596 JV. L. Bowen — Melting Phenomena of the 



reason a rearrangement of the formulae has been proposed by 

 Washington* to enable both to be written as salts of the same 

 acid, with, as far as present knowledge goes, extremely dissimi- 

 lar basic radicles. 



It is not, however, clear that a common acid radicle is more 

 favorable to complete miscibility than a common basic radicle, 

 for the pair CdEr 2 — Cdl 2 , and also the pair K 2 Cr0 4 — K 2 S0 4 , 

 form complete series of solid solutions. These facts, of course, 

 prove nothing relative to the feldspars, but they indicate that, 

 until more is known of the laws of miscibility in the solid 

 state, there is no compelling need to assume that albite and 

 anorthite are salts of the same acid. 



It has been shown in some cases that crystalline similarity 

 alone, even when very marked, f may not be sufficient to deter- 

 mine miscibility in the solid state, but that the molecular vol- 

 umes of the two substances must also be nearly equal, or, 

 stated in another form, the fundamental structural parallelo- 

 pipeda must be alike not only in shajDe but also in size. It 

 happens in the present case that the molecular volumes are 

 nearly identical and it may, therefore, be in part due to this 

 fact that complete miscibility is possible. The molecular vol- 

 ume is equal to the molecular weight divided by the density, 

 but since in general nothing is known of the molecular weight 

 of the solid, the molecular volume is equally indefinite. The 

 melting phenomena have shown, however, in the present case, 

 that in the solid state albite and anorthite consist either of 

 simple or of equally associated molecules. The molecular 

 volumes, calculated on the assumption that the molecules are 

 simple, will therefore give figures which represent the actual 

 ratio of these quantities. The molecular volume of anorthite, 



-^ = 100-8 and of albite ^~ 

 2-765 2-605 



practically identical values. 



Of the still more fundamental question as to the factor 

 which determines the crystallographic similarity and therefore, 

 ultimately, the complete miscibility there is very little positive 

 knowledge. It may be noted, however, that the valency- 

 volume theory of Barlow and Pope;}: is applicable to the 

 present pair, inasmuch as they have the same valency volume 

 (32) and could be derived from related, closely-packed assem- 

 blages of spheres of atomic influence. 



* This Journal, xxxiv, p. 555, 1912. 



fTatton, A. E. H., Crystalline Structure and Chemical Constitution, 

 p. 128. Loudon, 1910. 



X Jour. Chem. Soc, lxxxix, p. 1727, 1906. 



so calculated, is ^_ Mn „ = 100*8 and of albite „.„„„ = 100-9, 



