May 1, 1908] 



SCIENCE 



711 



II III III 



Anoithite CaAlAlS^Oa. 



In this series the valences of some of the 

 individual elements vary, but their total and 

 also the number of atoms in each compound 

 are the same. 



The galena group illustrates an isomorphous 

 series in which the individual valences and 

 'the total number of atoms may both vary. 



II 



Galena PbS 



I 

 Argentite Ag.S. 



(For other examples of isomorphous series 

 see Brauns's " Ohemische Mineralogie," Arz- 

 runi's " Physikalische Chemie der Krystalle," 

 and Groth's " Tabellarische Uebersicht der 

 Mineralien," preferably the Erench edition of 

 1904.) 



The similarity of beryl and benitoite is also 

 to be noted by comparing their axial ratios. 

 Both minerals crystallize in the hexagonal 

 -system. 



Beryl 1 : 0.4989 



Benitoite 1 : 0.4230. 



The value for benitoite is one half of one of 

 those suggested by Louderback and is, no 

 doubt, to be considered only as approximate, 

 -since no data concerning the reliability of the 

 readings are given. 



It is well known that TiO^ has either acidic 

 or basic properties. In the case under con- 

 sideration, the quantitative analysis simply 

 reveals the presence of TiO, as such. As to 

 whether it is acid or basic, depends upon 

 "which of these assumptions will allow of the 

 ■simplest explanation and yet be in strict ac- 

 cord with the observed facts. 



The similarity of the composition of beryl 

 and benitoite indicates a metasilicate and that 

 TiO, is to be considered basic in this instance. 

 The metasilicates are very common in nature, 

 which is not the case with the salts of H,Si^O„ 

 a derivative of the tetrasilicic acid H^Si^O^,, 

 to which Louderback would refer benitoite. 

 Up to the present time only one mineral, 

 lorenzenite with the composition Na,(Ti, Zr),- 

 Si,0„, has been observed which can be referred 

 ■to H,Si,0„. In lorenzenite, Ti and Zr replace 



two atoms of silicon, but in benitoite, accord- 

 ing to Louderback's interpretation, one atom 

 of silicon would be replaced by titanium. 

 Thus, in both cases, H,SijO„ is to be consid- 

 ered the basis. Hence, even though Louder- 

 beck's interpretation be correct his statement, 

 page 152, " Benitoite . . . stands in a class by 

 itself, both as regards acid silicates and titano- 

 silicates," needs to be modified. 



In a very recent paper by Ralph Arnold,'' 

 it is pointed out that benitoite occurs in an 

 area of basic rocks, such as serpentine and 

 glaucophane schists. In fact, benitoite is 

 found in cracks and cavities in the latter 

 and is always associated with the hydrated 

 basic metasilicate natrolite, Na,Al(A10)- 

 (8103)3.211,0. According to Arnold, benitoite 

 may have crystallized before the natrolite, but 

 some occurrences seem to indicate that prob- 

 ably the crystallization of these minerals was 

 jjractically contemporaneous. 



Therefore, the interpretation suggested 

 above, which is based (1) upon the similarity 

 of the chemical composition and axial ratios 

 of beryl and benitoite; (2) upon the fact that 

 TiO^ may act as a base; (3) that benitoite oc- 

 curs in an area of basic rocks, and (4) is 

 always associated with the hydrated basic 

 rnetasilicate, natrolite, seems amply justified. 

 Accordingly, from what has been published, 

 benitoite is not to be considered " a very acid 

 titanosilicate of barium," but rather a 

 metasilicate of barium and titanium. 



Edward H. Kraus 



mineealogical laboratory, 

 Univeksitt OF Michigan 



SOME PHYSICAL CHAEACTERISTICS OF COLLEGE 

 STUDENTS ' 



A PHYSICAL examination is required of all 

 freshmen in Columbia College and the Schools 

 of Applied Science during the fijst month of 

 the academic year. The examination includes 



' " Notes on the Occurrence of the Recently 

 Described Gem Mineral, Benitoite," by Ralph 

 Arnold, Science, N. S., Vol. XXVII., pp. 312-314, 

 1908. 



'Read before the New York Academy of Sci- 

 ences, Section of Anthropology and Psychology, 

 on October 28, 1907. 



