196 SEVENTH REPORT—1837. 
the external characters of true crystals. Among the latter may 
be mentioned the cubes of quartz found at Rochette, in the pro- 
vince of Liege, which are so perfect as to have been mistaken 
by Haiiy for the primary rhomboids*, and which are inferred 
to be parasitic chiefly from the occurrence in the same locality 
of hollow prisms, obviously casts of previous crystals of calc 
spar. Similar observations apply to many of the quartz cry- 
stals found at Haytor, while the want of internal structure is the 
chief reason why the hornstones and steatites of Germany, the 
cubical chalcedonies of Transylvania, and the rhomboidal from 
Iceland, are classed among pseudomorphous crystals. 
The octohedral peroxide of iron (Martite) is one of those 
minerals which retains the cleavage as well as the form of the 
mineral (magnetic iron) from which it is derived. The perfec- 
tion of these crystals has induced Kobellt to consider them as 
an example of dimorphism, though, perhaps, rather hastily. It 
is not unlikely that some of the supposed parasitic may be true 
crystals ; but the possession of a distinct cleavage is not alone 
sufficient to prove that any given crystals are so. Calc spar, 
after being calcined and deprived of its carbonic acid, still re- 
tains its form and cleavages. 
42. We can imitate nature in the production even of apparently 
perfect changed (epigene) crystals. Native crystals of peroxide 
of iron, heated in a current of sulphuretted hydrogen, give at 
212° F. sesquisulphuret Fe,S,, and at a higher temperature. 
Bisulphuret of iron FeS,, and the new compounds retain the 
lustre and cleavage of the original crystals.{ A similar result, 
without change of form, is obtained from the carbonate of iron. 
Crystals of bicyanide of mercury at ordinary temperatures may 
by the same means be converted into black shining crystals of 
bisulphuret. By simple exposure of the salt to the air, me- 
tallic gold may be obtained in the form of the double chloride 
of gold and ammonia. Nitrate of silver occasionally undergoes 
a similar decomposition. Many of the salts of lead, silver, and 
other metals may likewise, by the agency of sulphuretted hy- 
drogen, be converted into sulphurets without losing their form, 
and very many of the hydrated salts of the earths and metallic 
oxides part with their water without suffering disintegration. 
Still, in connexion with these numerous changes, natural 
and artificial, one question suggests itself. Are there any limits 
to the number of forms which the same substance, a metallic. 
* Geologie de Liege. Par Dumont. P.147. 
¢ Neues Jahrbuch der Chim. und Phys, (1831) vol. ii. p. 195, 
* Berzelius, Arsberiittelse, 1826. 
