By the expression _diseases_ of wort and beer, we mean radical changes which so affect the nature of those liquids as to make them unpalatable, especially if they are kept; such changes produce beer which is sharp, sour, turned, oily, putrid, and otherwise bad. It would be unreasonable to apply the term _disease_ to certain modifications in the quality of beer, which may be produced by practices more or less commendable. Such modifications, too, may result from want of skill in the brewer, from the composition of the wort, from the specific nature of the yeast, or from the inferior quality of ingredients. It is a well-known fact that “low beer,” if manufactured according to the ordinary process, has not that same delicacy of flavour which characterizes beer fermented at a lower temperature than 10° C. (50° F.). Fermented at 10° C. (50° F.), or 12° C. (53° F.) or at a higher temperature, it loses the peculiar properties which consumers prize. Nevertheless, in point of soundness it may be as good a beer as one which has been fermented at 6° C. (43° F.), or 8° C. (46° F.). One might say of the former beer that it is inferior to the latter in estimation; but we could not rightly call it diseased, for we are supposing a case in which disease does not actually exist. § I.—Every Unhealthy Change in the Quality of Beer coincides with a Development of Microscopic Germs which are alien to the Pure Ferment of Beer. Our proposition concerning the causes of the diseases of wort and beer might be demonstrated in several ways. The following is one of the simplest:—Take a few bottles of sound beer, say, for instance, that which is known in Paris as Tourtel’s, Grüber’s, or Dreher’s, from the name of the brewer who manufactures it. Place some of these bottles in a hot-water bath and raise the temperature to about 60° C. (140° F.). Permit them to cool, and then place them by the side of the other bottles that have not been heated. In every case, especially if we conduct this experiment in summer, we shall find that in the course of a few weeks—the length of time varying according to the temperature and the quality of the beer—all the bottles which have not been heated will have become diseased, in some cases even to the extent of being undrinkable. Let us next examine, by way of comparison, the deposits in the heated and non-heated bottles. We shall find associated with the pure alcoholic ferment other organisms, filiform and for the most part very slender, and either simple or articulated, as represented in Plate II., the design of which is taken from actual deposits occurring in beer that had been kept for some time at the ordinary temperature. A number of bottles of beer which had been heated on October 8th, 1871, were compared with those of an equal number of bottles of the same beer which had not been heated. The examination took place on July 27th, 1872. The beer, which had been heated to 55° C.