250 PHENOMENA, ATOMS, AND MOLECULES 



TABLE VI 

 Sulfates and Phosphates, Type 14 (b). Triclinic Pinacoid 



a. 



NaHSO^^ 0.6460 



CaHPOi 0.6467 



SrHAsO* 0.6466 



theory (see Table II ) , we find that the data available in Groth's "Chemische 

 Kristallographie" indicate an almost complete identity of crystalline form. 

 The agreement between the pairs of substances given in Table II is in fact 

 usually rather better than among most of the classical examples of iso- 

 morphism. For example, the agreement of the ratios of the axes for 

 potassium perchlorate and strontium sulfate (Table V) is much closer than 

 among the various chlorates or among the sulfates. Sodium nitrate crystals 

 (Table III) are more nearly like calcium carbonate than these are like 

 magnesium carbonate. Potassium nitrate (Table IV) shows better agree- 

 ment with strontium carbonate than calcium carbonate (aragonite) does 

 with barium carbonate. Sodium hydrogen sulfate (Table VI) agrees 

 slightly better (in axial angles) with calcium hydrogen phosphate than this 

 does with strontium hydrogen arsenate. 



In several of the cases in Table II we find that no data are given by 

 Groth b}' which a comparison between the crystalline form of similarly 

 constituted compounds can be made. For example (Type 15a), no an- 

 hydrous sulfites of divalent metals and no hydrated chlorates of univalent 

 metals are given. Although sodium hydrogen sulfite is given, no phosphites 

 of divalent metals have been measured. 



It seems safe to predict that nearly all the remaining pairs of substances 

 given in Table II will be found to be isomorphous when data become 

 available. By Hull's method of X-ray analysis these comparisons are now 

 possible even with powdered materials, so that most of the difficulties dis- 

 appear that have heretofore prevented the accumulation of such data. 



We have thus far considered cases in which the octet theory leads us 

 to expect isomorphism not predicted by the ordinary theory of valence. 

 Let us now consider those cases where the ordinary theory indicates iso- 

 morphism, but the octet theory does not. Examples of this kind are, (a) 

 carbonates and sulfites, (b) chlorates and nitrates, (c) chlorates and meta- 

 phosphates. 



According to the ordinary theory the sulfur in sulfites has a valence of 



^ Incomplete cleavage. 



^ Axes b and c have been interchanged in order to correspond to those used to 

 form the other compounds. Where the angles a, (3, y, were greater than 90° their 

 supplementary angles were chosen. 



