Art. XIX. — On the Comptonite Vesuvius^ the Brewsterite 
(^Scotland, the Stilbite and the Healandite. By H. J. Brooke, 
Esq. F. R. S. Lond. M. G. S. &c. &c. Communicated by the 
Author. 
In the Edinburgh Philosophical Journal, Vol. IV. p. 131. Dr 
Brewster has described a new mineral, and given it the name of 
Comptmiite. I have found the crystals of this substance cleave 
parallel to the planes M and T, Plate V. Fig. 1. the cleavage 
planes meeting at an angle of 90°. This circumstance, com- 
bined with the existence of the planes o, o', which meet at an 
angle of about 177° 35', shews that the primary form is a rect- 
angular prism, the terminal edges of which are, however, very 
nearly equal. On three crystals of my own, I have found M on a 
measure 135° 30', 135° 30', and 135° 45'; and on the same crys- 
tals respectively, a on T measured 134° 30', 134° 45', 135° 15', 
If 135° 30' and 134° 30' be taken as the true measurements, the 
edges of the base terminating the planes M and T, will be to 
each other respectively very nearly as 56 to 55. 
A mineral from Strontian, which has been called in France 
Primitive Stilbite^ and was at one time considered to be Apo- 
phylllte, is certainly a distinct substance. 
I have therefore given it the name of Brewsterite^ on account 
of the many important discoveries connected with crystallo- 
graphy? which have resulted fi'om the experimental researches 
of Dr Brewster. The primary form of the Brewster'ite is a right 
pT’ism, Fig. 2. whose bases are oblique-angled parallelograms^ 
M on T measuring 93° 40', as deduced from the inclination of 
a on a\ c on o', and a on c. Fig. 3. I have not been able to 
cleave the crystals with certainty in any other direction than pa- 
rallel to the plane P. Yet when an attempt is made to divide 
them perpendicularly to P, and parallel to T? the new surfaces 
exhibit traces of cleavage planes. 
The inclination of the edge h on the edge i being 93° 40', it 
was necessary to adopt a prism oblique in one direction, as the 
primary form ; and I have preferred placing that prism in the' 
position I have just described, from its agreement with Sulphate 
of Lime, Euclase, and some other substances belonging to that 
