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THE PROBLEM OF RADIOACTIVE

LEADI
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.

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ciated rather with the philosophical conception ease by any given atom of radium. In the of indivisibility than with the idea of chem- end most, indeed probably all, of the residual ical combination in definite proportions. To- part of the radium appears to have been day many chemists and physicists think that converted into the peculiar kind of metallic the chemical atoms of the last century are no lead with which we are concerned to-night. longer to be considered as indivisible. In The nature of the end-product was first sugthat case, the old Greek name atom" is no gested by Boltwood, who pointed out the inlonger fitting, because it denotes indivisibility. variable presence of lead in radium minerals. Some one has even facetiously suggested that Thus we must accept a kind of limited transthe word “tom”-indicating divisibility- mutation of the elements, although not of the would be more appropriate! Moreover, if our immediately profitable type sought by the anso-called atoms are really divisible, we can not cient alchemists. but be somewhat doubtful as to our definition Interesting and significant as all of this is, of the ultimate elements of the universe. The nevertheless the whole story has not yet been reason for this new turn of thought is due, told. Radium itself appears to come from as you all know, to the discovery of the un- the exceedingly slow decomposition of uraexpected and startling phenomena of radio- nium, an inference drawn from the fact that activity.

radium is found only in conjunction with To-night we have to deal with a substance the uranium, which even after careful purifidirectly concerned with the iconoclastic radio- cation soon becomes radioactive and gives active changes—with the very phenomena every indication of suffering slow disintegrawhich cause us to stop and think about our tion. Moreover, uranium is not the only definitions of atoms and elements. For the other heavy element which appears to be lead obtained from radioactive minerals ap- capable of decomposing and yielding elements pears to have resulted, together with helium, of lower atomic weight. Another, thorium, from the radioactive decomposition of ele- has a like propensity, although the steps in ments of higher atomic weight. Skeptical at

this case are perhaps not so fully interpreted, first, the whole chemical world has now come

nor so generally accepted. In the process of to acknowledge that the well-defined element,

disintegration all these heavy atoms yield helium (discovered by Sir William Ramsey

strange radiations, some of them akin to, or

identical with X-rays, which bear away that twenty-three years ago), is one of the decomposition products of radium. Radium itself

part of the colossal energy of disintegration

not made manifest as heat. These facts have is a substance which, in many respects, acts as

been proved beyond doubt by the brilliant an element, with 226 as its atomic weight,

work of Madame Curie, Sir Ernest Rutherand must be considered as the heaviest mem

ford, and others. ber of the well-known calcium family; but

The nature of the rays, and of the highly its atoms appear to be so big and so complex interesting evanescent transition products and as to disintegrate because of lack of stability.

their relation to one another is too complex The disintegration is slow, and not to be

for discussion now. We are concerned rather hastened or retarded by any agency known to

with the nature of the more permanent of the man; 1,670 years are demanded for the

substances concerned-especially with the decomposition of half of any given portion starting point, uranium (possessing the heavof radium, acc

ccording to the exact measure- iest of all atoms), radium, and the lead which ments of Professors Boltwood and Ellen Gled

seems to result from their disintegration. itsch. Moreover, we have reason to believe Omitting the less stable transition products, that this decomposition proceeds in a series the most essential outcomes are roughly inof stages, successive atoms of helium (five in dicated by a sort of genealogical tree herewith all) being evolved with different degrees of shown:

HYPOTHESIS CONCERNING THE DISINTEGRATION OF

URANIUM

1 1 Helium

is entrenched to-day more firmly than ever

before in its history. Uranium

Interesting speculations by Drs. Russell, 3 Helium

Fleck, Soddy and Fajans and others have inRadium

terpreted in extremely ingenious and plausible 8 Helium

fashion the several transitory steps of the Emanation

changes, and indicate the reasons why the end1

4 Helium Lead (Isotopic)

products of the decomposition both of uranium

and thorium should be very similar to lead, if Thus each atom of uranium is supposed not identical with it. Therefore a careful to be converted into radium by losing three

study of the properties of lead of indubitably atoms of helium, and each atom of radium is radioactive origin became a matter of great supposed to be converted into a kind of lead

interest, as a step toward confirming these by losing five more, as already stated.

speculations, especially in comparison with the If uranium can thus disintegrate, should we

properties of ordinary lead. Such investigacall it an element? and should we call its

tions should throw light on the nature of smallest particles atoms? The answers de- radium and uranium and the extraordinary pend upon our definition of these two words.

changes which those metals suffer. Moreover, If the word “element” is supposed to des- by analogy, the resulting conclusions might ignate a substance incapable of disintegration,

be more or less applicable to the relations of apparently it should not be applied to ura- other elements to each other; and the comnium; neither should the word “atom" be

parison of this new kind of lead with ordinary applied to the smallest conceivable particles of

lead might afford important information as to this substance. But no one would now main

the essential attributes of elementary subtain that any element is really incapable of

stances in general, in case any differences bedisintegration. A method of still retaining tween the two kinds should be found. the terms in this and analogous cases is to Before the subject had been taken up at define an element as a substance which has Harvard University, chemists had already not yet been decomposed artificially," that is recognized the fact that the so-called uraniumto say, by the hand of man-and an atom as lead is indeed qualitatively very like ordinary “the smallest particle of such a substance, lead. It yields a black sulphide, a yellow inferred from physicochemical behavior.” The

chromate, and a white sulphate, all very sparatom, then, is not to be considered as wholly ingly soluble in water, just as ordinary lead indivisible, but only as indivisible (or at least,

does. Continued fractional crystallization or as not yet divided) by artificial means. For,

precipitation had been shown by Professor as in the case of radium, the disintegration of

Soddy and others to separate no foreign suburanium can not be hastened or retarded by

stance. Hence great similarity was proved;

but this does not signify identity. Identity any known earthly agency. So long as it

is to be established only by quantitative restays intact, the atom of uranium behaves

searches. Plato recognized, long ago, in an quantitatively in the same fashion as any

often-quoted epigram, that when weights and other atom: Dalton's laws of definite and

measures are left out, little remains of any multiple combining proportions apply without

art. Modern science echoes this dictum in its exception to its compounds. In this connec

insistence on quantitative data; science betion one should remember that the atomic

comes more scientific as it becomes more extheory, as a whole, including Dalton's and

actly quantitative. Avogadro's generalizations, is not in the least

One of the most striking and significant of invalidated by the new discoveries of radio- the quantitative properties of an element is activity. On the contrary, the atomic theory its atomic weight-a number computed from

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