POS 
by which the garrison may march in and 
out nnperceived by tlie enemy, eitlier to 
relieve the works, or to make private sal- 
lies, &c. 
POSTIL, a name anciently given to a 
note in the margin of the bible, and after- 
wards to one in any other book posterior to 
the text. 
POSTING, among merchants, the put- 
ting an account forward from one book to 
another, particularly from the journal or 
waste-book to the ledger. 
POSTULATE, in mathematics, &c. is 
described to be such an easy and self-evi- 
dent supposition, as needs no explication 
or illustration to render it intelligible; as, 
that a right line may be drawn from one 
point to another. That a circle may be 
described on any centre given, of any mag- 
nitude, &c. ; however, authors are not well 
agreed as to the signification of the term 
postulatum ; some make the difference be- 
tween axioms and postulata to be the same 
as that between theorems and problems; 
axioms, according to those authors, being 
truths that require no demonstration. But 
others wilt have it, that axioms are primitive 
and common to all things, partaking of 
the nature of quantity, and which therefore 
may become the objects of mathematical 
science: such as number, time, extension, 
weight, motion, &c. and that postulata re- 
late particularly to magnitudes, strictly so 
called, as to things having local extension, 
such as lines, surfaces, and solids ; so that in 
this sense of the word postulatum, Euclid, 
besides axioms, or those principles which 
are cornmon to all kinds of quantities, has 
assumed certain postulata to be granted 
him peculiar to extensive magnitude. Hence 
several of the principles assumed in his 
Elements, and ranked among the axioms 
by the moderns, are by Proclus ranked 
among the postulata, which has induced 
Dr. Wallis to judge, that the last of the two 
senses given to the term postulatum is most 
agreeable to the meaning of tlie ancient 
geometers. 
POSTURE, in painting and sculpture, 
the situation of a figure with regard to the 
eye, and of the several principal members 
thereof with regard to one another, whei e- 
by its action is expressed. A considerable 
part of the art of a painter consists in ad- 
justing the postures, in giving the most 
agreeable postures to his figures, in accom- 
modating them to the characters of the re- 
spective figures, and the part each has in 
POT 
the action, and in conducting and pursuing 
them throughout. 
POTAMOGETON, in botany, pond- 
weed, a genus of the Telrandria T^ragynia 
class and order. Natural order of Inun- 
datse. Naiades, Jussieu. Essential cha- 
racter ; calyx none ; petalsfour ; style none ; 
seeds four. There are fourteen species; 
these are perennial, herbaceous plants, in- 
habitants of the water. 
POTASH, in chemistry, a substance 
which is procured from the burnt ashes of 
vegetables, hence the termination ash; the 
prefix pot was given on account of its being 
prepared in iron pots. It obtained the 
name of vegetable alkali, because it was 
supposed to exist only in vegetable sub- 
stances : and being prepared from nitre 
and tartar, it was called the “ alkali of 
nitre,” and likewise “ salt of tartar,” a name 
■by which it is still known in the shops. By 
some it is distinguished by the name of 
“ kali,” the plant from which it was origi- 
nally procured. This substance, in its rough 
state, is prepared by burning wood, or other 
vegetable matter, and tlius reducing them 
to ashes. The ashes are washed repeatedly 
with fresh waters, till the liquid comes off 
pel fectly tasteless. The liquid thus obtained 
is evaporated, and the salt obtained is pot- 
ash. If this substance is exposed to a red 
heat, many of the substances which are 
mixed with it are driven off, and what re- 
mains is much whiter, and on account of its 
colour it is called “ pearl-ash.” In this 
state it is deemed sufficiently pure for the 
ordinary purposes of life, though by no 
means adapted to the purposes of the expe- 
rimental chemist. Even when apparently 
freed from all extraneous substances, it is 
found to possess very different properties 
after having been subjected to certain pro- 
cesses. In one state it is mild and inactive • 
in another extremely acrid and corrosive. 
In the former case it is united with carbonic 
acid gas, and is a carbonate of potash, and 
not pure potash. When deprived of this 
acid gas, it is powerful, corrosive, and high- 
ly caustic. Different methods have been 
proposed by different chemists to obtain 
this substance quite pure: we shall trans- 
cribe that given by Professor Lowitz, of 
Petersburg!]. He boils in an iron pot for 
two or three hours any quantity of potash 
with double its weight of quicklime, and 
eight times the weight of the whole mixture 
of distilled or rain water. The liquor is to 
be set by to cool, and then filtered and 
