it would appear from the recent
experiments of Cloeez, which, should they be confirmed by farther
enquiry, will be of much importance, that this compound is also
produced without electrical action when air is passed over certain
porous substances, saturated with alkaline and earthy compounds.
Fragments of calcined brick and pumice stone were saturated with
solution of carbonate of potash, with carbonates of lime and magnesia
and other mixtures, and a current of air freed from nitric acid and
ammonia passed over them for a long period, at the end of which notable
quantities of nitric acid were detected.

_Source of the Inorganic Constituents of Plants._--The inorganic
constituents of plants being all fixed substances, it is sufficiently
obvious that they can only be obtained from the soil, which, as we shall
afterwards see, contains all of them in greater or less abundance, and
has always been admitted to be the only substance capable of supplying
them. The older chemists and physiologists, however, attributed no
importance to these substances, and from the small quantities in which
they are found in plants, imagined that they were there merely
accidental impurities absorbed from the soil along with the humus, which
was at that time considered to be their organic food. This opinion,
sufficiently disproved by the constant occurrence of the same substances
in nearly the same proportions, in the ash of each individual plant, has
been further refuted by the experiments of Prince Salm Horstmar, who has
established their importance to vegetation, by experiments upon oats
grown on artificial soils, in each of which one inorganic constituent
was omitted. He found that, without silica, the grain vegetated, but
remained small, pale in colour, and so weak as to be incapable of
supporting itself; without lime, it died when it had produced its second
leaf; without potash and soda, it grew only to the height of three
inches; without magnesia, it was weak and incapable of supporting
itself; without phosphoric acid, weak but upright; and without sulphuric
acid, though normal in form, the plant was feeble, and produced no
fruit.

_Manner in which the Constituents of Plants are absorbed._--Having
treated of the sources of the elements of plants, it is necessary to
direct attention to the mode in which they enter their system.

_Water._--The absorption of water by plants takes place in great
abundance, and is connected with many of the