er, but a very long period of daylight.

[23] A point of great interest which these experiments elucidated is
that nocturnal repose is not absolutely necessary for the growth and
development of all plants.

[24] See pp. 15 and 22.

[25] See p. 22.

[26] See Chapter III., pp. 120 and 131.

[27] Further reference is made to this subject in Chapter III., p. 136.

[28] See p. 6.

[29] See Phil. Trans., Part II., 1861, pp. 444-446. Lawes & Gilbert.
Schloesing has found in the air in the neighbourhood of Paris 1 lb. of
ammonia in 26,000,000 cubic yards; while Muentz found only about half
that amount in a similar quantity of air on the top of the Pic du Midi.

[30] See Chapter III., pp. 119, 120; Appendix, p. 155.

[31] Some recent experiments by Dyer and Smetham would seem to show that
comparatively small quantities of ammonia in the air prove actually
hurtful to plant-life. Thus they found that one volume of ammonia in
1000 volumes of air was fatal to hardy plants; while one volume in 3000
volumes killed tender ones.

[32] According to the experiments of Hellriegel and Wollny. The
quantity, it may be added, varies with the leaf-surface and the length
of the period of growth of the plant. It is greatest with clovers and
grasses, and least in the potatoes and roots.

PART II.

PRINCIPLES OF MANURING

CHAPTER I.

FERTILITY OF THE SOIL.

It is necessary to clearly understand to what the fertility of a soil is
due ere we can hope to master the theory of manuring.

_What constitutes Fertility in a Soil._

The question, What constitutes fertility in a soil? is by no means an
easy one to answer. If we say, The presence of a plentiful supply of the
constituents which form the plant's food, our answer will be incomplete.
Similarly, if we reply, A certain physical condition of the soil--here,
again, it will be found equally unsatisfactory; for fertility of a soil
depends both on its physical condition and on its chemical composition,
and indeed even on other circumstances. It may be well, then, before
proceeding to treat of the nature and action of the different manures,
to offer a brief statement of the conditions of fertility so far, at any
rate, as we at present know them. For it may be well to warn the reader
that, despite the great amount of work carried out on this subject by
experimenters, we still have much to learn before we shall be in a
position fully and clearly to understand the subj