Biographical Memoir of M. Homy, ^ll 



AU is found ! he exclaimed : the molecules of calcareous spar 

 have but one and the same form ; it is by being varioudy 

 grouped that they compose those crystals which deceive us by 

 the variety of external forms which they assume. Proceeding 

 from this idea, it was very easy for him to imagine that the 

 layers of these molecules being heaped upon each other, and 

 contracting proportionally, should form new pyramids or new 

 polyhedrons, and envelope the first crystal as with another crys- 

 tal, in which the number and figure of the external surfaces 

 would differ much from the original surfaces, according as the 

 new layers might be diminished on a particular side, or in a par- 

 ticular direction. 



If this were the true principle of crystallization, it could not 

 fail to be observed in the crystals of other substances. Each 

 of them would have identical constituent molecules, a nucleus 

 always similar to itself, and laminae or accessory layers, produ- 

 cing all the varieties. M. Haiiy did not hesitate about break- 

 ing his small collection to pieces. His crystals, those which he 

 liad obtained from his friends, were reduced to fragments under 

 the hammer, and in all of them he found a structure founded 

 on the same laws. In garnet, it was a tetrahedron ; in fluor- 

 spar, an octahedron ; in pyrites, a cube ; in gypsum and heavy 

 spar, oblique four-sided prisms, but with bases of different angles, 

 that formed the constituent molecules. The crystals always 

 broke in laminae parallel to the faces of the nucleus ; the ex- 

 ternal surfaces could always be conceived as resulting from the 

 decrement of the superimposed laminae, a decrement varying in 

 its rapidity, and sometimes taking place by the angles, some- 

 times by the edges ; the new faces are but small stairs or small 

 rows of j)oints produced by the retreat of these laminae, but which 

 appear smooth to the eye on account of their great tenuity. None 

 of the crystals which he examined offered an exception to his 

 law. He exclaimed a second time, and with more confidence, 

 all is found. 



But to make assurance complete, a third condition was to be 

 fulfilled. The nucleus and constituent molecule having each a 

 fixed form, geometically determinable in its angles and in the 

 relation of its lines, each law of decrement ought also to pro- 

 duce determinable secondary faces ; and even, the nucleus and 



