158 Intelligence and Miscellaneous Articles. 



outside of wliicli there are only a small number of forms pretty distant 

 from one another, and which will perhaps some day become types of 

 new genera. 



It would be interesting to discover the common cause which 

 determines the analogy of form in the different substances of the 

 same group. If it may be explained sometimes with Mitscherlich, 

 by analogy of atomic constitution, and sometimes also perhaps by a 

 relation between the atomic volumes, most frequently the cause of 

 this analogy cannot be suspected. 



The rhombohedric system contains one of these groups which is 

 very remarkable. Bromate of potash, besides the cubic fonn of 

 which M. Rammelsberg has recently expressed some doubts, cry- 

 talHzes in rhombohedra of 86° 18'. This form causes it to enter 

 into a very numerous group, including very various simple and com- 

 pound bodies, which, however, nearly all present a common and 

 curious character, namely that by their nature, or by the analogy 

 of their constitution with that of other compounds, one would be 

 led to suppose that they had a cubic crystallization, but for the 

 power of determining their forms. 



The following are the bodies composing this group : — 



Arsenic, tellurium, antimony, bismuth : their quality of simple 

 bodies, and, especially as regards two of them, of simple metals, 

 rendered it probable that they would have a form belonging to the 

 regular system ; 



Peroxide of iron, oxide of chrome, alumina and glucina, corre- 

 sponding in constitution with oxide of antimony and arsenious acid ; 



Bromate of potash ; the corresponding iodate is cubic. 



The preceding substances particularly affect rhombohedric forms, 

 and are all derived from rhombohedra of which the angle is comprised 

 between 85° 4' and 87° 4'. The following crystallize rather in hexa- 

 gonal forms, but are naturally derived from rhombohedra of which 

 the angle would be between 83° and 86° : 



Oxide of zinc, sulphurets of cadmium and nickel, magnetic pyrites, 

 iodide of silver, fluosilicate of soda, and bromate of didymium with 

 6 equivs. of water. Now we find in the regular system, — magnesia 

 (periclase), sulphurets of zinc and cobalt, chloride and bromide of 

 silver, the fluosilicates of potash and ammonia, and the bromates of 

 magnesia, zinc and cobalt with 6 equivs. of water. 



Besides these sixteen substances, the relations of which with 

 bodies belonging to the regular system cannot be overlooked, there 

 are ten others of which the forms are included in the same group, 

 but for which we do not clearly perceive similar relations, although 

 it is not impossible that they may exist. These are, — 



Arseniuret and antimoniuret of nickel, osmiuret of iridium, 

 stannate of soda, suljihate of lithia, the hyposulphates of strontia 

 and lead, aldehydate of ammonia, nepheline and Cancrinite. 



At the first glance this curious relation might seem to be explained 

 by the ordinary theory of isomorphism, — the substances above men- 

 tioned being forms very nearly allied to the cube, and consequently 

 isomorphous with the cube. 



