432 SCIENCE OF GARDENING. P a r t II.
ÎÎ'T;
Sect. I. O f the Elements o f ihe .
1352. Oxygen, -nitrogen, water, and carbonic acid gas, are the principal substances
composing the atmosjihere; but more minute inquiries respecting thefr nature and
agencies are necessary to afford con-ect views of its uses in vegetation.
1353. That water exists in the atmosphere is easily proved. I f some of the salt, caUcd
muriate of lime, wliich has been just heated red, be exposed to the air, even in the di-iest
and coldest weather, it will increase in weight, and become moist ; and in a certain time
will be converted into a fluid. I f put into a retort and heated, it will yield pure water;
will gradually recover its pristine state, and, if heated red, its former weight; so that it
is evident that the water united to it was derived from the afr. That it existed in tho
air in an invisible aud elastic form, is proved by the cfrcumstanccs, tliat if a given
quantity of air be exposed to the salt, its volume and weight will diminish, provided the
experiment be correctly made.
1354. The quantity o f water which exists in ihe air, as vapour, vai-ies with the temperature.
In proportion as the weather is hotter, tlio quantity is greater. A t 50° of Ealircnheit,
air contains about 5*5 of its volume of vapour; and, as the specific gi-avity of vapour is to
that of air nearly as 10 to 15, this is about of its weiglit. A t 100°, supposing that
there is a free communication witli water, it contains about part in volume, or in
weight. I t is the condensation of vapour, by diminution of the temperature of the
atmosphere, wliich is probably the principal cause of the foi-mation of clouds, and of the
deposition of dew, mist, snow, or hail.
1355. The power o f different substances to absorb aqueous vapour fi-om the atmosphere
by cohesive attraction has been afready rcfen-ed to. The leaves of living plants appeai-
to act upon tliis vapour in its elastic form, and to absorb it. Some vegetables increase
in weight from this cause, when suspended in the atmosphere and unconnected with the
soil; such are the house-leek, and different species of the aloe. In very intense heats,
and when the soil is dry, the life of plants seems to he preserved by the absorbent
power of thefr leaves; and it is a beautiful cii-cumstancc in the economy of nature,
that aqueous vapour is most abundant in the atmosphere when it is most needed for the
purposes of life ; and that, when other sources of its supply ai-e cut off, this is most
copious.
1356. The existence o f carbonic acid gas in ihe atmosphere is proved by the following
process : if a clear solution of lime water be exposed to the air, a pellicie will speedily
form upon it, and a solid matter will gradually fall to the bottom of the water, and in a
certain time the water wiU become tasteless : tliis is owing to the combination of the lime
which was dissolved in the water with cai-bonic acid gas, wliich existed in the atmosphere,
as may be proved by collecting the film and the solid matter, and igniting them
strongly in a little tube of platina or iron ; they will give out carbonic acid gas, and will
become quicldimc, which, added to the same water, will again bring it to the state of
lime-water.
1357. The quantity o f carbonic acid gas in the atmosphere misiR. I t is not easy to
determine it with precision, and it varies slightly in different situations ; but, where there is
a fi-ee cfrculation of air, it is probably never more than one 1000th, nor less than cne 2000th,
of the volume of air. Carbonic acid gas is neai-ly one-third heavier than the other elastic
pai-ts of the atmosphere in their mixed state; hence, at first view, it might be supposed
that it would be most abundant in the lower regions of the atmosphere; but, unless it has
been immediately produced at the suiface of the earth by some chemical process, this does
not seem to be the case ; elastic fluids of different specific gi-avities have a tendency to
equable mixture by a species of attraction, and the different parts of the atmosphere are
constantly agitated aud blended together by winds or other causes. De Saussure found
lime-water precipitated on Mont Blanc, the highest point of land in Europe; and carbonic
acid gas has been always found, apparently in due proportion, in the afr brought
down fi-om great heights in the atmosphere by aeronautic adventui-ers.
1358. The principal consumption o f the carbonic acid in the atmosphere seems to be in
affording nourishment to plants; aud some of them appear to be supplied with carbon
chiefly from this source.
1359. The formation o f carbonic acid gas takes place during fermentation, combustion,
putrefaction, respiration, and a number of operations taking place upon the surface of
the earth; and there is no other extensive operation known in natm-e, by which it can be
destroyed but by vegetation.
1360. Oxygen and azote, or nitrogen, are the remaining constituents of the atmosphere.
After a given portion of common afr has been deprived of aqueous vapour and carbonic
acid gas, it appears little altered in its properties; it remains a mixture of oxygen and
azote, which supports combustion and animal life. There are many modes of separating
these two gases from each other. A simple one is by bm-ning phosphorus iu a confined
volume of afr; this absorbs the oxygen and leaves the azote; and 100 parts in a volume
Book II.
t h e ATM0S3TIEJÎE. 433
n i l ' T'antities tlian either oxya-cn or hydroo-en
S S S S k H S * “
aanndd vaties with the various stages o fth e development and decay of theior f othrgeairn sa iwIft“h ,’
f ft ft, ™fr exposed to air at a tcmperatm-e not bolmv 45° it soo?
I f t L ah-fth “ which rises upwai'ds, and n radicle which d’creonds
ootf I¡tL, iIsS aLbLsorrbbtead . 'T4h e a'fzt o?te "re mains "u’ n“alftt ered; noo fc agrebromniinc ataicoind tihse toaxkyoghe ani,v oarva pfrm,.-?i
the an-; on the contrary, some is added. Seeds are incapable of L r Z L i L
when oxygen is pre.scnt. In the exliausted receiver of the a i r - p um L T X ® a z L e L
m pure carbomc acid, when moistened they swcU, but do not vegetate- and if kent in
these gases, lose their living powers, and undergo piiti-ofaction. I f a seed hi, examined
before gemmation, it wUl be found more or less insipid, at least not sweet - but a fter
germination it is always sweet. Its eoagidated m n d laL , or s t a r ^ is eoL ra te a t S
îb " P'ooo]®; a substance difficult of solution is changed into one easüy soluble-
and the sng™ ean-md thr-ough the ceUs or vessels of the ootykdons is t Z n o X C e m r f
nL cd t f if, ”b ? "“ "T tio n of oxygen by the seed iu germination has been compared
to its absorption in producing the evolution of foetal life in the eg»-- but thi,
X L f '■‘" " ‘’“ta fro” ntost to the least p c rfo c fe laZ s lenMe
a e T a L t k n T t e ill X “ “ ft to Pfosate tiU it ceases’ to beat,
acid ii X n L - ? constant and the function of respiration invariable: carbonk
acid IS given off in the process; bnt the chemical change produced in the hknd
X r ™ ! ¡rerid oL f a Z ''ft^ft" “ ' Z ‘’'-ft “ -ft ®°™"*on of any .substance simüar to
f X i x n X t e iS l ’ / ? f t ' ft ®«™“ tion, the seeds should be sown so as to be
tuny exposed to the influence of the air; and one cause o fth e unproductivone,, nf eelH
soikThZftfi™- 'ft’T' ooated with matter impci-mcabk to air. In sandy
t e , e sufficiently penetrable by the atmosphere; but in clayey soik
there can seai-cely be too great a mechanical dh-ision of parts. A iiL fe c l L t M lv
t e f L b u u -?w ay s produces a weak and diseased plant. We have already seen
Xoxoyg-oenZ iss Zadr ddeLd Xto bth“e arétm osp7her“e a“t ftth;™e s ame“ tfrt nPi™c. "®I'®t oisf wveogrethtayt ioofn rinei nsuarnks hitnhea;t atnhde
absence of light is necossai-y to the fom.ation of sugar in the gci-mination of seeds- and
presence to the production of sugar in fi-idts. The foUowing is the late D r Mm-r.ay’s
mgemons ex pM atio " of those remarkable facts. The seed consists chiefly of fai-iuaceoLs
matter, which rcqiures oxygen to convert it into sugar. Now, hvlng vcLtablcs anneai-
to absorb oxygen in the dai-k; unripe fruits usuaUy contain an acid, tliat is have an
excess f oxygen ; and hght is favourable to the evolution of oxygen from living plants.
( 71) Plants absorb the greater part of their carbon fi-om the atmosphere. Chcmlsk have
found by experiments that theoai-th mwhich plants had grown did not contain one-tcnth
part of the carbon afterwards found in those plants, and hence were led to the sun
t e Z f i L ffrfrfoonffi quantity of carbon which they contained was obtained by
them from the atmosphere, and on foUoiving up this train of enquii-y they found
that certain substances m the soil had the power of attracting cai-bonic acid from the
L m Tv aréré b"d it to the plants in such a fom and such proportions as to he
r é t e b L b ? by them. In this manner it was found that burnt clay and charcoal
^ L S g L tX a T e ! X e L r X p l “ ® ft“ “ ”“ '’ P™’
1364. T im e changes in the atmosphere which constitute the most important meteoroloaical
p / _ u may bo classed under flve distinet heads: the alterations that o c e L t t e
t a X L L X X ' ’ ft? L"ftft '" the eh.anges produced
rébke? X d ™.fr ‘ho excessive agitation to which it is frequently
subject; and the phenomena arising from electric and other causes, which at particular
times occasion or attend the precipitations and agitations aUuded to. All th r lb o v è
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