THEORY OF ISOMORPHISM AND DIMORPHISM. 93 



It has been recently discovered by the re- 

 searches of chemists, that many bodies composed 

 of different elements, the atoms or particles of 

 which vary in size, exhibit the same crystalline 

 form, as in the phenomena of isomorphism, from 

 laos, equal, and /uop</>i7, shape.* It has been also 

 ascertained that other bodies composed of the 

 same elements of ponderable matter in the same 

 proportions, assume totally different crystalline 

 forms, as in the phenomena of dimorphism. 



For example, it was found by Scoresby, 

 that in the arctic regions water congeals in an 

 almost endless variety of geometrical figures, of 

 which he enumerates five classes; the lamellar, 

 the stelliform, (which is the most general, and 

 occurs chiefly when the temperature is near 32) ; 



than an equal weight of water, sulphur, phosphorous, or any of 

 the compounds of oxygen, carbon, hydrogen, and nitrogen. 



* In regard to the rationale of isomorphism, Mitscherlich laid 

 it down as a law, that " the same number of atoms combined in 

 the same way, produce the same crystalline form, which is deter- 

 mined by the number and position of the atoms, independent of 

 their chemical nature." According to this view, the arseniate 

 and phosphate of soda, the protoxides of iron, zinc, copper, 

 nickel, manganese, and some other salts, are of the same form, 

 because composed of the same number of particles arranged in the 

 same manner. But the atomic weight of arsenic acid is thirty- 

 eight, and that of phosphoric acid, forty. The seleniate of soda 

 is also identical in form with the sulphate of soda, although the 

 atomic weight of selenic acid is sixty-four, and that of sulphuric 

 acid, forty. It is therefore not easy to comprehend why they 

 produce the same form on uniting with soda, without supposing 

 that sulphuric acid contains a larger amount of caloric around its 

 particles, the volume of which would be thus increased, so as to 

 approximate the magnitude of those of the selenic acid. 



