In ordinary language, the term fermentation is employed to signify the peculiar changes which take place when a solution of sugar, or any vegetable substance containing saccharine matter, is converted into spirit - this, however, is only one of many of such kinds of action, which are well known to chemists. The most important fermentations are the saccharine fermentation, when sugar is formed by a change taking place in starch; the vinous, when spirit is formed from sugar; and the acetous, in which vinegar or acetic acid is formed from spirit.

The saccharine fermentation, or the formation of sugar from starch, is interesting, though it does not influence such operations as are included in our Domestic Manipulations. If starch is dissolved in water, a little wheat flour added, and the whole exposed to a moderately warm temperature, it will be found that after a few days, varying in number with the degree of warmth, the starch has disappeared, and the liquid has become sweet from the formation of sugar. The same change takes place with much greater rapidity if starch is boiled with a solution of malt, which contains an active principle called diastase, capable of bringing about this fermentation in a short time-The formation of sugar from starch is an operation which constantly occurs in all growing seeds, the effect being to change an insoluble substance, such as starch, into one which is capable of being dissolved in the juices of the young plant, and nourishing it during the early stages of its growth. In the operation of malting barley, the change is induced artificially, for the purpose of producing sugar in the malt, which is afterwards made to undergo the second kind of fermentation, namely, the vinous, or that in which spirit is produced.

When sugar, either that which naturally exists, in many plants, or as formed from starch as just mentioned, is dissolved in water, so as to form a moderately weak solution, and the whole exposed to a degree of warmth varying from seventy to eighty degrees, it rapidly undergoes a remarkable change, provided a small quantity of any vegetable ferment is present - such as yeast, or the juice of the grape, or of many other fruits. The sugar wholly disappears, and is resolved into two substances - one a gas, termed carbonic acid, which escapes, giving rise to a slow effervescence; and the other, a portion of spirit, which remains in the liquid. This kind of fermentation is much more difficult to prevent than to establish; in making syrups, it is found especially annoying; for if the quantity of sugar used is too small, the syrup is certain to ferment and spoil; and if too much is "added, it crystallizes out in the solid form; as a general rule, however, it is found that two parts, by weight, of sugar, to one part, by weight, of water, or other liquid - such as the juice of fruits, made into a syrup by boiling for a short time - neither ferments nor crystallizes.'

In the act of fermentation the spirit produced by the process last described is changed into acetic acid, or vinegar. Here, also, the presence of some substance capable of commencing the fermentation is requisite, for pure spirit and water will not undergo the change. The ferment employed may be the vinegar-plant - or it may be a little vinegar, which may have been previously formed.

For the rapid progress of the acetous fermentation, a high temperature is requisite - even as great as about eighty-six degrees - and free exposure to air is essential. The best vinegar is that made from weak wine, at Orleans; the plan followed is to introduce a portion of vinegar into the vessels, adding the wine at intervals, and never quite emptying them. In this country, a weak beer is brewed, without hops, for the purpose of making vinegar, and a small quantity of dilute oil of vitriol is added, after the vinegar is formed, to destroy the mouldiness that is otherwise apt to be present. Vinegar may be formed from any weak spirituous liquid; but it should be borne in mind that two circumstances are essential to success - namely, a high summer temperature, either natural or artificial, and free exposure to air.

The process of distillation is one which is used for separating liquids from each other that boil at different degrees of heat. In domestic economy, it is most frequently employed to obtain spirit, more or less flavored, or scented, with some volatile essential oil. The apparatus commonly used is the Still, for boiling the liquid to generate the vapor, and a long spirally twisted tube termed the worm, which is placed in a tub of cold water, and through which the steam passes to be condensed. The worm is the most objectionable part of the modern still; its great evil is the difficulty with which it is cleaned, so as to prevent one strong-flavored substance spoiling those which are distilled afterwards. If the coils of the worm are not very numerous, a bullet, with a string attached, may be passed through it, and a sponge or small bottle-brush, fastened to the string, may be worked backwards and forwards; but if there are several coils, it will be found impossible to do this, from the resistance caused by friction. In this case, the only plan is to close one end of the worm with a cork, and fill it with a solution of caustic alkali, allowing it to remain for some hours, and repeating the application with fresh liquid, if it be required.

Fig. 1.

In Germany, the worm is being superseded by an excellent condenser, which is so superior that we are induced to give a sketch of it, hoping that it may lead to its adoption in this country. The vapors from the still pass into the tube A (Fig 1), by which they are conducted into B, a hollow globe, made to unscrew at its centre. The vapors, passing along the tabes C, are condensed, and the distilled liquid drops from D. The pipe E should convey a constant stream of cold water to the bottom of the tub, and this, rising as it is warmed by abstracting heat from the tubes and globe, should escape by F. All the tubes being straight, it is obvious that they can be readily cleaned from their ends.

In the laboratory, distilling is most frequently performed with vessels termed retorts, or even from flasks; but as these are not very applicable to domestic purposes, we pass them over.

In domestic practice, the still is usually employed to* obtain some water or spirit flavored with essential oil, or the oil itself, and the process should be slight'y modified so as to suit each case. The vegetable substance should not be placed on the bottom of the still itself, as in that case it might become burnt, and so give an unpleasant flavor to the whole; but a bottom of wicker-work should be placed in the still in the first instance for it to rest upon, or a perforated board. The substance to be distilled should be placed in the still, covered with water, for some hours before the fire is lighted; no more water being added than sufficient to cover it, if the preparation of oil is the object.

Herbs, for distilling, should be collected on a dry day, and - unless the oil resides in the seeds, as in the case of caraway, anise, etc, or in the flowers, as in the rose, lavender, etc. - just before the flowers have opened, as at that period there is the greatest quantity of essential oil in the plant. All plants cultivated for distillation, should be grown in a situation where they can receive a full amount of sun-light, as shade or darkness very much tends to prevent the formation of essential oil.

The liquid which comes out of the worm, is a mixture of water highly flavored with the substance, and some undissolved oil - this latter is sometimes heavier and sometimes lighter than water, either sinking or floating; in the latter Case the oil may be readily separated by filling a bottle with the mixture, and when the oil has collected at the top. carrying it off by a few threads of cotton placed as in the figure (Fig. 2)

Fig. 2.

taking care that they are moistened with oil before arranging them; the cotton acts as a syphon, and removes the whole of the oil. If the object of the operation is to obtain the oil and not the distilled water, the latter should be preserved, and used again and again with fresh herbs, because having in the first operation dissolved as much oil as it is capable of doing, it causes no loss in the subsequent distillations.