H. E. Merwin — Density of Minerals. 425 



Art. XLV. — A Method of Determining the Density of Min- 

 erals by means of RohrbacKs Solution having a Standard 

 Refractive Index; by H. E. Merwin. 



Two methods of finding the density of minerals or glasses 

 which are in granular form, or which require to be granulated 

 in order to be made homogeneous, are in general use — the pyc- 

 nometer method and the suspension method. 1 Fine powders 

 (grains less than abont - 1 or •2 mm in diameter) cannot be used 

 successfully with the suspension method on account of the dis- 

 turbing effects of convection currents and viscosity in the sus- 

 pending liquid, and of flocculation of the solid particles. The 

 pycnometer method is a universal method if sufficient material 

 (a few grams) is available. The method is laborious, and 

 requires much skill and standardized apparatus. The suspen- 

 sion method, with the same skill and care as to the adjustment 

 of the instrument used, should yield equally accurate results 

 for" much less labor, provided the material may be obtained in 

 sufficiently large homogeneous grains. 



The difficulty of convection currents in the liquid can be 

 largely overcome by preventing evaporation and by surround- 

 ing the cylinder in which the liquid is held with a closed vessel 

 of air or water. The proper adjustment and standardization 

 of the specific gravity balance by means of which the density 

 of the liquid is obtained are, of course, essential. Some of 

 the more sensitive balances cannot be adjusted and tested 

 without the use of special devices not furnished with the 

 balances. If properly adjusted, the best of these balances are 

 sensitive to '0003, but errors in the graduation of the beam 

 often amount to "001. The density of a liquid may be cor- 

 rectly determined within ±*001 with such a balance. If the 

 grains of material suspended in the liquid are not moved about 

 by convection currents, then the density of the grains may be 

 matched by the liquid much closer than ±"001. A disadvan- 

 tage of the specific gravity balance is that the column of liquid 

 must be deep enough to receive the sinker; this depth increases 

 the possibility of convection currents. 



An alternative method of finding the density of the liquid 

 in which the grains are suspended, which depends upon the 

 relation of refractive index to density, has been found to be 

 expeditious and accurate. The liquid is contained in the small 

 glass cell accompanying the standard refractometers, and its 



1 Recently J. L. Andreae has described a method of finding the density of 

 suitable solids correctly within ± '0001 by suspending a fragment of the 

 solid in a heavy solution in a dilatometer. Zeitschr. phys. Chem./lxxvi, 

 491-496, 1911. 



