276 Formation of Compound or Twin Crystals. 



which have resulted in the erection of the most splendid and lasting 

 monuments of the age, the various sciences, have occasioned besides 

 other effects, the complete overthrow of theories so weakly found- 

 ed. It is no longer considered indispensable to provide atoms with 

 hooks, to account for their various combinations ; nor to suppose 

 them either "round, oval, lenticular, flat, gibbous, oblong, conical, 

 smooth, rough, bristly, quadrangular/'* &c, to account for the dif- 

 ferent phenomena in which they are the agents. 



Yet our knowledge of the nature of the ultimate particles of bod- 

 ies is still quite limited. The rapid advancement however of the 

 various sciences, affords from time to time some slight elucidation of 

 iheir secret nature, and leads us to expect that ere long we may 

 speak of them, as we do of the natural objects apparent to our 

 senses. 



Attempts have of late been frequently made to advance one step 

 towards a complete understanding of these bodies by a determina- 

 tion of their form, which accomplished would be a valuable acces- 

 sion to the few ideas of their nature already acquired. The present 

 state of Science seems to render this a favorable time for a consider- 

 ation of this subject, and if I mistake not, the time has already arri- 

 ved, when correct conclusions may be deduced not only respecting 

 their form, but also other qualities of these invisible constituents of 

 our world. If the truth is attained on these points, there will ne- 

 cessarily flow from it a satisfactory explanation of the formation 

 and structure of those Siametic prodigies in Mineralogy, called 

 Compound or Twin Crystals, otherwise, although improperly, He- 

 mitropes. 



Among the theories that have been proposed relative to the forms 

 of atoms or molecules, the following are the principal. Many oth- 

 ers have at different times been brought forward, but like the Leu- 

 cippian, they appear to be too absurd, viewed with the present light 

 of science, to deserve enumeration. - 



According to one author ,f Molecules have the same form as those 

 polyhedral solids that may be obtained by the cleavage of crystals. 



Another,J seeing the inability of the Haiiyan hypothesis to ac- 

 count for the very facts that gave rise to it, adopts the conclusion, 



* Epicurus. 



t UAbbi HaUy. His integrant molecules were however only three in number, 

 the Tetrahedron, cube, and three sided prism. 

 t Wollaston in the Phil Trans, of the Rov. Sor. of T.nr.Hnr. for 1813. 



