550 PRINCIPLES OF CHEMISTRY 



A. Scott, is equal to 19 (if that of 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 other inetals, 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 45000 heat units per atomic weight. 

 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- 



under these conditions (the temperature at which it was determined) a portion of the 

 FeCls was decomposed. If it was not decomposed, then a density 81 would correspond 

 with the formula FeCls, and if the decomposition were Fe.^Clg = '2FeClo + Clo, then the 

 .density should be 54. With regard to the silver chloride, there is reason to think that 

 the platinum decomposed this salt. 



The majority of Scott's results so closely correspond with the formulae that a better 

 concord cannot be expected in such determinations. 



19 The molecules of non-metals are more complex for instance, H.,, O r ,, CL, &c. 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 4 . 

 With respect to the vapour of iodine, see Chap. VII. p. 313. 



20 As the atomic weight of potassium is greater than that of sodium, therefore 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 augment. We cite for comparison the 

 volumes of the corresponding compounds 



Na 24 NaHO 18 NaCl 28 NaNO 5 37 Na 2 SO 4 54 

 K 45 KHO 27 KC1 39 KNO 3 48 K 2 SO t <>(> 



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

 to be observed with sodium (Chap. 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 + C = 

 K 2 + CO is reversible and the relation is the same in this case as between hydrogen and 

 zinc. Nothing of the kind could be expected from a comparison of the quantities of 

 heat evolved in the formation of these compounds, for charcoal in combining with 

 oxygen to form carbonic oxide evolves (for its molecular weight) only about 30000 

 heat units (Chap. IX. Note 25), whilst potassium in forming the oxide K 2 O gives 

 about 100000 units, and iron in forming ferrous oxide, FeO, evolves about 70000 units. 

 It is evident that the decomposition of potassium oxide by charcoal is accompanied by 

 the absorption of a large amount of heat, and the reverse reaction which, however, 

 proceeds with greater difficulty with the development of heat, which again shows 

 the impossibility of judging the direction in which a reaction will proceed from 

 thermal data. 



