THEORISS OF CRYSTALLOGRAPHY, ^5 



Other muft be fuppofed finite. This material diflference will Laws of the 

 neceffarily caufe a difFerence between the forms of their aggre- f^'yi^rs^and 

 gates. Thofe formed with the particles of a fluid will be theory thence 

 bounded by curved lines; the cryftalline aggregates, on the "^^^"^^'"S* 

 contrary, will be terminated by ftraight lines; and when thefe 

 ftraight lines are not too fmall, the boundaries will be fenfibly 

 rectilinear. 



To afcertain what the power is that holds the particles in 

 the ftate of refl, though not in clofe contaft, is not the quef- 

 tion ; but the form of the polyedrons which they produce,, 

 The clofer adhefion of the particles to be obtained by the fub- 

 tra6tion of caloric, fufficiently demonftrates that the particles. , 

 ^re not in clofe contad with each other, and the conftancy of 

 the cryftalline forms equally proves that they are in equilibrio. 

 We will now proceed to the conftrudion of a cryftal with 

 thefe cryftalline particles. That the conftancy of the form in 

 tJie large cryftal be preferved, the particles muft be in equi" 

 librio. That the equilibrium be preferved, the forces that fo-, 

 licit the particles to motion muft mutually deftroy each other. 

 That the mutual deftrudion of thofe forces be effedled, thefe 

 forces after having been decompofed into other relatively pa- 

 rallel to three axes perpendicular to each other, and having a 

 common point of interfe6tion, muft each meet in its.direftion 

 another force equal and diametrically oppofed to it. This will 

 be obtained if the fimilar particles are arranged on ftraight 

 lines parallel two and two at equal oppofite diftances from the 

 common centre, and bifedted by lines pafling through that 

 centre ; but if the particles are thus arranged, they muft pro- 

 duce fymmetrical folids bounded by planes; and they are thus 

 arranged : for if a foreign force, an excefs of caloric for ex- 

 ample, does not impede the free arrangement of the particles 

 in the formation of the cryftal, their exterior difpofition will- 

 follow as much as poffible their interior arrangement j but their 

 interior arrangement muft be on ftraight lines, or the cryftal- 

 would ceafe to be homogeneous; their exterior difpofition 

 will therefore be on ftraight lines. 



As the circumftances giving rife to the approach of the par-- 

 tides maybe in the higheft degree variable, it muft follow 

 that the forms .produced may be diverfified in the extreme. 

 Such, Sir, is the anfwer I fliould fubmil for the folution of 

 the queftion propofed, 



5 When 



