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THE PROBLEM OF RADIOACTIVE

LEAD1

WE meet to-day with happiness which six
months ago would have seemed beyond the
bounds of reasonable hope. After anxious
months, the confidently awaited victory, which
last spring still seemed far away, has crowned
the cause of justice, truth and liberty. We
in America rejoice that this cause is our
cause, and that at the most critical time we
were able to render effective help to the
staunch and brave allied forces which had
fought so long and so nobly.

The object of this address is not, however,

to appraise the military issues of the great war

so fortunately ending, nor to deal with the

weighty international problems now faced by

the world, but rather to bring before you

other considerations, having to do with the

advancement of science.

The particular subject chosen, namely, the

problem of radioactive lead, is one of peculiar
and extraordinary interest, because it involves
a readjustment and enlargement of many
rather firmly fixed ideas concerning the chem-
ical elements and their mutual relations, as
well as the nature of atoms.

Within the last twenty years the definition
of these two words, "elements" and "atoms,"
has been rendered somewhat uncertain, and
bids fair to suffer even further change. Both
of them are ancient words, and both even a
century since had acquired meanings different
from those of long ago. Thales thought of
but one element, and Aristotle's elements-
earth, air, fire, water and the quintessence,
derived perhaps from yet more ancient phi-
losophy-were not plentiful enough to account
for all the manifold phenomena of nature.
Democritus's old idea of the atom was asso-

1 Address of the President of the American Asso-

ciation for the Advancement of Science, Baltimore,

December, 1918.

ciated rather with the philosophical conception of indivisibility than with the idea of chemical combination in definite proportions. Today many chemists and physicists think that the chemical atoms of the last century are no longer to be considered as indivisible. In that case, the old Greek name 66 atom" is no longer fitting, because it denotes indivisibility. Some one has even facetiously suggested that the word "tom "-indicating divisibilitywould be more appropriate! Moreover, if our so-called atoms are really divisible, we can not but be somewhat doubtful as to our definition of the ultimate elements of the universe. reason for this new turn of thought is due, as you all know, to the discovery of the unexpected and startling phenomena of radioactivity.

The

To-night we have to deal with a substance directly concerned with the iconoclastic radioactive changes-with the very phenomena which cause us to stop and think about our definitions of atoms and elements. For the lead obtained from radioactive minerals appears to have resulted, together with helium, from the radioactive decomposition of elements of higher atomic weight. Skeptical at first, the whole chemical world has now come to acknowledge that the well-defined element, helium (discovered by Sir William Ramsey twenty-three years ago), is one of the decomposition products of radium. Radium itself is a substance which, in many respects, acts as an element, with 226 as its atomic weight, and must be considered as the heaviest member of the well-known calcium family; but its atoms appear to be so big and so complex as to disintegrate because of lack of stability. The disintegration is slow, and not to be hastened or retarded by any agency known to man; 1,670 years are demanded for the decomposition of half of any given portion of radium, according to the exact measurements of Professors Boltwood and Ellen Gleditsch. Moreover, we have reason to believe that this decomposition proceeds in a series of stages, successive atoms of helium (five in all) being evolved with different degrees of

ease by any given atom of radium. In the end most, indeed probably all, of the residual part of the radium appears to have been converted into the peculiar kind of metallic lead with which we are concerned to-night. The nature of the end-product was first suggested by Boltwood, who pointed out the invariable presence of lead in radium minerals. Thus we must accept a kind of limited transmutation of the elements, although not of the immediately profitable type sought by the ancient alchemists.

Interesting and significant as all of this is, nevertheless the whole story has not yet been told. Radium itself appears to come from the exceedingly slow decomposition of uranium, an inference drawn from the fact that radium is found only in conjunction with the uranium, which even after careful purification soon becomes radioactive and gives every indication of suffering slow disintegration. Moreover, uranium is not the only other heavy element which appears to be capable of decomposing and yielding elements of lower atomic weight. Another, thorium, has a like propensity, although the steps in this case are perhaps not so fully interpreted, nor so generally accepted. In the process of disintegration all these heavy atoms yield strange radiations, some of them akin to, or identical with X-rays, which bear away that part of the colossal energy of disintegration not made manifest as heat. These facts have been proved beyond doubt by the brilliant work of Madame Curie, Sir Ernest Rutherford, and others.

The nature of the rays, and of the highly interesting evanescent transition products and their relation to one another is too complex for discussion now. We are concerned rather with the nature of the more permanent of the substances concerned-especially with the starting point, uranium (possessing the heaviest of all atoms), radium, and the lead which seems to result from their disintegration. Omitting the less stable transition products, the most essential outcomes are roughly indicated by a sort of genealogical tree herewith shown:

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Thus each atom of uranium is supposed to be converted into radium by losing three atoms of helium, and each atom of radium is supposed to be converted into a kind of lead by losing five more, as already stated.

If uranium can thus disintegrate, should we call it an element and should we call its smallest particles atoms? The answers depend upon our definition of these two words. If the word "element" is supposed to designate a substance incapable of disintegration, apparently it should not be applied to uranium; neither should the word "atom" be applied to the smallest conceivable particles of this substance. But no one would now maintain that any element is really incapable of disintegration. A method of still retaining the terms in this and analogous cases is to define an element as "a substance which has not yet been decomposed artificially," that is to say, by the hand of man-and an atom as "the smallest particle of such a substance, inferred from physicochemical behavior." The atom, then, is not to be considered as wholly indivisible, but only as indivisible (or at least, as not yet divided) by artificial means. For, as in the case of radium, the disintegration of uranium can not be hastened or retarded by any known earthly agency. So long as it stays intact, the atom of uranium behaves quantitatively in the same fashion as any other atom: Dalton's laws of definite and multiple combining proportions apply without exception to its compounds. In this connection one should remember that the atomic theory, as a whole, including Dalton's and Avogadro's generalizations, is not in the least invalidated by the new discoveries of radioactivity. On the contrary, the atomic theory

is entrenched to-day more firmly than ever before in its history.

Interesting speculations by Drs. Russell, Fleck, Soddy and Fajans and others have interpreted in extremely ingenious and plausible fashion the several transitory steps of the changes, and indicate the reasons why the endproducts of the decomposition both of uranium and thorium should be very similar to lead, if not identical with it. Therefore a careful study of the properties of lead of indubitably radioactive origin became a matter of great interest, as a step toward confirming these speculations, especially in comparison with the properties of ordinary lead. Such investigations should throw light on the nature of radium and uranium and the extraordinary changes which those metals suffer. Moreover, by analogy, the resulting conclusions might be more or less applicable to the relations of other elements to each other; and the comparison of this new kind of lead with ordinary lead might afford important information as to the essential attributes of elementary substances in general, in case any differences between the two kinds should be found.

Before the subject had been taken up at Harvard University, chemists had already recognized the fact that the so-called uraniumlead is indeed qualitatively very like ordinary lead. It yields a black sulphide, a yellow chromate, and a white sulphate, all very sparingly soluble in water, just as ordinary lead does. Continued fractional crystallization or precipitation had been shown by Professor Soddy and others to separate no foreign substance. Hence great similarity was proved; but this does not signify identity. Identity is to be established only by quantitative researches. Plato recognized, long ago, in an often-quoted epigram, that when weights and measures are left out, little remains of any art. Modern science echoes this dictum in its insistence on quantitative data; science becomes more scientific as it becomes more exactly quantitative.

One of the most striking and significant of the quantitative properties of an element is its atomic weight-a number computed from

the proportion by weight in which it combines with some other element, taken as a standard. There is no need, before this distinguished audience, of emphasizing the importance of the familiar table of atomic weights; but a few parenthetical words about their character is perhaps not out of place. As has been more than once said, the atomic weights of the relatively permanent elements, which constitute almost all of the crust of the earth, seem to be concerned with the ultimate nature of things, and must have been fixed at the very beginning of the universe, if indeed the universe ever had any beginning. They are silent, apparently unchanging witnesses of the transition from the imagined chaos of old philosophy to the existing cosmos. The crystal of quartz in a newly hewn piece of granite seems, and probably is, as compact and perfect as it was just after it was formed, eons ago. We can not imagine that any of its properties have essentially changed during its protracted imprisonment; and, so far as we can guess, the silicon and oxygen of which it was made may have existed for previous eons, first as gas, and then as liquid. The relative weights in which these two elements combine must date at least from the inconceivably distant time when the earth "was without form and void."

Although, apparently, these numbers were thus determined at the birth of our universe, they are, philosophically speaking, in a different class from the purely mathematical constants such as the relation of circumference to the diameter of a circle. 3.14159 . . . is a geometrical magnitude entirely independent of any kind of material, and it therefore belongs in the more general class of numbers, together with simple numerical relations, logarithmic and trigonometric quantities, and other mathematical functions. On the other hand, the atomic weights of the primeval elements, although less general than these, are much more general and fundamental than the constants of astronomy, such as the so-called constant of gravity, the length of the day and year, the proper motion of the sun, and all the other incommensurable magnitudes which have

been more or less accidentally ordained in the cosmic system. The physicochemical constants, such as the atomic weights, lie in a group between the mathematical constants and the astronomical "constants," and their values have a significance only less important than the former.

In the lead from uranium, we have a comparatively youthful elementary substance, which seems to have been formed since the rocks in which it occurs had crystallized. Is the atomic weight of this youthful lead identical with that of the far more ancient common lead, which seems to be more nearly contemporary as to its origin with the silicon and oxygen of quartz?

The idea that different specimens of a given element might have different atomic weights. is by no means new-it far antedates the discovery of radioactivity.

Ever since the discovery of the definite combining proportions of the elements and the ascription of these proportions to the relative weights of the atoms, the complete constancy of the atomic weights has occasionally been questioned. More than once in the past investigators have found apparent differences in the weights of atoms of a single kind, but until very recently all these irregularities have been proved to be due to inaccurate experimentation. Nevertheless, even thirty years ago the question seemed to me not definitively answered, and careful experiments were made with copper, silver and sodium, obtained from widely different sources, in the hope of finding differences in the atomic weights, according to the source of the material. No such differences whatever were found. More recently Professor Baxter, of Harvard, compared the atomic weights of iron and nickel in meteorites (from an unknown, perhaps inconceivably distant source) and the same terrestrial metals. In these cases also the results were negative. Thus copper, silver, sodium, iron and nickel all appeared to be perfectly definite in nature, and their atoms, each after its own kind, all alike.

The general question remained, nevertheless, one of profound interest to the theoretical

chemist, because it involved the very nature of the elements themselves; and in its relation to the possible discovery of a difference between uranium lead and ordinary lead, it became a very crucial question.

Early in 1913, when the hypothesis of radioactive disintegration had assumed definite shape, Dr. Fajans's assistant, Max Lembert, journeyed to Cambridge, bringing a large quantity of lead from Bohemian radioactive sources, in order that its atomic weight might be determined by Harvard methods, with the precision attainable there. The Carnegie Institution of Washington gave generous pecuniary assistance toward providing the necessary apparatus, in this and subsequent investigations.

The most important precautions to be taken in such work are worthy of brief notice, because the value of the results inevitably depends upon them. The operation consists in weighing specimens of a salt of the element in question, and then precipitating one of the constituents in each specimen, determining the weight of the precipitate, and thus the composition of the salt. 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 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. Especial care was taken that each specimen was as pure as it could be made, for impurity in one would vitiate the whole comparison.

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. 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.

After all these digressions, made in order that the problems concerned should be clearly recognized, let us turn to the main object of our quest. In the present case, each form of lead was first weighed as pure chloride, and the chlorine in this salt after solution was precipitated as silver chloride, the weight of which was determined. Precautions too numerous to mention were observed. Thus the weight of chlorine in the salt was found, and by difference the weight of the lead. From the ratio of weights, the atomic weight of lead was easily calculated.

The outcome of the first Harvard trials, published in July, 1914, brought convincing evidence that the atomic weight of the specimen of uranium-lead from Bohemia is really less than that of ordinary lead, the value found being 206.6, instead of 207.2-a difference of 0.3 per cent., far beyond the probable error of experiment. Almost simultaneously preliminary figures were made public by Drs. Hönigschmid and St. Horovitz and Maurice Curie, pointing toward the same verdict.

This result, interesting and convincing as it was, was only a beginning. Other experimenters abroad have since confirmed it, especially Professor Hönigschmid, who had studied at Harvard and understood the necessary refinements of analysis; and many new determinations have been made at the Wolcott Gibbs Memorial Laboratory, with the assistance of Dr. Charles Wadsworth, 3d, and Dr. Norris F. Hall, upon various samples of lead

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