166 SEVENTH REPORT—1837. 
stances was considered by many chemists to afford a plausible 
explanation of what was then regarded as a very singular ano- 
maly. A few years after the publication of this opinion, how- 
ever, Mitscherlich observed a similar difference between the 
form of sulphur crystallized from fusion, and that in which it 
occurs in the mineral kingdom* ; and as in this case it was easy 
to prove the absence of any foreign body, it became necessary to 
attribute the difference to some other cause than that advanced 
by Stromeyer, to explain the production of arragonite. The pro- 
secution of the inquiry soon put into the hands of Mitscherlich 
other examples, and since that period scarcely a year has passed 
without adding some new facts to our growing knowledge. 
10. The following table contains a list of all the substances 
hitherto described as dimorphous, and it embodies nearly every 
thing we at present know in regard to the chemical and physical 
differences which the several forms of these substances present. 
See opposite table. 
11. To this list might have been added anatase and rutile, were 
it not that some doubt still exists as to whether both of these 
minerals consist of titanic acid only. They crystallize in square 
prisms of different dimensions and having different cleavages. 
The bichromate of potash appears also to be dimorphous, cry- 
stallizing from fusion, in a form which it does not retain on 
coolingt. I have also obtained from a London manufacturer 
crystals of iodide of potassium in square prisms three-eighths 
of an inch (? in.) in length, which are frequently deposited 
along with the ordinary cubical crystals from the concentrated 
solution. On resolution and evaporation they give only cubes. 
They exhibit. traces of double refraction, which, however, the 
opacity of the crystals renders very indistinct. Mr. Brooke, to 
whom I have submitted them, is unable to pronounce decidedly 
as to their form, from the want of well-defined secondary faces. 
Like the capillary red oxide of copper from Cornwall, they may 
be only an aggregation of cubes. Dufrenoy{ states that cast 
iron has been observed in cubes and in rhomboids, but the 
statement is of too uncertain a kind to be deserving of much 
confidence §. Among the ordinary crystals of sulphate of pot- 
ash with two axes, Sir David Brewster states that he observed 
some six-sided prisms with only one axis of double refraction. — 
* Poggend. Annalen VII. p. 528, (1826.) 
+ See Table LV. p. 26. 
t An. de Chim. et de Phys., LVI. p.198. ' 
§ It was formerly considered that the sulphates of zinc and magnesia belonged 
to the group of dimorphous sulphates, but later observations of Mitscherlich 
have shown that the supposed second form contains only 6 atoms of water. 
