3rd. That this oxide is very easily reducible by the action of hydrogen. There may perhaps be two objections raised against the conclusions I have made the first, that these phenomena which I have described may be due to the solution of the gases in the liquids; the second, that they are only the result of a simple adherence, sometimes of oxygen, sometimes of hydrogen, at the surface of the platinum. We will now examine successively these two objections. I ought to acknowledge that the acidulated water may dissolve a part of the gases which are disengaged at each of the poles of the pile, and more particularly a little oxygen: it must also be admitted that the gas, by being developed on a large surface, facilitates this solution. But the whole of the phenomena cannot be thus explained. When after having put the platinum plate in contact with the positive pole, and having observed the disappearance of a certain quantity of oxygen, I repeated the same experiment, after taking great care to completely renew that part of the liquid which was in contact with the poles, I found that the oxygen disengaged very nearly the requisite proportion. If the absence of a part of the oxygen in the former case was due to its solution in the ambient liquid, how is it that this solution and consequently this same absence did not take place in the second case in the same manner? How is it that a platinum plate put to the pole, after having been cleansed and washed, gives hydrogen in the requisite proportion, whilst the same plate after having been exposed to the action of oxygen, causes a portion of the hydrogen which is disengaged at its surface, to disappear in the same liquid? The following observation, which is very easy, shows very well that it is impossible to attribute these phenomena which we have just described, at least totally, to the absorption of the gases by the liquid. When we take a plate of platinum which has been exposed for some time to the air, or to the action of oxygen, and which we plunge into a liquid conductor, making use of it as the negative pole of a pile, the current from which traverses this liquid, some seconds, sometimes up to twenty, elapse, before the hydrogen makes its appearance, whilst the oxygen is disengaged immediately on a similar plate placed at the positive pole. When, on the contrary, we put the poles in communication with two plates of platinum perfectly cleansed, the hydrogen appears on the negative plate, whilst the oxygen does not show itself until some seconds later on the positive plate. It is then evident that the differences in the precise periods when the gases become visible on each plate, are due even to the state of the surfaces of the plates, and not to the faculty which the liquid has for dissolving the gas, a faculty which should have acted in the same manner in the experiments which we have just reported. In fine, the secondary polarites which the platinum plates acquire, polarities whose true natures vary according as the plates have disen gaged hydrogen or oxygen at their surface, are a further proof that this surface has been modified by these gases, and that it has been so we shall see instantly, by the effect due to an oxidation or a reduction. Here the second objection presents itself. The platinum plates retain some oxygen at their surface it is said: but this gas is only adherent, it does not form any chemical combination, and it is the same with the hydrogen, which also adheres in its form to the surface of the platinum. These phenomena are due to an action purely physical, that of adhesion; and it is added, that when the plate which has been in communication with the positive pole is transported to the negative pole, the disappearance of a part of the hydrogen is due to this: that in its nascent state it is combined with the oxygen which remains adhering to this plate. I have likewise examined with care this part of the question, the more so as the opinion which I have just developed is the same as that adopted by M. Matteucci, and, as it appears to me, by M. Schoenbein, in the experiments which these two philosophers have made, each on his own part, on the second polarities. These are, now, the motives which have led me to believe that it is not a simple adhesion, but a true chemical combination between the gases and the surface of the platinum : 1st. When the surface of the platinum is newly cleansed and scoured with care, the oxygen alone adheres, and not the hydrogen. If, in putting a plate of platinum in a tube (eprouvette) full of hydrogen, M. Matteucci saw the volume of the gas diminish, it was because this plate had had its surface slightly oxidized, by its exposure to the air or the direct action of oxygen. The plate, in fact, when it has been deoxidized and washed, takes up no hydrogen, whether it is placed at the negative pole of a pile, or put into a tube filled with hydrogen. 2nd. The pulverulent state which a surface of platinum takes, when it has been exposed during a long time to the alternative action of oxygen and hydrogen, is a proof that the adhesion of the oxygen has been a true chemical combination, and that of the hydrogen a veritable decomposition of the oxide. For the same thing takes place with platinum exactly as with other metals, such as copper, for instance, when their surface undergoes a succession of oxidations and reductions, this surface becomes pulverulent. 3rd. It is impossible to perceive, even by the aid of a powerful magnifier, the least bubble of gaseous oxygen on the surface of the platinum which has been exposed to the action of this gas. The friction of a linen cloth will no longer remove this oxygen; it requires, in order to effect its disappearance, either a mechanical action which will renew the surface, or a strong and boiling acid which will dissolve the oxygen formed. This is, then, another proof that it is a true chemical combination: it is true that it is only superficial, and if we esteem this circumstance as sufficient to render the phenomenon, then the question is no more than a question of words. But it is not thus, it seems to me, that it can be understood: a simple physical adhesion implies the idea that the adhering bodies preserve their own individual properties. Now, it is not so in this case, since the oxygen is no longer gaseous, and the surface of the platinum finishes by becoming reduced into a powder by the successive action of the gases: thus the phenomenon has all the characters of a veritable chemical phenomenon. The experiments which I have just reported were made in the spring and summer of the year 1838. They formed the subject of a memoir which I read to the Society of Physical and Natural History of Genoa, the 4th of September of the same year; I did not then print this Memoir, but I addressed an extract of it to M. Becquerel, which he communicated to the Academy of Sciences at Paris, and it was published in the Compte Rendu of the Sittings of that Academy, December, 1838. I have repeated frequently, since that time, the experiments I have just described, and they have constantly given me the same results. Still more recently (the 28th of April, 1841), I obtained the following result, with a pile of constant force, consisting of ten elements, the poles of which communicated with a plate of platinum, formed into a helix, and which had been perfectly cleansed, (decapée) and also with a very short platinum wire; the plate and wire were immersed in sulphuric acid carefully purified, and diluted with nine times its volume of distilled water; the gases disengaged from each of the poles were received in graduated tubes. At the commencement, the pole being brought into communication with the plate, the gases were in their right proportions. The plate was now brought to the + pole, the wire to the pole; there were twenty centimetres of hydrogen and only six of oxygen in place of ten. On changing the poles anew, I obtained ten centimetres of oxygen and only fifteen of hydrogen; there ought to have been wanting eight in place of four of it, to represent the equivalent of oxygen which had disappeared. But, as I have already said, it is very rarely that there disappears in these experiments a quantity of hydrogen equivalent to that of oxygen. I will return to this case further on, in indicating one of the courses of which I have not yet spoken, which may contribute to occasioning this difference between the two gases. SECTION II. On the Influence which the Oxidation of Platinum exerts on the production of Electric Currents, by Couples composed uniquely of Metals. I am the first, I believe, who has shown that when two platinum wires have served to decompose water by the effect of a voltaic current, these two wires may form a voltaic pair which gives birth to a current from the wire which has served at the positive pole; and at which, consequently, oxygen has been disengaged, is the negative element of the pair; the wire used at the negative pole, and at the place where the hydrogen was disengaged, is the positive element. This current lasts for some instants, and ceases more or less quickly according to the nature of the liquid in which the two wires were immersed during the decomposition of water produced by the current, and according to the nature of the liquid into which they are afterwards plunged to form the pair. I have attributed this phenomenon, in the memoir in which I have described and studied it,* to a particular electrical state into which the molecules of the metallic wires were constituted, during the time that they transmitted the current, and I have supported my hypothesis on the fact that those parts of the platinum wires which are not immersed in the liquid while the current passed, manifest, though in a less degree, the same properties as those corresponding portions of the wires on which were deposited the elements of the liquid decomposed. M. Becquerel, at a later period, has shown that this explanation was not the true one, and that the effect observed was due to an excessively thin deposit of the elements of the substances decomposed by the current, which deposit takes place on the surface of the platinum wires. When the current ceases passing, the action of the liquid on this deposit determines the current called secondary, in which the platinum wires play the part, the one of a positive element, and the other that of a negative element. Subsequent experiments, however, have proved to me that, even as I have already advanced, the wires and plates of platinum which have not received this deposit on their surface, may give rise to a secondary current, much more feeble, it is true, but still perceptible. I will only add that the most profound study that I have made of these phenomena, have led me to believe that they are entirely due to a chemical effect of the liquid on the surface of the platinum, not only in the case where there is a deposit, as M. Becquerel has indicated, but also in the case where the deposit has not taken place. MM. Matteucci, Peltier, and Schoenbein, who have been occupied in examining this subject, have shown that the plates of platinum and of gold are capable of developing a secondary current, by the single fact of their having served in the decomposition of water slightly acidulated, and even of water perfectly pure, without the products of the decomposition having been any other than oxygen on one of the plates, and hydrogen on the other. M. Matteucci attributes this phenomenon to the adherence of the oxygen and hydrogen on each of the plates where these gases are disengaged. He proves this adherence by introducing the plate to which oxygen is conveyed into a graduated measure filled with hydrogen, and that on which hydrogen has been disengaged into a graduated measure • Mem. de la Soc. Phys. et d'Hist. Nat. de Geneve, t. iii (2 partie), et Ann. de Ch. et de Phys., t. xxxvi, p. 34. filled with oxygen, and by proving that the volume of the gases is diminished a little in each measure, he has succeeded in producing the same secondary polarities, by placing for some moments the plates of platinum, the one in an atmosphere of oxygen, the other in an atmosphere of hydrogen, before plunging them into the liquid conductor, which ought to form the circuit, in place of putting them to the positive and negative poles of the pile.* In M. Peltier'st estimation, the effect is due to the gases which remain dissolved in the water; and he has succeeded in producing a secondary current, by causing the hydrogen to pass directly into the water, and by putting on one of the extremities of the platinum a galvanometer into this hydrogenated water, and the other extremity likewise of platinum in ordinary water, in contact with the former. Lastly, M. Schoenbeint has made a great number of experiments by immersing platinum wires, or gold and silver ones, in different gases, and determining the polarities which these wires acquired or lost by the action of these gases; he likewise employed different liquid conductors to complete the circuit of the pair formed by the metallic wires, and has studied the influence of these liquids. He appears to attribute all the effects which he obtained to chemical action, without well explaining these actions, for he adds: I dare not yet say that these secondary currents are entirely due to an ordinary chemical action. The conclusions at which I have arrived at the close of the researches contained in the paragraph preceding, have made me presume that it is possible that the oxidation and deoxidation of platinum may perform an important part in the production of secondary currents. The experiments of the philosophers above named agree with the explanation, and I have sought to verify it by studying with care the phenomenon in question. I felt asssured from the first that wires and plates of platinum which have served in the decomposition of water slightly acidulated, give birth to a secondary current extremely strong, and stronger in proportion as the surfaces are greater. The most decided intensity was obtained by making use of a piece of spongy platinum. A wire whose surface had been rendered pulverulent by magneto-electric currents, also gives a secondary current much more intense than a wire whose surface is united, probably because it presents, like the sponge, a much greater number of points of contact with the ambient medium. This, in my opinion, is the cause of the phenomenon: the platinum which has been exposed to the action of hydrogen being perfectly cleansed, is attacked slightly by the acidulated water, and even by the air dissolved in the water when it is plunged in pure water; it then gives birth to a current when we form a pair with it, and another morsel of platinum whose surface has been oxidized, • Bibl. Univ., t. xvii (October, 1838), p. 378. |