plied with purified and sterilized air. This
principle has already been applied to the refrigerator, and apparently with
success. In America the cooler is frequently replaced by a cooling tank, an
enclosed vessel of some depth, capable of artificial aeration. It is not
practicable, in any case, to cool the wort sufficiently on the cooler to
bring it to the proper temperature for the fermentation stage, and for this
purpose, therefore, the _refrigerator_ is employed. There are several kinds
of refrigerators, the main distinction being that some are vertical, others
horizontal; but the principle in each case is much the same, and consists
in allowing a thin film or stream of wort to trickle over a series of pipes
through which cold water circulates. Fig. 5, Plate I., shows refrigerators,
employed in Messrs Allsopp's lager beer brewery, at work.

_Fermenting_.--By the process of fermentation the wort is converted into
beer. By the action of living yeast cells (see FERMENTATION) the sugar
contained in the wort is split up into alcohol and carbonic acid, and a
number of subsidiary reactions occur. There are two main systems of
fermentation, the _top fermentation_ system, which is that employed in the
United Kingdom, and the _bottom fermentation_ system, which is that used
for the production of beers of the continental ("lager") type. The wort,
generally at a temperature of about 60 deg. F. (this applies to all the systems
excepting B [see below], in which the temperature is higher), is "pitched"
with liquid yeast (or "barm," as it is often called) at the rate of,
according to the type and strength of the beer to be made, 1 to 4 lb to the
barrel. After a few hours a slight froth or scum makes its appearance on
the surface of the liquid. At the end of a further short period this
develops into a light curly mass (_cauliflower_ or _curly head_), which
gradually becomes lighter and more solid in appearance, and is then known
as _rocky head_. This in its turn shrinks to a compact mass--the _yeasty
head_--which emits great bubbles of gas with a hissing sound. At this point
the _cleansing_ of the beer--_i.e._ the separation of the yeast from the
liquid--has fairly commenced, and it is let down (except in the skimming
and Yorkshire systems [see below]) into the pontos or unions, as the case
may be. During fermentation the temperature rises considerably, and in
order to prevent an excessive temperature being obtained (70-75 deg. F. shoul