136 ISOMORPHISM. 



The densities of vapours, taken a few degrees above their con- 

 densing points,, are generally a little greater than the truth, 

 owing to a peculiarity in their physical constitution which was 

 formerly explained (page 68). Of such bodies, therefore, the 

 theoretical is a necessary check upon the experimental density. 

 Indeed, the calculated should in all cases be considered and 

 used as the true density. 



RELATION BETWEEN THE CRYSTALLINE FORM AND ATOMIC 

 CONSTITUTION OF BODIESDOCTRINE OF ISOMORPHISM. 



Bodies on passing from the gaseous or liquid to the solid state, 

 generally present themselves in crystals, or regular geome- 

 trical figures, which are the larger and more distinct, the more 

 slowly and gradually they are produced. Their formation, is 

 readily observed in the spontaneous evaporation of a solution 

 of sea-salt, or in the slow cooling of a hot and saturated solu- 

 tion of alum, which salts assume the forms of the cube and 

 regular octohedron. The crystalline form of a body is con- 

 stant, or subject only to certain geometrical modifications 

 which can be calculated, and is most serviceable as a physical 

 character for distinguishing salts and minerals. Between 

 bodies of similar atomic constitution, a relation in form has 

 been observed of great interest and beauty, which now 

 forms a fundamental doctrine of physical science, like the 

 subjects of atomic weights and volumes just considered. 



Gay-Lussac first made the remark that a crystal of potash- 

 alum transferred to a solution of ammonia-alum, continued to 

 increase without its form being modified, and might thus be 

 covered with alternate layers of the two alums, preserving its 

 regularity and proper crystalline figure. M. Beudant after- 

 wards observed that other bodies, such as the sulphates of 

 iron and copper, might present themselves in crystals of the 

 same form and angles, although the form was not a simple 

 one like that of alum. But M. Mitscherlich first recognised 

 this correspondence in a sufficient number of cases, to prove 

 that it was a general consequence of similarity of composition 

 in different bodies. To the relation in form, he applied the 

 term isomorphism (from laos, equal, and ^op</>?), shape), and 

 distinguished bodies which assume the same figure as isomor- 

 phous } or (in the same sense) as similiform bodies. The law 



