POTASSIUM, KUBIDIUM, CESIUM, AND LITHIUM.. 559 



hydrogen = 1). This shows that the molecule of potassium (like that 

 of sodium, mercury, and zinc) contains but one atom. This is also the 

 case with many other metals, judging by recent researches. 19 The 

 specific gravity of potassium at 15 is 0'87, and is therefore less than 

 that of sodium, as is also the case with all its compounds. 20 Potassium 

 decomposes water with great ease at the ordinary temperature, evolving 

 45,000 heat units per atomic weight in grams. The heat evolved is 

 sufficient to inflame the hydrogen, the flame being coloured violet from 

 the presence of particles of potassium. 21 



With regard to the relation of potassium to hydrogen and oxygen, 

 it is closely analogous to sodium in this respect. Thus, with hydrogen 

 it forms potassium hydride, K 2 H (between 200 and 411), and with 

 oxygen it gives a suboxide K 4 O, oxide K 2 O, and peroxide, only more 

 oxygen enters into the composition of the latter than in sodium per- 

 oxide ; potassium peroxide contains K0 2 , but it is probable that in the 

 combustion of potassium an oxide KO is also formed. Potassium, 

 like sodium, is soluble in mercury. 22 In a word, the relation between 

 sodium and potassium is as close- as that between chlorine and bromine; 

 or, better still, between .fluorine and chlorine, as the atomic weight of 



w The molecules of non-metals are more complex for instance, H 2 , O 3 , C1 2 , &o. But 

 arsenic, whose superficial appearance recalls that of metals, but whose chemical proper- 

 ties approach more nearly to the non-metals, has a complex molecule containing As^ 



20 As the atomic weight of potassium is greater than that of sodium, the volumes 

 of the molecules, or the quotients of the molecular weight by the specific gravity; for 

 potassium compounds are greater than those of sodium compounds, because both the 

 denominator and numerator of the fraction increase. We cite for comparison the volumes 

 of the corresponding compounds 



Na24 NaHOlS NaCl 28 NaNO 3 87 Na 2 S0 4 54 

 K 45 KHO 27 KC1 89 KNO 3 48 K 2 SO 4 66 



21 The same precautions must be taken in decomposing water by potassium as have 

 to be observed with sodium (Chapter II., Note 8). 



It must be observed that potassium decomposes carbonic anhydride and carbonic, 

 oxide when heated, the carbon being liberated and the oxygen taken up by the metal, 

 whilst on the other hand charcoal takes up oxygen from potassium, as is seen from the 

 preparation of potassium by heating potash with charcoal, hence the reaction K 2 O + 

 = ILj + CO is reversible and the relation is the same in this case as between hydrogen 

 and zinc. 



23 Potassium forms alloys with sodium in all proportions. The alloys containing 1 and 

 8 equivalents of potassium to one equivalent of sodium are liquids, like mercury at the 

 ordinary temperature. Joannis, by determining the amount of heat developed by these 

 alloys in decomposing water, found the evolution for Na 2 K, NaK, NaK 2 , and NaK 3 to be 

 44-5, 44-1, 48'8 and 44'4 thousand heat units respectively (for Na 42'6 and for K 45'4). 

 The formation of the alloy NaK 2 is therefore accompanied by the development of heat, 

 whilst the other alloys may be regarded as solutions of potassium or sodium in this alloy. 

 In any case a fall of the temperature of fusion is evident in this instance as in the alloys 

 of nitre (Note 14)* The liquid alloy NaK 2 is now used for filling thermometers employed 

 lor temperatures above 860, when mercury boils. 



*\2 



