duced, must be taken as solely indicating those changes in chemical composition merely produced by the presence of certain substances, not undergoing any change themselves, or in any way chemically active in producing the effects observed. The decomposition of oxywater by the metals, and of fused chlorate of potash by oxides of manganese and copper, are good illustrations of catalytic action. Those who advocate the contact theory of electricity, or believe that the mere contact of dissimilar kinds of matter gives rise to the evolution of electricity, may explain these phenomena on the supposition, that, as the electrical equilibrium of matter is disturbed by the contact of dissimilar matter, so the chemical powers of any substance will be more or less modified by the contact of other substances, even though they have no affinity or tendency to combine with the substance which thus influences them.

The decomposition of a substance is generally not caused by one of these forces alone, but by a combination of two or more of them together, the facility with which they act depending on the strength which binds together the elements of the compound. A very great difference is found to exist between even the same class of compounds: thus potassium and oxygen combine with the greatest facility and under any circumstances, however unfavorable; but are separated with very great difficulty, there being a wide range between the point of combination and that of decomposition. Oxygen and mercury, on the other hand, exhibit comparatively but little attraction for each other, and when combined, a very slight determining cause will produce their separation. In some cases, chemical affinity and the opposing forces are so nearly balanced, that combination or decomposition may be apparently produced at pleasure. The decomposition of steam by hot iron, oxide of iron, and hydrogen being produced, and the decomposition of hot oxide of iron and hydrogen, steam and iron resulting, is a familiar illustration of this balance of power.

In effects of this kind it is obvious some part of the action is produced by mass, as distinct even from that due to mere rapidity of action, or perfect mixture. When a solution of sulphuret of potassium or calcium is poured into excess of muriatic acid, a very different effect is produced to that caused by pouring muriatic acid into the sulphuret; in the former case bisulphuretted hydrogen is formed, in the latter sulphuretted hydrogen is evolved, and sulphur deposited.

Berthollet, on studying the nature of decomposition, was led to the conclusion, that when a compound was decomposed by the exertion of superior affinity, the result was not merely in accordance with the ratio of affinity, but depended also on several other conditions, and on the nature of the substances present. When, for example, a strong acid is added to a saline solution, he conceived that the base would not entirely separate from the weaker acid, and combine with the stronger one alone, but that it would be equally divided between the two, in the proportion of their relative mass. He conceived that single or double decomposition was never complete where no change of form took place that two salts when mixed together would only half decompose each other, unless one of the resulting compounds was in

soluble; when this was the case, or in any other way one of the products could be removed, as the remainder of the salts would then be half decomposed, and, as this action would continue with infinite. rapidity, the whole of the compound would finally be decomposed.

Subsequent experiments showed, however, that this view was not correct, and that in many cases, Berthollet was misled by the formation of acid or basic salts. Some interesting experiments on this subject were made by Mr. Phillips in 1816 (Quart. Jour., i, 80) showing the mutual action of carbonate of potash on sulphate of baryta, and carbonate of baryta on sulphate of potash. There are, however, many points connected with the decomposition of salts, requiring further investigation.

For more than fifty years it has been known, that under certain circumstances, steam possesses considerable power in aiding the decomposition of many compounds. This has, for the most part, been either attributed to superior affinity, or to mere mechanical action-it has not been recognised as a distinct and definite mode of decomposition, and, consequently, the recent experiments of Mr. R. A. Tilghman, whilst they unite a number of known phenomena under one general head, and exhibit a series of new and unexpected decompositions, show that the force which produces them is one of very high and peculiar power. Mr. Tilghman has found that a large number of salts, including some of the most fixed and stable compounds known are readily decomposed by a current of steam, at a high temperature. It appears essential to this action, that the acid itself should be either volatile or decomposable at a high temperature, and the action is probably facilitated by any affinity which the base may have for water, the tendency to form a hydrate assisting in the expulsion of the acid. The temperature and other conditions requisite for this effect vary with different salts, depending on various circumstances, such as the affinity to be overcome, &c. In many cases also, decomposition is masked, or even wholly prevented, by interfering circumstances; yet these present no exception to the general rule, for it may readily be shown. that decomposition takes place as soon as these interfering causes are removed. Amer. Phil. Trans. x. 167.

The interesting experiments of Gay Lussac and Thenard, which arise out of their investigations into the nature of chlorine and muriatic acid, showed many remarkable instances of decomposition under the agency of steam. When, for example, dry chlorides were treated with dry boracic acid, they were not decomposed, though they were readily acted on when exposed to the influence of steam. This, it was at first supposed, depended on the fact that the acid could not exist in the dry state, but afterwards was shown to result from the want of oxygen to combine with the metal of the chloride, and form an oxide which could unite with the boracic acid.

These experiments, and the consideration they give rise to, were of considerable practical interest in connexion with lime burning, for it was shown that carbonate of lime is far more easily decomposed when heated in steam than in dry air, and on the other hand, that hydrate of lime is readily converted into carbonate when heated in carbonic

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acid, bearing out the well-known fact that damp chalk or limestone is easier converted into quick lime than it is when quite dry. This action of water has been variously explained. Dumas regards it as either caused by the superior affinity of lime for water at a high temperature, which causes it to combine with the latter and set free the carbonic acid, or else that the steam being partially decomposed by the ignited fuel in the lime kiln, generates a portion of hydro-carbon, which decomposes the carbonic acid of the limestone, reducing it to the state of carbonic oxide (Traité ii, 247). Gay Lussac considers that the first explanation is inadmissable, because hydrate of lime is quite as easily decomposed by heat as carbonate, and shows the incorrectness of the second explanation by some excellent experiments, in which decomposition is shown to be wholly caused by the steam, quite independently of the products of combustion.-(Ann. de Ch. et de Phys. Ixiii, 219).

Although it is perfectly true, that under similar circumstances, the carbonate and hydrate of lime are decomposed with equal facility by heat, yet it is also evident that the affinities are so nearly balanced, that some influence may be reasonably attributed to mass, and that the affinity of lime for water would be rather greater than for carbonic acid when the former was present in large excess. Another very important consideration is, the facility of diffusion. The mere mechanical effect of diffusion would facilitate the decomposition of carbonate in an atmosphere of steam, or hydrate in an atmosphere of carbonic acid. The conclusion drawn by Gay Lussac from his experiments, was, that the influence of steam in assisting the decomposition of the carbonates, was due to the increased facility given for the diffusion or escape of the carbonic acid.

The most important compounds which Mr. Tilghman has decomposed by the influence of hot steam are the earthy and alkaline chlorides, fluorides, sulphates, silicates, borates, phosphates, and chromates. Two interfering effects require to be guarded against in experiments of this sort, namely, the fusibility or volatility of the base when set free; those compounds which are fusible, or the base of which is fusible, are of course more difficult to decompose than others, as becoming superficially fused they are then for the most part protected from further action. On the other hand, when the base is as volatile as the acid, the two though separated by the steam, reunite to form the original compound nearly as rapidly as they are separated.

The decomposition of such substances by steam, is very greatly facilitated by the admixture of some infusible inert porous substance, which shall act as a retainer, mechanically preventing loss by evapo

ration or fusion.

Common salt, sulphate of soda, sulphate of baryta, &c., may be partially decomposed by steam alone, when sufficiently heated, but the interfering causes just mentioned, prevent more than a small portion from being decomposed; by the use of retaining substances, however, they may be rapidly decomposed to a very considerable amount, muriatic acid, or sulphuric acid mixed with sulphurous acid and oxygen, being given off, whilst caustic soda or baryta remains.

Several different theories, or modes of explaining this action, naturally suggest themselves; amongst these are, superior affinity, diffusion, and catalytic action. It is evident that in all feeble combinations, and especially those where both acid and base have a tendency to form hydrates, the affinity of the acid and base for each other may be weakened by heat, whilst that of both to form hydrates may not, or may be, proportionably less weakened. It is evident that in the case. of all haloid salts, the presence of water must be essential to their decomposition by heat, whether alone or in the presence of another acid, as a source of oxygen to the metal, and hydrogen to the chlorine; and it is plain that, in most cases the separation of an oxide from muriatic acid, must be far more easy than that of a metal from chlorine. This action of water does not apply to the decomposition of other salts, unless we adopt the view of the composition of salts first advanced about 1815, by Davy, and afterwards brought forward by Dulong and others, according to which all salts are assumed to be formed on a similar type, the salts of the oxy acids being considered as consisting of the metals united to the acids + oxygen: thus chloride of sodium being Na, C, sulphate of soda would be Na. So.. Perhaps the strongest argument in favor of this hypothesis is the manner in which it explains the relation of acid to the oxygen in salts of sesqui or deutoxides. There are, however, serious objections in the number of hypothetical substances the existence of which must be assumed, and in the fact that two distinct bases must be assumed to explain the composition of bi-salts.

There can be little doubt that in several cases these decompositions are materially facilitated by the freedom with which the substances evolved are carried away by diffusion. It is, however, hardly possible to conceive that these effects are wholly caused in this manner; because, if they were, dry air should be expected to produce some effect in decomposing salts like the sulphates, which, however, appears not to be the case; and further, in some cases, the substance evolved has no tendency to diffuse, being at once decomposed, like chromic acid, which is readily evolved from its compounds with the alkalies, at a high temperature by steam, though at once resolved into oxide of chromium and oxygen.

In some respects the process of decomposition by steam approaches more nearly to the effects of catalytic action than to those of any other known mode of decomposition; at all events, though it may not be clear that they depend on the same power, yet it is evident that no other explanation can be considered as altogether comprehending the phenomena in question.

Independently of the scientific interest of these experiments, in connexion with the laws of affinity and decomposition, they possess great value in a practical point of view, both as indicating new processes of manufacture, and as explaining the nature of many geological phenomena which are of practical importance in the arts.

One of the most remarkable of the new manufacturing processes, and for which Mr. Tilghman has obtained a patent, is the direct decomposition of soda salts, and especially common salt. The experi

ments of Gay Lussac and Thenard, as well as others, have long since shown that common salt may be decomposed by the action of various substances, particularly silex, when exposed to the influence of steam at a high temperature; but these cases appear to have been merely considered as decompositions by superior affinity or substitution, the result being only the conversion of common salt into some oxygen salt, in which the soda is strongly combined to an acid, from which it can only be obtained by a new process. In Mr. Tilghman's mode of working, common salt is mixed with some porous retaining substance, such as alumina, phosphate of lime, &c., decomposed by steam at a high heat, and the product simply lixiviated, when it at once yields a strongly alkaline lye. It must be borne in mind, that the retaining substances do not act merely by chemical affinity, but that their action is chiefly mechanical; this constitutes an important distinction between this and all other processes, and, as a necessary result, the soda obtained is nearly pure, and requires very little further manipulation.

The decomposition of salt in this mode, may easily be carried on continuously in a kiln, like lime-burning, balls of a mixture of salt and alumina being thrown in above, and withdrawn from the furnace below, when the operation is judged complete. The accompanying wood-cut represents the kind of kiln which has hitherto been employed, though there is little doubt that it will soon be modified and improved. C is the fire fed with coal or coke; A the kiln through which the whole products of combustion pass; the salt balls are thrown in above, and withdrawn from time to time at the discharge door D; B, the steam pipes through which a constant current of hot steam is allowed to pass over the ignited fuel, thus insuring a uniformly damp atmosphere in the body of the kiln.