CHAPTER IX 

 OPTICAL PROPERTIES OF CRYSTALS 



IT has been shown that crystal forms are dependent upon and 

 are the result of a definite arrangement of the molecules. In some 

 cases substances which differ chemically may crystallize with al- 

 most the same angles and forms. Again, substances which, upon 

 chemical analysis, as pyroxene and amphibole, may yield the same 

 percentage result, crystallize with angles which are different. Such 

 substances may be easily identified when comparatively large 

 specimens and well-developed forms are at hand ; but when in small 

 fragments, even chemical analysis will fail, and yet each fragment 

 will possess the peculiar molecular arrangement in which the one 

 species will differ from the other, and in this case pyroxene from 

 amphibole. 



It is well known that light in its passage through any medium is 

 modified in its velocity, direction, and vibrations. These various 

 modifications of transmitted light are the effect, in part, of the 

 molecular arrangement, and these effects are constant and char- 

 acteristic. They are therefore reliable when used in the identifi- 

 cation of crystalline compounds, and just as much so as chemical 

 tests, while in many cases they are much less troublesome in their 

 application. According to the accepted theory, light is propagated 

 in a medium which heretofore has been purely imaginary, but at 

 the present time evidence is being brought forward, and from sev- 

 eral sources, which would seem to prove the actual existence of this 

 imaginary medium, the ether. Light is propagated and is the 

 effect of very rapid oscillations or electric polarization of the ether. 

 These oscillations are periodical and transverse to the ray of light 

 or direction of transmission. They are exceedingly rapid altera- 

 tions of the electromagnetic conditions of the ether, which vibrate 

 back and forth, or rotate in a plane at right angles to the direction 

 of propagation. 



That light is an electric effect is substantially proven from its 

 analogy to the electric waves used in the transmission of wireless 



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