The science which gave an insight into the cause of the ‘rising’ of bread, and suggested substitutes for the ordinary fermenting materials, is but of recent date. These ferments operate by generating an infinity of gas bubbles, which honeycomb the dough. The earliest process was to employ leaven, which is still largely used in the manufacture of the black rye bread of the Continent, and consists of dough which has become more or less sour by over-fermentation. This is kept from one baking to another, to inoculate a fresh bulk of paste with its fermenting influence. No sooner does it come into contact with the fresh dough than it communicates its own properties, as by contagion. Probably the discovery of leavening has, in many countries, been owing to accident, through neglected paste having been attacked by the fungus which is the cause of fermentation. Many of my readers probably do not know that yeast is a plant. It belongs to the class of _fungi_, and, in accordance with the general habit of its kind, it differs from the green forms of vegetable life by feeding upon organic substances. The yeast plant represents one condition of a species of fungus remarkable for the diversity of forms it exhibits, its wide, nay, universal distribution, and the magnitude of the effects, sometimes beneficial, sometimes mischievous, which it is capable of producing. The forms in which it is familiar to most persons, although its nature may be unsuspected, are yeast, the gelatinous vinegar plant, the ‘mother’ of vinegar, and many decomposing vegetable infusions, and the common blue or green mould (_penicillium glaucum_) which occurs everywhere on sour paste, decaying fruits, and, in general, on all dead organic matters exposed to combined moisture and moderate heat. Yeast and the vinegar plant are the forms in which it vegetates under various circumstances when well supplied with food. Mildew is its fruit, formed on the surfaces exposed to air at certain epochs, like the flowers and seeds of the higher plants, to enable it to diffuse itself, which it does most effectually, for the microscopic germs, invisible singly to the naked eye, are produced in myriads, and are so diminutive that ordinary motes floating in the atmosphere are large in comparison.