1818.] Crystalline Form and Chemical Composition. 2G7 



were obtained ; and when the proportions were equal, the form 

 was that of the sulphate of copper ; but on adding a very minute 

 quantity of sulphate of iron, the rhomboidal crystal was imme- 

 diately produced. 



On mixing various other salts, equally remarkable results 

 were not obtained ; and it was generally necessary to add, at 

 least, one half of any given salt, in order to obtain its peculiar 

 crystal. 



In the course of these experiments, it was remarked that com- 

 pounds of the same acid, with different bases, mixed more 

 readily than compounds of the same base with different acids. 

 Where the bases and acids both differed, the mixture took place 

 iu only small quantity. 



These results appear to lead to the following important con- 

 clusions, relative to the classification of minerals. 



In a chemical compound where no indication of mechanical 

 mixture * appears, it has been shown that a foreign ingredient 

 may be present in only indefinite and small proportions, and yet 

 exercise the important function of determining the crystalline 

 form of the compound. The same law may be regarded as 

 applying to mineral compounds also, and will explain the anoma- 

 lies frequently observed to exist between their chemical analysis 

 and crystallographical character ; incompatible crystallizations 

 having been found composed of similar elements, and identical 

 forms of widely differing elements. 'And it is not extraordinary 

 that this should be the case, considering the variety of substances 

 connected together in the same formation. 



But as the chemical products of the experiments already 

 detailed were mere mixtures of the several salts employed, and 

 as the mineral whose analysis is at variance with its crystalline 

 form may also be considered as a mixed compound, how are 

 these to be arranged in our cabinets ? Let us first consider the 

 saline crystals already described. These may be regarded as 

 composed of sulphuric acid, and the oxides of copper and iron, 

 or zinc and iron, or copper, zinc, and iron. The chemist may 

 consider these as double or triple salts ; but as it is known that 

 the single salts employed may be in solution together without 

 mutual decomposition, and as the sulphuric acid in the mixture 

 is found in the proportion necessary to saturate separately 

 the oxides employed, the compound salts produced must be 

 regarded as simple mixtures only. In classing these chemically, 

 the place assigned to each would be regulated by the salt found. 

 in excess ; but in the instance of mixtures of equal parts of two 

 or more salts, the chemist would be reduced to the necessity of 



* In the term mechanical mixture, that species is not included of which the 

 Fontainbleau saudstone may be considered a type: the grains of quartz there 

 being merely invested, aud held together by the carbonate of lime, which has crys- 

 tallized round them 



