MR. DANIELL ON VOLTAIC COMBINATIONS. 
155 
we see at once that a current is established, which, setting out from the iron, passes 
through the electrolyte to the copper, and completes its circuit through the galvano- 
meter in the direction 1, 2, 3, 4, 5, 6, 7? 8, 9. In fig. 6. the connection with the iron 
remaining the same, the battery cell is connected by its zinc rod with the zinc cup 
of the galvanometer, and we have a powerful reversed current, which we must sup- 
pose to set out from the zinc, and to pass through a portion of the electrolyte to the 
copper, and from the copper through another portion of electrolyte to the iron, and 
to complete its course in the direction 1, 2, 3, 4, 5, 6, 7 , 8, 9. In fig. 7 * the main 
battery circuit is likewise completed, and the primary current will flow in the direc- 
tion I., II., III., IV. ; while the extra circuit, although apparently connected, as in 
fig. 6, with the zinc, is in fact connected with the copper, as in fig. 5, by means of the 
main battery connection, and will convey the extra current from the iron through the 
electrolyte to the copper, and from the copper through the galvanometer back again 
to the iron in the direction 1, 2, 3, 4, 5, 6, 7 , 8, 9. 
It will be seen that the two currents coincide in their direction in that part of their 
circuits which is common to both, viz. III., IV., and 3, 4. 
There is no difficulty at all in understanding how an extra current is established 
from the iron to the copper in addition to the main current from the zinc to the cop- 
per ; but I was for a long time puzzled to make out how an extra current could 
pass from the zinc through the electrolyte to the copper, and from the copper 
through the electrolyte to the iron : it seemed to me that the interposed copper must 
act as a retarding plate, upon the opposite surfaces of which hydrogen and oxygen 
must be evolved ; and that the intensity of a single circle could not be sufficient to 
force this passage. You will, I dare say, remember suggesting an experiment which 
led to the explanation of the difficulty. 
Some dilute sulphuric acid was poured into a basin, and a platinum crucible con- 
taining some solution of sulphate of copper was placed in it. An amalgamated zinc 
rod wrapped in filtering paper moistened with dilute acid, to prevent it from precipi- 
tating the copper by its local action, was held in the crucible. A plate of iron was 
also immersed in the acid in the basin ; contact with the platinum being carefully 
avoided. A metallic communication was then made by means of a wire with the 
zinc and the iron, and we had thus the exact circumstances of the battery cell in the 
iron case, except that platinum was substituted for copper. No current, however, was 
thus formed, and no copper was precipitated upon the platinum from the solution of 
sulphate. A piece of copper-plate was now placed under the platinum crucible, and 
in contact with it : the current immediately passed, and copper was precipitated upon 
the interior surface of the crucible from the sulphate. The current of the single 
circle could not pass by the retarding plate of platina, when oxygen must have been 
evolved on one side and hydrogen on the other ; but when the oxygen was absorbed 
by the copper, and the hydrogen by the oxide of copper, these concurring affinities 
enabled the current affinity to make good its circuit. 
To vary the experiment with regard to the metallic part of the combination, three 
x 2 
