and pointed out some really striking relationships, such as the following: I = 127 = (2 × 19) + (2 × 16·5) + (2 × 28). A. Strecker, in his work Theorien und Experimente zur Bestimmung der Atomgewichte der Elemente (Braunschweig, 1859), after summarising the data relating to the subject, and pointing out the remarkable series of equivalents Cr = 26.2 Mn = 27·6 Fe = 28 Ni = 29 Cu 31.7 Zn = 32.5 = Co= 30 remarks that: "It is hardly probable that all the above-mentioned relations between the atomic weights (or equivalents) of chemically analogous elements are merely accidental. We must, however, leave to the future the discovery of the law of the relations which appear in these figures.' In such attempts at arrangement and in such views are to be recognised the real forerunners of the periodic law; the ground was prepared for it between 1860 and 1870, and that it was not expressed in a determinate form before the end of the decade may, I suppose, be ascribed to the fact that only analogous elements had been compared. The idea of seeking for a relation between the atomic weights of all elements was foreign to the ideas then current, so that neither the vis tellurique of De Chancourtois, nor the law of octaves of Newlands, could secure anybody's attention. And yet both De Chancourtois and Newlands, like Dumas and Strecker, more than Lenssen and Pettenkofer, had made an approach to the periodic law and had discovered its germs. The solution of the problem advanced but slowly, because the facts, and not the law, stood foremost in all attempts; and the law could not awaken a general interest so long as elements, having no apparent connection with each other, were included in the same octave, as for example: Ist octave of Newlands... HF CI | Co and Ni 7th Ditto....... OS Fe Se Br Pd I Pt & Ir Rh & Ru Te Au Os or Th Au Analogies of the above order seemed quite accidental, and the more so as the octave contained occasionally ten elements instead of eight, and when two such elements as Ba and V, Co and Ni, or Rh and Ru, occupied one place in the octave. Nevertheless, the fruit was ripening, and I now see clearly that Strecker, De Chancourtois, and Newlands stood foremost in the way towards the discovery of the periodic law, and that they merely wanted the boldness necessary to place the whole question at such a height that its reflection on the facts could be clearly seen. A third circumstance which revealed the periodicity of chemical elements was the accumulation, by the end of the sixties, of new information respecting the rare elements, disclosing their many-sided relations to the other elements and to each other. The researches of Marignac on niobium, and those of Roscoe on vanadium, were of special moment. The striking analogies between vanadium and phosphorus on the one hand, and between vanadium and chromium on the other, which became so apparent in the investigations connected with that element, naturally induced the comparison of V = 51 with Cr 52, Nb: 94 with Mo= 96, and Ta 192 with W 194; while, on the other hand, P= 31 could be compared with S = 32, As = 75 with Se = 79, and Sb = 120 with Te 125. From such approximations there remained but one step to the discovery of the law of periodicity. = = = = = The law of periodicity was thus a direct outcome of the stock of generalisations and established facts which had accumulated by the end of the decade 1860-1870: it is the embodiment of those data in a more or less systematic expression. ELECTROCHEMISTRY THE great advance in our knowledge of atomic theory which has distinguished recent years has been made by means of electrical evidence. We must therefore now turn back our attention to 1834, when Michael Faraday applied the new science of current electricity to chemical phenomena. Faraday was assistant to Sir Humphry Davy, Professor of Chemistry at the Royal Institution, and succeeded his master in that Chair. Soon after Volta invented the galvanic battery in 1800, Davy, among the first, turned it to account. He decomposed the alkalies soda and potash, and extracted from them the new metals sodium and potassium. Probably it was these striking results which afterwards led Faraday to his researches on this subject. At first, the ideas of static electricity and magnetism were used in examining the effects of currents. The metallic plates, by which the voltaic current enters and leaves a solution of a salt or acid, were regarded as centres of force analogous to the poles of a magnet, and it was Faraday who showed the advantage of a good nomenclature by inventing a new terminology and using it to develop new conceptions. ON ELECTROCHEMICAL DECOMPOSITION By MICHAEL FARADAY (Philosophical Transactions of the Royal Society, 1834.) THE theory which I believe to be a true expression of the facts of electrochemical decomposition, and which I have therefore detailed in a former series of these Researches, is so much at variance with those previously advanced that I find the greatest difficulty in stating results, as I think, correctly, whilst limited to the use of terms which are current with a certain accepted meaning. Of this kind is the term pole, with its prefixes of positive and negative, and the attached ideas of attraction and repulsion. The general phraseology is that the positive pole attracts oxygen, acids, etc., or more cautiously, that it determines their evolution upon its surface; and that the negative pole acts in an equal manner upon hydrogen, combustibles, metals, and bases. According to my view, the determining force is not at the poles, but within the body under decomposition; and the oxygen and acids are rendered at the negative extremity of that body, whilst hydrogen, metals, etc., are evolved at the positive extremity. To avoid, therefore, confusion and circumlocution, and for the sake of greater precision of expression than I can otherwise obtain, I have deliberately considered the subject with two friends, and with their assistance and concurrence in framing them, I purpose henceforward using certain other terms, which I will now define. The poles, as they are usually called, are only the doors or ways by which the electric current passes into and out of the decomposing body; and they of course, when in contact with that body, are the limits of its extent in the direction of the current. The term has been generally applied to the metal surfaces in contact with the decomposing substance; but whether philosophers generally would also apply it to the surfaces of air and water, against which I have effected electrochemical decomposition, is subject to doubt. In place of the term pole, I propose using that of electrode*, and I mean thereby that substance, or rather surface, whether of air, water, metal, or any other body, which bounds the extent of the decomposing matter in the direction of the electric current. The surfaces at which, according to common phraseology, the electric current enters and leaves a decomposing body are most important places of action, and require to be distinguished apart from the poles, with which they are mostly, and the electrodes, with which they are always, in contact....The anodet is therefore that surface at which the electric current, according to our present expression, enters: it is the negative extremity of the decomposing body; is where oxygen, chlorine, acids, etc., are evolved; and is against or opposite the positive electrode. The cathode is that surface at which the current leaves the decomposing body, and is its positive extremity; the combustible bodies, metals, alkalies, and bases are evolved there, and it is in contact with the negative electrode. ἤλεκτρον, and ὁδός, a way. I shall have occasion in these Researches, also, to class bodies together according to certain relations derived from their electrical actions; and wishing to express those relations without at the same time involving the expression of any hypothetical views, I intend using the following names and terms. Many bodies are decomposed directly by the electric current, their elements being set free; these I propose to call electrolytes*.... Finally, I require a term to express those bodies which can pass to the electrodes, or, as they are usually called, the poles. Substances are frequently spoken of as being electro-negative or electro-positive, according as they go under the supposed influence of a direct attraction to the positive or negative pole. But these terms are much too significant for the use to which I should have to put them; for, though the meanings are perhaps right, they are only hypothetical, and may be wrong; and then, through a very imperceptible, but still very dangerous, because continual, influence, they do great injury to science by contracting and limiting the habitual views of those engaged in pursuing it. I propose to distinguish such bodies by calling those anions t which go to the anode of the decomposing body; and those passing to the cathode, cations‡; and when I have occasion to speak of these together, I shall call them ions. Thus, the chloride of lead is an electrolyte, and when electrolyzed evolves the two ions, chlorine and lead, the former being an anion, and the latter a cation. These terms, being once well defined, will, I hope, in their use enable me to avoid much periphrasis and ambiguity of expression. I do not mean to press them into service more frequently than will be required, for I am fully aware that names are one thing and science another. It will be well understood that I am giving no opinion respecting the nature of the electric current now, beyond what I have done on former occasions; and that though I speak of the current as proceeding from the parts which are positive to those which are negative, it is merely in accordance with the conventional, though in some degree tacit, agreement entered * ектρov, and λúw, solvo. Noun, electrolyte; verb, electrolyze. távv, that which goes up. [Neuter participle.] ‡ kariwv, that which goes down. |