216 PROFESSOR DANIELL ON THE ELECTROLYSIS OF SECONDARY COMPOUNDS. 
the additional energy of zinc in a voltaic circuit, is beautifully exemplified in the fol- 
lowing experiment. I took ten cells of a small constant battery and charged them 
in the usual way, only substituting three amalgamated rods of grain tin, which had 
been previously weighed, for three of the zinc rods. The circuit was completed with 
a voltameter, and twenty-five cubic inches of mixed gases were collected in one hour; 
and the three rods were found to have lost respectively twenty-two grains, twenty-two 
grains, and twenty-one grains, or about an equivalent each for the gas. The circuit 
was immediately closed with the voltameter and the seven remaining zinc rods, and 
the same quantity of gas was collected in eight minutes. Now with regard to the 
mere oxidation of the metal and the removal of the oxide by the acid, everything pro- 
ceeded apparently the same with the tin rods as with the zinc, and yet we find that 
instead of adding anything to the energy of the current, they reduced its efficiency 
to less than one seventh. 
I cannot help remarking, by the way, that this is a result which the advocates of 
the contact theory would find it very hard to reconcile to their principles. According 
to their views the electromotive force of tin and copper is very little, if anything, 
inferior to that of zinc and copper ; the resistances of all parts of the circuit are the 
same, and yet the addition of a few more couples of tin and copper instead of adding 
anything to the intensity of the current would stop it altogether. 
But to return to the more immediate object of this paper. We have seen that 
whatever be the metal of the zincode, the quantity of acid which travels to it in the 
electrolysis of dilute sulphuric acid is sensibly the same, and we have yet found 
nothing to explain the extraordinary proportion which it bears to the gases evolved, 
or to make it harmonize with the results of other electrolvtes. 
Experiment 32. — Dilute phosphoric acid was next submitted to similar experiments. 
Some pure acid was prepared for the purpose ; but I was at a loss for some time for 
a convenient and accurate method of determining the quantity of acid in any solu- 
tion. This I at last satisfactorily effected by mixing it with pure oxide of lead, care- 
ful evaporation and ignition, according to the method of Professor Rose. 
In the first experiment, the solution consisted of one part by weight of the glacial 
acid to sixteen parts of water. It conducted badly, and only 1 7‘75 cubic inches of 
mixed gases (or one fourth of an equivalent) were collected in three hours and twenty- 
five minutes. The result was accompanied by a circumstance which I must make the 
subject of a future communication. 
Experiment 33. — The experiment was repeated with a solution which consisted of 
one part glacial acid to eight of water. Twenty-four cubic inches of the gases were 
now collected in 2f hours, and the quantity of acid which had travelled from the 
platinode to the zincode was found to be 3'6 grs., which again is in about the propor- 
tion of one quarter of an equivalent to one equivalent of the gases. So that with 
phosphoric acid we have the same anomalous proportions as with the sulphuric acid. 
Experiment 34. — In a postscript to my last letter I informed you that I had ascer- 
