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and in sudi casos the supply of it to tho soil will not only bo useless, bnt may bo injurious.
In great stonns the spray of the sea has been can-led more than fifty miles from
the shore ; so that from tliis source salt must be often supplied to the soil. Salt is found
in almost aU sandstone rochs, and it must exist in tho soil derived from these rocks. I t
is a coiistitnoiit likewise of almost every kind of aiihnal and vegetable manure. ’
132,3. Nitric acid, as might be expected fr-om its composition, is a very powerful
mamu'o when combined with some alkaline base. Nitric acid is never found in its
free or iincombined state, but always in combination with some base ; being a very
powerful acid, it is easy to understand that as soon as formed it seizes upon some base
to combine with, and forms a neutral salt. The commonest salts contammg nitric acid,
or nitrates, as they are called, are the nitrates of potash, soda, and lime, which are found
native abundantly in different places. Of the salts tlms formed, nitrate of soda appears
to be the most powerful as a maiim*e. Its eifeets, however, are to produce a gi-eat
abundance of leaves, whicli are of an intensely deep green ; but it does not appear to
have so good an effect in producing either seeds or fruits. Nitrate of potash has been
found to make plants grow luxuriantly when dissolved in water.
1324. ^ Soot owes its eiBcacy to the ammoniacal salts it contains. The liquor produced
by the distillation of coal contains carbonate and acetate of ammonia, aud is a very
good manure. The ammonia which it contains being partly in tbe free or caustic state,
it must either be diluted with water, or saturated with some acid. This prevents it
from destroying the plants mainrred with it, or being dissipated by evaporation. It
may also be used with great advantage as an addition to composts or decaying vegetahlo
matters of all kinds, as it assists in thcir decomposition, aud reduces them raptdly to
the condition of well-fennented muck.
1325. Soapers’ waste is veiy efficacious as a mamrro, and its efficacy depends upon
the different saline matters it contains, wliich arc in a state of minute subdivision. It
also contains lime.
1326. All salts, when used as manures, must be given in very small quantities, and in a
state o f minute subdivision. Solutions of saline substances were used twice a week, in
the quantity of two ounces, on spots of grass and corn, suificicntly remote from each
other to prevent any interference of results. Tlie substances tried were bicai-bonate,
siilphatc, acetate, nitrate, and muriate of potassa ; sulphate of soda ; and sulphate,’
nitrate, muriate, and carbonate of ammonia. I t was found, that, in all cases when the
quantity of the salts equalled one thirtieth part of the weight of the water, the effects
were injurious; but least so in the instance of the carbonate, sulphate, and muriate of
ammoma. When the quantities of the salts were one three-hundredth pai-t of the
solution, the effects were different. The plants watered with the solutions of the sulphates
grew just ill the same manner as similar plants watered with rain-water. Those
acted on by the solution of nitre, acetate and cai’bonate of potassa, and mm-iate of
ammonia, grew rather better. Those treated with the solution of carbonate of ammonia
gi-ew the most luxuriantly of all. This last result is what might be expected; for carbonate
of ammonia consists of cai-bon, hydrogen, azote, and oxygen. There was, however
another result which was not anticipated; the plants watered with the solution of nitrate
of ammonia did not grow better than those watered with rain-water. The solution
reddened litmus paper ; and probably tlie free acid exerted a prejudicial effect and
interfered with the result.
1327. Composts formed o f different kinds o f earths arealways more adapted forgrowing
plants than any simple soil, as tlie number of substances which they contain have all
then- pai-ticles in a state, of minute-subdivision.
Cmvp. III.
O f the Agency o f Heat, Light, Electricity, and Water, in Vegetable Culture.
1328. The piuticular iipcKcy o f heat, light, and taaier, in vegetation and culture, has
been so frequently illustrated, that it only remains to give a general idea of the nature of
tliese agents, and to offer some remai-ks on electricity.
Sect. I. O f Heat and Light.
1329. The heat o f the sun is Hie cause o f growth, and its light the cause o f maturity, in
the vegetable kmg-dom. This is universally acknowledged: animals will live without
light, or with veiy little; but no plants whatever can come to perfection without the presence
of liglit. The agency of electricity in vegetation is less known.
1330. Two opinions are current respecting the nature o f heat. Bv some philosophers
it IS conceived to be a pcouliai- subtile fluid, of wliich the particles repel each other, bnt
H E A T A N D LIGHT.
iiave a strong attraction for tho particles of other matter; by others it is considered as a
motion or vibration of the particles of matter, which is supposed to differ in velocity in
different cases, and thus to produce. the different degrees of temperature. Whatever
decision be ultimately made respecting these opinions, it is certain that there is matter
moving m the space between us and the heavenly bodies capable of communicating heat;
the motions of which are rectilineal: thus the solar rays produce heat in acting on the
surface of the earth. The beautiful experiments of Br. Herschcl have shown that there
are rays transmitted from the sun which do not illuminate, and which yet produce more
heat than the visible rays ; and Ritter and Br. Wollaston have shown that there are
other invisible rays distinguished by their chemical effects.
1331. Heat is radiated by the sun to the earth, and if suffered to accumulate, Dr. Wells
observes, would quickly destroy the constitution of our globe. This evil is prevented*
by evaporation and the radiation of heat from the earth to the heavens, during the
night, when it receives httle or no heat in return. But through the wise economy of
means, which is witnessed in all the operations of nature, the prevention of this evil is
made the soui-ce of great positive good; for the surface of the eaith, having thus become
colder than the neighbouring air, condenses a part of the watery vapour of the atmosphere
into dew, the utility of which is too manifest to require elucidation. This fluid
appears chiefly where it is most wanted, on herbage and low plants, avoiding, in a great
measure, rocks, bare earth, and considerable masses of water. Its production, too, tends
to prevent the injury that might ai-ise from its own cause; since the precipitation of
water, upon the tender parts of plants, must in them lessen the cold which occasions it.
The prevention, either wholly or in part, of cold, from radiation, in substances on the
ground, hy the interposition of any solid body between them and the sky, ai-ises in the
following manner: the lower body radiates its heat upwards, as if no other intervened
between it and the sky; but the loss, which it hence suffers, is more or less compensated
by what is radiated to it, ft-om the body above, the under surface of ■which possesses
always the same, or very nearly the same temperature as the air. The manner in which
clouds prevent, or gi-e_atly diminish, the appearance of cold at night, upon the surface
of the earth, is by radiating heat to the earth, in return for that which they intercept
in its progress from the earth towards the heavens. F o r although, upon the sky
becoming suddenly cloudy during a. calm night, a naked thei-moraeter, suspended in
the air, commonly rises 2 or 3 degrees, little of this rise is to be attributed to the heat
evolved hy the condensation of watery vapoui- in the atmosphere; for the heat so
extricated must soon be dissipated, whereas the effect of greatly lessening, or preventing
altogether, the appearance of a superior cold on the earth to that of the afr, will be
produced by a cloudy sky, during the whole of a long night.
1332.^ Dense clouds, near the earth, reflect back the heat they receive from it by radiation.
But similar dense clouds, if very high, though they equally intercept the communication
o fth e earth with the sky, yet being, from their elevated situation, colder than the earth,
will radiate to it less heat than they receive from it, and may, consequently, admit of
bodies on its surface becoming several degrees colder than the air. Islands, and parts of
continents close to the sea, being, by thefr situations, subject to a cloudy sky, will, from
the smaller quantity of heat lost by them tlu-ough radiation to the heavens, at night, in
addition to the reasons commonly assigned, be less cold in winter than countries considerably
distant from any ocean. But the chief cause why islands, and the coasts of
the ocean, are more temperate than continents and inland situations, is, that the temperature
of the ocean a little from the surface, and where not cooled by contact with
ice, is very uniformly about 54° Falir. in all latitudes. The ocean is the ereat cqualiser
ofheat. (71)
1333. Fogs, like clouds, will arrest heat, which is radiated upwards by the earth, and if
they are very dense, and of considerable perpendicular extent, may remit to it as much
as they receive. Fogs do not, in any instance, fiirnisli a real exception to the general
rule, that whatever exists in the atmosphere, capable of stopping or impeding the passag-e
of radiant heat, will prevent or lessen the appearance at night of a cold on the surface
of the earth, greater than that of the neighbouring air. The water deposited upon the
earth, during a fog at night, may sometimes be derived from two different sources, one
of which is a precipitation of moisture from a considerable part of the atmosphere, in
consequence of its general cold; the other, a real formation of dew, from the condensation,
by means of the superficial cold of the ground, of tho moisture of that portion of
the afr which comes in contact with it. In such a state of things, all bodies will become
moist, but those especially which most readily attract dew in clear weather.
1334. When bodies become cold by radiation, the degree of effect observed must depend,
not only on thefr radiating power, hut in part also on the greater or less ease with which
they can derive heat, by conduction, from waimer substances in contact with them.
Bodies, exposed in a clear night to the sky, must radiate as much heat to it dui-ing the
prevalence of wind, as they would do if the aii- were altogether still. But in the former
A f I