ADHESION SURFACE ACTIONS.

867

frequently favours the mutual action of dry gases upon each other. For example, sulphurous anhydride and sulphuretted hydrogen may be mixed when dry without acting upon each other, but if water be present, the mutual decomposition of the two gases is the result. In like manner, when dry gaseous sulphurous anhydride and dry peroxide of nitrogen are mixed together, no combination takes place between them; the addition of a few drops of water, howL ever, causes them immediately to condense and to form the white · crystalline compound which has been spoken of when treating of the manufacture of sulphuric acid (412, and note p. 179). If the elasticity of these gases be overcome by other means-if, for instance, they be liquefied by exposing them to a low temperature -combination occurs without the intervention of moisture.

Water, by overcoming the self-repulsion of the gases, favours their chemical action upon solid bodies. Hydrochloric acid, and ammonia, in their gaseous form, generally exert comparatively little influence upon the metals or upon their salts, although when in solution their action upon them is rapid and powerful.

Surface Actions.-The adhesion of gases to solids produces many curious phenomena :-for example, let a piece of charcoal be thoroughly saturated with hydrogen by attaching it to the negative wire of the voltaic battery, and employing it as the platinode in the decomposition of acidulated water: this charcoal, if now detached from the battery and thrown into a solution of sulphate of copper, or of nitrate of silver, will effect the decomposition of these salts, and their respective metals will be thrown down upon the charcoal in the reduced state; the charcoal and condensed hydrogen appearing to act the part of a voltaic circuit, in which the hydrogen supplies the place of the electro-positive or oxidizable metal, and the charcoal that of the electro-negative metal or conducting plate.. If a plate of platinum, rendered chemically clean,* be introduced into a mixture of pure oxygen and hydrogen, in the proportions to form water, the gases become condensed upon the surface of the plate, and being brought within the sphere of each other's attraction, begin to unite; at first slowly, but during the act of combination heat is extricated, and the action proceeds more quickly, until at last the plate be

This may be effected by holding the plate over the flame of a spirit-lamp and rubbing it, when hot, with a stick of caustic potash; the potash is to be maintained in a fused state upon its surface for a second or two; the alkali is then to be washed off completely in distilled water, and the plate is to be immersed for a minute in hot oil of vitriol; after which it is to be freed from adhering acid by immersion for a quarter of an hour in a large bulk of distilled water.

868

SURFACE ACTIONS OF PLATINUM.

comes red hot, and an explosion of the gas ensues (Faraday, Phil. Trans., 1834, p. 55). By employing the metal in a disintegrated or spongy form, the surface exposed is greater, and the action much more rapid: the metal conducts away but little of the heat which is generated, and soon becomes red hot; whilst in the condition of platinum black (968) this activity attains its maximum. On throwing a little of this black powder into a mixture of oxygen and hydrogen it immediately becomes incandescent, and the gases combine with a loud report. Platinum may be obtained in a convenient state of fine subdivision for experiments of this nature, by moistening asbestos with a solution of perchloride of platinum and exposing it to a red heat; the chlorine is expelled, and a film of minutely divided platinum is left upon the surface of each fibre of asbestos.

From its inalterability by ordinary chemical agents, platinum in this finely divided form has been used to effect various combinations which cannot otherwise readily be procured between vaporized and gaseous bodies:-For instance, if ammonia be mixed with atmospheric air, and transmitted over spongy platinum gently heated, its nitrogen becomes converted into nitric acid, and its hydrogen into water; H2N+20,=HNO2+H2O: but this transformation cannot be effected by heat, unless some substance analogous to spongy platinum be used, since nitric acid is decomposed at a temperature which, under ordinary circumstances, is required to effect the combustion of ammonia. On the other hand, ammonia may be formed from the oxides of nitrogen, by mixing them with hydrogen and transmitting the gases over platinum sponge gently heated; 2 NO+5 H2=2 H2N +2 H ̧Ð. Nitrate of ammonium, when heated with platinum black, yields nitric acid, nitrogen, and water, instead of nitrous oxide; for instance, 5 HNN→ ̧=2 HNO3+4 N2+9 H2→. A variety of other interesting changes may be effected. According to Döbereiner, (who first pointed out the remarkable power which finely divided platinum possesses of effecting combinations of this kind,) a mixture of cyanogen and hydrogen when in contact with spongy platinum is partially converted by the aid of a gentle heat into cyanide of ammonium. In a mixture of nitric oxide and olefiant gas, carbonate of ammonium is produced; and in a mixture of the vapour of alcohol and nitric oxide,-cyanide and carbonate of ammonium, olefiant gas, water, and a deposit of carbon are formed. In like manner, sulphurous anhydride may be rapidly converted into sulphuric acid, if it be driven, in a moist state, mingled with air, through tubes containing spongy platinum: this method was

3

1

SURFACE ACTIONS OF PLATINUM.

869 even proposed as a manufacturing process for obtaining oil of vitriol, but it was abandoned in consequence of a gradual alteration in the platinum, by which it is deprived of this power of effecting combination. Platinum black produces with the vapours of alcohol in contact with atmospheric air, a series of compounds which are finally converted into acetic acid and water:

For the success of these experiments, it is necessary that the surface of the platinum be chemically clean, otherwise the combination of the gases does not take place. Faraday considers that these actions are owing to the adhesion of the gases to the surface of the metal, by which the particles of one gas are brought into chemical contact with those of the other. He observed that the admixture of small quantities of carbonic oxide, or of the vapour of bisulphide of carbon, or of olefiant gas,* prevented the platinum from effecting the combination of the oxygen and hydrogen, but did not deprive the metal of its activity, as was ascertained by afterwards plunging it into a mixture of pure oxygen and hydrogen. On the other hand, the addition of sulphuretted or of phosphuretted hydrogen to an explosive mixture of oxygen and hydrogen, not only prevented the combination from being produced by the platinum, but it effected such an alteration of the surface of this metal that when it was plunged into a fresh portion of mixed oxygen and hydrogen, no combination of the gases occurred. Hydrochloric acid also rapidly destroys the peculiar properties of finely divided platinum; according to Döbereiner, the preventive action of this gas depends upon the decomposition of the hydrochloric acid by the oxygen condensed upon the platinum: water is formed, whilst chlorine is liberated, and this chlorine, by converting the platinum superficially into chloride, destroys its power; its activity, however, can be restored by treating it with boiling oil of vitriol. Hydrochloric acid is in this case expelled, and a small quantity of protoxide of platinum is dissolved; the metal is then to be well washed in distilled water. (1005) Other finely divided substances besides platinum possess this property of favouring the combination of oxygen and

Graham finds that in the case of carbonic oxide a gradual oxidation of the carbonic oxide takes place, but that this action is much slower than the oxidation of the hydrogen: the oxidizing influence is wholly concentrated on the carbonic oxide, and until this gas is entirely oxidated the hydrogen remains unaltered in the mixture.

hydrogen in an inferior degree; even pounded glass, porcelain charcoal, pumice, and rock-crystal, if warmed to 600°, produce this effect. Finely divided palladium, rhodium, and iridium also determine the combination of oxygen and hydrogen with explosion at ordinary temperatures. Gold and silver effect the combination of hydrogen with oxygen quietly, at temperatures far below the boiling-point of mercury (Dulong and Thénard). Metals which have a strong chemical attraction for oxygen cannot be used, because they immediately become oxidized upon their surface. (1006) Catalysis.-The remarkable actions produced by the agency of finely divided platinum have in the foregoing paragraphs been attributed to the force of adhesion, which is supposed to bring the different gaseous bodies within the sphere of mutual action; but they were viewed by Berzelius as arising from a new force, which he termed catalysis, in virtue of which, he says, "Certain bodies exert, by their contact with others, such an influence upon these bodies, that chemical action is excited; compounds are destroyed, or new ones are formed, although the substance by which these actions are induced does not take the slightest part in these changes." This catalytic force, however, is probably purely imaginary; most of the phenomena which have hitherto been referred to its agency being occasioned by several different causes, which often admit of being distinguished from each other, and which may, as in the case of the action of pla tinum, be explained by the active operation of other known forces.

One class of these phenomena is that included under the term fermentation. Fermentations are peculiar to the products of organic chemistry; such for instance, as the change of solution of sugar into alcohol and carbonic anhydride, under the influence of yeast: the change of starch into sugar in the operation of mashing wort, or in the germination of seeds, owing to the presence of a peculiar albuminous substance termed diastase: and the gradual conversion of amygdalin, the bitter principle in the bitter almond, into hydrocyanic acid, oil of bitter almonds, sugar, and formic acid, when it is dissolved in water, and mixed with synaptase, or the albuminous substance contained in the pulp of the seed. In all these cases, however, although the constituents of the yeast, the diastase, or the synaptase, do not enter into the formation of the new products, yet these bodies disappear during the change, and during the whole time are undergoing a series of specific alterations, which stand in intimate but as yet unexplained relation to the metamorphosis of the sugar, the starch, or the amygdalin. One of the most remarkable features of these decompositions is the

LIEBIG'S THEORY OF CATALYSIS.

871

small proportion of the ferment, or catalytic body as Berzelius termed it, which is required to produce the change: for instance, I part of yeast, calculated in its dry state, is sufficient to convert 60 parts of sugar into alcohol and carbonic anhydride; and a still smaller quantity is required in the case of diastase, I part of which is able to effect the transformation of more than ICOO times its weight of starch into sugar. The consideration of these remarkable metamorphoses must however be deferred until the organic bodies themselves have been described.

Liebig's theory of catalysis is, " that a body in the act of combination or decomposition enables another body with which it is in contact to enter into the same state. It is evident," says he, "that the active state of the atoms of one body has an influence upon the atoms of a body in contact with it, and if these atoms be capable of the same change as the former, they likewise undergo that change, and combinations and decompositions are the conse*** This influence exerted by one compound upon quence. the other, is exactly similar to that which a body in the act of combustion exercises upon a combustible body in its vicinity; with this difference only, that the causes which determine the participation and duration of these conditions are different."

These explanations have been found insufficient to account for the phenomena of fermentation, as the bodies which are undergoing fermentation do not "enter into the same state" as the particles of the ferment; though they apply admirably to many of the illustrations cited by Liebig in support of his theory. Amongst these illustrations is an experiment by Saussure, who observed that moist woody fibre, if placed in contact with oxygen, gradually converts the oxygen into carbonic anhydride. On adding a certain quantity of hydrogen to a measured bulk of oxygen, which was undergoing this change, he observed a diminution in the volume of the two gases immediately after making the mixture; a portion of oxygen had thus been caused to enter into combination with the hydrogen, and a true gradual combustion of the hydrogen had been effected, analogous to that produced by platinum, owing to its contact with vegetable matter which was itself undergoing slow oxidation.

Again, it has been observed in the case of certain alloys, that the compound is entirely soluble in an acid which may be unable to attack one of the components of the alloy when in a separate form. Platinum, for instance, is not soluble in nitric acid, but if it be alloyed with 10 or 12 parts of silver, the acid dissolves it readily. In like manner, copper is insoluble in diluted sulphuric acid;