es .28
lb. of digestible proteids daily. The chief sources of our proteid foods
are meats, fish, beans, etc. It has been as a proteid food that
mushrooms have been most strongly recommended. Referring to Table I, it
will be seen that nitrogen constituted 5.79 per cent. of the total dry
substance of _Coprinus comatus_. This high nitrogen content, which is
common to the mushrooms in general, was formerly taken to indicate a
very unusual richness in proteid materials. It is now known, however,
that there were several sources of error in this assumption.

Much of the nitrogen is present in the form of non-proteid substances of
a very low food value. Another and very considerable portion enters into
the composition of a substance closely related to cellulose. A third
source of error was the assumption that all the proteid material was
digestible. It is now known that a very considerable portion is not
digestible and hence not available as food. Thus, notwithstanding the
5.79 per cent. of nitrogen in _Coprinus comatus_, we find but .82 per
cent. in the form of actually available (i. e., digestible) proteids, or
approximately one-seventh of what was formerly supposed to be present.

The digestibility of the proteids varies very greatly with the species.
Moerner found the common field mushroom, _Agaricus campestris_, to have a
larger amount of proteids available than any other species studied by
him. Unfortunately, the digestibility of the American plant has not been
tested. There is great need for further work along this line. Enough has
been done, however, to demonstrate that mushrooms are no longer to be
regarded as a food of the proteid class.

The energy for the muscular exertion and heat is most economically
derived from the foods in which the carbohydrates and fats predominate.

The common way of comparing foods of the first two classes
scientifically is to compare their heat-giving powers. The unit of
measurement is termed a _calorie_. It represents the amount of heat
required to raise a kilogram of water 1 deg. Centigrade. (This is
approximately the heat required to raise one pound of water 4 deg.
Fahrenheit.) A man at moderately hard muscular labor requires daily
enough food to give about 3500 _calories_ of heat-units. The major part
of this food may be most economically derived from the foods of the
second class, any deficiency in the .28 lb. of digestible protein being
made up by the addition of some food rich in