14 LOUIS PASTEUK. 



case such a plane ; but a straight one has. You see 

 this? 



' It would have been truly extraordinary, would it 

 not, if the various kinds of minerals, such as sea salt, 

 alum, the diamond, rock crystal, and so many others 

 which illustrate the great law of crystallisation, and 

 which clothe themselves in geometric forms, should 

 not present to us examples of the two categories of 

 which we have just been speaking ? They do so in 

 fact. Thus a cube, which has the form of a player's 

 die, has a plane of symmetry ; it has indeed several 

 planes. The form of the diamond, which is a regular 

 octahedron, has also several planes of symmetry. It 

 is thus also with the great majority of the mineral 

 forms met with in nature or in the laboratory. They 

 have generally one or several planes of symmetry. 

 There are, however, exceptions. Kock crystal, which 

 is found in prisms, often of large volume, in the 

 fissures of certain primitive rocks, has no plane of 

 symmetry. This crystal exhibits certain small facets, 

 distributed in such a manner that in their totality 

 they might be compared to a helix, or spiral, or 

 screw, which are all objects not possessing a plane of 

 symmetry. 



* Every object which has a plane of symmetry, when 

 placed before a looking-glass, has an image which is 

 rigorously identical with the object itself. The image 

 can be superposed upon the reality. Place a chair 



