Mr. Brooke on Isomorphism. 169 



with the substances already enumerated, ought therefore to 

 replace or be replaced by any of them without occasioning 

 any change in the form of the compound. Therefore eicdya- 

 lite, whose form is a rhomboid, might be composed wholly of 

 silicate of zirconia, a compound which occurs as a square prism 

 in zircon; or it might be composed wholly of silicate of' iron 

 or magnesia, which is the composition of olivine, whose cry- 

 stalline form is a right rhombic prism. Hence the same che- 

 mical formula, denoting a silicate of any or all of nine ele- 

 ments which are assumed to be isomorphous, represents the 

 composition of at least three different minerals, whose re- 

 spective forms are a square prism, a right rhombic prism, and 

 a rhomboid. 



But there are other difficulties in the way of isomorphism, 

 which do not appear to have been considered, and which arise 

 out of results that are at variance with the assumption of any 

 fixed relation between the crystalline form of a mineral and 

 the forms of its constituent molecules. 



The crystalline form of sulphur is either a right or an 

 oblique rhombic prism, according to the circumstances under 

 which it crystallizes. Hence, imless some new element not 

 hitherto discovered enters into the composition of one of these 

 forms, the crystalline form of even a simple substance de- 

 pends upon the mode of arrangement of the molecules in the 

 crystal as well as upon their figure. 



The crystalline form of silver, of copper, and of bismuth, is 

 a cube, and the sulphurets of these metals, (as sulphur must 

 be regarded as isomorphous in respect of each,) might be ex- 

 pected to present similar forms. But the sulphuret of silver 

 is a cube, that of copper a right rhombic prism, and that of 

 bismuth also a right rhombic prism, but differing in its angle 

 from the sulphuret of copper. — Arsenic combines with sul- 

 phur in two different proportions, and producing different 

 primary forms. Hence the proportion of a common element, 

 and therefore an isomorphous one, occasions a change, even 

 in the system of crystallization. 



Sulphuret of silver and sulphuret of lead are both cubes, 

 and hence silver and lead should be isomorphous. But chlo- 

 ride of silver is a cube, and cliloride of lead a rhombic prism. 



These remarks arc thiown together for the purpose of 

 calling the attention of those by whom the theory has been 

 perhaps over-hastily received, to a closer investigation of its 

 merits; that if it be really founded on sound principles, its ap- 

 parent inconsistencies may be explained, and if otherwise, that 

 it may not remain an impediment to the progress of analytical 

 research into the true chemical composition of minerals. 



iV. .S'. Vol. 10. No. 57. Sept. 1831. Z XXII. Ex- 



