PASTEUR'S STUDY OF MICROBES 139 



life or endanger the products of human industry are the 

 beneficent forms, some of which have in all ages co- 

 operated with man, \vhile 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 Amoeba 

 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 in- 

 directly to Pasteur. 



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