extraordinary figure of Calcareous Spar. 315 



in hexagons whose sides are straight. And I have often ob- 

 served within water which begins to freeze, a species of flat and 

 thin leaves of ice, whose middle radius throws out branches 

 inclined at an angle of 60°. All these things deserve to be 

 carefully investigated, in order to ascertain how and by what 

 process nature produces them. It seems to me that in general 

 the regularity which is found in these productions arises from 

 the arrangement of the small equal and invisible particles of 

 which they are composed. 



In respect to calcareous spar, I say that if there was a pyra- 

 mid ABCD, Plate IV, Fig. 6, composed of small corpuscles, 

 round and not spherical, but flat spheroids, such as would be 

 formed by the revolution of an ellipse GH, Fig. 7, round its 

 lesser axis EF, whose ratio to the greater axis is very nearly that 

 of one to the square root of eight (or 1 to 2.8) : I say then that 

 the solid angle of the point D, will be equal to the obtuse and 

 equilateral angle of this crystal. I say, besides, that if these 

 corpuscles are slightly cemented together, they would, in touch- 

 ing the pyramid, cleave in the direction of faces parallel to 

 those which form the solid angle, and that, by this means, as 

 may be easily seen, they would yield prisms similar to those 

 shown in the figure. The reason is, that in splitting in this 

 manner, every stratum separates itself easily from the neigh- 

 bouring stratum, because each spheroid detaches itself only 

 from three spheroids of the other stratum, out of which three 

 there is only one which touches with the flat surface, and the 

 two others only at their edges. It is this which causes the sur- 

 faces to separate so distinct and well polished, for if any one 

 spheroid of the neighbouring stratum left its place to detach 

 itself from the separated surface, it must detach itself from six 

 other spheroids which keep it shut in, and four of which touch 

 by their flat surfaces. Since, therefore, the angles of our cry- 

 stal, as well as its manner of cleavage, agree exactly with what 

 should take place in a body composed of such spheroids, we 

 have great reason to believe that its particles are formed and 

 arranged in a similar manner. 



There is also some appearance that the prisms of this crystal 

 are formed by the rupture of the pyramids, since M. Bartho- 



