Formation of Compound or Twin Crystals. 287 



of this compound, — that is, of its acid and base, or salts, — either at 

 the time of crystallization or previously at the formation of the com- 

 pound itself. For the union of the compound molecules as such, if 

 the requisite form could be produced, could not give rise to regular 

 symmetrical molecules. The compound molecule of Sulphuric 

 acid and Lime for instance, would contain different atoms as to kind 

 and number in similar parts. Such molecules seem to be incon- 

 sistent with the regularity of the Crystalline solids, and more par- 

 ticularly with the occurrence of similar secondary planes on homo- 

 logous parts. But can it indeed be believed that the molecule of~ 

 the above acid placed along side of the base, should produce a Right 

 Rhomboidal Prism of the same angles and proportional dimensions 

 as in a crystal of Sulphate of Lime ? If this meihod is then not 

 practicable, a rearrangement is necessary, and this seems to have 

 been the principle on which M. Gaudin has proceeded. But it may 

 be objected to this, that the power which causes sulphuric acid to 

 unite with Lime resides in these two compounds as such, and would 

 not exist were the sulphur separated from its oxygen or the ele- 

 ments of Lime to be disunited. 



But were all other difficulties removed, it might be asked how can 

 an atom of Sulphur united to an atom of Lead — probably dissimilar 

 in size and form — produce a molecule having any similarity to the 

 Cube of Galena. At least eight equal spheres would be required. 

 A remark that has been already made may hence be here repeat- 

 ed ; the relation between the primary form and the molecule cannot 

 be supposed to exist elsewhere, if not in instances similar to the 

 above, and consequently we are forced again to the improbable de- 

 duction that they never exist, a deduction which appears 'to be general- 

 ly rejected by Chemists, judging from their explanations of Isomor- 

 phism, although perhaps not always as much disapproved of in ac- 

 counting for other occurrences, as a general survey of facts and an 

 extended view of the simplicity of the operations of nature, lead us 

 to suppose would be consistent with truth. 



Admitting however that the molecule could be formed of the re- 

 quisite dimensions and angles, as might possibly take place provided 

 the molecules of the elements are supposed to be sufficiently poly- 

 atomic, (a supposition that has been advanced,) how is it possible that 

 several additional atoms can be united to such a molecule without 

 changing in the least the ratio of its edges, or the inclinations of its 

 faces ? Instances of this fact are numerous. 



