combustion.

MRS. B.

Potash derives its name from the _pots_ in which the vegetables, from which it was obtained, used formerly to be burnt; the alkali remained mixed with the ashes at the bottom, and was thence called potash.

EMILY.

The ashes of a wood-fire, then, are potash, since they are vegetable ashes?

MRS. B.

They always contain more or less potash, but are very far from consisting of that substance alone, as they are a mixture of various earths and salts which remain after the combustion of vegetables, and from which it is not easy to separate the alkali in its pure form. The process by which potash is obtained, even in the imperfect state in which it is used in the arts, is much more complicated than simple combustion. It was once deemed impossible to separate it entirely from all foreign substances, and it is only in chemical laboratories that it is to be met with in the state of purity in which you find it in this phial. Wood-ashes are, however, valuable for the alkali which they contain, and are used for some purposes without any further preparation.

Purified in a certain degree, they make what is commonly called _pearlash_, which is of great efficacy in taking out grease, in washing linen, &c.; for potash combines readily with oil or fat, with which it forms a compound well known to you under the name of _soap_.

CAROLINE.

Really! Then I should think it would be better to wash all linen with pearlash than with soap, as, in the latter case, the alkali being already combined with oil, must be less efficacious in extracting grease.

MRS. B.

Its effect would be too powerful on fine linen, and would injure its texture; pearlash is therefore only used for that which is of a strong coa.r.s.e kind. For the same reason you cannot wash your hands with plain potash; but, when mixed with oil in the form of soap, it is soft as well as cleansing, and is therefore much better adapted to the purpose.

Caustic potash, as we already observed, acts on the skin, and animal fibre, in virtue of its attraction for water and oil, and converts all animal matter into a kind of saponaceous jelly.

EMILY.