(FARADAY LECTURE.)

THE FUNDAMENTAL PROPERTIES OF THE ELEMENTS.1

By THEODORE WILLIAM RICHARDS,

Professor of Chemistry, Harvard University.

We meet to-night to honor the memory of Michael Faraday. It is fitting that we should come to this historic place, for here were his home and his laboratory, and in this room he lectured. Science is one of the great influences promoting the solidarity of mankind; it is world embracing, and recognizes no bounds of nationality. Faraday's work especially was a message to the whole world, and has grown into a priceless heritage for all humanity. Therefore, from time to time the generous guardians of this famous lectureship have called chemists and physicists from many lands to honor his unique genius. England, Germany, France, Italy, Russia, have all sent eminent representatives; and now from across the sea there comes a pilgrim who is proud indeed to bring the homage of the New World to this shrine of cherished memories. The many ties which bind together our two nations add especial pleasure to the fulfillment of the trust.

The mystery that enshrouds the ultimate nature of the physical universe has always stimulated the curiosity of thinking man. Of old, philosophers sought to solve the cosmic problem by abstract reasoning, but to-day we agree that the only hope of penetrating into the closely guarded secret lies in the precise estimation of that which is tangible and visible. Knowledge of the actual behavior of material and of energy provides the only safe basis for logical inference as to the real essence of things. Faraday was deeply imbued with this conviction, and it is widely recognized as the basis of all modern experimental science. The subject of my lecture to-night concerns the methods and general results of several extended series of investigations, planned with the hope of adding a little to the foundations of human knowledge by means of careful experiment.

1 Delivered before the Fellows of the Chemical Society in the theater of the Royal Institution, London, on Wednesday, June 14, 1911. Printed in the Journal of the Chemical Society, London, vol. 99, p. 1201, 1911. See also Proceedings of the Chemical Society vol. 27, p. 177, 1911. Printed also in Science, Oct. 27, 1911. Reprinted by permission, after author's revision.

At the outset let me remind you of an old saying of Plato's, for it sounds the keynote of the lecture: "If arithmetic, mensuration and weighing be taken away from any art, that which remains will not be much." In other words, the soundness of all important conclusions of mankind depends on the definiteness of the data on which they are based.

1

Lord Kelvin said: "Accurate and minute measurement seems to the nonscientific imagination a less lofty and dignified work than looking for something new. But nearly all the grandest discoveries of science have been the rewards of accurate measurement and patient, long-continued labor in the minute sifting of numerical results." The more subtle and complicated the conclusions to be drawn, the more exactly quantitative must be the knowledge of the facts.

2

Measurement is a means, not an end. Through measurement we obtain data full of precise significance, about which to reason; but indiscriminate measurement will lead nowhere. We must choose wisely the quantities to be measured, or else our time may be wasted.

Among all quantities worthy of exact measurement the properties of the chemical elements are surely some of the most fundamental, because the elements are the vehicles of all the manifold phenomena within the range of our perception.

Weight is clearly one of the most significant of these properties. The eighty or more individual numbers which we call the atomic weights are perhaps the most striking of the physical records nature has given us concerning the earliest stages of the evolution of the universe. They are mute witnesses of the first beginnings of the cosmos out of the chaos, and their significance is one of the first concerns of the chemical philosopher.

Mankind is not yet in a position to predict any single atomic weight with exactness. Therefore the exact determination of atomic weights rests upon precise laboratory work; and in order to arrive at the real values of these fundamental constants, chemical methods must be improved and revised so as to free them from systematic or accidental errors.

What, now, are the most important precautions to be taken in such work? These are worthy of brief notice, because the value of the results inevitably depends upon them. Obvious although they may be, they are often disregarded.

In the first place, each portion of substance to be weighed must be free from the suspicion of containing unheeded impurities; otherwise its weight will mean little. This is an end not easily attained, for liquids often attack their containing vessels and absorb gases, crystals

1 Plato, Philebus (trans. Jowett), 1875, vol. 4, p. 104.

Sir W. Thomson (Lord Kelvin), Address to British Association, August, 1871, Life, vol. 2, 600.

include and occlude solvents, precipitates carry down polluting impurities, dried substances cling to water, and solids, even at high temperatures, often fail to discharge their imprisoned contaminations.

In the next place, after an analysis has once begun, every trace of each substance to be weighed must be collected and find its way in due course to the scale pan. The trouble here lies in the difficulty in estimating, or even detecting, minute traces of substances remaining in solution, or minute losses by evaporation at high temperatures. In brief, "the whole truth and nothing but the truth" is the aim. The chemical side of the question is far more intricate and uncertain than the physical operation of weighing. For this reason it is neither necessary nor advisable to use extraordinarily large amounts of material. From 5 to 20 grams in each experiment is usually enough. The exclamation, "What wonderfully fine scales you must have to weigh atoms" indicates lack of knowledge. The real difficulties precede the introduction of the substance into the balance case. Every substance must be assumed to be impure, every reaction must be assumed to be incomplete, every measurement must be assumed to contain error, until proof to the contrary can be obtained. Only by means of the utmost care, applied with ever-watchful judgment, may the unexpected snares which always lurk in complicated processes be detected and rendered powerless for evil.

2

Among all the possibilities of error, the unsuspected presence of water is perhaps the most frequent and most insidious. Hence, I shall show you a device for overcoming this potent source of confusion, a device which has played a great rôle in the recent researches concerning atomic weights at Harvard, and is in large measure responsible for such value as the results may possess. The instrument enables one to dry, inclose, and weigh an anhydrous substance in such a manner as to preclude the admission of a trace of water from the atmosphere. It might well find applications in every quantitative laboratory. The simple device consists of a quartz ignition tube fitted to a soft-glass tube which has a projection or pocket in one side (fig. 1). A weighing bottle is placed at the end of the latter tube, and its stopper in the pocket. The boat containing the substance to be dried is heated in the quartz tube, surrounded by an atmosphere consisting of any desired mixture of gases. These gases are displaced after partial cooling, first by nitrogen and then by pure dry air, and the boat is pushed past the stopper into the weighing bottle,

1 Richards, Methods Used in Precise Chemical Investigation, published by the Carnegie Institution of Washington, 1910, No. 125, p. 97.

Richards, Zeitsch. anorg. Chem., 1895, vol. 8, p. 267; also Richards and Parker, ibid., 1897, vol. 13, p. 86. Two forms of apparatus are shown in this diagram; the upper drawing depicts the earlier form, suitable for a hard glass or porcelain ignition tube, whereas the lower drawing illustrates a form slightly different from the original arrangement, although the main idea is the same. The flat ground joint between quartz and glass allows for their different coefficients of expansion, and makes a quartz tube interchangeable with any other in case of breakage.

the stopper being then forced into place and the substance thus shut up in an entirely dry atmosphere. The weighing bottle may now be removed, placed in an ordinary desiccator and weighed at leisure. The substance is really dry, and its weight has definite significance.

Mention may be made also of another instrument, which likewise has greatly facilitated the recent work at Harvard, namely, the nephelometer.1 With the nephelometer, minute traces of suspended precipitate may be approximately determined from the brightness of the light they reflect. The construction is very simple. Two test tubes, near together and slightly inclined toward one another, are arranged so as to be partly shielded from a bright source of light by sliding screens. The tubes are observed from above through two thin prisms, which bring their images together and produce an appearance resembling that in the familiar half-shadow polarimeter. The

unknown quantity of dissolved substance is precipitated as a faint opalescence in one tube by means of suitable reagents, and a known amount, treated in exactly the same way, is prepared in the other. Each precipitate reflects the light; the tubes appear faintly luminous. If the tubes show like tints to the eye when the screens are similarly placed, the precipitates may be presumed to be equal in amount. In case of inequality of appearance, the changed positions of the screens necessary to produce equality of tint give a fairly accurate guide as to the relative quantities of precipitate in the two tubes. Traces of substance, which are too attenuated to be caught on any ordinary filter, may thus be estimated.

The two errors obviated by these simple devices, namely, the presence of residual water and the loss of traces of precipitate, respectively, have perhaps ruined more previous investigations than any

1 Richards, Zeitsch. anorg. Chem., 1895, vol. 8, p. 269; Richards and Wells, American Chemical Journal, 1904, vol. 31, p. 235; Richards, Ibid., 1906, vol. 35, p. 510.

other two causes, unless the inclusion of foreign substances by precipitates may be ranked as an equal vitiating effect. But these are merely details. The scope and method of the recent work on this subject at Harvard (in the course of which 30 atomic weights have been redetermined) may be seen in their full bearing only in the original papers.1

That the atomic weights may be connected by precise mathematical equations seems highly probable; but, although many interesting attempts have been made to solve the problem,' the exact nature of such relationships has not yet been discovered. No attempt which takes liberties with the more certain of the observed values is worthy of much respect. It seems to me that the discovery of the ultimate generalization is not likely to occur until many atomic weights have been determined with the greatest accuracy. No trouble being too great to attain this end, the Harvard work will be continued indefinitely, and attempts will be made to improve its quality, for the discovery of an exact mathematical relationship between atomic weights would afford us an immeasurably precious insight into the ultimate nature of things.

But weight is only one of the fundamental properties of an element Volume is almost, if not quite, as important in its own way, although far more variable and confusing. All gases, indeed, approach closely to a simple relationship of volumes, defined by the law of Gay Lussac and the rule of Avogadro, and well known to you all. In the liquid and solid state, however, great irregularities are manifest, and very little system as regards volume is generally recognized. About 12 years ago, the study of such small irregularities as exist among gases led me to the suspicion of a possible cause for the greater irregularities in liquids and solids." On applying van der Waals's well-known equation to several gases, in some tentative and unpublished computations, it seemed clear that the quantity b is not really a constant quantity, but is subject to change under the influence of both pressure and temperature. This conclusion has also been reached independently by van der Waals himself. But if the quantity b (supposed to be dependent upon the space

1 An important part in these researches has been taken by G. P. Baxter, and many able students also have assisted the author in the work. A complete bibliography is given in Publications Carnegie Institution of Washington, 1910, No. 125, p. 91. Most of the papers are reprinted in full in a volume entitled, "Experimentelle Untersuchungen über Atomgewichte," by the author and his collaborators (Hamburg, 1909). The Carnegie Institution of Washington has generously subsidized the work in recent years. * See especially Rydberg, Zeitsch. anorg. Chem., 1897, vol. 14, p. 66.

Richards, The Significance of Changing Atomic Volume, Proceedings American Academy, 1901, vol. 37, p. 1; 1902, vol. 37, p. 300; 1902, vol. 38, p. 293; 1904, vol. 39, p. 581; Zeitsch. physikal. Chem., 1902, vol. 40, pp. 169, 597; 1903, vol. 42, p. 129; 1904, vol. 49, p. 15.

1 Van der Waals, Zeitsch. physikal. Chem., 1901, vol. 38, p. 257. His earlier publication on this topic (Proc. R. Akad. Wetensch. Amsterdam, 1898, vol. 29, p. 138) was unknown to me at that time. See also Lewis, Proceedings American Academy, 1899, vol. 35, p. 21.