436 



NATURE 



[July 26, 19 17 



Determination of Atomic Weights. 



The results enumerated thus prove that the atomic 

 ■weight can no longer be regarded as a natural con- 

 stant, or the chemically pure element as a homo- 

 geneous type of matter. The latter may be, and 

 doubtless often is, a mixture of isotopes varying in 

 atomic weight over a small number of units, and the 

 former then has no exact physical significance, being 

 a mean value in which the proportions of the mixture 

 as well as the separate atomic weights are both un- 

 known. New ideals emerge and old ones are resusci- 

 tated by this development. There may be, after all, a 

 very simple numerical relation between the true atomic 

 weights. The view that seems most probably true at 

 present is that while hydrogen and helium may be the 

 ultimate constituents of matter in the Proutian sense, 

 and the atomic weights therefore approximate mul- 

 tiples of that of hydrogen, sjnall deviations, such as 

 exist between the atomic weights of these two con- 

 stituent elements themselves, may be due to the 

 manner in which the atom is constituted, in accordance 

 with the principle of mutual electromagnetic mass, 

 developved by Silberstein and others. The electro- 

 magnetic mass of two charges in juxtaposition would 

 not be the exact sum of the masses when the charges 

 are separated. The atomic weight of hydrogen is 

 1-0078 in terms of that of helium as 399, and that the 

 latter is not exactly four times the former may be 

 the expression of this effect. Harkins and Wilson 

 have recently gone into the question with some 

 thoroughness, and the conclusion of most interest in 

 the present connection which appears to emerge is in 

 favour of regarding most of the effect to occur in the 

 formation of helium from hydrogen, and very little 

 in subsequent aggregations of the helium. In the 

 region of the radio-elements, where we have abundant 

 examples of the expulsion of helium atoms as a par- 

 tides, it seems as if we could almost safely neglect 

 this effect altogether. Thus radium has the atomic 

 weight almost exactly 226, and the ultimate product 

 almost exactly 206, showing that in five o- and four 

 y8-ray changes the mean effect is nil, and the atomic 

 weights are, moreover, integers in terms of oxygen 

 as 16, or helium 4. It is true that the atomic weights 

 of both thorium and uranium are between o-i and 02 

 greater than exact integers, but it is difficult to be 

 sure that this difference is real. 



When, among the light elements, we come across 

 a clear case of large departure from an integral value, 

 such as magnesium, 24-32, and chlorine, 3546, we 

 may reasonably suspect the elements to be a mixture 

 of isotopes. If this is true for chlorine, it suggests a 

 most undesirable feature in the modern practice of 

 determining atomic weights. More and more the one 

 method has come to be relied upon, the preparation 

 of the chloride of the element and the comparison , 

 of its weight with that of the silver necessary to com- 

 bine with the chlorine, and with the weight of the 

 silver chloride formed. 



Almost the only practical method, and that a very 

 laborious and imperfect one, which may be expected 

 to resolve a mixture of isotopes is bj' long-continued 

 fractional gaseous diffusion, which is likely to be the 

 more effective the lower the atomic weight. Assume, 

 for example, that chlorine were a mixture of isotopes of 

 separate atomic weights 34 and 36, or 35 and 36. 

 The 34 isotope would diffuse some 3 per cent, faster 

 than the 36, and the 35 some 1-5 per cent, faster. 



The determination of the atomic weight of chlorine 

 in terms of that of silver has reached now such a 

 pitch of refinement that it should be able to detect a 

 difference in the end fractions of the atomic weight 

 of chlorine, if chlorine or hydrogen chloride were 

 systematically subjected to diffusion. It is extremely 



NO. 2491, VOL. 99"] 



desirable that such a test of the homogeneity of this 

 gas should be made in this way. 



Clearly a change must come in this class of work. 

 It is not of much use starting with stuff out of a 

 bottle labelled ■" purissimum," or "garantirt," and 

 determmmg to the highest possible degree of accuracy 

 the atomic weight of an element of unknown origin. 

 The great pioneers in the subject, like Berzelius, were 

 masters of the whole domain of inorganic chemistry, 

 and knew the sources of the elements in Nature first- 

 hand. Their successors must revert to their practice 

 and go direct to Nature for their materials, must 

 select them carefully with due regard to 

 what geology teaches as to their age and his- 

 tory, and, before carrying out a single determination, 

 they must analyse their actual raw materials com- 

 pletely, and know exactly what it is they are dealing 

 with. Much of the work on the atomic weight of 

 lead from mixed minerals is useless from failure 

 to do this. Workers must rely more on the agree- 

 ment, or disagreement, of a great variety of results 

 by methods and for materials as different as 

 possible than on the result of a single method 

 pushed to the limit of refinement for an element pro- 

 visionally purified by a dealer from quite unknown 

 materials. The preconceived notion that the results 

 must necessarily agree if the work is well done must 

 be replaced by a system of co-operation between the 

 workers of the world checking each other's results for 

 the same material. A year ago anyone bold enough 

 to publish atomic weight determinations which were 

 not up to the modern standards of agreement among 

 themselves would have been regarded as having mis- 

 taken his vocation. If these wider ideals are pursued, 

 all the labour that has been lavished in this field, and 

 which now seems to have been so largely wasted, may 

 possibly bear fruit, and where the newer methods fail, 

 far beyond the narrow belt of elements which it is 

 possible to watch changing, the atomic weight worker 

 may be able to pick up the threads of the great story. 

 No doubt it is writ in full in the natural records pre- 

 served by rock and mineral, and the evidence of the 

 atomic weights may be able to carry to a triumphant 

 conclusion the course of elementary evolution, of 

 which as yet only an isolated chapter has been de- 

 ciphered. 



The Structure of the Atom. 



The third line of recent advance which does much 

 to explain the meaning of isotopes and the periodic 

 law starts from Sir Ernest Rutherford's nuclear theory 

 of the atom, which is an attempt to determine the 

 nature of atomic structure, which again is the neces- 

 sary preliminary to the understanding of the third 

 aspect in which the elements are, or may be, complex. 

 That uranium and thorium are built up of different 

 isotopes of lead, helium, and electrons is now an ex- 

 perimental fact, since they have been proved to change 

 into these constituents. But the questions how they 

 are built up, and what is the nature of the non- 

 radio-active elements, which do not undergo changes, 

 remain unsolved 



Prof. Bragg showed in 1905 that the a particles 

 can traverse the atoms of matter in their path almost 

 as though they were not there. So far as he could 

 tell — and the statement is still true of the vast majority 

 of a particles colliding with the atoms of matter — the 

 a particle ploughs its way straight through, pursuing 

 a practically rectilinear course, losing slightly in 

 kinetic energy at each encounter with an atom until 

 its velocity is reduced to the point at which it can no 

 longer be detected. From that time the a particle 

 became, as it were, a messenger that could penetrate 

 the atom, traverse regions which hitherto had been 

 bolted and barred from human curiosity, and on re- 



