POLARISATION OF LIGHT. 



was highly interesting, as it displayed to 

 the eye not only the law according to 

 which the intensity of the polarising 

 forces varied in different parts of the 

 crystal, but also the variation in the na- 

 ture of the tints, and the connection be- 

 tween these two classes of phenomena. At 

 the points in the diagonal m n, opposite 

 to a and b of the crystal, the tints rose to 

 the seventh order of colours ; at other 

 two places, opposite to c and d, they rose 

 only to the sixth ; while near the sum- 

 mits, at m and n, they descended so low 

 as the fourth order. Hence it follows 

 that the four curvilineal segments, fig. 

 72, are next to these in intensity ; that 

 the central portions of the squares are 

 again inferior to these; and that the 

 weakest polarising force is near the sum- 

 mits of the prism. At a and b, the fourth, 

 fifth, and sixth fringes have a singularly 

 serrated outline, exhibiting in a very in- 

 teresting manner the sudden variations 

 which take place in the polarising forces 

 of the successive laminae. 



"Having thus described the structure 

 and properties of the tesselated apophyl- 

 lite, it becomes interesting to inquire how 

 far such a combination of structures is 

 compatible with the admitted laws of 

 crystallography. The growth of a crys- 

 tal, in virtue of the aggregation of minute 

 particles endowed with polarity and pos- 

 sessing certain primitive forms, is easily 

 comprehended, whether we suppose the 

 particles to exist in a state of igneous 

 fluidity or aqueous solution. But it is a 

 necessary consequence of this process 

 that the same law presides at the forma- 

 tion of every part of it, and that the 

 crystal is homogeneous throughout, pos- 

 sessing the same mechanical and physi- 

 cal properties in all parallel directions. 



" The tesselated apophyllite, however, 

 could not have been formed by this pro- 

 cess. It resembles more a work of art, 

 in which the artist has varied, not only 

 the materials, but the laws of their com- 

 bination. 



" A foundation appears to be first laid 

 by means of an uniform homogeneous 

 plate, the primitive form of which is 

 pyramidal. A central pillar, whose sec- 

 tion is a rectangular lozenge, then rises 

 perpendicularly from the base, and con- 

 sists of similar particles. Round this 

 pillar are placed new materials, in the 

 form of four trapezoidal solids, the pri- 

 mitive form of whose particles is pris- 

 matic, and in these solids the lines of 

 similar properties are at right angles to 



each other. The ciystal is then made 

 quadrangular by the application of four 

 triangular prisms of unusual acuteness. 

 ,The nine solids, arranged in this sym- 

 metrical manner, and joined by trans- 

 parent veins, performing the functions 

 of a cement, are then surrounded by a 

 wall, composed of numerous films, de- 

 posited in succession, and the whole of 

 this singular assemblage is finally roofed 

 in by a plate exactly similar to that 

 which formed its foundation. 



" The second variety of the tesselated 

 apophyllite is still more complicated. 

 Possessing the different combinations of 

 the one which has just been described, 

 it displays, in the direction of the length 

 of the prism, an organization of the most 

 singular kind. Forms, unknown in crys- 

 tallography, occupy its central portion; 

 and on each side of it particles of simi- 

 lar properties take their place, at similar 

 distances, now forming a zone of uniform 

 polarising force, now another increasing 

 to a maximum, and now a third, de- 

 scending in the scale by regular grada- 

 tions. The boundaries of these corre- 

 sponding though distant zones are 

 marked with the greatest precision, and 

 all their parts as nicely adjusted as 

 if some skilful workman had selected the 

 materials, measured the spaces they 

 were to occupy, and finally combined 

 them into the finest specimen of natural 

 mosaic. 



" The irregularities of crystallisation, 

 which are known by the name of Made, 

 orHemitrope forms, and those compound 

 groups which arise from the mutual 

 penetration of crystals, are merely acci- 

 dental deviations from particular laws, 

 which govern the crystallisations in which 

 they occur. The aberrations themselves 

 testify the predominance of the laws to 

 which they form exceptions, and they are 

 susceptible of explanation, by assuming 

 certain polarities in the integrant mole- 

 cules. The compound structure of the 

 apophyllite, however, cannot be referred 

 to these capricious formations. It is it- 

 self the result of a general law, to which 

 there are no exceptions, and when more 

 deeply studied and better understood, it 

 must ultimately lead to the introduction 

 of some new principle of organisation, 

 of which crystallographers have at pre- 

 sent no conception. 



" The difficulty of accounting for the 

 formation of apophyllite is in no way 

 diminished by giving the utmost licence 

 to speculation, We cannot even avail 



