122 The Ottawa Naturalist. 



uously of the " brise-cristaitx" or "crystalloclastes." But Bergman did 

 not p'oceed far enough, and it remained for another to fully develop 

 the theory of the structure of crystals as indicated by their cleavage. 



In 17S4 the Able Haiiy made his remarkable discovery, which, 

 like Newton's immortal one, was the result of a mere accident. 



A six-sided prism of calcite (carbonate of lime) had been broken 

 from a large group in the cabinet of M. Defrance, and he noticed that 

 the fractures were smooth and polished, not irregular as in the case of 

 ken glass. He then commenced splitting-up the crystal with his 

 knife and finally reduced the six-sided prism to a rhombohedron 

 Extending his experiment to other minerals Haiiy arrived at the con 

 elusion that the kernel obtained from a mineral by cleavage was to he 

 regarded as its true primitive form. 



E. S. Dana defines cleavage as the tendency to break or cleave 

 along certain planes due to regularity of internal structure and fracture, 

 produced, in addition to external symmetry of form, by crystallization; 

 and he states two principles : 



(1) In any species, the direction in which cleavage takes place 

 is always parallel to some plane which either actually occurs in the 

 crystals or may ex ; st there in accordance with certain general laws. 



(2) Cleavage is uniform as to ease parallel to all like planes. That 

 is to say that if it may be obtained parallel to one of the faces of a regular 

 octahedron, for instance, it may be obtained with the same facility 

 parallel to each of the remaining octahedral faces. 



Haiiy's primitive forms were ten in number, four more than those 

 of Rome de ITsle . They were : 

 The cube. 



2. The regular octahedron. 



3. The regular tetrahedron. 



4. The rhombic dodecahedron. 



5. The rhombohedron, obtuse or acute. 



6. The octahedron, with square, rectangular, or rhombic base. 



7. The four-sided prism, with edges at right angles to the base, the 

 base being either a square, a rectangle, a rhomb, or merely a parallelo- 

 gram. 



