- 
98 AIR. 
and malaria which originate in conditions of ter- 
ritory produced by human guilt or by processes 
of manufacture rendered necessary by human 
folly and wickedness, almost constantly diminish 
the air’s healthy power over both vegetables and 
animals, and often become concentrated into the 
very virus of epidemic and pestilential diseases. 
Air, or pure air, is the constant gift of the Divine 
bounty; and impurity of air is the product, di- 
rectly or indirectly, of man’s sin. 
The necessity of ventilation, or of constant or 
at least frequent accession of pure air to confined 
apartments occupied by man or by inferior ani- 
mals, is now so well understood that it does not 
need to be explained. Almost every farmer 
knows—and certainly every farmer ought to 
know—that animals shut up in a house send off 
large volumes of effluvia which require to be 
carried away, and consume large quantities of 
oxygen which require to be renewed,—and that, 
in consequence, both his family and his live stock 
will maintain vigour of vital action, and escape 
languor and feebleness and disease, exceedingly 
much in the proportion in which all the houses 
of the farmery are kept sweet and balmy by con- 
stant or frequent circulation of fresh air—Nor 
is the necessity of a constant ventilation of soil, 
or of the circulation of fresh air through its pores 
and mimic channels, less urgent in order to the 
vigorous germination of seeds, or the healthy 
growth of plants. A calculation has been made 
that a properly pulverized and porous soil is oc- 
cupied with air throughout one-fourth of its 
whole bulk; that, therefore, every imperial acre 
of land, duly tilled to the depth of eight inches, 
contains beneath its surface 12,545,280 cubic 
inches of air; and that every additional inch in 
depth of tillage achieved by thorough ploughing, 
brings into activity, upon each imperial acre, 
2353 tons of soil, and brings into play within 
that soil, for the uses of vegetation, 1,568,160 
additional cubic inches of air.—The comparative 
purity of the air, too, just as truly as tempera- 
ture, exposure, or climate, affects the health and 
even the vitality of plants through the medium 
of their leaves. Some plants languish and others 
die in the atmosphere of a crowded city, while 
both flourish in uncontaminated air in the neigh- 
bouring country; some languish and others die 
in the midst of plains affected with malaria, while 
both flourish in the freer circulation of neigh- 
bouring hills and slopes ; some languish and others 
die amid such bituminous and sulphurous exhala- 
tions as those of the shores of the Dead sea, while 
both flourish in the unmedicated air of the neigh- 
bouring districts; and some languish and others 
die amid the attenuated air of an alpine elevation, 
while both flourish in denser air of the same tem- 
perature in valleys or upon plains. In the air 
of London, alpine plants scarcely ever produce 
flowers, snow-drops die away, privets and china 
roses exist in health during only a short time, 
AIRA. 
laburnum continues in perfect health during only 
a few years; and yet, in that air, most bulbous 
and tuberous rooted plants, the sycamore, the 
elm, the ivy, the oriental plane, the vine, and the 
mulberry, grow freely and prosper. See articles 
Axnration, Air-Cetis, Arr-PLants, ATMOSPHERE, 
GaAsEs, Oxyern, Azotz, Hyprocren, Ammonia, and 
CarBonic Actp. 
AIRA—popularly Harr-Grass. 
grasses of the bromus tribe. They are distin- 
guished by their aquatic habits, and by the at- 
tenuated, filamentous, hairy form of their leaves, 
They flower from the first week till the third - 
week of July, and ripen from the beginning till 
the end of August. They are very far inferior to 
some other grasses, both in the bulk of their green 
produce and in the quantity of-their nutritious 
matter; and even if they were intrinsically de- 
serving the attention of the farmer, they are, in 
a great measure, rendered unfit for field culture 
by their aquatic habits. Yet they possess suffi- 
cient features of interest to demand from us a 
comparatively full notice. The species of them, 
examined and analyzed in connexion with agri- 
cultural inquiry, are four in number, and bear 
the designations of Aquatica, Cristata, Cespitosa, 
and Hlexuosa. 
Aira aquatica, or water hair-grass,—now some- 
times called Catabrosa aquatica, or water food- 
grass,—grows naturally on the margins of pools 
and in the mud of slowly running water, but is 
easily and profitably cultivated on imperfectly 
drained fens. It is said to contribute much to 
the fine quality of the Cambridgeshire butter ; 
and it has been pronounced by Mr. Curtis the | 
sweetest of the British grasses; yet it is excelled, 
in the proportion of sugar to other nutritious 
matters, by the grasses Glyceria fluitans, Poa 
aquatica, Elymus arenarius, and Poa nemoralis 
angustifolia. It flowers in the second and third 
weeks of July; and when grown on mud covered 
with water, it yields per acre when in flower 
10,890 lbs. of green produce, 3,267 lbs. of dry pro- 
duce, and 382? lbs. of nutritious matter. 
Aira cristata, or crested hair-grass—called by 
the older botanists Poa cristata or crested mea- 
dow-grass, and now sometimes called A@leria cris- 
tata or crested kceleria—is plentiful in some dis- 
tricts upon dry pastures, and may be cultivated 
as easily and successfully as Pestuca ovina on any 
kind of dry soil; but it thrives best, and remains 
permanent, on moist and clayey lands. It is ne- 
glected by cattle so long as they can obtain Loliwm 
perenne, Cynosurus cristatus, Festuca ovina, Hordeum 
pratense, Avena flavescens, or even Aira flexuosa ; 
but the cause of their dislike to it seems to be, not 
its taste or composition, but the soft, hairy char- 
acter of its foliage. It differs little in nutritious 
power from the grasses which cattle prefer to it; 
and approaches nearest to Aira fluxuosa, but has 
a smaller proportion of bitter extractive matter, 
and a greater proportion of tasteless mucilage. It 
A genus of | 
and so hardy and accommodating a tree as the | flowers about the first week of July, and ripens 
