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and which is most readily explained as consisting of two parts proceeding simultaneously

$(1) Cu+H.S0 =CuSO4+H2).

(2) H,+H,50,= 2H,0+S0.J Tin and lead are dissolved by hot concentrated sulphuric acid, with production of sulphates and evolution of hydrogen and sulphur dioxide, sometimes accompanied by sulphuretted hydrogen, and with separation of sulphur. With more dilute acid tin evolves hydrogen, and as temperature is increased, sulphuretted hydrogen also. The action of zinc on sulphuric acid is broadly analogous to that of tin.

Quantitative analysis of the products of reduction of nitric acid by magnesium, zinc, and cadmium respectively, shews that reduction is carried furthest by magnesium, and further by zinc than by cadmium. Now the ‘heats of formation' (see Chap. IV) of the oxides of these metals are, for Mg 147,132, for Zn 88,244, and for Cd 30,364 thermal gram-units, hence it is almost certain that that reaction of metal on acid in which the greatest amount of heat is evolved is accompanied by the greatest reduction of acid.

The following numbers representing quantities of heat evolved in the chemical changes formulated were obtained by Thomsen (see Chap. IV.).

H2, S, 0,, aq. = 210,760 gram-units +.
H, N, O3, aq.= 34,270

+.
Zn, H.S04, aq. = 106,090

Zn, 2HNO3, aq.=136,340 Berthelot also gives the thermal value of 21,500 gramunits to the chemical change

HNO3 (dilute) +8H=NH, (dilute) + 3H,0. From these numbers we should expect sulphuric acid to be more stable, towards heat, than nitric acid, and we should expect the action of zinc on these acids to result in a more complete deoxidation of nitric than of sulphuric acid.

In the action of metal on nitric acid at ordinary temperatures, we have then, an unstable acid, a considerable

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heat evolution, and the production of hydrogen in contact with the acid, we have conditions eminently favourable to deoxidation. In the action of metal on dilute sulphuric acid, on the other hand, we have a more stable acid and a smaller heat evolution, consequently the hydrogen escapes unchanged; but when the acid is so concentrated that addition of heat from without is required to start the reaction, and when the acid is therefore in a condition more comparable with that of nitric acid at ordinary temperatures, a portion of the hydrogen then evolved undergoes oxidation at the expense of the oxygen of the acid. If however hydrogen is evolved-as in the experiments of Gladstone and Tribe-in contact with concentrated acid at ordinary temperatures, a part of this hydrogen is always oxidised'.

The facts, that hot sulphuric acid is deoxidised by carbon, and apparently by phosphorus also?, and that it is possible by heat alone to decompose this acid into sulphur dioxide, oxygen, and water, have caused some chemists to regard the actions of metals on this acid as simply cases of direct deoxidation: but it seems to me that the facts enumerated—both chemical and physical, with regard to the action of metals on this acid and on nitric acid-are more in keeping with that hypothesis according to which hydrogen plays an essential part in the series of changes, than with any other hitherto advanced. There may be, indeed there undoubtedly is, more than one process of chemical change resulting in the deoxidation of sulphuric acid, in some cases direct deoxidation preponderates, in others hydrogen plays the more important part.

Experiments recently conducted by Thorpes on the reducing action of zinc, magnesium and tin on acidulated solutions of ferric sulphate, shewed that whatever condition tends to give greater chances of contact between the hydrogen produced in the liquid and the ferric sulphate in solu

1 When however vapour of sulphuric acid mixed with hydrogen is passed through a hot tube, sulphuretted hydrogen is produced.

Cross, C. S. Journal Trans. for 1879. 253. 3 C. S. Journal Trans. for 1882. 289.

tion, increases the rate of reduction; that increase of the rate at which hydrogen is evolved, other conditions remaining constant, is accompanied by decrease of the amount of reduction in unit of time; and that the presence of certain salts, e.g. zinc sulphate, causes a decrease in the rate of reduction. Thorpe's results also established a distinct connection between the nature of the metal used and the influence on the rate of reduction of the varying conditions under which the experiments were conducted.

These experiments, and indeed all experiments on the action of metals on acids, emphasise the necessity that exists for considering all the reacting substances which take part in a process of reduction by hydrogen, and not confining attention to the hydrogen alone. The results of experiments by Tommasi' also shew this need : Tommasi found that potassium chlorate was not deoxidised by hydrogen evolved by the action of sodium-amalgam, but was reduced by hydrogen evolved by the action of 'zinc on diluted sulphuric acid, but that the latter agents failed to remove oxygen from potassium perchlorate.

43. The conception which underlies such expressions as nascent actions, action of nascent hydrogen, &c., is that implied in the distinction drawn between atom and molecule. That this distinction is one not merely of terminology' but based on actual reactions, is rendered apparent by the results of recent experiments by Traube on the conditions under which hydrogen peroxide-H,0,-is produced.

Hydrogen peroxide has been very generally regarded as oxidised water; Traube says it is rather reduced oxygen. The production of hydrogen peroxide during processes of oxidation, occurring in presence of water, has been sought to be explained by assuming that the oxidising substance decomposes oxygen molecules, retains some of the atoms, and sets the others free under conditions favourable to the production of triatomic molecules of ozone, and that the ozone thus produced then reacts on water molecules and con

See especially Pogg. Beiblätter, 2. 205.

Ber. 15. 659, 2421, 2434: 16. 1201.
M. C.

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verts these into molecules of hydrogen peroxide. Traube's hypothesis regards the oxidising substance as decomposing the water molecules present, withdrawing oxygen and part of the hydrogen, and setting free the remainder of the hydrogen, which thereupon combines with oxygen molecules and so produces hydrogen peroxide.

Thus, the production of hydrogen peroxide by the mutual action of zinc, oxygen, and water might be represented, on the first hypothesis, as occurring in two stages,

(32n + 30,=3ZnO +03, (a)

13ZnO +3H,0=3Zn(OH)2,

(6) 3H,0+0,=3H,O,; and on the second hypothesis, as occurring essentially in one stage, thus,

(a) 32n +60H, +30,=3Zn(OH)2+3H,02%. The following numbers, representing the thermal values of various changes, some of which may occur in the complete reaction now under consideration, are taken from Naumann's Thermochemie.

[H2, 0]=68.360 gram-units +. [H,0, 0]=23,070 gram-units [H2, 02, aq]=45,290 » +. [H,Ogaq, H,]=91,430 » »

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1 The two hypotheses may be more clearly grasped if these reactions are represented graphically thus:

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1. (a) Zn)

Zn+ 0 0=32n0+ 0 [3Zn0+ 34,0= 32n (OH)2].
Zn) 100

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an explanation of these thermal measurements see post, chap. IV., par. 118.

These numbers are opposed to the supposition that water should be readily changed into hydrogen peroxide by a process of direct oxidation. Traube's results, especially those connected with his experiments on electrolysis, seem to shew that neither water nor hydrogen can be directly oxidised to hydrogen peroxide (see pp. 100—101).

Certain metals, e.g. zinc, decompose water only in presence of oxygen, forming hydroxides and hydrogen peroxide ; other metals, e.g. sodium, decompose water in absence of oxygen, forming hydroxides and evolving hydrogen; there are other substances, for instance palladium charged with hydrogen, the action of which on water shews that they belong to the same class of substances as zinc. Traube formulates the actions of zinc and hydrogenised palladium (which he regards as a definite compound Pd,H) on water in presence of oxygen thus,

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When the quantity of hydrogen peroxide reaches a certain limit a secondary action begins, resulting in the decomposition of the peroxide; thus with zinc,

Zn+H,02=Zn(OH)2. Traube thus regards the formation of each molecule of hydrogen peroxide as the result of the action of two atoms of hydrogen on one molecule of oxygen; he does not suppose that the molecule of oxygen is shattered and that its constituent atoms combine with the atoms of hydrogen, but rather that the two hydrogen atoms join themselves on to the already formed oxygen molecule. If this be granted, it seems to follow that, were atoms of oxygen presented to the hydrogen atoms as they escape in pairs from the water molecules, water, and not peroxide of hydrogen, would be produced. Traube says that this supposition is shewn to be correct by the reaction of

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