314 Count de Bournon's Description of 
angle of the summit of the pyramid at 136° 54/ 41"* Pome 1, 
de Lisle's measure is nearly the same 5 namely, 137°. I have 
measured this angle with more than usual care, (on account of 
my not agreeing with these two celebrated naturalists,) having 
taken the precaution of using several different goniometers, and 
I have constantly found it to be 139 0 ; which would make the 
angles of the rhombic planes 114 0 12', and 6 f 48', instead of 
113 0 34' 41", and 66 ° 25' 19", as stated by the Abbd Hauy. 
The second of the forms abovementioned is a prism, either 
hexhedral/enneaedral, or dodecaedral, of which the terminal faces 
are perpendicular to the axis. This variety is produced in the 
following manner, viz. the plane that has replaced the solid 
angle of the summit of the pyramid, (which plane is represented 
by the Abbe Hauy in Figs. 119 and 120, Plate LH. of his 
Mineralogy,) has acquired an increase of sufficient extent to 
cause the planes of the pyramid entirely to disappear. 
I think it right to add here, a variety of this substance, which 
also comes from Ceylon, and has not yet been described, namely, 
a prism which has become of a triedral form, with equilateral 
bases, by the enlargement of the planes that have replaced the 
three alternate edges ; the formation of which planes is known 
to change the hexaedral prism into an enneaedral one ; and the 
enlargement is such as to cause the six others entirely to disap- 
pear. The tourmalins of Ceylon are not the only ones in which 
I have observed this triedral prism; I have also met with it 
among the tourmalins of Saxony, and among those of Bohemia. 
Lastly, I shall add, as forms not yet described, (although 
they do not belong to tourmalins of Ceylon,) two complete 
triedral pyramids, which, if they were not separated by an inter- 
mediate prism, .would produce two secondary rhomboids, the 
