zinc on an aqueous solution of potassium nitrate; the products of this action are zinc hydroxide, potassium nitrite and water; thus, Zn+ он H он H +ONO,K=Zn(OH)2 + H2O + NO2K. Zinc has no action on an aqueous solution of potassium nitrite, and we know that this salt in the solid form is unchanged at temperatures whereat potassium nitrate parts with one-third of its oxygen. If the conditions of the preceding action-that viz. of zinc on an aqueous solution of potassium nitrate in presence of oxygen-are arranged so that hydrogen is freely evolved. during the change [this can be most readily done by using zinc and iron in place of zinc only], then some of the potassium nitrite is further reduced with production of ammonia. It would appear then that in certain reactions, when hydrogen is separated from water, and oxygen is separated from another compound in contact with the water,-the two gases being brought within the sphere of mutual action in the proportion of two atoms of hydrogen to one atom of oxygen,— water is produced; but that when the hydrogen and oxygen, produced as before, react in the proportion of two atoms of hydrogen to one molecule (not two atoms) of oxygen, hydrogen peroxide is the product. Traube got some very interesting and important results from experiments on the electrolysis of water, using electrodes of different materials. He found that no hydrogen peroxide was produced when the electrodes were composed of one of those metals which readily produce hydrogen peroxide by their action on water and oxygen (or in some cases dilute acid and oxygen); but that when the electrode consisted of a metal which does not yield hydrogen peroxide under the conditions named, hydrogen peroxide was formed, in greater or less quantity, during electrolysis. As a typical member of the latter class of metals, Traube chose palladium. When palladium is charged with hydrogen and made the positive pole of the battery, no hydrogen peroxide is produced, but the oxygen which is being evolved is absorbed by the palladium and is combined with the occluded hydrogen to form water. When however the hydrogenised palladium is made the negative pole, a little hydrogen peroxide is produced; and the quantity of this compound may be considerably increased by causing bubbles of air to rise through the liquid near the negative pole. If however no air is passed through the water, and at the same time the transference of oxygen from the positive pole (where it is being liberated) through the liquid to the negative pole is mechanically prevented, no hydrogen peroxide, or only a trace of this compound, is produced. Further, if hydrogenised palladium is made the positive pole, and bubbles of air are at the same time caused to rise through the liquid around the pole, a little, but only a little, hydrogen peroxide is produced. Finally if the electrodes are made of palladium uncharged with hydrogen the maximum yield of hydrogen peroxide is obtained (entirely at the negative pole) by arranging the rate of electrolysis so that the whole of the hydrogen produced is occluded by the palladium; the more rapid the evolution of hydrogen from the liquid the smaller is the quantity of hydrogen peroxide produced'. Now it is generally supposed that the greater part of the oxygen or hydrogen liberated during electrolysis of water is at the moment of its production in the state of atoms, and that the greater part of the oxygen in ordinary air is composed of molecules; if this be granted, it follows that Traube's experiments establish a marked difference between the reactions of oxygen atoms and oxygen molecules: by their action on hydrogen occluded by palladium, the former produce water, the latter produce hydrogen peroxide; if a few atoms and many molecules of oxygen are present much peroxide and little water are the products, while if many atoms and few molecules of oxygen are brought into contact with the hydrogen, much water and little peroxide is the result. 1 These results are strictly confirmatory of those obtained by Gladstone and Tribe in their electrolytic experiments on the reduction of acids. See ante, p. 93. 44. But the experiments of Traube also shew that the direction and final goal of the chemical change depends not only on the structure of the particles of oxygen, but also on the source and conditions of supply of the hydrogen. If the hydrogen is produced by rapid electrolysis little peroxide is formed; indeed if the hydrogen is produced, rapidly or slowly, by electrolysis with carbon poles no peroxide is obtained. The chemical nature, and the masses, of all the members of the changing system influence the final configuration. The importance of considering the conditions under which hydrogen is produced when we are attempting to explain any of the phenomena classed together as those of nascent state, is emphasised by the fact, already alluded to, that the metals which decompose water in absence of oxygen, do not give rise to the production of hydrogen peroxide by their action on water in presence of oxygen: for instance, hydrogen peroxide is never produced by the action of sodium on water. It is not enough then that oxygen molecules should be present in contact with atoms of hydrogen as these are liberated from water. The peroxide results from the mutual actions and reactions of the three substances, metal, water, oxygen; if the water is decomposed by the metal alone, hydrogen is evolved rapidly and escapes the pursuit of the oxygen molecules; the peroxide appears to be a product of the joint action of the metal and oxygen on the molecules of water. This conception of a joint action of metal and oxygen may be carried over to explain some of the phenomena exhibited when metals and acids react. Traube seeks to explain many of these reactions in this way. Copper does not remove oxygen from an aqueous solution of potassium nitrate as zinc does; but if copper is brought into contact with dilute sulphuric acid in presence of oxygen, hydrogen peroxide is produced. The joint action of copper and potassium nitrate is not sufficient to decompose watermolecules; but copper and oxygen aided by a little sulphuric acid suffice to complete this change. The action in question is represented thus by Traube, (b) (when a certain amount of H2O2 is produced) Cu+H2O2 Cu(OH)2 (c) Cu(OH)2+H2SO1 = CuSO4+2H2O. If some compound which is readily acted on by hydrogen be substituted for oxygen in this series of changes, then copper and dilute sulphuric acid form a reducing agent; ferric sulphate e.g. is reduced under these conditions to ferrous sulphate, Cu + JOH OH H +Fe2. 3SO4 + xH2SO1=Cu(OH)2+2FeSO4+(x+1)H2SO [= CuSO1+2FeSO4+2H2O+H2SO] Similarly the action of copper on dilute nitric acid would be represented thus, (OH H 3Cu+3 } (OHH) +3(O NO,H)=3Cu(OH)2+3H2O+NO2H [but 3NO2H rapidly decomposes to give HNO3+2NO+H2O]. As thus regarded, these actions of metals on acids are complex changes; at one stage or other of the complete change hydrogen plays an important part, and it does this in virtue of being itself a product of another part of the whole reaction. Hydrogen imported from without the system fails to accomplish actions which are brought about by hydrogen generated within the system, provided this hydrogen be produced at the proper rate, and under conditions generally favourable to the action it is to perform. The investigation of Divers' 'On the production of hydroxylamine from nitric acid' is an interesting and instructive example of the need of considering all the members of a changing system in attempting to find an explanation of the change. Hydroxylamine is one of the products of the action of tin, zinc, and some other metals on nitric acid; ammonia is also 1 C. S. Journal Trans. for 1883. 443. produced in these reactions; if the action continues for some time no hydroxylamine, but only ammonia can be detected. Addition of hydrochloric or sulphuric acid causes a marked increase in the yield of hydroxylamine. Divers shews that the production of hydroxylamine by the direct action of tin, &c. on nitric acid is most probably preceded by separation of hydrogen from the acid, and that the action of this hydrogen on another portion of the acid is the immediate cause of the formation of hydroxylamine. He also regards his experiments (which it must be admitted are not very numerous) as pointing to the conclusion that the reason why silver, mercury, copper, and bismuth do not produce hydroxylamine or ammonia, when they act on nitric acid, is, that these metals do not displace hydrogen in nitric acid, but rather combine with the nitroxyl and hydroxyl groups, (NO2 and OH) forming nitrites and hydroxides (MNO, and MOH), which then mutually react to produce nitrous acid, metallic nitrate, and water. The tin and zinc metals, on the other hand, probably directly produce metallic nitrates, the subsequent formation of nitrites being due to reactions between the metal and its nitrate. Hydroxylamine is an unstable substance; Divers thinks he has experimentally shewn that the increase in the yield of this compound, when nitric and hydrochloric (or sulphuric) acids act on tin, &c. over the yield obtained by the action of nitric acid alone, is chiefly due (1) to production of chloride, or sulphate, of hydroxylamine, both of which salts are more stable than the nitrate, (2) to prevention, by the hydrochloric or sulphuric acid, of formation of nitrous acid, which readily decomposes hydroxylamine, and (3) to production and maintenance of a reducing environment (hydrogen) around the hydroxylamine, by virtue of direct action between the metal and the second acid. Under these circumstances the greater part of the hydroxylamine, produced by the action of the nitric acid on the metal, remains undecomposed. Divers does not find it necessary to suppose that the second acid directly supplies hydrogen for reduction of nitric acid, but, at the same time, he thinks that this reduction is indirectly assisted by the second acid |