Conversations on Chemistry Part 91

MRS. B.

The common temperature of the atmosphere in the cellars in which the juice of the grape is fermented is sufficiently warm for this purpose; and as the juice contains an ample supply of water, there is no occasion for any addition of it. But when fermentation is produced in dry malt, a quant.i.ty of water must necessarily be added.

EMILY.

But what are precisely the changes that happen during the vinous fermentation?

MRS. B.

The sugar is decomposed, and its const.i.tuents are recombined into two new substances; the one a peculiar liquid substance, called _alcohol_ or _spirit of wine_, which remains in the fluid; the other, carbonic acid gas, which escapes during the fermentation. Wine, therefore, as I before observed, in a general point of view, may be considered as a liquid of which alcohol const.i.tutes the essential part. And the varieties of strength and flavour of the different kinds of wine are to be attributed to the different qualities of the fruits from which they are obtained, independently of the sugar.

CAROLINE.

I am astonished to hear that so powerful a liquid as spirit of wine should be obtained from so mild a substance as sugar.

MRS. B.

Can you tell me in what the princ.i.p.al difference consists between alcohol and sugar?

CAROLINE.

Let me reflect . . . . . Sugar consists of carbon, hydrogen, and oxygen.

If carbonic acid be subtracted from it, during the formation of alcohol, the latter will contain less carbon and oxygen than sugar does; therefore hydrogen must be the prevailing principle of alcohol.

MRS. B.

It is exactly so. And this very large proportion of hydrogen accounts for the lightness and combustible property of alcohol, and of spirits in general, all of which consist of alcohol variously modified.

EMILY.

And can sugar be recomposed from the combination of alcohol and carbonic acid?

MRS. B.

Chemists have never been able to succeed in effecting this; but from a.n.a.logy, I should suppose such a recomposition possible. Let us now observe more particularly the phenomena that take place during the vinous fermentation. At the commencement of this process, heat is evolved, and the liquor swells considerably from the formation of the carbonic acid, which is disengaged in such prodigious quant.i.ties as would be fatal to any person who should unawares inspire it; an accident which has sometimes happened. If the fermentation be stopped by putting the liquor into barrels, before the whole of the carbonic acid is evolved, the wine is brisk, like Champagne, from the carbonic acid imprisoned in it, and it tastes sweet, like cyder, from the sugar not being completely decomposed.

EMILY.

But I do not understand why heat should be evolved during this operation. For, as there is a considerable formation of gas, in which a proportionable quant.i.ty of heat must become insensible, I should have imagined that cold, rather than heat, would have been produced.

MRS. B.

It appears so on first consideration; but you must recollect that fermentation is a complicated chemical process; and that, during the decompositions and recompositions attending it, a quant.i.ty of chemical heat may be disengaged, sufficient both to develope the gas, and to effect an increase of temperature. When the fermentation is completed, the liquid cools and subsides, the effervescence ceases, and the thick, sweet, sticky juice of the fruit is converted into a clear, transparent, spirituous liquor, called wine.

EMILY.

How much I regret not having been acquainted with the nature of the vinous fermentation, when I had an opportunity of seeing the process!

MRS. B.

You have an easy method of satisfying yourself in that respect by observing the process of brewing, which, in every essential circ.u.mstance, is similar to that of making wine, and is really a very curious chemical operation.

Although we cannot actually make wine at this moment, it will be easy to show you the mode of a.n.a.lyzing it. This is done by distillation. When wine of any kind is submitted to this operation, it is found to contain brandy, water, tartar, extractive colouring matter, and some vegetable acids. I have put a little port wine into this alembic of gla.s.s (PLATE XIV. Fig. 1.), and on placing the lamp under it, you will soon see the spirit and water successively come over--

[Ill.u.s.tration: Plate XIV. Vol. II. p. 213.

Fig. 1.

A Alembic.

B Lamp.

C Wine gla.s.s.

Fig. 2. Alcohol blowpipe.

D the Lamp.

E the vessel in which the Alcohol is boiling.

F a safety valve.

G the inflamed jet or steam of alcohol directed towards a gla.s.s tube H.]

EMILY.

But you do not mention alcohol amongst the _products_ of the distillation of wine; and yet that is its most essential ingredient?

MRS. B.

The alcohol is contained in the brandy which is now coming over, and dropping from the still. Brandy is nothing more than a mixture of alcohol and water; and in order to obtain the alcohol pure, we must again distil it from brandy.

CAROLINE.

I have just taken a drop on my finger; it tastes like strong brandy, but it is without colour, whilst brandy is of a deep yellow.

MRS. B.

It is not so naturally; in its pure state brandy is colourless, and it obtains the yellow tint you observe, by extracting the colouring matter from the new oaken casks in which it is kept. But if it does not acquire the usual tinge in this way, it is the custom to colour the brandy used in this country artificially, with a little burnt sugar, in order to give it the appearance of having been long kept.

CAROLINE.

And is rum also distilled from wine?

MRS. B.

By no means; it is distilled from the sugar-cane, a plant which contains so great a quant.i.ty of sugar, that it yields more alcohol than almost any other vegetable. After the juice of the cane has been pressed out for making sugar, what still remains in the bruised cane is extracted by water, and this watery solution of sugar is fermented, and produces rum.

The spirituous liquor called _arack_ is in a similar manner distilled from the product of the vinous fermentation of rice.

EMILY.

But rice has no sweetness; does it contain any sugar?

MRS. B.