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XI. Second Letter on the Electrolysis of Secondary Compounds. Addressed to 
Michael Faraday, Esq. D.C.L. F.R.S. , Fullerian Prof. Chem. Royal Institu- 
tion , fyc. fyc. fyc. By J. Frederic Daniell, Esq. F.R.S. For. Sec. R.S., Prof 
Chem. in King's College, London. 
Received April 30, — Read May 21, 1840. 
My dear Faraday, 
If OU will not, I think, be surprised or displeased at my addressing you again upon 
the Electrolysis of Secondary Compounds. The whole of my very limited leisure, since 
my last Letter which the Royal Society did me the honour to publish in the Philo- 
sophical Transactions for 1839, has been occupied with experiments upon the subject; 
and I have obtained some results which I trust will not be found unworthy of the 
continued attention of yourself and the Society. The mode of investigation which I 
have adopted seems to me calculated not only to throw light upon the nature of elec- 
trolytes, but upon the mode in which the chemical elements group themselves together 
to constitute Radicles or Proximate Principles, the question which now seems uni- 
versally to occupy the attention of chemists. I feel more than ever satisfied that the 
laws of electrolysis will be found to lead to the solution of this great problem. 
Upon reflecting upon the constitution of the oxyacid salts, as developed in my last 
Letter, I conceived that it might be possible to obtain further evidence that the 
simple cathion travelled as a metal to the platinode, while the compound anion was 
passing in the opposite direction ; and that means might be devised of stopping it, as 
it were, in transitu. Your beautiful experiment*, which I have often repeated with 
success, of precipitating the magnesia from a solution of Epsom salt against a surface 
of pure water, in the course of a voltaic current, suggested the mode of proceeding. 
According to my view of that experiment, the first electrolyte was resolved into 
the compound anion, sulphuric acid + oxygen, which passed to the zincode; and 
the simple cathion, magnesium, which on its passage to the platinode was stopped at 
the surface of water, from not finding any ion, by temporarily combining with which 
it could be further transferred according to the laws of electrolysis. At this point, 
therefore, it gave up its charge to the hydrogen of the water, which passed in the usual 
manner to the platinode ; and the circuit was completed by the decomposition of this 
second electrolyte. The corresponding oxygen, of course, met the magnesium at the 
point where it was arrested in its progress, and, combining with it, magnesia was 
precipitated. 
mdcccxl. 
* Experimental Researches, § 494. 
2 E 
