THEODORE WILLIAMRICHARDS
INVESTIGATIONS OF ATOMIC WEIGHTS BY THEODORE WILLIAM RICHARDS
"The atomic weights . . . are certainly concerned in determining the composition of every compound substance in the heavens above, on the earth beneath, or in the waters under the earth Every protein in each muscle of our body, every drop of liquid in the ocean, every stone on the mountain top bears within itself the stamp of the influence of this profoundly significant and impressive series of numbers." While Richards was an undergraduate at Haverford College, and undecided regarding his life work, he fell under the influence of Josiah P. Cooke, renowned and stimulating professor of chemistry at Harvard University. As early as 1854, fifteen years before the great achievement of Mendel&, Cooke had classified the elements into six series, in each of which the properties of the elements followed a law of progression. For years, also, Cooke had wondered whether the atomic weights were even multiples of the atomic weight of hydrogen, or of one-sixteenth the atomic weight of oxygen. After graduation, Richards was found in Cooke's laboratory, revising the atomic weights of hydrogen and copper. Decisive evidence was soon forthcoming that oxygen was as low as 15.88 referred to hydrogen as unity. After attaining the degree of Ph.D. in 1888 he studied with three eminent German analysts-Jannasch in Gottingen, Kritss in Munich, and Hempel in Dresden. Returned to Harvard, he again applied himself to the study of atomic weights "not merely because I felt more competent in that direction than in any other (having already shown the atomic weight of hydrogen distinctly too high and that of copper distinctly too low) but also because atomic weights seemed to he one of the primal mysteries of the universe." He continued his work with copper, and was impressed by the obstinacy with which copper oxide prepared by ignition of nitrate retained occluded gases. The fact that metallic chlorides and bromides could he fused (in an atmosphere of hydrogen chloride or bromide) without decomposition led him to use these substances as starting points; to precipitate silver chloride or bromide from them in water solution, and to collect and fuse the product. Both the initial reactants and the final products could then he assumed free from water and occluded gases. A variation in the above method involved the addition of weighed quantities of pure silver (converted into nitrate) to the solution of the dissolved halide until small samples of the supernatant liquid produced equal opalescence with equal additions of silver salt or halide. The six years following his first period of European study produced investigations upon halides of barium, strontium, zinc, and magnesium, uniformly resulting in marked improvement of the atomic weights previously accepted. 453
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At this point occurred an event of the greatest significance in Richards' scientific life. The late eighties had witnessed, in the rise of physical chemistry, the beginnings of a scientific revolution analogous to that taking place at the present time. Cooke had always advocated this sort of an approach to chemistry, and had rattled the dry bones of collegiate instruction with such books as "Elements of Chemical Physics" (1860) and "Principles of Chemical Philosophy" (1868). Upon the decease of Cooke in 1895, Richards was chosen to continue his work. The better to prepare himself, he revisited Europe, to study with Ostwald in Leipzig and with Nemst in Gottingen. From this time on his interest in the subject continually increased, and the critical study of analytical processes from the physico-chemical standpoint was a decisive factor in the success of all his atomic weight determinations. Upon his return, analyses of the halides of nickel, cobalt, iron, calcium, uranium, and cesium followed in rapid succession. In 1901 came the first of many great honor-the offer of a full professorship of chemistry at Gottingen. He accepted, instead, a corresponding appointment at Harvard, and remained in its service until his death in 1928. The great Belgian chemist, Jean Servais Stas (1813-91), had been an authority beyond compare in the field of atomic weights. His purifications had been so elaborate and his analytical precautions so exhaustive that everybody including Richards supposed his values to represent the ultimate in precision and reliability. In 1894 Richards had been puzzled by the f a d that the atomic weight of strontium was higher by 0.033 unit when referred to Stas' figure for chlorine than when calculated by reference to bromine. At that time Richards suspected his work on strontium chloride to be at fault, and left his results unpublished for eleven years. But in 1904 the analysis of an unusually pure sample of sodium bromide convinced him that Stas' figure for sodium must be in error. The masterly revision (1905) of sodium and chlorine demonstrated the errors which had caused Stas to find an atomic weight for sodium which was too high, and one for chlorine which was too low. Substituting 35.473 for 35.455 in his former calculation of the atomic weight of strontium, the latter became 87.661 in agreement with 87.663 as found by analyzing the bromide. These papers established the supremacy of Richards' methods, and attracted wide attention. To this period belong his important researches on nitrogen whose atomic weight he showed to be 14.008, in agreement with the values determined from gas densities. In 1907 he was visiting professor a t the University of Berlin, where he lectured on exact quantitative analysis and directed several advanced workers in research problems. In 1911 he delivered in London the Faraday Lecture of the Chemical Society on "The Fundamental Properties of the Elements," and received the Faraday Medal, an event to which he afterward referred as the climax
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of his scientific career. In 1914 he was notified of the award of the Nobel Prize in chemistry for 1915. He was the first (and only, so far) American chemist to be so honored. Honorary degrees from twelve great universities, membership in an equal number of the most important learned societies, as well as medals and decorations rarely bestowed, attested the world's appreciation of his achievements. The following extract from his Nobel lecture on atomic weights expresses well the principles upon which his experiments were based. Experimental work of great refinement is necessary in order t o determine atomic weights. No relationships between them have yet been certainly found which make i t possible for us t o compute by any sort of calculation exactly the value of any one atomic weight from any other. We must find by actual experiment the amount of one element which actually combines with the given amount of some other element, producing a pure compound of definite composition. The ex~erimentalwork usuallv resolves itself naturallv in several s u ~ ~ e s s i vprocesses. e In the first place, substances to 'be weighed must all he canable of actual isolation in a nure state. uncontaminated by any kind ck admixture. This is no eas; task. ðer we weigh the elements in their uncombined state or weigh them in the form of some compound of known composition, we must be very sure that conditions are such as to make possible the exclusion of all complicating impiuities from the scale pan. Thus it comes t o pass that comparatively few compounds of any given element are fit to serve as a means of determining its atomic weight, for the reason that comparatively few substances may he prepared in a perfectly pure state. The choice of the compounds to be employed is in some ways the most crucial part of the whole process, for with some compounds no result worthy of consideration could be obtained, even using the greatest care possible. To repeat, then, the first task is the choice of materials to be employed. The second task is a corollary of the first. Having chosen wisely, the experimenter must then prepare the substances, whatever they may he, in a state of the greatest possible purity. He must never foreet that e v e n nrecinitate carries down with i t contaminatine imiurities adsorded'or iiclnded by the substance as it separates fro& the solution. He must remember alwavs that no receotacle necessarv to contain the substance is free iron, thh possibility of being dissolveh and thus affectine the result. hloreo\.er. nrecinitatcs are never whollv insoluble; and Gost substances will voiaiilize' and lose some of the& weizht if heated to an excessive temuerature. These comulicatine ci&mstances combine often in unexpected ways t o intro&uce imy purity into one's preparation, and the experimenter must not only guard against these dangers, but must prove by adequate and satisfactory tests that no such complication has occurred. Moreover, above all he must not forget that oxygen, nitrogen, and water are almost omnipresent, and continual care must he exercised lest in some way one of these impurities may affect the substance which is serving as the basis of the work.
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These difficulties which hamper progress are serious enough even in the first part of the preparation, during which the substance needed to serve as the starting part of the research is made ready; but they are multiplied tenfold or one hundredfold during the latter part of the work. This is because during the first part of the work the hampering influences are mitigated by the circumstance that much of the material may be sacrificed during purification, whereas after the beginning of the quantitative experiment, not only must the substance be kept in a pure state, but also it must be collected to the last trace and brought on to the balance pan. If any, even a tenth of a milligram, escapes collection, the loss must be estimated by careful experiments, so that its exact amount may be known. I n this work, as a t a trial in court, the witness must testify as t o the whole truth, and nothin? but the truth. Such an investigation to have merit must be conducted with ceaseless attention to these rules of procedure. I have tried always to be sure that the substance being weighed represented all the substance which I was seeking to weigh, and nothing more; and whenever possible I have not contented myself with a hypothetical presumption that such was the case, but have endeavored to prove by special experiments, first, that nothing was lost, and, secondly, that no foreign substance had been unintentionally included. Usually, if the experimenter gives this matter sufficient thought, and if he is sufficiently impressed with the importance of certainty on these points, a fairly satisfactory proof is obtainable. He does well to discover such a proof before publication, and not to leave the matter to the subsequent investigation of others. The theory of isotopes announced in 1913, and the apparent variations in atomic weights of samples of lead from radioactive minerals aroused Richard's keenest interest. I n 1914 Fajans sent one of his ablest students to Richards with a sample of uranium lead for final decision. This material, as well as other preparations from uranium or thorium minerals, yielded atomic weights ranging from 206.40 to 206.86. Later, samples of uranium lead examined in Richards' laboratory were found to fall as low as 206.07, or 206.02 if corrected for their probable content of thorium lead, in harmony with the isotopic theory. I n this connection Richards proved that while the densities of the samples varied in proportion to their atomic weights, as did the solubilities of their salts on a percentage basis, the atomic volume and the wiohl solubilities are identical. The following summary of Richards' atomic weights is taken from Sir Harold Hartley's Memorial Lecture. Atomic Weights Determined by Richards and His Pupils at Hanard Elcntcnl
Doll of PrUzraimn
PTCDIOUS Valw
Haroord Value
Plesntl
12.005 14.008 22.995
12.00 14.008
Vnlrr
Hydrogen Lithium Carbon Nitrogen
Sodium
1915 1907 1905
12.0 14.04 23.05
22.997
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Magnesium
Aluminum Sulfur Chlorine Potassium Calcium Iron Nickel
Cobalt Copper zinc Gallium Bromine Rubidium Strontium Silver Cesium Barium Lead (uranium) Uranium
Of his many pupils in research (about ninety in number) Baxter in America and Houigschmidt in Germany have been the most active and snccessful in atomic weight work. These three together have covered no less than fifty-five elements. Let all who are tempted to abstain from scientific work because their laboratories seem inadequate remember that the f i s t twenty-four years of Richards' chemical work were spent in Boylston Hall, a building originally constructed for miscellaneous purposes and peculiarly unsuitable for exact chemical work. Crowds of students, dust, fumes, cramped quarters, and vibration had caused him incalculable worry and loss of time. But in 1912, graduates and friends of the University provided and endowed a modem research laboratory for the exclusive use of Richards and his collaborators. Bgaring the name of Wolcott Gibbs, pioneer in American chemistry, this building embodies in every detail the experience and the foresight of its first director. Fifteen years later he cobperated in designing the new Mallinckrodt and Converse Laboratories a t Harvard, and lived to see them far advanced toward completion. Few undertakings could be so profitable as a thorough study of Richards' papers, of which over 300 are to be found in the chemical literature. His physico-chemical studies are more extensive than his revisions of atomic weights, and possibly of even greater ultimate significance. His writings cannot convey an idea of his delightful personality, his kindliness, helpfulness, enthusiasm, and good humor which never failed in trying circumstances. But they do reveal his extraordmary experimental skill, in-
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genuity, critical judgment, and his unsurpassed standards of scientific integrity. The following is perhaps the most widely known and highly prized of all his sayings: "First and foremost I should emphasize the overwhelming importance of perfect sincerity and truth; one must purge oneself of the very human tendency to look only a t the favorable aspects of his work, and be ever on the lookout for self deception (which may be quite unintentional). Next, one should never be content with a conventional experimental method or scientific point of view; one should be open-minded as to the possibility that the procedure or hypothesis may be incomplete. Each step should be questioned, and each possibility of improvement realized. And then, patience, patience! Only by unremitting, persistent labor can a lasting outcome be reached."
