74 Mr HAIDINGER on the Parasitic Formation 



decomposition of the substances themselves, will effloresce, and 

 yield sulphate of iron. Heat, and the disengagement of power- 

 ful acids, in the neighbourhood of active volcanoes, and burning 

 coal-seams, give rise to the formation of a number of new sub- 

 stances, while those which existed before are destroyed. Usually 

 even the last trace which could lead us to discover, from what 

 source the new substances draw their origin is lost ; but there 

 are examples in which the form, peculiar to the crystals of the 

 decomposed substances, is entirely preserved, while the rest of 

 their properties undergo more or less notable changes. The 

 consideration of these constitutes the especial object of this 

 communication. 



Mineral productions of the description alluded to, have been 

 comprised by most authors under the idea of pseudomorphoses, a 

 name expressive of their nature, if we attend only to the etymo- 

 logy of that word, since, indeed, the form is not the one be- 

 longing to the substance ; but not agreeing with the definition 

 given of them, which requires that they should be produced by 

 the deposition of crystals in an empty mould, left in the sur- 

 rounding mass, by a decomposed crystal of another species. 

 The names proposed by HAUY, epigenies > and by BREITHAUPT, 

 tnetamorphous crystals, are more objectionable than the usual 

 denomination, if we regard etymology ; and as they were nei- 

 ther circumscribed by accurate definitions, nor applied exclu- 

 sively to this kind of formation of substances, we need not be 

 over careful in making use of any of them, by preference, 

 particularly since difficulties might arise from the circum- 

 stance, that the effect of the decomposition is not always the 

 same, and that only some cases will be found, in which the en- 

 tire form is preserved, while it is considerably impaired, though 

 still recognizable in others, and frequently altogether lost. If 

 we were to select a particular word for this kind of formation, 

 the most appropriate expression would be parasitic, to denote the 



