eit Fs Scale 
Review of Dana's Mineralogy. 367 
same is described by Beck as occurring in the apatite of St. Lawrence 
Co., N. Y. Six-sided prisms of cale spar are occasionally curved in the 
same manner, 
_ “In many species the crystals appear as if they had been broken trans- 
versely into many pieces, a slight displacement of which has given a 
curved form to the prism. This is common in tourmaline and beryl. 
The beryl from Monroe, Ct., often presents these interrupted curvatures 
— te in figure 7.” 
bi “In Vol. xxi, p. 206, of this Journal, Dr. John Locke described 
some very curious instances of curved crystallizations of gypsum, 
from Mammoth Cave, in Kentucky. Mr. Dana has given two 
excéllent figures of this species of distortion, drawn from speci- 
mens in the collections of the National Institute at Washington. 
Sir John Herschel has also described similar forms in ice on the 
stalks of plants, (Phil. Mag. 1833, IL, 110,) and we had the pleas- 
ure of observing the same phenomenon recently on the stalks of 
the Helianthemum corymbosum and H. Canadense. 
“Variations in the Angles of Crystals.—Variations in the angles ari- 
sing from curvatures and imperfections of surface have been alluded to. 
Other variations are owing to impurities in the crystal. Calcareous spar 
is one of the most noted instances of this variation; it varies from 105° 
to 105° 17’. Pure crystals have the constant angle 105° 5’. These 
variations are in general so small as seldom to cause any difficulty in ~ 
practice. Secondary planes, lustre, cleavage, and other peculiarities, 
will always distinguish a cube from a square prism, although the angles 
differ but 1” from one another. 
“From the investigations of Mitscherlich it is ascertained that the an- 
gles of crystals vary with the temperature. In passing from 32° to 
212° F., the angle of calc spar was diminished 8}’, thus approaching 
the form of a cube as the temperature increased. Dolomite, in the 
Same range of temperature, diminished 4’ 6”. The angle of the prism 
of arragonite was increased 2’ 46” while passing from 63° to 212° F.” 
3. Internal imperfections and impurities. —This is a head ca- 
Pable of much expansion. The controlling influence exerted 
by the menstruum or medium, while minerals are taking on 
their forms, particularly as regards the chemical constitution of 
Species, has not hitherto received sufficient attention. We have 
no doubt that when this subject has been thoroughly inves- 
tigated, much of: the present complexity of the formulas given 
for many species will vanish, and the small per cent. of many 
Matters discovered by chemical analysis and not essential to 
