506 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.47. 



Thus Dr. J. T. Singewald, jr./ concluded that the presence of 

 titanium in magnetites showing no visible ilmenite intergrowths 

 proved the existence of a " titanomagnetite " ; but he had found the 

 lameUas of ilmenite to vary continuously from 4 mm. down to 0.001 

 mm. in length, and there is no reason at all why they should cease 

 to exist at just the latter size, for the limit of microscopic visibiHty 

 is determined by the wave length of hght, and has no significance 

 as far as the molecules are concerned, a particle of this size containing 

 thousands of molecules. So submicroscopic inclusions may well 

 account for much of the titanium. Stopford Brunton,^ from similar 

 studies, assumed the titanium to be present in "sohd solution," 

 but evidently used this term to cover aU cases where no inclusions 

 are visible, making no attempt to differentiate the various possibiU- 

 ties Usted above. It seems to the writer that we should be more 

 specific in stating just what mode of combination the evidence 

 favors in any particular case, so the data in regard to wolframite 

 will be further discussed here. 



The absence of the elements columbiimi and tantalum from some 

 specimens of the latter mineral, and their variable amount when foimd, 

 clearly inchoate that they are not chemically combined with the tungs- 

 tates. That then* presence might be due to inclusions of columbite 

 and tantalite has been suggested,'' without defhiite proof; and only 

 the existence of visible inclusions would justify the assumption of 

 submicroscopic ones, as in the titaniferous magnetites above men- 

 tioned. There remain, therefore, only the possibihties of sohd solu- 

 tion and adsorption, 3, 4, and 5. 



A sohd solution, according to van't Hoff, the first to employ the 

 term, is a sohd homogeneous complex of two or more substances, the 

 relative proportions of which may vary, but the homogeneous charac- 

 ter be retained."* Two principal types can be distinguished, isomor- 

 phous replacement, where the substances are closely related chemi- 

 cally and may be regarded as taking one another's places in the point 

 system constituting the crystal structure; and mix-crystal forma- 

 tion, where the substances are so different in character that mutual 

 replacement is out of the question. The formation of mix crystals, 

 in which, as has been shown more especially by O. Lehmami,^ two or 

 more crystalline substances grow together so intimately that they 

 appear homogeneous under the microscope, yet mutually affect one 

 another as to crystallization, producing changes in habit, crystal 

 angle, optical properties, etc., is probably a much more frequent and 

 important phenomenon than is ordinarily supposed. It takes the 



1 Econ. Geol., vol. 8, 1913, p. 207; U. S. Bureau of Mines, Bull. 64,. 1913. 



2 Idem, p. 670. 



« Damour, Bull. soc. g^ol. France, ser. 1, vol. 2, 1848, p. 108; Hess, loc. cit. 



* Zeits. phys. Chemie, vol. 5, 1890, p. 323. 



' Zeits. Kryst. Min., vol. 1, 1877, p. 453; vol. 6, 1882, p. 48, p. 580; vol. 12, 1887, p. 399, etc. 



