680 REPORT OF NATIONAL MUSEUM, 1897. 



magnet, or diamagnetic, that is, repelled by the poles of a magnet. 

 These characters can be illustrated properly only in the laboratory, 

 and only those minerals possessing polarity, that is, being themselves 

 magnetic, or those that are sensibly affected by an ordinary magnet, 

 can be shown. 



ELECTRICITY. 



The electrical properties of minerals are distinguished by the follow- 

 ing terms: Erictional electricity, or the power of becoming electrified 

 by friction. Pyroelectricity, or tlie development of electricity through 

 change of temperature. Thermoelectricity, due to differences of tem- 

 perature at the point of contact with another substance, and is the 

 electro-motive force developed when two or more minerals which are at 

 different temperatures are brouglit in contact and establish an electric 

 circuit. 



E — CHARACTERS DEPENDING UPON THE ACTION OF LIGHT. 



LIGHT. 



Light is the sensation produced upon the eye resulting from the 

 excitation of a vibratory motion by a luminous body in the particles of 

 a highly elastic imponderable medium called ether, which is assumed 

 to pervade all space, including the most minute pores of all matter, 

 whether solid, li(iuid, or gaseous. These vibrations are propagated in 

 straight lines and in all directions from the luminous point. The small- 

 est portion of light which can be separated is called a ray, and it may 

 be considered as a combination of two vibrating motions, one of which 

 for the sake of convenience may be regarded as vertical and the other 

 horizontal. A ray of light will pass through any medium of the same 

 density in a perfectly straight line, but if it pass out of that medium 

 into another of different density it may be disposed of in different ways, 

 being either transmitted, absorbed, reflected, refracted, or polarized. 



TRANSMISSION OF LIGHT. 



Diaphaneity. — All bodies transmit light to a greater or less degree. 

 Among minerals the amount of light transmitted, or the degrees of 

 transparency, are classed as: 



Transparent — when the outline of an object seen through the min- 

 eral is perfectly distinct. Subtransparent — when an object may be 

 seen but its outline is indistinct. Translucent — when light is trans- 

 mitted but objects are not seen. Subtranslucent — when merely the 

 edges are translucent. Opaque — when no light is transmitted. The 

 property of diaphaneity occurs in every degree among minerals and is 

 here represented in its several shades by five specimens of quartz. 



ABSOHrTION OF LIGHT. 



Color. — The color of a mineral depends upon its power of absorbing 

 certain portions of light; that is, absorbing certain rays of the spec- 



