l-".i> //. /:. Merwi/n — Quartz and Fluorite as 



•0015 g. each, an error which affects the density determinations 

 less than '0003. The heam was found to he inaccurate hy as 

 much as ±-0007. After adjusting the balance to read unity 

 for water at 18°, it read 2-653 for the quartz at 20°. ' Beducing 

 this reading to true density at 20°, we obtain 2-6495. A 

 second determination with a different combination of riders 

 gave 2 - 649. Standardizing the balance by means of several 

 fragments of glasses, the densities of which had been deter- 

 mined by E. S. Larsen in the Geophysical Laboratory, a value 

 of 2 - 650 was obtained. The density of any one of the twelve 

 quartzes investigated is thus accurately expressed by the figures 

 2-6495 ±'0010. This value may be taken as the density at 20° 

 of clear quartz from any locality. As a check upon the 

 suspension method, L. H. Adams and B. B. Sosman of the 

 Geophysical Laboratory determined the density of two of' the 

 fragments of quartz, using the Principle of Archimedes. The 

 fragments weighed about 30 g. The values found were 

 2-6495 and 2-6496. By the pyenometer, the Earl of Berkeley* 

 obtained, for powdered and ignited quartz, the mean value 

 2-6486. 



According to Fizeau and others 3 the decrease in the density 

 of quartz between 10° and 30° amounts to about -001 for 10 

 degrees rise of temperature. At ordinary laboratory temper- 

 atures the density of any clear colorless quartz is therefore 

 2-6495 ± -0010. 



Fragments of nearly colorless fluorite from nine different 

 localities 4 in Europe and America, furnished to the writer by 

 the United States National Museum, were suspended in 

 Bohrback's solution. 6 A maximum difference of density of 

 ■001 was observed. The mean density as found by the West- 

 phal balance was 3*180, with a probable error of ±'001. 



The density of three pale-colored fluorites from different 

 localities was found by Dudenhausen, 6 using the Principle of 

 Archimedes. The values twice determined at 14° to 16° all 

 came within the limits 3*181 ± -001. At 20° this density, cal- 

 culated from the coefficient of expansion of fluorite as given by 

 Kopp,' is 3-180 d= '001. This is the value found by the writer. 

 Further evidence of the approximate identity of density of 

 various fluorites is found in the uniformity of their indices of 

 refraction. 



5 Jour. Chem. Soc, Trans., xci, 60, 1907. 



3 Smithsonian Physical Tahles, 215-216, 1904. 



4 Durham, Weardale and Cumberland in England : Freiberg, Saxony ; 

 Rosiclare, 111. (from three different occurrences) ; Macomb and Muscalonge 

 Lake, N. Y.; Pike's Peak, Col.; Shenandoah, Va. 



6 The temperature of the solution was kept constant within •3° by placing 

 the cylinder containing the solution in a covered vessel of water at the air 

 temperature. 



6 N. Jahrb. f. Min., i, 8-29, 1904. 



7 Pogg. Ann., lxxxvi, 156, 1852. 



