THE PROPERTIES OF MINERALS. 683 



Asterism: Conuulum, Ceylon (Cat. No. 82892, U.S.N.M.). 



Ph'ochroism : lolito, Haddam, Counecticut (Cat. No. 83616, U.S.N.M.). 



Fluoreacetur : Fluorite, Euglaud (Cat. No. 49492, U.S.N.M.). 



EMISSION OF LIGHT — I'lIOSPIIORESCENCE. 



Phosphorescence, or the emission of Iijj:ht, may ))e produced in differ- 

 ent ways — by rise of temperature, by mechanical effects (such as fric- 

 tion, percussion, or cleav^ age) and by insolation — that is, by exj)0sure to 

 the direct action of sunliglit. 



Piiosphorescence by rise of temperature is best seen in fluorite (Cat. 

 No. 49598, U.S.N.M.), when heated to 145° or 150^, and especially in 

 the variety chlorophane. 



Phosphorescence by friction will result from rubbing the specimen of 

 sphalerite (Cat. No. 125G2, U.S.N.M.) in the dark. 



Pho.sphorescence by insolation is seen in some diamonds, such as the 

 specimen shown, and in some varieties of fluorite, such as chloroj)hane. 



REFLKCTIOX OF LIGHT. 



When a ray of light passes from one medium to another, a i)ortion of 

 it is always thrown back or reflected. Every substance in nature pos- 

 sesses in a greater or less degree the power of reflecting the light 

 rays which fall upon it. The amount of light reflected depends mainly 

 upon the condition of the surface of the substance. 



Luster. — Among minerals luster is that character depending upon 

 the power and manner of reflecting light, and is dependent upon the 

 nature of the reriecting surface and the quantity or intensity of the 

 light reflected. The kinds of luster are: Metallic, submetallic, adaman- 

 tine, vitreous, resinous, pearly, and silky. The degrees of intensity 

 are: Splendent, shining, glistening, glimmering, and dull. 



The specimens showing the different kinds and degrees of luster are : 



Metallic: Galena, Herniosa, New Mexico (Cat. No. 48173, U. S.N.M.); collected by 

 W. F. Hillcbraud. 



Adamantine: Cerussite, county Yancowinna, New South Wales (Cat. No. 82480, 

 U.S.N.M.). 



ntreoio^: Quartz, Eldorado County, California (Cat. No. 16052, U.S.N.M.). 



Ecfiiiioiis: Sphalerite, Cartagena, Murcia, Spain (Cat. No. 18309, U.S.N.M.). 



Peurhj : Brucite, Texas, Lancaster County, Penusylvauia(Cat. No. 81884, U.S.N.M.) ; 

 the \1. S. Geological Survey. 



Silky: Selenite, England (Cat. No. 16786, U.S.N.M.). 



Splendent: Hematite, Isle of Elba (Cat. No. 49971, U.S.N.M.); the Lea Collection. 



Shinin;/: Celestite, Girgenti, Sicily (Cat. No. 49789, U.S.N.M.,); the Lea Collectiou. 



Glistening: Tale, Edwards, St. Lawrence County, New York (Cat. No. 48227, 

 U.S.N.M.); collected by S. L. Pentield. 



Glimmerinij : Chalcedony, Faroe Islands (Cat. No. 82224, U.S.N.M.). 



Dull: Kaolin, Berks County, Pennsylvania (Cat. No. 9783, U.S.N.M.). 



REFRACTION OF LIGHT. 



A ray of light passing from one medium to another which is of 

 different density, and in a direction other than that of a perpendicular to 



