Hardly less important than the bacteria which destroy life or endanger the products of human industry are the beneficent forms, some of which have in all ages co-operated with man, while others can only be employed by those who possess knowledge and skill. None are so important to our welfare as the bacteria which renew the fertility of the soil. But for the soil-bacteria farm-yard manure would be useless to the crop, for it is they which render it fit for assimilation. Now the bacteria of the soil have their natural enemies, the most mischievous being certain Protozoa, such as Amœba and its kindred. As soon as this fact was grasped, likely remedies were thought of; indeed, one remedy was suggested without any guidance from theory by a vine-grower of Alsace, who treated his soil with carbon disulphide to destroy phylloxera, and found that in so doing he had notably enhanced its fertility. Heating to the temperature of boiling water destroys the soil-protozoa and at the same time the bulk of the soil-bacteria. The bacteria, however, soon multiply more than ever by reason of the absence of their enemies, and a soil cleared of protozoa yields for a few years appreciably richer crops. Of other useful bacteria the briefest notice must suffice. Wine, beer, cheese, and tobacco owe to certain of them distinct flavours, for which the customer is willing to pay high. Leather in certain stages of manufacture, indigo, and woad require the access of other forms. If we also bear in mind the part which yeast plays in the every-day manufacture of bread, wine, and beer, and the part which the vinegar-mould plays in the manufacture of acetic acid, we shall get some notion of the industrial importance of the various micro-organisms. Not a little of the control which we exercise over them we owe directly or indirectly to Pasteur.