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myself* for the last three, that these several metals do not dissolve in nitric acid, but if the reduction-product of nitr acid, be it nitrous acid or nitrogen peroxide, is present, then these metals dissolve. To put the facts in a logical form :(i.) These metals do not dissolve in nitric acid; (ii.) these metals do dissolve in nitric acid containing nitrous acid t. Unfortunately at present there seems to be no means of proving the third term, namely, that these metals do not dissolve in nitrous acid, though it is clearly demonstrated that the amount of metal dissolved varies not with the amount of nitric acid within certain limits, but with the amount of nitrous acid. It must therefore be concluded that the presence of the nitrous acid is the cause, or a necessary part of the cause, of the phenomenon of the solution; and it would follow that, whatever be the correct interpretation of the chemical change, nitric acid is not the actual solvent of the metal. Indeed my own experiments have shown that, within limits, the presence of an excess of nitric acid not only does not increase, but actually retards the solution of the three metals copper, mercury, and bismuth. But Armstrong conceives that "the hydrogen of the acid is virtually directly displaced by the metal with the assistance, however, of the current-energy derived from its own oxidation by the nitrous compound". There are, therefore, at least two interpretations of the problem: First, it is supposed that the hydrogen of the acid is displaced, and this in the nascent condition, or somehow or another, reduces the nitric to nitrous acid, thus:2HNO2+M = M(NO3)2 + H2,

HNO2+H2 = HNO2+H2O;

or

2HNO3 + H2

=

N2O4+2H2O.

Secondly, it may be supposed that the nitrous acid or nitrogen peroxide is formed from the nitric acid per se, and then the metal can be dissolved thus :—

(i.)

HNO3 = HNO2+0,

(ii.) 2HNO2+M = M(NO2)2+2H2O+2NO ;

or

(i.)

2HNO3

=

N2O4 + H2O+0,

(ii.)

M+N2O4 = M(NO2) 2

* Phil. Trans. 1891 [A], pp. 279-317.

I use the term "nitrous acid" rather for the sake of brevity, and without wishing to exclude the possibility that the active material is really nitrogen peroxide.

Armstrong, Proc. Chem. Soc. 1892 ("On the Nature of Depolarisers") Phil. Mag. S. 5. Vol. 37. No. 225. Feb. 1894,

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182

Mr. V. H. Veley on the Phases and

The first view is based upon the analogy of the solution of certain metals in sulphuric acid, whereby hydrogen is set free, and also upon the argument that this same gas is set free, when metallic magnesium is dissolved in nitric acid; but it appears that, if we accept this view, a certain property, namely that of reducing nitric acid, is attributed to the element hydrogen when in a certain condition (to which also is superadded a currentenergy), which it does not possess in the state or condition in which we know it. The general conception is that the element hydrogen in the former case is "atomic" and in the latter is "molecular," though it is to be observed that the relations of atoms to molecules are in obscurity. That is to say, four distinct hypotheses are involved:-(i.) that of nascent hydrogen; (ii.) that nascent hydrogen is atomic; (iii.) that nascent or atomic hydrogen differs in its attributes from hydrogen as we know it; and (iv.) that hydrogen, as we know it, is molecular. Further, it does not seem safe to argue from the reaction of magnesium with nitric acid to those of copper &c. with the same acid, for at present there is no evidence to show that the solution of magnesium in the acid is dependent upon the presence of nitrous acid. If this is shown not to be the case, cadit quæstio.

The often-cited argument in favour of the nascent hydrogen hypothesis, namely, that no reduction ensues when hydrogen gas is passed through nitric acid, appears to be rather beside the question, for on the one hand the contact between a gas and a liquid is very incomplete, only the outer shell of each bubble being affected, and on the other the stream of the gas would mechanically blow off any nitrous acid (or nitrogen peroxide) which might be formed.

The argument from the reactions supposed to take place in the Grove's cell seems also to be beside the point, since there must be a certain amount of interdiffusion between the nitric and sulphuric acids which would complicate the chemical changes. But, on the other hand, the following facts support the view that the nitrous acid or nitrogen peroxide is formed initially from the nitric acid per se:-First, the more concentrated acid very rapidly turns yellow from the formation of nitrogen peroxide, when exposed to the sunlight, but more slowly when the acid is kept in the dark or diluted. Further, my experience has shown that even the jolting of a railway journey is sufficient to increase the proportion of the nitrous compound even in the case of acids of about 30 per cent. concentration. Secondly, the same change is very readily effected by the presence of organic matter, which it is well-nigh impossible to eliminate from the air of laboratories.

In the course of the investigations conducted by Burch and myself on the E.M.F. of certain cells containing nitric acid, it was continually found that if the containing vesssel was not clean, if the metallic strips were touched with the fingers, or if the strips were not thoroughly cleansed, the E.M.F. of the cell rose to its maximum at once. It is quite clear, therefore, that a trace of the nitrous compound could be formed in numerous ways, and, when once formed, would tend to increase in the presence of the metal. The recent investigations of Sabatier and Sanderens have shown that finely divided copper readily absorbs nitrogen peroxide at ordinary temperatures to produce a nitro-copper of composition Cu2NO2, upon which water reacts with violence to give a solution of copper nitrate and nitrite, nitric-oxide gas being evolved.

Though it is not wholly safe to argue from the results of experiments conducted under one set of conditions to those conducted under another, yet it is remarkable that the products of the reaction of copper with diluted nitric acid are in the main identical with those of water with the nitro-copper. The conclusion is not, therefore, wholly improbable that the nitrogen peroxide present in the nitric acid combines with the copper to form intermediately the nitro-copper, which in its turn is decomposed by the water to re-form the nitrous compound. The researches of Freer and Higley † have further shown that in the case of concentrated acids the main product of the reaction of metallic copper is nitrogen peroxide.

It is also remarkable that metallic lead dissolves very rapidly in the liquid obtained by the addition of nitrogen peroxide to water, even though of a less degree of acidity than other mixtures of nitric acid and the nitrous compound, though here again it is not wholly safe to argue from the results obtained in the case of one metal to those of another. At present I am engaged upon this point for metals other than lead. If, then, the formation of nitrogen peroxide be accomplished, then not only will nitric oxide result, but also the more deoxidized products such as nitrous oxide and nitrogen will be produced from subsidiary reactions between the nitric acid and the metallic nitrates. The formation, therefore, of all the products in the case of the metals mercury, copper, silver, and bismuth can readily be explained without recourse to the convenient hypothesis, nascent hydrogen. But at present there is no satisfactory explanation of the formation of ammonia and hydroxylamine, and this mainly for the *Bull. Soc. Chim. [3] ix. pp. 669–674. † Amer. Chem. Journ. 1893, p. 71. Journ. Soc. Chem. Indust. 1891.

reason that the reactions between nitric oxide and metallic nitrates have hitherto received but little attention.

The reactions of metals with sulphuric acid have not been the subject of many investigations, partly, no doubt, from their assumed simplicity. Even at present there is no exact knowledge as to the nature of the intermediate substance formed from the sulphuric acid, which enables the metal to dissolve and to continue dissolving; probably it is some product of electrolysis, for it has been shown that pure zinc dissolves in diluted sulphuric acid through which an electric current has been passed, though no reaction ensued between the same samples of zinc and sulphuric acid previous to the passage of the current. Yet, whatever this substance may be, if the metal dissolves in the dilute acid, there are always formed traces, however small, of products other than hydrogen, and especially of hydrogen sulphide, while cadmium (which in many other respects resembles zinc) gives such reductionproducts that the liquid turns milky with formation of sulphur.

The reaction of copper and allied metals with concentrated sulphuric acid is generally represented as taking place in two successive stages, namely, first the direct replacement of the hydrogen by the metal, and, secondly, the reduction of the sulphuric acid by the nascent hydrogen, thus:

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But here, again, there seems to be but little necessity for the hypothesis of nascent hydrogen, for the reaction only ensues with more concentrated acid, in which there is a greater tendency towards a decomposition of the acid into the trioxide and water, and the metallic copper may reduce the former with formation of cupric oxide and sulphur dioxide. Or, if the solution of the metal, as that of zinc in sulphuric acid, is dependent upon some product of electrolysis, the metal per se may directly reduce this. Though neither of these views is as yet confirmed by experimental investigation, yet both appear to be more probable than that of nascent hydrogen.

Investigations upon the conditions of formation and function of these intermediate compounds are greatly wanted, especially as the chemical changes between metals and acids resemble others discussed above, in that they pass through the same successive phases.

The University Museum, Oxford.

XIV. A Study of the Polarization upon a Thin Metal Partition in a Voltameter.-Part I. By JOHN DANIEL*.

THER

[Plates III. & IV.]

Introduction.

HE curious polarization phenomena of very small electrodes in a sulphuric-acid voltameter bearing a strong current, accounts of which appeared in the Annalen during the winter of 1892, suggested to Dr. L. Arons, of the University of Berlin, to try a very thin metal partition in a voltameter, expecting, as he said, that there would be a development of heat at this partition, resulting in the destruction of the partition. Dr. Arons used gold-leaf as a partition. The gold-leaf was pasted with canada balsam over a hole about 15 millim. in diameter bored in a glass plate, which was slipped into the groove of a wooden or vulcanite frame in the middle of the voltameter. This did not give a tight partition, but sufficed for the observation that there was not even visible development of gas on the gold-leaf; whereas platinum 02 millim. thick, substituted for the gold-leaf, showed profuse development of gas with the current-strength used, even when punctured with a hole 2 millim. square. I think Dr. Arons also tried the gold-leaf partition in a solution of Cu or Ag salt, and observed that the metal was deposited on the partition.

At the suggestion and under the kind direction of Professor A. Kundt and Dr. L. Arons I undertook a more thorough investigation of the subject. Several problems presented themselves :

(1) By quantitative measurement to determine whether there be a critical thickness below which there will be no polarization at the partition, and, if so, to determine this thickness.

(2) To determine the other critical thickness for which the polarization is as great as for very thick plates.

(3) The quantitative measurement of this polarization with the same current-strength for plates of various thicknesses between these limits.

(4) By varying the current-strength in these different cases, to determine what function the polarization is of the currentstrength.

(5) To learn how the polarization in these cases varies with the time during which a given current-strength flows, readings mmunicated by Prof. O. J. Lodge, D.Sc., F.R.S.

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