342 Mr. William Phillips on the Form of some — [Nov. 
On the Form of the before-mentioned Crystals. By William Phil- 
lips, FLS. MGS. &c. 5 
Treturn the crystals with drawings of their forms (figs. 1 to 6 
inclusive, Pl. XVI), of about the size of the crystals themselves, 
but made somewhat roughly, not having been done according to 
rule, but only by the eye. 
These crystals yield to mechanical division parallel to the 
planes M, T, and P, and the following admeasurements by means 
of the reflective goniometer of Dr. Wollaston, show that the 
primary form is a right rectangular, but not square prism, and 
especially the angles of c on c’, and c’, fig. 3, on the adjacent 
plane over the edge x. 
The constant convexity of the plane b (which, however, from 
that circumstance, may rather be considered as a series of 
planes than as one plane) has rendered it impossible to obtain 
any measurement from its surface with another plane on any one 
of the crystals ; while the nearly constant concavity of the plane 
m has rendered it almost equally impossible to ascertain its angle 
with any other plane accurately. Arguing, however, from ana- 
logy, we should assume that the plane 6 probably consists of a 
combination of the planes m and n, especially since the planes 
aandc appear at both extremes of the crystals, figs. 2 and 3. 
All the crystals of these forms show the same convexity on the: 
planes on one side, and concavity on those of the other side, of 
the plane d. 
The ordinary crystals of the sulphate of potash are in the form 
of two six-sided pyramids united, at a common base, but some- 
times separated by a short intervening prism, and variously modi- 
fied, fig. 7. These crystals cleave parallel to the planes of the 
prism, and to that replacing the summit, that is, parallel to all 
the planes of a regular six-sided prism measuring on the lateral 
planes precisely 120° by the reflective goniometer. This circum- 
stance, apparently so decidedly at variance with the cleavage of 
the crystals figs. 1 to 6, and the apparent incompatibility of their 
external forms with that of fig. 7, induced me to conclude that 
this substance must be peculiar in possessing two primary forms 
having no relation to each other ; for it appeared certain, that, as 
the double six-sided pyramid may be cleaved parallel to all the 
sides of aregular hexadral prism, M M M, T, fig. 7, that that solid 
must be the primary form of these crystals, notwithstanding the 
almost constant appearance of a line passing along the plane o, 
in the direction of the dotted line on that plane of fig. 7, and 
which had tempted me to suspect the possibility of these crys- 
tals being macles, or hemitrope crystals. 
After arriving at the fore-mentioned conclusion, [ showed the 
sketches to my friend H. J. Brooke, Esq. who perceived a cir- 
cumstance that had not made asuflicient impression on my 
mind ; namely, that the measurement of the planes M on d, figs. 
