THE PROPERTIES OF MINERALS. 665 



stance at the same time losing its crystalline form and falling- to 

 powder; in other cases the compound may absorb water from moist 

 air to such an extent as to form a solution. These conditions are 

 respectively known as efflorescence and deliquescence. Further, the 

 jn^eseuce of water combined with a salt often influences its color; and 

 finally, those minerals containing water are, as a rule, lighter and 

 softer than anhydrous minerals of an otherwise similar composition. 



These effects are illustrated by a series of pairs of minerals and 

 laboratory compounds, each pair consisting of aualagous hydrous and 

 anhydrous compounds, the differences in each case being stated on the 

 accompanying label. 



RELATION OF COMPOSITION TO PHYSICAL PROPERTIES. 



The comparison of almost any mineral with the elements of which it 

 is composed shows that its more fundamental properties, such as specific 

 gravity, luster, etc., are, to a limited extent, the average of those of its 

 constituent elements. Three series of specimens illustrative of this are 

 shown. 



I. Density series : This series illustrates the relations between the 

 specific gravity of minerals and their composition. Those minerals in 

 which the heavy elements predominate are themselves heavy ; and those 

 composed chiefly of the lighter elements have a correspondingly low 

 specific gravity. 



JI. Magnetism series: Those minerals which are magnetic are rich 

 in the magnetic elements, and their magnetic properties increase with 

 the increase of their magnetic constituents. 



III. Luster series: Those minerals in which elements having a metal- 

 lic luster i^redomiuate usually possess a metallic luster; and those com- 

 posed chiefly of the nonmetallic elements have a nonmetallic luster. 



These relations are illustrated by the following series of minerals: 



I. DENSITY SERIES. 



Galena, density 7.6, composed chiefly of tlie heavy element lead. Utah. (Cat. 

 No. 18213, U.S.N. M.) 



Cinnabar, density 8.2, composed chiefly of the heavy element mercurj-. Mercur 

 mine, Mercur, Utah. (Cat. No. 81321, U.S.N.M.) 



Chalcopyrite, density 4.3, composed chiefly of the heavy elements copper and 

 iron. Copiapo, Chile. (Cat. No. 12989, U.S.N. M.) 



Wulfenite, density 7, contains the heavy element lead. Eureka County, Nevada. 

 (Cat. No. 15923, U.S.N.M.) 



Bauxite, density 2.5, composed chiefly of the light clement aluminum. Bartow 

 County, Georgia. (Cat. No. 83239, U.S.N.M.) 



Kaolin, density 2.5, contains the light element aluminum. Aiken, Aiken County^ 

 South Carolina. (Cat. No. 10G94, U.S.N.M.) 



Quartz, density 2.6, composed chiefly of the light element silicon. Crystal Moun- 

 tain, Arkansas. (Cat. No. 45885, U.S.N.M.) 



Magnesite, density 3.4, composed chiefly of the light (dement magnesium. Napa 

 County, California. (Cat. No. 18710, U.S.N.M.) 



