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LXVi On the Deformation of the " Rings and Brushes" 

 as observed through a Spath Hemitrope. By B. N. 

 Chuckerbutti, M.Sc, Assistant Professor of Physics, 

 Calcutta University *. 



TPlate IX.] 

 1. Introduction. 



(CRYSTALS are frequently found which are obviously of 

 J a composite character, that is, are composed of more 

 than a single individual crystal of the same substance and in 

 which the parts belonging to different individuals are united 

 in a definite and regular manner, the peculiar mode of union 

 being characteristic of the substance. The true nature of 

 the composite structure is often betrayed by the presence 

 of re-entrant angles, forming notches (as in Diamond 

 crystals), arrow-head shapes (as in Grypsum), knee-shapes 

 (as in Oassiterite), cruciform (as in Staurolite), or heart- 

 shapes (as in Fluorspar) ; but not in frequently the two or 

 more individuals are so intimately blended, that the ap- 

 pearance at first sight is that of a single individual crystal. 



The law defining the manner in which the composition 

 occurs is usually stated as follows : — 



The two individuals are first supposed to be arranged 

 parallel to each other, with one face of each in mutual con- 

 tact, and then in order to produce the twin, one individual 

 is supposed to be rotated to 180°, upon a plane, which is 

 called the twin-plane, and about the normal to the plane as 

 axis, which is called the twin-axis. But it is necessary 

 to point out here, that in the actual process of the forma- 

 tion of the twins, no such two different crystals are brought 

 into contact and then one turned through 180°, as stated 

 above. The orientation takes place amongst the crystalline 

 molecules of their own accord during the process of for- 

 mation. Hence, the twin-plane is not necessarily always the 

 plane of contact. In general, the plane of twinning may 

 be any actual or possible face, except obviously a plane of 

 symmetry of the crystal. 



The twinning may be either of Primary or of Secondary 

 origin. The former refers to the case when the formation 

 takes place of itself without the intervention of some 

 external agency during the process ; but when the twin- 

 ning takes place subsequently to the original formation of 



* Communicated by the Author. 



