DIFFUSION 219 tiori, and radiation in a still atmosphere. It differs from the formula of the convection theory only by the factor in the last term. TT The first part of this factor is certainly less than unity, and probably about 77. If the bulb is spherical and of radius r, A = 47ir 2 and C~>r, so that the second part is - . pSD Hence, the larger the wet bulb, the greater will be the ratio ot the effect of radiation to that of conduction. If, on the other hand, the air is in motion, this will increase both conduction and diffu sion, so as to increase the ratio of the first part to the second. By comparing actual observations of the dew-point with Apjohn s formula, it has been found that the factor should be somewhat greater than unity. According to our theory it ought to be greater if the bulb is larger, and smaller if there is much wind. Relation betiveen Diffusion and Electrolytic Conduction. Electrolysis (see separate article) is a molecular movement of the constituents of a compound liquid in which, under the action of electromotive force, one of the components travels in the positive and the other in the negative direc tion, the flow of each component, when reckoned in electro chemical equivalents, being in all cases numerically equal to the flow of electricity. Electrolysis resembles diffusion in being a molecular movement of two currents in opposite directions through the same liquid; but since the liquid is of the same compo sition throughout, we cannot ascribe the currents to the molecular agitation of a medium whose composition varies from one part to another as in ordinary diffusion, but we must ascribe it to the action of the electromotive force on particles having definite charges of electricity. The force, therefore, urging an electro-chemical equiva lent of either component, or ion, as it is called, in a given direction is numerically equal to the electromotive force at a given point of the electrolyte, and is therefore comparable with any ordinary force. The resistance which prevents the current from rising above a certain value is that arising from the encounters of the molecules of the ion with other molecules as they struggle forward through the liquid, and this depends on their relative velocity, and also on the nature of the ion, and of the Liquid through which it has to flow. The average velocity of the ions will therefore increase, till the resistance they meet with is equal to the force which urges them forward, and they will thus acquire a definite velocity proportional to the electric force at the point, but depending also on the nature of the liquid. If the resistance of the liquid to the passage of the ion is the same for different strengths of solution, the velocity of the ion will be the same for different strengths, but the quantity of it, and therefore the quantity of electricity which passes hi a given time, will be proportional to the strength of the solution. Now, Kohlrausch has determined the conductivity of the .solutions of many electrolytes in water, and he finds that for very weak solutions the conductivity is propor tional to the strength. When the solution is strong the liquid through which the ions struggle can no longer be considered sensibly the same as pure water, and conse quently this proportionality does not hold good for strong solutions. Kohlrausch has determined the actual velocity in cen timetres per second of various ions in weak solutions under an electro-motive force of unit value. From these velocities he has calculated the conductivities of weak solutions of electrolytes different from those of which he made use in calculating the velocity of the ions, and he finds the results consistent with direct experiments on those electrolytes. It is manifest that we have here important informa tion as to the resistance which the ion meets with iu travelling through the liquid. It is not easy, however, to make a numerical comparison between this resistance and any results of ordinary diffusion, for, in the first place, we cannot make experiments on the diffusion of ions. Many electrolytes, indeed, are decomposed by the current into components, one or both of which are cap able of diffusion, but these components, when once sepa rated out of the electrolyte, are no longer ions they are no longer acted on by electric force, or charged with definite quantities of electricity. Some of them, as the metals, are insoluble, and therefore incapable of diffusion ; others, like the gases, though soluble in the liquid electro lyte, are not, when in solution, acted on by the current. Besides this, if we accept the theory of electrolysis pro posed by Clausius, the molecules acted on by the electro motive force are not the whole of the molecules which form the constituents of the electrolyte, but only those which afc a given instant are iu a state of dissociation from molecules of the other kind, being forced away from them tempo rarily by the violence of the molecular agitation. If these dissociated molecules form a small proportion of the whole, the velocity of their passage through the medium must be much greater than the mean velocity of the whole, which is the quantity calculated by Kohlrausch. On Processes by which the Mixture, and Separation of Fluids can be effected in a Reversible Manner. A physical process is said to be reversible when the material system can be made to return from the final state to the original state under conditions which at every stage of the reverse process differ only infinites imally from the conditions at the corresponding stage of the direct process. All other processes are called irreversible. Thus the passage of heat from one body to another is a reversible process if the temperature of the first body exceeds that of the second only by an infinitesimal quan tity, because by changing the temperature of either of the the bodies by an infinitesimal quantity, the heat may be made to flow back again from the second body to first. But if the temperature of the first body is higher than that of the second by a finite quantity, the passage of heat from the first body to the second is not a reversible process, for the temperature of one or both of the bodies must be altered by a finite quantity before the heat can be made to flow back again. In like manner the iuterdiffusion of two gases is in general an irreversible process, for in order to separate the two gases the conditions must be very considerably changed. Fur instance, if carbonic acid is one of the gases, we can separate it from the other by means of quicklime ; but the absorption of carbonic acid by quicklime at ordinary temperatures and pressures is an irreversible pro cess, for in order to separate the carbonic acid from the lime it must be raised to a high temperature. In ell reversible processes the substances which are in contact must be in complete equilibrium throughout the process ; and Professor Gibbs has shown the condition of equilibrium to be that not only the temperature and the pressure of the two substances must be the same, but also that the potential of each of the component substances must be the same in both compounds, and that there is an additional condition which we need not here specify. Now, we may obtain complete equilibrium between quicklime and the mixture containing carbonic acid if we raise the whole to a temperature at which the pressure of dissociation of the carbonic acid in carbonate of lime is equal to the pressure of the carbonic acid in the mixed gases. By altering the temperature or the pressure very
slowly we may cause carbonic acid to pass from the mix-