I 



THE PROPERTIES OF MINERALS. 681 



tium that pass throu<j:li or fall upon its surface. A yellow mineral, 

 for example, will absorb all the rays of the spectrum except the yellow; 

 a green mineral will retiect chiefly green rays, while a white mineral 

 will reflect all, and a black mineral will absorb all the rays. The color, 

 then, is the result of the mixture of those rays of light which are not 

 absorbed. The color of a mineral is of two kinds — essential and non- 

 essential. 



EHseiitinl color, — The essential color is that of the mineral itself in 

 its purest state, and belongs not only to the mass, but to the flnest par- 

 ticle that can be mechanically divided. The essential color is deter- 

 mined by the color of the fine powder of the mineral, or by rubbing 

 it on a surface of unglazed porcelain. The color of this powder or 

 mark is known as the streak. This character is illustrated by a series 

 of twenty-two minerals. To the left of each specimen is a small vial 

 containing the powder of the mineral. It will be observed that although 

 in many cases the essential color and that of the mineral are the same, 

 in others the color of the streak differs from that of the mass. 



Nonessential eolor. — The nonessential color is in general that of the 

 impurities contained in the mineral and the color of the mass will difter 

 from that of the streak; that is, the same mineral species may display 

 several diflerent colors, all of which disappear in powder. 



This nonessential character of the color is seen in specimens of quartz 

 and fluorite, in which the several distinct colors of the individual 

 masses all disappear in the powder. 



Varieties of color. — The variations in color are classed, flrst, as 

 metallic and non metallic, and all shades are referred to eight funda- 

 mental colors: white, gray, black, blue, green, yellow, red, and brown. 

 Second, according to peculiarities in the arrangement of colors, as play 

 of color, o])alescence, iridescence, tarnish, and asterism. Third, as to 

 the ditterence in color shown for light transmitted in different direc- 

 tions through the crystal. This case of color absorption is called 

 pleochroism and is peculiar to doubly-refracting minerals. Further, 

 certain minerals when viewed under given conditions i)resent a bluish 

 appearance, resulting from the absorption of certain rays of light. This 

 property is called fluorescence. 



The princii)al metallic colors are showu in the following series of 

 minerals. These are: 



Copper red : Native copper, Longfellow Miue, Clifton, Arizona (Cat. No. 83558, 

 IJ.S.N.M.). 



liron:e yelloiv: Pyrrhotite, Siuiilaud, Sweden (Cat. No. 46593, U.S.N. M.)- 



BrnsH yellow: Chalcopyrite, Shimoteuke Province, Japan (Cat. No. 47154, 

 U.S.N.M.). 



Gold tjellow: Gold, Nova Scotia (Cat. No. 82217, U.S.N.M.). 



Silver white: Silver, Chile (Cat. No. 11994, U.S.N. M.) ; collected by J. M. Gilliss. 



Tin white: Arsenopyrite, Criklova, IJiinat, Hungary (Cat. No. 48672, U. S. N.M.). 



Lead yrny : Molybdenite, Altrnbcrg, Saxony (Cat. No. 8129, U.S.N. M.) ; collected 

 by F. M.Endlicli. 



ateel (jray : Smaltite, Schnecberg, Saxony (Cat. No. 45691, U.S.N. M.). 



