64 PHYSICAL PROPERTIES OF MINERALS. 



equal to the weight of the same bulk of water as the 

 mineral.* 



A better and more simple process than the above, and one 

 available for porous as well as compact minerals, is per- 

 formed with a light glass bottle, capable of holding exactly a 

 thousand grains (or any known weight) of distilled water. 

 The specimen should be reduced to a coarse powder. Pour 

 out a few drops of water from the bottle, and weigh it ; then 

 add the powdered mineral till the water is again to the brim 

 and reweigh it : the difference in the two weights, divided 

 by the loss of water poured out, is the specific gravity sought 

 The weight of the glass bottle itself is here supposed to be 

 balanced by an equivalent weight in the other scale. 



HARDNESS. 



The comparative hardness of minerals is easily ascer- 

 tained, and should be the first character attended to by the 

 student in examining a specimen. It is only necessary to 

 draw the file across the specimen, or to make trials of scratch- 

 ing one with another. As standards of comparison, the 

 following minerals have been selected, increasing gradually 

 in hardness from talc, which is very soft and easily cut with 

 a knife, to the diamond, which nothing will cut. This table 

 is called the scale of hardness. 



1, talc, common foliated variety ; 2, rock salt; 3, calc spar, 

 transparent variety ; 4, jluor spar, crystallized variety ; 5, 

 apatite, transparent crystal ; 6, feldspar, cleavable variety ; 

 7, quartz, transparent variety ; 8, topaz, transparent crystal ; 

 9, sapphire, cleavable variety ; 10, diamond. 



If on drawing a file across a mineral, it is impressed as 

 easily as jluor spar, the hardness is said to be 4 ; if as easily 

 as feldspar, the hardness is said to be 6 ; if more easily than 



What other mode is fitted for porous as well as compact minerals ? 

 How is the hardness of minerals ascertained ? What is the scale of 

 hardness? Explain its use. What directions are given for trials of 

 hardness 1 



* For perfectly accurate results, the most delicate scales and weights 

 should be used, and great care be observed in the trial. The purity and 

 temperature of the water should also be attended to, and the height of 

 the barometer. For the latter, an allowance is made for any variation 

 from a height of 30 inches. The temperature of water at its marimum 

 density, or at 39° 1 F., is recommended as preferable to 60° F. 



