bread, the dough is made to ferment by the action of leaven or yeast, previous to its introduction into the oven. Three materials, therefore, are indispensably necessary to bread, namely, meal, leaven or yeast, and water. To these salt may be added as equally essential.

Leaven, much used in France, Germany, and Switzerland, consists of a portion of the dough already undergoing fermentation; and, according to Liebig's views, it has the property of inducing by its presence a similar molecular change and chemical decomposition in the fresh dough with which it is mixed. Yeast, employed in England instead of leaven, is obtained from the fermentation of the wort in the manufacture of beer; its chemical composition bears a strong analogy to that of leaven; and it possesses the same power of inducing the fermentation of the dough. Fermentation raises the dough by the evolution of gas through its entire mass, the gas in question being carbonic acid; a small quantity of sugar and some alcohol also result from the same process, these various products owing their formation to the decomposition of a part of the starch, and a little sugar preexisting in the flour, from the action of the ferment. During the process of baking, the whole of this alcohol and some carbonic acid are given out by the dough, and consequently a loss of flour is incurred. The reader will be astonished at the amount of alcohol, or spirits of wine, dissipated by the heat of the oven; thus, in London, the quantity of bread consumed annually, supposes the evolution of about three hundred thousand gallons of spirits, which, at the price of nineteen shillings per gallon, would be worth two hundred and eighty-five thousand pounds. All the experiments undertaken with the view of preventing the loss of this alcohol have failed; and in the military bakehouse at Chelsea, twenty thousand pounds have

been expended in vain upon a collecting and condensing apparatus.

For the purpose of avoiding the loss of substance resulting from the fermentation of the dough, the addition of carbonate of soda and hydrochloric acid were recommended first by Henry, as a substitute for fermentation, and the process was patented, in 1837, by Whiting. Some time afterwards, Dr. R. Thomson observed, that a sack of flour yields only one hundred loaves, by the fermentation process, while, on the improved system, it would yield one hundred and seven loaves. According to other experiments by Frickinger, the saving would not be so considerable; but still it would be sufficient to cover the cost of the new raising material. The chemical action resulting from the addition of bicarbonate of soda and hydrochloric acid to the dough, is interesting; the bicarbonate of soda is decomposed, yielding free carbonic acid, which, from its gaseous form, permeates and raises the mass, and chloride of sodium or common salt, a substance added at all events during the baking in the fermentation process. In Germany, carbonate of ammonia is occasionally used, instead of hydrochloric acid and bicarbonate of soda. The heat of the oven induces the escape of this substance in the form of a gas, and it is entirely volatilized after the baking, leaving wide interstices in the dough. Unfortunately, bread prepared by this method is tough, and unequally inflated; and consequently heavy and difficult to digest.

During the process of baking, the dough becomes much lighter from the loss of water; when, therefore, the bread is required to be of a certain weight, this loss must be calculated upon in forming the loaf. According to Accum, seven pounds of flour yield ten pounds of dough, and eight pounds three quarters of bread.

According to Herbstädt, three pounds of flour yield four pounds of bread. In the military bakehouse at Hanover, eight pounds five ounces of dough are required for a seven pounds loaf. I might add many interesting details upon the manufacture of bread, but they would be altogether misplaced in this little work, where I propose to call the attention of the reader more particularly to the adulteration of food.

Flour may be obtained from any kind of seed containing much starch, and in the majority of cases, the different meals thus prepared will exhibit the same characters to the naked eye. It is obvious, therefore, that the methods we possess for detecting the characteristic properties of different flours are of the highest value, as they enable the observer not only to determine which is the flour under examination, but also to find out with the greatest accuracy if a particular flour is perfectly pure, or has been mixed up with other meals previous to its being converted into bread.

Moreover, not only are flours to be met with fraudulently mixed together, but also entirely foreign substances are occasionally added to them; both these adulterations escape completely the scrutiny of the naked eye, and would remain unknown, were it not for the methods of investigation we possess, depending upon the following chemical and microscopical properties of flour.

2. Chemical composition and analysis of flour. The normal constituents of any kind of flour are water, starch, gluten or fibrin, lignine from the bran, a small quantity of albumen, of sugar, of dextrine, and finally, inorganic or mineral salts.

The qualitative analysis of flour, or the determination of

the presence of the substances just enumerated, is best. effected as follows:

A sample of the flour being gently heated in a glass tube, water will be disengaged, and the vapours by collecting on the cold upper part of the tube, will show that it contains more or less moisture. The flour is next to be kneaded with enough water to make it into a semifluid paste, and tightly tied up in a towel or piece of clean calico; this dough when squeezed in pure water, with the thumb and index-finger, will give out starch under the form of a granular substance, falling down to the bottom of the vessel. On boiling this, or any other fluid which contains starch in suspension, the granules will be noticed to swell and finally dissolve, yielding a solution, which if considerably diluted with water and mixed with weak nitric acid and iodide of potassium, acquires a blue colour.

The dough having been thoroughly expressed under water, nothing is left in the calico but the fibrin or gluten, (with perhaps a few particles of bran,) whose chemical properties may now be examined; its fibrinous consistence will suffice, however, to show its presence; after a few hours the starch obtained by the above process, and suspended in the water with which the flour has been washed, subsides to the bottom of the vessel; the upper part of the fluid, now become clear, is decanted and tested, first, for albumen; for this purpose, heat is applied to a sample of the clear fluid and when boiling it will become turbid and deposit flakes, owing to the presence of this substance. The above result is confirmed if a few drops of nitric acid added to the turbid liquid do not dissolve the deposit. The fluid also usually contains a little grape sugar, which substance is detected by means of sulphate of copper and caustic potash,

dissolved in water. Enough sulphate of copper is added to a sample of the clear aqueous extract to give it a decidedly blue colour; the potash solution poured into this mixture will first induce a precipitate, disappearing, however, when an excess has been used, and a dark blue fluid is thus prepared, which is now to be heated; as soon as it commences to boil (if any sugar be present,) a red precipitate will occur in the liquid, subsiding to the bottom of the test tube, and consisting of suboxide of copper.

In order to avoid the trouble of preparing solutions of sulphate of copper and caustic potash as often as a test for grape sugar is wanted, a test fluid can be conveniently made by adding, first, a small quantity of potash to solution of sulphate of copper, and then adding enough tartaric acid to dissolve the precipitate thus obtained. By mixing, subsequently, a large quantity of potash with this solution, it will acquire a deep blue colour, and may be preserved in this state as an excellent means of detecting the presence of grape sugar.

Another sample of the clear fluid extract from the flour deprived of albumen by boiling and subsequent filtration, is now evaporated to dryness, and the residue boiled with alcohol to remove all the sugar; the substance insoluble in alcohol consists of dextrine, which differs but slightly from gum, or mucilage. When dissolved in water, and boiled with a diluted mineral acid, dextrine as well as gum is converted into grape sugar; thus, with the tartrate of copper and potash test, the presence of these substances may also be determined. A small quantity of bran is usually found in flour; it is obtained by mixing the flour with a large quantity of water, and then decanting the fluid before the starch has time to subside. The bran will then be observed settled at the bottom of the vessel.