te
into distinct and perfect cells. The cavities of the cells include, in
the form of little drops, a considerable quantity of aromatic volatile
oil, on the presence of which the fragrance and many of the active
principles of the berry depend" (see fig. 3).

[Illustration: FIG. 3.--Microscopic structure of Coffee.]

_Physiological Action._--Coffee belongs to the medicinal or auxiliary
class of food substances, being solely valuable for its stimulant effect
upon the nervous and vascular system. It produces a feeling of buoyancy
and exhilaration comparable to a certain stage of alcoholic
intoxication, but which does not end in depression or collapse. It
increases the frequency of the pulse, lightens the sensation of fatigue,
and it sustains the strength under prolonged and severe muscular
exertion. The value of its hot infusion under the rigours of Arctic cold
has been demonstrated in the experience of all Arctic explorers, and it
is scarcely less useful in tropical regions, where it beneficially
stimulates the action of the skin.

The physiological action of coffee mainly depends on the presence of the
alkaloid caffeine, which occurs also in tea, Paraguay tea, and cola
nuts, and is very similar to theobromine, the active principle in cocoa.
The percentage of caffeine present varies in the different species of
_Coffea_. In Arabian coffee it ranges from about 0.7 to 1.6%; in
Liberian coffee from 1.0 to 1.5%. Sierra Leone coffee (_C. stenophylla_)
contains from 1.52 to 1.70%; in _C. excelsa_ 1.89% is recorded, and as
much as 1.97% in _C. canephora_. Four species have been shown by M. G.
Bertrand to contain no caffeine at all, but instead a considerable
quantity of a bitter principle. All these four species are found only in
Madagascar or the neighbouring islands. Other coffees grown there
contain caffeine as usual. Coffee, with the caffeine extracted, has also
been recently prepared for the market. The commercial value of coffee is
determined by the amount of the aromatic oil, caffeone, which develops
in it by the process of roasting. By prolonged keeping it is found that
the richness of any seeds in this peculiar oil is increased, and with
increased aroma the coffee also yields a blander and more mellow
beverage. Stored coffee loses weight at first with great rapidity, as
much as 8% having been found to dissipate in the first year of keeping,
5% in the second, and 2% in the third; but such loss of weight is more
than co