408 Prof. Johnston on apparent exceptions to the Ima that 



the positive elements together R^ ^s^ when the ratio is the 

 same as in nitre on the above supposition. This would imply 

 that compounds may be isomorphous, may possibly replace 

 each other, of which the formulae are only so far analogous 

 that the ratio between the positive and negative elements is 

 the same in both. How far this is true must be determined 

 by experiment ; it at least points out a new line along which 

 our inquiries may proceed. 



6. In the sixth group the formulas have precisely the same 

 relation as in the fifth. 



7. In the seventh group the formula Na S has to be assi- 

 milated to Ba Mn. This Dr. Clark proposes to do by doubling 

 the atom of sodium, and representing the anhydrous sulphate 



of soda by Na S. The ratio of the positive to the negative 



elements in this formula is the same (3 : 8) as in the perman- 

 ganate of baryta. 



The analogy thus established between the formulas is of the 

 same kind as that above arrived at in regard to the members 

 of the fifth group. It is obtained however by an inverse 

 process by doubling instead of halving the atom of sodium. 

 It is of importance to mark this fact, as it shows that besides 

 the changes in received views which the reconciling of these 

 formulae would require, they do not all suggest Wee changes, 

 and therefore any they do indicate ought to be looked upon 

 with great suspicion. Dr. Clark has not remained satis- 

 fied with the distant analogy obtained by his method of re- 

 conciling these two formulae. He pushes it still further, and 

 to bring, not the whole formula merely, but each member of 

 them, into exact correspondence, he represents the composition 



of his two salts by the more rational (?) formulae Na S and 



Ba Mn, supposing that the whole of the oxygen enters into 



the composition of the acid, and that this acid thus sz^roxidized 

 combines directly with the metal. It is unnecessary thus to 

 strain isolated facts, or to attempt to remodel our entire views 

 in regard to the salts on the faith of one solitary exception. 

 The most important conclusion at which we shall arrive from 

 the study of the relations among the several groups in our 

 table will be, that from none of them can we draw any x>osi- 

 iive knowledge, though all of them afford hints which may 

 lead to new inquiries. 



8. The identity of the forms of euclase and zoisite, accord- 

 ing to the measurements of Mr. Brooke, has given rise to 



