312 THE PRINCIPLES OF SCIENCE. 



true faces, but surfaces produced by the orderly junction 

 of an immense number of distinct thin crystalline plates, 

 each plate being in fact a separate crystal, in which the 

 laws of crystallography are strictly observed. The rough- 

 ness of the supposed face, the striae detected by the 

 microscope, or inference by continuity from other speci- 

 mens where the true faces of the plates are clearly seen, 

 prove the purely mistaken character of the supposed 

 exception. 



In tracing out the isomorphic relations of the elements, 

 great perplexity has often been caused by mistaking 

 one substance for another. It was pointed out that 

 though arsenic was supposed to be isomorphous with 

 phosphorus, the arseniate of soda crystallized in a form 

 distinctly different from that of the corresponding phos- 

 phate. Some chemists held this to be a fatal objection 

 to the doctrine of isomorphism ; but it was 'afterwards 

 pointed out by Clarke, that the arseniate and phosphate 

 in question were not corresponding compounds, as they 

 differed in regard to the water of crystallization . Vana- 

 dium again appeared to be an exception to the laws of 

 isomorphism, until it was proved by Professor Roscoe, that 

 what Berzelius supposed to be metallic vanadium was 

 really an oxide of vanadium d . 



In the science of crystallography many other apparent 

 exceptions present themselves, and sometimes cause con- 

 siderable perplexity. Four of the faces of a regular octa- 

 hedron may become so enlarged in the crystallization of 

 iron pyrites and some other substances, that the other 

 four faces become altogether imperceptible and a regular 

 tetrahedron appears to be produced, contrary to the laws 

 of crystallographic symmetry. Many other crystalline 



c Daubeny's ' Atomic Theory,' p. 76. 



d 'Bakerian Lecture/ 'Philosophical Transactions,' (1868) vol. clviii. 

 p. 2. 



