Count de Bournon on the Laumonite. *79 



Specific Characters of the Laumonite. 

 Essential Characters. 



A. CRYSTALLOGRAPHICAL. 



1. Primitive Crystal. A rhomboldal tetrahedral prism with 

 rhombic bases, the sides of the prism meet at angles of 92° 30' and 

 87° 30' ; the bases are inclined upon the edges of 92° 30', so as to 

 form with them angles of SS" and of 125% fig. 1. The height of 

 the prism is to the edges of the terminal faces, in the ratio of eight 

 to seven. This prism divides in a direction parallel to all its planes, 

 but much more easily longitudinally than on its terminal surfaces ; 

 this division takes place also with greater facility on two of the op- 

 posite sides than on the two others. It is likewise more readily 

 effected, but at the same time less neatly, when the substance has 

 undergone alteration. 



2. Integrant Molecule. This slightly rhomboidal tetrahedral 

 prism is besides divisible parallel to its axis, and to the greater dia- 

 gonal of its rhombic terminal planes. I have not perceived any 

 natural joints in the direction of the other diagonal. This second 

 division shews that the integrant molecule of the laumonite is a tri- 

 hedral prism, the exact half of the rhomboidal tetrahedral prism, 

 fig. 2. 



3. Fracture. Lamellar. 



B. PHYSICAL CHARACTERS. 



1. Specific gravity. Taken with a piece slightly changed, but 

 still preserving in some degree its transparency 22,34.* The variety 



* Water being considered as lOjOC. 



