276 Mr Haidinger on the Parasitic Formation of Minerals, 



stance; but the definition given of them requires that they 

 should be produced by the deposition of crystals in an empty 

 mould, left in the surrounding mass, by a decomposed crystal 

 of another species. The names proposed by Haiiy, epigenies, 

 and by Breithaupt metamorphous crystals, are more objection- 

 able, in an etymological point of view, than the usual denomi- 

 nation; and as they were neither circumscribed by accurate 

 definitions, nor applied exclusively 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 circumstance, that the effect of the decomposition 

 is not always the same, and that only some cases will be found 

 in which the entire 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 intrusive nature of the new compounds, 

 in prej udice of those which existed before. 



The facts met with in nature are highly interesting, and de- 

 serve the particular attention of naturalists, in order to com- 

 plete the series in which they are here considered. 



I. Changes in Substances having the same composition. 

 The chemical mixture, essential to the common vitriol of 

 zinc, is a dimorphous one, or one of those which are capable of 

 crystallizing in two different kinds of forms, incompatible with 

 each other. The most common of them is derived from a 

 scalene four-sided pyramid, which has its three axes perpendi- 

 cular to each other, and is comprised in the prismatic system. 

 It is deposited from solutions not sufficiently concentrated to 

 form a crystalline skin on their surface, and at temperatures 

 below 126° Fahrenheit. Above that temperature a highly con- 

 centrated liquid yields crystals of another species, whose forms 

 are derived from a scalene four-sided pyramid, having its axis 

 inclined on the base, and belonging to the hemi-prismatic 

 system. The chemical composition of both substances is ex- 

 pressed in the formula by Berzelius, of Zn S* + 14Aq, which 

 is derived from Mitscherlich's analysis of the prismatic species, 

 giving oxide of zinc 27.67, sulphuric acid 27.57, and water 

 44.76. 



