12<i //. E. Merwvn — Density of Minerals. 



refractive index is determined while the grains are suspended 



in it. 



A liquid having as great a range of refractive index as pos- 

 sible for a given difference of density is most advantageous 

 for accuracy. The liquid should have also a wide range of 

 density. Further requirements are convenience in attaining a 

 standard condition, and permanency. Rohrbach's solution of 

 barium-mercuric iodide meets these requirements. 



Rohrbach's solution as ordinarily prepared 2 and as sold by 

 the dealers has a maximum density of about 3'5. To stand- 

 ardize the liquid it is diluted, at a temperature of 19° to 21°, 

 with water, to the density of clear crystals of sulphur (about 

 2*07), or until the refractive index for sodium light is 1'510 to 

 1/520. The solution is cleared of the precipitated mercuric 

 iodide, a part is left dilute and the remainder concentrated by 

 evaporation. But if crystals are present in the cold concen- 

 trated solution, enough water should be added to dissolve 

 them, for they differ in composition from the liquid above 

 them. Mixtures of these solutions thus prepared have a fixed 

 density (correct to =b - 001) for a given index of refraction. If 

 desired, the concentrated solution may be diluted with water 

 if it is stirred rapidly to prevent precipitation of mercuric 

 iodide ; it is to be noted, however, that a very slight precipi- 

 tate on the bottom of the cell of the refractometer greatly 

 obscures the signal. 



The prepared Rohrbach's solution or the materials for mak- 

 ing it may be obtained from the dealers sufficiently pure 3 for 

 the preparation of the standard solution. As a check the 

 purity of the solution should be tested by bringing it to the 

 density of clear quartz, 2"6195, and determining its index of 

 refraction <o for sodium light at 20°. This index should be 

 1-6208. 



The relation of density to refractive index of this solution 

 at 20° was found by means of a standard refractometer giving 

 results correct to ±'00015, and indicators of standard density, 

 correct to =b'0015. 



The results are contained in Table I. 



These results may be plotted on coordinate paper and con- 

 nected by a curve; or their relations may be stated in a simple 



2 By rapidly mixing 100 g. Bal 2 and 130 g. Hgl 2 , adding 20 cc of water, then 

 placing immediately in an oil bath at 150° to 160°, and agitating till boiling 

 and solution take place. The solution may be prepared equally well by 

 rubbing the constituents together in an evaporating disk over a hot water- 

 bath. 



8 Barium iodide may be strongly colored by iodine. Washing with ben- 

 zene, xylol or ether will remove the iodine. 



If the heavy liquid becomes discolored it may be cleared by shaking with 

 mercury, Bestandardizing is not necessary unless the solution was deeply 

 colored. 



