that the heating of the ammonium chloride takes place in the neighbourhood of a porous diaphragm, more of the light ammonia gas will diffuse through in a given time than of the heavier hydrochloric acid, so that a partial separation of these gases will be effected. Fig. II shows a convenient arrangement for carrying out the experiment. A fragment of ammonium chloride is heated in a short glass tube through which passes the stem of an ordinary clay tobacco pipe. As the dissociation takes place, both of the gaseous products begin to diffuse into the interior of the porous clay pipe, but owing to their greater rate of diffusion, a larger number of ammonia molecules will pass in, than of hydrochloric acid, in the same time; consequently, when the gases pass away from the heated region and once more recombine, there will be a surplus of ammonia molecules within the porous pipe, and for the same reason an excess of hydrochloric acid molecules outside. If the gaseous contents of the porous tube be driven out by means of a stream of

FIG. II.

air from an ordinary bellows, the presence of the free ammonia may be recognised by allowing the air to impinge upon a piece of paper, coloured yellow with turmeric, which is instantly turned brown by ammonia. The excess of hydrochloric acid within the glass tube may also be proved by placing a piece of blue litmus paper in the tube before heating the compound, and it will be reddened by the free hydrochloric acid.

In all cases of dissociation we may imagine two opposing forces in operation, one being the external force supplying the energy which tends to bring about the disruption of the molecules, and the other being the force of the chemical affinity existing between the disunited portions of the molecule, which tends to bring about their reunion. When these forces are equally balanced, the same number of molecules are dissociated as are recombined in a given

unit of time, and the system is said to be in a state of equilibrium. If by any means the balance between the two opposing forces is disturbed, by augmenting or lessening either one or the other of them, the equilibrium of the system will also be disturbed and a new condition of equilibrium will be set up, in which again an equal number of molecules undergo dissociation and combination in a given time, but in which the ratio of the number of united and disunited molecules is different from that which obtained under the former condition of equilibrium. The relation between these two forces may be most readily disturbed, by either a change of temperature or pressure. Thus, in the case of nitrogen peroxide, N2O4, when this gas is at a temperature of 26.7°, 20 per cent. of it is dissociated into molecules having the composition NO; and so long as this temperature is maintained this ratio of the weight of the dissociated molecules to the total weight of the system (known as the fraction of dissociation) still subsists.

When the temperature of the gas is raised to 60.2°, the state of equilibrium existing at the lower temperature is disturbed, and the system gradually assumes a new condition of equilibrium, where once more the actual number of molecules undergoing dissociation and recombination in a given unit of time is the same, but where the percentage of dissociated molecules in the gaseous mixture is now 52.04.

It might at first be supposed when such a gas is heated, and a temperature is reached at which the molecules are dissociated, that they would all dissociate, and that the process once begun would rapidly proceed until the decomposition was complete; instead of which, we find a definite fraction of dissociation corresponding to a particular temperature. This may be explained on the basis of the kinetic molecular theory. Let us imagine the gas nitrogen peroxide to be at a temperature below that at which dissociation begins, when all the molecules will have the composition NO4. The molecules of the gas are in a state of rapid movement, and the rapidity of their movement is increased by rise of temperature. But the molecules in a given volume of the gas do not all move at the same velocity, and therefore they have not all the same temperature. On account of the infinite complications in their movements, caused by their impacts against one another, some will be moving at a speed considerably greater than that of the average, and will have a temperature proportionally higher, while others again will have a velocity and a temperature below the average.

The observed temperature of the gas, therefore, is not that of the molecules having the highest or the lowest velocity and temperature, but is the average or mean temperature between, possibly, a very wide range.

On the application of heat to the gas, the observed or mean temperature rises, but the velocity of some of the molecules, and consequently their temperature, may have been thereby raised to the point at which dissociation takes place, and they consequently separate into the simpler molecules. Let us suppose that the observed temperature of the nitrogen peroxide is 26.7°, and that it is maintained at this point. Although this temperature may be below the dissociation temperature of the molecules, it must be remembered that it only represents the mean temperature, and that while some of the molecules have a lower, some also have a higher temperature. As already mentioned, at the temperature of 26.7°, 20 per cent. of the molecules are dissociated; that is to say, at any given instant one-fifth of the total number of molecules reach a velocity which causes them to break down into the simpler NO. molecules, which themselves then take up independent movements. If, in the process of their movements, two of these disunited molecules come into contact with each other at a moment when their velocities are lower than that at which they dissociated, they at once reunite, so that at the same instant some are uniting and others are dissociating, and, the two processes going on equally, the percentage of disunited molecules at any moment is the same, although the actual molecules which are dissociated at one point of time may not be the identical ones that are in this state at another time. Let us now suppose the gas to be heated until the registered (ie. the mean) temperature reaches 60.2°, and that it be maintained at this point. At this higher temperature a much larger proportion of the molecules will acquire a velocity at which tey are unable to hold together, namely, 52.04 per cent.; but the remainder, amounting to nearly one-half, are still at a temperature below that at which dissociation takes place. Under these altered conditions a greater number of disunions and reunions takes place during a given interval of time, but the numbers are equal, and therefore the equilibrium exists. If once more the gas be further heated, until the indicated temperature is 140°, then it is found that the whole of the NO, molecules have dissociated into NO molecules; that is to say, when the mean temperature has reached 140°, then even those molecules that are moving with the slowest

speed have reached the temperature of dissociation. It will be evident that the rate at which the fraction of dissociation increases, as the temperature of a gas is gradually raised, will be greatest when the mean temperature approaches the real dissociation temperature of the gas, for the temperature of the greater number of the molecules will be coincident with, or very closely approximating to, that point.

The vapour density of nitrogen peroxide, if it could be ascertained when all the gaseous molecules had the composition N2O4, would be 46; while that of the gas, when entirely dissociated into NO2 molecules, is 23. At temperatures between these extremes, the gas, consisting of mixtures of both molecules, will have a density lying between these figures, thus at 27.6° and 60.2° the density is 38.3 and 30.1 (see Nitrogen Peroxide, and also Phosphorus Pentachloride).

The effect of increased pressure upon a gas being to diminish the mean free path of the molecules, and thereby increase the number of molecules in a given space, the number of impacts between the molecules in a given time will be increased. If, therefore, while the nitrogen peroxide is maintained at a constant temperature, say 62.2°, the pressure be increased, the dissociated molecules, having shorter distances to travel, and making more frequent impacts in a given time, will unite more quickly than others are being disunited, and a fresh condition of equilibrium will be established for any particular pressure.

The case of phosphonium chloride already mentioned may be referred to as an illustration. This compound is completely dissociated into molecules of phosphoretted hydrogen, PH, and hydrochloric acid, below a temperature of o°. If, while at this temperature, it be subjected to pressure, the dissociated molecules are caused to unite, and at a pressure of thirteen atmospheres the union is complete, the whole of the disunited molecules having combined to form molecules of phosphonium chloride, PH,Cl.

If in the process of dissociation one of the products be withdrawn from the sphere of action, then the process may be carried on to completion. For example, in the case of calcium carbonate already quoted, if this substance is heated in such a manner that as fast as it dissociates, the gaseous product, namely the carbon dioxide, is allowed to escape and so pass away from the sphere of action, the change expressed by the equation

will proceed until the whole of the carbonate has been converted

into oxide. But if, on the other hand, the action is made to take place in a closed vessel, so that the carbon dioxide remains in contact with the lime, then the reverse action comes into operation, namely

CaO + CO, = CaCO3,

and a condition is arrived at in which the one action proceeds at the same rate as the other. The pressure exerted by the carbon dioxide under these circumstances is spoken of as the dissocia tion pressure of the calcium carbonate for that particular temperature.

If, now, when this condition of equilibrium is established the temperature be raised, the balance will be disturbed, and the materials will readjust themselves to a fresh condition of equilibrium at the higher temperature in which the dissociation pressure will also be greater. For any given temperature, therefore, the dissociation pressure is the only possible pressure at which a state of equilibrium can be established between carbon dioxide, calcium carbonate, and calcium oxide; for if while the temperature is constant the pressure upon the gas were to be increased by external means and maintained at a higher point, union between the carbon dioxide and lime would proceed until the whole of the lime was converted into the carbonate. On the other hand, if the pressure were to be reduced and maintained at a lower point, then dissociation would go on until the action was complete and once more one of the three interacting substances would cease to exist.

Increasing and diminishing the pressure upon a gas is obviously synonymous with increasing and diminishing the number of molecules in a given volume. This in modern phraseology is called the molecular concentration of the gas, which embodies the same idea as the expression active mass. From the above illustration, therefore, it will be clear that there is some connection between the molecular concentration (or active mass) of the carbon dioxide and the rate of the chemical actions in question. This connection is thus formulated (Guldberg and Waage): the rate of chemical action is proportional to the active mass (molecular concentration) of each of the reacting substances. Advantage is sometimes taken of these facts in determining the vapour-density of a substance which when heated dissociates into two gaseous constituents. For example, phosphorus pentachloride when heated