We may still obtain similar effects without touch ing the plate of zinc with the fingers, but merely by holding it between sticks of glass, or of any other insulating substance. But as this plate now com municates no more with the ground, it must be brought in contact with some body of a great capa city, from which it may receive the electricity to fur nish to the collecting plate of the condenser. This is done, either by using a plate of zinc of a large sur face, or what is still better, by making it touch the inside of a large Leyden jar, coated within with a leaf of zinc, and of which the external surface, coated also with the same metal, is in communication with the ground.
This experiment being finished, repeat it in a re verse order. Take between the fingers that extremi ty C of the plate which is of copper, and touch with the zinc extremity the upper plate of the condenser, which is also of copper. Fig. 5, Plate LXXXI. • When we put an end to this contact, and raise up the plate which was applied to the condenser, it does not acquire any electricity, although the inferior plate communicates with the ground. In this expe riment, nevertheless, the copper and the zinc still communicate together, and still touch eachother as at first ; the only difference consists in this, that the two pieces of copper, which communicate with the zinc, were then placed end to end, while, in the se.. cond experiment, they were placed between the op.. posite ends of the zinc. Whatever be the cause, then, which developes the electricity, it acts like an at tractive or repulsive force, which is exerted recipro cally by the zinc on the copper, and the copper on the zinc. In the first experiment, where the two pieces of copper are on the same side with the zinc, this force is allowed to exert itself, and the electri city which it disengages, spreads over the plate of copper of the condenser. But, in the second ex periment, where the zinc is situated between the two plates of copper, the electromotive action, what ever be its nature, is exerted equally on the two sides of the zinc, and cannot, therefore, develope any elec tricity.
The metals, and a great number of non-metallic substances, act in this manner on their natural trickles when we bring them in contact with each other ; and it is extremely probable, that this pro perty extends in different degrees to every body in nature. Among all the combinations, then, that may arise from it, there will be some where the pro duction of electricity will be more powerful, and others where it will be feebler and even insensi ble. In the first class are the heterogeneous metals,
when they are brought in contact with each other ; in the last are found pure water, saline solutions, and even liquid acids, brought in contact with each other, or with metals.
To verify this property, take a tube of glass open at its two extremities : shut one of them with a stopper of copper, terminated below by a stick of the same metal, which is prolonged within, as is re presented in fig. 6, and fill this tube with any of the liquids above mentioned, with water, for example, with saline solutions, or even with an acid ; we shall then have an arrangement exactly similar to that of the plates of zinc and of copper soldered end to end. But the electromotive property will be incompara bly weaker. For, if we try it in the same manner, by touching with the finger the liquid in the tube, and carrying the stick of copper to the plate of the condenser, which is precisely the same mode as in the first experiment ; then, how often soever we repeat this contact, the plate, thus touched, will never re ceive any sensible quantity of electricity. The same thing will happen, even if the liquid contained in the tube should act chemically on the stopper of copper; at least if we do not employ very great masses of liquid and of metal acting violently on each other; for, in that case, it is well known, that the chemical combination of two substances developes electricit oisier and Laplace observed, in disso • g several pounds of iron filings in sulphuric • . But, it is evident, that the electricity deve• loped in this case is totally different from the phe nomenon produced by the contact of metals or of heterogeneous substances in general • since, in the last case, the smallest quantities of these substances soldered together, and which, by their chemical ac tion, do not produce on each other any sensible al teration, exert as much power as the largest mas ses ; and, what finally indicates a very decided dis tinction between these two classes of phenomena,—if we try to estimate on the condenser the effects of the mutual contact of metals with metals, and of metals with the most powerful acids, using in both cases equal masses, and for the liquids the little apparatus above described, it will be found, that the electromotive force, exerted by the immediate contact of the metals, and the liquid conductors, is absolutely imperceptible.