426 KANSAS CITY REVIEW OF SCIENCE. 



1. About the first characteristic of a mineral to engage our attention, is its^ 

 color. Colors, as relating to minerals, are of two kinds, essential and non-essen- 

 tial. The essential color of a mineral is its color when in a pure state. The non, 

 essential is mainly the color of the impurities contained in the mineral. The 

 essential color is found by powdering the mineral or rubbing it on any hard sur- 

 face, as unglazed porcelain. The powder thus obtained is called the streak, and 

 although the non-essential color may vary greatly, its streak is always nearly uni- 

 form, A mineral shows its true color when powdered, for the same reason that 

 muddy water becomes white when beaten into foam and made opaque. 



The essential color or streak of limestone is white or grayish white ; its non- 

 essential colors range from red, green and yellow to blue, brown and black. 

 Common feldspar (orthoclase) may be white, gray, flesh-red, or even green, as- 

 in Amazon stone, but its streak is uncolored. 



Metallic minerals, those in which metallic elements predominate, are always 

 opaque and generally have essential colors, while vitreous or glassy minerals, 

 which are more or less transparent, often have non-essential colors, because we 

 can see into them and discern the impurities. Magnetite (an ore of iron) is a 

 metallic mineral, and its color and streak are both black, 



2. Closely related to color is the property termed luster, by which is meant 

 the quality of the light reflected by a mineral as determined by the character of 

 its surface. The two principal kinds of luster are the metallic and vitreous. 

 The former is the luster of all true metals, and of nearly all minerals which are 

 chiefly composed of metallic elements. An example may be seen in galena. 

 The vitreous luster is the luster of minerals in which the non-metallic elements 

 preponderate, as in vitreous quartz. There are various other kinds of luster, as 

 adamantine, the luster of the diamond ; resinous, the luster of resin ; pearly, like 

 pearl, as talc, pearl spar; and silky, as satin spar. When luster is entirely want- 

 ing a mineral is said to be dull, as chalk and kaolin. 



3. After the color, streak and luster have been determined, the hardness is 

 the next property that commands attention. In minerals there are all grades of 

 hardness, from talc, which is impressible by the finger-nail, to the diamond, the 

 hardest of all known substances. To facilitate the determination of this charac- 

 teristic a scale of hardness has been devised, as follows, beginning with the 



softest : 



1. Talc, 6. Orthoclase. 



2. Gypsum. 7. Quartz. 



3. Calcite. 8. Beryl or Topaz. 



4. Fluorite. 9, Corundum, 



5. Apatite. 10. Diamond. 



Of any two minerals that which scratches the other is the harder, and by 

 testing an unknown mineral by those given in this scale its degree of hardness 

 can be ascertained. For instance, if we have a specimen that scratches calcite, 

 but is scratched by apatite, we estimate its hardness at 4, but if it should also be 

 scratched by fluorite we would place it at 3.5. The hardness of all common 



