hillebrand: mineral separations 



137 



The following are the values of the resistance units and corre- 

 sponding audibility for 2500 ohm telephones at 1000 sparks per 

 second in an audibility meter designed at my suggestion by G. W. 

 Pickard : 



MTNERALOGY.^ — A danger to be guarded against in making 

 mineral separations by means of heavy solutions. W. F. Hille- 

 brand, Bureau of Standards. To appear in the American 

 Journal of Science and the Zeitschr. Kryst. Mineral. 



The occasional action of heavy solutions on minerals is usually 

 evident to the eye. When using Thoulet's solution of mercury 

 and potassium iodides to separate from its gangue a carnotite 

 from Paradox Valley, Montrose County, Colorado, carrying cal- 

 cium instead of potassium (probably identical with the tuy- 

 amunite of Nenadkevich,i the author found that the calcium of the 

 mineral was largely, if not wholly, displaced by potassium, without 

 visible alteration, altho Mr. H. E. Merwin observed a large de- 

 crease in the optic axial angle to have resulted. The observation 

 shows how important it is to assure oneself, when using heavy 

 solutions, that such chemical changes are not incurred. Question 



1 Bull. Acad. Sci. St. Petersburg, 1912, p. 945. 



