ACHANIA. 
ACHANIA. A genus of evergreen shrubs of 
the mallow family. They comprise about fifteen 
species, and grow spontaneously in the West 
Indies and America; but are known in Great 
Britain only as hothouse plants. 
ACHE. A very painful local affection in ani- 
mals, unaccompanied by any visible symptom in 
or near its seat. Aches are very apt to arise in 
the limbs of horses from hard riding or from 
exposure to cold; but they can, at any time, be 
detected only by the general symptoms of pain. 
ACHILLE’A. A numerous genus of herbace- 
ous plants of the composite tribe, chiefly vile 
weeds, but partially of doubtful character be- 
tween the noxious and the useful. Several of 
the species are well known to farmers under the 
name of Yarrow [which see], and others are 
popularly known under the names of sneezewort 
and sweet maudlin. ‘The total number of species 
is about seventy ; and all are perennial herbs of 
the colder climates of the northern hemisphere. 
The Achillea millefolium, though long regarded 
as a nuisance on British pastures, has of late 
years found great and general favour with some 
agriculturists as an agreeable condiment to cat- 
tle, and with others as one of the most valuable 
ingredients in nutritious herbage. 
ACHILLE'S. A beautiful species of barley 
| mentioned by Theophrastus and Gallen. 
ACHNODONTON. A small genus of exotic 
grasses, of the agrostis tribe. They comprise 
only two species, and have their name from the 
toothed form of their paleze. The bulbous spe- 
cies is a perennial, and a native of Spain; and 
the slender species is an annual, and a native of 
Mesopotamia. 
ACHRAS— popularly Sapora. A genus of ever- 
green fruit-trees of South America. Its species 
are four in number; and two of them are popu- 
larly called the mammee and the naseberry trees. 
The bark of some of the species has the same 
astringent and medicinal properties as the well- 
known Peruvian bark. 
ACICULA. A weed whose organs of fructifi- 
cation bear some resemblance to needles; either 
the wild chervil, cherophyllum,—or the shepherd’s 
needle, scandiz. 
ACIDIFIABLE. Capable of being converted 
into an acid: such as sulphur and carbon, which 
by combination with oxygen may be converted 
into respectively sulphuric and carbonic acid. 
ACIDS. The most important class of com- 
pound substances known to vegetable physiology, 
to scientific agriculture, or to general chemistry. 
The name acid, as originally and for a long time 
used, meant strictly a sour substance; but, as 
chemical discoveries have expanded and multi- 
plied, it has come to be applied to several liquid, 
solid, or gaseous substances quite destitute of 
sourness, and to some others whose sourness is 
barely perceptible. The characteristic property 
of an acid, as now understood, is its property of 
uniting with alkalies, earths, or metallic oxides, 
ACIDS. 
to form some of the very numerous ana impor- 
tant class of substances called salts. But by far 
the greater number of the acids also possess the 
original characteristic of sourness, and some pos- 
sess it to a degree highly acrid and even perfectly 
corrosive; most of them combine in any propor- 
tions with water, and, in the act of combining 
with it, decrease in volume and send out heat; 
all, with a few exceptions, are converted into 
vapour or decomposed into simple substances by 
the action of moderate heat ; and very nearly all 
have the power of changing the purple colours 
of vegetables into a bright red. Some of the 
acids, as the carbonic and the chloric, are gases ; 
some, as the sulphuric and the acetic, are liquids ; 
and some, as the tartaric and the citric, are so- 
lids. Some are strictly natural products; some 
are the results of chemical agency ; and some are 
both natural and artificial, Some exist or can 
be obtained in great abundance; and others are 
obscure and very rare, or can be obtained only 
in small quantities and with considerable diffi- 
culty. Some, as the nitric, can be retained only 
in water or ina base; a few are evanescent or 
very easily decomposible; and many have an in- 
dependent and very sturdy subsistence. 
A classification of acids which has been very 
generally adopted, distributes them into mineral, 
vegetable, and animal,—or such as are derivable 
from respectively mineral, vegetable, and animal 
substances ; but though this classification is fa- 
cile, popular, and apparently quite clear, and 
though, for these reasons, it will be adopted in 
much which we may have to say respecting agri- 
cultural chemistry, yet it is neither sufficiently 
analytical, nor scientifically correct. Another 
classification distributes acids into such as are 
simply compound, and such as are doubly com- | 
pound, or into those which have only one acidi- | 
fied basis, and those which have two or more | 
bases ; but this is at once vague in its character, 
uncertain in its application, and obscure in its 
comprehension. A much preferable classifica- 
tion, for combining clearness and facility with 
correctness and comprehensiveness, divides acids 
into organic and inorganic, and subdivides inor- 
ganic acids into such as contain neither hydrogen 
nor oxygen, such as contain hydrogen, such as 
contain oxygen with metallic bases, and such as 
contain oxygen with non-metallic bases. Oxygen 
is a simple gaseous substance, one of the most 
pervading and valuable in the world, forming 
the vital air of the atmosphere, and acting as the 
chief agent in combustion, and in animal and 
vegetable decomposition ; and this gas received 
its name of oxygen—which means the generator 
of acid—from its great power of forming acids 
by entering into combination with earths and 
metals, and from its having been originally sup- 
posed to be the only substance by which this 
power is possessed. Hydrogen is another simple 
gaseous substance, of widely different properties 
from oxygen, and forming a principal constituent 
