COMPOUND MINERALS. 83 



On the other hand the composition is irregular, if the forms are 

 not connected in the manner above described ; but rather in such 

 a manner as not to give rise to any regular or symmetrical forms. 

 Individuals joined in this way are said to be aggregated. 



There are compound minerals, which affect regular external forms, 

 although their composition is in fact irregular. The regularity of 

 the form in such cases evidently does not follow from the composi- 

 tion, but it must originate from something which is foreign to the 

 mineral. Compositions of this sort cannot be called regular in the 

 sense of the word now explained. 



. 72. REGULAR COMPOSITION OF TWO INDIVIDUALS. 



The regular composition of two homogeneous individu- 

 als, joined in one crystalline form, has been called a Tivin 

 crystal. 



The peculiar character of Twin crystals is, that the face of com- 

 position is in close and exact connexion, and that the plane in which 

 they unite is either parallel to the face, or perpendicular to the edge 

 of a form belonging to the series of crystallization of the species. 

 The situation of the two individuals connected, is conceived of, if 

 we suppose both to be in a parallel situation, and then one of them 

 to be turned round a certain line under an angle of 180, while the 

 other remains unmoved. This line is termed the axis of Rev- 

 olution, and it is possessed of a determined direction, being either 

 perpendicular to the face of composition, or it coincides with 

 this face, while it is paiallel to one of the ordinary axes of the 

 individual. The angle of 180 is the angle of Revolution. The 

 resulting form is frequently called a hemitrope; a term expressive 

 of the idea of the demi-revolution which is supposed to have taken 

 place.* The term made .was first applied to such forms by Rome 



* It is scarcely necessary to say that it is not really believed that reg- 

 ular compound crystals were actually formed in this way ; our only rea- 

 son for making use of such language is the facility it affords in compre- 

 hending the structure of these crystals. 



The term hemitrope, however, is rather confined in its application 

 to twin crystals, including only such as are capable of being explained, 

 by supposing a single crystal to be bisected in a determined direction, 

 and one of the halves to be turned in the plane of bisection through 180. 



