Nevertheless, we sometimes find in nature that crystals, after arriving at what may be regarded as, in some sort, their maturity, are, owing to a change of the conditions under which they were formed, gradually decomposed. 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 circumstances. We find, for example, Pseudomorphism, (or _false-form_), as this class of phenomena is named, occurring 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 examination 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, produce 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 (_like-form_)[37] peculiarities are exceedingly common, and the discoverer of the phenomena, Mitscherlich, announced the above law. It cannot, however, be regarded as a philosophical expression of the fact, and requires reconsideration--chemical elements of a dissimilar character may have the same law of aggregation, and thus produce the same form, without having any relation to the number of atoms.