ver, that it is also partly of a physical nature.

With regard to the absorption of phosphoric acid, this has been shown to
be a chemical act, and depends on the formation of insoluble phosphates
of calcium, iron, aluminium, and magnesium, the percentage of iron
especially determining this.

Much analytical work has been accomplished of late years with a view of
ascertaining the amount of ash in different kinds of plants, and in the
different parts of the plant.

_Action of Manures._

The department of agricultural chemistry which has been most largely
developed of late years is that connected with the problems of
_manuring_. It is, from a practical point of view, of most value. It is
some considerable time since we have recognised that the only three
ingredients it is, as a rule, expedient to apply as artificial manures,
are _nitrogen_, _phosphoric acid_, and _potash_. The nature, mode of
action of the different compounds, and properties of these three
substances, and their comparative influence in fostering plant-growth,
together with the economic question of which form is, under various
circumstances, the most economical for the farmer to use, have together
given rise to a large number of "field" and "pot" experiments. As the
principles underlying this practice form the subject of the following
treatise, any further discussion of the question must be left to the
following chapters.

_Note._--The reader interested in the historical development of
agricultural chemistry is referred to Sir J. H. Gilbert's
Presidential Address to the Chemical Section of the British
Association, 1880.

FOOTNOTES:

[1] The History of the Chemical Elements. By Sir Henry E. Roscoe, F.R.S.
(Wm. Collins, Sons, & Co.)

[2] Van Helmont's science was, however, of an extremely rudimentary
nature, as may be evidenced by the belief he entertained that the smells
which arise from the bottom of morasses produce frogs, slugs, leeches,
and other things; as well as by the following recipe which he gave for
the production of a pot of mice: "Press a dirty shirt into the orifice
of a vessel containing a little corn, after about twenty-one days the
ferment proceeding from the dirty shirt, modified by the odour of the
corn, effects a transmutation of the wheat into mice." The crowning
point in this recipe, however, lay in the fact that he asserted that he
had himself witnessed the fact, and, as an interesting and corroborative
d