ATTRACTION. 149 



It is worthy of observation, that Professor Mitscherlich of Berlin, 

 in 1819,* discovered "that certain substances are capable of being 

 substituted for each other in combination, without influencing the 

 form of the compound. The neutral phosphate and biphosphate of 

 soda, have exactly the same form as the arseniate and binarsemate of 

 soda ; the phosphate and biphosphate of ammonia with the arseniate 

 and binarsemate of ammonia, the biphosphate and binarsemate of 

 potash ; each arseniate has a corresponding phosphate, possessed of 

 the same form and containing the same number of equivalents of 

 acid, alkali and water, and differing in nothing but in one's containing 

 arsenic, and the other phosphoric acid." 



It appears then that certain substances, when combined in the same 

 manner with the same body, are disposed to assume the same crys- 

 talline form, and this discovery has given origin to the phrase 

 isomorphous crystals. The arseniates are isomorphous with the 

 phosphates ; the oxide of lead and baryta and strontia form iso- 

 morphous salts with the same acid. The isomorphous crystals ap- 

 pear to contain the same quantity of waterf of crystallization, and 

 there are many other very curious circumstances in the constitution 

 of these bodies, which are too minute to be introduced into this work, 

 but which are thought to give great support to the atomic theory to 

 be mentioned hereafter. 



THEORY OF DR. WOLLASTON. 



It has been already remarked, that among solids bounded by plane 

 faces, the tetrahedron, the triangular prism, and the cube, are the 

 simplest ; these are the three integrant molecules of Hauy, and it 

 would seem that their simplicity and their capability of being so ar- 

 ranged as to produce, perhaps, all other solids, afforded a strong pre- 

 sumption in favour of their being the real integrant particles of bodies. 

 But a different view has been taken of this subject by Dr. Wollaston ; 

 for this reason among others, that in " crystallograpy we meet with 

 appearances which Haiiy's theory but imperfectly explains. A slice 

 of fluor spar, for instance, obtained by making two successive and 

 parallel sections, may be divided into acute rhomboids ; but these 

 are not the primitive forms of the spar, because by the removal of a 

 tetrahedron from each extremity of the rhomboid, an octohedron is 

 obtained. Thus, as the whole mass of fluor may be divided into te- 

 trahedra and octohedra, it becomes a question which of these forms 



* Ann. de Chimie and de Physique, Vol. XIV, p. 172, XIX, p. 850, and XXIV, 

 pp 264 and 355, Turner. 



t And when the quantity of water is different, the crystals assume a different 

 form. Turner. 



