POTASSir.M, KTBIDIUM, CAKSITM, AND LITHIUM 549 



Potassium was obtained like sodium ; first by the action of a gal- 

 vanic current, then by reduction of the hydroxide by means of metallic 

 iron, and lastly, by the action of charcoal on the carbonate at a high 

 temperature. The preparation of metallic potassium differs, however, 

 from that of sodium in that it easily combines with carbonic oxide, 

 forming an explosive and inflammable mass. 17 



However, in its essential points the method of its extraction is 

 not different from that of sodium, because potassium is quite as volatile 

 as sodium, if not more so. At the ordinary temperature, potassium 

 is even softer than sodium, its freshly- cut surfaces present a whiter 

 colour than sodium, but, like the latter, and with even greater ease, it 

 oxidises in moist air. It is brittle at low temperatures, but is quite 

 soft at 25, and melts at 58. At a low red heat (720) it distils with- 

 out change, forming a green vapour, whose density, 18 according to 



the entire process will be different in different cases. The difference in the composition 

 of the gases and residue depends, as the researches of Gay-Lussac, Shishkoff and Bun- 

 sen, Noble and Abel, Federoff, &c., show, on the conditions under which the combustion 

 of the powder proceeds. When gunpowder bums in an open space, the gaseous products 

 which are formed do not remain in contact with the residue, and then a considerable 

 portion of the charcoal entering into the composition of the powder remains unburnt 

 1 "realise the charcoal burns after the sulphur at the expense of the oxygen of the nitre. 

 In this extreme case the commencement of the combustion of the gunpowder may be 

 expressed by the equation. -JKNO 5 + 3C -f S = 2C + K 2 SO 4 + CO., + N 2 . The residue in a 

 blank cartridge often consists of a mixture of C, K. 2 SO 4 , KoCO 3 , and K 2 S 2 O 3 . If the 

 combustion of the gunpowder be impeded if it take place in a cartridge in the barrel of 

 a gun the quantity of potassium sulphate will first be diminished, then the amount 

 of sulphite, whilst the amount of carbonic anhydride in the gases and the amount of 

 potassium sulphide in the residue will increase. The quantity of charcoal entering into 

 the action will then be also increased, and hence the amount in the residue will de- 

 crease. Under these circumstances the weight of the residue will be less for example, 

 4K a CO 3 + 4S = KoSO 4 -(-8K2S-l-4CO.,. Besides which, carbonic oxide has been found in 

 the gases, and potassium bisulphide, K 2 S 2 , in the residue of gunpowder. The amount 

 of potassium sulphide. K.,,S, increases with the completeness of the combustion, and 

 is formed in the residue at the expense of the potassium sulphite. In recent times the 

 knowledge of the action of gunpowder and other explosives has made much progress, and 

 has developed into a vast province of artillery science. 



17 The substances obtained in this case are mentioned in Chap. IX. Note 81. 



18 A. Scott (1887) determined the vapour densities of many of the compounds of the 

 alkali elements in a platinum vessel heated in a furnace and previously filled with nitro- 

 gen. But these, the first data concerning a subject of great importance, have not yet 

 been sufficiently full} 1 described, nor have they received as much attention as could be 

 desired. Taking the density of hydrogen as unity, Scott found the vapour densities of 

 the following substances to be 



Na 12-75 (12-5). KI 92 (84). 



K 19 (19-5). RbCl 70 (60). 



CsCl 89-5 (84-2). Csl 188 (180), 



FeCl 5 68 AgCl 80 (71'7). 



In brackets are given the densities corresponding with the formulae, according to 

 Avogadro-Gerhardt's law. This figure is not given for FeClj, because in all probability 



