ACH 
-bending the millefolium and ptarmica .of 
Tuurnefort. It is of the class and order syn- 
genesis polygamia superfiua. The corolla is 
compound radiate: and the essential character 
is, calyx ovate, imbricate; florets of the ray 
about four ; down none ; recept. chaffy. 
It includes twenty-seven species, most of 
them hardy, though natives of the south of 
Europe, only two being indigenous here. 
The achillea ptarmica, or sneezevvort, is of 
this description. It is used in medicine as a 
sternutatory ; and in Siberia in decoction, to 
stop haemorrhages. A beautiful double vari- 
ety is cultivated in our gardens under the 
name of white bachelors’ buttons. The other 
English milfoil is the yarrow, well known . in 
our meadows. This has also by some been re- 
commended in haemorrhages, dysenteries, &c. 
. ACHILLES, an appellation . sometimes 
given to the principal argument, made use of 
by each sect of antient philosophers, in defence 
of their system. It has obtained this name in 
allusion to the strength of Achilles ; and is 
particularly used for Zeno’s argument against 
motion, which consisted in making a compa- 
rison between the swiftness of Achilles and 
the slowness of a tortoise ; whence lie inferred, 
that a slow body, if but ever so small a dis- 
tance before a swift one, could never be over- 
Uiken by it. 
Achilles, tendon of. See Anatomy. 
ACHIMBASSJ, the name of an officer 
who presides over the practice of medicine at 
Cairo. 
AGHIROPO ETOS, in church history, a 
name given to certain pictures of Christ, sup- 
posed to have been painted in a miraculous 
manner. 
ACHOR, a kind of running ulcer on the 
face, chiefly infesting children, but sometimes 
also grown persons. 
ACHRAS, the wild pear-tree, a genus of 
plants of the class and order hexandria mono- 
gynia, the flower of which consists of five 
erect petals, of a cordated shape; and the 
fruit is an oval berry divided into ten cells. 
The essential character is, calyx six leaved ; 
corolla ovate, sexlid, with six scales alternate, 
more within ; pome celled ; seeds solitary. 
There are four species, and they are called 
mammee, sapoti, and bully tree. They are 
all natives of the West Indies, where the fruit 
is eaten. 
ACHROMATIC, a term used to denote 
telescopes contrived to remedy the aberration 
in colours. See Optics. 
ACHTEL1NG, a measure for liquids used 
in Germany. 
ACH YRANTHES, a genus of the pen- 
tandria monogynia class of plants, belonging 
to the natural order of misal lane*, Linn. 
The characters are : The calyx is a double pe- 
rianthium ; the exterior one consisting of three 
lanced acute leaves, which are persistent ; the 
interior of five leaves, also persistent. No co- 
rolla ; the nectarium is five-valved, surround- 
ing the germen, bearded at the top, concave, 
and killing off. The stamina consist of five 
filaments the length of the coralla; the an- 
ther* are ovate and incumbent. The pistil- 
lum has a lop-shaped germen ; the stylus is 
filiform, and the length of the stamina ; the 
stigma is villous, and divided into two seg- 
ments. The perianthium is a roundish _one- 
Celled capsule, not gaping. The seed is single 
and oblong. Of this genus eleven species are 
enumerated ; but the" character of tiro genus 
A C I 
does not agree in them all. They are all na- 
tives of the Indies. Only one of them, the 
altissima, is commonly cultivated in botanic 
gardens, and that more for the sake of variety 
than beauty, it grows to the height of three 
feet, with oblong pointed leaves. The flowers 
come out in long spikes, from the extremities 
of the branches, and appear in July, the seeds 
ripening in September. Plants of this kind 
must be reared in a hotbed, and may be 
transplanted when they have acquired suffi- 
cient strength. If kept in pots, and shelter- 
ed during the winter in a warm greenhouse, 
they will five two or three years. 
ACIA, a genus of the monadelphia dode- 
candria class and order, and natural order of 
pomace*. The essential character is, calyx 
live-parted; coralla five petalled, unequal; 
drupe full of chinks. W e know of only one 
species, a native of Guiana, which rises to a 
tree of sixty feet in height. The' fruit is eaten 
by the Creoles, and accounted pleasant. 
ACICULiE, certain small spikes, or pric- 
kles, in form of needles, wherewith nature has 
armed several animals, as the hedgehog, echi- 
nus marinas, <yc. 
ACID, in chemistry. The word acid, ori- 
ginally synonymous with sour, and applied 
only to bodies distinguished by that taste, has 
been gradually extended in its signification 
by chemists, and now comprehends under it 
all substances possessed of the following pro- 
perties : 
1. When applied to the tongue, they ex- 
cite that sensation which is called sour or 
acid. 
2. They change the blue colours of vege- 
tables to a red. The vegetable blues em- 
ployed for this purpose are generally tincture 
of litmus and syrup of violets or radishes, 
which have obtained the name of re-agents or 
tests. If these colours have been previously 
converted to a green by alkalies, the acids re- 
store them again. 
3. They unite with water in almost any pro- 
portion. 
4. They combine with all the alkalies, and 
most of the metallic oxides and earths, and 
form with them those compounds which are 
called neutral salts. 
It must be remarked, however, that every 
acid does not possess all these properties, but 
they all possess a sufficient number by which 
they may be distinguished from other sub- 
stances. 
The acids are by far the most important 
class of bodies in chemistry. It was by their 
means indeed, by studying their properties, 
and by employing them as instruments in the 
examination of other bodies, that men of sci- 
ence laid the foundation of chemistry, and 
brought it to that state in which we find it at 
present, lire nature and composition of 
acids, therefore, became a very important point 
of discussion, and occupied the attention of the 
most eminent cultivators of the science. 
Paracelsus believed that there was only one 
acid principle in nature, which communicated 
taste and solubility to the bodies in which it 
was combined. Beecher embraced the same 
opinion ; and added to it, that this acid prin- 
ciple was a compound of earth and water, 
w hich he considered as two elements. Stahl 
adopted the theory of Beecher, and endea- 
voured to prove that this acid principle is sul- 
phuric acid; of which, according to him, all 
the other acids are mere compounds* But 
13 2 
a c r u 
life arguments were only conjectures or vague 
experiments, from which nothing could lie 
deduced. Nevertheless, his opinion, like 
every other which he advanced in chemistry, 
continued to have supporters for a long tune, 
and was even countenanced by Macquer.’ "At 
last its defects began to be perceived : Berg- 
man and Scheele declared openly against it : 
and their discoveries, together with those of 
Lavoisier, demonstrated the falsehood of both 
parts of the theory, by showing that sulphuric 
acid does not exist in the other acids; and that 
it is not composed of water and earth, but of 
sulphur and oxygen. 
The opinion, however, that acidity is owing 
to some principle common to all the salts, was 
not abandoned. Wallerius, Meyer, and Sage, 
had advanced different theories in succession 
about the nature of this principle ; but as they 
were founded rather on conjecture and ana- 
logy than direct proof, they obtained but few 
advocates. At last Mr. Lavoisier, by a 
number of ingenious and accurate experi- 
ments, proved that several combustible sub 4 
stances w'hen united with oxygen form acids % 
that a great number of acids contain oxygen j 
and that w'hen this principle is separated from 
them, they lose their acid properties. Thus 
phosphorus, charcoal, and sulphur, being se- 
parately intlamed in oxygen gas, com bind 
with its base ; acquire an additional weight, 
equivalent to that of the air consumed, or ab- 
sorbed ; and are converted into phosphoric-, 
carbonic, and sulphuric acids. 
The Lavoiserian theory is further support- 
ed bv experiments, in which the known acids 
are decomposed into oxygen and one or more 
combustible bases. If purified nitre (nitrat of 
potash), previously deprived of its w'atev of 
crystallization, is exposed in a silver retort to 
a low red heat, a large quantity of gas, con- 
sisting of oxygen and azote, in the proportion 
of four of the former to one of the latter, will 
be given out, and pure potash will remain in 
the retort; the weight of which, with that of the 
ga v ses, will be equivalent to that of the original 
nitre. But force the gases into union, byV 
means of the electric spark, their volume is 
gradually diminished, and at length the whole 
is reduced to an acid liquor, possessing all the 
qualities of nitrous acid ; if this, and the potash 
remaining in the first process, are mixed to- 
gether, chemical union immediately ensues* 
and nitre is reproduced. He concluded, 
therefore, that the acidifying principle is oxy- 
gen, and that acids are nothing but combus- 
tible substances combined with oxygen, and 
differing from one another according to the 
nature of the combustible base. 
This conclusion, as far as regards the greater 
number of acids, is certainly true. All the 
simple combustibles, except hydrogen, are 
convertible into acids; and these acids are 
composed of oxygen and the combustible body 
combined. This is the case also with four of 
the metals. It must not, however, be admit- 
ted w'ithout some limitation. 
1. When it is said that oxygen is the acidh- 
fying principle, it is not meant to atfirm that 
oxygen possesses the properties of an acid, 
which would be contrary to truth ; all that can 
be meant is, that it enters as a component 
part into acids, or that acids contain it as an es- 
sential ingredient. v 
2. But, even in t his sense, the assertion can- 
not be admitted : for it is not true that oxygen 
is an essoutiai ingredient ia all acids, or that am 
