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ether, to indicate its origin, and to distinguish it from other analogous compounds called ethers.

When poured on the hand, ether produces a strong sensation of cold, owing to the rapidity with which it evaporates, rendering heat latent. It is very inflammable, and burns with a white flame. Its vapor forms explosive mixtures with air or oxygen. It dissolves oils, fats, and resins far more readily than alcohol does. It also dissolves sulphur, phosphorus, and iodine. When inhaled in sufficient quantity, its vapor is an anesthetic, and was the first substance used for that purpose by physicians.

Acetic ether is obtained by distilling a mixture of alcohol, sulphuric acid, and acetic acid. It is used in medicine. If nitric acid be used, we get nitric ether, which is well known under the name of sweet spirits of nitre. Several of the ethers exist in a natural state in fruits, giving them their peculiar flavors.

206. Acetic Acid. - If alcohol, diluted with water, be mixed with a ferment, as yeast, and exposed to the air at ordinary temperatures, it is soon converted into acetic acid, or vinegar. The alcohol absorbs 2 atoms of oxygen, forming water and acetic acid :—

CH¿O+02=H2O+ C2H2O2 (acetic acid).

2

2

This process constitutes what is commonly called the acetous fermentation. Since the alcohol is oxidized, it is in a certain sense a combustion that takes place. It is, however, brought about by the influence of a ferment, known as the vinegar-plant, or mother of vinegar.

There are a great many methods of making vinegar on the large scale; but, for the most part, they are only different devices for exposing to the air as large a surface as possible of the alcoholic mixture. In one process, the liquid is allowed to trickle down through vats, filled with

flat pieces of wicker-work, piled one upon another. These are first watered with vinegar, or liquid partially converted into vinegar, and thus become covered with the germs of the vinegar-plant. Air is admitted through holes in the bottom of the vats, and passes out at the top, after giving up a part of its oxygen to the alcohol. Heat is produced by the oxidation, and this causes an upward draught, and quickens the circulation of air through the

vats.

In another process, the weak alcoholic liquor trickles down through perforated casks, filled with beech shavings, which have been thoroughly scalded to remove all soluble matter. Air is admitted by holes around the lower part of the cask, and passes out at the top.

This

When vinegar is purified, and concentrated by distillation, it constitutes the acetic acid of commerce. is prepared on a large scale from pyroligneous acid, or wood vinegar (174), which is obtained by the destructive distillation of wood.

207. Bread-making.—The making of bread in the ordinary way, by means of leaven or yeast, is an example of alcoholic fermentation.

When the grain of wheat is ground in a mill, and then sifted, it is separated into two parts, the bran and the flour. The bran is the outside harder part of the grain, which is not ground so readily, and, when ground, makes the flour darker. Both the bran and the flour consist mainly of gluten, starch, and water.

It

If the flour be mixed with water enough to moisten it thoroughly, the particles cohere and form a dough, which can be kneaded or moulded with the hand. is the gluten of the flour which gives the tenacity to the dough.

A little leaven or yeast is added to the flour, either before it is mixed with water into a dough or in the

course of this process; and the dough is then placed for some hours in a warm atmosphere, in order that it may rise. This rising is a fermentation, caused by the leaven. or yeast. Leaven is the primitive ferment, and is simply a piece of moistened dough which has begun to putrefy. When brought in contact with a fresh portion of flour and water, it very quickly acts as a ferment, and develops fermentation in the whole mass. Yeast, as has been stated, is one of the vegetable growths which are ferments (200, 201).

The leaven or yeast soon begins to act on the gluten, starch, and sugar of the flour, and a portion of the sugar is converted into carbonic acid and alcohol in every part of the dough. The bubbles of gas, thus disengaged in the mass, form innumerable cavities, and make it light and porous.

The spongy dough is now put into a hot oven, where the fermentation and swelling are at first increased by the heat; but when the whole has been heated to about 212°, the fermentation is suddenly arrested, and the mass becomes fixed in the form it has then attained. In the baking, some of the water is dissipated from the dough, the starch and gluten are partially boiled, and some of the starch is converted into dextrine. The brown, glossy appearance of the crust is due to this formation of dextrine (162). Most of the alcohol is driven off by the heat; but a large amount of water (about 45 per cent of the weight of the bread) remains in the loaf after the baking.

There are various methods of making bread without fermentation, most of which depend on the liberation of carbonic acid from one of its compounds, by means of an acid. Bicarbonate of soda and muriatic acid are the materials sometimes used. Cream of tartar, which is a compound of tartaric acid (165), is often used as the acid.

The salt and acid are usually mixed with the dough, and the carbonic acid there set free; but in the manufacture of the so-called aerated bread, water charged with the gas is used in making the dough.

SUMMARY.

Fermentation is the breaking up of complex organic compounds into simpler ones, by the agency of certain plants called ferments.

The alcoholic fermentation takes place in sugar, and produces alcohol and carbonic acid.

In making alcoholic liquors from grain, the starch of the grain is converted into sugar, by the action of diastase, as in the germination of seeds. The sugar is then converted into alcohol by the use of a ferment, as yeast.

The alcohol of fermented liquors may be separated from the water, and concentrated by distillation.

By the action of acids upon alcohol, important compounds called ethers are obtained.

The acetous fermentation takes place in alcohol, which undergoes oxidation, producing vinegar, or acetic acid.

The making of bread with yeast is an example of alcoholic fermentation.

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