depending on their Internal Changes. • 277 



To Prof. Mitscherlich we are likewise indebted for the fol- 

 lowing curious fact, (Edin. Journal of Science, vol. iv. p. 301.) 

 When a crystal of the salt, with a form belonging to the pris- 

 matic system, is heated above a temperature 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 surrounding mass while 

 they continue to grow. In a short time the whole is converted 

 into an aggregate of those crystals, diverging 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 lamellae of the specimen. 

 This circumstance proves the identity of the chemical compo- 

 sition 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 to crystallization on a warm 

 stove a highly concentrated solution of the salt, well covered 

 and wrapped up. The remaining liquid having been decanted, 

 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 

 of the solution remaining on the surface of the hemi-prismatic 

 crystals. Change of temperature is the only agent upon which, 

 in both cases, the change of the position of particles within the, 

 solid mass depends. 



The isomorphism of zinc and magnium is remarkably dis- 

 tinct in the regular forms, with all their peculiarities, and in the 

 cleavage of their sulphates. But it extends even to the phe- 

 nomena described above of sulphate of zinc. They both give 

 exactly the same results. 



The specific gravity of the hemi-prismatic species has not 

 been ascertained. It is very probable that it does not mate- 



