480 RKPOKT OF NATIONAL MUSEUM, 1900. 



thu.s found will bo the divisor of the weight of the specimen and the 

 quotient will 1)«^ the .specific gmvity. Thus: 



The suhstaiKH' is placed in the weight eui). 



The Uiiiit of the apparatus is - 100. Ml i 



Weight added to sink instrument to mark 22. oi ) 



Weight of specimen in air is then 77. "ii ) 



Specimen placed in lower i)an requires, to sink instrument to mark, the 



weight 35. 50 



Now% 35.50—22.50=13, the weight of a like bulk of water. Then 

 77.50-^13=5.9615, the specific gravity sought. 



The speeitie gra\'itv test is one of the most important and reliable 

 means of identifying a gem, since it is practically a constant ft)r all 

 individuals of the same kind, while the difference between individuals 

 of different kinds is often considerable. Thus: 



Zircon 4. 6 t Topaz .3. 65 



Almandite 4. 2 Diamond 3. 52 



Sapphire 4. I Beryl 2. 70 



Further, if the specific gravities of two substances are known, by 

 taking the specific gravity of their compound the relative weights of 

 the components may be found. Thus, having the w^eight of a diamond 

 and gold ring we can find the weight of the diamond. 



Let A be the weight of the stone, a its specific gravity; B be the 

 weight of the gold, h its specific gravity; C be the weight of the ring, 

 c its specific gravity: 



ThenA + B = Cand ABC 



a b c 

 (c-b) a 



From which we obtain A = C (a-b) c • 



Electricity. — The electrical properties of minerals are distinguished 

 by the following terms: Frictional electricity, pyroelectricity, and 

 thermoelectricity. Of these we have to do only Avith frictional elec- 

 tricity, or the power of becoming electrified by friction, and as a 

 result attracting or repelling certain substances. This property is 

 especially noticeable in the topaz, tourmaline, and amber. 



(JlemHK/e. — This is that tendency of a mineral to break in the direc- 

 tion of minimum cohesion, and that direction is always parallel to some 

 plane which occurs or may occur in the crystal. The cleavage is char- 

 acterized, first, according to direction, that is, when parallel to certain 

 faces or planes; second, according to the ease with which it ma}" be 

 obtained, as perfect, imperfect, interrupted, or difficult. 



Frad\u'<>. — This i.s that surface obtained b}' breaking the mineral in 

 a direction other than that of the cleavage. It ma}" be designated as 

 conchoidal, that is, breaking with cavities more or less deep; even, 

 when the fracture approximates a plane surface; uneven, when the 

 surface is ii-regular; hackly or splintery, when the surface is jagged. 



