Count de Bournon on the Laumomte. 85 



tive crystal, and the two others to the planes of the sixth modifica- 

 tion, meet at an angle of 127" 40'; but these angles are never 98° 

 12' and 121° 34 as is said in the work which I have just noticed. 

 The rhomboidal tetrahedral prism of 92' 30', and 87° 30' of this 

 substance, approaches so nearly to the rectangular form that it is 

 extremely easy to conceive how the Abbe Haiiy in his first deter- 

 mination of the primitive crystal of the laumonite (a determination 

 which he has given us to understand was merely a first glance of the 

 subject) might have been led to consider it as being in reality rect- 

 angular ; but he certainly would not have committed that error, 

 if the angles of this prism had really differed more than eight de- 

 grees from a right angle. It is on the supposition of the octohedron 

 being the primitive form, that the Abbe Haiiy has stated, that the 

 crystal, fig. 40, pi. III. of his " Tableau Comparatif," is a variety- 

 produced from that figure. This form is the only variety which he 

 has given. I have represented it at fig. 33, keeping on its planes the 

 letters of indication which he employs. In this figure the planes 

 M belong to the longitudinal planes of the primitive rhomboidal tetra- 

 hedral prism ; one of the planes P to the terminal face of this prism, 

 and the other to the plane of the sixth modification. The plane 1 

 belongs to the first modification, and that indicated by S, which I 

 have never perceived in any of the crystals that have come under my 

 own observation, w^ould belong to a fifteenth modification, which 

 would be the result of a retrogradation by a single row along the 

 edges of 87° 30' of the prism. 



This difference in opinion between the Abbe Haiiy and myself, 

 with regard to the true primitive form of the laumonite may perhaps 

 again give rise to the unjust reproaches which a similar diversity 

 under circumstances nearly alike has already occasioned. By render- 

 ing the science responsible for the errors which those who cultivate 



