independent of the way they are arranged, provided the number in unit volume remains constant, that is the mean free path will depend only on the density of the medium traversed by the corpuscles, and will be independent of its chemical nature and physical state: this from Lenard's very remarkable measurements of the absorption of the cathode rays by various media, must be a property possessed by the carriers of the charges in the cathode rays. Thus on this view we have in the cathode rays matter in a new state, a state in which the subdivision of matter is carried very much further than in the ordinary gaseous state: a state in which all matter—that is, matter derived from different sources such as hydrogen, oxygen, &c.—is of one and the same kind; this matter being the substance from which all the chemical elements are built up.

POSITIVE RAYS AND ISOTOPES

SIR J. J. THOMSON showed that negative or cathode rays were flights of the negative corpuscles, called by others electrons. He also investigated the nature of the positive rays which passed in the other direction in a vacuum tube. These positive rays are much less easily deflected, but by using very powerful electric and magnetic fields they have been examined and shown to consist of electrified atoms, the mass and charges of which may be estimated from the deflexions—a new and surprising method of chemical analysis.

In examining the gas neon, Thomson saw indications that it consisted of two constituents with atomic weights 20 and 22. Similar conclusions about other elements had already been reached from radio-active experiments. Soddy named such elements, similar in properties but different in atomic weight, isotopes.

Dr F. W. Aston, improving Thomson's method, has proved that many so-called elements really consist of mixtures of isotopes. The old puzzle of atomic weights not whole numbers thus finds its solutionthey are average values between those of their constituents. All real elements have atomic weights that are very nearly indeed whole numbers if oxygen be taken as 16. For this work, Dr Aston was awarded the Nobel Prize for Chemistry in 1922, a distinction which had previously been given to Sir J. J. Thomson and Sir E. Rutherford.

ISOTOPES AND ATOMIC WEIGHTS

By Dr F. W. ASTON

Nature: 105, p. 617.

In the atomic theory put forward by John Dalton in 1801 the second postulate was: "Atoms of the same element are similar to one another and equal in weight." For more than a century this was regarded by chemists and physicists alike as an article of scientific faith. The only item among the immense quantities of knowledge acquired during that productive period which offered the faintest suggestion against its validity was the inexplicable mixture of order and disorder among the elementary atomic weights. The general state of opinion at the end of last century may be gathered from the two following quotations from Sir William Ramsay's address to the British Association at Toronto in 1897:

"There have been almost innumerable attempts to reduce the differences between atomic weights to regularity by contriving some formula which will express the numbers which represent the atomic weights with all their irregularities. Needless to say, such attempts have in no case been successful. Apparent success is always attained at the expense of accuracy, and the numbers reproduced are not those accepted as the true atomic weights. Such attempts, in my opinion, are futile. Still, the human mind does not rest contented in merely chronicling such an irregularity; it strives to understand why such an irregularity should exist....The idea...has been advanced by Prof. Schutzenburger, and later by Mr Crookes, that what we term the atomic weight of an element is a mean; that when we say the atomic weight of oxygen is 16, we merely state that the average atomic weight is 16; and it is not inconceivable that a certain number of molecules have a weight somewhat higher than 32, while a certain number have a lower weight."

This idea was placed on an altogether different footing some ten years later by the work of Sir Ernest Rutherford and his colleagues on radio-active transformations. The results of these led inevitably to the conclusion that there must exist elements which have chemical properties identical for all practical pur

WLS

ΤΟ

poses, but the atoms of which have different weights. This conclusion has been recently confirmed in a most convincing manner by the production in quantity of specimens of lead from radio-active and other sources, which, though perfectly pure and chemically indistinguishable, give atomic weights differing by amounts quite outside the possible experimental error. Elements differing in mass but chemically identical and therefore occupying the same position in the periodic table have been called "isotopes' by Prof. Soddy.

At about the same period as the theory of isotopes was being developed by the radio-chemists at the heavy end of the periodic table an extremely interesting discovery was made by Sir J. J. Thomson, which carried the attack into the region of the lighter and non-radio-active elements. This was that, when positive rays from gases containing the element neon were analysed by electric and magnetic fields, results were obtained which indicated atomic weights roughly 20 and 22 respectively, the accepted atomic weight being 20-2. This naturally led to the expectation that neon might be a mixture of isotopes, but the weight 22 might possibly be due to other causes, and the method of analysis did not give sufficient accuracy to distinguish between 20 and 20.2 with certainty. Attempts were made to effect partial separation first by fractionation over charcoal cooled in liquid air, the results of which were absolutely negative, and then by diffusion, which in 1913 gave positive results, an apparent change in density of 0.7 per cent. between the lightest and heaviest fractions being attained after many thousands of operations. When the war interrupted the research, it might be said that several independent lines of reasoning pointed to the idea that neon was a mixture of isotopes, but that none of them could be said to carry the conviction necessary in such an important development.

By the time work was started again the isotope theory had been generally accepted so far as the radio-active elements were concerned, and a good deal of theoretical speculation had been made as to its applicability to the elements generally. As separation by diffusion is at the best extremely slow and laborious, attention was again turned to positive rays in the hope of increasing the accuracy of measurements to the required degree. This was done by means of the arrangement illustrated in Fig. 1.