The law is one of experience; instances of its application are given below. Its probable theoretical significance may be explained mechanically with the aid of the kinetic theory of gases and solutions, as follows: If chloroform is added to a solution of bromine in water, the chloroform takes up part of the bromine and, if the mixture is vigorously shaken, a condition of equilibrium and a definite distribution of bromine between the two solvents will result (‹exp.›[228]). Now, if one imagines a liter of the aqueous solution to contain ‹one mole› of bromine at some given temperature and to cover a liter of chloroform, the whole system being left to itself, then all the conditions affecting the migration of the bromine into the chloroform will be definite ones—the concentration of the bromine, the temperature, the surface between the two solvents—and bromine will pass from the aqueous solution into the chloroform solution at a definite speed. We may call the ‹quantity› (in moles) of bromine which would enter the chloroform in one minute, if the concentration of the bromine in the water were kept constant (one mole) throughout the minute, the ‹velocity of migration› of the bromine—this velocity, like chemical velocity, representing a quantity, not a distance. The velocity being a definite one under these conditions, we have ‹v›_{1} = ‹k›_{1}. Now, if all the conditions are left unaltered, except that the concentration of the bromine is changed, say kept at one-hundredth its original value, then only one one-hundredth as many molecules of bromine as in the first case will come into contact with the chloroform surface in unit time. The chances for migration are one one-hundredth as great, and the quantity entering the chloroform in unit time—the velocity of the [p120] change—will be one one-hundredth of the original velocity. In general, ‹the velocity will be proportional to the concentration of the bromine› [Br]_{aq.} ‹in the water at any moment› and to the characteristic constant ‹k›_{1}. ‹v›_{1} = [Br]_{aq.} × ‹k›_{1}. On the other hand, if a solution of bromine in chloroform is covered with water, bromine enters the water (‹exp.›).[229] We would find, by the method of analysis used before, and for the same conditions, that the velocity of migration, ‹v›_{2}, of the bromine into the water is also proportional to a characteristic constant, ‹k›_{2}, and to the concentration of the bromine in the chloroform [Br]_{ch.}. We have, therefore ‹v›_{2} = [Br]_{ch.} × ‹k›_{2}. Equilibrium between the two solutions will be reached when ‹v›_{1} = ‹v›_{2} or [Br]_{aq.} × ‹k›_{1} = [Br]_{ch.} × ‹k›_{2}, from which follows that[230] for the condition of equilibrium [Br]_{aq.} / [Br]_{ch.} = ‹k›_{2} / ‹k›_{1} = ‹k›.