Review of Dana's Mineralogy. 371 
attending crystallization. When a right rhombic prism of sulphate of 
zinc is heated to 126° F., certain points in its surface become opaque, 
and from these points bingiles of crystals shoot forth, in the interior of 
the specimen ; and ina short time, the whole is converted into an ag- 
gregate of these crystals diverging from several centres on the surface 
of the original crystal. These small crystals thus formed at 126° F., 
are oblique rhombic prisms; and the same form may be obtained by 
evaporating a solution, at this temperature, or above it. Sulphur crys- 
~fallizes from fusion in oblique rhombic prisms, while the common form > 
obtained by evaporation is a rhombic octahedron. Rose has obtained 
crystals of arragonite by evaporating a solution of carbonate of lime 
to dryness by means of a water bath, and crystals of cale spar by per- 
mitting the solution to evaporate in an open vessel at the ordinary tem- 
perature. The crystals of arragonite were minute six-sided prisms 
and double six-sided pyramids. They change to rhombohedrons of 
calc spar if left moist; but if washed and dried at once, they remain 
permanent. By exposing arragonite to a low temperature, the crystal 
falls to pieces, in consequence of the change to calc spar which takes 
place ; or if the prisms hold together, they consist, after the change, of 
an aggregate of minute particles of calc spar.* Artificial arragonite 
has been observed in the interior of a copper boiler used to supply hot 
water for household purposes at Port Eliot Cornwall. The crystals © 
Were minute six-sided prisms, and were attached at base to the surface 
Supporting them.* ‘Breithaupt has described a carbonate of lime from 
@ greenstone rock near Zwickau, which consists of alternations of lay- 
ers of arragonite and calc spar; and he suggests that the one may id 
4 winter and the other a summer deposit.} 
*Dimorphism appears therefore to be owing to the different circum- 
stances attending crystallization. Temperature appears to be the main 
cause ; but it is possible that the nature of the solvent, or the presence 
of some accidental ingredient in the solution, or the electrical state of 
the support, may have some effect in changing the molecules; but in 
general the only effect of these causes is to produce secondary planes. 
Rose did not succeed in obtaining arragonite crystals by mixing a stron- 
tian salt with the solution of lime, and supposes that the strontia in ar- 
Tagonite has nothing to do with producing the rhombic form.” 
The foregoing views are worthy of the most careful attention, 
Particularly in some cases, where their application has not here- 
tofore been looked for. We might cite for instance the species 
URES Bikgiege ce a 
* Rose, Lond. and Ed. Phil. Mag. 3d ser. XII, 465. 
t Lond. and Ed. Phil. Mag. 3d ser. XII, 330; 1841. 
” $ Pogg..LI, 506; 1840. 
