54 PSEUDOMORPHISM, ETC. 



ditions under which they were formed, gradually decom- 

 posed. In our mines we discover skeletons of crystals, 

 and within the hollow shell thus formed, other crystals 

 of a different constitution and figure find nuclei, and the 

 conditions required for their development. Again, to 

 give a striking instance, the felspar crystals of the 

 granitic formations are liable to decomposition in a 

 somewhat peculiar manner. In decomposing, these 

 crystals leave moulds of their own peculiar forms, and it 

 not unfrequently happens, in the stanniferous districts 

 of Cornwall, that oxide of tin gradually fills these moulds, 

 and we procure this metallic mineral in the form of the 

 earthy one. Then we have the curious instances of 

 bodies crystallising in a false form under change of cir- 

 cumstances. We find, for example, Pseudomorphism, (or 

 false-form], as this class of phenomena is named, occur- 

 ring by the removal of the constituent atoms of one 

 crystal, while another set which naturally assumes a 

 different form takes their place, yet still preserving 

 the original shape. It often happens that copper pyrites 

 will, in this manner, exhibit the angles of an ordinary 

 variety of crystallised carbonate of iron. These curious 

 changes may be familiarised by supposing a beautiful 

 statue of gold, from which some skilful mechanic removes 

 particle by particle, and so skilfully substitutes a grain of 

 brass for every one of gold removed, that the loss of the 

 precious metal cannot be detected by any mere examina- 

 tion of its form. 



Crystalline form is not strictly dependent upon the 

 chemical nature of the parts forming the crystal. The 

 same number of atoms, arranged in the same way, pro- 

 duce the same form. Substances much unlike each 

 other will assume the same crystalline arrangement. 

 Magnesia, lime, oxide of cadmium, the protoxides of 

 iron, nickel, and cobalt, combined with the same acid, 

 present similarly formed bodies. These Isomorphic 



