. 138. OF COMBINATIONS. 149 



A mineral occurring in a compound form is nevertheless 

 a simple mineral (. 21.), and may be an individual; for 

 that composition, which is the subject of our present consi- 

 deration, regards only the external form of the mineral. 

 Yet an individual, affecting a compound form, or more than 

 one simple form at a time, may be imagined to represent two, 

 three, or more individuals, if we suppose that all the na- 

 tural historical properties of the individual, with the ex- 

 ception of the geometrical ones, are connected with every 

 one of the simple forms contained in the combination. 



A combination may, in some instances, assume the as- 

 pect of a simple form, being contained under faces which 

 are equal and similar to each other (. 35.). This takes 

 place when two equal and similar forms combine in differ- 

 ent positions, which positions, nevertheless, are always pe- 

 culiar to the system in which these forms are found. Com- 

 binations of this kind may be ascertained to be such, and dis- 

 tinguished from really simple forms, either by the number 

 of their faces, which is greater than that produced by deri- 

 vation, or by their relations and the position, which exclude 

 these apparently simple forms from the series to which, as 

 simple forms, they would necessarily belong. 



The form of a combination is the space contained at the 

 same time within all the simple forms constituting it. 

 Hence, none of the angles of incidence of a combination 

 can be greater than 180, or re-entering angles. Such angles 

 are produced, though not always, if two or more indivi- 

 duals, of the same or of different forms, are connected in 

 different positions; these compositions will be properly 

 considered in . 178. &c. 



The number of forms entering into a combination, is un- 

 determined. There may be only two, but there also may 

 be a great number of them. A combination containing two 

 simple forms is also termed a Unary combination ; one con- 

 taining three simple forms, a triple combination, &c. The 

 exact knowledge of binary combinations is the most in- 

 teresting department of Crystallography, in as far as it 

 refers to compound forms. The knowledge of binary com- 



