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QUANTITATIVE AGRICULTURAL ANALYSIS

is the loss of essential oils and other volatile bodies during the
drying process. Partly compensating this is a possible gain in
weight due to oxidation of fats and sugars, when drying takes
place in the air. As these changes are variable the method of
drying in air at elevated temperatures has been abandoned. If
a temperature of 100° is to be used it is necessary to have avail-
able an oven for drying at reduced pressure or in an atmosphere
of an indifferent gas, such as hydrogen.

FIG. 40.—One form of grinder for coarse feeds. (Shown disassembled*)
Feeds dried at ordinary temperature under reduced pressure
usually show about 1 per cent less moisture than is found by a
direct heating method. About four to six days is usually required
to obtain constant weight by the reduced pressure method, even
if the sulphuric acid used in the desiccator is changed several
times. The special advantage of the method lies in the thorough
desiccation of the sample without the possibility of chemical
changes brought about by heating and oxidation but the length
of time required for the experiment makes the method imprac-
ticable for all work except that requiring a high degree of refine-
ment and accuracy.
Determination of Moisture: At 100°.—Weigh about 2 gm of the feed
in a weighed flat dish or, in case the fat is to be extracted, in a weighed
alundum cup. Place in an oven which can be exhausted or through which
dry hydrogen can be circulated and heat at 100° for at least five hours or