as may be realized in the gamy taste of meat which is in the incipient stages of putrefaction.  By purely empirical means mankind has learned methods of encouraging the development of some of these products, and is to-day making practical use of this power, possessed by bacteria, of furnishing desired chemical compounds.  Industries involving the investment of hundreds of millions of dollars are founded upon the products of bacterial life, and they have a far more important relation to our everyday life than is commonly imagined.  In many cases the artisan who is dependent upon this action of microscopic life is unaware of the fact.  His processes are those which experience has taught produce desired results, but, nevertheless, his dependence upon bacteria is none the less fundamental.

Bacteria in the fermentative industries.

We may notice, first, several miscellaneous instances of the application of bacteria to various fermentative industries where their aid is of more or less value to man.  In some of the examples to be mentioned the influence of bacteria is profound and fundamental, while in others it is only incidental.  The fermentative industries of civilization are gigantic in extent, and have come to be an important factor in modern civilized life.  The large part of the fermentation is based upon the growth of a class of microscopic plants which we call yeasts.  Bacteria and yeasts are both microscopic plants, and perhaps somewhat closely related to each other.  The botanist finds a difference between them, based upon their method of multiplication, and therefore places them in different classes (Fig. 2, page 19).  In their general power of producing chemical changes in their food products, yeasts agree closely with bacteria, though the kinds of chemical changes are different.  The whole of the great fermentative industries, in which are invested hundreds of millions of dollars, is based upon chemical decompositions produced by microscopic plants.  In the great part of commercial fermentations alcohol is the product desired, and alcohol, though it is sometimes produced by bacteria, is in commercial quantities produced only by yeasts.  Hence it is that, although the fermentations produced by bacteria are more common in Nature than those produced by yeasts and give rise to a much larger number of decomposition products, still their commercial aspect is decidedly less important than that of yeasts.  Nevertheless, bacteria are not without their importance in the ordinary fermentative processes.  Although they are of no importance as aids in the common fermentative processes, they are not infrequently the cause of much trouble.  In the fermentation of malt to produce beer, or grape juice to produce wine, it is the desire of the brewer and vintner to have this fermentation produced by pure yeasts, unmixed with bacteria.  If the yeast is pure the fermentation is uniform and successful.