tive calculation of the total quantity of humus on the
earth's surface, and of the carbon annually converted into carbonic acid
by the respiration of man and animals, the combustion of wood for fuel,
and other minor processes; and he draws the conclusion that, if there
were no other source of carbon except humus, the quantity of that
substance existing in the soil would only support vegetation for a
period of sixty years.

The particular phenomena of vegetation also afford abundant evidence
that humus cannot be the only source of carbon. Thus Boussingault has
shown that on the average of years, the crops cultivated on an acre of
land remove from it about one ton more organic matter than they receive
in the manure applied to them, although there is no corresponding
diminution in the quantity of humus contained in the soil. An instance
which leads still more unequivocally to the same conclusion is given by
Humboldt. He states that an acre of land, planted with bananas, yields
annually about 152,000 pounds weight of fruit, containing about 32,000
pounds, or almost exactly 14 tons of carbon; and as this production goes
on during a period of twenty years, there must be withdrawn in that time
no less than 280 tons of carbon. But the soil on an acre of land weighs,
in round numbers, 1000 tons, and supposing it to contain 4 per cent of
humus, the total weight of carbon in it would amount to little more than
20 tons.

It is obvious from these and many other analogous facts that humus
cannot be the only or even a considerable source of the carbon of
plants, although it is still contended by some chemists that it may be
absorbed to a small extent. But even this is at variance with many
well-known facts. For if humus were absorbed, it might be expected that
vegetation would be most luxuriant on soils containing abundance of that
substance, especially if it existed in a soluble and readily absorbable
form; but so far from this being the case, nothing is more certain than
that peat, in which these conditions are fulfilled, is positively
injurious to most plants. On the other hand, our daily experience
affords innumerable examples of plants growing luxuriantly in soils and
places where no humus exists. The sands of the sea-shore, and the most
barren rocks, have their vegetation, and the red-hot ashes which are
thrown out by active volcanoes are no sooner cool than a crop of plants
springs up on them.

The conclusions to be drawn fr