rilYSIC'AL IMinl'UJTII.S 



L'lil 



place. Various specimens of the same species will differ 

 according to their pnritio. but chemically pure specimens of a sub- 

 stance drtrniiiiu'd under the same conditions will not vary in their 

 -prcilic gravity. 



The specific gravity of most minerals, including most of the sili- 

 cates, will lie between 2.25 and 3.5. Minerals with metallic luster 

 are usually high and will lie between 4.5 to 10, 

 while the specific gravity of the naturally occur- 

 ring metals reaches as high as 23 in iridium. 

 The specific gravity of ice is 0.92. 



Two general methods are followed in the de- 

 termination of the specific gravity: (a) weighing 

 the substance in air and weighing it in water ; 

 (6) suspending the substance in a liquid of the 

 same specific gravity, then determining the spe- 

 cific gravity of the liquid by weighing. 



(a) There are several modifications of this first 

 method, depending upon the accuracy required, 

 or the solubility and physical condition of the 

 material. 



I. Joly balance. A quick but only an ap- 

 proximate method. The balance, Fig. 374, con- 

 sists of a spiral spring S, two scale pans P and 

 P', a long mirror graduated in units and tenths, 

 a white bead b, just above the top scale pan, to 

 assist in reading the scale, and a movable bracket, 

 which supports a beaker of water. If it is wished 

 to determine the specific gravity of a crystal of 

 quartz, as an example, three readings are neces- 

 sary, and in each reading the lower glass scale 

 pan P' should be submerged to the same depth 

 in the water, should not touch the walls of 

 should be free of air bubbles. 



FIG. 374. Joly Bal- 

 ance. 



the beaker, and 



The first reading is taken with both 

 pans empty and the lower one submerged in the beaker of water ; 

 the eye is held in such a position that the bead b will exactly cover 

 its image in the mirror = C = 2.8 in this case. The specimen is 

 now placed in the top pan P, care being taken that with the extra 

 weight it does not sink in the water and get wet, increasing its 

 weight; the beaker is now lowered until the pan P' rests at the 

 same depth in the water as before, when the second reading is 

 taken = 16.1. The specimen thoroughly wet is now placed in the 



