043 



CHAPTER XIII 



POTASSIUM, RUBIDIUM, CAESIUM, AND LITHIUM. SPECTRUM ANAEYSI8 



JUST as the series of halogens, fluorine, bromine and iodine correspond 

 with the chlorine contained in common salt, so also there exists a cor- 

 responding series of elements : lithium, Li = 7, potassium, K = 39, 

 rubidium, Bb = 85, and caesium, Cs = 133, which are analogous to the 

 sodium in common salt. These elements bear as great a resemblance 

 to sodium, Na = 23, as fluorine, F = 19, bromine, Br = 80, and iodine, 

 Issl27, do to chlorine, 01 = 35-5. Indeed, in a free state, these 

 elements, like sodium, are soft metals which rapidly oxidise in moist 

 air and decompose water at the ordinary temperature, forming soluble 

 hydroxides having clearly-defined basic properties and the composition 

 RHO, like that of caustic soda. The resemblance between these metals 

 is sometimes seen with striking clearness, especially in compounds such 

 as salts. 1 The corresponding salts of nitric, sulphuric, carbonic, and 

 nearly all acids with these metals have many points in common. The 

 metals which resemble sodium so much in their reactions are termed 

 the metals of the alkalis. 



1 Tutton' s researches (1894) upon the analogy of the crystalline fonns of K 2 SO 4 ,Eb 2 S0 4 

 and Cs3SO 4 may be taken as a typical example of the comparison of analogous compounds. 

 We cite the following data from these excellent researches : the sp. gr. at 20/4 of K 2 SQ 4 

 is 2-6683 of Kb 2 S0 4 , 8-6118, and of C8SO4, 4'2434. The coefficient of cubical expansion 

 (the mean between 20 and 60) for the K salt is 0'0053, for the Eb salt 0-0052, for the 

 Cs salt 0*0051. The linear expansion (the maximum for the vertical axis) along the axis 

 of crystallisation is the same for all three salts, within the limits bf experimental error. 

 The replacement of potassium by rubidium causes the distance between the centres of 

 (he molecules in the direction of the three axes of crystallisation to increase equally, and 

 less than with the replacement of rubidium by caesium. The index of refraction for all 

 rays and for every crystalline path (direction) is greater for the rubidium salt than for 

 (he potassium salt, and less than for the caesium salt, and the differences are nearly in 

 the ratio 2 : 5. The lengths of the rhombic crystalline axes for KsSC^ are in the ratio 

 0-5727 : 1 : D'7418, for Rb 2 SC>4, 0-5723 : 1 : 0-7485, and for Cs-jSO* 0'57l2 : 1 : 0'7521. 

 The development of the basic and brachy-pinacoids gradually increases in passing from 

 K to Bb and Cs. The optical properties also follow the same order both at the ordinary 

 and at a higher temperature. Tutton draws the general conclusion that the crystallo* 

 graphic properties of the isomorphic rhombic sulphates BgSO 4 are a function of the 

 atomic weight of the metals contained in them (see Chapter XV.) Such researches as 

 these should do much towards hastening the establishment of a (roe molecular mechanics 

 of physico-chemical phenomena. 



