214 PROFESSOR DANIELL ON THE ELECTROLYSIS OF SECONDARY COMPOUNDS. 
You will doubtless remember that in my last letter I communicated to you the 
anomalous and perplexing 1 results which I obtained from the electrolysis of different 
mixtures of sulphuric acid and water. Every analogy led me to expect that the hy- 
drated acid would prove an oxysulphion of hydrogen , and that hydrogen would be 
evolved at the platinode, and an equivalent of sulphuric acid with an equivalent of 
oxygen at the zincode. But after a great number of experiments, the results of 
which were consistent within certain limits, I was obliged to conclude that, for each 
equivalent of oxygen and hydrogen evolved, only one fourth equivalent of sulphuric 
acid passed to the zincode. Nov/ it would be an extravagant hypothesis to suppose 
that another compound anion was evolved upon this occasion, consisting of one equi- 
valent of sulphur and seven equivalents of oxygen, as the result of the decomposition 
of an electrolyte consisting of one equivalent of sulphuric acid united to four equi- 
valents of water. The subject obviously required further investigation; and as the 
question appeared to me to be of first-rate importance, I bestowed much time and 
labour in repeating and modifying the experiments*. It would uselessly occupy your 
time to enter upon the details of this course of investigation ; but I will briefly de- 
scribe my mode of proceeding, some of the difficulties which I met with, and the 
means by which I attempted to obviate them. 
Experiment 2/. — The apparatus which I employed was of the same general nature 
as that which I have previously described. The gases evolved were measured and 
estimated in the usual manner ; and the changes which took place in the relative pro- 
portions of acid and water in the two cells were approximately determined by weight, 
by neutralization with carbonate of soda, or by precipitation with baryta. The con- 
necting tube (Experiment 8.) was improved by a small perpendicular tube inserted at 
the top of the bend, which allowed room for the expansion of the included liquid 
from the heat evolved, which was at times not much short of the boiling point of 
water. Without this precaution there was danger of some of the liquid being me- 
chanically forced through the diaphragm, and even of rupture of the membranes. 
The determinations bywmight were rendered uncertain from the evaporation which 
took place from the surface of the liquid in the two cells at the high temperature ; 
and it was not easy to avoid loss from the fine spray which was forcibly thrown up 
by the escape of the gases at the electrodes. Upon the whole, however, there was no 
reason to conclude that water passed to the platinode while the sulphuric acid accu- 
mulated at the zincode. 
The determinations by neutralization and precipitation were much more satis- 
factory, and generally consistent with those which I had previously obtained. Upon 
a review of the whole I am quite certain that in the electrolysis of dilute sulphuric 
acid the quantity of acid which passes to the zincode with one equivalent of oxygen, 
* In all these experiments .1 was greatly indebted to the able assistance of my friend and pupil Mr. W. A. 
Miller, without which it would have been impossible for me to have gone through the almost numberless 
operations. 
