240 DISCOURSE ON THE STUDY 



of crystal might be built up of thin laminae ranged 

 in a certain order, and following certain rules of 

 superposition. He failed, however, in deducing just 

 and general conclusions from this remark, which, 

 correctly viewed, is the foundation of the most im- 

 portant law of crystallography, that which connects 

 the primitive form with other forms capable of being 

 exhibited by the same substance, by a certain 

 fixed relation. An idea may be formed of what is 

 meant by this sort of connection of one form with 

 another, by considering a pointed pyramid built 

 up of cubic stones, disposed in layers, each of 

 which separately is a square plate of the thickness 

 of a single stone. These layers, laid horizontally 

 one on the other, and decreasing regularly in size 

 from the bottom to the top, produce a pyramidal 

 form with a rough or channeled surface ; and if the 

 layers are so extremely thin that the channels cease 

 to be visible to the eye, the pyramid will seem 

 smooth and perfect. 



(262.) Very shortly after this, and without 

 knowledge of what had been done by Gahn and 

 Bergmann, the Abbe Haiiy, instructed by the ac- 

 cidental fracture of a fine groupe of crystals, made 

 the remark noticed already (in 67.), and reasoning 

 on it with more caution and success, and pur- 

 suing it into all its detail, developed the general 

 laws which regulate the superposition of the layers 

 of particles of which he supposes all crystals to 

 be built up, and which enable us, from knowing 

 their primitive forms, to discover, previous to trial, 

 what other forms they are capable of assuming; 

 and which, according to this idea, are called deriv- 



