in Reply to Mr. Brooke. 409 



each other in that species. The general laws of isomorphous 

 elements ai"e as yet imperfectly understood; and when we 

 have but one analysis, it must often be difficult or impossible 

 to decide whether any two ingredients are in definite propor- 

 tions, or in isomorphous or plesiomoi'phous relation. 



There are many, therefore, of the general arguments em- 

 ployed by Mr. Brooke which involve reasonings unauthorized 

 by anything yet established as part of the doctrine of isomor- 

 phism. 



IV. On Plesiomorphous Gtoups. 



We find in many cases minerals which, agreeing in the 

 form of their chemical formula, and differing in one or more 

 of their elements, agree also in their system of crystallization, 

 but differ slightly in their angles and in their physical pro- 

 perties. Thus carbonate of lime, of iron, of magnesia, 

 (C + 2c', Fe + 2c', M + 2c',) and mixtures of these, all belong 

 to the rhombohedral system, and have a certain approximate 

 agreement of hardness, scratch, lustre, cleavage, &c. which 

 gives them a general resemblance; while their angles vary 

 from 105° 5' to 107° 40', according to the ingredients. Such 

 minerals have been termed plesiomorphous j and it is evi- 

 dent, as Mr. Brooke allows, that if we can range minerals 

 into such groups, we shall have made an important Step 

 in mineralogy. " If," he says, " the class of primary forms 

 can be indicated with certainty by the chemical composition 

 of a crystallized bod}', a benefit will so far have been conferred 

 on science." Several such groups appear to have been already 

 detected : thus, besides the rhombohedral group of the form 

 R + 2c' already noticed, we have the carbonates of baryta, 

 strontia, lead, and lime (arragonite), which are prismatic ; the 

 sulphates of the same bases, which are also prismatic ; and 

 we have a similar group in the various species into which 

 mineralogists have subdivided the siliceous minerals formerly 

 included under the name Jelspar. Mr. Miller has suggested 

 the possibility of another gi'oup, containing the silicates of 

 zinc, of iron (yenite), of magnesia and iron (peridot) (Camb. 

 Trans, vol. iii. p. 419). A circumstance which gives additional 

 importance to these groups is, that besides the agreement in 

 the system of crystallization, and the close approximation of 

 their angles, they are always Ibund to possess several other 

 physical properties in common, or with slight differences. 



M. Beudant has asserted a proposition concerning the 

 dependance of the angle on the chemical constitution in these 

 groups, which is probably not true; as Mr. lirooke has very 

 satisfactorily shown. He has maintained, that the angle of 



N.S. Vol. 10. No. CO. Dec. 1831. 3 G C + 2c' 



