It was discovered in 1833 by Michael Faraday, Professor of Chemistry in the Royal Institution in London, that if an electric current be passed simultaneously through different solutions, the weights of metals deposited or of elements or groups of elements liberated are proportional to their equivalents (see p. I 5 ). If the same cur rent be passed, for example, through a solution of dilute sul phuric acid, copper sulphate, and iodide of potassium, each contained in its own vessel, provided with plates of platinum or some other unattackable metal dipping into the solution, for every gram of hydrogen evolved from the cathode in the vessel containing sulphuric acid, 8 grams of oxygen are evolved from the anode ; 32.7 grams of copper are de posited on the cathode dipping into the copper solution, while 8 grams of oxygen rise in bubbles from the anode ; and lastly, 127 grams of iodine are liberated from the anode in the vessel containing potassium iodide, I gram of hydrogen rising from the cathode. The evolution of hydrogen instead of the deposition of potassium is due to the fact that the metal potassium is unable to exist in presence of water, but immediately displaces its equivalent of hydrogen. All these numbers are in the proportions of the equivalents of the elements. And without the liberation of these elements no current passes. The elements may, there fore, in a certain sense, be said to convey the electricity ; and as the same quantity of electricity passes through each solution, liberating equivalents of the elements in each case, it would appear that the same quantity of electricity is con veyed by quantities of elements proportional to their equivalents. The equivalent of an element, it will be remembered, is the weight of the element which can combine with or re place one part by weight of hydrogen ; it may be identical with, or it may be a fraction of the atomic weight. In the instances given above, the equivalents of iodine and of potassium are identical in numerical value with their atomic weights ; but those of oxygen and of copper, 8 and 32.7, are half their atomic weights, which are respectively 1 6 and
63. 4. It would follow, therefore, that an atom of copper or of oxygen is capable of conveying a quantity of electricity twice as great as that conveyed by an atom of hydrogen or of iodine.
But how is it known that the atoms " convey " quantities of electricity ? Must they be imagined as like boats, taking in their load of electricity at one pole, and ferrying it over to the other, and there discharging ? It was at one time held that the process rather resembled the method of loading a barge with bricks, where a row of men, who may stand for the atoms, pass bricks, representing the electricity, from one to the other. But it was proved by Hittorf that the charged atoms actually travel or "migrate" from one pole to the other, carrying with them their electric charges. And the charged atoms, for which the name " ions," or "things which go," was de vised by Faraday, do not always move at the same rates. The rate of motion depends on the friction which the ions undergo on moving through the water or other solvent in which the salt is dissolved. This friction is different for different ions ; it also depends on the particular solvent employed ; and it is diminished if the temperature is raised. The force which impels the ions is the same as that commonly known as electric attraction and repulsion ; the negatively charged atoms or "cations" being repelled from the negative and attracted by the positive electrode dipping into the solu tion, while the positively charged atoms or "anions" are repelled by the anode and attracted by the cathode.
When the anions touch the cathode, they are discharged ; and similarly, when the cations touch the anode, they lose their charge. And for every anion discharged, a cation must simultaneously lose its charge. The result of this is that the number of anions remaining in solution must always be equivalent to the number of cations. It need not always be the same, for it is possible for a cation like copper to carry twice the charge of an anion like chlorine ; but the number of " electrons," or electric charges, must always be the same, although some ions are capable of carrying more than one electron. There can never, therefore, be an excess of, say, copper ions in solution ; for they are always balanced by the requisite number of anions. Thus, if the solution be evaporated, the remaining salt has its usual composition ; though, of course, there is less of it than if none had been decomposed.
Hittorf s Migration Constants
The fact that ions move at different rates can be demonstrated in two ways, one direct, the other indirect. The indirect method was devised by Hittorf; the direct method, which is much more recent, was first suggested by Lodge.
It is always advisable to form a mental picture, if possible, of any physical phenomenon, pour preciser lesidee*) as the French say ; and a trivial illustration will be now given which may render HittorFs conception clearer. Imagine a ball-room with a door at each end. Suppose the partners to be all separated from each other ; and suppose an order to be given that the men shall march to one door at twice the rate at which the ladies make for the other door ; but that at the same time, for every man who passes through the one door, only one lady shall pass through the other door. At a given signal, say when half the ballroom has escaped, let the condition of the room be examined. It is easy to see that there will be an equal number of men and women in the room, but that there will be a greater number round the door at which the men issue than round that at which the ladies are trying to escape. And the rates of motion will be proportional to the relative numbers in each half of the ballroom, for the greater the rates at which the men move proportionally to the ladies, the greater will be the number in that part of the room at which the men are escaping.
This is a conception in close analogy with Hittorf's. The men and women are the anions and cations ; and on analysing the solutions round the anode and cathode, he found that the concentration was, as a rule, altered, so that he was forced to conclude that the rate of motion towards the pole at which the concentration was increased was more rapid than that towards the pole at which he found the concentration to be diminished. By comparing the concentrations, too, he calculated the relative rates of motion of the anions and the cations towards the cathode and the anode respectively.
Lodge's direct method has recently been improved by Orme Masson, and very accurate results have been ob tained by him. His plan is to trace the rate of motion of the anions by following them up with a coloured anion, such as the copper ion, which is blue, and can be seen, while the rate of motion of the cation is indicated by following it up with a coloured cation : the one he used for this purpose is the chromate ion, which is orange-yellow. The apparatus which Masson employed consisted of two flasks connected together by a narrow tube. This tube is rilled with a solution of the salt of which the rate of migration of the ions is to be determined, but in order to prevent diffusion of the liquid, or escape owing to currents produced by differences of temperature, the water in which the salt is dissolved contains enough gelatine to make it set into a jelly when cold. It is found that the gelatine does not appreciably interfere with the motion of the ions. The one flask was charged with a solution of copper chloride, and the anode plate was of copper. The other flask was charged with a dilute solution of a mixture of chromate and bichromate of potassium, and the cathode was of platinum. The connecting tube was filled with a warm solution of the salt to be examined, say potassium chloride, in water containing gelatine, and after it had cooled and set it was placed in position. On passing the current, the potassions migrate towards the cathode, and are followed closely by the blue cuprions, which serve to mark the position of the rearmost of the potassions. The chlorions, on the other hand, migrate towards the anode, followed by the orange-yellow chromations, which reveal their position. The rates can be measured by following the advance of the colour in the tubes. If the ions have equal velocity, as is nearly the case with potassions and chlorions, the meeting-place of the blue and the orange is nearly at the middle of the tube ; but if, as in most other cases, the rates are different, the point of junction will be at one side or other of the middle point of the tube. The distances traversed in the same time give a direct measure of the relative velocities of the anion and cation. Having established this ratio, another salt, say sodium chloride, having a different anion but the same cation, can be employed, and so the relative rates of potassion and sodion may be compared.
The table which follows gives the rates of migration of a few ions compared with that of potassion, which is taken as 100.
K Na Li NH 4 Mg/2 Cl SO 4 /2 100 65.6 45*0 100 40.5 97-O 87.7
As the conductivity for a current depends on the velocity both of the anion and the cation, relative numbers for the conductivity may be obtained for any salt by adding the numbers of the individual ions given above. Thus, if it is required to find the conductivity of lithium sulphate, which
SO has the formula Li 2 SO 4 , we have Li = 45, and ^=Sj. F j t
