heat is not always the one which actually occurs. Decompositions accompanied by negative calorific effects are more frequent at high than at low temperatures. In this connection it must be remembered that heat is not the only form in which energy can be lost by a changing chemical system, as the change may give rise to electrical phenomena. A redistribution of atoms in the molecule may take place, and although the energy is not lost to the system it may not be possible for it to directly appear as heat. In some cases the only outward sign of change is the passage of an element from a normal to an allotropic modification.

Van't Hoff has pointed out that modern chemical theory had two weak points: it expressed itself neither as to the relative position of the atoms nor as to their movement. It is known that many active organic bodies lose their activity by being heated, and this fact, which is recognised as being of much importance in organic chemistry, can hardly be without significant relations in the inorganic reactions with which the metallurgist has to deal.

The atoms in the molecule have, as has already been stated, movements of their own, and, as Lothar Meyer has urged "if these movements of the atoms are to be considered, then we must investigate what part in the observed calorific effects is to be attributed to them, and how much is due to the potential energy of the hypothetical force of affinity. It is doubtful, however, whether the atoms do possess powers of attraction and consequently potential energy; it is more probable that the whole of the kinetic energy the atoms give out is already their own as such." But suppose atoms do possess "potential energy" of their own to which they owe their "affinities," then when the system undergoes change which satisfies the affinities of the atoms without the addition of energy from an external source, this change must be attended by a "locking up" of energy which must have been kinetic, and consequently there is less kinetic energy available in the system to appear as heat, so that although in a particular reaction but little heat may be evolved, there may nevertheless be a considerable degradation of chemical energy.

In chemical operations generally, and especially in metallurgical ones, there is another consideration of much importance. If two compound bodies react one upon the other, the presence of the products of the reaction will bring it to an end, and a state of equilibrium will be established, so that both the original and the newly formed substances are present in definite quantities that

*La Chimie dans l'espace (original edition).

remain the same so long as the conditions of temperature and pressure do not undergo further change. If this is the case the reaction can only be resumed if the products are eliminated from the system as fast as they are formed. In "wet" chemical processes this removal is effected by the precipitation of a product, or by its evolution as a gas. In metallurgical operations conducted by the aid of fire the products are often gaseous, and are swept away by the draught of the furnace. In other cases the fusion of the products enables this separation to be effected, as the products either flow away or arrange themselves in layers under the influence of gravity.

Again, in very many metallurgical processes, reactions are rendered incomplete by the limitations imposed by the presence of bodies which cannot be speedily eliminated from the system, and the result may be to greatly retard the completion of an operation. The time has come when the principles of dynamic chemistry must be applied to the study of metallurgical problems if they are to be correctly understood, and it is, moreover, necessary to remember the part played by the surface separating the different aggregates in contact with one another. When, for instance, a reaction has to take place accompanied by the evolution of gas, there must be space into which the gas can pass, and the rate at which change takes place will obviously depend on the state of division of the mass.

One of the most remarkable points in the whole range of chemistry is the action engendered between two elements capable of reacting, by the presence of a third body. It may be, and this is the most wonderful fact of all, that merely a trace of a third body is necessary to induce reaction, or to profoundly modify the structure of a mass. H. le Chatelier and Mouret have pointed out that in certain cases it is inaccurate to say that the third body causes the reaction to take place, because, after it has destroyed the inter-molecular resistances which prevented the reaction taking place, the third body ceases to intervene. This is apparently the case when platinum sponge effects the union of oxygen and hydrogen, or conversely, when very hot platinum splits up water vapour into its constituent gases. Future investigation will, it is to be hoped, show whether the platinum does not exert some direct action in such cases. We can no longer neglect the study of such questions from the point of view of their practical application. The manufacture of red-lead presents a case in point. In "drossing" molten lead, the oxidation of the lead is greatly promoted by the presence of a trace of antimony, and conversely, in the separation of silver from molten

lead, by the aid of zinc, H. Roessler and Endelmann have recently shown that aluminium has a remarkable effect in protecting the zinc from loss by oxidation, and, further, the presence of one-thousandth part of aluminium in the zinc is sufficient to exert this protecting action on that metal.

An examination of the following thermal equations and the remarks which precede them, will show how frequently, in conducting metallurgical operations, demands are made upon energy, in the form of heat, from a source external to the particular "chemical system" which is undergoing change. It has been urged that if it is the energy of the external heat which dissociates a compound and enables a reaction to take place, then chemical equilibrium such as is revealed by experiments on dissociation, is really equilibrium between the energies of affinity and of heat. It follows, as M. Duhem* has pointed out, that the third law of thermo-chemistry is greatly weakened, if not rendered absurd, by the necessity for bringing external heat to a chemical system. This law states that, "Every chemical change which is accomplished without the intervention of external energy, tends towards the production of a body which evolves the most heat." If it be necessary to import external heat, the law admits of being reduced to the useless expression, "Every reaction which does not absorb heat evolves it."

But the more ardent members of the school of Deville, which has rendered such splendid services to metallurgy and to physics, do not advocate the employment of the mechanism of atoms and molecules in dealing with chemical problems, but would simply accumulate evidence as to the physical circumstances under which chemical combination and dissociation take place. They do not even insist upon the view that matter is minutely granular, but, in all cases of change of state, make calculations on the basis of work done, viewing "internal energy" as a quantity which should reappear when the system returns to the initial state; and they view chemical combination and dissociation as belonging to the same class of phenomena as solidification, fusion, condensation, and evaporation. As yet the study of chemical equilibrium is not sufficiently advanced to afford a basis for building a theory of metallurgy, and, moreover, enough has been adduced in this book to show the author's belief in the existence of molecular movement and atomic grouping, and if there is evidence of the existence of atoms and molecules it is not advisable to ignore their existence when dealing with metallurgical problems.

*Duhem, Introduction a la Mèchanique Chimique, Paris, 1893, p. 79.

There is one important theorem developed by M. Moutier which must not be overlooked. He has shown that under a given pressure there is only one temperature at which transformations are reversible. If the conditions of equilibrium are such that the transformation occurs below the critical temperature, it is attended with evolution of heat, while, on the other hand, if the transformation occurs above this temperature, it is attended with absorption of heat. Take the case of a mixture of carbonic anhydride, lime and calcium carbonate, exposed to such a pressure and temperature that the system is in equilibrium. It is stable, more carbonic anhydride cannot combine with the lime, nor can fresh calcium carbonate be decomposed. But destroy the equilibrium by an elevation of temperature, carbonic anhydride and lime will be the result, but the reaction is attended with absorption of heat. On the other hand, destroy the equilibrium by lowering the temperature, lime and carbonic anhydride will combine, and the reaction will be attended with an evolution of heat. According to this law, the occurrence of a definite chemical change will not be determined by the fact that much or little work would be done (law of maximum work), but by the relation between the temperature and pressure to which the substances are subjected. This, and other theorems of Moutier, apply, however, only to reversible reactions, and the student should be warned that the theorems as to the conditions of chemical equilibrium lead in many cases to the expectation of reactions or transformations which are not found to occur exactly as anticipated. Thus reactions occur suddenly at an abnormally high temperature, and with explosive violence, whilst the theory of chemical equilibrium indicates that reactions should tend to check themselves, and that therefore there should be no tendency to explosion. Other cases are presented by the phenomena of surfusion, supersaturation, and delayed ebullition. It would thus appear that a position of unstable or false equilibrium may be established, but if the equilibrium be destroyed, it may be by a further rise in temperature, by the presence of a minute trace of impurity or even by a mechanical shock, the change is propagated rapidly through the mass.

Thermo-chemists are reproached for having neglected the study of reactions at high temperatures, for the measurement of which, until recently, no simple methods were available, but now, as has been shown in these pages, that high temperatures can be measured with facility, it is to be hoped this reproach may be removed. When this is done, it will be interesting to compare the new thermal equations representing reactions at high temperatures with those now in use. Such an investigation will be very tedious,

but there are many reactions at high temperatures the study of which can be undertaken, though they present great difficulty.

The student may fairly ask why a series of equations are presented which are confessedly based on numbers that were obtained from experiments conducted at temperatures differing from those employed in practice. The answer is that these equations do enable him to know the quantity of heat that will be required to obtain a certain result, and also indicate the probable temperature at which an operation can be effected. - A reaction, the final result of which is represented by a minus number which is large when considered in relation to the quantity of material involved, generally means that a high temperature is necessary to effect it, but much will depend upon the melting points of the members of the particular chemical system.

A reaction between sulphide of lead and sulphate of lead is attended with a large absorption of heat (- 180 large units), the mass involved being considerable. The reduction of ferric oxide by carbon is attended with an absorption of heat (-112 large units). Both these reactions require very high temperatures, the mass in the latter case is relatively small, but the melting point of the reduced iron is high.

The ordinary atomic equation

Fe,O,+3C2Fe + 3CO,

shows that the reduction of one ton of ferric oxide should be effected by four and a half hundredweights of carbon, leaving the CO produced to reduce still more ferric oxide. The thermal equation, with its very large minus number (-112), prepares the student for the fact, well known to the blast furnace manager, that the reduction of one ton of ferric oxide in accordance with the above equation is a very difficult operation, which would require a large amount of fuel, and, moreover, the student is led to expect that the reaction represented by the above equation is evidently not the one that does happen, for he will see that the reaction

Fe,0,+3CO2 Fe + 3CO,

which is accompanied by a disengagement of heat (+4.6) is probably the main reaction that takes place in the blast furnace.

Whenever an equation shows that a reaction is accompanied by the evolution of much heat, it is safe to conclude that it will take place either at a low temperature, or will be effected with ease.

Thermal Equations.-In using these, the student must bear in mind that all compounds that have to be decomposed will absorb as much heat during decomposition as they evolved while they were being formed, so that it is necessary to take the algebraic sum of each side of the equation, and to algebraically