410 Prof. Whewell 07i Isomorphism, 



C + 2c' being 105° 5', and that of M + 2c' being 107° 25', 

 the angle of any mineral of the kind C, M + 2c', will be in- 

 termediate between the two angles, exactly in proportion to 

 the mixture of C and M in its composition. Mr. Brooke 

 has pointed out that this doctrine is not reconcilable with the 

 measured angle and ascertained composition of Breiinnerite. 

 Indeed it does not appear that M. Beudant has attempted to 

 establish his law by a series of measures and analyses, and it 

 certainly cannot be established in any other way. It would 

 be highly interesting and important to mineralogy to deter- 

 mine the rule according to which the alteration of the ingre- 

 dients affects the angle of the crystal ; but it is not likely that 

 this will be done by assuming the first arbitrary conjecture 

 as the true law of natui'e. 



It is possible that some of the groups held to be isomor- 

 phous may be plesiomorphous only; and that more exact 

 measurements may detect differences in angles hitherto sup- 

 posed to be equal. This discovery would by no means di- 

 minish the importance of the study of those substitutions of 

 ingredients by which resembling minerals are related; and 

 this study is undoubtedly one on which the progress of mi- 

 neralogy will much depend. 



V. On Dimorphous Substances. 



It is well known that certain substances exhibit the curious 

 phenomenon of tv/o different fundamental crystalline forms, 

 without our being able to detect any difference in the che- 

 mical constitution of the bodies. Thus C + 2c' appears as a 

 rhombohedron in calc-spar, and as a prism in arragonite. 

 Sulphur is either a right or an oblique prism *. Such dimor- 

 phous substances certainly offer a difficulty in any attempt to 

 connect the crystalline form with the chemical constitution ; 

 but they present no peculiar difficulty to the theory of isomor- 

 phous or plesiomorphous substitution. The theory of essential 

 ingredients is at least as much embarrassed by these cases. 

 If all that is essential to rhombohedral calc-spar be a certain 

 proportion of lime and carbonic acid ; — if this essential com- 

 position can impress the character of such calc-spar on 50 per 

 cent, of silica; — how is it that sometimes, when there is not 

 \ or \ per cent, of foreign admixture, we have quite a different 

 crystalline form, as in arragonite? Certainly the theory of ac- 

 cidental mixture has here no superiority ; while that of iso- 

 morphism does on the other hand fall in exactly with the fact, 



• From Mr. Brooke's observations (Ann. Phil. Dec. 1823) it appears to 

 follow that sulphate of nickel is dimorphous, crystallizing both in right 

 rhombic prisms and in square prisms. 



that 



