214 Specific Gravity of Soluble Salts by 



case of the salts of potassium and rubidium crystallisation is 

 accompanied by considerable expansion, and this is what is 

 usually met with. In the case of the caesium salts the reverse 

 is the case, and very decidedly so in the case of the chloride 

 and of the iodide, much less so in the case of the bromide, 

 which, in this, as in other particulars, maintains its singular 

 position. 



TABLE VI. The Salt in Crystal and in Mother -liquor. 



K Rb Cs 



, MR v 



(a) Values of -^--- 



7-010 6-641 - 6-711 Cl 



5-422 7-422 - 0-317 Br 



5-074 3-411 -10-237 I 



(b) Values of 



1-225 I<J 75 ' 86 3 C1 



1-139 1-168 0-993 Br 



1-103 1-059 0-849 I 



In this connection it should be noted that among the ratios 

 ' . j given in compartment (b), the two which are nearest 



to unity are those for Rbl (1-059) an< ^ f r CsBr (0-993) respec- 

 tively; and their molecular weights are almost identical. 

 Further, the salts situated co-diagonally to them, namely RbBr 

 and Csl, have ratios whose difference from unity are almost 

 equal, namely + 0-168 for RbBr and 0-151 for Csl. 



Taking a general view of the figures in (b) which give the 

 ratios of displacement in crystal and in mother-liquor, we see 

 great differences. The most striking examples are, as in the 

 case of solubility, the extreme members of the ennead KC1 

 and Csl. The former expands by more than 25 per cent., and 

 the latter contracts by 15 per cent, on crystallising. 



These figures accentuate the peculiarity of the caesium 

 salts, that crystallisation is accompanied by contraction. An 

 interesting conclusion can be drawn from the behaviour of the 

 different salts in this respect, namely, that the crystallisation of 

 the potassium and rubidium salts of the ennead must be hindered 



