II. E. Merwin— Standards of Density. 



429 



Art. XLVI. — Quartz and Fluorite as Standards of Density 

 and Refractive Index • by H. E. Merwin. 



For checking the accuracy and adjustment of instruments 

 easily available standards are desirable. Such standards should 

 be as nearly as possible independent of other standards. 

 Standards of density correct to ±-001, and of refractive index 

 correct to ±-0001 are particularly useful to the mineralogist. 



Pure water is a most important standard, although its prop- 

 erties vary with temperature beyond these limits under ordi- 

 nary laboratory conditions. The density of water at 15° is 

 •999; at 20° is -998; at 25° is -997. Its refractive indices 

 according to H. Dufet's 1 tables are as follows : 





15° 



20° 



25° 



Li 



Na 



Tl.... ... 



1-3312 

 1-3334 

 1-3353 



1-3308 . 



1-3330 



1-3349 



1-3303 

 1-3325 + 

 1-3344 



The effects of dissolved air and of ordinary impurities may 

 be neglected. 



There are very few natural solids of wide distribution that 

 are uniform enough in composition to serve as standards of the 

 above specified accuracy. Quartz and fluorite approach such 

 conditions. 



The relative density of twelve fragments of clearest quartz 

 about l cm in diameter, from different localities, was investigated 

 by suspending the fragments in Thoulet's solution. The 

 solution was brought to the density of Brazilian pebble quartz, 

 then the other fragments were put in. Only two fragments 

 differed sensibly in density : one was quartz from Herkimer 

 Co., New York, which slowly settled ; the other, of unknown 

 locality, came slowly to the top. The experiment was repeated 

 with other fragments from the same localities with the same 

 result. A balance of the Westphal type sensitive to -0003 

 barely detected the difference of density of the solutions 

 matching the densities of the two fragments. 



The density of the Herkimer Co. quartz was determined by 

 means of a Ruepreckt balance with a JEteiman's plummet, 

 which by test was found to displace 5 grams of water at 18°. 

 The riders were weighed and found to be in error less than 



1 Eecueil de Donnees Numeriques, Optique, p. 87, 1900. 



