43o Doctor Clark to Professor Mitschcrlich, 



to inquire, whether the atomic weight of sodium should 

 be halved or doubled ? So that, in regard to these two salts. 

 First, We find, in the one, two atoms of manganese, in 

 the other, two atoms of sulphur— bodies proved to be iso- 

 morphous. 



Second, We find eight atoms of oxygen in each. 

 Third, Therefore, the question to be resolved shrinks 

 into this one point : In two salts of a similar form, and 

 otherwise composed alike, are we to regard barium, which 

 is in the proportion of one atom, as being substituted' by 

 sodium, in the proportion of one atom, or in the proportion 

 of tiro atoms ? 



Now if, as I think, justly, we regard 

 The oxymanganate of potash, as containing K + 8 O + 2 Mn 

 And the oxychlorate, as containing . . K + 8 + 2 CI 

 And not (assuming 2 CI = Ch) . . . . K + 8 + Oh 

 How can we hesitate in considering 



The oxymanganate of barytes, as containing Ba + 8 O + 2 Mn 

 And waterless sulphate of soda, as contng. So +8 + 2 S 

 And not 2Na + 8 + 2S 



Without renouncing, or at least, disregarding the known 

 analogies of chemistry, I own I do not perceive how to 

 evade the conclusion, that sodium and silver should each 

 have its atomic weight doubled, and that the oxymanga- 

 nate of barytes is not only similar in form, but analogous 

 in composition, to the waterless sulphate of soda, and, of 

 course, to the waterless seleniate of socla, and to the sul- 

 phate and the seleniate of silver. 



But, even in two salts of a like form, we cannot regard 

 analogy as fulfilled, merely by their containing in each a 

 like number of corresponding ultimate components, partly 

 of the same kind, and the rest isomorphous. Those ulti- 

 mate components may form intermediate compounds, which, 

 and not elementary bodies that are mere ultimate compo- 

 nents, may be the real constituents of the salts. Besides, 

 in isomorphous compounds, we should look, not so much 

 for analogy of composition, as for analogy of constitution. 

 When scrutinized for this purpose, the salts in question 

 present a difficulty that chemists can overcome, as far as I 

 can see, only by taking up a position, less repugnant to 

 reason, than it may be to their habits of thinking. 



