AERIAL ROOTS. 
Air is supplied, however, not only in a direct 
manner by the atmosphere, but indirectly and 
quite as necessarily and efficiently by water. The 
free circulation of water in the soil, by its ready 
descent as liquid and its ready ascent as vapour, 
|| is essential for the sake of the water’s own agency 
upon plants,—so much so that the farmer must 
provide for both a free filtration at the surface, 
and a free drainage in the subsoil; but this cir- 
culation of water—at least downward—is not less 
important for the conveyance into the soil of 
oxygen, carbonic acid, and ammoniacal gas, the 
last contained in rain-water itself, and the first 
and second contained in the atmospheric air. All 
| water which is exposed to the atmosphere con- 
tains a sufficient proportion of the atmospheric 
gases to be distinctly perceived by the human 
palate ; and hence running waters, taken directly 
from fountain or river, have always an agreeable 
| gout or flavour, while the same waters, when de- 
| prived of their atmospheric gases by boiling or 
| distillation, are always disagreeable and vapid. 
Any running water, therefore, when permitted to 
circulate freely in the soil, carries along with it a 
certain amount of aeration. But rain-water is 
most eminently aerative; for it descends so far 
' and in such small drops, and is so tossed by the 
| wind or moving air, that it completely saturates 
_ itself with the atmospheric gases; and, as already 
hinted, it also brings down the ammoniacal va- 
pours which escape from animal decompositions, 
| and, by depositing these in the soil, supplies one 
of the most fertilizing and vital of manures. The 
plentiful collection or drinking up of air, and the 
carrying of this into the soil to operate upon the 
| spongioles of plants, isa main reason why artifi- 
| cial waterings with a very fine rose are much 
more beneficial than waterings with a rose of 
comparatively larger orifices. See articles Arr, 
ATMOSPHERE, GERMINATION, and ELABORATION. 
AERIAL ROOTS. Roots which issue from the 
upper stem, and descend through the air to the 
soil, either in virtue of the natural habits of a 
plant, or in consequence of its growing in extra- 
ordinary circumstances. The stem of the plant 
called Pandanus odoratissimus, is of comparatively 
small diameter at the base, but widens as it 
ascends; and as it is arborescent, and cannot 
readily admit the internal descent of root fibres 
originating in its wider part, it sends out these 
fibres from buds considerably above the surface 
of the ground, to descend through the air to the 
soil, and there to accomplish the double purpose 
of giving the plant stability, and of supplying it 
with nourishment——In 1817, at Newabbey in 
Kirkcudbrightshire, a plane-tree, 20 feet in height, 
was growing on the top of a stone-wall, which 
measured 10 feet from the ground. “On this 
bare and scanty soil, it could not originally have 
found much nourishment, and could not send 
down its roots through stone and mortar. Ac- 
cordingly it had been compelled, many years be- 
ee to protrude them into the open air. They 
AEROLITES. 49 | 
elongated by descent, clinging to the side of the 
wall, and throwing out neither bud nor branch 
till they reached the ground, which they did after 
a period of several years: Here, having plunged 
their extremities into the soil, they found and 
transmitted the necessary nourishment, and the 
tree grew with vigour.” [Keith’s Lexicon, on the 
authority of the Philosophical Magazine.] Aerial 
roots have, in some recent instances, been forced 
from some hothouse plants in the conducting of 
experiments for ascertaining how far vegetation 
may be maintained in independence of the soil; 
but they have been produced only by slow de- 
grees, in successive stages, and in the course ofa 
prolonged, cautious, and piecemeal process of de- 
tachment from the soil and suspension in the air. 
The only practical lesson of any consequence 
taught by these experiments, is the important one 
that all plants derive a large proportion of their 
nourishment from the air, that some may be 
trained to nourish themselves wholly from it, and 
that most employ the soil more for their me- 
chanical fixture or for the preparation of their 
food, than for any purposes of proper nourish- 
| 
| 
ment, 
AERIDES. See Arr-Prants. 
AERIE. The nest of eagles, hawks, and other 
birds of prey. 
AERIFORM BODIES. A term of frequent oc- 
currence in chemistry and physics. Bodies exist 
in three states; as solids, liquids, or aeriform 
bodies; and these last are divided into vapours 
and gases. Vapours are characterized by the 
readiness with which they return to a liquid or 
a solid state, at or above ordinary temperature 
and pressure. Thus the vapour of mercury 
readily condenses into the liquid metal; and the 
vapour of camphor easily returns to its solid state, | 
Gases, on the other hand, are always aeriform at | 
common temperature ; nor can they be rendered | 
liquid or solid without a considerable increase of 
pressure, or reduction of temperature, or both 
conjoined, Some of them, indeed, have never | 
| 
yet been condensed, But the distinction between 
vapours and gases is not absolute, but one of con- 
venience only: for in the only point of difference, 
—the relative forces which they oppose to con- 
densation,—they graduate into each other. In 
common with liquids, they are also termed fluids ; 
and aeriform bodies are elastic fluids without co- 
hesive but probably with adhesive force; while 
liquids are inelastic fluids under the laws of co- 
hesion. 
AEROLITES. Meteoric stones. They are of 
various sizes, and fall sometimes singly, some- 
times in showers, Most of them vary in size from 
the bulk of a pin’s head to that of a pea; and few 
are more than three or four pounds in weight ; 
but one which fell in Bahia, in Brazil, weighs 
about 14,000 pounds. Their fall is usually pre- 
ceded or accompanied by meteoric lights and 
sounds. They bear a close resemblance to each 
other in both texture and mineral composition, 
D 
