Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 3, 1802).djvu/422

400&#93; 400 ] PL A sum, which are obtained prliicipnlly from Mont Martre, in the vicinity of thnt city. The best plaster is hard, white, and somewhat resembles marble : it neither ettervesces with aqua- forti-;, -nor emits sparks, when stricken against steel ; but it is easily calcined in the fire ; and, when palverizrd, is of great use for casting statues, forming ceil- ings, &c. — It pays, on importation, the sura of Is. 24d. per cwt. — See Alabaster and Gyfsum. Plate. See Silver. PLATINA, one of the most pre- cious metals that was discovered in South America, about the middle of the eighteenth century. It is found chiefly in the river Pinto, and near Carthagena, in small, ir- regular grains, which are always combined with iron. Platina retains its metallic lustre, in a manner similar to gold ; nor does it become tarni.shcd on expo- sure to the air : it is, however, ex- tremely difficult effusion, requir- ing an intense he^t to reduce it to a fluid state. "When properly re- tJued, its colour is between that of iron and silver : it emits no smell ; is reputed to be the most ponder- ous body hitherto known ; and its specilic gravity is, according to Mr. KiHWAV, to that uf water, as 23 to 1.1 — Farther, platina is consi- derably harder than iron, and, re- maining equally exempt from the etI'eAs of fire and air, it forms the best material for making crucibles. It resists the action ot acids, sul- phur, and alkalies ; and, notwith- standing its uncommon hardness, is extremely ductile, so that it may be rolled into plates, or leaves, like those manutaftured from gold ; and Dr. Withering observed, tha.t the wire ot platina is much PLE .stronger thnn that of silver or gold of a siinilar thickness. Lastly, platina posse.sses the pro- perty of soldering, or weUing, with- out mixture, on vdiich account it is preferable to gold. "When form- ed into a mirror, the former metal reflecls one image only, while it remains unchangeable like glass. PLETHORA, or Fulness, is that state of the human fluids, and particularly of the blood, in which they abound to such a degree, as to prove hurtful to the animal func- tions. Young persons that lead an in- dolent or luxurious life, especially females ; as well as those who have hastily suppressed any of the larger evacuations ; or whose pursuits have been suddenlv changed flora an a6live to a sedentary employ- ment, are particulerly subject to plvthnra ; — which is likewise often induced by the indulgence in sleep- ing, longer than Nature seems to require. Hence the patient be- comes languid, weak, and unable to take the necessary exercise for promoting the regular circulation of the blotxl ; the pulse sinks, the action of the heart and arteries is speedily lessened ; he becomes af- fecfed with violent palpitation, and oppressed with great vertigo, or duiness. Such are the symptoms that ge- nerally prevail in this complaint ; beside which, the vessels are fre- quently distended, and tlirown into dangerous commotions. Thus, ple- thcne patients are peculiarly dispos- ed to fevers, inflammations, haemor- rhages, and a long train of chroni- cal and other diseases ; of which they are not easily cured, on ac- count of tiieir singular dislike to avail themselves of proper exercise. Aitlicugh plethora cannot with strict