boiling is continued generally from an hour to three hours, according
to the type of beer. The objects of boiling, briefly put, are: (1)
sterilization of the wort; (2) extraction from the hops of substances that
give flavour and aroma to the beer; (3) the coagulation and precipitation
of a part of the nitrogenous matter (the coagulable albuminoids), which, if
left in, would cause cloudiness and fret, &c., in the finished beer; (4)
the concentration of the wort. At least three distinct substances are
extracted from the hops in boiling. First, the _hop tannin_, which,
combining with a part of the proteids derived from the malt, precipitates
them; second, the _hop resin_, which acts as a preservative and bitter;
third, the _hop oil_, to which much of the fine aroma of beer is due. The
latter is volatile, and it is customary, therefore, not to add the whole of
the hops to the wort when it commences to boil, but to reserve about a
third until near the end of the copper stage. The quantity of hops employed
varies according to the type of beer, from about 3 lb to 15 lb per quarter
(336 lb) of malt. For mild ales and porters about 3 to 4 lb, for light pale
ales and light stouts 6 to 10 lb, and for strong ales and stouts 9 to 15 lb
of hops are employed.

_Cooling_.--When the wort has boiled the necessary time, it is turned into
the _hop back_ to settle. A hop back is a wooden or metal vessel, fitted
with a false bottom of perforated plates; the latter retain the spent hops,
the wort being drawn off into the coolers. After resting for a brief period
in the hop back, the bright wort is run into the _coolers_. The cooler is a
very shallow vessel of great area, and the result of the exposure of the
hot wort to a comparatively large volume of air is that a part of the hop
constituents and other substances contained in the wort are rendered
insoluble and are precipitated. It was formerly considered absolutely
essential that this hot aeration should take place, but in many breweries
nowadays coolers are not used, the wort being run direct from the hop back
to the refrigerator. There is much to be said for this procedure, as the
exposure of hot wort in the cooler is attended with much danger of
bacterial and wild yeast infection, but it is still a moot point whether
the cooler or its equivalent can be entirely dispensed with for all classes
of beers. A rational alteration would appear to be to place the cooler in
an air-tight chamber sup