76 Mr HAIDINGER on the Parasitic Formation 



substances is expressed in the formula by BERZELIUS, of Zn S 2 

 + 14 Aq, 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. 



To Professor MITSCHERLICH we are likewise indebted for the 

 following curious fact *. When a crystal of the salt, with a 

 form belonging to the prismatic system, is heated above a tem- 

 perature of 126, we may observe certain points at its surface 

 become opaque, and then bunches of crystals shoot out from these 

 points in the interior of the original specimen. Since this is 

 transparent, and the newly formed crystals almost opaque, or of 

 a milky whiteness, they are easily distinguished from the sur- 

 rounding 'mass, while they continue to grow. In a short time, 

 the whole is converted into an aggregate of those crystals, di- 

 verging from several centres, that are situated on the surface 

 of the original crystal. No water escapes during this process, 

 except what may have been accidentally included in the lamel- 

 lae of the specimen. This circumstance proves the identity of 

 the chemical composition of the two species, one of which is 

 formed within that space, which is occupied by the other up to 

 the very moment of the decomposition of the latter, which gives 

 rise to the new substance. 



I have obtained crystals of the hemi-prismatic species, more 

 transparent than usual, by exposing, on a warm stove, a highly 

 concentrated solution of the salt, well covered and wrapt up, 

 to crystallization. The remaining liquid having been decant- 

 ed, the crystals obtained were dried and slowly cooled in the 

 same manner. If they are taken out of the solution singly, 

 and cooled rapidly, they soon lose their transparency, and, when 

 broken, frequently present an aggregate of crystals of the pris- 

 matic species, which is likewise immediately produced by drops 



* Edinburgh Journal of Science, vol. iv. p. 301. 



