Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 1, A-AME

AGRICULTURE. 455
had been impoveriflied by the growth of feveral crops :
I aifo tried dung, reduced more or lefs into the Hate of
mould j and likewife the molt active manures, fuch as
the ofl'al of animal fubftances rotted by putrefadion j
but in none of thefe, however carefully analyzed, were
found any falts in a free Hate. They contain indeed
the materials proper tor forming lalts, but it they con¬
tain any ready formed, it is merely by accident.
“ The refearches of Kraft, and thole of Alfton, were
not attended with different refults. Having fotvn tome
oats in allies, not lixiviated, and in land ilrongly im¬
pregnated with potafli and with fiiltpetre, and having
found that the oats did not grow, they concluded that
neutral falts, and alkalies, not only retarded the growth
of vegetables, but that they abfolutely prevented it. It
is well known that in Egypt there are diflricts where
the earth is entirely covered with fea-lalt, and thefe di-
ilribls are quite barren. It is probably' owing to this
property of fea-falt, that the Homans were accuftomed
to fcatter large quantities ot it over fields where any
Great crime had been committed, and of which they
wilhed to perpetuate the remembrance, by rendering
the part barren for a certain time.
“ The idea that falts had great influence in vegeta¬
tion ought to have been greatly weakened by the fol¬
lowing iimple refleftion. Suppofmg that talts exifted
in garden mould, they would very loon be diffolved by^
the rain, and carried away, towards the lower ftrata of
the earth, to a depth to which the longeft roots would
not reach. Indeed the famous experiment of Van Hel-
mont would have been lufficient to have deftroyed the
above opinion, if it did not generally happen that we
are no fooner fet free from one error than we fall into
another not lefs extraordinary. The lurprifing effects
of vegetation brought about by the overflowing of wa¬
ter, and in the neighbourhood of fait marfhes, and the
infinite number of inhaling capillary tubes obferved up¬
on the furface of vegetables, led to an opinion that the
air and water, abforbed by the roots and leaves of
plants, were only vehicles loaded with faline matter,
analogous to the vegetables nourilhed by them.
“ To the experiment of Van Ilelmont, which was re¬
peated by many accurate obfervers, fucceeded thofe of
modern plniofophers } from which it clearly appeared,
that plants could grow, and produce fruit, in the air of
the atmofphere, and in dillilled water, alfo in pure fand,
in powdered glafs, in wet mofs or fponge, in the cavity
of fleftiy roots, &c. and that plants which had nothing
but the above-mentioned fluids for their nourifhment,
gave, when fubmitted to chemical analyfis, the fame
produdts as thofe which had undergone their procefs of
vegetation in a foil perfectly well manured. It was alfo
obferved, that the moft barren foils were rendered fer¬
tile when they were properly fupplied with water by
canals } and the efficacy of irrigation was repeatedly
evinced in different ways: from thefe obfervations was
formed the following fyftcrn, that water riles in plants
in the form of vapour, as in dillillation $ that air intro¬
duces itfelf into their pores ; and that if falts contribute
to the fruitfulnefs of foils, it is only in confequence oi
their containing the two fluids above mentioned in great
abundance.”
Our author, after making many experiments upon
various foils and falts, maintains “ that faline fubftan¬
ces have no fenfible efferis in promoting vegetation,
3
except inafmuch as they are of a dcliquefcent nature, Manures,
have an earthy bafis ealily decompofed, and are ufed v "■
only in fmall quantity. In thofe circumftances they
have the power of at trailing, from the immenfe refer-
voir of the atmofphere, the vapours which circulate in
it} thefe vapours they retain, along with the moifture
that is produced from rain, fnow, dew, fog, &c. which
moifture they prevent from running together in a mafs,
or from being loft, either by exhaling into the air of
the atmofphere, or by filtering itfelf through the infe¬
rior ftrata of the earth, and thereby leaving the roots,
of vegetables dry •, they diftribute that moifture uni¬
formly, and tranfmit it, in a ftate of great divifion, to
the orifices of the tubes deftined to carry it into the
texture of the plant, where it is afterwards to undergo
the laws of aflimrlation. As every kind of vegetable
manure poffeffes a vifeous kind of moifture, it thereby
partakes of the property of deliquefeent falts. In fliort,
the preparation of land for vegetation has no other ob-
je£t in view but to divide the earthy particles, to foft-
en them, and to give them a form capable of pro¬
ducing the above-mentioned efferis. It is fufficient,
therefore, that water, by its mixture with the earth
and the manure, be divided, and Ipread out fo as to
be applied only by its furface, and that it keep the
root of the plant always wet, without drowning it, in
order to become the effential principle of vegetation.
But as plants which grow in the (hade, even in the heft
foil, are weakly, and as the greater part of thofe which
are made to grow in a place that is perfectly dark, nei¬
ther give fruit nor flowers, it cannot be denied that the
influence of the fun is of great importance in vegetable
economy.”
Such was the opinion of M. Parmentier while the
old theory of chemiftry prevailed j but when it ap¬
peared, by more recent difeoveries, that air and water
are not fimple but compound bodies, made up of oxy¬
gen, hydrogen, and azote, and that they are refolved
into thefe principles by many operations of nature and
of art, he fo far altered his theory of vegetation as to
admit that air and water a6t their part in that procefs,
not in a compound ftate, but by means of the princi¬
ples of which they confitt. He now concluded that
the value of manured earth conlifts of its tendency to
refolve water into gaffes which give out heat while
they are abforbed by the plants. As he thus fuppofes
that the gaffes conftitute the food of plants, it follows,
that the moft aerated waters will be the moft favour¬
able to vegetation } and hence arifes the value of thofe
in which putrid animal matters are diffolved. Salts
and dung a£t as leavens in bringing on a ftate of fer¬
mentation in the fubftances with which they are ming¬
led, and operating the decompofition of water, which
along with the carbon exifting in the atmofphere, he
imagines contains the whole materials of the more
limple vegetables. Too great a quantity of falts pre¬
vents fermentation, or the decompofition of water, and
hence is prejudicial to vegetation, while a fmall quan¬
tity is more advantageous, as more favourable to that
proceis of putrefaction. Different manures alfo give
forth gaffes which are abforbed by plants, and give
them a peculiarity of character : hence, in a foil com-
pofed of mud and dung, cabbages acquire a bad tafte,
from the hepatic gas, or fulphurated hydrogen gas,
which is there evolved. In addition to thefe chemical
properties