321. Finally, the experiment was extended into the following form, supplying in this case the tidiest analogy between common and voltaic electricity. Three compound pieces of litmus and turmeric paper (319.) were moistened in solution of sulphate of soda, and arranged on a plate of glass with platina wires, as in fig. 45. The wire _m_ was connected with the prime conductor of the machine, the wire _t_ with the discharging train, and the wires _r_ and _s_ entered into the course of the electrical current by means of the pieces of moistened paper; they were so bent as to rest each on three points, _n, r, p; n, s, p_, the points _r_ and _s_ being supported by the glass, and the others by the papers; the three terminations _p, p, p_ rested on the litmus, and the other three _n, n, n_ on the turmeric paper. On working the machine for a short time only, acid was evolved at _all_ the poles or terminations _p, p, p_, by which the electricity entered the solution, and alkali at the other poles _n, n, n_, by which the electricity left the solution. 322. In all experiments of electro-chemical decomposition by the common machine and moistened papers (316.), it is necessary to be aware of and to avoid the following important source of error. If a spark passes over moistened litmus and turmeric paper, the litmus paper (provided it be delicate and not too alkaline,) is reddened by it; and if several sparks are passed, it becomes powerfully reddened. If the electricity pass a little way from the wire over the surface of the moistened paper, before it finds mass and moisture enough to conduct it, then the reddening extends as far as the ramifications. If similar ramifications occur at the termination _n_, on the turmeric paper, they _prevent_ the occurrence of the red spot due to the alkali, which would otherwise collect there: sparks or ramifications from the points _n_ will also redden litmus paper. If paper moistened by a solution of iodide of potassium (which is an admirably delicate test of electro-chemical action,) be exposed to the sparks or ramifications, or even a feeble stream of electricity through the air from either the point _p_ or _n_, iodine will be immediately evolved. 323. These effects must not be confounded with those due to the true electro-chemical powers of common electricity, and must be carefully avoided when the latter are to be observed. No sparks should be passed, therefore, in any part of the current, nor any increase of intensity allowed, by which the electricity may be induced to pass between the platina wires and the moistened papers, otherwise than by conduction; for if it burst through the air, the effect referred to above (322.) ensues.