The yeast with which we raise our bread is a minute plant belonging to the division of the Conferva. It grows in certain substances - in the juice of ripe grapes, in the syrup of sugar-cane, in beer, etc. etc. "The minuteness of the yeast plant," says Professor Johnston in his admirable "Chemistry of Common Life," (to which we refer our readers for much valuable information), "the minuteness of the yeast plant, consisting in its simplest form of only a single cell, long prevented it from being generally regarded as a form of living matter. But the changes it undergoes in the fermenting tub, day by day, as shown by the microscope, prove it to be unquestionably a growing vegetable."*

"If we filter the juice of ripe grapes we obtain a clear transparent liquid. Within half an hour this liquid begins to grow first cloudy, and afterwards thick ; to give off bubbles of gas or to ferment; and in three hours a greyish yellow layer of yeast has collected on its surface. In the heat of the fermentation the plants are produced by millions ; a single cubic inch of such yeast, free from adhering water, containing 52,000,000 of the minute organisms".

Yeast has the power of changing sugar into alcohol or pure spirit, and of raising bread by fermentation.

Distillers do not wait for the natural appearance of the plant; they add a little existing yeast to the liquor as soon as it is ready for fermentation, because they know how swiftly this mysteious plant will grow and propagate itself. The making of yeast is one branch of business itself now. It is sold as dry yeast for the use of the private baker or brewer. The process is thus given in the " Chemistry of Common Life," vol. i. p. 86.

"Crushed rye is mashed with the proper quantity of barley-malt, and the wort when made cooled to the proper temperature. For every hundred pounds of the crushed grain, there are now added half a pound of carbonate of soda and six ounces of oil of vitriol (sulphuric acid) diluted with much water, and the wort is then brought into fermentation by the addition of yeast. From the strongly fermenting liquid the yeast is skimmed off and strained through a hair-sieve into cold water, through which it is allowed to settle. It is afterwards washed with one or two waters, and finally pressed in cloth-bags till it has the consistency of dough. It has a pleasant fruity smell, and in a cool place may be kept for two or three weeks. It then passes into a putrefying decomposition, acquires the odour of decaying cheese, and like decaying cheese, has now the property of changing sugar into lactic acid, instead of into alcohol as before. A hundred pounds of crushed grain will yield six to eight poinds of the pressed yeast.

It is made largely at Rotterdam, and is imputed thence to this country through Hull".

Many families prefer making their own yeast to buyng the German or patent yeast. We give three recipes for so doing in the next page. Brewer's yeast is apt sometimes to be bitter - yeast from porter "is always so ; to prevent the bitterness spoiling the bread, it is well to put a red hot poker for a minute or two into the yeast, or to threw in a few blades of grass which can afterwards be strained off, but except in the country or where the family brews at home, brewer's yeast is seldom used ; the German and patent yeasts are used. The dried yeasts consist of the sporules of the yeast plant, freed from gas and moisture by filtering or evaporating away the fermented liquid. When required for making, this yeast is dissolved in lukewarm water. A common method of drying yeast is to dip twigs in the yeast and dry them in the air; or the yeast is stirred round with a whisk till it is thin, and then spread with a brush over a clean piece of board, and dried in the air. When it is dry more yeast is brushed over it. and it is again dried. This process is repeated till the yeast is a good thickness on the board.

It may be kept on it, and cut off as required.

* "Chemistry of Common Life," vol. i. p. 29.

Both brewer's yeast and artificial yeasts must be kept in a cool cellar or any very cold place to preserve them good.