On Crystallography, 357 



B D C (tig. 45), which was one of the halves of the supe- 

 rior base (rtij. 41), is transported into the lower part (fig. 

 45), and forms a re-entering angle, with the fixed triangle 

 abd. 



We can easily conceive that the plane of junction D B Zi cf 

 of the two halves of a rhomboid, is situated iikc a face pro- 

 duced in virtue of a decrement by one range on one or other 

 ofthe ridges Art, Cc, (fie. 44); and thus the manner in v;hich 

 these two halves join is in strict relation to the structure. 



Now if we imagine a secondarv form which has for its 

 nucleus the same prism, and if we suppose that it has been 

 cut in tlie directions of the plane D B Id, and that one of 

 its halves is reversed in such a manner that the half of the 

 nucleus which corresponds with it assumes the same posi- 

 tion as in the preceding case, the assortment might be such 

 that there is still a re-entering angle on one hand and a 

 saliant ancjle on the other, which will result from the mu- 

 tual incidences of the faces produced by decrements. 



In certain cases the |)lane of junction, on which the two 

 halves of the crystal are jonied, is situated parallel to one 

 of the faces of tlxe nucleus, and the assortment does not 

 admit of presenting a rerentering angle opposed to a saliant 

 angle. 



J have given to these reversed crystals the name of hemi- 

 tropes*, and I call hem'itrope crystals such as are thus re- 

 versed. They seem to indicate a polarity in the integral 

 uiolecules, as I shall explain more at length under the head 

 of spiiiell. We shall also find uilder the articles/tW 5par, 

 pyiQxtne, oxidated tin, &:c., remarkable examplc^i of hemi- 

 tropes. 



Another accident extremely common, is the manner in 

 which grouped crystals are inserted into each otht-rf. This 

 kind of apparent penetration is subject to so many diver- 

 sities, that frequently, among crystals of the same group, 

 vve do not find two relative positions resembling each 

 other. We must except, however, staurotide, the prisms 

 of which, as we shaU see, have their junction Imuled to 

 two particular cases, which we shall make known when 

 treating of the crystallization of lliis substance. 



But although in general the positions in grouped crystals 

 are infinitely variable, we find, on a closer examination, 



• Romd de rule calls them nirt</ej. But this tiame bein^ already applied 

 to a very co.v.irum species of m!iiiTMl, I have tluuiglit p"i)pcr to avoid tlie 

 double a|)plio.itioii oj titc term. 



■f 1 he German word d.use it sometimes used to de!ij;:ia;e a group of 

 crytiaLt. 



Z 3 that 



