214 MR J. Y. BUCHANAN ON THE 



the salt in crystal from its mean molecular increment of displacement of the water in 



/MR m\ 

 the mother-liquor. In compartment (o) we have the ratio / ^=- . — j of these quantities. 



Taking the figures in compartment (w), we see that in the 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 that of the chloride and of the iodide, but much less so in the 

 case of the bromide, which, in this, as in other particulars, maintains its singular position. 



In this connection it should be noted that among the ratios (-i=^ ,— ) given in com- 

 partment (o), the two which are nearest to unity are those for Rbl (r059) and for CsBr 

 (0"993) respectively ; and their molecular weights are almost identical. Further, the 

 salts situated co-diagonally to them, namely RbBr and Csl, have ratios whose differences 

 from unity are, numerically, almost equal, namely + 0"168 for RbBr and — 0'151 for Csl. 



Taking a general view of the numbers in (o) 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, KCl 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 csesium 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 m^ust he hindered by increased pressure, 

 while that of the caesium salts must be helped by the same agency. 



§ 131. Extension of the Research to the Salts of the Ennead MRO^, or the 

 Oxyhalides of Potassium, Rubidium, and Caesium. — It appeared to be interesting to 

 extend this work so as to include the salts of the ennead of the oxyhalides, having 

 the general formula MROg, in which M may be K, Rb, Cs, and ROg may be CIO3, 

 BrOg, IO3. 



In contrast with the salts of the ennead MR, which are very soluble, the oxyhalides 

 are only sparingly soluble. The determination of the specific gravity of the crystals in 

 their mother-liquors is therefore much easier, and was effected quite successfully by my 

 assistant, Mr H. F. Fermor. The results so obtained are given in Table IV., which 

 is identical in form with Table II., dealing with the salts of the halides, which has 

 already been explained. 



§ 132. The results of the discussion of the observations made with the salts of the 

 ennead MRO3 are given in Tabic V., which is constructed on the same plan as Table III. 

 It consists of a number of sub-tables, (a), {b), (c), etc., and the nature of each is 

 specified in its title. The molecular weight of each salt, represented by the general 

 formula MRO3, differs from that of the corresponding salt of the general formula MR 

 by O3 = 48. Therefore the differences between the molecular weights in the same 



