Various preparations of farinaceous substances bear this denomination; but those which are chiefly used in this country may be distinguished into three principal kinds. In the first, called unleavened breed, either flour and water alone are mixed, or with the addition of some other substance, such as butter, eggs, sugar, and afterwards baked, by which the mass is reduced into a solid state, sometimes flakey, but never cellular or spongy. Biscuit, or sea-bread, is of the unleavened kind, for the process of making which see Biscuit In the second kind of bread, called leavened bread, the flour and water being mixed together, is either left for some hours in a thin and almost liquid state to ferment, that the saccharine matter contained in the flour may be spontaneously changed into alcohol and carbonic acid gas, which expand by the heat of the oven, and render the bread vesicular or spongy. Although this intestine change will take place naturally at the temperature employed in the vinous fermentation, it is usual to add certain substances, termed ferments, of which the barm of beer, or yeast, is preferred, where it can be obtained.

These accelerate the fermentation of the dough, and cause it to take place simultaneously throughout the whole mass of dough.

In the third kind of bread, a vesicular appearance is given to it by the addition to the dough of some ammoniacal salt, (usually the sub-carbonate,) which becomes wholly converted into a gaseous substance during the process of baking, causing the dough to swell out into little air vessels, which finally bursting, allow the gas to escape, and leave the bread exceedingly porous. Mr. Accum, in his Treatise on Culinary Poisons, has stigmatized this process as "fraudulent," but, in our opinion, most unjustly. The bakers would never adopt it but from necessity: when good yeast cannot be procured, it forms an admirable and perfectly harmless substitute; costing the baker more, it diminishes his profit, while the consumer is benefited by the bread retaining the solid matter, which by the process of fermentation is dissipated in the form of alcohol and carbonic acid gas. To persons unacquainted with the nature of the flour of wheat, it is necessary to state that it is composed of three distinct substances, which are easily separable by art: first, a mucilaginous saccharine matter, soluble in cold water; much starch, which will scarcely combine with water without the aid of heat; and an elastic adhesive grey substance, called gluten, which is insoluble in cold water, alcohol, oil, or ether, and resembles an animal substance in many of its properties.

To the gluten in wheat flour is supposed to be owing its property of making so tenacious a paste, and its peculiar facility of rising or expanding by the addition of leaven. From the flour of barley, rye, oats, or potatoes, no gluten has been extracted, probably from their containing too small a quantity; and as these substances are very difficult of fermentation by any of the ordinary processes, it has been supposed that the presence of gluten is the cause of fermentation. It is, however, more reasonable to suppose that the tenacious dough formed by the admixture of gluten prevents the escape of the gaseous products of the fermentation, and that these, by their expansion, swell the dough into a vesicular mass, producing what is technically called light bread. M. Beccari, of Bologna, and Dr. Cullen, inform us that by the addition of gluten to barley and potatoes, they produced better bread from each than could be obtained without this addition. Parmentier asserts that bread may be made from potatoes alone; but Mr. Edlin and Dr. Pearson, two very enlightened experimentalists, state decidedly that this root cannot be fermented so as to make bread, without the addition of wheaten flour; and that no farinaceous substance can be made into good bread that has not the three constituent parts of wheat before-mentioned: for if to the starch of potatoes some of this glutinous substance be added, with yeast and water, it will not form a bread, owing to the absence of the saccharine or sugary extract on which the process of fermentation depends, and which, if this last substance be added, even in a concentrated state, will immediately commence. Although we have no knowledge of Par-mentier's process, we believe that his assertion is perfectly correct; for since the experiments were made by Mr. Edlin and Dr. Pearson, the farina of potatoes has been converted, on a great scale, into sugar and alcohol; and it has been long known that potatoe starch may be transformed, by the application of dry heat, into a species of tapioca, of great tenacity and elasticity. With such materials, we should imagine the skill of a Parmentier would hardly be necessary in order to make good bread.

In the making of leavened bread without the addition of any article for exciting the speedy fermentation of the paste, a great deal of attention and skill are requisite. The spontaneous decomposition is extremely slow; the various parts of the mass are differently affected, according to the humidity, the thickness or thinness of the part, the vicinity or remoteness of fire, and other circumstances less easily investigated. The saccharine part is disposed to become converted into alcohol; the mucilage has a tendency to become sour and mouldy; while the gluten, in all probability, verges towards the putrid state. An entire change in the chemical attractions of the several component parts must then take place in a progressive manner, not altogether the same in the internal and more humid parts, as in the external parts, which not only become dry by simple evaporation, but are acted upon by the surrounding air. The outside may therefore become mouldy or putrid, while the inner part may be only advanced to an acid state. Occasional admixture of the mass would of course not only produce some change in the rapidity of this alteration, but likewise render it more uniform throughout the whole.