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CHEMISTRY 735 Zn + H2S04<—>H2 + ZnS04, and that the extent to which occurrence of electrolysis possible—whatever these may be reversal takes place depends on the power possessed by in the case of gases. the positive electrode of conditioning the return of hydrogen In order to test whether the high stability of the oxygen into circuit. molecule renders a mixture with carbonic oxide inexplosive, In cells of the Daniell type a constant electromotive and whether oxygen in the “ nascent ” state is more force is secured owing to the complete prevention of pol- active, Dixon has made the interesting experiment of arization and its consequences, but the depolarizer not exploding a well-dried mixture of carbonic oxide and only prevents back action and maintains the resistance chlorine peroxide, the result being that much of the former constant by acting as a scavenger, but also always con- remained unburnt just as when well-dried oxygen is used tributes to the development of energy in the circuit. (ibid. 1896, p. 605). As it must be supposed that at the Change, in fact, is often conditioned by the presence of a moment of explosion oxygen atoms are liberated, it might depolarizer, an exothermic series of changes being thereby be expected that the oxygen from such a source should be converted into an endothermic series. Thus copper is specially active. On the other hand, when cyanogen is insoluble in dilute sulphuric acid, its “ heat of dissolution ” exploded with an excess of oxygen, it is entirely burnt to in such acid being a negative value, which may readily be carbon dioxide, and Dixon supposes that the carbonic calculated from the known heat of formation of copper oxide formed in the wave front is slowly burnt behind the oxide and that of copper sulphate and water from the wave front. It should be mentioned that a spark of conoxide and dilute sulphuric acid; thus siderable intensity is required to initiate an explosion in such a mixture, and that the presence or absence of Cu + y = CuO = 37,160 gram. deg. C units moisture apparently makes no difference in the readiness CuO + H2S04, aq. = CuS04, aq. + H20 = 18,800 units with which the mixture is inflamed. As it cannot be assumed that Dixon dealt with really dry gases, this probH2 +y= H20 = 68,360 units ably means that a much smaller proportion of moisture Cu + H2S04, aq. = H2 + CuS04, aq. = — 12,400 units. suffices in the case of cyanogen to promote the interchange, In presence of oxygen, however, the metal readily dis- and the completeness of the oxidation may be ascribed to the fact that cyanogen has a very high “heat of combustion.” solves, the gas acting as the depolarizer, In the case of the explosion with chlorine peroxide, the Cu + H2S04, aq. + — H20 4- CuS04, aq. = 55,960 units. chlorine would interfere by decomposing a portion of the

water, and the effect of the atomic oxygen in promoting

A cell consisting of copper, dilute sulphuric acid, and the change would be counterbalanced by the withdrawal of oxygen, therefore, has an electromotive force of about moisture. In general chemical behaviour, carbonic oxide |-|g-jy§=l'2 volts. Actually, copper sulphate is manu- is a far more saturated and less active substance than factured on a very large scale by the action of dilute cyanogen, and water molecules would have less tendency sulphuric acid on granulated copper in presence of air. perhaps to couple up with it than with cyanogen. It is In the case just considered, the third term of the desirable to call attention to this point in order to lay emsystem is not a substance which acts as a conductor in phasis on the need, in discussing chemical interchanges, of* ordinary circumstances; but it is merely one which can taking into account both the compatibility of the various become included in the system. It would therefore seem molecules present and the stability of the loose molecular to be necessary, in order that chemical change may take complexes which may form prior to the occurrence of any place, that a circuit be established in a system composed of interchange. at least three “elements,” which is conducting as a whole, Although it may be said that a fairly general agreement and which contains, together with an electrolyte and a exists to regard chemical changes as essentially electrolytic substance which sooner or later is attacked by the electro- : in their nature, there is far from being any Mechanism lyte, a third substance which may or may not take a agreement as to what really takes place—as to of chemical material part in the change. the character of the “ mechanism ” concerned in interIn the case of the combustion of carbonic oxide by moist their occurrence. Williamson, in 1851, in his chan£eoxygen, Dixon has argued that since carbonic oxide is paper on the “ Theory of Etherification,” put forward in inert towards oxygen, its oxidation in presence explanation of “ the general process of chemical decomGaseous 0f moisture is a secondary effect, the steam act- position,” the view that in an aggregate of molecules of any 'actions catalytically as a carrier of oxygen. It is compound there is an exchange constantly going on between well known that at a high temperature carbonic the elements which are contained in it. Thus in a drop of oxide decomposes steam (CO + OH2 = H2 + C02), and Dixon hydrochloric acid, supposed to be made up of a great supposes that the whole of the oxidation is effected in this number of molecules of the composition C1H, each atom way, the hydrogen being alternately oxidized and deprived of hydrogen does not remain quietly in juxtaposition with of oxygen (Trans. Chem. Soc. 1898, p. 774). That a part the atom of chlorine with which it first united, but on the of the carbonic oxide may be oxidized in this simple manner contrary is constantly changing places with other atoms of cannot be denied, but probably it is only a minor part; hydrogen. Supposing the hydrochloric acid to be mixed there must be some collisions in which only carbonic oxide with some sulphate of copper (of which the component and water molecules meet, but considering the proportions atoms are undergoing a similar change of place), the in which carbonic oxide and oxygen are present, it must hydrogen does not merely move from one atom of chlorfrequently happen that their molecules will together ine to another, but in its turn also replaces an atom of collide with water molecules, in which case the interaction copper. Thus it is that at any moment of time at which we examine the mixture the bases are divided between the CO + OH., + ~ = C02 + H20 is bound to occur ; especially acids (Quart. Journ. Chem. Soc. 4, 110). Practically the as it is a change in which heat is liberated, i.e. exothermic, same view was applied by Clausius in 1857 in explanation whereas the interaction of carbonic oxide and water is one of the phenomena of electrolysis; but whereas Williamson in which heat is absorbed, i.e., endothermic. But apart had spoken only of exchanges going on, Clausius ascribed from the question which is the preponderating change, the occurrence of these to collisions taking place between they may equally be regarded as electrolytic changes, i.e., the molecules of sufficient violence to cause some to split as taking place only under the conditions which render the up into their constituents, which then wandered about till