With this arrangement Messrs Gay-Lussac and Thenard have observed, that the quantity of gas dis engaged in a given time by the same pile, whether constructed with moistened cloths or with troughs, varies considerably according to the nature of the substances dissolved in the water with which the funnel is filled. Concentrated saline solutions, and compounds of water and acids, give the most abun dant and most rapid disengagement. This pheno menon diminishes as the proportions of salt or of acid become smaller ; and lastly, when the funnel con tains only boiled and perfectly pure water, al most no more gas is disengaged. Thus pure water, which transmits powerfully the electricity which is excited by our ordinary machines, becomes almost an insulating substance in the case of the weak re pulsive forces to which the electromotive appara tus gives rise. This result is conformable to the ge neral laws which have been observed in regard to imperfectly conducting substances. For, with all supports of this kind, the state of perfect insulation _takes place at a certain degree of repulsive force, which is reciprocally as the square roots of their lengths. For a given distance of wires then, the insulation of the two poles of the pile can only be perfect with a certain degree of repulsive force, de termined by the number of plates of the appara tus ; and for each electromotive apparatus, there must be a certain distance between the wires, at which the communication may be entirely inter rupted. We may also perceive, in these experi ments, the influence which the more or less extended contact of the support with the insulated body ex.. erts, in general, upon the state of insulation. For Messrs Gay-Lussac and Thenard have remarked, that, in shortening the wires beyond a certain point, the quantities of gas disengaged in the same liquid have considerably diminished, but have again aug mented by substituting in the funnel a more con ducting liquid. This imperfect conductibility of water may be rendered sensible by a very simple experiment. Having insulated a pile, and placed conducting wires at its two poles, plunge these wires into a cup of glass partly filled with common water, immediately the gas will rise in abundance. If one of the wires be drawn out of the water, and holding it in one hand, the other be plunged in the water of the cup, the ordinary shock will be felt. But, in stead of this, form the communication by a co lumn of water, of one or two-tenths of an inch in diameter, and an inch or an inch and a half in length, which may be done by drawing up the water of the cup into a tube of these dimensions, held in the mouth. In that case, although the most sensible organs now form part of the arc of commu nication, a very slight taste may be felt, but not the least shock. We have arranged, in this manner, a pile of 68 pairs, and of which the poles communi cated with tubes, not capillary, filled with distilled water, and about 89 inches in length. The appara tus remained thus fitted up, during 24 hours, with out an atom of gas being disengaged ; and in at tempting to communicate from the one pole of the pile to the other, by means of the columns of water contained in the tubes, none of the sensations which the electromotive apparatus usually produces were any more felt. In a word, every thing happen ' ed as if an insulating body had been interposed between the two poles. But all the effects reappear ed whenever an immediate communication was made along the free surface of the water. For this rea son, it could have been wished, that, in the experi ments of Messrs Gay-Lussac and Thenard, the at tempt had been made to extend the wires along the surface of the water itself; for we are of opi nion, that, in this case, the communication between the two poles of the pile would have been establish ed.
Messrs Gay-Lussac and Thenard have tried if they could discover any relation between the quan tities of gas disengaged by a pile, and the quantities of salt dissolved in the water of the funnel ; but they have not found any simple relation except for the sulphate of soda. The quantities of gas disengaged in a given time are very nearly proportional to the cube roots of the quantities of this salt contained in the water, whose decomposition is going on. The solution of nitre presents an opposite effect. Sa turated with salt it produces less gas than when not saturated. On this subject, we ought to con sider two things,----the decomposition which the water suffers, and that which the salt also suffers in its elements. The phenomenon being corn. pound, it is clear that the result must also be corn. pound.
Much research has been spent in order to die. cover how the decomposition of the water, in the circumstances that we have described, is effected ; for it cannot be doubted that the water is decom posed, since the proportions of gas disengaged are always in the ratio of its constituent principles. In the absence of any thing decisive, an opinion has been proposed which seems extremely plausible, namely, that the particles of water situated be. tween the two wires, being influenced by the opposite electricities which emanate from them, arrange them selves, one after the other, like a row of condensers, or of electrical plates, in each of which there is a treous and a resinous pole; so that each resinous pole of one particle touches a vitreous pole of the other ; and at the extremities of the chain, the metallic wire, which possesses the vitreous electricity, cont• municates with a resinous pole of one of the par ticles, and reciprocally. Suppose that, in this po lariaation, the oxygen of the water possesses the resin,. ous electricity, and the hydrogen the vitreous elec tricity ; then, if the energy of the pile is so powerful as to decompose the first particle of water, this will suffice chain. The oxygen of this particle being set at liberty, will rise under the form of gas, or will combine with the vitreous wire, and oxidate it. The hydrogen of the same particle will then be also set at liberty ; but as it possesses the vitreous electricity, it will be attracted and re tained by the oxygen of the following particle, which possesses the resinous electricity. It will thus de cide in its turn the decomposition of this parti cle; will combine with its oxygen, and will form a new molecule of water. This combination will set
at liberty the hydrogen of the second particle, which will act in the same manner on the following, until at last the decomposition will be transmitted to the particle of water which is in immediate contact with the resinous wire. The electrical action of the mole cules upon each other will be prolonged no farther; and the hydrogen of the last particle, not finding any more electrified oxygen with which it can com bine, will consequently disengage itself on this wire or combine with it.
What we have said concerning water will apply to every other compound which the electromotive apparatus decomposes. The possibility of this phe nomenon will then depend in general on three ele ments; 1st, On the greater or leis disposition of the constituent principles to assume in each particle of the compound the opposite states of electricity. Sldly, On the greater or less energy of this oppo sition. 8dly, On the relation between this energy and the chemical affinity which the principles of the compound exert on each other. If we ope .rate, for example, on a body, of which •the princi ples take with facility a very opposite state of elec tricity, it may happen that •the pile will decompose this body, although the chemical affinity which unites its principles be very powerful. If, on the -contrary, the affinity is very weak, but the constitst. east principles of the substance have at the same time, very Little tendency to run into the contrary states of electricity, it is very possible that the de. composition will not be effected. Lastly, as in the friction of bodies against each other, we see very often the same substance take successively the state of vitreous and resinous electricity, accord ing to the different nature of the rubber to which it is applied ; in like manner it may happen, that the same chemical principle may take suc cessively the one state or the other, according to the combinations into which it enters ; and al though, in general, every principle must carry in to all its combinations the same natural disposi tions, yet the final result may depend also on the dispositions, similar or different, of the prin ciples with which it may be united. In all the experiments which have hitherto been made with the electromotive apparatus, the oxygen has ap peared to have preserved this disposition to the re sinous state, which has been recognized in it in the case of water, and which is also remarked in expe riments made with ordinary electricity, Fhere the oxygen of the air always attaches itself to surfaces electrified vitreously. It even happens when bodies are found to be composed of several principles, some of which have strong affinities for oxygen, that the latter communicates to them the resinous disposi tion, and draws them towards the vitreous pole ; while the other principles, on the contrary, then take the vitreous state, and are carried towards the resinous pole. By this law, all the oxides, and the acids which contain oxygen, have been decomposed by the electromotive apparatus, and the principle which was united to the oxygen has been transported to the resinous pole ; while the oxygen, according to its constant disposition, has moved to the vitreous one. These fine observations were first made by Messrs Hisinger and Berzelius. Sir Humphrey Davy, in varying and extending them, was led to try the action of the electromotive apparatus on the alkalies, which had hitherto been regarded as simple bodies ; and it was then that he observed bubbles of oxygen rising at the vitreous pole, while there were collected at the resinous pole shining substances of a metallic aspect and yet extremely light; burning in the air with violence, and even possessing the singular property of tak ing fire under water. Such were the metallic bases of soda and of potash, which have since been call ed Sodium and Potassium. But these properties did not permit the new substances to be obtained, ex cept in very small portions, which were no sooner formed than they were dissipated in the air. It was necessary, therefore, to obtain, if possible, some method of preserving them from the destroy ing contact of the air, and Dr Sebeck conceived for this purpose a very simple process, which consists in combining sodium or potassium, as they are disengaged, with mercury. In a small piece of soda or potash we form a hollow to contain the mer cury ; we place this on a metallic plate, and immerse in the mercury the resinous wire of the electromotive apparatus, which must contain at least 200 pairs of plates. We make the other wire communicate with the metal support, and then the soda or the potash is decomp, as well as the water which it con. tains. The oxygen of each of them moves towards the vitreous pole, to which its electrical state attracts it ; the hydrogen and the sodium or potassium which quits it, proceed, on the other hand, to the resinous pole ; the hydrogen there rises under the form of gas, and the potassium or the sodium combine with the mercury, which thus preserves them from the action of the air. The amalgam is poured from time to time into oil of naphtha, and the mercury is re. newed. When a certain quantity of amalgam is collected, it is distilled in a retort with the• smallest quantity of air possible. The oil first evaporates, then the mercury, and the sodium or potassium re main at last pure. In order that the decomposition of.the potash or the soda may be effected by the pro. cess which we have described, these alkalies must contain a sufficient quantity of water to transmit the electricity of the pile ; not, however, so much as to require, for the decomposition of this water, the whole effect of the electricity transmitted, for then the potash and the soda would not be decomposed. By a process of this kind, Sir Humphrey Davy and Dr Sebeck have succeeded in obtaining in the other alkalies undoubted signs of decomposition. But any more details on the subject would be foreign to the present article. We shall only add, that, setting out from the first discovery of Sir Hunvhrey Davy, on the composition of potash and soda, Messrs Gay-Lussac and Thenard have succeeded in depriving these sub. • stances of their oxygen, by the action of ' chemical affinities alone.