From this we see that the changes which the manufacture of beer has undergone during the present century have been based mainly on the diseases to which beer is liable, either during or after the process of brewing. The fact that English brewers have not as yet adopted “low fermentation” may be accounted for, in a great measure, by the difficulty of enlarging existing breweries, in cities like London, to the extent required for the new method of manufacture. Even in the event of public taste demanding a “low beer,” English brewers will hesitate a long time before converting their breweries. Such conversion would impose upon them expenses and difficulties of a very serious nature. If ever such a change should take place, it will probably be inaugurated out of London. It is, however, worthy of remark that English brewers, without adopting “low fermentation,” have introduced considerable improvements in brewing, especially in the management of the temperature during fermentation; this must be preserved within narrow and exact limits, for fear of injury to the product. It might easily be shown that these improvements have resulted from the liability of the beer to contract diseases, although this fact may not have been recognized by the brewers who have introduced them. Besides the yeasts which belong to the two principal kinds of fermentation, there exist many varieties of alcoholic ferment that produce, each of them, a special kind of beer. Among these special beers some are deficient in taste, others in aroma, others in brilliancy. Let us suppose that in the manufacture of a beer with one of these yeasts, from which a peculiar flavour is derived, a different and inferior variety is accidentally mixed with that which we intend to use; in such a case, the inferior variety, the product of which will possess an inferior quality, will exercise such an influence on the brewing as to induce the belief that disease must be present. The microscope, if consulted, will reveal no special organism, nor any of those diseased ferments of which we have given specimens. It is in the study of yeast that we must endeavour to find the cause of the results we observe. This point, which is of the greatest importance to brewers, will become clearer as we proceed. If we examine the practices of the beer trade, in its retail as well as in its export branches, we shall find that many of them afford evidence of the liability of the beer to deteriorate. We may cite some of these. When taken out of the ice-cellars, the beer is kept in casks of small capacity, that it may be the sooner consumed; when exposed to a high temperature, the beer will not keep sound for any length of time, but will speedily effloresce with _mycoderma vini_ or _mycoderma aceti_.