THE 



PHILOSOPHICAL MAGAZINE 



AND 



ANNALS OF PHILOSOPHY. 



[NEW SERIES.] 



SEPTEMBER 1831. 



XXI. On Isomorphism. By H. J. Brooke, Esq. F.R.S. L.S. 

 4' G.S.* 



SOME new and important theoretical doctrines relative to 

 the chemical composition of minerals have, within a very 

 few years, been advanced, and very generally adopted, on the 

 continent. They have also been favourably received by some 

 persons in this country, apparently on the credit of the pro- 

 posers, and without much, if any, inquiry into the merits of 

 the doctrines themselves. 



The theory of Isomorphism was first introduced by Mr. 

 Mitscherlich, and was founded upon the observation of a very 

 near agreement in the forms of crystals of certain chemical 

 compounds. Thus sulphate of iron and sulphate of cobalt 

 crystallize in oblique rhombic prisms, and apparently of the 

 same measurements ; and hence the atoms or molecules of the 

 oxides of iron and of cobalt are assumed to be isomorphous, or 

 to possess the same identical forms ; and the same apparent 

 agreement having been found to subsist among the arseniates 

 and phosphates of lead, the molecules of the arsenic and phos- 

 phoric acids are also assumed to be isomorphous. 



From other observations it appeared, that barytes, strontian, 

 and oxide of lead ought to be isomorphous ; and hence that 

 the salts of those substances, when produced by the same 

 acid, ought also to be isomorphous. 



But on examining the sulphates and acetates it was disco- 

 vered that their respective angular measurements were not 



• Communicated by the Author. 

 N.S. Vol. 10. No. 57. Sept. 1831 . Y alike, 



