II. Appearance under the microscope of the deposit from “turned” beer 20 III. _Torulæ_ in process of development 72 IV. _Mycoderma vini_ functioning as an alcoholic ferment.—Right half, showing appearance of spores just sown; left half, their appearance after an interval of submerged life 116 V. _Mucor_, vegetating submerged, in deficit of air 136 VI. Ferment of _mucor_ 138 VII. Yeast-cells—worn out and dissociated (left), after revival in a sweet wort (right) 148 VIII. Fertile mould-cells from the outer surface of grapes 152 IX. Various examples of the mode of growth of mould-cells from the outer surface of grapes 154 X. One of the ferments of acid fruits at the commencement of fermentation in its natural _medium_ 166 XI. _Saccharomyces pastorianus_, in course of regular growth 168 XII. Ferment-cells from a spontaneous fermentation just starting 184 CHAPTER I. On the Intimate Relation existing between the Deterioration of Beer, or the Wort from which it is Made, and the Process of Brewing. At the outset of these “Studies,” let us briefly consider the nature of beer and the methods of its manufacture. Beer is a beverage which has been known from the earliest times. It may be described as an infusion of germinated barley and hops, which has been caused to ferment after having been cooled, and which, by means of “settling” and racking, has ultimately been brought to a high state of clarification. It is an alcoholic beverage, vegetable in its origin—a _barley wine_, as it is sometimes rightly termed.[1] Beer and wine, however, differ widely in their composition. Beer is less acid and less alcoholic than wine; it holds more ingredients in solution, and the nature of these ingredients is by no means similar to that of those which are found in wine. These differences in the component parts of wine and beer give rise to corresponding differences in the keeping qualities of the two liquids. The small amount of acidity in beer, its poverty in alcohol, and the presence of matter that is saccharine, or liable to become so, all operate in imparting to beer a tendency to change, which wine does not possess. That this unequal resistance to the aggression of diseases is due to such differences, may be proved by the fact that wine could be made much more liable to change than it actually is, by a diminution of its acidity and its usual proportion of alcohol, or by increasing the proportion of viscid or saccharine matters,[2] modifications which would tend to assimilate its composition to that of beer.