tion was not true.

The nitrogenous constituents of the food did doubtless go to repair
the waste of muscle, which, like every other portion of the body,
needed renewal, while the function of the non-nitrogenous food was not
only to supply the animal heat, but also to furnish, by its oxidation,
the muscular energy of the body. We thus came to the conclusion that
it was the potential energy of the food which furnished the actual
energy of the body, expressed in terms either of heat or of mechanical
work.

But there was one other factor which came into play in this question
of mechanical energy, and must be taken into account; and this factor
we were as yet unable to estimate in our usual terms. It concerned the
action of the mind on the body, and although incapable of exact
expression, exerted none the less an important influence on the
physics and chemistry of the body, so that a connection undoubtedly
existed between intellectual activity or mental work and bodily
nutrition. What was the expenditure of mechanical energy which
accompanied mental effort was a question which science was probably
far from answering; but that the body experienced exhaustion as the
result of mental activity was a well-recognized fact.

CHEMISTRY OF VEGETATION.

The phenomena of vegetation, no less than those of the animal world,
had, however, during the last fifty years been placed by the chemist
on an entirely new basis.

Liebig, in 1860, asserted that the whole of the carbon of vegetation
was obtained from the atmospheric carbonic acid, which, though only
present in the small relative proportion of four parts in 10,000 of
air, was contained in such absolutely large quantity that if all the
vegetation on the earth's surface were burned, the proportion of
carbonic acid which would thus be thrown into the air would not be
sufficient to double the present amount. That this conclusion was
correct needed experimental proof, but such proof could only be given
by long-continued and laborious experiment.

It was to our English agricultural chemists, Lawes and Gilbert, that
we owed the complete experimental proof required, and this experiment
was long and tedious, for it had taken forty-four years to give a
definite reply.

At Rothamsted a plot was set apart for the growth of wheat. For
forty-four successive years that field had grown wheat without the
addition of any carbonized manure, so that the only possible source
from which the pla