ought up to a boil, does not subject the coffee to so
strenuous conditions. Local overheating and hydrolysis occur, but
not to so great an extent as in boiling; and most of the effects of
oxidation and volatization of caffeol are absent. However,
extraction is rather incomplete, due to lack of thorough admixture
of the water and coffee.

When coffee is to be made under the best conditions, the
temperature of the water used and of the extract after it is made
should not fluctuate. In the pumping percolator, as in the steeping
method, the temperature varies greatly from the time the extraction
is started to the completion of the operation. This is deleterious.
Also, local overheating of the infusion occurs at the point of
application of the heat; and because of the manner in which the
water is brought into contact with the coffee, the degree of
extraction shows inefficiency. Spraying of the water over the
coffee never permits the grounds to be completely covered with
water at any one time, and the opportunity offered for channeling
is excessive. The principle of thorough extraction demands that, as
the substance being extracted becomes progressively more exhausted,
fresh solvent should be brought into contact with it. In the
pumping percolator the solution pumped over the grounds becomes
more concentrated as the grounds become exhausted; so that the time
taken to reach the degree of extraction desired is longer, and an
appreciable amount of relatively concentrated liquor is retained by
the grounds.

[Illustration: MEDIUM GRIND UNDER THE MICROSCOPE]

The simplest procedure to follow is that in which boiling water is
poured over ground coffee suspended on a filtering medium in such a
manner that the extracting water will slowly pass through the
coffee and be received in a containing vessel, which obviates
further contact of the beverage with the grounds. The water as it
comes into contact with the ground coffee extracts the soluble
material, and the solution is removed by gravity. Fresh water takes
its place; so that, if the filter medium be of the proper fineness,
the water flows through at the correct rate of speed, and complete
extraction is effected with the production of a clear solution.
Thus a maximum extraction of desirable materials is