', 



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66 Connection of Chemical Forces with Polarization of Light 



color, blue, green, orange, red. It was shown, however, tfiat 

 another specimen exhibited these colors in the reverse order of 

 red, orange, green, blue, plum-color, red; in which order the 

 former spechnen of quartz produced these colors when the tour- 

 maline was turned in the opposite direction. - Hence these are 

 termed right and left-handed polarizations. The whole of these 

 phenomena were attributed to a complicated set of movements 

 of the light within the crystal, the resultant of which was prac- 

 tically a rotation of the plane in which ,the ray was capable of 

 being reflected, — so that the thicker the crystal, the further round 

 the tourmaline had to be turned to permit the ray to pass it, or 

 to be eclipsed, as the case might be. The opposite order of tha 

 colors was explained by the fiction of supposing the one to be 

 the effect of a left-handed thread to the screw and the other of a 

 right-handed thread characterizing the spiral in which the plane 

 of polarization was supposed to rotate. Of course this was only 

 a popular way of explaining the phenomenon, it being really due 

 to a more complicated series of movements which were explained 

 by Fresnel in the most triumphant manner by the wave theory* 

 The color was accounted for by the idea of the red following 

 a longer spiral (having a coarser thread to the screw) than that of 

 the orange, this than the yellow, and so on up to the violet. 

 Without the tourmaline in front all would emerge and form white 

 light ; but the tourmaline only allows such rays to pass it as are 

 capable of passing it in its particular position ; i e. only such, the 

 rotation of whose plane has brought them round to the position 

 of the plane in which the tourmaline lets the light through. 



The singular fact of amethyst being a combination of alternate 



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layers of right and left-handed quartz was then exhibited, both 

 by throwing the image of the alternate layers on the screen, and 

 afterwards by showing that the general effect of a traversing po- 

 larizing beam was to produce a neutrality of action. Other sub- 

 stances, however, produce phenomena of circular polarization. 

 Uncrystallized, fused tartaric acid, and barley sugar, &-c., produce 

 them; and these bodies when dissolved, and many more in the 

 form of liquids also, do so, some of which were exhibited. But 

 the silica of which quartz consists entirely loses this property 

 when divested of its crystalline character, whether artificially or 

 in its natural state, as calcedony, opal, <fcc. All other bodies re- 

 tain it so long as their chemical molecule retains its individuality 

 of character. 



The next point to be made clear, was the meaning of that form 

 of crystallographic development called "Hemihedrism." Haiiy's 

 great law was, that similar edges or angles were always similarly 

 modified. The nature of similarity in edges Of* angles was then 

 pointed out, and the general idea of many crystallographers of a 

 sort of nucleus or primitive form existing on which the crystal 



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