44S 



POSIDONIA. 



POTENTILLA. 



446 



aa. Last lateral tooth stouter and much more projecting than the 

 others. 



T. Callianatsa (Cancer Callianatsa, Herbst). Length about an 

 inch. Found in the Indian Ocean. 



6. Species having the latero-anterior border of the carapace armed 

 with seven teeth, two of which are rudimentary. 



T. erythrodactyla (Portunus erythrodactylus, Lam.). Length 2^ inches. 

 Found in Australasia. 



Podophthalmus (Lam.). M. Milne-Edwards justly observes, that of 

 all the Portunians the Podophthalmi have the most remarkable aspect, 

 and characters the most easy to seize. The enormous length of their 

 ocular peduncles, which are very short in the other Swimming 

 Brachyura, is sufficient to distinguish them at first sight. 



Eye, Orbit, Jaw-Foot, &c. of Putloi>ht!talir,us. 



The eyes are carried on delicate peduncles of extreme length ; and 

 these osseous stems are inserted near the median line of the front, 

 and earry at their extremity the second ocular piece, whilst in the 

 Ocypodians, where the eyes are also very much developed, it is on the 

 development of thia second piece, and not the first, that the length 

 depends. The ocular bulb is not very large, and reaches the lateral 

 extremity of the carapace. The internal antennae are situated below 

 the origin of the eyes, a disposition which is met with in no other 

 Portunian, and their stem cannot bend itself back into the cavity 

 where they are lodged. The external antennae are also below the 

 eyes ; they are placed between the antennary fossets and the orbits, at 

 the external side of the first, and their basilary joint is soldered with 

 the borders of these two cavities, so as to complete their walls and to 

 separate them from each other ; the moveable stem which terminates 

 these antenna) is formed of two small peduncular joints and of a 

 slender and rather short mnltiarticulate filament. 



P. vigil (Portunut vigil, Fabr. ; Podophthalmut spinosus, Lam.). 

 The only species known. Length from two to four inches. Found 

 in the Indian Ocean. 



POSIDO'NIA. [MAIXEACEA.] 



POSIDONOMY'A. [MALLEACEA.] 



PO'SYDON, a genus of Criutacca. 



POTADO'MA (Swainson), a sub-genus of Fluviatile Shells, belong- 

 ing to the genus Melania, and the sub-family Melaniana, and the 

 family Turlndte. 



POTAMIANS. [CHELONIA.] 



PO'TAMIS, a genus of Fresh-Water Shells. 



POTAMO'BIA (Leach), a genus of Crustacea. 



POTAMO'QETON (from trorafiot, a river, and the termination 

 yerov, which probably means 'produced'), a genus of Plants belong- 

 ing to the Endogenous clarni, and the natural order Potamacece. It has 

 a perfect flower, a 4-parted perianth, 4 sessile anthers opposite to the 

 divisions of the perianth, 4 ovaries, and 4 drupes or nuts. The species 

 are water-plant*. 



P. natani is a British plant, with floating ovate stalked leaves, the 

 petioles plano-concave above, the nuts large, rounded on the back 

 when fresh, keeled ; when dry the peduncles are equal. There is a 

 creeping rhizoma at the bottom of the water. The roots are some- 

 time* eaten in the wilds of Siberia by men, but in more temperate 

 regions are fed on only by swans, who devour them with avidity. 



P. critpus has a compressed stem, pellucid oblong linear leaves, 

 sessile and wavy, the nuts with long beaks, keeled on the back ; when 

 dry the peduncles are equal. It is a native of Great Britain. 



P. densus has its leaves all opposite, pellucid, clasping, elliptical- 

 lanceolate, or lanceolate ; the spikes shortly stalked, ultimately 

 reflexed ; the spike is 4-flowered. It is found in ditches and ponds of 

 water hi England. Ducks feed on the seeds and leaves of both these 

 species, but they serve a more important purpose in giving out oxygen, 

 and rendering the water respirable for fish and other aquatic animals. 

 There are twenty British species of Potamogeton, and Haller says that 

 in the Swiss lakes P. serratum grows to the length of from 10 to 20 

 fathoms, forming extensive sub-aquatic forests. 



(Burnett, Outlines of Botany ; Babington, Manual of British Botany.) 



POTAMOMY'A, a genus of Fresh-Water Shells. 



POTAMOMY'A, a genus of Fossil Conchifera. 



POTAMON (Savigny), a genus of Crustacea. 



POTAMOTHILA (Sowerby), a genus of Conchifera. 



POTAMO'PHILUS (Latreille), a genus of Crustacea. 



POTASH. [POTASSIDM.] 



POTASSIUM, a Metal, the base of the alkali Potash, in which it 

 exists, combined with Oxygen. Potash is in fact an oxide of potas- 

 sium. On account of its intense affinity for oxygen, potassium is not 

 found pure in nature. In a mineral form it occurs in a great number 

 of minerals. It is present in considerable quantities in Mica [MICA] 

 and Felspar [FELSPAR], which enter, with quartz, into the compo- 

 sition of granite. Its most abundant form in nature is what is called 

 Nitre. 



Jfitre is a nitrate of potash, and occurs in modified right rhombic 

 prisms; usually in thin white subtransparent crusts, and in needleform 

 crystals on old walls and in caverns. Its taste is saline and cooling. 

 It has the following composition : 



Potassa 46-56 



Nitric Acid 53-44 



100 



It burns vividly on a live coal. It is distinguished readily by its 

 taste ; and from nitrate of soda, which it most resembles, by its not 

 becoming liquid on exposure to the air. 



Nitre, called also Saltpetre, is employed in making gunpowder, 

 forming 75 to 78 per cent, in shooting powder, and 65 per cent, in 

 mining powder. The other materials are sulphur (12 to 15 per cent.), 

 and charcoal (9 to 12J per cent, for shooting, and 20 per cent, for 

 mining powder). It is also extensively used in the manufacture of 

 nitric and sulphuric acids ; also for pyrotechnic purposes, fulminating 

 powders, and sparingly hi medicine. [NITRE, in ARTS AND So. Div.J 



It occurs in many of the caverns of Kentucky and other Western 

 States of America, scattered through the earth that forms the floor of 

 the cave. In procuring it the earth is lixiviated, and the lye, when eva- 

 porated, yields the saltpetre. India is its most abundant locality, where 

 it is obtained largely for exportation. It readily dissolves in water, 

 lowering its temperature, and is used in India as a frigorific agent. 



Spam and Egypt also afford large quantities of nitre for commerce. 

 This salt forms on the ground in the hot weather succeeding copious 

 rains, and appears in silky tufts or efflorescences ; these are brushed 

 up by a kind of broom, lixiviated, and after settling, evaporated and 

 crystallised. In France, Germany, Sweden, Hungary, and other 

 countries, there are artificial arrangements called ' nitriaries,' or nitre- 

 beds, from which nitre is obtained by the decomposition mostly of 

 the nitrates of lime and magnesia which form in these beds. Refuse 

 animal and vegetable matter putrified hi contact with calcareous soils 

 produces nitrate of lime, which affords the nitre by re-action with 

 carbonate of potash. Old plaster lixiviated affords about 5 per cent. 

 Thia process was employed extensively in France, -when the French 

 were prevented from obtaining nitre from foreign sources during the 

 last war with England. Nitre is also employed in the manufacture 

 of glass, and a large consumption of it takes place in this country for 

 this purpose. 



Carbonate of Potath is obtained from the ashes of land plants, just 

 as carbonate of soda is obtained from the ashes of sea-shore plants 

 and sea-weeds. When the ashes are purified they form first the 

 potash of commerce, and when further purified, pearlash. 



Sylvine is a chloride of Potassium, which has been found as a mineral. 

 For the further compounds of Potassium, see POTASSIUM, in ARTS AND 

 Sc. Div. 



(Dana, Manual of Mineralogy.) 



POTATO. [SOLAHUM.1 



POTATO, SWEET. [BATATAS.] 



POTENTILLA ('potens,' powerful, from the supposed medica 

 qualities of some of the species), a genus of Plants belonging to the 

 natural order Jlosacew. It has a concave calyx, 4-5-parted, with 4-5- 

 bractlets ; there are 4-5-petals, numerous stamens, a lateral or nearly 

 terminal style. The fruit consists of numerous small nuts placed on 

 a flattish dry receptacle ; the seeds pendulous or ascending, the radicle 

 superior; the flowers white or yellow, rarely red. 



P. reptam, Creeping Cinquefoil, is a common British species : it is 

 found likewise in other parts of Europe. The stem is filiform, pro- 

 cumbent, and creeping ; the leaves quinato and stalked ; the leaflets 

 obovate, serrated ; the peduncles solitary ; carpels granulate, scabrous. 

 The flowers are large, bright yellow, and on long hairy axillary stalks. 



