POʻSTAGE. s. (from post.) Money paid for conveyance of a letter (Dryden).

POSTBOY. s. (post and boy.) Courier ; boy that rides post (Tatler).

To POSTDATE. v. a. (post, after, Latin, and date.) To date later than the real time. POSTDILUVIAN. a. (post and diluvium, Latin.) Posterior to the flood (Woodward). POSTDILUVIAN. s. (post and diluvium, Latin.) One that lived since the flood (Grew). POʻSTER. s. (from post.) A courier; one that travels hastily (Shakspeare).

POSTERIOR. a. (posterior, Latin.) 1. Happening after; placed after; following. 2. Backward (Pope).

POSTERIOR ANNULARIS, in anatomy, an external interosseal muscle of the hand, that extends and draws the ring finger inwards. P. INDICIS, an internal interosseal muscle of the hand, that extends the fore finger obliquely, and draws it outwards.

P. MEDII, an external interosseal muscle of the hand, that extends the middle finger, and draws it outwards.

POSTERIORS. s. (posteriora, Latin.) The hinder parts (Swift).

POSTERIORITY. s. (posteriorité, French; from posterior.) The state of being after : opposite to priority (Hale).

POSTERITY. s. (posteritas, Latin.) Succeeding generations; descendants (Smalridge). PO'STERN. s. (posterne, Dutch.) A small gate; a little door (Fairfax).

POSTEXISTENCE. s. (post and existence.) Future existence (Addison). POSTHACKNEY. s. (post and hackney.) Hired posthorses (Wotton).

POSTHA'STE. s. (post and haste.) Haste like that of a courier (Hakewill).

PO'STHORSE. s. (post and horse.) A horse stationed for the use of couriers (Shak speare).

PO'STHOUSE. s. (post and house.) Postoffice; house where letters are taken and dis patched (Watts).

PO'STHUMOUS. a. (posthumus, Latin.) Done, had, or published after one's death (Addison).

POSTIC. a. (posticus, Latin.) Backward (Brown).

POSTIL. s. (postille, Fr. postilla, Lat.) Gloss; marginal notes.

To PO'STIL. v. a. (from the noun.) To gloss; to illustrate with marginal notes (Ba

POʻSTILLER. s. (from postil.) One who glosses or illustrates with marginal notes (Br.). POSTPLION. s. (postillon, French.) 1. One who guides the first pair of a set of six horses in a coach (Tatler). 2. One who guides a postchaise.

POSTLIMINIOUS. a. (postliminium, Lat.) Doue or contrived subsequently (South). POSTMASTER. s. (post and master.) One who has charge of public conveyance of letters (Spectator).

POSTMASTER-GENERAL. s. He who presides over the posts or letter-carriers.

POSTMERDIAN. a. (postmeridianus, Lat.) Being in the afternoon (Bacon).

POSTOFFICE. s. (post and office.) Office where letters are delivered to the post; a posthouse (Swift).

To POSTPONE. v. a. (postpono, Latin.) 1. To put off; to delay (Rogers). 2. To set in value below something else (Locke).

PO'STSCRIPT. 8. (post and scriptum, Lat.) The paragraph added to the end of a letter (Addison).

To PO'STULATE. v. a. (postulo, Latin.) postuler, French.) To beg or assume without proof (Brown).

POSTULATE, in mathematics, &c. is described to be such an easy and self-evident supposition, as needs no explication or illustration to render it intelligible; as that a right line may be drawn from one point to another.

POSTULATION. s. (postulatio, Latin.) The act of supposing without proof; gratuitous assumption (Hale).

POʻSTULATORY. a. (from postulate.) 1. Assuming without proof. 2. Assumed without proof (Bacon).

POSTULATUM. s. (Latin.) Position as sumed without proof (Addison).

PO'STURE. s. (posture, French; positura, Latin.) 1. Place; situation (IIale). 2. Volun tary collocation of the parts of the body with respect to each other (South). 3. State; disposition (Clarendon).

To PO'STURE. v. a. (from the noun.) To put in any particular place or disposition (Grew).

POSTUREMAʼSTER. s. (posture and master.) One who teaches or practises artificial contortions of the body (Spectator).

PO'SY. s. (contracted from poesy.) 1. A motto on a ring (Addison). 2. A bunch of flowers (Swift).

4. A

POT. s. (pot, French; potte, Islandic.) 1. A vessel in which meat is boiled on the fire (Dryden). 2. Vessel to hold liquids (John) 3. Vessel made of earth (Mortimer). small cup (Prior). 5. To go to POT. To be destroyed or devoured. A low phrase (L'Estrange).

To POT. v. a. (from the noun.) 1. To preserve seasoned in pots (Dryden). 2. To enclose in pots of earth (Evelyn).

POTABLE. a. (potable, French; potabilis, Latin.) Such as may be drank; drinkable (Philips).

POTABLENESS. s. Drinkableness. POTAGER s. (from pottage.) A porringer (Grew).

POTAMOGETON.

Pond-weed. In botany, a genus of the class tetrandria, order tetragynia. Calyxless; petals four; styleless; seeds four. Thirteen species; almost all of them common to the streams, ponds, muddy or stagnant ditches of our own country.

POTAMON, or POTAMO, was a philosopher of Alexandria. He kept a middle course between the scepticism of the Pyrrhonists and the presumption of the dogmatists; but attached himself to none of the schools of philosophy

of his time. He was the first projector of the eclectic sect; for though that mode of philosophising had been pretty common before, he was the first that attempted to institute a new sect on this principle. Diogenes Laertius relates, that not long before he wrote his Lives of the Philosophers, an eclectic sect, xxxx Tito, had been introduced by Potamo, of Alexandria, who selected tenets from every former sect. He then proceeds to quote a few particulars of his system from his eclectic institutes, respecting the principles of reasoning, and certain general topics of philosophical inquiry; from which nothing further can be learned, than that Potamo endeavoured to reconcile the precepts of Plato with those of other masters. As nothing remains concerning this philosopher besides the brief account just referred to in Laertius, an obscure passage in Suidas, and another still more obscure in Porphyry; it is probable that this attempt to institute a school upon the eclectic plan proved unsuccessful. The time when Potamo flourished is uncertain. Suidas places him under Augustus; but it is more probable, from the account of Laertius, that he began his undertaking about the close of the second century.

POTASH, in chemistry, one of the three fixed alkalies, procured from the burnt ashes of vegetables, by combustion in iron or other pots, whence the compound pot-ash. It was long distinguished by the name of vegetable alkali, in consequence of its being chiefly obtained from plants, and being at one time supposed to be peculiar to the vegetable kingdom, which is now well known to be a mistake. It was also called salt of tartar, because it may be procured by burning the salt called tartar. Mr. Kirwan has given it the name of tartarin: Klaproth has called it kali; Dr. Pearson, vegalkali; and Dr. Black, lixiva. By most British chemists it is called as above, potash; although, in common language, this last term rather signifies the carbonat of potash, or potash of commerce.

That potash was known to the ancient Gauls and Germans cannot be doubted, as they were the inventors of soap, which Pliny informs us they composed of ashes and tallow. These ashes (for he mentions the ashes of the beechtree particularly) were nothing more than potash, though not pure potash. The xovie, moreover, referred to by Aristophanes and Plato, appears to have been a ley made of the same kind of ashes. The alchymists were well acquainted with it; and, in every period, it has been very largely employed in chemical researches. It may with justice, however, be affirmed that till Berthollet published his process in the year 1786, chemists had never examined potash in a state of complete purity.

This substance, in its rough state, is prepared by burning wood, or other vegetable matter, and thus reducing them to ashes. The lashes are washed repeatedly with fresh waters, till the liquid comes off perfectly tasteless. The liquids thus obtained are evaporated, and the salt pro

cured is potash. If this substance be exposed to a red heat, many of the materials which are mixed with it are driven off, and what remains 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 experimental chemist. Even when apparently freed from all extraneous substances, it is found to possess very different properties after having been subjected to certain processes. In one state it is mild and inactive; in another extremely acrid and corrosive. In the former case it is united with carbonic acid gass, and is hence a carbonat of potash, and not pure potash. When deprived of this acid gass, it is powerful, corrosive, and highly caustic. Different methods have been proposed by different chemists to obtain this substance quite pure: we shall transcribe that given by professor Lowitz of Petersburgh. 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 evaporated, till a thick pellicle arises on the surface. It is then left quiescent till crystals are formed on it, which are crystals of extraneous salts, that are to be separated. The evaporation is to be continued, and the several pellicles removed as fast as they are formed. When the fluid ceases to boil, and no more pellicles arise, it is taken from the fire, and kept stirring till cold. It is then dissolved in double its weight of water the solution is filtered and evaporated in a glass retort, till regular crystals begin to be deposited. When a sufficient quantity has been formed, the liquid is decanted, and the salt is re-dissolved after it is suffered to drain, in the same quantity of water. The decanted liquor is preserved in a well-closed bottle for several days, till it subsides and becomes clear. It is then re-decanted, evaporated, and crystallized again, and the process repeated as long as the crystals afford with the least quantity of water solutions that are perfectly limpid.

Potash thus obtained is a white solid substance, susceptible of crystallization in long compressed quadrangular prisms, terminating in sharp-pointed pyramids. These crystals, which are only obtained from very concentrated solutions, are soft and deliquescent. The taste is extremely acrid; and so corrosive, that it destroys the texture of the skin the moment it touches it: hence it has derived the name of caustic, and is employed in surgery under the name of potential cautery, for the purpose of opening abscesses, or destroying excrescences. Its specific gravity is about 1.7. By a similar mode to the above, pure soda may be prepared, substituting the carbonat of soda for the pearlash. They both possess the following proper ties:-1. They convert vegetable blues into a green colour. 2. They powerfully attract moisture 3. They readily dissolve in water, and produce heat during the solution. They are not volatilized by a moderate heat, and

atral termination which, by common cons been applied to other newly discovered n examination of the volatile alkali, and great number of complex and tedious exits, Mr. Davy saw reason to conclude that la contains oxygen as an essential ingrend that this cannot well be estimated at a 7 or 8 parts in the hundred: this body refore, in his opinion, be considered as the e of alkalcscence, with as much reason as ich have made it the principle of acidity. aking some general remarks on the prefacts, he suggests the probability that the c, fluoric, and boracic acids all contain as one of their constituent principles. ths of barytes and strontian, as being most us to the alkalies, were likewise examined, th yielded oxygen. In concluding this very ant communication, Mr. Davy remarks n immense variety of objects of research is ted in the powers and affinities of the new produced from the alkalies. In themselves ill undoubtedly prove powerful agents for is; and having an affinity for oxygen, er than any other known substances, they ossibly supersede the application of electrito some of the undecompounded bodies. v rate to use the words of the professor him"In sciences kindred to chemistry, the knowof the nature of the alkalies, and the analorising in consequence, will open many new ; they may lead to the solution of many ins in geology, and show that agents have operated in the formation of rocks and which have not hitherto been suspected

"

st."

M. Gay Lusac and Thenard have since dised that potassium may be obtained much more y and in larger quantities by the simple acof ignited iron upon potash, which separates xygen as effectuaily as the voltaic circuit. e these chemists conceived, and at first rly asserted, that potassium, instead of being pie substance, is a compound of potash and ozen, the last being obtained from the iron. have since, however, been sufficiently cond by other experiments of Mr. Davy's and eir own, and have liberally acknowledged their

P

various subsequent papers introduced into Palosophical Transactions this able indagator pressed the same or similar trains of experits into the different kinds of earths, and ough by no means with equal success, with it ingenuity and many valuable elucidations. general result of the whole appears to be as

WS.

seems to be sufficiently proved that the fixed les and the alkaline earths are metallic oxyds; the proportion of their bases are nearly as well ertained as those of several metals known for to philosophers, and in common life. That nine, zircone, glucine, and silex are also meic oxyds, seems highly probable; but their omposition has not yet been so completely etained as to render this point altogether tain; while, as to the metals which, in this e, constitute their bases, we can scarcely be d to have precisely discovered any thing. It is nonstrated that ammonia is a compound of ygen with hydrogen and nitrogen; and that en the oxygen is removed the hydrogen and rogen are capable of entering into a true che VOL. IX.

mical union with mercury, forming a substance peculiarly similar to the amalgams of that body with other metals. It is very probable that the hydrogen and nitrogén are combined as a chemical compound which thus unites with mercury, and that the same compound unites with oxygen to form ammonia. The appearance of amalgamation as well as the analogy of the other alkaline bodies, leads us to suspect that this compound basis is truly of a metallic nature; and that the volatile like the fixed alkalies, and the alkaline earths, is a metallic oxyd: but this basis has not yet been separately exhibited.

Such, in general, is the state of our knowledge upon the constitution of the alkalies and the earths, as extended by the late wonderful discoveries: and such is the line to be drawn between what we have strictly learned as physical truths, aud what we have been taught to conjecture upon evidence of a lower nature than that of legitimate induction.

The last of these wonders, the constitution of ammonia, gives rise to various hypotheses. To account for the phænomena of amalgamation with mercury and of reproduction of the alkali, three different theories have been stated. Mr. Davy himself seems to think it possible that hydrogen and nitrogen are both metals, acriform at common temperatures, as zinc and mercury are when ignited. M. Berzelius suggests that they may be simple bodies not metallic, but forming a metal when united without oxygen, and an alkali when united and oxygenated. Mr. Cavendish submitted a third conjecture, which was that these gasses, in their common form, may be oxyds, which when farther oxygenated may become metallic.

The singular facts which abound in the decomposition of ammonia appear to strike Mr. Davy as capable of leading to some degree of scepticism respecting the phlogistic and antiphlogistic theories: but he clearly shows that they leave the latter in a much better state than the former. He sees some difficulty in reconciling them to either; but far more difficulty in adjusting them with the former. If," says he, "we assume the phlogistic hypothesis, then we must assume that nitrogen by combining with one-fourth of its weight of hydrogen can form an alkali, and by combining with one-twelfth more can become metallic. If we reason on the antiphlogistic hypothesis, we must assert that though nitrogen has a weaker affinity for oxygen than hydrogen has, yet a compound of hydrogen and nitrogen is capable of decomposing water." He proceeds to show that the latter difficulty is the lesser one, and though he thinks it cannot be wholly removed, it may yet be diminished by chemical analogies; as for example, by the superior inflammability of certain compounds and the greater oxydability of alloys.

In the formation of an amalgam with ammonia and mercury, the following are very curious facts.

First, mercury is rendered solid by a combination with about part of its weight of new matter, while its specific gravity is diminished from 13.5 to less than 3, and it retains all its metallic characters, colour, lustre, opacity, and conducting power.

Secondly, it is scarcely possible to conceive that the substance thus producing the amalgam is not metallic in its own nature; yet if so, the hydrogen and nitrogen (as Mr. Davy argues in a subsequent article) must be metallic.

In the Phil. Trans. for 1809, part I. the same subject is again pursued through a variety of exPP