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Sect. VIIL] 



MINERALOGY. 



239 



this 



pro 



already remarked that carhonate of lime has 



perty, and that a difference in the heat of the solution 



whence it raa^ 



formed 



to accompany this 



variation of shape. This apparent influence of difference 

 in temperature upon the arrangement of the component 

 particles becomes the more interesting when we couple it 



with the experiments of Mitscherlich.* Right rhomboidal 

 crystals of sulphate of nickel exposed in a vase to the 

 sun, were found changed in the interior, without passing 

 through the liquid state, into octahedrons with a square 

 base, the exterior crust of the original crystals retaining 

 its first form. As previously observed, the natural mi- 

 nerals hitherto ascertained to have two different primitive 

 forms amount to so few as scarcely to interfere materially 

 with the determination of minerals by their crystalline 



form.t 



With isomorphism^ or the replacement of one substance 



by another, so that the resulting crystals are not altered, 



as before stated, the voyager will not be embarrassed 



when he becomes familiar with such substances. 



Mag- 



nesia, lime, protoxide of iron, and protoxide of manganese 

 thus replace each other in any proportion. With the 

 crystals termed pseudomorphotis^ or those which do not 

 belong to the chemical character of the substance thus 

 presenting them, the case is different. There would ap- 

 pear two classes of these bodies ; at least it is convenient 



4 



"^ Mitscherlich, ' Annales de Chimie et de Physique/ tome xxxvii 

 t M. Dufrenoy points out that the two forms of carbonate of lime, 

 carbonate of iron, and carbonate of lead are of the same nature, aU 

 occurring both as vhombohedrons and as right rhomboidal prisms, and 

 adverts to the probability of the carbonates of baryta and of strontia 

 being dimorphous also. ' Traite de Mineralogie/ tome i. p. 204. 



;vi^ 





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