POSTULATE 



branches of P.O. work was also 

 constituted. 



In most countries the postal ser- 

 vices are managed by the state. 

 In Australia, on June 30, 1919, 

 there were 8,334 post and receiving 

 offices, receipts from which, ex- 

 cluding those from telegraphs and 

 telephones, were 2,998,724. Can- 

 ada also has a very efficient ser- 

 vice, the number of post offices on 

 March 31, 1919, being 12,290, and, 

 in addition, there were -3,705 

 rural mail delivery routes, on which 

 were erected 181,505 boxes. As 

 for the U.S.A., in 1775 Congress 

 undertook the direction of the 

 postal services, and in 1847 stamps 

 were introduced. The revenue for 

 the year ending June 30, 1920, 

 showed an increase of nearly 

 15,000,000 on the preceding year. 



Bibliography. Her Majesty's Mails, 

 W. Lewins, 2nd ed. 1865 ; Life of 

 Sir R. Hill and History of Penny 

 Postage. Sir R. and G. B. Hill, 

 1880 ; The Royal Mail, J. W. Hyde. 

 1885; History of the British P.O., 

 H. Joyce, 1893 ; Forty Years at 

 the P.O., 1895 ; On the Track of 

 the Mail Coach, 1895, F. E. Baines ; 

 History of the British P.O., J. C. 

 Hemmeon, 1912. 



Postulate (Lat. postulatum, 

 something demanded). In scienti- 

 fic language, the demand that the 

 truth of a principle which has not 

 been, or cannot be proved, should 

 be taken for granted ; e.g. in 

 geometry, the claim for the ad- 

 mission of the assumption that a 

 straight line can be drawn between 

 any two points, or, two straight 

 lines cannot enclose a space. 



P6sty6n. Town in the Slovakia 

 div. of the Czecho-Slovak republic, 

 also known as Piestany. It was 

 formerly in the kingdom of Hun- 

 gary. Situated on the river Vag, 

 30 m. by rly. N. of Galanta, it is 

 a spa with hot sulphur springs. 

 Most of the people are Roman 

 Catholics, three-quarters are Slo- 

 vaks, the rest Magyars. Pop. 7,400. 



Potamogetonaceae. Botanical 

 name for the pond-weed family. 

 In modern botanical systems it is 

 included in the natural order 

 Naiadaceae (q.v.). 



Potash. Names given to various 

 chemical combinations of potas- 

 sium (q.v.). Potassium salts is 

 used in the same sense. Potash 

 manures are manures originally 

 obtained from plant ashes, but 

 now mostly derived from natural 

 deposits. The chief forms are 

 muriate or chloride of potash, 

 sulphate of potash, and kainit, 

 which is a mixture of sulphate and 

 chlorides of potash with other 

 salts. See Manures ; Potassium. 



Potash and Perlmutter. 

 Comedy based by Charles Klein 

 and Montague Glass on short 



6291 



stories by the latter. It was pro- 

 duced at the Queen's Theatre, 

 London, April 14, 1914, where it 

 had a run of 665 performances. 

 Its success was largely due to 

 Robert Leonard and Augustus 

 Yorke, who assumed the charac- 

 ters of Jews, partners in a ladies' 

 clothing business in New York. 



Potassium. One of the metallic 

 elements. Its chemical symbol is 

 K (Arabic, Kali) ; atomic weight, 

 39-03 ; specific gravity, 0'87 ; 

 melting point, 62 C (143-6F.); 

 in colour, silver white with a touch 

 of violet and a brilliant metallic 

 lustre. At normal temperatures it 

 is soft and malleable, easily cut 

 with a knife or moulded into any 

 form. At C. (32 F.) it is brittle 

 and crystalline ; while at a red 

 heat it volatilises in a green vapour. 

 Being lighter than water it will 

 float on its surface, but it instantly 

 decomposes the water with which 

 it is in contact owing to its great 

 affinity for oxygen. A freshly-cut 

 surface is immediately covered 

 with a film of oxide. It is therefore 

 necessary to keep the metal under 

 naphtha or rock oil ; though even 

 so protected it will tarnish in time, 

 and the only way of preserving its 

 brilliant lustre indefinitely is to 

 seal it in a high vacuum tube. It 

 is the metallic base of potash (q.v.). 



Potassium was first isolated by 

 Sir Humphry Davy in 1807. Man- 

 kind had long been familiar with 

 the carbonates of potassium and 

 sodium, but had regarded them as 

 identical. They were called fixed 

 alkali by the Arabian alchemist 

 Geber to distinguish them from 

 ammonium carbonate, which he 

 called volatile alkali, and it was 

 not till 1736 that the French 

 chemist Duhamel showed that the 

 alkali of common salt was a dif- 

 ferent thing from that of wood 

 ashes, potash, and from that time 

 the latter was considered and de- 

 scribed as vegetable alkali and 

 soda as mineral alkali. 



Later Klaproth, the Austrian 

 chemist, showed that the so-called 

 vegetable alkali was found in 

 various mineral substances, and 

 the term potash was given to this 

 alkali, and natron or soda to the 

 mineral or common salt alkali. 

 Both these substances, however, 

 continued to be regarded as ele- 

 ments until Davy's discovery. 

 The French chemist Lavoisier and 

 others had for some time suspected 

 their compound character. Davy 

 obtained the metal by passing an 

 electric current through a slightly 

 moistened piece of caustic potash 

 contained in a platinum dish ; the 

 metal appearing at the negative 

 pole, while oxygen was evolved at 

 the positive. 



POTASSIUM 



Potassium does not occur native 

 in the crust of the earth, but is very 

 widely distributed, as a constituent 

 of felspar and mica, and thus of 

 all granite rocks ; the mineral syl- 

 vine or silvite is a chlorite, leucite a 

 silicate, of potassium ; nitre or salt- 

 petre (q.v. ) is crude nitrate of po- 

 tassium ; it is found as carbonate 

 mostly in the ashes of all plants, 

 particularly in those of seaweeds ; 

 in the residues of the manufacture 

 of beet sugar, and in the " suint " 

 or grease of wool ; carnallite, the 

 mineral of the famous potash for- 

 mations of Prussia, consists of 

 from 16 p.c. to 27 p.c. of potassium 

 chloride ; while sea water contains 

 sufficient of this latter salt to make 

 its extraction from that source 

 profitable in certain suitable locali- 

 ties, as on the shores of the Medi- 

 terranean Sea. 



The metal may be obtained by 

 reducing the carbonate by means 

 of carbon and distilling the result- 

 ing porous mass ; the process most 

 generally now used, devised by the 

 French chemist Castner, consists in 

 reducing the hydrate caustic pot- 

 ash in an iron crucible by means 

 of carbon and a finely divided 

 metal or a metallic carbide, the 

 potassium being distilled over 

 from the crucible as the reduction 

 proceeds, and received in a sep- 

 arate vessel under petroleum. Po- 

 tassium is a valuable chemical re- 

 agent in analytical work, but has 

 no direct metallurgical uses, though 

 in the form of cyanide of potassium 

 it is the second most important 

 agent in the extraction of gold 

 from its ores. 



POTASSIUM SALT. Caustic pot- 

 ash, or potassium hydroxide, is 

 prepared by the interaction of po- 

 tassium carbonate and milk of 

 lime, the clear solution which re- 

 sults being evaporated in a silver 

 vessel until all the water is driven 

 off. The caustic potash, whilst still 

 liquid, is poured into moulds to 

 form, when cold, white sticks, 

 which are characterised by their 

 deliquescent properties. The name 

 caustic potash is derived from the 

 fact that this substance rapidly 

 destroys both animal and vege- 

 table substances with which it is 

 brought into contact. Caustic pot- 

 ash is employed for making soft 

 soap. Potassium bromide, much 

 used in medicine, is made by de- 

 composing bromide of iron by 

 potassium carbonate, the iodide of 

 potassium, another medicinal salt, 

 being prepared in a similar man- 

 ner. Potassium chlorate is used as 

 an oxidising agent, and for many 

 purposes in industrial chemistry, 

 such as the manufacture of 

 matches, fireworks, and in calico- 

 printing. The method of making it 



