REPORT ON MINERALOGY. 
349 
sible, more unknown and obscure than those which determine 
different fundamental forms to different compounds. He has 
examined the circumstances which determine the various modi¬ 
fications which a given fundamental form undergoes ; and has 
proceeded so far as to he able to produce at will one or other 
of certain possible modifications. Thus (. Mineralogie , i. 190.) 
common salt crystallizing in pure water affected almost always 
the cubical form ; if it crystallized in a solution of boracic acid, it 
assumed the form of the cube with truncated angles. Alum in 
nitric acid had the same form; in muriatic acid it was a figure 
of twenty sides, the octohedron and dodecahedron combined, 
the faces of the former being much the larger. An addition 
of alumine to the liquor, produced, in addition to the former 
faces, those of the cube ; in pure water this salt is the simple 
octohedron. Sulphate of iron has commonly a simple form ; 
by adding a few drops of sulphuric acid, more complex forms 
are obtained ; and this rule respecting the effect of the addition 
of acid appears to be extensively true. The sulphate of iron 
mixed with sulphate of copper has its simple form, an oblique 
rhombic prism ; the mixture of sulphate of nickel produced 
the same effect, but that of zinc an opposite one, the crystals 
becoming less simple. It has long been known that common 
salt mixed with urea, affects the octohedron instead of its usual 
form, the cube ; and that in similar circumstances, sal ammoniac 
becomes the cube instead of the octohedron. Alum in a con¬ 
centrated solution of alumine assumes the cubical form ; an oc- 
tohedral crystal of alum placed in such a solution soon assumes 
a cubical form; by being placed again in a solution adapted to 
give octohedral crystals it may be made to assume the octohe¬ 
dron. It is impossible not to be tempted to refer phenomena 
similar to these, occurring, as they so often do, in natural cry¬ 
stals, to similar circumstances which have prevailed when the 
crystals have been forming. 
Several statements of a curious kind have been made con¬ 
cerning the recent crystallization of substances which we can¬ 
not cause to crystallize in our laboratories. Thus (Brewster s 
Journal , vol. x.) Repetti observed quartz in a pasty state and in 
the act of crystallizing. The same kind of occurrence is said 
to have been observed of various other substances, as beryl, 
opal, heavy spar. 
The chemical changes to which minerals are subject have been 
well described by Haidinger, (Brewster s Journal , vol. x.) who 
applies the name of parasitic minerals to those which retain the 
form of a substance while the substance has undergone a change, 
particle by particle. This change is of various kinds; weaker 
