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THE OLD CHRONICLE
Hand-painted Photogravure from the Painting by Eduard Griitzner

The creator of this composition was actuated by an impulse suddenly
formed while examining some antique records in Rome, the atmosphere of his
environment being strongly suggestive of monkish incident and life. The
painting, renowned for its faithful portraiture, is reproduced and shown
here as an example of a study in the Middle Ages, before printing was in
common use. as compared with the modern library, enriched with the accu-
mulated wisdom of all time and freely accessible to even the poorest. The
picture is instructive for its revelation of a phase of life many centuries ago,
and it is a masterpiece of delineation.

INTERNATIONAL CONGRESS

OF

ARTS AND SCIENCE

EDITED BY

HOWARD J. ROGERS, A.M., LL.D.

DIRECTOR OF CONGRESSES

VOLUME VII
PHYSICS AND CHEMISTRY

COMPRISING

Lectures on Physics of Matter, Physics of Ether, Physics of

the Electron, Carbon Chemistry, Inorganic Chemistry,

Organic Chemistry, Physical Chemistry

and Physics and Chemistry in the

Nineteenth Century

UNIVERSITY ALLIANCE

LONDON NEW YORK

COPYRIGHT 1906 BY HOUGHTON, MIFFLIN & Co.

ALL RIGHTS RESERVED
COPYRIGHT 1908 BY UNIVERSITY ALLIANCE

ILLUSTRATIONS

VOLUME VII

FACING
PAGE

THE OLD CHRONICLE Frontispiece

Photogravure from the painting by EDUARD GRUTZNER

COLUMBUS AT SALAMANCA . . •

Photogravure from the painting by NICCOLO BARABINO

SIR WILLIAM RAMSAY 190

Photogravure from a photograph

GOLDEN AGE OF THE MEDICI 324

Photogravure from the painting by ANDREAS MULLEK

TABLE OF CONTENTS

VOLUME VII
PHYSICAL SCIENCE

The Unity of Physical Science ....... 3

BY PRESIDENT ROBERT SIMPSON WOODWARD, Ph.D., LL.D.

PHYSICS.

The Fundamental Concepts of Physical Science .... 18
BY PROF. EDWARD LEAMINGTON NICHOLS, Ph.D.

The Progress of Physics in the Nineteenth Century . . .29

BY PROP. CARL BARUS, Ph.D.

PHYSICS OF MATTER.

The Relations of the Science of Physics of Hatter to Other Branches
of Learning .......... 69

BY PROF. ARTHUR LALANNE KIMBALL, Ph.D.

Present Problems in the Physics of Matter ..... 87
BY PROF. FRANCIS EUGENE NIPHER, LL.D.

PHYSICS OF ETHER.

The Ether and Moving Matter ....... 105

BY PROF. DEWITT BRISTOL BRACE, Ph.D.

PHYSICS OF THE ELECTRON.

The Relations of Physics of Electrons to Other Branches of Science . 121
BY PROF. PAUL LANGEVIN, Ph.S.D.

Present Problems of Radioactivity . . . . . . . 157

BY PROF. ERNEST RUTHERFORD, D.Sc.

Bibliography: Department of Physics ...... 188

CHEMISTRY

The Progress of Chemistry ........ 193

BY PROF. JAMES M. CRAFTS, LL.D.

On the Fundamental Conceptions Underlying the Chemistry of the
Element Carbon ......... 195

BY PROF. JOHN ULRIC NEF, Ph.D.

TABLE OF CONTENTS

The Progress and Development of Chemistry during the Nineteenth

Century ... 221

BY PROF. FRANK WIGGLESWORTH CLARKE, D.Sc.

INORGANIC CHEMISTRY.

Inorganic Chemistry: Its Relations with the Other Sciences . . 243
BY PROF. HENRI MOISSAN, D.S., LL.D.

The Present Problems of Inorganic Chemistry .... 258
BY SIR WILLIAM BAMSAY, Ph.D., LL.D.

ORGANIC CHEMISTRY.

The Relations of Organic Chemistry to Other Sciences . . . 276
BY PROF. JULIUS STIEGLITZ, Ph.D.

Present Problems of Organic Chemistry ..... 285
BY PROF. WILLIAM ALBERT NOYES, Ph.D.

i
PHYSICAL CHEMISTRY.

The Relations of Physical Chemistry to Physics and Chemistry . . 304
BY PROF. JACOBUS HENRICUS VAN 'T HOFF, Ph.D., LL.D.

The Physical Properties of Aqueous Salt Solutions in relation to the
Ionic Theory . . . . ... . . . 311

BY PROF. ARTHUR A. NOYES, Ph.D.

PHYSIOLOGICAL CHEMISTRY.

Problems in Nutrition ......... 327

BY PROF. OTTO COHNHEIM, M.D.

The Present Problems of Physiological Chemistry .... 335
BY PROF. EUSSELL HENRY CHITTENDEN, LL.D., Sc.D.

Bibliography: Department of Chemistry ...... 352

Special Worls of Reference for Address of Professor Frank W. Clarke 354
Special Works of Reference for Physiological Chemistry . . . 355

COLUMBUS AT SALAMANCA

Photogravure from the Painting by Niccolo Barabino

Columbus discovering a new world, which has been the theme of many
artists, is the antithesis of the subject chosen by Barabino, and to the
credit of this very famous painter it may be said that his wonderful
composition 'has probably won greater sympathy and pity for the explorer
than any history however eloquent in describing his wrongs. The sneer of
nobles, the calumny of enemies, the disrespect of the learned, are pictured
by the artist with such pathos and genius that he has left a perpetual memo-
rial of his talent in this masterful representation of the famous discoverer's
tribulations.

DIVISION C— PHYSICAL SCIENCE

DIVISION C — PHYSICAL SCIENCE

(Hall 4, September 20, 10 a. TO.)
SPEAKER: PROFESSOR ROBERT S. WOODWARD, Columbia University.

THE UNITY OF PHYSICAL SCIENCE

BY ROBERT SIMPSON WOODWARD

[Robert Simpson Woodward, Ph.D., Sc.D., LL.D., President of the Carnegie In-
stitution of Washington, b. Rochester, Mich., 1849. C.E. University of Michi-
gan, 1872; Ph.D. University of Michigan, 1892; Honorary LL.D. University
of Wisconsin, 1904; Sc.D., University of Pennsylvania, and Columbia Univer-
sity, 1905. Assistant engineer, U. S. Lake Survey, 1872-82; assistant astro-
nomer, U. S. Transit of Venus Commission, 1882-84; astronomer, geographer,
and chief geographer, U. S. Geological Survey, 1884-90; assistant, U. S. Coast
and Geodetic Survey, 1890-93; Professor of Mechanics and Mathematical
Physics, Columbia University, 1893-1905; Dean of School of Pure Science,
ibid., 1895-1905; President of Carnegie Institution of Washington, 1905.
Member of National Academy of Sciences; Past President and Treasurer (since
1894) of American Association for the Advancement of Science; Past President
of American Mathematical Society and of New York Academy of Sciences ; mem-
ber of Astronomical and Astrophysical Society of America, Geological Society
of America, Physical Society of America, and Washington Academy of Sciences.
Author of Smithsonian Geographical Tables ; Higher Mathematics (with Mansfield
Merriman); also of many Government reports and numerous papers and ad-
dresses on subjects in astronomy, geodesy, mathematics, mathematical physics,
and education.]

THERE is a tradition, still tacitly sanctioned even by men of science,
that there have been epochs when the more eminent minds were able
to compass the entire range of knowledge. Amongst the vanishing
heroic figures of the past it seems possible, indeed, to discern, here
and there, a Galileo, a Huygens, a Descartes, a Leibnitz, a Newton,
a Laplace, or a Humboldt, each capable, at least, of summing up with
great completeness the state of contemporary knowledge. Traditions,
however, are generally more or less mythical, and the myth in this
case seems to be in flat contradiction with the fact that there never
was such an epoch, that the great masters of our distinguished pre-
decessors were, after all, much like the masters of to-day, simply
the leading specialists of their times. But however this may be, if
we grant the possibility of the requisite attainments, even in a few
individuals at any epoch, we shall speedily conclude that there never
was an epoch so much in need of them as the immediate present,
when the divisional speakers of this Congress are called upon to
explain the unities which pervade the ever-widening and largely
diverse fields of their several domains.

4 PHYSICAL SCIENCE

The domain of physical science, concerning which I have the
honor to address you to-day, presents peculiar and peculiarly for-
midable difficulties in the way of a summary review. While we may
not be disposed to limit the wide range of inclusion specified by our
programme, we must at once disclaim any attempt to speak author-
itatively with respect to most of its details. There is, in fact, such
a vast array of knowledge now comprehended under any one of the
six Departments of our Division, that the boldest author must hesi-
tate to enter on a limited discussion with respect to any of them.
But if it is thus difficult to consider any department of physical
science, it appears incomparably more difficult to contemplate all
of them in the bewildering complexity of their interrelations and
in the bewildering diversity of their subject-matter. What, for
example, could seem more appalling to the average man of science
than the duty of explaining the connections of archeology and astro-
physics, or those of ecology and electrons?

Happily, however, the managers of the Congress have provided
an adequate division of labor, whereby the technical details of the
various Departments are allotted to experts, giving thus to a divis-
ional speaker a degree of freedom with respect to depth in some
way commensurate with the breadth of his task. Presuming, there-
fore, that I may deal only with the broader outlines and salient
features of the subject, I invite your attention to a summary view
of the present status and the apparent trend of physical science.

Whatever may be affirmed with respect to science in general,
there appears to be no doubt that all of the physical sciences are
characterized by three remarkable unities, — a unity of origin, a
unity of growth, and a unity of purpose. Physical science originates
in observation and experiment; it rises from the fact-gathering
stage of unrelated qualities to the higher plane of related quantities,
and passes thence on to the realm of correlation, computation, and
prediction under theory ; and its purpose is to interpret in consistent
and verifiable terms the universe, of which we form a part. The re-
cognition of these unities is of prime importance ; for it helps us to
understand and to anticipate a great diversity of perfection amongst
the different branches of science, and hence leads us to appreciate the
desirability of hearty cooperation on the part of scientific workers in
order that progress may be ever positive towards the common goal.

Glancing rapidly seriatim at the different departments of physical
science as specified by our programme, we come first to a consider-
ation of formal physics, and we may most quickly orient ourselves
aright in this department by trying to state in what respects the
physics of to-day differs from the physics of a hundred years ago.

In spite of the extraordinary perfection of the work of Lagrange,
Laplace, Fourier, Young, Fresnel, Poisson, Green, Gauss, and others

THE UNITY OF PHYSICAL SCIENCE 5

of the early part of the nineteenth century, it will be at once admitted
that great progress has been made. In addition to noteworthy ad-
vances and improvements along the lines laid down by these mas-
ters, there have been developed the relatively new fields of elasticity,
electromagnetics, thermodynamics, and astrophysics; and there has
been discovered the widest of all generalizations in physical science,
— the law of conservation of energy. Whereas it was easy a century
ago to conceive, as in gravitational astronomy, of action at a dis-
tance across empty space, the universe in the mean time has come
to appear more and more plethoric not only with "gross matter,"
but with that most wonderful entity we call the ether. The astro-
nomers have shown us, in fact, that the number of molar systems in
the universe is enormously greater than was supposed possible a cen-
tury ago; while the physicists have revealed to us molecular systems
rivaling our solar system and its Jovian and Saturnian subsystems,
and they have loaded down the ether with a burden of properties
and relationships which its usual tenuity seems scarcely fitted to
bear. Whereas, also, a century ago the tendency of thought, under
the stimulus of the remarkable developments of the elastic solid
theory of light and the fluid theories of electricity, was chiefly to-
wards an ether whose continuity would have pleased Anaxagoras,
the tendency to-day is chiefly towards an ether whose atomicity
would have pleased Democritus,

On the whole, it must be said that the advances of the past cen-
tury, and especially those of the past half -century, have been mainly
along the lines of molecular physics. The epoch of Laplace was dis-
tinctly an epoch of molar physics; the epoch of to-day is distinctly an
epoch of molecular physics. Light, heat, electricity, and magnetism
have been definitely correlated as molecular and ethereal pheno-
mena; while the recently discovered X-rays and the wonders of
radioactivity, along with the " electrons," the "corpuscles" and the
"electrions" of current investigations, all point towards a molecular
constitution of the ether. Thermodynamics, likewise, large as it has
grown in recent decades, is essentially a development of the mole-
cular theory of gases. It would be too bold, perhaps, to assert that
the trend of accumulating knowledge is towards an atomic unity of
matter, but the day seems not far distant when there will be room
for a new Principia and for a treatise which will accomplish for
molecular systems what the Mecanique Celeste accomplished for the
solar system.

One of the most important advances of recent decades is found
in the fixation of ideas with respect to the units of physical science,
and in the great improvements which have been wrought in metro-
logy by the "International Bureau of Weights and Measures." Our
standards of length, mass, and time are now fixed with a degree

6 PHYSICAL SCIENCE

of precision which leaves little to be desired for the present; and
the capital resources of measurement and calculation are now avail-
able to an extent never hitherto approached.

It should be noted, however, that confidence in the stability of
our standards is by no means comparable with the perfection of
their current applications. Indeed, we may raise with respect to
them the question so long mooted with regard to the motions of
the members of the solar system: namely, are they stable? Not-
withstanding the admirable precision of the intercomparisons of the
prototype meters and prototype kilograms and the equally admir-
able precision of Professor Michelson's determination of the length
of the meter in terms of wave-lengths of cadmium light, we cannot
affirm that these observed relations will hold indefinitely. Our
inherited notions of mass have been rather rudely shaken, also, by
the penetrating criticisms of Mach, and it appears possible even
that the law of conservation of mass may need modification in the
light of pending researches. But worst of all, our time-unit, the
sidereal day, is so far from possessing the element of constancy that
we may affirm with practical certainty that it is secularly variable.
Having realized, through Professor Michelson's superb determination
just referred to, the cosmic standard of length suggested by Max-
well thirty years ago, we are npw much more in need of an equally
trustworthy cosmic standard of time.

If the progress of physics during the past century has been chiefly
in the direction of atomic theory, the progress of chemistry has been
still more so. Chemistry is, in fact, the science of atoms and mole-
cules par excellence, a distinction it has maintained for well-nigh
a full century under the dominance of the fruitful atomic and mole-
cular hypotheses of Dalton and of Avogadro and Ampere, and under
the similarly fruitful laws of gases established by Dalton and Gay-
Lussac. Perhaps the most striking feature of this progress, in a
general way, is the gradual disappearance it has entailed of the
imaginary lines which have been long thought to separate the fields
of chemistry and physics. Through the remarkable discoveries of
Faraday the two fields have been found to overlap in actual electrical
contact. Through the wonderful revelations of spectrum analysis,
originating with Bunsen and Kirchhoff, they have been proved to
be very largely common ground. And through the broader generaliz-
ations inaugurated by Willard Gibbs, Helmholtz, and others, they are
now both somewhat in danger of being annexed as a sub-province
of rational mechanics.

To one whose work has fallen more especially in the fields of pre-
cise astronomy, geodesy, or metrology, it might seem a just reproach
to chemistry that it is a science whose measurements and calcul-
ations demand, as a rule, no greater arithmetical resources than

THE UNITY OF PHYSICAL SCIENCE 7

those of four-place tables of logarithms and anti-logarithms. The
so-called "Constants of Nature" supplied by chemistry are, in fact,
known with a low degree of certainty; a degree expressed, say, by
three to five significant figures. A small amount of reflection, how-
ever, will convince one that the phenomena with which the chemist
has to deal are usually far more complex than those which have
yielded the splendid precision of astronomy, geodesy, and metro-
logy. Moreover, it should be observed that the certainties even of
these highly perfected sciences are very unequal in their different
branches. It appears more correct, therefore, as well as more just,
considering the central position it occupies and the wide range of
its ramifications, along with the vast aggregate of qualitative and
quantitative knowledge it has massed, to assert that the precision
of chemistry affords the best numerical index of the present state of
physical science. That is, when reduced to the most compact form
of statement, the certainties of physical science are best indicated,
in a general way, by a table of the combining weights of the eighty-
odd chemical elements.

When one contemplates the numbers of such a table, and when
one adds to its suggestions those which flow from the various peri-
odic groupings of the same numbers, he can hardly avoid being in-
spired by the day-dreams of those who have looked long for the
atomic unity of matter. But however the grand problem which thus
obtrudes itself may be resolved finally, it appears certain that this
table must stand as one of the great landmarks along the path of
progress in physical science.

It was justly remarked by Laplace in his Systeme du Monde that
''L' Astronomic, par la dignit6 de son objet et par la perfection de ses
theories, est le plus beau monument de 1'esprit humain, le titre le
plus noble de son intelligence " ; and we must all admit that subse-
quent progress has gone far to maintain this high position for the
most ancient and interesting of the older sciences. One finds little
difficulty in accounting for the early rise of astronomical science
and for the universal interest in celestial phenomena. Their im-
manence and omnipresence appeal even to the dullest intellects. But
it is not so easy to account for the remarkable fact that although
astronomy deals chiefly with the relations of bodies separated by
immense distances, progress in its development has thus far been
at least equal to, if not in advance of, the progress of physics and
chemistry, which have to deal with matter close at hand. Without
attempting a full explanation of this fact, it may suffice to observe
that the principal phenomena of astronomy thus far developed
appear to be relatively simple in comparison with those of the other
physical sciences; and that the immense distances which separate
the celestial bodies, instead of being an obstacle to, are a fortunate

8 PHYSICAL SCIENCE

circumstance directly in favor of, the triumphant advances which
have distinguished astronomical science from the epoch of Galileo
down to the present day.

Not less noteworthy than his high estimate of the position of
astronomy in his time are Laplace's anticipations of the course of
future progress. Our admiration is kindled by the clearness of his
vision with respect to ways and means, and by the penetration of
his predictions of future discoveries. Advances in sidereal astro-
nomy, he rightly thought, would depend chiefly on improvements
in telescopes; while advances in dynamical astronomy were to
come along with increased precision in the observed places of the
members of the solar system and along with the growing perfection
of analysis. It is almost needless to say that Laplace's brilliant anti-
cipations have been quite surpassed by the actual developments.
Observational astronomy has become one of the most delicately
perfect of all the sciences; dynamical astronomy easily outstrips all
competitors in the perfection of its theories and in the certainty of
its predictions; while the newly developed branch of astrophysics
supplies the last link in the chain of evidence of the essential unity
of the material universe.

The order of the dimensions and the order of the mass contents
of the visible universe, at any rate, have been pretty clearly made
out. In addition to the vast aggregate of direct observational evi-
dence collected and recorded during the past century, numerous
theoretical researches have gone far, also, to interpret the laws which
reign in the apparent chaos of the stars. The solar system, with its
magnificent subsystems, has been proved to exhibit the type of
stellar systems in general.

In a profound investigation recently published, Lord Kelvin
has sought to correlate under the law of gravitation the principal
observed data of the visible universe. Assuming this universe to
lie within a sphere of radius equal to the distance of a star whose
parallax is one thousandth of a second of arc, he concludes that
there must be something like a thousand million masses of the mag-
nitude of our sun within that sphere. Light traveling at the rate of
300,000 kilometers per second would require about six thousand
years to traverse the diameter of this universe, and while the aver-
age distance asunder of the visible stars is considerably less, it is
still of the same order. It is only essential, therefore, to imagine
our luminary surrounded by a thousand million such suns, most
of which are, in all probability, attended by groups of planets, to
get some idea of the quantity of matter within visual range of our
relatively insignificant terrestrial abode. And the imposing range of
the astronomer's time-scale is perhaps impressively brought home to
us when we reflect that a million years is the smallest convenient

THE UNITY OF PHYSICAL SCIENCE 9

unit for recording the life-history of a star, while the current events
in that history are transmitted across the interstellar medium by
vibrations which occur at the rate of about six hundred million
million times per second. Measured by its accumulation of achieve-
ments, then, the astronomy of to-day fulfills the requirements of a
highly developed science. It is characterized by a vast aggregate
of accurately determined facts related by theories founded on a
small number of hypotheses. In the past it has called forth the two
greatest of all systematic treatises, the Principia of Newton and
the Mecanique Celeste of Laplace. It has probably done more also
than any other science, up to the present time, to illuminate the dark
periods during which man has floundered in his struggle for advance-
ment; and the indications are that its prestige will long continue.

•But there are spots on every sun; and lest some may infer, even
humorously, as Carlyle did seventy-odd years ago, that our system
of the world is "as good as perfect," attention should be called to
some noteworthy defects in astronomical data and to some singular
obscurities in astronomical theory. Here, however, great caution
and brevity are essential to avoid poaching on the preserves of our
colleagues of the Sections. It may suffice, therefore, merely to
mention, under the head of defective data, the low precision of the
solar parallax, the aberration constant, the masses of the members
of the solar system, and the uncertainty of our time-unit, already
referred to. Two instances, likewise, which belong to the general field
of physics as well, may suffice as illustrations of obscurities in astro-
nomical theory. Stated in the order of their apparent complexity,
these obscurities refer to the law of gravitation and to the phenome-
non of stellar aberration. Probably both are related, and one may
hope that any explanation of either will throw light on the other.

So long as no attempt is made to reconcile the law of gravitation
with other branches of physics, progress, up to a certain point, is easy ;
and probably great advantage has resulted from the fact that dynam-
ical astronomers have not been seriously disturbed by a desire to
harmonize this law with the more elementary laws of mechanics.
Perhaps they have unconsciously rested on the platform that gravi-
tation is one of the " primordial causes " which are impenetrable to
us. There are some indications that even Laplace and Fourier did
so rest. However this may be, it has grown steadily more and more
imperative during the past century to explain gravitation, or to dis-
cover the mechanism which provides that the force between two
widely separated masses is proportional to their product directly and
to the square of the distance between them inversely. All evidence
seems to indicate that the ether must provide this mechanism;
but, strangely enough, so far, the ether has baffled all attempts to
reveal the secret. The problem has been attacked also on the purely

10 PHYSICAL SCIENCE

observational side of the numerical value of the gravitation constant.
But the splendid experimental researches for this purpose throw no
light on the mechanism in question, and, unfortunately, they bring
out values for the constant of a low order of precision.

With regard to stellar aberration, it must be at once admitted that
we have neither an adequate theory nor a precisely determined fact.
The astronomer has generally contented himself with the elementary
view that aberration is a purely kinematical phenomenon; that the
earth not only slips through the ether without sensible retardation,
but that the ether slips through the earth without sensible effects.
This difficulty was recognized, in a way, by Young and Fresnel, and,
although the subject of elaborate investigation in recent decades, it
has proved equally baffling with Newtonian gravitation. As in the
case of the latter also, the numerous attempts made to determine
the constant of aberration by observational methods have been re-
warded by results of only meagre precision. Possibly the time has
arrived when one may raise the question, Within what limits is it
proper to speak of a gravitation constant or of an aberration con-
stant?

If we agree with Laplace that astronomy is entitled to the highest
rank among the physical sciences, we can accord nothing short of
second place to the sciences of the earth. Most of them are, indeed,
intimately related to astronomy; and some of them are scarcely
less ancient in their origins, less dignified in their objects, or less
perfect in their theories. Primarily, also, it should be observed, geo-
physics is not simply a part of, but is the very foundation of, astro-
nomy; for the earth furnishes the orientation, the base-line, and the
timepiece by means of which the astronomer explores the heavens.
Geology, likewise, in the broader sense of the term, as we are now
coming to see, is a fundamental science not only by reason of its
interpretations of terrestrial phenomena, but also by reason of its
parallel interpretations of celestial phenomena; for there is little
doubt that in the evolution of the earth we may read a history which
is in large degree typical of the history of celestial bodies. In any
revised estimate, therefore, of the relative rank of the physical
sciences, while it would be impossible to lower the science of the
heavens, it would appear essential to raise the sciences of the earth
to a much higher plane of importance than was thought appropriate
by our predecessors of a hundred years ago.

As with physics, chemistry, and astronomy, the wonderful progress
of the nineteenth century in geophysical science has been along
lines converging towards the more recondite properties of matter.
All parts of the earth, through observation, experiment, induction,
and deduction, have yielded increasing evidence of limited unities
amid endless diversities. Adopting the convenient terminology of

THE UNITY OF PHYSICAL SCIENCE 11

geologists for the different shells of the earth, let us glance rapidly in
turn at the sciences of the atmosphere, the hydrosphere or oceans,
the lithosphere or crust, and the centrosphere or nucleus.

The atmosphere is the special province of meteorologists, and
although they are not yet able to issue long-range predictions, like
those guaranteed by our theories of tides and terrestrial magnetism,
it must be admitted that they have made great progress towards
a rational description of the apparently erratic phenomena of the
weather. One of the peculiar anomalies of this science illustrates
in a striking way the general need of additional knowledge of the
properties of matter; in this case, especially, the properties of gases.
It is the fact that in meteorology greater progress has been made,
up to date, in the interpretation of the kinetic than in the inter-
pretation of the static phenomena of the atmosphere. Considering
that static properties are usually much simpler than kinetic proper-
ties, it seems strange that we should know much more about cyclones,
for example, than we do about the mass and the mass distribution of
the atmosphere. In respect to this apparently simple question meteor-
ology seems to have made no advance beyond the work of Laplace.
There are indications, however, that this, along with many other
questions, must await the advent of a new Principia.

The geodesists, who are the closest allies of the astronomers, may
be said to preside over the hydrosphere, since most of their theories
as well as most of their observations are referred to the sea level.
They have determined the shape and the size of the earth to a sur-
prising degree of certainty; but they are now confronted by pro-
blems which depend chiefly on the mass and mass distribution of
the earth. The exquisite refinement of their observational methods
has brought to light a minute wandering in the earth of its axis of
rotation, which makes the latitude of any place a variable quantity ;
but the interpretation of this phenomenon is again a physical and
not a mensurational problem. They have worked improvements
also in all kinds of apparatus for refined measurements, as of base-
lines, angles, and differences of level; but here, likewise, they appear
to approach limits set by the properties of matter.

The lithosphere was once thought to be the restricted province
of geologists, but they now lay claim to the entire earth, from the
centre of the centrosphere to the limits of the atmosphere, and they
threaten to invade the region of the astronomers on their way toward
the outlying domain of cosmogony. Geology illustrates better than
any other science, probably, the wide ramifications and the close
interrelations of physical phenomena. There is scarcely a process,
a product, or a principle in the whole range of physical science, from
physics and chemistry up to astronomy and astrophysics, which is
not fully illustrated in its uniqueness or in its diversity by actual

12 PHYSICAL SCIENCE

operations still in progress on the earth, or by actual records pre-
served in her crust. The earth is thus at once the grandest of labor-
atories and the grandest of museums available to man.

Any summary statement, from a non-professional student, of the
advances in geology during the past century, would be hopelessly in-
adequate. Such a task could be fitly undertaken only by an expert,
or by a corps of them. But out of the impressive array of achieve-
ments of this science, two seem to be especially worthy of general
attention. They are the essential determination of the properties
and the role of the lithosphere, and the essential determination of
the time-scale suitable for measuring the historical succession of ter-
restrial events. The lithosphere is the theatre of the principal activ-
ities, mechanical and biological, of our planet; and a million years
is the smallest convenient unit for recording the march of those activ-
ities. When one considers the intellectual as well as the physical
obstacles which had to be surmounted, and when one recalls the
bitter controversies between the Neptunists and the Vulcanists and
between the Catastrophists and the Uniformitarians, these achieve-
ments are seen to be amongst the most important in the annals of
science.

The centrosphere is the terra incognita whose boundaries only
are accessible to physical science. It is that part of the earth con-
cerning which astronomers, geologists, and physicists have written
much, but concerning which, alas! we are still in doubt. Where direct
observation is unattainable, speculation is generally easy, but the
exclusion of inappropriate hypotheses is, in such cases, generally diffi-
cult. Nevertheless, it may be affirmed that the range of possibilities
for the state of the centrosphere has been sharply restricted during
the past half -century. Whatever may have been the origin of our
planet, whether it has evolved from nebular condensation or from
meteoric accretion; and whatever may be the distribution of tem-
perature within the earth's mass as a whole ; it appears certain that
pressure is the dominant factor within the nucleus. Pressure from
above, supplied in hydrostatic measure by the plastic lithosphere,
supplemented by internal pressure below, must determine, it would
seem, within narrow limits, the actual distribution of density through-
out the centrosphere, regardless of its material composition, of its
effective rigidity, or of its potential liquidity. Here, however, we
are extending the known properties of matter quite beyond the
bounds of experience, or of present possible experiment; and we
are again reminded of the unity of our needs by the diversity of our
difficulties.

In his recently published autobiography, Herbert Spencer asserts
that at the time of issue of his work on biology (1864) "not one
person in ten or more knew the meaning of the word . . . and

THE UNITY OF PHYSICAL SCIENCE 13

among those who knew it, few cared to know anything about the
subject." That the attitude of the educated public towards biolog-
ical science could have been thus indifferent, if not inimical, forty
years ago, seems strange enough now even to those of us who have
witnessed in part the scientific progress subsequent to that epoch.
But this was a memorable epoch, marked by the advent of the great
intellectual awakening ushered in by the generalizations of Darwin,
Wallace, Spencer, and their coadjutors. And the quarter of a cen-
tury which immediately followed this epoch appears, as we look back
upon it, like an heroic age of scientific achievement. It was an age
during which some men of science, and more men not of science,
lost their heads temporarily, if not permanently; but it was also an
age during which most men of science, and thinking people in gen-
eral, moved forward at a rate quite without precedent in the history
of human advancement. A new, and a greatly enlarged, view of the
universe was introduced in the doctrine of evolution, advanced and
opposed, alike vigorously, chiefly by reason of its biological appli-
cations and implications. Galileo, Newton, and Laplace had given
us a system of the inorganic world; Darwin, Spencer, and their
followers have foreshadowed a system which includes the organic
world as well.

The astonishing progress of biology in recent times furnishes the
most convincing evidence of the unity and the efficiency of the
methods of physical science in the interpretation of natural phe-
nomena. For the biologist has followed the same methods, with
changes appropriate to his subject-matter only, as those found
fruitful in astronomy, chemistry, and all the rest. And whatever
may be the increased complexity of the organic over the inorganic
world, or however high the factor of life may seem to raise the pro-
blems of biology above the plane of the other physical sciences,
there has appeared no sufficient reason, as yet, to doubt either the
validity or the adequacy of those methods.

Moreover, the interrelations of biology with chemistry and phys-
ics especially are yearly growing more and more extended and in-
timate through the rapidly expanding researches of bacteriology,
physiology, and physiological chemistry, plant and animal patho-
logy, and so on, up through cytology to the embryology of the higher
forms of life. Through the problems of these researches also we
are again brought face to face, sooner or later, with the problems of
molecular science.

And finally, what may be said of anthropology, which is at once
the most interesting and the most novel of the physical sciences, —
interesting by reason of its subject-matter, novel by reason of its
applications? Some of us, perhaps, might be inclined to demur from
a classification which makes man, along with matter, a fit object

14 PHYSICAL SCIENCE

of investigation in physical science. Granted even that he is usually
a not altogether efficient thermodynamic engine, it may yet appear
that he is worthy of a separate category. Fortunately, however, it
is not a rule of physical science to demand immediate answers to
such ulterior questions. It is enough for the present to know that
man furnishes no exception, save in point of complexity, to the mani-
festations of physical phenomena so widely exhibited in the animal
kingdom.

But whatever may be our inherited prejudices, or our philosophic
judgments, we are confronted by the fact that the study of man
in all his attributes is now an established domain of science. And
herein we rise to a table-land of transcendent fascination; for, to
adapt a phrase of an eminent master in physical science, the instru-
ments of investigation are the objects of research. Herein also we
find the culminating unity, not only of the physical sciences, but of
all of the sciences; and it is chiefly for the promotion of these higher
interests of anthropology that we are assembled in this cosmopoli-
tan congress to-day.

It has been our good fortune to witness in recent decades an un-
paralleled series of achievements in the fields of physical science.
All of them, from anthropology and astronomy up to zoology, have
yielded rich harvests of results; and one is prone to raise the question
whether a like degree of progress may be expected to prevail during
the century on which we have now entered. No man can tell what
a day may bring forth; much less may one forecast the progress of
a decade or a century. But, judging from the long experience of the
past, there are few reasons to doubt and many reasons to expect
that the future has still greater achievements available. It would
appear that we have found the right methods of investigation. Phil-
osophically considered, the remarkable advances of the past afford
little cause for marvel. On the contrary, they are just such results
as we should anticipate from persistent pursuit of scientific investi-
gation. Conscious of the adequacy of his methods, therefore, the
devotee to physical science has every inducement to continue his
labors with unflagging zeal and confident optimism.

DEPARTMENT IX — PHYSICS

DEPARTMENT IX — PHYSICS

(Hall 6, September 20, 2 p. m.)

CHAIRMAN: PROFESSOR HENRY CREW, Northwestern University.
SPEAKERS: PROFESSOR EDWARD L. NICHOLS, Cornell University.
PROFESSOR CARL BARUS, Brown University.

THE Chairman of the Department of Physics was Professor Henry
Crew, of Northwestern University, who opened the proceedings of
the Department by saying: " Whatever views we may entertain con-
cerning the classification of the sciences which Professor Miinster-
berg has proposed for the guidance of this congress, we will, I believe,
all concur in the opinion that it is full of suggestion and very instruct-
ive. For my own part, I think it gives a really profound glimpse
into the relationships of the various departments of human learn-
ing. You will recall that the first main division is between the pure
and applied sciences. We have come together this afternoon to con-
sider a subject which lies in the former group. But physics is not the
only pure science: it is merely one belonging to that subdivision
which deals with phenomena. Again, there are two classes of phe-
nomena, the mental and the physical: and physics has to do only
with the latter class. Indeed, it does not cover the entire field of
physical phenomena, but constitutes merely one of the six Depart-
ments in this Division. Physics is, however, the most general and
most fundamental of this group of six. It is properly found, there-
fore, at the head of the list. Our theme this afternoon, then, is that
fundamental science which deals with the general properties of
matter and energy and which includes the general principles of all
physical phenomena. We are fortunate in having with us men who,
by wide experience gained in their own researches, and by a thor-
ough study of the philosophy of the subject, are eminently fitted to
treat this topic."

THE FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE

BY EDWARD LEAMINGTON NICHOLS

[Edward Leamington Nichols, Professor of Physics, Cornell University, and
Editor-in-chief of the Physical Review, b. September 14, 1854, Leamington,
England. B.S. Cornell University, 1875; Ph.D. Gottingen, 1879; Fellowship in
Physics, Johns Hopkins University, 1879-80; Professor of Physics and Chem-
istry, Central University, 1881-83; Professor of Physics and Astronomy, Uni-
versity of Kansas, 1883-87. Member of National Academy of Science, American
Academy of Arts and Sciences, American Institute of Electrical Engineers,
American Philosophical Society, American Physical Society. Author of A Lab-
oratory Manual of Physics andApplied Electricity; The Outlines of Physics, etc.]

ALL algebra, as was pointed out by von Helmholtz 1 nearly fifty
years ago, is based upon the three following very simple proposi-
tions :

Things equal to the same thing are equal to each other.

If equals be added to equals the wholes are equal.

If unequals be added to equals the wholes are unequal.

Geometry, he adds, is founded upon a few equally obvious and
simple axioms.

The science of physics, similarly, has for its foundation three funda-
mental conceptions: those of mass, distance, and time, in terms of
which all physical quantities may be expressed.

Physics, in so far as it is an exact science, deals with the relations
of these so-called physical quantities; and this is true not merely
of those portions of the science which are usually included under
the head of physics, but also of that broader realm which consists
of the entire group of the physical sciences, viz., astronomy, the
physics of the heavens ; chemistry, the physics of the atom; geology,
the physics of the earth's crust; biology, the physics of matter im-
bued with life; physics proper (mechanics, heat, electricity, sound,
and light).

The manner in which the three fundamental quantities L, M, and
T (length, mass, and time) enter, in the case of a physical quantity,
is given by its dimensional formula.

Thus the dimensional formula for an acceleration is LT~2 which
expresses the fact that an acceleration is a velocity (a length di-
vided by a time) divided by a time. Energy has for its dimensional
formula L2MT~2; it is a force, LT~2M (an acceleration multiplied
by a mass), multiplied by a distance.

Not all physical quantities, in the present state of our knowledge^
can be assigned a definite dimensional formula, and this indicates
that not all of physics has as yet been reduced to a clearly established
1 Von Helmholtz, Populare Wissenschaftliche Vortrage, p. 136.

FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 19

mechanical basis. The dimensional formula thus affords a valuable
criterion of the extent and boundaries of our strictly definite know-
ledge of physics. Within these boundaries we are on safe and easy
ground, and are dealing, independent of all speculation, with the
relations between precisely defined quantities. These relations are
mathematical, and the entire superstructure is erected upon the three
fundamental quantities, L, M, and T, and certain definitions; just as
geometry arises from its axioms and definitions.

Of many of those physical quantities, for which we are not as yet
able to give the dimensional formula, our knowledge is precise and
definite, but it is incomplete. In the case, for example, of one import-
ant group of quantities, those used in electric and magnetic measure-
ments, we have to introduce, in addition to L, M, and T, a constant
factor to make the dimensional formula complete. This, the sup-
pressed factor of Riicker,1 is /*., the magnetic permeability, when the
quantity is expressed in the electromagnetic system, and becomes k,
the specific inductive capacity, when the quantity is expressed in
terms of the electrostatic system.

Here.the existence of the suppressed factor is indicative of our
ignorance of the mechanics involved. If we knew in what way a
medium like iron increased the magnetic field, or a medium like glass
the electric field, we should probably be able to express p. and k in
terms of the three selected fundamental dimensions and complete the
dimensional formulae of a large number of quantities.

Where direct mechanical knowledge ceases, the great realm of
physical speculation begins. It is the object of such speculation
to place all phenomena upon a mechanical basis; excluding as unsci-
entific all occult, obscure, and mystical considerations.

Whenever the mechanism by means of which phenomena are pro-
duced is incapable of direct observation either because of its remote-
ness in space, as in the case of physical processes occurring in the
stars, or in time, as in the case of the phenomena with which the
geologist has to do, or because of the minuteness of the moving parts,
as in molecular physics, physical chemistry, etc., the speculative ele-
ment is unavoidable. Here we are compelled to make use of analogy.
We infer the unknown from the known. Though our logic be without
flaw, and we violate no mathematical principle, yet are our con-
clusions not absolute. They rest of necessity upon assumptions,
and these are subject to modification indefinitely as our knowledge
becomes more complete.

A striking instance of the uncertainties of extrapolation and of the
precarious nature of scientific assumptions is afforded by the various
estimates of the temperature of the sun. Pouillet placed this tempera-
ture between 1461°C. and 1761°C.; Secchi at 5,000,000°; Ericsson
1 Rucker, Philos. Mag., 27, p. 104. 1889.

20 PHYSICS

at 2,500,000°. The newer determinations * of the temperature of the
surface are, to be sure, in better agreement. Le Chatelier finds it to be
7600°; Paschen, 5400°; Warburg, 6000°. Wilson and Gray publish
as their corrected result 8000°. The estimate of the internal temper-
ature is of a more speculative character. Schuster's computation
gives 6,000,000° to 15,000,000°; that of Kelvin, 200,000,000°; that
of Ekholm, 5,000,000°.

Another interesting illustration of the dangers of extrapolation
occurs in the history of electricity. Faraday, starting from data con-
cerning the variation between the length of electric sparks through
air with the difference of potential, made an interesting computation
of the potential difference between earth and sky necessary to dis-
charge a cloud at a height of one mile. He estimated the difference of
potential to be about 1,000,000 volts. Later investigations of the
sparking distance have, however, shown this function to possess a
character quite different from that which might have been inferred
from the earlier work, and it is likely that Faraday's value is scarcely
nearer the truth than was the original estimate of the temperature of
the sun, mentioned above.

Still another notable instance of the errors to which physical re-
search is subject when the attempt is made to extend results beyond
the limits established by actual observation occurs in the case of the
measurements of the infra-red spectrum of the sun by Langley. His
beautiful and ingenious device, the bolometer, made it possible to
explore the spectrum to wave-lengths beyond those for which the law
of dispersion of the rock-salt prism had at that time been experi-
mentally determined. Within the limits of observation the dispersion
showed a curve of simple form, tending apparently to become a
straight line as the wave-length increased. There was nothing in the
appearance of the curve to indicate that it differed in character from
the numerous empirical curves of similar type employed in experi-
mental physics, or to lead even the most experienced investigator to
suspect values for the wave-length derived from an extension of the
curve. The wave-lengths published by Langley were accordingly ac-
cepted as substantially correct by all other students of radiation; but
subsequent measurements of the dispersion of rock salt at the hands
of Rubens and his co-workers showed the existence of a second sudden
and unlooked-for turn of the curve just beyond the point at which the
earlier determinations ceased; and in consequence Langley 's wave-
lengths and all work based upon them are now known to be not even
approximately accurate. The history of physics is full of such ex-
amples of the dangers of extrapolation, or, to speak more broadly, of
the tentative character of most of our assumptions in experimental
physics.

1 See Arrhenius, Kosmische Physik, p. 131.

FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 21

We have, then, two distinct sets of physical concepts. The first of
these deals with that positive portion of physics, the mechanical basis
of which, being established upon direct observation, is fixed and defi-
nite, and in which the relations are as absolute and certain as those of
mathematics itself. Here speculation is excluded. Matter is simply
one of the three factors, which enters, by virtue of its mass, into our
formulae for energy, momentum, etc. Force is simply a quantity of
which we need to know only its magnitude, direction, point of appli-
cation, and the time during which it is applied. The Newtonian con-
ception of force — the producer of motion — is adequate. All
troublesome questions as to how force acts, of the mechanism by
means of which its effects are produced, are held in abeyance.

Speculative physics, to which the second set of concepts belongs,
deals with those portions of the science for which the mechanical
basis has to be imagined. Heat, light, electricity, and the science of
the nature and ultimate properties of matter belong to this domain.

In the history of the theory -of heat we find one of the earliest
manifestations of a tendency so common in speculative physics that
it may be considered characteristic: the assumption of a medium.
The medium in this case was the so-called imponderable caloric; and
it was one of a large class, of which the two electric fluids, the mag-
netic fluid, etc., were important members.

The theory of heat remained entirely speculative up to the time
of the establishment of the mechanical equivalent of heat by Joule.
The discovery that heat could be measured in terms of work in-
jected into thermal theory the conception of energy, and led to the
development of thermodynamics.

Generalizations of the sort expressed by TyndalPs phrase, heat
a mode of motion, follow easily from the experimental evidence of the
part which energy plays in thermal phenomena, but the specification
of the precise mode of motion in question must always depend upon
our views concerning the nature of matter, and can emerge from the
speculative stage only, if ever, when our knowledge of the mechanics
of the constitution of matter becomes fixed. The problem of the
mechanism by which energy is stored or set free rests upon a similar
speculative basis.

These are proper subjects for theoretical consideration, but the
dictum of Rowland J that we get out of mathematical formulae only
what we put into them should never be lost from sight. So long as
we put in only assumptions we shall take out hypotheses, and useful
as these may prove, they are to be regarded as belonging to the realm
of scientific speculation. They must be recognized as subject to modi-
fication indefinitely as we, in consequence of increasing knowledge,
are led to modify our assumptions.

1 Rowland, President's Address to the American Physical Society, 1900.

22 PHYSICS

The conditions with which the physicist has to deal in his study
of optics are especially favorable to the development of the scientific
imagination, and it is in this field that some of the most remarkable
instances of successful speculative work are to be found. The emission
theory died hard; and the early advocates of the undulatory theory of
light were forced to work up, with a completeness probably without
parallel in the history of science, the evidence, necessarily indirect,
that in optics we have to do with a wave-motion. The standpoint of
optical theory may be deemed conclusive, possibly final, so far as the
general proposition is concerned that it is the science of a wave-
motion. In a few cases, indeed, such as the photography of the actual
nodes of a standing wave-system, by Wiener ,we reach the firm ground
of direct observation.

Optics has nevertheless certain distinctly speculative features.
Wave-motion demands a medium. The enormous velocity of light
excludes known forms of matter; the transmission of radiation in
vacuo and through outer space from, the most remote regions of the
universe, and at the same time through solids such as glass, demands
that this medium shall have properties very different from that of
any substance with which chemistry has made us acquainted.

The assumption of a medium is, indeed, an intellectual necessity,
and the attempt to specify definitely the properties which it must
possess in order to fulfill the extraordinary functions assigned to it
has afforded a field for the highest display of scientific acumen.
While the problem of the mechanism of the Ivrminiferous ether has not
as yet met with a satisfactory solution, the ingenuity and imaginative
power developed in the attack upon its difficulties command our
admiration.

Happily the development of what may be termed the older optics
did not depend upon any complete formulation of the mechanics
of the ether. Just as the whole of the older mechanics was built up
from Kepler's laws, Newton's laws of motion, the law of gravitational
attraction, the law of inverse squares, etc., without any necessity of
describing the mechanics of gravitation or of any force, or of matter
itself, so the system of geometrical relations involved in the con-
sideration of reflection and refraction, diffraction, interference, and
polarization was brought to virtual completion without introducing
the troublesome questions of the nature of the ether and the consti-
tution of matter.

Underlying this field of geometrical optics, or what I have just
termed the older optics, are, however, a host of fundamental questions
of the utmost interest and importance, the treatment of which de-
pends upon molecular mechanics and the mechanics of the ether. Our
theories as to the nature and causes of radiation, of absorption, and of
dispersion, for example, belong to the newer optics, and are based

FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 23

upon our conceptions of the constitution of matter; and since our
ideas concerning the nature of matter, like our knowledge of the
ether, is purely speculative, the science of optics has a doubly specu-
lative basis. One type of selective absorption, for example, is as-
cribed to resonance of the particles of the absorbing substance, and
our modern dispersion theories depend upon the assumption of nat-
ural periods of vibration of the particles of the refracting medium
of the same order of frequency as that of the light-waves. When the
frequency of the waves falling upon a substance coincides with the
natural period of vibration of the particles of the latter, we have
selective absorption, and accompanying it, anomalous dispersion.
For these and numerous other phenomena no adequate theory is
possible which does not have its foundation upon some assumed
conception as to the constitution of matter.

The development of the modern idea of the ether forms one of
the most interesting chapters in the history of physics. We find at
first a tendency to assume a number of distinct media corresponding
to the various effects (visual, chemical, thermal, phosphorescent, etc.)
of light-waves, and later the growth of the conception of a single
medium, the luminiferous ether.

In the development of electricity and magnetism, meantime, the
assumption of media was found to be an essential — something with-
out which no definite philosophy of the phenomena was possible. At
first there was the same tendency to a multiplicity of media — there
were the positive and negative electric fluids, the magnetic fluid, etc.
Then there grew up in the fertile mind of Faraday that wonderful
fabric of the scientific imagination, the electric field; the conception
upon which all later attempts to form an idea of a thinkable mechan-
ism of electric and magnetic action have been established.

It is the object of science, as has been pointed out by Ostwald,
to reduce the number of hypotheses; the highest development
would be that in which a single hypothesis served to elucidate the
relations of the entire universe. Maxwell's discovery that the whole
theory of optics is capable of expression in terms identical with those
found most convenient and suitable in electricity, in a word, that
optics may be treated simply as a branch of electromagnetics, was
the first great step towards such a simplification of our fundamental
conceptions. This was followed by Hertz's experimental demonstra-
tion of the existence of artificially produced electromagnetic waves
in every respect identical with light-waves, an achievement which
served to establish upon a sure foundation the conception of a single
medium. The idea of one universal medium as the mechanical basis
for all physical phenomena was not altogether new to the theoretical
physicist, but the unification of optics and electricity did much to
strengthen this conception.

24 PHYSICS

The question of the ultimate structure of matter, as has already
been pointed out, is also speculative in the sense that the mechanism
upon which its properties are based is out of the range of direct
observation. For the older chemistry and the older molecular physics
the assumption of an absolutely simple atom and of molecules com-
posed of comparatively simple groupings of such atoms sufficed.
Physical chemistry and that new phase of molecular physics which
has been termed the physics of the ion demand the breaking up of
the atom into still smaller parts and the clothing of these with an
electric charge. The extreme step in this direction is the suggestion
of Larmor that the electron is a " disembodied charge " of negative
electricity. Since, however, in the last analysis, the only conception
having a definite and intelligible mechanical basis which physicists
have been able to form of an electric charge is that which regards it
as a phenomenon of the ether, this form of speculation is but a return
under another name to views which had earlier proved attractive to
some of the most brilliant minds in the world of science, such as
Helmholtz and Kelvin. The idea of the atom, as a vortex motion
of a perfect fluid (the ether), and similar speculative conceptions,
whatever be the precise form of mechanism imagined, are of the
same class as the moving electric charge of the later theorists.

Lodge. x in a recent article in which he attempts to voice in a pop-
ular way the views of this school of thought, says:

"Electricity under strain constitutes 'charge'; electricity in loco-
motion constitutes light. What electricity itself is we do not know,
but it may, perhaps, be a form or aspect of matter. . . . Now we can
go one step further and say, matter is composed of electricity and of
nothing else. . . ."

If for the word electricity in this quotation from Lodge we substi-
tute ether, we have a statement which conforms quite as well to the
accepted theories of light and electricity as his original statement
does to the newer ideas it is intended to express.

This reconstructed statement would read as follows:

Ether under strain constitutes "charge "; ether in locomotion con-
stitutes current and magnetism; ether in vibration constitutes light.
What ether itself is we do not know, but it may, perhaps, be a
form or aspect of matter. Now we can go one step further and say :
"Matter is composed of ether and of nothing else."

The use of the word electricity, as employed by Lodge and others,
is now much in vogue, but it appears to me unfortunate. It would
be distinctly conducive to clearness of thought and an avoidance of
confusion to restrict the term to the only meaning which is free from
criticism; that in which it is used to designate the science which
deals with electrical phenomena.

1 Lodge, Harper's Magazine, August, 1904, p. 383.

FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 25

The only way in which the noun electricity enters, in any definite
and legitimate manner, into our electrical treatises is in the designa-
tion of Q in the equations — •

Q =fldt, C = Q\E, W = QE, etc.

Here we are in the habit — whether by inheritance from the age
of the electric fluid, by reason of the hydrodynamic analogy, or as
a matter of convention or of convenience merely — of calling Q the
quantity of electricity.

Now Q is " charge " and its unit, the coulomb, is unit-charge. The
alternative expression, quantity of electricity, is a purely conventional
designation and without independent physical significance. It owes
its prevalence among electricians to the fact that by virtue of long
familiarity we prefer to think in terms of matter, which is tangible,
rather than of ether. Charge is to be regarded as fundamental, and
its substitute, quantity of electricity, as merely an artificial term of
convenience; because of the former we have a definite mechanical con-
ception, whereas we can intelligently define a quantity of electricity
only in terms of charge.

In the science of heat the case differs, in that the term heat is used,
if not as precisely synonymous with energy, at least for a quantity
having the same dimensions as energy and having as its unit the erg.
It might easily have happened, as has happened in electrical theory,
that the ancient notion of a heat substance should survive, in which
case we should have had for the quantity of heat not something
measured in terms of energy, but, as in the case of electricity, one of
the terms which enter into our expression for energy. We should
then have had to struggle continually, in thermodynamics, as we
now do in electrical theory, against the tendency to revert to an
antiquated and abandoned view.

It would, I cannot but think, have been fortunate had the word
electricity been used for what we now call electrical energy; using
charge, or some other convenient designation, for the quantity Q.
That aspect of the science in accordance with which we regard it
as a branch of energetics in which movements of the ether are pri-
marily involved would have been duly emphasized. We should have
been quit forever of the bad notion of electricity as a medium, just
as we are already freed from the incubus of heat as a medium. We
should have had electricity — a mode of motion (or stress), ether, as
we have heat — a mode of motion of matter. When our friends asked
us: "What is electricity?" we should have had a ready answer for
them instead of a puzzled smile.

One real advance which has been attained by means of the theory
of ionization, and it is of extreme significance and of far-reaching
importance, consists in the discovery that electrification, or the pos-
session of charge, instead of being a casual or accidental property,

26 PHYSICS

temporarily imparted by friction or other process, is a fundamental
property of matter. According to this newer conception of matter,
the fruit of the ionic theory, the ultimate parts of matter are elec-
trically charged particles. In the language of Rutherford: 1

"It must then be supposed that the process of ionization in gases
consists in a removal of a negative corpuscle or electron from the
molecule of gas. At atmospheric pressure this corpuscle immediately
becomes the centre of an aggregation of molecules which moves
with it and is the negative ion. After removal of the negative ion
the molecule retains a positive charge and probably also becomes
the centre of a cluster of new molecules.

"The electron or corpuscle is the body of smallest mass yet known
to science. It carries a negative charge of 3.4 X 10~~10 electrostatic
units. Its presence has only been detected when in rapid motion,
when it has for speeds up to about 1010 cms. a second, an apparent
mass m given by e/m — 1.86X107 electromagnetic units. This
apparent mass increases with the speed as the velocity of light is
approached."

At low pressures the electron appears to lose its load of cluster-
ing molecules, so that finally the negative ion becomes identical
with the electron or corpuscle, and has a mass, according to the
estimates of J. J. Thomson*, about one thousandth of that of the
hydrogen atom. The positive ion is, however, supposed to remain
of atomic size even at low pressures.

The ionic theory and the related hypothesis of electrolytic dis-
sociation afford a key to numerous phenomena concerning which
no adequate or plausible theories had hitherto been formed. By
means of them explanations have been found, for example, of such
widely divergent matters as the positive electric charge known to
exist in the upper atmosphere, and the perplexing phenomena of
fluorescence.

The evidence obtained by J. J. Thomson and other students of
ionization, that electrons from different substances are identical,
has greatly strengthened the conviction which for a long time has
been in process of formation in the minds of physicists, that all
matter is in its ultimate nature identical. This conception, neces-
sarily speculative, has been held in abeyance by the facts, regarded
as established, and lying at the foundation of the accepted system
of chemistry, of the conservation of matter and the intransmut-
ability of the elements. The phenomena observed in recent investi-
gations of radioactive substances have, however, begun to shake our
faith in this principle.

If matter is to be regarded as a product of certain operations
performed upon the ether, there is no theoretical difficulty about
1 Rutherford, Radioactivity, p. 53. 1904.

FUNDAMENTAL CONCEPTS OF PHYSICAL SCIENCE 27

transmutation of elements, variation of mass, or even the complete
disappearance or creation of matter. The absence of such phe^
nomena in our experience has been the real difficulty, and if the
views of students of radioactivity concerning the transformations
undergone by uranium, thorium, and radium are substantiated, the
doctrines of the conservation of mass and matter which lie at
the foundation of the science of chemistry will have to be modified.
There has been talk of late of violations of the principle of the con-
servation of energy in connection with the phenomena of radio-
activity, but the conservation of matter is far more likely to lose its
place among our fundamental conceptions.

The development of physics on the speculative side has led, then,
to the idea, gradually become more definite and fixed, of a universal
medium, the existence of which is a matter of inference. To this
medium properties have been assigned which are such as to enable
us to form an intelligible, consistent conception of the mechanism by
means of which phenomena, the mechanics of which is not capable of
direct observation, may be logically considered to be produced. The
great step in this speculation has been the discovery that a single
medium may be made to serve for the numerous phenomena of optics,
and that, without ascribing to it any characteristics incompatible
with a luminiferous ether, it is equally available for the description
and explanation of electric and magnetic fields, and finally may be
made the basis for intelligible theories of the structure of matter.

To many minds this seemingly universal adaptability of the ether
to the needs of physics almost removes it from the field of specu-
lation; but it should not be forgotten that a system, entirely imagin-
ary, may be devised, which fits all the known phenomena and appears
to offer the only satisfactory explanation of the facts, and which
subsequently is abandoned in favor of other views. The history of
physics is full of instances where a theory is for a time regarded
as final on account of its seeming completeness, only to give way to
something entirely different.

In this consideration of the fundamental concepts I have attempted
to distinguish between those which have the positive character of
mathematical laws and which are entirely independent of all theories
of the ultimate nature of matter, and those which deal with the latter
questions and which are essentially speculative. I have purposely
refrained from taking that further step which plunges us from the
heights of physics into the depths of philosophy.

With the statement that science in the ultimate analysis is nothing
more than an attempt to classify and correlate our sensations the
physicist has no quarrel. It is, indeed, a wholesome discipline for
him to formulate for himself his own relations to his science in terms
such as those which, to paraphrase and translate very freely the

28 PHYSICS

opening passages of his recent Treatise on Physics, Chwolson 1 has
employed.

"For every one there exist two worlds, an inner and an outer, and
our senses are the medium of communication between the two. The
outer world has the property of acting upon our senses, to bring about
certain changes, or, as we say, to exert certain stimuli.

"The inner world, for any individual, consists of all those phe-
nomena which are absolutely inaccessible (so far as direct observa-
tion goes) to other individuals. The stimulus from the outer world
produces in our inner world a subjective perception which is de-
pendent upon our consciousness. The subjective perception is made
objective, viz., is assigned time and place in the outer world and given
a name. The investigation of the processes by which this objectiv-
ication is performed is a function of philosophy."

Some such confession of faith is good for the man of science, — lest
he forget; but once it is made he is free to turn his face to the light
once more, thankful that the investigation of objectivication is, indeed,
a function of philosophy, and that the only speculations in which he,
as a physicist, is entitled to engage are those which are amenable at
every step to mathematics and to the equally definite axioms and
laws of mechanics.

1 Chwolson, Physik, vol. i, Introduction.

THE PROGRESS OF PHYSICS IN THE NINETEENTH

CENTURY

BY CARL BARUS

[Carl Barus, Dean of the Graduate Department, Brown University, b. February 19,
1856, Cincinnati, Ohio. Ph.D. Columbia University, University of Wiirzburg, Ba-
varia. Physicist, U. S. Geological Survey; Professor of Meteorology, U. S.
Weather Bureau; Professor of Physics, Smithsonian Institution; Member of the
National Academy of Science of the United States; Vice-President of American
Association for the Advancement of Science; Corresponding Member of the
British Association for the Advancement of Science; Honorary Member of the
Royal Institution of Great Britain; President of American Physical Society;
Rumford Medalist. Author of The Laws of Gases; The Physical Properties of the
Iron Carburets ; and many other books; contributor to the standard magazines.]

You have honored me by requesting at my hands an account of
the advances made in physics during the nineteenth century. I have
endeavored, in so far as I have been able, to meet the grave respon-
sibilities implied in your invitation; yet had I but thought of the
overwhelmingly vast territory to be surveyed, I well might have
hesitated to embark on so hazardous an undertaking. To mention
merely the names of men whose efforts are linked with splendid
accomplishments in the history of modern physics would far exceed
the time allotted to this address. To bear solely on certain subjects,
those, for instance, with which I am more familiar, would be to de-
velop an unsymmetrical picture. As this is to be avoided, it will be
necessary to present a straightforward compilation of all work above
a certain somewhat vague and arbitrary lower limit of importance.
Physics is, as a rule, making vigorous though partial progress along
independent parallel lines of investigation, a discrimination between
which is not possible until some cataclysm in the history of thought
ushers in a new era. It will be essential to abstain from entering
into either explanation or criticism, and to assume that all present
are familiar with the details of the subjects to be treated. I can
neither popularize nor can I endeavor to entertain, except in so
far as a rapid review of the glorious conquests of the century may be
stimulating.

In spite of all this simplicity of aim, there is bound to be distortion.
In any brief account, the men working at the beginning of the cen-
tury, when investigations were few and the principles evolved neces-
sarily fundamental, will be given greater consideration than equally
able and abler investigations near the close, when workers (let us be
thankful) were many, and the subjects lengthening into detail.
Again, the higher order of genius will usually be additionally exalted
at the expense of the less gifted thinker. I can but regret that these
are the inevitable limitations of the cursory treatment prescribed.

30 PHYSICS

As time rolls on, the greatest names more and more fully absorb the
activity of a whole epoch.

Metrology

Finally, it will hardly be possible to consider the great advances
made in physics except on the theoretical side. Of renowned experi-
mental researches, in particular of the investigations of the con-
stants of nature to a degree of ever-increasing accuracy, it is not prac-
ticable to give any adequate account. Indeed, the refinement and
precision now demanded have placed many subjects beyond the
reach of individual experimental research, and have culminated in
the establishment of the great national or international laboratories
of investigation at Sevres (1872), at Berlin (1887, 1890), at London
(1900), at Washington (1901). The introduction of uniform inter-
national units in cases of the arts and sciences of more recent develop-
ment is gradually, but inexorably, urging the same advantages on
all. Finally, the access to adequate instruments of research has
everywhere become an easier possibility for those duly qualified, and
the institutions and academies which are systematically undertaking
the distribution of the means of research are continually increasing
in strength and in number.

Classification

In the present paper it will be advisable to follow the usual pro-
cedure in physics, taking in order the advances made in dynamics,
acoustics, heat, light, and electricity. The plan pursued will, there-
fore, specifically consider the progress in elastics, crystallography,
capillarity, solution, diffusion, dynamics, viscosity, hydrodynamics,
acoustics; in thermometry, calorimetry, thermodynamics, kinetic
theory, thermal radiation; in geometric optics, dispersion, photo-
metry, fluorescence, photochemistry, interference, diffraction, polar-
ization, optical media; in electrostatics, Volta contacts, Seebeck
contacts, electrolysis, electric current, magnetism, electromagnetism,
electrodynamics, induction, electric oscillation, electric field, radio-
activity.

Surely this is too extensive a field for any one man! Few who are
not physicists realize that each of these divisions has a splendid and
voluminous history of development, its own heroes, its sublime class-
ics, often culled from the activity of several hundred years. I repeat
that few understand the unmitigatedly fundamental character, the
scope, the vast and profound intellectual possessions, of pure physics;
few think of it as the one science into which all other sciences must
ultimately converge — or a separate representation would have been
given to most of the great divisions which I have named.

PROGRESS IN NINETEENTH CENTURY 31

Hence even if the literary references may be given in print with
some fullness, it is impossible to refer verbally to more than the chief
actors, and quite impossible to delineate sharply the real significance
and the relations of what has been done. Moreover, the dates will in
most instances have to be omitted from the reading. It has been my
aim, however, to collect the greater papers in the history of physics,
and the suggestion is implied that science would gain if by some
august tribunal researches of commanding importance were formally
canonized for the benefit of posterity.

Elastics

To begin with elasticity, whose development has been of such
' marked influence throughout the whole of physics, we note that the
theory is virtually a creation of the nineteenth century. Antedating
Thomas Young, who in 1807 gave to the subject the useful concep-
tion of a modulus, and who seems to have definitely recognized the
shear, there were merely the experimental contribution of Galileo
(1638), Hooke (1660), Mariotte (1680), the elastic curve of J. Ber-
noulli (1705), the elementary treatment of vibrating bars of Euler
and Bernoulli (1742), and an attempted analysis of flexure and tor-
sion by Coulomb (1776).

The establishment of a theory of elasticity on broad lines begins
almost at a bound with Navier (1821), reasoning from a molecular
hypothesis to the equation of elastic displacement and of elastic po-
tential energy (1822-1827); yet this startling advance was destined
to be soon discredited, in the light of the brilliant generalizations of
Cauchy (1827). To him we owe the six component stresses and the six
component strains, the stress quadric and the strain quadric, the
reduction of the components to three principal stresses and three
principal strains, the ellipsoids, and other of the indispensable con-
ceptions of the present day. Cauchy reached his equations both by
the molecular hypothesis and by an analysis of the oblique stress
across an interface, — methods which predicate fifteen constants of
elasticity in the most general case, reducing to but one in the case
of isotropy. Contemporaneous with Cauchy's results are certain in-
dependent researches by Lam6 and Clapeyron (1828) and by Poisson
(1829).

Another independent and fundamental method in elastics was
introduced by Green (1837), who took as his point of departure
the potential energy of a conservative system in connection with the
Lagrangian principle of virtual displacements. This method, which
has been fruitful in the hands of Kelvin (1856), of Kirchhoff (1876),
of Neumann (1885), leads to equations with twenty-one constants
for the seolotropic medium reducing to two in the simplest case.

32 PHYSICS

The wave-motion in an isotropic medium was first deduced by
Poisson in 1828, showing the occurrence -of longitudinal and trans-
verse waves of different velocities; the general problem of wave-
motion in seolotropic media, though treated by Green (1842), was
attacked with requisite power by Blanchet (1840-1842) and by
Christoffel (1877).

Poisson also treated the case of radial vibrations of a sphere (1828),
a problem which, without this restriction, awaited the solutions of
Jaerisch (1879) and of Lamb (1882). The theory of the free vibra-
tions of solids, however, is a generalization due to Clebsch (1857-58,
Vorlesungen, 1862).

Elasticity received a final phenomenal advance through the long-
continued labors of de St. Venant (1839-55), which in the course of
his editions of the work of Moigno, of Navier (1863), and of Clebsch
(1864), effectually overhauled the whole subject. He was the first to
assert adequately the fundamental importance of the shear. The pro-
found researches of de St. Venant on the torsion of prisms and on the
flexure of prisms appeared in their complete form in 1855 and 1856.
In both cases the right sections of the stressed solids are shown to
be curved, and the curvature is succinctly specified; in the former
Coulomb's inadequate torsion formula is superseded, and in the latter
flexural stress is reduced to a transverse force and a couple. But
these mere statements convey no impression of the magnitude of
the work.

Among other notable creations with a special bearing on the theory
of elasticity there is only time to mention the invention and applica-
tion of curvilinear coordinates by Lame" (1852) ; the reciprocal the-
orem of Betti (1872), applied by Cerruti (1882) to solids with a plane
boundary — problems to which Lam6 and Clapeyron (1828) and
Boussinesq (1879-85) contributed by other methods; the case of
the strained sphere studied by Lame" (1854) and others; Kirchhoff's
flexed plate (1850); Rayleigh's treatment of the oscillations of
systems of finite freedom (1873); the thermo-elastic equations of
Duhamel (1838), of F. Neumann (1841), of Kelvin (1878); Kelvin's
analogy of the torsion of prisms with the supposed rotation of an
incompressible fluid within (1878); his splendid investigations
(1863) of the dynamics of elastic spheroids and the geophysical
applications to which they were put.

Finally, the battle royal of the molecular school following Navier,
Poisson, Cauchy, and championed by de St. Venant, with the disciples
of Green, headed by Kelvin and Kirchhoff , — the struggle of the fif-
teen constants with the twenty-one constants, in other words, —
seems to have temporarily subsided with a victory for the latter
through the researches of Voigt (1887-89).

PROGRESS IN NINETEENTH CENTURY 33

Crystallography

Theoretical crystallography, approached by Steno (1669), but
formally founded by Haiiy (1781, Traiti, 1801), has limited its
development during the century to systematic classifications of
form. Thus the thirty- two type sets of Hessel (1830) and of Bravais
(1850) have expanded into the more extensive point series involving
230 types due to Jordan (1868), Sohncke (1876), Federow (1890),
and Schoenfliess (1891). Physical theories of crystalline form have
scarcely been unfolded.

Capillarity

Capillarity antedated the century in little more than the provi-
sional, though brilliant, treatment due to Clairaut (1743). The
theory arose in almost its present state of perfection in the great
memoir of Laplace (1805), one of the most beautiful examples of
the Newton-Boscovichian (1758) molecular dynamics. Capillary
pressure was here shown to vary with the principal radii of curva-
ture of the exposed surface, in an equation involving two constants,
one dependent on the liquid only, the other doubly specific for the
bodies in contact. Integrations for special conditions include the
cases of tubes, plates, drops, contact angle, and similar instances.
Gauss (1829), dissatisfied with Laplace's method, virtually repro-
duced the whole theory from a new basis, avoiding molecular forces
in favor of Lagrangian displacements, while Poisson (1831) obtained
Laplace's equations by actually accentuating the molecular hypo-
thesis; but his demonstration has since been discredited. Young
in 1805 explained capillary phenomena by postulating a constant
surface tension, a method which has since been popularized by Max-
well (Heat, 1872).

With these magnificent theories propounded for guidance at
the very threshold of the century, one is prepared to anticipate the
wealth of experimental and detailed theoretical research which
has been devoted to capillarity. Among these the fascinating mono-
graph of Plateau (1873), in which the consequences of theory are
tested by the behavior both of liquid lamellse and by suspended
masses, Savart's (1833), and particularly Rayleigh's, researches
with jets (1879-83), Kelvin's ripples (1871), may be cited as typ-
ical. Of peculiar importance, quite apart from its meteorological
bearing, is Kelvin's deduction (1870) of the interdependence of sur-
face tension and vapor pressure when varying with the curvature of
a droplet.

34 PHYSICS

Diffusion

Diffusion was formally introduced into physics by Graham (1850).
Fick (1855), appreciating the analogy of diffusion and heat conduc-
tion, placed the phenomenon on a satisfactory theoretical basis, and
Pick's law has since been rigorously tested, in particular by H. F.
Weber (1879).

The development of diffusion from a physical point of view fol-
lowed Pfeffer's discovery (1877) of osmotic pressure, soon after
to be interpreted by van 't Hoff (1887) in terms of Boyle's and
Avogadro's laws. A molecular theory of diffusion was thereupon
given by Nernst (1887).

Dynamics

In pure dynamics the nineteenth century inherited from the
eighteenth that unrivaled feat of reasoning called by Lagrange
the Mecanique analytique (1788), and the great master was present
as far as 1813 to point out its resources and to watch over the legit-
imacy of its applications. Throughout the whole century each new
advance has but vindicated the preeminent power and safety of
its methods. It triumphed with Maxwell (1864), when he deduced
the concealed kinetics of the electromagnetic field, and with Gibbs
(1876-78), when he adapted it to the equilibrium of chemical sys-
tems. It will triumph again in the electromagnetic dynamics of
the future.

Naturally there were reactions against the tyranny of the method
of "liaisons." The most outspoken of these, propounded under the
protection of Laplace himself, was the celebrated Mecanique phy-
sique of Poisson (1828), an accentuation of Boscovich's (1758)
dynamics, which permeates the work of Navier, Cauchy, de St.
Venant, Boussinesq, even Fresnel, Ampere, and a host of others.
Cauchy in particular spent much time to reconcile the molecular
method with the Lagrangian abstractions. But Poisson's method,
though sustained by such splendid genius, has, nevertheless, on
more than one occasion — in capillarity, in elastics — shown itself
to be untrustworthy. It was rudely shaken when, with the rise of
modern electricity, the influence of the medium was more and more
pushed to the front.

Another complete reconstruction of dynamics is due to Thomson
and Tait (1867), in their endeavor to gain clearness and uniformity
of design, by referring the whole subject logically back to Newton.
This great work is the first to make systematic use of the doctrine
of the conservation of energy.

Finally, Hertz (1894), imbued with the general trend of con-
temporaneous thought, made a powerful effort to exclude force

PROGRESS IN NINETEENTH CENTURY 35

and potential energy from dynamics altogether — postulating a uni-
verse of concealed motions such as Helmholtz (1884) had treated
in his theory of cyclic systems, and Kelvin had conceived in his
adynamic gyrostatic ether (1890). In fact, the introduction of con-
cealed systems and of ordered molecular motions by Helmholtz and
Boltzmann has proved most potent in justifying the Lagrangian
dynamics in its application to the actual motions of nature.

The specific contributions of the first rank which dynamics owes
to the last century, engrossed as it was with the applications of the
subject, or with its mathematical difficulties, are not numerous.
In chronological order we recall naturally the statics (1804) and
the rotational dynamics (1834) of Poinsot, all in their geometrical
character so surprisingly distinct from the contemporary dynamics
of Lagrange and Laplace. We further recall Gauss's principle of
least constraint (1829), but little used, though often in its appli-
cations superior to the method of displacement; Hamilton's prin-
ciple of varying action (1834) and his characteristic function (1834,
1835), the former obtainable by an easy transition from D'Alem-
bert's principle and by contrast with Gauss's principle, of such
exceptional utility in the development of modern physics; finally
the development of the Leibnitzian doctrine of work and vis viva
into the law of the conservation of energy, which more than any
other principle has consciously pervaded the progress of the nine-
teenth century. Clausius's theorem of the Virial (1870) and Jacobi's
(1866) contributions should be added among others.

The potential, though contained explicitly in the writings of
Lagrange (1777), may well be claimed by the last century. The
differential equation underlying the doctrine had already been
given by Laplace in 1782, but it was subsequently to be completed
by Poisson (1827). Gauss (1813, 1839) contributed his invaluable
theorems relative to the surface integrals and force flux, and Stokes
(1854) his equally important relation of the line and the surface
integral. Legendre (published 1785) and Laplace (1782) were the
first to apply spherical harmonics in expansions. The detailed devel-
opment of volume surface and line potential has enlisted many
of the ablest writers, among whom Chasles (1837, 1839, 1842),
Helmholtz (1853), C. Neumann (1877, 1880),Lejeune-Dirichlet (1876),
Murphy (1833), and others are prominent.

The gradual growth of the doctrine of the potential would have
been accelerated, had not science to its own loss overlooked the
famous essay of Green (1828), in which many of the important
theorems were anticipated, and of which Green's theorem and
Green's function are to-day familiar reminders.

Recent dynamists incline to the uses of the methods of modern
geometry and to the vector calculus with continually increasing

36 PHYSICS

favor. Noteworthy progress was first made in this direction by
Moebius (1837-43, Statik, 1838), but the power of these methods
to be fully appreciated required the invention of the Ausdehnungs-
lehre, by Grassmann (1844), and of quaternions, by Hamilton (1853).

Finally the profound investigations of Sir Robert Ball (1871,
et seq., Treatise') on the theory of screws with its immediate dynamical
applications, though as yet but little cultivated except by the author,
must be reckoned among the promising heritages of the twentieth
century.

On the experimental side it is possible to refer only to researches
of a strikingly original character, like Foucault's pendulum (1851)
and Fizeau's gyrostat; or like Boys's (1887, et seq.} remarkable
quartz-fibre torsion-balance, by which the Newtonian constant
of gravitation and the mean density of the earth originally deter-
mined by Maskelyne (1775-78) and by Cavendish (1798) were evalu-
ated with a precision probably superior to that of the other recent
measurements, the pendulum work of Airy (1856) and Wilsing
(1885-87), or the balance methods of Jolly (1881), Konig, and
Richarz (1884). Extensive transcontinental gravitational surveys
like that of Mendenhall (1895) have but begun.

Hydrodynamics

The theory of the equilibrium of liquids was well understood
prior to the century, even in the case of rotating fluids, thanks to
the labors of Maclaurin (1742), Clairaut (1743), and Lagrange (1788).
The generalizations of Jacobi (1834) contributed the triaxial ellip-
soid of revolution, and the case has been extended to two rotating
attracting masses by Poincar6 (1885) and Darwin (1887). The
astonishing revelations contained in the recent work of Poincar6
are particularly noteworthy.

Unlike elastics, theoretical hydrodynamics passed into the nine-
teenth century in a relatively well-developed state. Both types of
the Eulerian equations of motion (1755, 1759) had left the hands
of Lagrange (1788) in their present form. In relatively recent times
H. Weber (1868) transformed them in a way combining certain
advantages of both, and another transformation was undertaken
by Clebsch (1859). Hankel (1861) modified the equation of con-
tinuity, and Svanberg and Edlund (1847) the surface conditions.

Helmholtz in his epoch-making paper of 1858 divided the subject
into those classes of motion (flow in tubes, streams, jets, waves)
for which a velocity potential exists and the vortex motions for
which it does not exist. This classification was carried even into
higher orders of motion by Craig and by Rowland (1881). For cases
with a velocity potential, much progress has been made during

PROGRESS IN NINETEENTH CENTURY 37

the century in the treatment of waves, of discontinuous fluid motion,
and in the dynamics of solids suspended in frictionless liquids.
Kelland (1844), Scott Russel (1844), and Green (1837) dealt with
the motion of progressive waves in relatively shallow vessels, Ger-
ster (1804) and Rankine (1863) with progressive waves in deep water,
while Stokes (1846, 1847, 1880), after digesting the contemporaneous
advances in hydrodynamics, brought his powerful mind to bear
on most of the outstanding difficulties. Kelvin introduced the case
of ripples (1871), afterwards treated by Rayleigh (1883). The soli-
tary wave of Russel occupied Boussinesq (1872, 1882), Rayleigh
(1876), and others; group-waves were treated by Reynolds (1877)
and Rayleigh (1879). Finally the theory of stationary waves re-
ceived extended attention in the writings of de St. Venant (1871),
Kirchhoff (1879), and Greenhill (1887). Early experimental guid-
ance was given by the classic researches of C. H. and W. Weber
(1825).

The occurrence of discontinuous variation of velocity within the
liquid was first fully appreciated by Helmholtz (1868), later by
Kirchhoff (1869), Rayleigh (1876), Voigt (1885), and others. It lends
itself well to conformal representations.

The motions of solids within a liquid have fascinated many inves-
tigators, and it is chiefly in connection with this subject that the
method of sources and sinks was developed by English mathema-
ticians, following Kelvin's method (1856) for the flow of heat. The
problem of the sphere was solved more or less completely by Poisson
(1832), Stokes (1843), Dirichlet (1852); the problem of the ellip-
soid by Green (1833),Clebsch (1858), generalized by Kirchhoff (1869).
Rankine treated the translatory motion of cylinders and ellipsoids
in a way bearing on the resistance of ships. Stokes (1843) and Kirch-
hoff entertain the question of more than one body. The motion
of rings has occupied Kirchhoff (1869), Boltzmann (1871), Kelvin
(1871), Bjerknes (1879), and others. The results of C. A. Bjerknes
(1868) on the fields of hydrodynamic force surrounding spheres,
pulsating or oscillating, in translatory or rotational motion, accent-
uate the remarkable similarity of these fields with the corresponding
cases in electricity and magnetism, and have been edited in a unique
monograph (1900) by his son. In a special category belong certain
powerful researches with a practical bearing, such as the modern
treatment of ballistics by Greenhill and of the ship propeller of
Ressel (1826), summarized by Gerlach (1885, 1886).

The numerous contributions of Kelvin (1888, 1889) in particular
have thrown new light on the difficult but exceedingly important
question of the stability of fluid motion.

The century, moreover, has extended the working theory of the

38 PHYSICS

tides due to Newton (1687) and Laplace (1774), through the labors
of Airy, Kelvin, and Darwin.

Finally the forbidding subject of vortex motion was gradually
approached more and more fully by Lagrange, Cauchy (1815, 1827),
Svanberg (1839), Stokes (1845); but the epoch-making integrations
of the differential equations, together with singularly clear-cut inter-
pretations of the whole subject, are due to Helmholtz (1858). Kelvin
(1867, 1883) soon recognized the importance of Helmholtz's work
and extended it, and further advance came in particular from J. J.
Thomson (1883) and Beltrami (1875). The conditions of stability
in vortex motion were considered by Kelvin (1880), Lamb (1878),
J. J. Thomson, and others, and the cases of one or more columnar
vortices, of cylindrical vortex sheets, of one or more vortex rings,
simple or linked, have all yielded to treatment.

The indestructibility of vortex motion in a frictionless fluid, its
open structure, the occurrence of reciprocal forces, were compared
by Kelvin (1867) with the essential properties of the atom. Others
like Fitzgerald in his cobwebbed ether, and Hicks (1885) in his vortex
sponge, have found in the properties of vortices a clue to the pos-
sible structure of the ether. Yet it has not been possible to deduce
the principles of dynamics from the vortex hypothesis, neither is the
property which typifies the mass of an atom clearly discernible.
Kelvin invokes the corpuscular hypothesis of Lesage (1818).

Viscosity

The development of viscous flow is largely on the experimental
side, particularly for solids, where Weber (1835), Kohlrausch (1863,
et seq.), and others have worked out the main laws. Stokes (1845)
deduced the full equations for liquids. Poiseiulle's law (1847), the
motion of small solids in viscous liquids, of vibrating plates, and other
important special cases, has yielded to treatment. The coefficients
of viscosity defined by Poisson (1831), Maxwell (1868), Hagenbach
(1860), O. E. Meyer (1863), are exhaustively investigated for gases
and for liquids. Maxwell (1877) has given the most suggestive and
Boltzmann (1876) the most carefully formulated theory for solids,
but the investigation of absolute data has but begun. The difficulty
of reconciling viscous flow with Lagrange's dynamics seems first to
have been adjusted by Navier.

Aeromechanics

Aerostatics is indissolubly linked with thermodynamics. Aero-
dynamics has not marked out for itself any very definite line of
progress. Though the resistance of oblique planes has engaged the

PROGRESS IN NINETEENTH CENTURY 39

attention of Rayleigh, it is chiefly on the experimental side that the
subject has been enriched, as, for instance, by the labors of Langley
(1891) and Lilienthal. Langley (1897) has, indeed, constructed a
steam-propelled aeroplane which flew successfully; but man himself
has not yet flown.

Moreover, the meteorological applications of aerodynamics con-
tained in the profound researches of Guldberg and Mohn (1877),
Ferrel (1877), Oberbeck (1882, 1886), Helmholtz (1888, 1889), and
others, as well as in such investigations as Sprung's (1880) on the in-
ertia.path, are as yet rather qualitative in their bearing on the actual
motions of the atmosphere. The marked progress of meteorology is
observational in character.

Acoustics

Early in the century the velocity of sound given in a famous equa-
tion of Newton was corrected to agree with observation by Laplace*
(1816).

The great problems in acoustics are addressed in part to the elas-
tician, in part to the physiologist. In the former case the work of
Rayleigh (1877) has described the present stage of development,
interpreting and enriching almost every part discussed. In the latter
case Helmholtz (1863) has devoted his immense powers to a like
purpose and with like success. Konig has been prominently con-
cerned with the construction of accurate acoustic apparatus.

It is interesting to note that the differential equation representing
the vibration of strings was the first to be integrated; that it passed
from D'Alembert (1747) successively to Euler (1779), Bernoulli
(1753) and Lagrange (1759). With the introduction of Fourier's series
(1807) and of spherical harmonics at the very beginning of the cen-
tury, D'Alembert's and the other corresponding equations in acous-
tics readily yielded to rigorous analysis. Rayleigh's first six chapters
summarize the results for one and for two degrees of freedom.

Flexural vibration in rods, membranes, and plates become pro-
minent in the unique investigations of Chladni (1787, 1796, Akustik,
1802). The behavior of vibrating rods has been developed by Euler
(1779), Cauchy (1827), Poisson (1833), Strehlke (1833), Lissajous
(1833), Seebeck (1849), and is summarized in the seventh and eighth
chapters of Rayleigh's book. The transverse vibration of membranes
engaged the attention of Poisson (1829). Round membranes were
rigorously treated by Kirchhoff (1850) and by Clebsch (1862); ellip-
tic membranes by Mathieu (1868). The problem of vibrating plates
presents formidable difficulties resulting not only from the edge con-
ditions, but from the underlying differential equation of the fourth
degree due to Sophie Germain (1810) and to Lagrange (1811). The

40 PHYSICS

solutions have taxed the powers of Poisson (1812, 1829), Cauchy
(1829), Kirchhoff (1850), Boussinesq (1871-79), and others. For the
circular plate Kirchhoff gave the complete theory. Rayleigh system-
atized the results for the quadratic plate, and the general account
makes up his ninth and tenth chapters.

Longitudinal vibrations, which are of particular importance in case
of the organ-pipe, were considered in succession by Poisson (1817),
Hopkins (1838), Quet (1855); but Helmholtz in his famous paper
of 1860 gave the first adequate theory of the open organ -pipe, involv-
ing viscosity. Further extension was then added by Kirchhoff (1868),
and by Rayleigh (1870, et seq.), including particularly powerful
analysis of resonance. The subject in its entirety, including the allied
treatment of the resonator, completes the second volume of Ray-
leigh's Sound.

On the other hand, the whole subject of tone-quality, of combin-
ation and difference tones, of speech, of harmony, in its physical,
physiological, and aesthetic relations, has been reconstructed, using
all the work of earlier investigators, by Helmholtz (1862), in his mas-
terly Tonempfindungen. With rare skill and devotion Konig contrib-
uted a wealth of siren-like experimental appurtenances.

Acousticians have been fertile in devising ingenious methods and
apparatus, among which the tuning-fork with resonator of Marloye,
the siren of Cagniard de la Tour (1819), the Lissajous curves (1857),
the stroboscope of Plateau (1832), the manometric flames of Konig
(1862, 1872), the dust methods of Chladni (1787) and of Kundt
(1865-68), Melde's vibrating strings (1860, 1864), the phonograph
of Edison and of Bell (1877), are among the more famous.

Heat: Thermometry

The invention of the air thermometer dates back at least to Amon-
tons (1699), but it was not until Rudberg (1837), and more thor-
oughly Regnault (1841, et seq.) and Magnus (1842), had completed
their work on the thermal expansion and compressibility of air,
that air thermometry became adequately rigorous. On the theoret-
ical side Clapeyron (1834), Helmholtz (1847), Joule (1848), had in
various ways proposed the use of the Carnot function (1894) for
temperature measurement, but the subject was finally disposed of
by Kelvin (1849, et seq.) in his series of papers on temperature and
temperature measurement.

Practical thermometry gained much from the measurement of the
expansion of mercury by Dulong and Petit (1818), repeated by
Regnault. It also profited by the determination of the viscous
behavior of glass, due to Pernet (1876) and others, but more from
the elimination of these errors by the invention of the Jena glass.

PROGRESS IN NINETEENTH CENTURY 41

It is significant to note that the broad question of thermal expan-
sion has yet no adequate equation, though much has been done
experimentally for fluids by the magnificent work of Amagat (1869,
1873, et seq.).

Heat Conduction

The subject of heat conduction from a theoretical point of view
was virtually created by the great memoir of Fourier (1822), which
shed its first light here, but subsequently illumined almost the whole
of physics. The treatment passed successively through the hands of
many of the foremost thinkers, notably of Poisson (1835, 1837),
Lame" (1836, 1839, 1843), Kelvin (1841-44), and others. With the
latter (1856) the ingenious method of sources and sinks originated.
The character of the conduction is now well known for continuous
media, isotropic or not, bounded by the more simple geometrical
forms, in particular for the sphere under all reasonable initial and
surface conditions. Much attention has been given to the heat con-
duction of the earth, following Fourier, by Kelvin (1862, 1878),
King (1893), and others.

Experimentally, Wiedemann and Franz (1853) determined the
relative heat conduction of metals and showed that for simple bodies
a parallel gradation exists for the cases of heat and of electrical con-
ductivity. Noteworthy absolute methods for measuring heat conduc-
tion were devised in particular by Forbes (1842), F. Neumann (1862),

o

Angstrom (1861-64), and a lamellar method applying to fluids by
H. F. Weber (1880).

Calorimetry

Practical calorimetry was virtually completed by the researches
of Black in 1763. A rich harvest of experimental results, therefore,
has since accrued to the subjects of specific, latent, and chemical
heats, due in particularly important cases to the indefatigable Reg-
nault (1840, 1845, et seq.). Dulong and Petit (1819) discovered the
remarkable fact of the approximate constancy of the atomic heats
of the elements. The apparently exceptional cases were interpreted
for carbon silicon and boron by H. F. Weber (1875), and for sulphur
by Regnault (1840). F. Neumann (1831) extended the law to com-
pound bodies, and Joule (1844) showed that in many cases specific
heat could be treated as additively related to the component specific
heats.

Among recent apparatus the invention of Bunsen's ice calorimeter
(1870) deserves particular mention.

42 PHYSICS

Thermodynamics

Thermodynamics, as has been stated, in a singularly fruitful way
interpreted and broadened the old Leibnitzian principle of vis viva
of 1686. Beginning with the incidental experiments of Rumford
(1798) and of Davy (1799) just antedating the century, the new
conception almost leaped into being when J. R. Mayer (1842, 1845)
defined and computed the mechanical equivalent of heat, and when
Joule (1843, 1845, et seq.} made that series of precise and judiciously
varied measurements which mark an epoch. Shortly after Helmholtz
(1847), transcending the mere bounds of heat, carried the doctrine
of the conservation of energy throughout the whole of physics.

Earlier in the century Carnot (1824), stimulated by the growing
importance of the steam engine of Watt (1763, et seq.), which Fulton
(1806) had already applied to transportation by water and which
Stephenson (1829) soon after applied to transportation by land,
invented the reversible thermodynamic cycle. This cycle or sequence
of states of equilibrium of two bodies in mutual action is, perhaps,
without a parallel in the prolific fruitfulness of its contributions to
modern physics. Its continued use in fifty years of research has
but sharpened its logical edge. Carnot deduced the startling doc-
trine of a temperature criterion for the efficiency of engines. Clapey-
ron (1834) then gave the geometrical method of representation
universally used in thermodynamic discussions to-day, though often
made more flexible by new coordinates as suggested by Gibbs (1873).

To bring the ideas of Carnot into harmony with the first law of
thermodynamics it is necessary to define the value of a transform-
ation, and this was the great work of Clausius (1850), followed very
closely by Kelvin (1851) and more hypothetically by Rankine (1851).
The latter's broad treatment of energetics (1855) antedates many
recent discussions. As early as 1858 Kirchhoff investigated the
solution of solids and of gases thermodynamically, introducing at
the same time an original method of treatment.

The second law was not generally accepted without grave mis-
giving. Clausius, indeed, succeeded in surmounting most of the
objections, even those contained in theoretically delicate problems
associated with radiation. Nevertheless, the confusion raised by the
invocation of Maxwell's " demon " has never quite been calmed; and
while Boltzmann (1877, 1878) refers to the second law as a case of
probability, Helmholtz (1882) admits that the law is an expression
of our inability to deal with the individual atom. Irreversible pro-
cesses as yet lie quite beyond the pale of thermodynamics. For these
the famous inequality of Clausius is the only refuge. The value of an
uncompensated transformation is always positive.

The invention of mechanical systems which more or less fully

PROGRESS IN NINETEENTH CENTURY 43

conform to the second law has not been infrequent. Ideas of this
nature have been put forward by Boltzmann (1866, 1872), by Clau-
sius (1870, 1871), and more powerfully by Helmholtz (1884) in his
theory of cyclic systems, which in a measure suggested the hidden
mechanism at the root of Hertz's dynamics. Gibbs's (1902) element-
ary principles of statistical mechanics seem, however, to contain the
nearest approach to a logical justification of the second law — an
approach which is more than a dynamical illustration.

The applications of the first and second laws of thermodynamics
are ubiquitous. As interesting instances we may mention the con-
ception of an ideal gas and its properties; the departure of physical
gases from ideality as shown in Kelvin and Joule's plug experiment
(1854, 1862); the corrected temperature scale resulting on the one
hand, and the possibility of the modern liquid air refrigerator of
Linde and Hampson (1895) on the other. Difficulties encountered
in the liquefaction of incoercible gases by Cailletet and Pictet (1877)
have vanished even from the hydrogen coercions of Olezewski (1895)
and of Dewar and Travers.

Again, the broad treatment of fusion and evaporation, beginning
with James Thomson's (1849) computation of the melting point of
ice under pressure, Kirchhoff's (1858) treatment of sublimation, the
extensive chapter of thermo-elastics set on foot by Kelvin's (1883)
equation, are further examples.

To these must be added Andrews's (1869) discovery of the continu-
ity of the liquid and the gaseous states foreshadowed by Cagniard
de la Tour (1822, 1823); the deep insight into the laws of physical
gases furnished by the experimental prowess of Amagat (1881, 1893,
1896), and the remarkably close approximation amounting almost to
a prediction of the facts observed which is given by the great work
of van der Waals (1873).

The further development of thermodynamics, remarkable for the
breadth, not to say audacity, of its generalizations, was to take
place in connection with chemical systems. The analytical power
of the conception of a thermodynamic potential was recognized
nearly at the same time by many thinkers:1 by Gibbs (1876), who
discovered both the isothermal and the adiabatic potential; by
Massieu (1877), independently in his Fonctions characteristiques ;
by Helmholtz (1882), in his Freie Energie; by Duhem (1886) and by
Planck (1887, 1891), in their respective thermodynamic potentials.
The transformation of Lagrange's doctrine of virtual displacements of
infinitely more complicated systems than those originally contem-
plated, in other words the introduction of a virtual thermodynamic
modification in complete analogy with the virtual displacement of
the mecanique analytique, marked a new possibility of research of
1 Maxwell's available energy is accidentally overlooked in the text.

44 PHYSICS

which Gibbs made the profoundest use. Unaware of this marshaling
of powerful mathematical forces, van 't Hoff (1886, 1888) consum-
mated his marvelously simple application of the second law; and
from interpretations of the experiments of Pfeffer (1877) and of
Raoult (1883, 1887) propounded a new theory of solution, indeed,
a basis for chemical physics, in a form at once available for experi-
mental investigation.

The highly generalized treatment of chemical statics by Gibbs
bore early fruit in its application to Deville's phenomenon of disso-
ciation (1857), and in succession Gibbs (1878, 1879), Duhem (1886),
Planck (1887), have deduced adequate equations, while the latter
in case of dilute solutions gave 'a theoretical basis for Guldberg and
Waage's law of mass action (1879). An earlier independent treat-
ment of dissociation is due to Horstmann (1869, 1873).

In comparison with the brilliant advance of chemical statics which
followed Gibbs, the progress of chemical dynamics has been less
obvious; but the outlines of the subject have, nevertheless, been suc-
cinctly drawn in a profound paper by Helmholtz (1886), followed
with much skill by Duhem (1894, 1896) and Natanson (1896).

Kinetic Theory of Gases

The kinetic theory of gases at the outset, and as suggested by
Herapath (1821), Joule (1851, 1857), Kronig (1856), virtually re-
affirmed the classic treatise of Bernoulli (1738). Clausius in 1857-62
gave to the theory a modern aspect in his derivation of Boyle's law
in its thermal relations, of molecular velocity and of the ratio of
translational to total energy. He also introduced the mean free
path (1858). Closely after followed Maxwell (1860), adducing the
law for the distribution of velocity among molecules, later critically
and elaborately examined by Boltzmann (1868-81). Nevertheless,
the difficulties relating to the partition of energy have not yet been
surmounted. The subject is still under vigorous discussion, as the
papers of Burbury (1899) and others testify.

To Maxwell (1860, 1868) is due the specifically kinetic interpret-
ation of viscosity, of diffusion, of heat conduction, subjects which
also engaged the attention of Boltzmann (1872-87). Rigorous data
for molecular velocity and mean free path have thus become avail-
able, and van der Waals (1873) added a final allowance for the size
of the molecules. Less satisfactory has been the exploration of the
character of molecular force for which Maxwell, Boltzmann (1872,
et seq.), Sutherland (1886, 1893), and others have put forward tenta-
tive investigations.

The intrinsic equation of fluids discovered and treated in the
great paper of van der Waals (1873), though partaking of the charac-

PROGRESS IN NINETEENTH CENTURY 45

ter of a first approximation, has greatly promoted the coordination
of most of the known facts. Corresponding states, the thermal
coefficients, the vapor pressure relation, the minimum of pressure-
volume products, and even molecular diameters, are reasonably in-
ferred by van der Waals from very simple premises. Many of the
results have been tested by Amagat (1896).

The data for molecular diameter furnished by the kinetic theory
as a whole, viz., the original values of Loschmidt (1865), of van der
Waals (1873), and others, are of the same order of values as Kelvin's
estimates (1883) from capillarity and contact electricity. Many
converging lines of evidence show that an approximation to the
truth has surely been reached.

Radiation

Our knowledge of the radiation of heat, diathermacy, thermo-
crosis, was promoted by the perfection which the thermopyle reached
in the hands of Melloni (1835-53). These and other researches set
at rest forever all questions relating to the identity of heat and light.
The subject was, however, destined to attain a much higher order
of precision with the invention of Langley's bolometer (1881). The
survey of heat spectra, beginning with the laborious attempts of
Herschel (1840), of E. Becquerel (1843, 1870), H. Becquerel (1883),
and others, has thus culminated in the magnificent development
shown in Langley's charts (1883, 1884, et seq.).

Kirchhoff's law (1860), to some extent anticipated by Stewart
(1857, 1858), pervades the whole subject. The radiation of the black
body, tentatively formulated in relation to temperature by Stefan
(1879) and more rigorously by Boltzmann (1884), has furnished
the savants of the Reichsanstalt with means for the development
of a new pyrometry whose upper limit is not in sight.

Among curious inventions Crooke's radiometer (1874) and Bell's
photophone may be cited. The adaptation of the former in case of
high exhaustion to the actual measurement of Maxwell's (1873)
light pressure by Lebedew (1901) and Nichols and Hull (1903) is
of quite recent history. ,

The first estimate of the important constant of solar radiation at
the earth was made by Pouillet (1838); but other pyrheliometric
methods have since been devised by Langley (1884) and more re-
cently by Angstrom (1886, et seq.).

Velocity of light

Data for the velocity of light, verified by independent astronom-
ical observations, were well known prior to the century; for Romer

46 PHYSICS

had worked as long ago as 1675, and Bradley in 1727. It remained
to actually measure this enormous velocity in the laboratory, appar-
ently an extraordinary feat, but accomplished simultaneously by
Fizeau (1849) and by the aid of Wheatstone's revolving mirror (1834)
by Foucault (1849, 1850, 1862). Since that time precision has been
given to this important constant by Cornu (1871, 1873, 1874), Forbes
and Young (1882), Michelson (1878, et seq.}, and Newcomb (1885).
Foucault (1850), and more accurately Michelson (1884), deter-
mined the variation of velocity with the medium and wave-length,
thus assuring to the undulatory theory its ultimate triumph. Grave
concern, however, still exists, inasmuch as Michelson and Morley
(1886) by the most refined measurement, and differing from the
older observations of Fizeau (1851, 1859), were unable to detect
the optical effect of the relative motion of the atmosphere and the
luminiferous ether predicted by theory.

Romer's observation may in some degree be considered as an
anticipation of the principle first clearly stated by Doppler (1842),
which has since become invaluable in spectroscopy. Estimates of
the density of the luminiferous ether have been published, in par-
ticular by Kelvin (1854).

Geometric optics

Prior to the nineteenth century geometric optics, having been
mustered before Huyghens (1690), Newton (1704), Malus (1808),
Lagrange (1778, 1803), and others, had naturally attained a high
order of development. It was, nevertheless, remodeled by the great
paper of Gauss (1841), and was thereafter generalized step by step
by Listing, Mobius (1855), and particularly by Abbe (1872), post-
ulating that in character, the cardinal elements are independent
of the physical reasons by which one region is imaged in another.

So many able thinkers, like Airy (1827), Maxwell (1856, et seq.},
Bessel (1840, 1841), Helmholtz (1856, 1867), Ferraris (1877, 1880),
and others have contributed to the furtherance of geometric optics,
that definite mention is impossible. In other cases, again, profound
methods like those of Hamilton (1828, et seq.}, Kummer (1859),
do not seem to have borne correspondingly obvious fruit. The fun-
damental bearing of diffraction on geometric optics was first pointed
out by Airy (1838), but developed by Abbe (1873), and after him by
Rayleigh (1879). An adequate theory of the rainbow, due to Airy
and others, is one of its picturesque accomplishments (1838).

The so-called astronomical refraction of a medium of continu-
ously varying index, successively treated by Bouguer (1739, 1749),
Simpson (1743), Bradley (1750, 1762), owes its recent refined de-
velopment to Bessel (1823, 1826, 1842), Ivory (1822, 1823, et seq.),

PROGRESS IN NINETEENTH CENTURY 47

Radau (1884), and others. Tait (1883) gave much attention to the
allied treatment of mirage.

In relation to instruments the conditions of aplantism were exam-
ined by Clausius (1864), by Helmholtz (1874), by Abbe (1873, et
seq.), by Hockin (1884), and others, and the apochromatic lens was
introduced by Abbe (1879). The microscope is still well subserved
by either the Huyghens or the Ramsden (1873) eye-piece, but the
objective has undergone successive stages of improvement, begin-
ning with Lister's discovery in 1830. Amici (1840) introduced the
principle of immersion; Stephenson (1878) and Abbe (1879), homo-
geneous immersion; and the Abbe-Zeiss apochromatic objective
(1886), the outcome of the Jena-glass experiments, marks, perhaps,
the high-water mark of the art for the microscope. Steinheil (1865,
1866) introduced the guiding principle for photographic objectives.
Alvan Clark carried the difficult technique of telescope lens con-
struction to a degree of astonishing excellence.

Spectrum — Dispersion

Curiously, the acumen of Newton (1666, 1704) stopped short of
the ultimate conditions of purity of spectrum. It was left to Wollas-
ton (1802), about one hundred years later, to introduce the slit
and observe the dark lines of the solar spectrum. Fraunhofer (1814,
1815, 1823) mapped them out carefully and insisted on their solar
origin. Brewster (1833, 1834), who afterwards (1860) published a
map of 3000 lines, was the first to lay stress on the occurrence of
absorption, believing it to be atmospheric. Forbes (1836) gave even
greater definiteness to absorption by referring it to solar origin.
Foucault (1849) pointed out the coincidence of the sodium lines
with the D group of Fraunhofer, and discovered the reversing
effect of sodium vapor. A statement of the parallelism of emission
and absorption came from Angstrom (1855) and with greater defin-
iteness and ingenious experiments from Stewart (1860). Never-
theless, it was reserved to Kirchhoff and Bunsen (1860, 1861) to
give the clear-cut distinctions between the continuous spectra and
the characteristically fixed bright-line or dark-line spectra upon
which spectrum analysis depends. Kirchhoff's law was announced
in 1861, and the same year brought his map of the solar spectrum
and a discussion of the chemical composition of the sun. Huggins
(1864, et seq.}, Angstrom (1868), Thalen (1875), followed with im-
proved observations on the distribution and wave-length of the solar
lines; but the work of these and other observers was suddenly over-
shadowed by the marvelous possibilities of the Rowland concave
grating (1882, et seq.). Rowland's maps and tables of the solar spec-
trum as they appeared in 1887, 1889, et seq., his summary of the

48 PHYSICS

elements contained in the sun (1891), each marked a definite stage
of advance of the subject. Mitscherlich (1862, 1863) probably was
the first to recognize the banded or channeled spectra of compound
bodies. Balmer (1885) constructed a valuable equation for recog-
nizing the distribution of single types of lines. Kayser and Runge
(1887, et seq.) successfully analyzed the structure of the spectra of
alkaline and other elements.

The modernized theory of the grating had been given by Rayleigh
in 1874 and was extended to the concave grating by Rowland (1892,
1893) and others. A general theory of the resolving power of pris-
matic systems is also due to Rayleigh (1879, 1880), and another to
Thollon (1881).

The work of Rowland for the visible spectrum was ably paral-
leled by Langley's investigations (1883 et seq.) of the infra-red, dating
from the invention of the bolometer (1881). Superseding the work
of earlier investigators like Fizeau and Foucault (1878) and others,
Langley extended the spectrum with detailed accuracy to over
eight times its visible length. The solar and the lunar spectrum, the
radiations of incandescent and of hot bodies, were all specified abso-
lutely and with precision. With artificial spectra Rubens (1892,
1899) has since gone further, reaching the longest heat-waves known.

A similarly remarkable extension was added for the ultra-violet
by Schumann (1890, 1892), contending successfully with the grad-
ually increasing opacity of all known media.

Experimentally the suggestion of the spectroheliograph by Lock-
yer (1868) and by Janssen (1868) and its brilliant achievement by
Hale (1892) promise notable additions to our knowledge of solar
activity.

Finally, the refractions of absorbing media have been of great
importance in their bearing on theory. The peculiarities of metallic
reflection were announced from his earlier experiments (1811) by
Arago in 1817 and more fully investigated by Brewster (1815, 1830,
1831). F. Neumann (1832) and MacCullagh (1837) gave sharper
statements to these phenomena. Equations were advanced by
Cauchy (1836, et seq.) for isotropic bodies, and later with greater
detail by Rayleigh (1872), Ketteler (1875, et seq.), Drude (1887, et
seq.), and others. Jamin (1847, 1848) devised the first experiments of
requisite precision and found them in close agreement with Cauchy 's
theory. Kundt (1888) more recently investigated the refraction of
metallic prisms.

Anomalous dispersion was discovered by Christiansen in 1870,
and studied by Kundt (1871, et seq.). Sellmeyer's (1872) powerful
and flexible theory of dispersion was extended to include absorp-
tion effects by Helmholtz (1874), with greater detail by Ketteler
(1879, et seq.), and from a different point of view by Kelvin (1885).

PROGRESS IN NINETEENTH CENTURY 49

The electromagnetic theory lends itself particularly well to the same
phenomena, and Koldzek (1887, 1888), Goldhammer (1892), Helm-
holtz (1892), Drude (1893), and others instanced its adaptation with
success.

Photometry, Fluorescence, Photochemistry

The cosine law of Lambert (1760) has since been interpreted in
a way satisfying modern requirements by Fourier (1817, 1824) and
by Lommel (1880). Among new resources for the experimentalist
the spectrophotometer, the Lummer-Brodhun photometer (1889),
and Rood's flicker photometer (1893, 1899), should be mentioned.

Fluorescence, though ingeniously treated by Herschel (1845,
1853) and Brewster (1846, et seq.), was virtually created in its philo-
sophical aspects by Stokes in his great papers (1852, et seq.) on the
subject. In recent years Lommel (1877) made noteworthy contribu-
tions. Phosphorescence has engaged the attention of E. Becquerel
(1859), among others.

The laws of photochemistry are in large measure due to Bunsen
and Roscoe (1857, 1862). The practical development of photography
from its beginnings with Daguerre (1829, 1838) and Niepce and
Fox-Talbot (1839), to its final improvement by Maddox (1871)
with the introduction of the dry plate, is familiar to all. Vogel's
(1873) discovery of appropriate sensitizers for different colors has
added new resources to the already invaluable application of photo-
graphy to spectroscopy.

Interference

The colors of thin plates treated successively by Boyle (1663),
Hooke (1665), and more particularly by Newton (1672, Optiks,
1704), became in the hands of Young (1802) the means of framing
an adequate theory of light. Young also discovered the colors of
mixed plates and was cognizant of loss of half a wave-length on
reflection from the denser medium. Fresnel (1815) gave an inde-
pendent explanation of Newton's colors in terms of interference,
devising for further evidence his double mirrors (1816), his biprism
(1819), and eventually the triple mirror (1820). Billet's plates and
split lens (1858) belong to the same classical order, as do also Lloyd's
(1837) and Haidinger's (1849) interferences. Brewster's (1817)
observation of interference in case of thick plates culminated in
the hands of Jamin (1856, 1857) in the useful interferometer. The
scope of this apparatus was immensely advanced by the famous
device of Michelson (1881, 1882), which has now become a funda-
mental instrument of research. Michelson's determination of the
length of the meter in terms of the wave-length of light with as-
tounding accuracy is a mere example of its accomplishments.

50 PHYSICS

Wiener (1890) in his discovery of the stationary light-wave intro-
duced an entirely new interference phenomenon. The method was
successfully applied to color photography by Lippmann (1891,
1892), showing that the electric and not the magnetic vector is
photographically active.

The theory of interferences from a broader point of view, and
including the occurrence of multiple reflections, was successively
perfected by Poisson (1823), Fresnel (1823), Airy (1831). It has
recently been further advanced by Feussner (1880, et seq.}, Sohncke
and Wangerin (1881, 1883), Rayleigh (1889), and others. The inter-
ferences along a caustic were treated by Airy (1836), but the
endeavor to reconstruct geometric optics on a diffraction basis
has as yet only succeeded in certain important instances, as already
mentioned.

Diffraction

Though diffraction dates back to Grimaldi (1665) and was well
known to Newton (1704), the first correct though crude interpret-
ation of the phenomenon is due to Young (1802, 1804). Independ-
ently Fresnel (1815) in his original work devised similar explanations,
but later (1818, 1819, 1826) gave a more rational theory in terms
of Huyghens's principle, which he was the first adequately to inter-
pret. Fresnel showed that all points of a wave-front are concerned
in producing diffraction, though the ultimate critical analysis was
left to Stokes (1849).

In 1822 Fraunhofer published his remarkable paper, in which,
among other inventions, he introduced the grating into science.
Zone plates were studied by Cornu (1875) and by Soret (1875).
Rowland's concave grating appeared in 1881 ; Michelson's echelon
spectrometer in 1899.

The theory of gratings and other diffraction phenomena was
exhaustively treated by Schwerd (1837). Babinet established the
principle bearing his name in 1837. Subsequent developments were
in part concerned with the improvement of Fresnel's method of
computation, in part with a more rigorous treatment of the theory
of diffraction. Stokes (1850, 1852) gave the first account of the
polarization accompanying diffraction, and thereafter Rayleigh
(1871) and many others, including Kirchhoff (1882, 1883), profoundly
modified the classic treatment. Airy (1834, 1838) and others elabor-
ately examined the diffraction due to a point source in view of its
important bearing on the efficiency of optical instruments.

A unique development of diffraction is the phenomenon of scat-
tering propounded by Rayleigh (1871) in his dynamics of the blue
sky. This great theory which Rayleigh has repeatedly improved
(1881, et seq.) has since superseded all other relevant explanations.

PROGRESS IN NINETEENTH CENTURY 51

Polarization

An infinite variety of polarization phenomena grew out of Bar-
tholinus's (1670) discovery. Sound beginnings of a theory were
laid by Huyghens (Traite, 1690), whose wavelet principle and ele-
mentary wave-front have persisted as an invaluable acquisition, to
be generalized by Fresnel in 1821.

Fresh foundations in this department of optics were laid by
Malus (1810) in his discovery of the cosine law and the further
discovery of the polarization of reflected light. Later (1815) Brewster
adduced the conditions of maximum polarization for this case.

In 1811 Arago announced the occurrence of interferences in con-
nection with parallel plane-polarized light, phenomena which under
the observations of Arago and Fresnel (1816, 1819), Biot (1816),
Brewster (1813, 1814, 1818), and others grew immensely in variety,
and in the importance of their bearing on the undulatory theory.
It is on the basis of these phenomena that Fresnel in 1819 insisted
on the transversality of light-waves, offering proof which was sub-
sequently made rigorous by Verdet (1850). Though a tentative
explanation was here again given by Young (1814), the first ade-
quate theory of the behavior of thin plates of asolotropic media
with polarized light came from Fresnel (1821).

Airy (1833) elucidated a special case of the gorgeously compli-
cated interferences obtained with convergent pencils; Neumann
in 1834 gave the general theory. The forbidding equations resulting
were geometrically interpreted by Bertin (1861, 1884), and Lommel
(1883) and Neumann (1841) added a theory for stressed media,
afterwards improved by Pockels (1889).

The peculiarly undulatory character of natural light owes its
explanation largely to Stokes (1852), and his views were verified
by many physicists, notably by Fizeau (1862) showing interferences
for path differences of 50,000 wave-lengths, and by Michelson for
much larger path differences.

The occurrence of double refraction in all non-regular crystals
was recognized by Haiiy (1788) and studied by Brewster (1818).
In 1821, largely by a feat of intuition, Fresnel introduced his gen-
eralized elementary wave-surface, and the correctness of his explan-
ation has since been substantiated by a host of observers. Stokes
(1862, et seq.) was unremittingly active in pointing out the theoret-
ical bearing of the results obtained. Hamilton (1832) supplied
a remarkable criterion of the truth of Fresnel's theory deductively,
in the prediction of both types of conic refraction. The phenomena
were detected experimentally by Lloyd (1833).

The domain of natural rotary polarization, discovered by Arago
(1811) and enlarged by Biot (1815), has recently been placed in

52 PHYSICS

close relation to non-symmetrical chemical structure by LeBel (1874)
and van 't Hoff (1875), and a tentative molecular theory was ad-
vanced by Sohncke (1876).

Boussinesq (1868) adapted Cauchy's theory (1842) to these phe-
nomena. Independent elastic theories were propounded by Mac-
Cullagh (1837), Briot, Sarrau (1868); but there is naturally no diffi-
culty in accounting for rotary polarization by the electromagnetic
theory of light, as was shown by Drude (1892).

Among investigational apparatus of great importance the Soleil
(1846, 1847) saccharimeter may be mentioned.

Theories

In conclusion, a brief summary may be given of the chief mechan-
isms proposed to account for the undulations of light. Fresnel sug-
gested the first adequate optical theory in 1821, which, though
singularly correct in its bearing on reflection and refraction in the
widest sense, was merely tentative in construction. Cauchy (1829)
proposed a specifically elastic theory for the motion of relatively
long waves of light in continuous media, based on a reasonable
hypothesis of molecular force, and deduced therefrom Fresnel's
reflection and refraction equations. Green (1838), ignoring molecular
forces and proceeding in accordance with his own method in elastics,
published a different theory, which did not, however, lead to Fresnel's
equations. Kelvin (1888) found the conditions implied in Cauchy's
theory compatible with stability if the ether were considered as
bound by a rigid medium. The ether implied throughout is to have
the same elasticity everywhere, but to vary in density from medium
to medium, and vibration to be normal to the plane of polariza-
tion.

Neumann (1835), whose work has been reconstructed by Kirchhoff
(1876), and MacCullagh (1837), with the counter-hypothesis of an
ether of fixed density but varying in elasticity from medium to
medium, also deduced Fresnel's equations, obtaining at the same
time better surface conditions in the case of seolotropic media. The
vibrations are in the plane of polarization.

All the elastic theories essentially predict a longitudinal light-wave.
It was not until Kelvin in 1889-90 proposed his remarkable gyro-
static theory of light, in which force and displacement become torque
and twist, that these objections to the elastic theory were wholly
removed. MacCullagh, without recognizing their bearing, seems
actually to have anticipated Kelvin's equation.

With the purpose of accounting for dispersion, Cauchy in 1835 gave
greater breadth to his theory by postulating a sphere of action of
ether particles commensurate with wave-length, and in this direction

PROGRESS IN NINETEENTH CENTURY 53

he was followed by F. Neumann (1841), Briot (1864), Rayleigh
(1871), and others, treating an ether variously loaded with material
particles. Among theories beginning with the phenomena observed,
that of Boussinesq (1867, et seq.) has received the most extensive
development.

The difficult surface conditions met with when light passes from
one medium to another, including such subjects as ellipticity, total
reflection, etc., have been critically discussed, among others, by Neu-
mann (1835) and Rayleigh (1888); but the discrimination between
the Fresnel and the Neumann vector was not accomplished without
misgiving before the advent of the work of Hertz.

It appears, therefore, that the elastic theories of light, if Kelvin's
gyrostatic adynamic ether be admitted, have not been wholly routed.
Nevertheless, the great electromagnetic theory of light propounded
by Maxwell (1864, Treatise, 1873) has been singularly apt not only
in explaining all the phenomena reached by the older theories and
in predicting entirely novel results, but in harmoniously uniting, as
parts of a unique doctrine, both the electric or photographic light
vector of Fresnel and Cauchy and the magnetic vector of Neumann
and MacCullagh. Its predictions have, moreover, been astonishingly
verified by the work of Hertz (1890), and it is to-day acquiring added
power in the convection theories of Lorentz (1895) and others.

Electrostatics

Coulomb's (1785) law antedates the century; indeed, it was known
to Cavendish (1771, 1781). Problems of electric distribution were
not seriously approached, however, until Poisson (1811) solved the
case for spheres in contact. Afterwards Clausius (1852), Helmholtz
(1868), and Kirchhoff (1877) examined the conditions for discs, the
last giving the first rigorous theory of the experimentally important
plate-condenser. In 1845-48 the investigation of electric distribu-
tion received new incentive as an application of Kelvin's beautiful
method of images. Maxwell (Treatise, 1873) systematized the treat-
ment of capacity and induction coefficients.

Riess (1837), in a classic series of experiments on the heat produced
by electrostatic discharge, virtually deduced the potential energy
of a conductor and in a measure anticipated Joule's law (1841). In
1860 appeared Kelvin's great paper on the electromotive force needed
to produce a spark. As early as 1855, however, he had shown that
the spark discharge is liable to be of the character of a damped vibra-
tion and the theory of electric oscillation was subsequently extended
by Kirchhoff (1867). The first adequate experimental verification
was due to Feddersen (1858, 1861).

The specific inductive capacity of a medium with its fundamental

54 PHYSICS

bearing on the character of electric force was discovered by Far-
aday in 1837. Of the theories propounded to account for this pro-
perty the most far-reaching is Maxwell's (1865), which culminates in
the unique result showing that the refraction index of a medium
is the square root of its specific inductive capacity. With regard
to Maxwell's theory of the Faraday stress in the ether as compared
with the subsequent development of electrostriction in other media
by many authors, notably by Boltzmann (1880) and by Kirchhoff
(1885), it is observable that the tendency of the former to assign
concrete physical properties to the tube of force is growing, partic-
ularly in connection with radioactivity. Duhem (1892, 1895) in-
sists, however, on the greater trustworthiness of the thermodynamic
potential.

The seemingly trivial subject of pyroelectricity interpreted by
^Epinus (1756) and studied by Brewster (1825), has none the less
elicited much discussion and curiosity, a vast number of data by
Hankel (1839-93) and others, and a succinct explanation by Kelvin
(1860, 1878). Similarly piezoelectricity, discovered by the brothers
Curie (1880), has been made the subject of a searching investigation
by Voigt (1890). Finally Kerr (1875, et seq.) observed the occurrence
of double refraction in an electrically polarized medium. Recent
researches, among which those of Lemoine (1896) are most accurate,
have determined the phase difference corresponding to the Kerr
effect under normal conditions, while Voigt (1899) has adduced
an adequate theory.

Certain electrostatic inventions have had a marked bearing on the
development of electricity. We may mention in particular Kelvin's
quadrant electrometer (1867) and Lippmann's capillary electrometer
(1873). Moreover, among apparatus originating in Nicholson's dupli-
cator (1788) and Volta's electrophorus, the Topler-Holtz machine
(1865-67), with the recent improvement due to Wimshurst, has
replaced all others. Atmospheric electricity, after the memorable
experiment of Franklin (1751), made little progress until Kelvin
(1860) organized a systematic attack. More recently a revival of
interest began with Exner (1886), but more particularly with Linss
(1887), who insisted on the fundamental importance of a detailed
knowledge of atmospheric conduction. It is in this direction that the
recent vigorous treatment of the atmosphere as an ionized medium
has progressed, owing chiefly to the indefatigable devotion of Elster
and Geitel (1899, et seq.) and of C. T. R. Wilson (1897, et seq.). Quali-
tatively the main phenomena of atmospheric electricity are now
plausibly accounted for; quantitatively there is as yet very little
specific information.

PROGRESS IN NINETEENTH CENTURY 55

Volta Contacts

Volta's epoch-making experiment of 1797 may well be added to
the century which made such prolific use of it; indeed, the Voltaic
pile (1800-02) and Volta's law of series (1802) come just within it.
Among the innumerable relevant experiments Kelvin's dropping
electrodes (1859) and his funnel experiment (1867) are among
the more interesting, while the Spannungsreihe of R. Kohlrausch
(1851, 1853) is the first adequate investigation. Nevertheless, the
phenomenon has remained without a universally acceptable explana-
tion until the present day, when it is reluctantly yielding to electronic
theory, although ingenious suggestions like Helmholtz's Doppel-
schicht (1879), the interpretations of physical chemistry and the
discovery of the concentration cell (Helmholtz; Nernst, 1888, 1889;
Planck, 1890) have thrown light upon it.

Among the earliest theories of the galvanic cell is Kelvin's (1851,
1860), which, like Helmholtz's, is incomplete. The most satisfactory
theory is Nernst 's (1889). Gibbs (1878) and Helmholtz (1882) have
made searching critical contributions, chiefly in relation to the
thermal phenomena. .

Volta's invention was made practically efficient in certain famous
galvanic cells, among which Daniell's (1836), Grove's (1839), Clarke's
(1878), deserve mention, and the purposes of measurement have
been subserved by the potentiometers of Poggendorff (1841), Bosscha
(1855), Clarke (1873).

Seebeck Contacts

Thermoelectricity, destined to advance many departments of
physics, was discovered by Seebeck in 1821. The Peltier effect fol-
lowed in 1834, subsequently to be interpreted by Icilius (1853). A
thermodynamic theory of the phenomena came from Clausius (1853)
and with greater elaboration, together with the discovery of the
Thomson effect, from Kelvin (1854, 1856), to whom the thermo-
electric diagram is due. This was subsequently developed by Tait
(1872, et seq.) and his pupils. Avenarius (1863), however, first
observed the thermoelectric parabola.

The modern platinum-iridium or platinum-rhodium thermo-
electric pyrometer dates from about 1885 and has recently been
perfected at the Reichsanstalt. Melloni (1835, et seq.) made the most
efficient use of the thermopyle in detecting minute temperature
differences.

Electrolysis

Though recognized by Nichols and Carlisle (1800) early in the
century, the laws of electrolysis awaited the discovery of Faraday

56 PHYSICS

(1834). Again, it was not till 1853 that further marked advances
were made by Hittorf's (1853-59) strikingly original researches
on the motions of the ions. Later Clausius (1857) suggested an ade-
quate theory of electrolysis, which was subsequently to be specialized
in the dissociation hypothesis of Arrhenius (1881, 1884). To the
elaborate investigations of F. Kohlrausch (1879, et seq.}, however,
science owes the fundamental law of the independent velocities of
migration of the ions.

Polarization discovered by Ritter in 1803 became in the hands
of Plante (1859-1879) an invaluable means for the storage of energy,
an application which was further improved by Faure (1880).

Steady Flow

The fundamental law of the steady flow of electricity, in spite
of its simplicity, proved to be peculiarly elusive. True, Cavendish
(1771-81) had definite notions of electrostatic resistance as depend-
ent on length section and potential, but his intuitions were lost to
the world. Davy (1820), from his experiments on the resistances of
conductors, seems to have arrived at the law of sections, though he
obscured it in a misleading statement. Barlow (1825) and Becquerel
(1825-26), the latter operating with the ingenious differential gal-
vanometer of his own invention, were not more definite. Surface
effects were frequently suspected. Ohm himself, in his first paper
(1825), confused resistance with the polarization of his battery, and
it was not till the next year (1826) that he discovered the true
law, eventually promulgated in his epoch-making Die galvanische
Kette (1827).

It is well known that Ohm's mathematical deductions were un-
fortunate, and would have left a gap between electrostatics and
voltaic electricity. But after Ohm's law had been further experi-
mentally established by Fechner (1830), the correct theory was
given by Kirchhoff (1849) in a way to bridge over the gap specified.
Kirchhoff approached the question gradually, considering first the
distribution of current in a plane conductor (1845-46), from which
he passed to the laws of distribution in branched conductors (1847-
48) — laws which now find such universal application. In his great
paper, moreover, Kirchhoff gives the general equation for the act-
ivity of the circuit and from this Clausius (1852) soon after deduced
the Joule effect theoretically. The law, though virtually implied
in Riess's results (1837), was experimentally discovered by Joule
(1841).

As bearing critically or otherwise on Ohm's law we may mention
the researches of Helmholtz (1852), of Maxwell (1876), the solution
of difficult problems in regard to terminals or of the resistance of

PROGRESS IN NINETEENTH CENTURY 57

special forms of conductors, by Rayleigh (1871, 1879), Hicks (1883),
and others, the discussion of the refraction of lines of flow by Kirch-
hoff (1845), and many researches on the limits of accuracy of the
law.

Finally, in regard to the evolution of the modern galvanometer
from its invention by Schweigger (1820), we may enumerate in suc-
cession Nobili's astatic system (1834), Poggendorffs (1826) and
Gauss's (1833) mirror device, the aperiodic systems, Weber's (1862)
and Kelvin's critical study of the best condition for galvanometry ,
so cleverly applied in the instruments of the latter. Kelvin's siphon
recorder (1867), reproduced in the Depretz-D'Arsonval system (1882),
has adapted the galvanometer to modern conditions in cities. For
absolute measurement Pouillet's tangent galvanometer (1837),
treated for absolute measurement by Weber (1840), and Weber's
dynamometer (1846) have lost little of their original importance.

Magnetism

Magnetism, definitely founded by Gilbert (1600) and put on a
quantitative basis by Coulomb (1785), was first made the subject
of recondite theoretical treatment by Poisson (1824-27). The inter-
pretation thus given to the mechanism of two conditionally separable
magnetic fluids facilitated discussion and was very generally used
in argument, as for instance by Gauss (1833) and others, although
Ampere had suggested the permanent molecular current as early
as 1820. Weber (1852) introduced the revolvable molecular magnet,
a theory which Ewing (1890) afterwards generalized in a way to
include magnetic hysteresis. The phenomenon itself was independ-
ently discovered by Warburg (1881) and by Ewing (1882), and has
since become of special practical importance.

Faraday in 1852 introduced his invaluable conception of lines of
magnetic force, a geometric embodiment of Gauss's (1813, 1839)
theorem of force flux, and Maxwell (1855, 1862, et seq.} thereafter
gave the rigorous scientific meaning to this conception which per-
vades the whole of contemporaneous electromagnetics.

The phenomenon of magnetic induction, treated hypothetically
by Poisson (1824-27) and even by Barlow (1820), has since been
attacked by many great thinkers, like F. Neumann (1848), Kirchhoff
(1854); but the predominating and most highly elaborated theory
is due to Kelvin (1849, et seq.}. This theory is broad enough to be
applicable to seolotropic media and to it the greater part of the not-
ation in current use throughout the world is due. A new method of
attack of great promise has, however, been introduced by Duhem
(1888, 1895, et seq.) in his application of the thermodynamic potential
to magnetic phenomena.

58 PHYSICS

Magneticians have succeeded in expressing the magnetic distri-
bution induced in certain simple geometrical figures like the
sphere, the spherical shell, the ellipsoid, the infinite cylinder, the
ring. Green in 1828 gave an original but untrustworthy treatment
for the finite cylinder. Lamellar and solenoidal distributions are
defined by Kelvin (1850), to whom the similarity theorems (1856)
are also due. Kirchhoff's results for the ring were practically utilized
in the absolute measurements of Stoletow (1872) and of Rowland
(1878).

Diamagnetism, though known since Brugmans (1778), first chal-
lenged the permanent interest of science in the researches of Becquerel
(1827) and of Faraday (1845). It is naturally included harmoniously
in Kelvin's great theory (1847, et seq.). Independent explanations of
diamagnetism, however, have by no means abandoned the field;
one may instance Weber's (1852) ingenious generalization of Ampere's
molecular currents (1820) and the broad critical deductions of Duhem
(1889) from the thermodynamic potential. For the treatment of
seolotropic magnetic media, Kelvin's (1850, 1851) theory seems to
be peculiarly applicable. Weber's theory would seem to lend itself
well to electronic treatment.

The extremely complicated subject of magnetostriction, originally
observed by Matteuci (1847) and by Joule (1849) in different cases,
and elaborately studied by Wiedemann (1858, et seq.}, has been
repeatedly attacked by theoretical physicists, among whom Helm-
holtz (1881), Kirchhoff (1885), Boltzmann (1879), and Duhem (1891)
may be mentioned. None of the carefully elaborated theories accoun s
in detail for the facts observed.

The relations of magnetism to light have increased in importance
since the fundamental discoveries of Faraday (1845) and of Verdet
(1854), and they have been specially enriched by the magneto-optic
discoveries of Kerr (1876, et seq.), of Kundt (1884, et seq.}, and more
recently by the Zeemann effect (1897, et seq.). Among the theorie
put forth for the latter, the electronic explanation of Lorentz (1898
1899) and that of Voigt (1899) are supplementary or at least not con
tradictory. The treatment of the Kerr effect has been systematized
by Drude (1892, 1893). The instantaneity of the rotational effect
was first shown by Bichat and Blondlot (1882), and this result has
since been found useful in chronography. Sheldon demonstrated
the possibility of reversing the Faraday effect. Finally terrestrial
magnetism was revolutionized and made accessible to absolute meas-
urement by Gauss (1833), and his method served Weber (1840, et
seq.) and his successors as a model for the definition of absolute units
throughout physics. Another equally important contribution from
the same great thinker (1840) is the elaborate treatment of the dis-
tribution of terrestrial magnetism, the computations of which have

PROGRESS IN NINETEENTH CENTURY 59

been twice modernized, in the last instance by Neumeyer l (1880).
Magnetometric methods have advanced but little since the time of
Gauss (1833), and Weber's (1853) earth inductor remains a standard
instrument of research. Observationally, the development of cycles
of variation in the earth's constants is looked forward to with eager-
ness, and will probably bear on an adequate theory of terrestrial
magnetism, yet to be framed. Arrhenius (1903) accentuates the
importance of the solar cathode torrent in its bearing on the earth's
magnetic phenomena.

Electromagnetism

Electromagnetism, considered either in theory or in its applica-
tions, is, perhaps, the most conspicuous creation of the nineteenth
century. Beginning with Oersted's great discovery of 1820, the
quantitative measurements of Biot and Savart (1820) and Laplace's
(1821) law followed in quick succession. Ampere (1820) without
delay propounded his famous theory of magnetism. For many years
the science was conveniently subserved by Ampere's swimmer (1820),
though his functions have since advantageously yielded to Fleming's
hand rule for moving current elements. The induction produced by
ellipsoidal coils or the derivative cases is fully understood. In prac-
tice the rule for the magnetic circuit devised by the Hopkinsons
(1886) is in general use. It may be regarded as a terse summary of
the theories of Euler (1780), Faraday, Maxwell, and particularly
Kelvin (1872), who already made explicit use of it. Nevertheless,
the clear-cut practical interpretation of the present day had to be
gradually worked out by Rowland (1873, 1884), Bosanquet (1883-
85), Kapp (1885), and Pisati (1890).

The construction of elementary motors was taken up by Faraday
(1821), Ampere (1822), Barlow (1822), and others, and they were
treated rather as laboratory curiosities; for it was not until 1857 that
Siemens devised his shuttle-wound armature, and the development
of the motor thereafter went pari passu with the dynamo, to be pre-
sently considered. It culminated in a new principle in 1888, when
Ferraris, and somewhat later Tesla (1888) and Borel (1888), intro-
duced polyphase transmission and the more practical realization of
Arago's rotating magnetic field (1824).

Theoretical electromagnetics, after a period of quiescence, was
again enriched by the discovery of the Hall effect (1879, et seq.), which
at once elicited wide and vigorous discussion, and for which Row-
land (1880), Lorentz (1883), Boltzmann (1886), and others put for-
ward theories of continually increasing finish. Nernst and v. Ettings-
hausen (1886, 1887) afterwards added the thermomagnetic effect.

1 Dr. L. A. Bauer kindly called my attention to the more recent work of A.
Schmidt summarized in Dr. Bauer's own admirable paper.

60 PHYSICS

Electrodynamics

The discovery and interpretation of electrodynamic phenomena
were the burden of the unique researches of Ampere (1820, et seq.,
Memoir, 1826). Not until 1846, however, were Ampere's results
critically tested. This examination came with great originality from
Weber using the bifilar dynamometer of his own invention. Grass-
mann (1845), Maxwell (1873), and others have invented elementary
laws differing from Ampere's; but as Stefan (1869) showed that an
indefinite number of such laws might be constructed to meet the
given integral conditions, the original law is naturally preferred.

Induction

Faraday (1831, 1832) did not put forward the epoch-making dis-
covery of electrokinetic induction in quantitative form, as the great
physicist was insufficiently familiar with Ohm's law. Lentz, how-
ever, soon supplied the requisite interpretation in a series of papers
(1833, 1835) which contain his wrell-known law both for the mutual
inductions of circuits and of magnets and circuits. Lentz clearly
announced that the induced quantity is an electromotive force, in-
dependent of the diameter and metal and varying, caeteris paribus,
with the number of spires. The mutual induction of circuits was
first carefully studied by Weber (1846), later by Filici (1852), using
a zero method, and Faraday's self-induction by Edlund (1849),
while Matteuci (1854) attested the independence of induction of the
interposed non-magnetic medium. Henry (1842) demonstrated the
successive induction of induced currents.

Curiously enough the occurrence of eddy currents in massive con-
ductors moving in the magnetic field was announced from a differ-
ent point of view by Arago (1824-26) long before Faraday's great
discovery. They were but vaguely understood, however, until Fou-
cault (1855) made his investigation. The general problem of the
induction to be anticipated in massive conductor is one of great
interest, and Helmholtz (1870), Kirchhoff (1891), Maxwell (1873),
Hertz (1880), and others have treated it for different geometrical
figures.

The rigorous expression of the law of induction was first ob-
tained by F. Neumann (1845, 1847) on the basis of Lentz's law, both
for circuits and for magnets. W. Weber (1846) deduced the law of
induction from his generalized law of attraction. More acceptably,
however, Helmholtz (1847), and shortly after him Kelvin (1848),
showed the law of induction to be a necessary consequence of the
law of the conservation of energy, of Ohm's and Joule's law. In
1851 Helmholtz treated the induction in branched circuits. Finally

PROGRESS IN NINETEENTH CENTURY 61

Faraday's " electrotonic state " was mathematically interpreted
thirty years later, by Maxwell, and to-day, under the name of elec-
tromagnetic momentum, it is being translated into the notation of
the electronic theory.

Many physicists, following the fundamental equation of Neumann
(1845, 1847), have developed the treatment of mutual and self in-
duction with special reference to experimental measurement.

On the practical side the magneto-inductor may be traced back
to d'al Negro (1832) and to Pixii (1832). The tremendous devel-
opment of induction electric machinery which followed the intro-
duction of Siemens's (1857) armature can only be instanced. In 1867
Siemens, improving upon Wilde (1866), designed electric generators
without permanent magnets. Pacinotti '(18(50) and later Gramme
(1871) invented the ring armature, while von Hefner-Alteneck (1872)
and others improved the drum armature. Thereafter further progress
was rapid.

It took a different direction in connection with the Ferraris (1888)
motor by the development of the induction coil of the laboratory
(Faraday, 1831; Neef, 1839; Ruhmkoff, 1853) into the transformer
(Gaulard and Gibbs, 1882-84) of the arts. Among special apparatus
Hughes (1879) contributed the induction balance, and Tesla (1891)
the high frequency transformer. The Elihu Thompson effect (1887)
has also been variously used.

In 1860 Reiss devised a telephone, in a form, however, not at once
capable of practical development. Bell in 1875 invented a different
instrument which needed only the microphone (1878) of Hughes
and others to introduce it permanently into the arts. Of particu-
lar importance in its bearing on telegraphy, long associated with
the names of Gauss and Weber (1833) or practically with Morse
and Vail (1837), is the theory of conduction with distributed capac-
ity and inductance established by Kelvin (1856) and extended by
Kirchhoff (1857). The working success of the Atlantic cable demon-
strated the acumen of the guiding physicist.

Electric Oscillation

The subject of electric oscillation announced in a remarkable paper
of Henry in 1842 and threshed out in its main features by Kelvin;
in 1856, followed by Kirchhoff's treatment of the transmission of
oscillations along a wire (1857), has become of discriminating im-
portance between Maxwell's theory of the electric field and the
other equally profound theories of an earlier date. These crucial
experiments contributed by Hertz (1887, et seq.) showed that elec-
tromagnetic waves move with the velocity of light, and like it
are capable of being reflected, refracted, brought to interference, and

62 PHYSICS

polarized. A year later Hertz (1888) worked out the distribution of
the vectors in the space surrounding the oscillatory source. Lecher
(1890) using an ingenious device of parallel wires, Blondlot (1891)
with a special oscillator, and with greater accuracy Trowbridge
and Duane (1895) and Saunders (1896), further identified the veloc-
ity of the electric wave with that of the wave of light. Simultan-
eously the reasons for the discrepancies in the strikingly original
method for the velocity of electricity due to Wheatstone (1834),
and the American and other longitude observations (Walker, 1894;
Mitchell, 1850; Gould, 1851), became apparent, though the nature
of the difficulties had already appeared in the work of Fizeau and
Gounelle (1850).

Some doubt was thrown on the details of Hertz's results by Sarasin
and de la Rive's phenomenon of multiple resonance (1890), but this
was soon explained away as the necessary result of the occurrence
of damped oscillations by Poincare" (1891), by Bjerknes (1891), and
others. J. J. Thomson (1891) contributed interesting results for
electrodeless discharges, and on the value of the dielectric constant
for slow oscillations (1889); Boltzmann (1893) examined the inter-
ferences due to thin plates; but it is hardly practicable to summarize
the voluminous historjr of the subject. On the practical side, we are
to-day witnessing the astoundingly rapid growth of Hertzian wave
wireless telegraphy, due to the successive inventions of Branly (1890,
1891), Popoff, Braun (1899), and the engineering prowess of Marconi.
In 1901 these efforts were crowned by the incredible feat of Mar-
coni's first message from Poldhu to Cape Breton, placing the Old
World within electric earshot of the New.

Maxwell's equations of the electromagnetic field were put for-
ward as early as 1864, but the whole subject is presented in its broad-
est relations in his famous treatise of 1873. The fundamental feature
of Maxwell's work is the recognition of the displacement current,
a conception by which Maxwell was able to annex the phenomena
of light to electricity. The methods by which Maxwell arrived at
his great discoveries are not generally admitted as logically binding.
Most physicists prefer to regard them as an invaluable possession
as yet unliquidated in logical coin; but of the truth of his equations
there is no doubt. Maxwell's theory has been frequently expounded
by other great thinkers, by Rayleigh (1881), by Poincare (1890),
by Boltzmann (1890), by Heaviside (1889), by Hertz (1890), by
Lorentz, and others. Hertz and Heaviside, in particular, have con-
densed the equations into the symmetrical form now commonly
used. Poynting (1884) contributed his remarkable theorem on the
energy path.

Prior to 1870 the famous law of Weber (1846) had gained wide
recognition, containing as it did Coulomb's law, Ampere's law,

PROGRESS IN NINETEENTH CENTURY 63

Laplace's law, Neumann's law of induction, the conditions of electric
oscillation and of electric convection. Every phenomenon in electric-
ity was deducible from it compatibly with the doctrine of the con-
servation of energy. Clausius (1878), moreover, by a logical effort of
extraordinary vigor, established a similar law. Moreover, the early
confirmation of Maxwell's theory in terms of the dielectric constant
and refractive index of the medium was complex and partial. Row-
land's (1876, 1889) famous experiment of electric convection, which
has recently been repeatedly verified by Fender and Cremieu and
others, though deduced from Maxwell's theory, is not incompatible
with Weber's view. Again the ratio between the electrostatic and
the electromagnetic system of units, repeatedly determined from
the early measurement of Maxwell (1868) to the recent elaborate
determinations of Abraham (1892) and Margaret Maltby (1897),
with an ever closer approach to the velocity of light, was at its incep-
tion one of the great original feats of measurement of Weber himself
associated with Kohlrausch (1856). The older theories, however,
are based on the so-called action at a distance or on the instantane-
ous transmission of electromagnetic force. Maxwell's equations, while
equally universal with the preceding, predicate not merely a finite
time of transmission, but transmission at the rate of the velocity
of light. The triumph of this prediction in the work of Hertz has
left no further room for reasonable discrimination.

As a consequence of the resulting enthusiasm, perhaps, there has
been but little reference in recent years to the great investigation
of Helmholtz (1870, 1874), which includes Maxwell's equations as
a special case; nor to his later deduction (1886, 1893) of Hertz's
equations from the principle of least action. Nevertheless, Helm-
holtz's electromagnetic potential is deduced rigorously from funda-
mental principles, and contains, as Duhem (1901) showed, the electro-
magnetic theory of light.

Maxwell's own vortex theory of physical lines of force (1861,
1862) probably suggested his equations. In recent years, however,
the efforts to deduce them directly from apparently simpler proper-
ties of a continuous medium, as for instance from its ideal elastics,
or again from a specialized ether, have not been infrequent. Kelvin
(1890), with his quasi-rigid ether, Boltzmann (1893), Sommerfeld
(1892), and others have worked efficiently in this direction. On the
other hand, J. J. Thomson (1891, et seq.), with remarkable intuition,
affirms the concrete physical existence of Faraday tubes of force,
and from this hypothesis reaches many of his brilliant predictions
on the nature of matter.

As a final commentary on all these divers interpretations, the
important dictum of Poincare* should not be forgotten: If, says
Poincare", compatibly with the principle of the conservation of energy

64 PHYSICS

and of least action, any single ether mechanism is possible, there
must at the same time be an infinity of others.

The Electronic Theory

The splendid triumph of the electronic theory is of quite recent
date, although Davy discovered the electric arc in 1821, and although
many experiments were made on the conduction of gases by Faraday
(1838), Reiss, Gassiot (1858, et seq.}, and others. The marvelous
progress which the subject has made begins with the observations
of the properties of the cathode ray by Pliicker and Hittorf (1868),
brilliantly substantiated and extended later by Crookes (1879).
Hertz (1892) and more specifically Lenard (1894) observed the pass-
age of the cathode rays into the atmosphere. Perrin (1895) showed
them to be negatively charged. Rontgen (1895) shattered them
against a solid obstacle, generating the X-ray. Goldstein (1886)
discovered the anodal rays.

Schuster's (1890) original determination of the charge carried by
the ion per gram was soon followed by others utilizing both the elec-
trostatic and the magnetic deviation of the cathode torrent, and by
Lorentz (1895) using the Zeeman effect. J. J. Thomson (1898) suc-
ceeded in measuring the charge per corpuscle and its mass, and the
velocities following Thomson (1897) and Wiechert (1899), are known
under most varied conditions.

But all this rapid advance, remarkable in itself, became startlingly
so when viewed correlatively with the new phenomena of radio-
activity, discovered by Becquerel (1896), wonderfully developed by
M. and Madame Curie (1898, et seq.}, by J. J. Thomson and his pupils,
particularly by Rutherford (1899, et seq.}. From the Curies came
radium (1898) and the thermal effect of radioactivity (1903), from
Thomson much of the philosophical prevision which revealed the
lines of simplicity and order in a bewildering chaos of facts, and
from Rutherford the brilliant demonstration of atomic disintegra-
tion (1903) which has become the immediate trust of the twentieth
century. Even if the ultimate significance of such profound re-
searches as Larmor's (1891) Ether and Matter cannot yet be dis-
cerned, the evidences of the transmutation of matter are assured,
and it is with these that the century will immediately have to reckon.

The physical manifestations accompanying the breakdown of
atomic structure, astoundingly varied as these prove to be, assume
fundamental importance when it appears that the ultimate issue
involved is nothing less than a complete reconstruction of dynamics
on an electromagnetic basis. It is now confidently affirmed that the
mass of the electron is wholly of the nature of electromagnetic
inertia, and hence, as Abraham (1902), utilizing Kaufmann's data

PROGRESS IN NINETEENTH CENTURY 65

(1902) on the increase of electromagnetic mass with the velocity of
the corpuscle, has shown, the Lagrangian equations of motion may be
recast in an electromagnetic form. This profound question has been
approached independently by two lines of argument, one beginning
with Heaviside (1889), who seems to have been the first to compute
the magnetic energy of the electron, J. J. Thomson (1891, 1893),
Morton (1896), Searle (1896), Sutherland (1899); the other with
H. A. Lorentz (1895), Wiechert (1898, 1899), Des Coudres (1900),
Drude (1900), Poincare (1900), Kaufmann (1901), Abraham (1902).
Not only does this new electronic tendency in physics give an accept-
able account of heat, light, the X-ray, etc., but of the Lagrangian
function and of Newton's laws.

Thus it appears, even in the present necessarily superficial sum-
mary of the progress of physics within one hundred years, that, curi-
ously enough, just as the nineteenth century began with dynamics
and closed with electricity, so the twentieth century begins anew
with dynamics, to reach a goal the magnitude of which the human
mind can only await with awe. If no Lagrange stands toweringly
at the threshold of the era now fully begun, superior \vorkmen abound
in continually increasing numbers, endowed with insight, adroit-
ness, audacity, and resources, in a way far transcending the early
visions of the wonderful century which has just closed.

SECTION A — PHYSICS OF MATTER

SECTION -A — PHYSICS OF MATTER

(Hall 11, September 23, 10 a. TO.)

CHAIRMAN: PROFESSOR SAMUEL W. STRATTON, Director of the National Bureau

of Standards, Washington.
SPEAKERS: PROFESSOR ARTHUR L. KIMBALL, Amherst College.

PROFESSOR FRANCIS E. NIPHER, Washington University.
SECRETARY: PROFESSOR R. A. MILLIKAN, University of Chicago.

THE RELATIONS OF THE SCIENCE OF PHYSICS OF
MATTER TO OTHER BRANCHES OF LEARNING

BY ARTHUR LALANNE KIMBALL

[Arthur Lalanne Kimball, Professor of Physics, Amherst College, b. October 16,
1856, Succasunna Plains, N. J. A.B. Princeton, 1881; Ph.D. Johns Hopkins
University, 1884; post-graduate, Johns Hopkins University; Associate Pro-
fessor of Physics, Johns Hopkins University, 1888-91; Fellow of American
Association for the Advancement of Science, and American Physical Society.
Author of Physical Properties of Gases.]

IT is evident at the outset that it is quite out of the question, in
the time at our disposal, to discuss adequately the relation of the
physics of matter to the other sciences, even if the speaker were
endowed with the requisite omniscience.

For matter is the very stuff in which the phenomena of all the
natural sciences are manifested, the chemist finds himself con-
fronted at every turn with physical relations which must be taken
into account, the astronomer finds his greatest triumph in exhibit-
ing the universe that he explores with the telescope as an harmonious
illustration of physical principles, the geologist also hardly faces a
single question that does not demand the aid of physics or chemistry
in its solution, and even in the biological sciences the laws of matter
still condition the phenomena of life.

Perhaps a brief consideration of the interrelations of these sciences
may aid us in a clearer perception of their dependence on the physics
of matter.

There are three sciences that may be said to be especially funda-
mental, in that they deal with the elements of the universe of phe-
nomena. These are physics, which, if we define it somewhat narrowly,
deals with all the phenomena that can be exhibited by and through
the means of any one kind of matter, as well as all interactions between
different kinds of matter in which each preserves its separate iden-
tity; chemistry, which has for its province those special phenomena
in which one kind of matter is broken up into two or more kinds,

70 PHYSICS OF MATTER

or in which the interactions between different kinds of matter result
in the formation of a substance different from either of the constitu-
ents; and that phase of biology which is concerned with the study
of the living cell and of the simplest conditions under which matter
exhibits the phenomena of life.

It might have been said that physics deals with those phenomena
exhibited by and through matter when molecular groupings of atoms
are not disturbed, while chemistry deals with the phenomena of the
formation and breaking-up of the molecules. But such a statement
is based upon a theory of the structure of matter which in itself calls
for explanation, and therefore the previous statement is preferred as
being more general and avoiding the theoretical assumptions that
are involved in those just given.

If it is asked what constitutes a particular kind of matter, why,
for instance, water-vapor is said to be the same substance as water
in the liquid form, it may be said that it is because one can be wholly
transformed into the other, each is homogeneous, and remains un-
changed in its properties during the transforming, and the trans-
formation is unique.

Professor Ostwald has recently given a most interesting statement
of the criterion by which a substance or chemical individual may
be recognized without the need of any atomic hypothesis. We may
summarize his presentation thus: Where two substances are com-
bined as in solution, there will be one and only one proportion be-
tween the quantities of the substances for which, on change of state,
such as evaporation or crystallization, the vapor or crystals will
have the same composition as the remaining substance, while with
a greater or less proportion of either ingredient, there will be a change
of concentration with change of state. When such a combination
retains this property under widely different conditions of tempera-
ture and pressure, it is known as a chemical individual or definite
compound. If under no circumstances it can be broken up into two
phases which differ in constitution, it is called an element.

Ostwald remarks, "The possibility of being changed from one
phase into another without variation of the properties of the residue
and of the new phase is indeed the most characteristic property of
a substance or chemical individual, and all our methods of testing
the purity of a substance, or of preparing a pure one, can be reduced
to this one property."

But returning to our classification, it is seen that physics, chemis-
try, and biology are the three fundamental natural sciences, each
having as its primary object not the mere arrangement and classi-
fication of phenomena, but the formation of such a concept of matter
in those relations with which it deals, that the varied facts of obser-
vation appear as natural and inevitable consequences.

RELATIONS TO OTHER SCIENCES 71

The other sciences are in a certain sense secondary to the three
that have been mentioned. Each is concerned with the investigation
of some system that is built up out of matter, and involves the same
fundamental relations which are the objects of study for the primary
sciences, but the secondary science finds its interest not in the ma-
terials of which the structure is made, but in the study of the result-
ing structure itself.

Thus astronomy seeks to describe and make out the past history
and future development of the universe of sun and star and planet.
The sciences of the earth are concerned with the history of the devel-
opment of our planet, with the present phenomena of its interior,
of its crust, of its surface, and of its atmosphere, while the secondary
biological sciences have as their aim to trace the relations of the
various forms of life and to follow out the developments of each.

But while each secondary science thus has an aim of its own quite
distinct from that of the primary sciences, nevertheless it must be
controlled and to some extent guided by the sciences of matter.
Thus in almost every science chemical phenomena play a part
which must be reckoned with, while physics, dealing as it does with
the most universal phenomena of matter, underlies and conditions
all the sciences without exception. Therefore it is to be expected
that with the development of physics both in discovery and theory
there should be a greater or less reaction on the other sciences, for
in so far as they depend for their development on the laws of matter
they are dependent on the labors of the physicist.

We might therefore expect to find in every science, if we only knew
it well enough, a response to every considerable advance in physics.
For the advances in a science result not from discovery alone, but
from new points of view taken by those who are thinking on its
problems; and the ideas of physics, bearing as they may be said to
do on the raw material of the other sciences, must in a preeminent
degree influence the thinking of workers in all fields.

It deserves to be emphasized that every science is an intellectual
structure. Only as this is conceded will science be yielded the lofty
and dignified position which is its due. Experiments may be multi-
plied, facts and data may be accumulated in bewildering numbers,
but there is no science without the clear intellectual vision that sees
the parts in their dependencies and relations one to another and
catches glimpses of the larger unities that run through all.

They are mistaken who think the true scientist less an idealist
than is the artist or student of literature, or who think the path of
experiment mere drudgery in the accumulation of insignificant facts.
The investigator lives in a world of ideas, and in every step of a dif-
ficult inquiry he has the buoyant consciousness that he is getting
a deeper, truer insight into his science.

72 PHYSICS OF MATTER

This intellectual character of scientific research is well illustrated
in the enthusiasm which marked the news of Hertz's discovery of
electromagnetic waves. The facts observed might easily have been
thought to be in themselves insignificant : a slight spark observed
between the ends of a bent wire near a discharging electrified system.
There was no thought of a practical application, and yet a wave of
almost unprecedented excitement spread among physicists the world
over. Nor was it alone admiration for the skill, the insight and grasp
of the great experimenter that won the victory, though this had its
effect. It was mainly an exultant enthusiasm over the triumph of
an idea, the unification of science in the confirmation of Maxwell's
great theory.

It is clear, then, that physics may react on the other sciences in a
variety of ways, in its methods and appliances, in its discoveries, and
in its ideas and generalizations; and it is evident, therefore, that we
must limit ourselves to a brief consideration of certain phases of the
subject. I have, therefore, chosen to present very briefly some con-
siderations relative to theories of matter, for here physics and chem-
istry come into the closest contact; also to touch upon some other
relations of chemistry and geology to physics, that are of particular
interest at this present time.

The fundamental problem in the physics of matter is the nature
of matter itself. Of course we recognize at the outset the limitations
that bound our attempts at a solution.- We may hope to reach event-
ually some conclusion as to the structure of matter, whether homo-
geneous or molecular or grained, also as to the relative motions of the
parts of the molecule and the law of variation of force between them
with the distance. But if we seek to go farther and explain the
forces acting in and between molecules in terms of what appear to
be more simple and general laws, it seems inevitable that a medium
must be assumed, the properties of which will depend on what is
assumed as a primary postulate. If we accept, as is usually done,
the postulate that forces in their last analysis can only be explained
when referred to pressures exerted between contiguous portions of
some underlying medium, it seems probable that a theory must be
adopted something like the vortex atom theory of Lord Kelvin,
with its continuous, incompressible, perfectly fluid 'medium in which
vertically moving portions constitute the atoms, or Osborne Rey-
nolds's theory of space as filled with fine hard spherical grains, in
which, regions with nonconformity in arrangement, are the atoms
of ordinary matter. Though it must be said that the assumed hard-
ness of the ultimate spherules in the latter theory is a property
which in itself needs explanation.

Perhaps, however, in laying down the postulate mentioned above
we are pushing too far inferences from our superficial experience.

RELATIONS TO OTHER SCIENCES 73

The idea that force must be a pressure between contiguous portions
of substance is derived directly from the notion of the impenetra-
bility of matter. This is why the incompressible medium of Lord
Kelvin's theory seems so simple a conception; it is the naked em-
bodiment of the idea of impenetrability associated with inertia.

It is entirely natural that such ideas as impenetrability and inertia,
borne in upon us as they are by our experience of matter in bulk,
should affect our theorizing, but it should never be forgotten that
as fundamental postulates they have no more authority than any
others that might be assumed that will coordinate the same facts
of observation.

But passing from this more speculative region we find a pretty
general agreement on the rough outlines of the structure of matter.
With one notable exception most physicists and chemists agree in
the idea that matter is atomic or molecular in structure, and that
these molecules are in a state of more or less energetic translator^
motion, bounding and rebounding from each other. This seems to
be the mechanical hypothesis which coordinates the largest number
of facts.

A portion of matter is conceived as in a condition of equilibrium
under three pressures: the cohesive pressure due to mutual attrac-
tion between all molecules which are not farther apart than 50 to
300 millionths of a millimeter; the external pressure, which also acts
to cause contraction; and the internal pressure, which balances the
two former, and is due to a repulsive force called the force of impact,
which is usually supposed to be exerted only between contiguous
molecules.

In the solid and liquid states the cohesive pressure is usually very
great compared with the external pressure. In case of gases it nearly
vanishes. The force between molecules is thus conceived as an attrac-
tion which increases rapidly as they approach, until at a certain dis-
tance it is balanced by a repulsive force which, increasing still more
rapidly, is the controlling force at all less distances.

Lord Kelvin has recently followed out a study of equilibrium con-
ditions in a group of atoms which are assumed to have no mutual
influence until within a certain distance, then to attract each other
with a force that increases as they approach still nearer, rising to
a maximum and then diminishing, and finally becoming a repulsion
when the atoms are very near. He remarks, "It is wonderful how
much toward explaining the crystallography and elasticity of solids,
and the thermo-elastic properties of solids, liquids, and gases, we find
without assuming in the Boscovitchian law of force more than one
transition from attraction to repulsion."

The fundamental soundness of the conception of matter as having
a grained structure of some sort seems to be established by the re-

74 PHYSICS OF MATTER

markable degree of agreement in the estimates by various physicists
of the size of these ultimate particles, meaning by that the smallest
distance between their centres as they rebound from each other,
especially when it is considered that these results have been reached
from so many different points of view, and are based on such a variety
of physical data.

As to the structure of the atom itself a most remarkable theory
has been recently developed. J. J. Thomson has marshaled the evi-
dence in favor of the theory proposed by Larmor that matter has an
electrical basis, and the theory has already been considerably devel-
oped by Lorentz and others. There appears to be reason for believing
that the corpuscles of the Kathode rays are simply moving charges
of negative electricity, their whole apparent mass being due to their
relation to the ether, in consequence of which there is a magnetic
field around the moving charge having energy dependent on the
square of its velocity. The corpuscle, therefore, effectively has mass
in consequence of this reaction between it and the ether.

The corpuscles are found always to carry the same charge, what-
ever the nature of the gas in which the Kathode rays are formed, and
whatever the nature of the electrodes — the charge being the same
as that given up by the hydrogen atom in electrolysis, while the mass
of the corpuscle is about one one-thousandth that of the hydrogen
atom.

The energy in the ether associated with the moving corpuscle
depends on the size of the corpuscle as well as upon its charge, and
it is found that to account for its apparent mass it must be of ex-
tremely small size relative to ordinary atomic dimensions.

Professor Thomson suggests that the primordial element of matter
is such a negative electron combined with an equal positive charge,
the latter being of nearly atomic dimension. An atom of hydrogen
may be thought of as made up of nearly a thousand such pairs, the
positive charge being distributed throughout a spherical region
giving rise to a field of force within it in which the force on a nega-
tive corpuscle will be towards the centre and proportional to its
distance from the centre. In this field of force the corpuscles are
conceived as describing closed orbits with great velocities.

The internal energy of such an atom is conceived as enormous.
In case of the atoms contained in a gram of hydrogen Thomson
reckons about 1019 ergs as the energy received from mutual attrac-
tions in the formation of the atoms, an amount of work that would
lift a hundred million kilograms, one thousand meters.

The whole mass of the atom is supposed to be due to the negative
electrons or corpuscles which it contains. As to the positive charge,
although it determines the apparent size of the atom, it appears to
make no contribution to its mass.

RELATIONS TO OTHER SCIENCES 75

When such an atom impacts against another, the corpuscles in
each will be disturbed by the jar in their orbital motion, and there
will be superposed oscillations which will cause radiation of energy.

If a corpuscle escapes from such an atom, the latter will be left
with a positive charge, while if an additional free corpuscle is en-
trapped, the atom will have a negative charge. The conditions of
stability of motion of the corpuscles in the atom would thus deter-
mine whether in case of electrolysis the substance would appear
electro-positive or electro-negative.

J. J. Thomson, Drude, and others have discussed the electric
conduction of metals from the standpoint of this theory. Drude
states that in non-conductors only bound electrons are present, that
is, positive and negative in combination; and that it is these that
determine the dielectric constant of the medium and consequently
its index of refraction and optical dispersion; while Langevin ex-
plains magnetism and diamagnetism.

Thus we have a theory already surprisingly developed which
appears to be applicable to explain many of the properties of matter,
though it is not clear that it can give an explanation of cohesion and
gravitation. A theory of matter, to be accepted as final, must offer
some explanation of the relation between the various elements. Many
thinkers have been led to look for some primordial element from
which the others are derived, influenced on the one hand by the
present evolutionary ideas of biology, and on the other by com-
parison of spectra and by the remarkable tendency towards whole
numbers observed in the atomic weights of the elements which
Strutt has discussed from the standpoint of the theory of probabili-
ties. Professor Thomson has accordingly shown how atoms of matter
containing great numbers of corpuscles may have been evolved from
a simpler primordial form containing fewer corpuscles. But though
he has made clear how the hydrogen atom with its thousand cor-
puscles might be the surviving atom having the least number of
corpuscles, it is not so clear why there might not be atoms having
any number of corpuscles greater than that of hydrogen, within
certain limits; why none should be found between hydrogen and
helium for example. Some kind of natural selection seems to be
needed to explain why some atoms having special numbers of cor-
puscles survive while intermediate ones are eliminated, though prob-
ably the answer is to be sought in the conditions of stability of the
motions of the corpuscles.

It is an interesting question what would be the effect of change of
temperature of the substance on the motions of the corpuscles in this
theory. If the corpuscles in the atom were very numerous, all moving
in the same orbit at equal distances apart, they would produce
almost the effect of a circular current of electricity, — a steady

76 PHYSICS OF MATTER

magnetic field and no radiation; and it seems probable that in the
actual case the radiation of internal energy is extremely small, and
the total internal energy may be supposed to be so enormous com-
pared with the energy of translation of the atom due to temperature
that we may expect no appreciable change in the radiation of internal
energy of the atom, whatever the temperature may be.

That component of the vibration of a corpuscle which is radial
within the atom, and is set up by the impact of one atom against
another, seems to furnish the great mass of radiated energy. This
radiation must also react on the motion of the atom as a whole, taking
away from the translatory energy of the atom.

The question how the Boltzmann law of partition of energy be-
tween the various degrees of freedom will apply to molecules made
up of such atoms as are here conceived is an interesting and im-
portant one. Is it possible that the cloud, as Lord Kelvin calls it,
resting on the kinetic theory of gases may be dissipated by the new
theory?

This theory of the atom seems also to explain the possibility of
the production of spectra of great complexity. It is to be hoped
that Balmer's formula and Rydberg's laws of the grouping of lines
in spectra may be shown to be the natural outcome of the system
of vibration possible in such an atom.

We are startled at first by the very audacity of this theory, seeming
as it does to upset the old point of view, and seek the explanation
of matter and its laws in terms of the properties of ether and elec-
tricity, instead of trying to unravel the secrets of electricity and
ether in terms of matter and motion.

Only a few years ago it was thought that the electromagnetic
theory of light must be rationalized by giving a mechanical explan-
ation of the various phenomena of the ether, or by showing at least
that such an explanation was possible. Witness Maxwell's won-
derfully ingenious mechanical model illustrating the phenomena
of magnetism, induced currents, and the propagation of electro-
magnetic waves.

But is it necessary to regard the mechanical explanation as the
only sound one ? If electricity and ether are fundamental entities
underlying all matter and material phenomena, is it not more logical
to find a basis for the mechanical laws in some more fundamental
laws of ether and electricity which must be accepted as the primary
postulates?

In all this development of the atomic view of matter, chemistry
and physics have gone hand in hand. The atomic theory of Dalton
has been the basis on which both sciences have worked. Avogadro's
law for gases has been reached not only by chemical evidence, but
has been raised to the rank of a mechanical deduction from the kinetic

RELATIONS TO OTHER SCIENCES 77

theory. The significance of the arrangement of atoms in the molecule
in determining chemical reaction was emphasized and developed by
Kekule", but it was not until 1874 that the space diagrams of mole-
cules of van't Hoff and Le Bel marked a full appreciation of the
possibilities of structure in explaining the differences of isomeric
forms.

All of these physical and chemical developments of the atomic
theory have been in accordance with a general method of scientific
procedure which may be called the method of mechanical models.
According to this method, an attempt is made to conceive a certain
mechanism by which the various phenomena sought to be explained
may be imagined to be brought about.

Such a theory of atoms, for example, if perfect, would exhibit all
the properties of atoms as direct consequences of the assumed struc-
ture. This cannot, however, be taken as proof that the assumption
is real, though for the purpose of our thinking such a theory would
have all the value of reality, since all consequences deduced from it
would conform to the facts of observation. And this suggests wherein
the great value of such a theory lies, not alone in the large number
of observations which it correlates and brings under a few general
principles, but in that it suggests the application of experiments
and tests of its sufficiency, thereby enlarging and making more pre-
cise our knowledge.

Perhaps the most remarkable instance of the application of this
method was Maxwell's development of a mechanical model to illus-
trate the reactions in the electromagnetic field. Working from this
model he developed the equations of the field, which later he deduced
in a more general way. And Hertz speaking of them says, "We can-
not study this wonderful theory without at times feeling as if an
independent life and a reason of its own dwelt in these mathematical
formulae; as if they were wiser than we were, wiser even than their
discoverer; as if they gave out more than had been put into them."

On which Boltzmann's comment is, "I should like to add to these
words of Hertz only this, that Maxwell's formulae are simple conse-
quences from his mechanical models; and Hertz's enthusiastic praise
is due in the first place, not to Maxwell's analysis, but to his acute
penetration in the discovery of mechanical analogies." Such an
example well illustrates the importance of the method.

But of recent years, the influence of quite a different method has
been strongly marked in chemical research. A method in which
certain general laws are established and then applied to particular
cases by a process of mathematical reasoning, deducing conclusions
quite independently of the particular details of the operation by
which they are brought about. This method is well illustrated in
Professor J. J. Thomson's work on the application of dynamics to

78 PHYSICS OF MATTER

problems in physics and chemistry, and in the deductions based on
the laws of thermodynamics that have marked the development of
the new physical chemistry.

It is under the influence of this method that Professor Ostwald
has been led to propose a theory of matter which does not recognize
the necessity of any atomic structure whatever. In a recent address,
he says, "It is possible to deduce from the principles of chemical
dynamics all the stoichiometrical laws; the law of constant proportion,
the law of multiple proportion, and the law of combining weights."
And he continues, "You all know that up to this time it has only been
possible to deduce these laws by the help of the atomic hypothesis.
Chemical dynamics has, therefore, made the atomic hypothesis un-
necessary for this purpose and has put the theory of the stoichio-
metrical laws on more secure ground than that furnished by a mere
hypothesis." And then farther on he continues, " What we call matter
is only a complex of energies which we find together in the same place.
We are still perfectly free if we like to suppose either that the energy
fills the space homogeneously, or in a periodic or grained way; the
latter assumption would- be a substitute for the atomic hypothesis."
And then he adds, "Evidently there exists a great number of facts —
and I count the chemical facts among them — which can be com-
pletely described by a homogeneous or non-periodic distribution of
energy in space. Whether there exist facts which cannot be de-
scribed without the periodic assumption, I dare not decide for want
of knowledge; only I am bound to say that I know of none."

It is interesting and remarkable that this challenge to the atomic
theories of matter should come from the side of chemistry, the very
science for which the atomic theory of Dalton was conceived. Espe-
cially is it remarkable, in view of the measure of success that has
attended the explanation of the differences between such forms as
right and left rotating tartaric acids on the basis of molecular struc-
ture. And it is difficult to see how it is possible to give any satisfac-
tory explanation of these differences, simply on the basis of the laws
of energetics applied to a conception of matter as homogeneous.

With reference to the view that " What we call matter is only a com-
plex of energies which we find together in the same place," it may be
said that we recognize different forms of energy only in association
with matter or ether; as heat, light, chemical energy, kinetical energy,
etc. Hence the term, "a complex of energies," can only mean the
total energy in a given region, unless we recognize some vehicle,
as matter or ether, in which the special manifestations of energy may
exist. This seems to be admitted tacitly by Ostwald himself, for a
little farther on he says, "The reason why it is possible to isolate a
substance from a solution is that the available energy of the sub-
stance is at a minimum." He thus distinguishes between the avail-

RELATIONS TO OTHER SCIENCES 79

able and the total energy of a portion of matter. But this discrimi-
nation can have no meaning unless it is granted that a portion of the
energy of a substance is not available. If we ask why it is not avail-
able, the answer may be that when a substance passes from one
state to another at constant temperature the work that it can do is
less than its total intrinsic energy as a consequence of the laws of
thermodynamics. The case must therefore be one to which the second
law of thermodynamics can apply. That is, it must involve flow of
energy by some such process as heat conduction.

It might perhaps be successfully argued that the very existence
of such a process implies grained structure of some sort to which a
statistical law may apply. However this may be, it is certainly diffi-
cult to conceive of energy as existing apart from some vehicle, matter
or ether or both as you will; but to conceive of this sublimated energy
as in part available and in part non-available is surely quite beyond
attainment.

It is with great diffidence that we dissent from the expressed views
of one who has done so much for the advance of physical chemistry,
and our excuse for entering on the discussion must be that as the
latest utterance with regard to matter, and coming from one who has
won the right to have his views given a respectful consideration, it
seemed more fitting to present this brief and imperfect discussion
than to pass them by without comment.

One of the most important reactions of physics upon the other
sciences has resulted from the extension of the thermodynamic laws
to chemical problems which has marked the new physical chemistry,
a science which has sprung into being within the last seventeen years
and has already, under the leadership of van't Hoff, Ostwald, Arrhe-
nius, and Nernst, attained a surprising development, and is making
itself felt in many other lines of scientific activity, notably in electro-
chemistry, geology, and biology. The starting-point in this devel-
opment was the idea conceived by van't Hoff that Avogadro's law
might be so extended as to apply to the case of substances in solu-
tion. Just as a gas expands and fills the containing vessel exerting
a pressure against its walls, so a salt dissolved in a liquid diffuses
uniformly throughout the liquid and exerts a pressure within the
liquid tending to expand it. This osmotic pressure, so called, had
been measured in certain cases by Pfeffer and de Vries, but it re-
mained for van 't Hoff to show that, as in case of a gas, the pressure
was proportional to the absolute temperature and to the number of
molecules of the dissolved substance contained in unit volume.

As has so often happened before, the study of the apparent ex-
ceptions to the rule led to a second great advance, the theory of
electrolytic dissociation proposed by Arrhenius, to account for the
observation that in solutions of electrolytes the osmotic pressure was

80 PHYSICS OF MATTER

greater than that reckoned on the basis of the number of molecules
present, but was to be explained by their dissociation into ions;
thus reaching the same conclusion which Clausius had announced in
1857, but affording a method by which the precise amount of the
dissociation might be measured. Additional evidence in favor of
this theory was afforded by the studies of the electrical conductivity
of dilute solutions of electrolytes made by Kohlrausch.

All this was accompanied by an increasing realization of the
important relations that might be established by an application of
the laws of thermodynamics to chemical problems. Thus van 't Hoff
showed in his paper of 1887 that the depression of the freezing-point
of a liquid due to a substance in solution depended directly on the
osmotic pressure and could be used to measure it; a result which
had already been experimentally reached by Raoul.

In this field, Professor J. Willard Gibbs, in whose recent death the
world of science has lost a most profound thinker, was a pioneer.
His most important contributions to the subject were in two ex-
traordinary papers, On the Equilibrium of Heterogeneous Substances.
The first of these related to chemical phenomena, while the second
was concerned especially with capillarity and electricity.

To quote from a recent writer, "The most essential feature of
Gibbs 's discoveries consisted in the extension of the notion of thermo-
dynamical potential to mixtures consisting of a number of com-
ponents, and the establishment of the properties that the potential
is a linear function of certain quantities which Gibbs has called the
potentials of the components, and that where the same component
is present in different phases, which remain in equilibrium with each
other, its potential is the same in all the phases, besides which the
temperatures and pressures are equal. The importance of these re-
sults was not realized for a considerable time. It was difficult for
the experimentalist to appreciate a memoir in which the treatment
is highly mathematical and theoretical, and in which but little at-
tempt is made to reduce conclusions to the language of the chemist;
moreover it is not unnatural to find the pioneer dwelling at consid-
erable length on comparatively infertile regions of the newly explored
territory, while fields that were to prove the most productive were
dismissed very briefly."

"It was largely due to Professor van der Waals that two new
and important fundamental laws were discovered in Gibbs 's paper,
namely, the phase rule and the law of critical states."

The phase rule has been the guiding principle in some most import-
ant studies of chemical equilibrium. It furnishes a clue by which the
polymorphism of such substances as sulphur and tin may be scien-
tifically investigated and the conditions of equilibrium between the
different polymorphic forms determined. The studies of the case

RELATIONS TO OTHER SCIENCES 81

of ferric chloride by Roozeboom, and of the crystallization out of
sea-water of the contained salts by van't Hoff and Meyerhoffer
indicates the great value of the phase rule in bringing scientific order
out of the complicated relations of the various components and
phases involved.

Speaking of this department of physical chemistry, van 't Hoff re-
marked, " Since the study of chemical equilibrium has been related
to thermodynamics, and so has steadily gained a broader and safer
foundation, it has come into the foreground of the chemical system,
and seems more and more to belong there." And Ostwald says in
answer to the question, " What are the most important achievements
of the chemistry of our day? I do not hesitate to answer: chemical
dynamics, or the theory of the progress of chemical reaction, and
the theory of chemical equilibrium."

These statements, coming from two masters in the field, are most
significant of the importance of the introduction of these ideas into
chemistry.

The conceptions and methods of physical chemistry have also been
most strongly felt in the field of electrochemical theory. To the
question what is the nature of electrolysis, Faraday and Hittorf
and Clausius had each contributed important elements of the final
answer, then came Arrhenius with the theory of electrolytic dissocia-
tion, which has proved so fruitful of consequences, not only in the
domain of chemistry, but also in biology and in physics.

One of the most interesting scientific questions connected with
electrochemistry is the relation between electromotive force and
electrolytic separation, and the development of the theory of the
voltaic cell. The question of the seat of electromotive force in the
cell was for many years the very storm-centre of physical discussion;
but from the standpoint of electrolytic dissociation Nernst has sup-
plemented the work of Helmholtz and Gibbs, and out of all has come
a theory which, while not perfect, seems to be in its main features on
the solid foundation of the conservation of energy and the laws of
thermodynamics.

Another important service for which the world of science is indebted
to physics is the determination of the absolute zero of temperature
in terms of degrees of the ordinary centigrade scale. About a century
ago, Dalton, in his new chemical philosophy, adopts — 3000° C. as the
probable zero of temperature. While Lavoisier and Laplace make
various estimates of the zero ranging from 1500 to 3000 degrees
below the freezing-point of water. But when the doctrine of energy
became firmly established together with the kinetic theory of gases, it
was natural that the condition of a gas in which the particles had no
energy of motion, and hence no pressure, should have been taken as
indicating the absolute zero. But it was Clausius and Lord Kelvin who

82 PHYSICS OF MATTER

based firmly on the laws of thermodynamics the absolute scale of
temperature, as we know it to-day.

The absolute zero of temperature has to the physicist all the fasci-
nation that the North Pole has to Arctic explorers, and is probably
even more difficult to attain. Yet steady progress has been made
in conquering the difficult territory that lies toward this goal. The
experimental efforts to liquefy the more refractory gases showed that
far lower temperatures than had previously been reached must be
employed; and step by step, following the suggestions of thermo-
dynamics, the means of attaining low temperatures have been im-
proved, at first cooling by adiabatic expansion of more compressible
gases, then aided by the sudden expansion of the gas itself which
had been compressed and cooled, and then by a continuous self-
intensive action, in which the cold produced by the expansion of one
portion of the compressed gas was made use of to cool the still unex-
panded gas as it approached the point of expansion.

The mere record of the temperatures reached marks a series of
triumphs of ingenuity and perseverance. Thus Faraday, in 1845,
reached a temperature of —110 by the use of solid carbon dioxide
and ether evaporated at low pressure. Pictet in 1877 reached — 140,
and liquefied oxygen under pressure. Olszewski in 1885 obtained a
temperature of —225 by the evaporation of a mass of solid nitrogen.
In 1898 Dewar obtained liquid hydrogen boiling at — 252, or only
20.5 above the absolute zero, and later by boiling at reduced pres-
sures he was able to obtain —259.5 or 13.5 degrees absolute scale,
at which point hydrogen is frozen solid.

The attainment of these low temperatures has not alone made
possible investigations of the greatest interest to the physicist,
such as studies of the magnetic and electric properties of bodies as
they approach the absolute zero, but has enabled the effect of extreme
cold on chemical actions to be determined, and has led to the inter-
esting conclusion that "The great majority of chemical interactions
are entirely suspended." Though it has been shown by Dewar and
Moissan that in case of solid hydrogen and liquid fluorine, violent
reaction still takes place even at that small remove from the absolute
zero.

A very interesting field has also been opened to biological research,
in the effect of extreme cold on the vitality of seeds and micro-
organisms. It was found, for example, that barley, pea, and mustard
seeds steeped for six hours in liquid hydrogen and thus kept at a tem-
perature of minus 252 degrees, showed no loss of vitality. So, also,
certain micro-organisms, among others the bacilli of typhoid fever,
Asiatic cholera, and diphtheria, were kept by MacFadyen for seven
days at the temperature of liquid air without appreciable loss of
vitality. It has been suggested by Professor Travers that, "It is

RELATIONS TO OTHER SCIENCES 83

quite possible that if a living organism were cooled only to temper-
atures at which physical changes, such as crystallization, take place
with reasonable velocity, the process would be fatal, whereas, if they
were cooled to the temperature of liquid air no such change would
take place within finite time, and the organism would survive."

Also the study of the various combinations of carbon and iron
that may exist in steel, and the conditions of equilibrium that
exist between them has proved a most important investigation in
the field of what van 't Hoff calls solid solutions.

Geology, dealing as it does with the greatest variety of physical
processes, such as changes of state, fusion, crystallization, solution,
conduction of heat, radiation, with complications depending on
variations of pressure and temperature, presents many problems
for the solution of which the resources of modern physics must be
taxed. The fusing-points of the different chief minerals of the earth's
crust, the effect of great pressure on their fusing-points and modes
of crystallization, the crystallization of the various elementary min-
erals out of a fused magma also studied at different pressures, the
effect of pressure not only on fusing-points, but .on the viscosity and
rigidity of minerals at high temperature, the heat conductivities of
the various substances making the bulk of the earth's crust, all these
are questions that must be thoroughly studied to enable the geologist
to determine the probable condition both of temperature and pres-
sure which prevailed during the formation of a given rock mass, and
to throw light on the great problem of geology, the age of the earth.

To this latter question, physics has already given a tentative
answer. Lord Kelvin's discussion, based on the assumption of the
earth as a mass cooling from a uniform high temperature, points
to a period of between twenty and one hundred million years, within
which geologic changes in the crust of the earth must have occurred;
while Helmholtz and Kelvin's deduction of the time during which
solar radiation can have been of such an intensity that life conditions
on the earth were possible gives about twenty million years as the
limit.

But later investigations giving new data as to the properties of
the materials of the earth's crust, as to the laws of variation of radi-
ation with temperature, and as to absorption and radiation by the
solar and earth's atmospheres, will all contribute to modify and make
more precise these methods. Already some progress in this direction
has been made. A few years ago, Clarence King gave a most inter-
esting and ingenious rediscussion of Kelvin's cooling of the earth
method, making use of the determinations made by Barus of the
fusing-points of diabase at different pressures, and gives as the most
probable result of the method the period of twenty-four million

84 PHYSICS OF MATTER

years, a period in close agreement with that found by Helmholtz
and Kelvin from the radiation of the sun.

It should be remarked, however, that in discussing the state of
things in the earth's interior, where the pressures so far transcend
anything that can be approached in the laboratory, such constants
as melting-points should be looked on with great suspicion.

Assuming Laplace's law of distribution of density in the earth, the
pressure at a depth of one two-hundredth of the earth's radius is
8600 atmospheres, while at the centre of the earth it becomes more
than three million atmospheres. Now the largest pressures that have
been used in high temperature experiments are less than three thou-
sand atmospheres. It is evident, then, that any conclusion as to
melting-points from laboratory data must be violent exterpolations,
if deduced for the enormous pressures at depths greater than one
one-hundredth of a radius within the earth, where the pressure will
be over 17,000 atmospheres.

But not only is there necessarily great uncertainty as to the
fusing-points at these great pressures, but it seems probable that such
a process as fusion marked by sudden increase in liquidity can hardly
take place at all. In the phenomenon of fusion, the equilibrium of
a substance may be regarded as conditioned by the external
pressure, the cohesive pressure, and the internal pressure due to
the translatory kinetic energy of the molecules, which may be
called the kinetic pressure. In a state of equilibrium, the external
pressure plus the cohesive pressure must equal the kinetic pressure,
the last tending to produce expansion, while the two former act to
cause contraction. At ordinary atmospheric pressures in the liquid
and solid state, the cohesive pressure is enormously greater than the
external pressure. In water at ordinary temperatures it is estimated
about 6500 atmospheres, while in a solid such as steel it may have
a value of perhaps 18,000 atmospheres. And not only is this cohesive
force great relatively to the external pressure, but it decreases writh
great rapidity as the substance expands. Under these conditions
it is easy to see that a slight rise in temperature with consequent
expansion and weakening of the cohesive pressure while the kinetic
pressure is increased may bring the substance to a point of trans-
ition, a melting-point or boiling-point where great changes occur
within narrow limits of temperature.

But if we conceive the external pressure to be so great that the
cohesive pressure is relatively insignificant, then we should not expect
to find any sharply marked changes of state for small changes of
temperature or pressure.

To make the case definite assume a temperature of 1000 degrees
absolute scale, and a pressure of 1,000,000 atmospheres, and suppose
the cohesive pressure is 10,000 atmospheres. Under these circum-

RELATIONS TO OTHER SCIENCES 85

stances a rise in temperature of ten degrees or a one per cent increase
in temperature may be expected to produce a one per cent increase
in the kinetic pressure at the original volume; but as the external
pressure is constant and the cohesion is insignificant, we may expect
a one per cent increase in the volume in which the molecular motions
take place or an increase in the mean distance between molecules of
one third of one per cent. Such an expansion will be accompanied
by slightly lessened cohesive force, less rigidity, and less viscosity,
probably; but nothing like a sudden change of state is suggested.
The fact that at pressures greater than the critical pressures there
can be observed no sharp transition from the liquid to the gaseous
state with rise of temperature is quite in accord with the above con-
siderations, and it seems probable that in case of solids under great
pressure nothing like melting will be observed, but rather a gradual
loss of rigidity or transition to great viscosity, and that the viscosity
will decrease steadily with rise in temperature.

But a new aspect is now given to the problem of the age of the
earth by the discovery of radioactivity and its attendant phe-
nomena. The earth, instead of being thought of as a cooling body,
is now conceived as having within itself a source of almost un-
limited energy. Locked up in each atom is believed to be a store of
energy so vast that the breaking down of comparatively few of them
in the radioactive process will supply the known outflow of heat
from the earth.

Rutherford has shown that the observed dissemination of radio-
active siibstances in the earth's crust is probably sufficient to ac-
count for the outflow of energy from its surface. Thus the method
of estimating the age of the earth from the consideration of it as a
cooling body, a method which until lately seemed to physicists to be
based on essentially sound premises, and deserving of confidence
because of its greater simplicity as compared with the methods by
which geological and biological estimates are obtained, is now by the
very progress of physics itself abandoned as unreliable.

So also has the study of radioactivity thrown new light on the
question of the maintenance of the sun's heat. It is now seen
that possible atomic transformations accompanied by the liber-
ation of the vast stores of energy locked up within the atoms of
matter may permit an enormous extension of the time during
which the sun may have been radiating with something like its
present intensity.

In conclusion it may be remarked that a new world is opened
to the investigator by the discovery of radioactivity. The atoms
of matter are no longer thought of as necessarily fixed and un-
changeable. Besides the older problems of matter questions now
arise as to evidences of atomic disintegration and change from

86 PHYSICS OF MATTER

more complex to less complex forms, and also the possible develop-
ment of more complex atoms from simpler ones.

Already we begin to see the effect of these recent discoveries
and ideas on other departments of science. The clue at last seems to
have been found to those long-standing enigmas of nature, thunder-
storms, the Aurora Borealis, the zodiacal light, and the tails of
comets. But these achievements belong perhaps rather to the
realm of the physics of the ether and of the electron, than to that
of the physics of matter.

PRESENT PROBLEMS IN THE PHYSICS OF MATTER

BY FRANCIS EUGENE NIPHER

[Francis Eugene Nipher, Professor of Physics, Washington University, St. Louis,
Mo. b. December 10, 1847, Port Byron, N. Y. Phil.B. State University of
Iowa, 1870; A.M. State University of Iowa, 1875; LL.D. Washington Uni-
versity, 1905. Instructor in Physics and Chemistry, State University of Iowa,
1870-74; Professor of Physics, Washington University, 1874. Member of
Academy of Science of St. Louis, American Physical Society; Fellow of Amer-
ican Society for the Advancement of Science. Author of Theory of Magnetic
Measurements; Introduction to Graphical Algebra; Electricity and Magnetism;
and many scientific papers.]

IN dealing with the subject allotted to me by the officers of the
Congress, I must say that I have not presumed to solve the problems
which present themselves at this time, nor do I feel competent even
to state many of them. But it is instructive, in a time like this, to
attempt a general survey of some of the great questions of the day,
with a view of noting their bearing upon the knowledge of the past.
We are continually made to feel that all of our inquiries and results
must be reexamined, and our conclusions broadened and modified
by new phenomena.

Charles Babbage, whose last published work was, if I mistake not,
a review of the London Exposition of 1851, in the Ninth Bridge water
Treatise, gave incidentally, by way of enforcing his thoughts, a review
of his earlier work on calculating-machines. His work covered the
simple case of a machine composed of wheels and levers, capable of
computing the successive terms of any series. The simplest case is
an arithmetical series, the differences between the successive terms
being unity. This is the device which we now use in the street-cars
for counting fares. He asserted the possibility of making a machine,
capable of computing the terms of such a series, or of any other,
continuing the operation for thousands of years; and pointed out
that the machine may be so designed that it will then compute one
single arbitrary term, having no relation to the series which had pre-
ceded. It may then resume the former series, or it may begin com-
puting a geometrical series, or a series of squares or cubes of the
natural numbers. A scientific investigator, who is not permitted to
see the mechanism, begins to observe and record the series of numbers
which are being disclosed on the dials. He soon learns the mathe-
matical law of the series. He observes the time-sequence of the suc-
cessive terms, and computes the date when this order of things began.
He then makes use of his knowledge of other machinery, and makes
a working drawing of the hidden mechanism which produces these
results. He verifies his work by years of subsequent observations.
With what amazement does he finally behold that single arbitrary

88 PHYSICS OF MATTER

term! With what amazement does he then see the machine begin to
compute the squares or the cubes of the numbers it had previously
disclosed! The date when that machine was created and set to work
has been rudely called in question by the new and seemingly lawless
behavior of which it appears to be capable. And yet the observer still
feels that the principles of mechanism have not been shaken by this
unlooked-for disclosure. He again begins his work, with broader
conceptions of the plan of this machine. And his subsequent work
is along precisely the same lines, and by the same methods as his
previous work.

It is in exactly this way that all scientific work has proceeded,
and I wish to point out a few interesting cases of this kind. I find it
impossible to do this without presenting the present aspect of these
problems in connection with the work of the past. This plan gives
a perspective which not only adds to the interest but to the clearness
of the presentation.

The nebular hypothesis was an attempt by Kant, Laplace, and
Herschel to trace the evolution of the solar system from a glowing
mass of incandescent vapor or gas. As the theory was considered
and developed, an immense number of correlated phenomena were
found to be in harmony with this hypothesis, and a few discordant
phenomena were also found. The operation was, moreover, based
on a few fundamental and well-established laws, governing the pre-
sent condition of the system; such as gravitation, radiation of heat,
etc. The case became more and more convincing, as the knowledge
of the last century was applied. All of this caused the astronomers
and physicists to find it very easy to give to the hypothesis their
tacit assent.

Later, Sir William Thomson, now Lord Kelvin, took up the ques-
tion of underground temperature, and determined the limit in time
since which the earth must have begun to solidify. He also assumed
that the present order of things had come down to us from the past,
and that the present order of things consisted in the radiation of
heat from a cooling earth.

The time-interval which Kelvin thus determined was in entire
harmony with the nebular hypothesis, but the results were received
with something like consternation by geologists, and those who had
followed Darwin in the study of the evolution of organic life upon
the earth. Afterwards Kelvin sought to show that the process of
solidification might have required but a short interval of time, and
the evolutionists have found that evolution goes on by steps or sudden
changes rather than by a continuous succession of imperceptible
increments.

The geologists have never been reconciled to Kelvin's results, and
their protests have of late seemed to be on the increase. Of late the

PRESENT PROBLEMS 89

situation has changed in various ways. The discovery of radioactive
matter in wide diffusion in the earth's crust has reopened the whole
question of underground temperature as related to the age of the
earth and its past history. Nevertheless, if the nebular theory in
any form, or any similar theory, represents the process of evolution
of the solar system, a large amount of heat due to gravitational
contraction must have resulted, and must have been disposed of by
radiation.

During several years I have been giving attention to the condi-
tions of evolution of a gaseous nebula. The equations of equilibrium
for such a mass have been developed.1 A cosmical mass of gas was
assumed, satisfying everywhere the Boyle-Gay-Lussac law, capable
therefore of expanding, of being compressed, and of transmitting
pressure, and having a centre towards which it gravitates.

Such a mass of gas is a simple heat-engine. The piston face is any
spherical concentric surface. The load on the piston is the wreight
of superposed layers, external to the piston face. The radially in-
wardly directed pressure is exactly that required to balance the
outward pressure of the inclosed mass. As radiation and contraction
proceed, the load on the piston increases, in a perfectly definite way,
due to increase in weight of each element of mass as it approaches
the gravitating centre. Whatever may be the nature of the gas, as
determined by the numerical value of the Boyle-Gay-Lussac con-
stant, at some time in its history contraction will have proceeded
until some fixed or definite mass shall have been compressed within
a fixed volume of definite radius. The equations show that the
pressure at the surface of this mass, that is to say, the load on the
piston, will then be entirely independent of the nature of the gas.

The difference between gases will only be shown in the time re-
quired for them to reach this assumed stage in their gravitational
history. A gas wrhich permits the heat of compression within the
piston face to escape most quickly into the refrigerator external
to the nebula will reach this stage most quickly. When this has
been done, pressures and densities at the piston face are wholly inde-
pendent of the nature of the gas. The total work of compression done
on the mass within the piston face up to this time is also independent
of the nature of the gas. But the temperatures at the piston face
will be inversely as the numerical value of the Boyle-Gay-Lussac
constant.

It is evident, therefore, that the law of contraction cannot be
indeterminate as in the case where the load is imposed by the hand
of man. There is, therefore, in addition to the Boyle-Gay-Lussac
law, another definite relation between any two of the three variables
involved in that law. The application of well-known equations of
1 Transactions, Academy of Science of St. Louis, xin, no. 3; xiv, no. 4.

90 PHYSICS OF MATTER

thermodynamics led to the result that the density at any such piston
face was directly proportional to the nth power of the pressure.
The value of n is found to be 0.908 for all gases like oxygen, hydrogen,
nitrogen, and air. The operation is, therefore, one lying between iso-
thermal and isentropic compression, and near to the former. The
specific heat of gravitational compression is therefore negative. The
unit mass of gas at any point rises in temperature during compres-
sion, and for a rise of temperature of 1°C., it gives off by radiation
a definite amount of heat.

If, now, such a nebula be supposed to extend to an infinite distance
from the gravitating centre, the mass of the nebula will be infinite.
Pressure, density, and temperature then all become zero at an infinite
distance. Suppose such a nebula to have reached such a stage in its
contraction that the mass of our solar system, 1.99X 1033 grammes,
is internal to Neptune's orbit, then it turns out that the pressure there
will be about what it is in Crookes tube, 1.74X 10~7 atmospheres. The
density will be far less than in a Crookes tube, viz.: 1.40X 10~12
c. G. s. The temperature for a hydrogen nebula will be 3000°C., and
for other gases it will be higher in inverse ratio as the value of the
Boyle-Gay-Lussac constant.

If the mass of the nebula be made finite, the conditions become
still more interesting. Let the condition be imposed that the mass of
the nebula is that of our solar system, and that it has so contracted
that Neptune's mass only is external to Neptune's orbit. Then the
temperature at Neptune's place drops to about 1900°C., for hydrogen,1
and both pressure and temperature become very much less than
before. P 1.49 X 10~10; d 1.93 X 10~15. The thickness of the spherical
shell which would contain Neptune's mass is about a million miles
(1.65X 10~u cm.). At the external surface of this nebula, the con-
dition imposed makes P, d, and T zero, as the equations show.
Nevertheless, a large fraction of Neptune's mass would be gaseous
and far above its critical temperature. It seems to me impossible to
think of a nebula having such properties generating by any reason-
able rotation a system of planetary bodies. With Neptune's mass
on the surface of such a nebula consisting of matter having a density
and pressure less than a thousandth of these values in a Crookes
tube vacuum, how could we conceive of this matter being gathered
into a single planet?

A much more reasonable hypothesis is one discussed by G. H.
Darwin in 1889, in the Philosophical Transactions of the Royal
Society.2 Darwin discussed the properties of a swarm of solid
meteoric masses, and gives very strong proof of the proposition that

1 In a nebula of mixed gases, each gas will, of course, have its own temperature,
as is well understood.

2 On the " Mechanical Conditions of a Swarm of Meteorites," and on "Theories
of Cosmogony," Phil. Trans. 1889.

PRESENT PROBLEMS 91

a system of planetary bodies may originate in this way, although he
is very cautious and conservative in stating conclusions. The great
importance of this theory of planetary origin from the standpoint of
planetary geology and the evolution theory seems to demand that
it should receive more attention than it has yet received. The tem-
perature of the great mass of such a swarm will be very much lower
than in the case of the gaseous nebula. The larger part of such a mass
will approach absolute zero in temperature. According to this hypo-
thesis, even Mercury may have been solid when it separated from the
parent mass, although in its later stages a large mass might become
a gaseous nebula, as the sun now is. But in case of a body like our
earth, of such relatively small size, and so far removed from the
heated core, there does not seem to be any necessity for the assump-
tion that it was ever in a fused condition.

In view of these new developments, it seems peculiarly important
that a discussion of the limits of maximum temperature which the
mass of our earth has reached in the past should now be taken in
hand again. Suppose a swarm of meteorites to fill the space internal
to the moon's orbit, having a total mass equal to that of our earth.
Assume that the mass is in rotation, so that the moon is about to
separate from the parent mass. It would probably be too radical
to assume that each element of mass has either the same actual
velocity or the same angular velocity. Various hypotheses, more or
less probable, are possible. Assume an initial temperature approach-
ing zero absolute. It seems clear that the highest temperature
reached in passing to the present condition of things may be far below
the temperature of fusion.

A body falling directly from the moon's distance to the earth will
develop 59/60 of the kinetic energy it would acquire in falling from
an infinite distance. The earth is yet being bombarded by meteoric
matter having such velocities. But the operation is taking place
so slowly that the heat has time to become dissipated by radiation, so
that no appreciable rise in temperature of the earth results. To what
extent may this condition have held in the past? Darwin discussed
the tendency of the larger masses in such a swarm to accumulate
towards the centre. It is a kind of sorting process. These larger
masses would be in general of a metallic character. The more brittle
rocks of smaller density would therefore form the outer layers of our
earth. May not the heterogeneous character of our so-called igneous
rocks be explained in this way? And the shrinking of the earth would
then perhaps be in part the flowing of this porous mass into con-
tinuity. And it may incidentally be pointed out that the existence
of the belt of meteorites known as the asteroids is a most significant
indication of the conditions which must have existed at a certain
stage in the history of our solar system.

92 PHYSICS OF MATTER

The problems of the present which have aroused general interest
are those which pertain to the physical constitution of matter. And
here we are at once confronted with the question, What do we mean
by matter? How is matter to be recognized? Of late we have been
hearing such phrases as "the electrical theory of matter." There
seems to be a marked tendency towards the idea that matter and its
properties are alike electrical phenomena. Some even intimate that
the molecular theory of gases, and the atomic theory of the chemist
are tottering to a fall. We have long known that matter in motion
is a form of energy. This energy of moving matter is continually
being converted into molecular or atomic vibration, and then escapes
from us, apparently, forever, in the form of ether waves. We have
also long known that electricity in motion is a form of energy, and
that the energy so manifesting itself is also all finally converted into
heat, and then into ether waves.

Now this parallel certainty suggests an electrical theory of matter,
but it also suggests, equally, a material theory of electricity. And so
far from being antagonistic, these two theories are identical. There
is nothing whatever to show that electricity has ever been separated
from something which has what we have been accustomed to call
mass. Rowland 1 found that when the charged sectors on his rotating
disk were rotated, a magnetic field was produced, corresponding
to that produced by a current of electricity. If the motion of the
matter which carries the positive electric charge is in a positive direc-
tion, the field is the same as that produced when a negative charge
is moved in a negative direction.

Rutherford has recently found phenomena of radioactive matter
which have a most vital interest in connection with Rowland's work.
The a and /? particles which are shot off from such matter are mov-
ing in the same direction, and they are oppositely deflected in a mag-
netic field. They behave like superposed or perhaps juxtaposed
electric currents of opposite sign flowing in the same direction. If
in these radiations the a and /? particles were moving in opposite
directions, then in a magnetic field they would be deflected in the
same direction. This at once raises a question concerning the nature
of an electric current in a conducting wire. Let us assume that we
start with the positive and negative charges on the terminals of the
Holtz machine. What is it that is taking place when the terminals
are joined by wires leading to a galvanometer? We get a current
which we are wont to say is due either to a positive current flowing
in a positive direction, or to a negative current flowing in an opposite
direction. If we cease to apply work to the rotating wheel, it comes
to rest, and the potential of the conducting wire becomes uniform
throughout. Its extremities which terminate in front of the charged
1 American Journal of Science, [3] xv, 30-38, 1878.

PRESENT PROBLEMS 93

inductors are therefore so charged as to produce this uniform poten-
tial in the presence of these charged inductors, and the polarized
glass of the rotor. The ends of the conductor are therefore oppositely
charged. There is on its surface a neutral line of no charge. During
the motion of the rotor these opposite charges are oppositely directed
in the conductor. They are continually being added together. Equal
quantities of unlike signs are continually being added together. Are
we to assume that equal currents of unlike signs are superposed? Is
a positive current in a positive direction identical with a negative
current in a negative direction? Mathematically we should say yes.
The resulting current, moreover, is uniform throughout the circuit,
when measured by its external electromagnetic effects. We may loop
in calibrated galvanometers at any point in the circuit, and they tell
the same story. But what do the results of Rowland and Rutherford
teach us? The /? particles carry the negative charge. The negative
charge is part and parcel of something which has a positive mass.
The a particles are perhaps a combination of more /? particles in
combination with other particles having (or being) a positive charge
of greater numerical value. We have found long ago that the pro-
ducts of an explosion are not necessarily composed of matter in its
most elementary form. But these a particles are also part and parcel
of something which has a positive mass.

Are we to think of this conductor as being the seat of some action
by which positive masses are being urged in a positive direction and
positive masses are also being urged in an opposite direction? Are
we to think that the mass of such a conductor, carrying a direct cur-
rent, is slowly increasing, and that after many thousands of years this
increase will become appreciable, resulting, perhaps, in a clogging of
the conductor, and a decrease in its conduction? In that case a cur-
rent of positive electricity moving in a positive direction is not a
current of negative electricity moving in a negative direction. In
that case the nature of positive and negative currents of electricity
flowing in opposite directions is fundamentally different from that
of the flow of heat and cold in opposite directions, for it involves
the motion of masses in opposite directions. It would be interesting
to examine whether the long-continued use of a conductor carrying
a continuous current may not result in conferring upon it radioactive
properties. The results of J. J. Thomson 1 on the phenomena shown
by a Geissler tube 15"meters in length are very significant in this con-
nection. He finds the positive luminescence to travel in a direction
opposite to that of the cathode stream in the Crookes tube, with a
velocity somewhat more than half that of light. The older results of
Wheatstone 2 also show that the current from a Leyden jar travels in

1 Recent Researches in Electricity and Magnetism, p. 116.
3 Phil. Trans., Royal Society, London, 1834.

94 PHYSICS OF MATTER

opposite directions within the conductor which joins its coatings.
The middle point of the conductor is last reached by the discharge.
If the discharge is maintained and a steady current is finally pro-
duced, this current must apparently consist of positive and negative
electricity flowing in opposite directions.

If air be pumped out of one boiler and into another, two kinds of
pressure are thus generated. If these pressures are added together,
by connecting the boilers by means of a conductor, these pressures
are added together, and both disappear. If we tap these charged
boilers, the discharge from one will attract, and from the other will
repel, an uncharged testing sphere. If the testing sphere be itself
charged, we shall find that like charges repel, if both are positive, and
attract, if both are negative.

It is unnecessary here to enlarge upon the well-known differences
between the positive and negative terminals of an exhausted tube.
All of these phenomena will finally be helpful in arriving at the nature
of the difference between positive and negative electricity. But I will
refer to certain phenomena which do not seem to be so well known.
Every one is familiar with the small points of light which may often
be seen dancing in a crazy fashion over the cathode knob of the Holtz
machine. A similar appearance can be seen on the negative carbon
of a direct current arc, and in the negative bulb of the mercury vapor-
lamp. These points of light may be made to pass from the cathode
knob of the Holtz machine to the surface of a photographic dry-
plate, exposed in open daylight.1 Separate the knobs so that no
spark will pass. Place the plate near or between them. Connect the
knobs with two small metal disks, each armed with a pin-point, so
bent that it makes contact with the film. The point of the pin may
rest upon the short mark of a lead pencil, drawn upon the film, the
pins pointing towards each other on the plate. Points of light, like
the so-called ball-lightning discharges, will come from the cathode
terminal and successively travel slowly over the plate, leaving a black-
ened trail of reduced silver behind. By means of a lead pencil held
in the hand with the point near the cathode pin-point, these dis-
charges may be induced to make their appearance on the film, and
may be deflected into various directions after they have appeared.
When left to themselves these minute specimens, of what may per-
haps be called ball-lightning, tend to follow the lines of the field, but
their paths are somewhat affected by the paths of prior discharges. If
one of these points of light is seen on the pin which arms the cathode
terminal, there will usually be none upon the film of the dry-plate. It
may be brought upon the plate by holding a pencil-point near it.

These ball discharges come from the cathode and travel to or
towards the anode. They cannot be induced to come from the anode,
1 Transactions of the Academy of Science of St. Louis, x, no. 6.

PRESENT PROBLEMS 95

or to travel against the negative current. The anode terminal has a
visible discharge which appears to pass from it, and the photographic
plate at the anode looks somewhat like a picture of a relief map of
the delta formation at the mouth of a river.

If a conductor be laid upon the plate between the two pin-points,
there are then two gaps in the circuit. Each has an anode and
a cathode. This conductor may be a metal disk armed with pins 180
degrees apart, which face the discharge points. It may be a pencil-
mark upon the film or even a spot of reduced silver on the film. The
same discharge will start from the cathode terminal of this inter-
mediate conductor and will travel slowly in the negative direction.

With an induction coil giving an eight-inch spark, these ball dis-
charges can be formed on the surface of wood. In all cases it is evi-
dent that chemical work is being done by the slowly advancing ball
or point of light, and it is interesting to observe that it is the cathode
discharge only which seems to be active. The reason for this may be
partly electrical and partly chemical. The anode terminal of the
machine may be grounded on a gas-pipe, and the cathode terminal
only armed with a point, and the plate may be placed far away from
the machine, connection being made between its cathode terminal
and the pin-point on the film, with the same results. It may be added
that these plates may be of the most sensitive character, and may
be freely exposed to daylight for days before they are used. They may
also be developed in the light in a bath not very strongly alkaline.
The plate will develop clear, with the discharge tracks dark. The
picture will not reverse photographically. It probably would do so
if the plate were exposed to direct sunlight wrhile the electrical ex-
posure is made.

With an induction coil having an alternating potential on its
terminals, these ball discharges may be obtained from both terminals.
They will travel towards each other if on the same plate, but they
will not unite.

In a closed circuit, one part of which is moved across the lines of
a magnetic field, as in the case of a dynamo, we must suppose that
the positive and negative currents, if both exist, are superposed in
that part of the wire in which the electromotive force originates.
The currents are superposed at their origin. The same ether machin-
ery which urges the positive current in one direction urges the nega-
tive current in the opposite direction. With the Holtz machine, we
have one half of the machine positively and the other half negatively
charged. If the knobs are widely separated, and conductors each
armed with the pin-point be led off in opposite directions, each ter-
minating on the film of a photographic plate, the cathode will
deliver a ball discharge upon its film, while the anode will not. The
machine terminal which is not being used may, if desired, be grounded

96

PHYSICS OF MATTER

on a gas-pipe. If a pencil-mark be made upon the plate near the anode
it will be acted upon inductively, and a ball discharge will pass from
it to the anode pin-point. The positive discharge will go in the oppo-
site direction from the pencil-mark, but it leaves no trace. It appears
that this ball discharge upon the surface, which results in a destruc-
tion of the insulation of the surface, is a characteristic of the negative
current.

What would be the result if a suspended Maxwell coil were to be
looped into either of these unipolar circuits? Would this case neces-
sarily give the same result that Maxwell obtained?1 Of course we
know that the result which Maxwell sought to detect is very small.
We are more particularly concerned with the nature of the action
than with the magnitude of the result. If the a particles are so large
that they can contribute little or nothing to the current through
a metallic conductor, then the positive current may practically be
left out of consideration. But it seems doubtful whether the a par-
ticles are ultimate in their character, and here is where experimental
work is yet needed. It would be exceedingly interesting to study
these ball discharges upon a photographic plate under diminishing
pressures, as they gradually become a cathode discharge, in a Crookes
tube. A Crookes tube may be connected by only one of its terminals
to the Holtz machine. The free terminals of the machine and tube
may be connected to wires hung on silk fibres and making contact
with 'many pointed ground plates hung on long silk fibres in air.
The terminals are then in fact grounded on the dust particles in the
air. Either one of the^se air contacts may be replaced by a ground on
the gas-pipe. In all of the possible arrangements covered in this
description the tube will give excellent X-ray pictures.

-H-

contact

One of these arrangements is represented in the annexed figure,
where the cathode terminal of the tube is grounded on the gas-pipe,

1 Electricity and Magnetism, u, p. 201 .

PRESENT PROBLEMS 97

and is therefore at zero potential. The ground contact of the tube
may be replaced by an air contact, and the negative terminal of the
machine may then be grounded on the gas-pipe if desired. In none
of these cases are the positive and negative currents delivered by the
machine superposed in the X-ray tube. In all these cases X-ray
effects are obtained, but in some respects the tube behaves very differ-
ently when in the positive current from what it does in the negative.
In the negative unipolar circuit, the cathode terminal of the tube
is in direct communication with the negative terminal of the machine.
When in the positive circuit, the anti-cathode terminal of the tube
is in direct communication with the positive terminal of the machine,
and the cathode terminal is acted upon inductively across the Crookes
tube vacuum. In the negative current the luminous appearances
are normal and stable. When in the positive current, the discharge
may be made to cease by holding the hands near the bulb, and the
luminous glow is affected by the motion of neighboring bodies. The
discharge is much more unstable. If the observer approaches the
suspended grounding device the face and the hands are covered by
luminous points of light, which characterize the cathode terminal.
This phenomenon is very striking under these conditions. Ball
discharges may be drawn from such point discharges on a metal point
to a photographic plate, moving on the plate towards the anode wire
or contact plate suspended in air. It is apparent that the wire, when
considerably removed from other bodies, is discharging upon the
dust particles in the air.

In 1879 Spottiswoode and Moulton1 published a paper containing
a great array of experiments upon the spark discharge through gases.
They there dealt with unipolar discharge, and their conclusions are
well worthy of notice in this connection. They conclude that " the
independence of the discharge from each terminal of the tube is so
complete that we can at will cause the discharges from the two
terminals to be equal in intensity but opposite in sign (as in the case
of the coil) or of any required degree of inequality (as in the case of
the coil with a small condenser). Or we can cause the discharge to
be from one terminal only, the other terminal acting merely recep-
tively (as in the case of the air-spark discharge with the Holtz ma-
chine) ; or we can cause the discharge to pass from one terminal only,
and return to it, the other terminal not taking any part in the dis-
charge; or finally, we can make the two terminals pour forth inde-
pendent discharges of the same sign, each of which passes back
through the terminal from whence it came." This work was done
before the Crookes tube had appeared. It is certainly interesting to
observe that when a high degree of rarefaction has been reached,
the activity within the tube is represented by the cathode stream,

1 Phil. Trans., 1879.

98 PHYSICS OF MATTER

even when the terminal from which it comes is acted upon only
inductively. The remarkable thing is that the X-ray effects and the
luminosity of the tube should then be so great. The unipolar positive
discharge in the positive direction and the unipolar negative dis-
charge in the negative direction give, in the same time, an X-ray
picture of the same intensity, when developed in the same bath,
although in the latter case the cathode is only acted on inductively.

Unquestionably the great problem of to-day is the determination
of the nature of positive electricity and its relation to what is left
when the /? particles have been removed. When the cathode particles
have left the induced cathode terminal it is positively charged, and
communicates that charge to the dust in the air, or to neighboring
bodies. It does this, however, by a similar inductive action, and the
ball discharge traveling over the photographic plate suggests that here
also the negative particles are the active -ones.

The few who have search-lights have of late been throwing them
upon the great mass of experimental work on the discharge through
gases, published during the last generation. It is most instructive
to remember that the Crookes tube was known for seventeen years
before Roentgen discovered that something was going on outside of
it. A repetition of some of the work done on spark discharge, and
in particular the work of Wheatstone, in the light of what is now
known, would be likely to yield results of the greatest value. It would
be of particular value to study by the Wheatstone method the
unipolar discharges of the" Holtz machine.

A few words only may be added respecting radioactive phenomena.

We have long been familiar with the changes in matter, of a char-
acter such as may perhaps be. described as spontaneous. Many
crystals slowly lose their water of crystallization. They give off eman-
ations. They explode very slowly. Now emanations, like all other
matters and things, have individual peculiarities which enable us to
recognize them. The emanation from crystallized sodium carbonate
is also given off by all animals and plants, and is evidently a very
useful and widely diffused substance. There are many substances
which go to pieces and give off energy. They explode. Many of them
give off more energy per gram per second than any radioactive body,
while radium gives off more energy per gram than any other body.
The radium explosion also goes on at a lower temperature than that
of any other body. It hardly seems to be warranted, to say that the
action is the same at the temperature of freezing hydrogen as at
ordinary temperatures, for it does not seem that any high degree of
precision has been attained in such a measurement. And certainly
it can hardly be claimed that we know what these radioactive bodies
would do at a temperature below 16 degrees absolute.

Seven years ago, an attempt was made in my laboratory to obtain

PRESENT PROBLEMS 99

X-ray effects from explosions. High-grade gunpowder was loaded
by strong compression into rifle-shells designed for a 40-grain charge.
This powder was discharged from a heavy rifle, against an oak target
six inches from the muzzle of the gun. The target was faced with thin
plates of aluminum, which required frequent renewal. The concus-
sion was sufficient to extinguish a gas-flame seven feet from the line
of discharge. The plate used was one which would yield distinct
X-ray effects from an exposure of one second to a Crookes tube,
operated by an eight-plate Holtz machine. The dry plate was placed
behind the target, and was subjected to the discharge of twenty-five
pounds of powder, the operation requiring the spare time of the ex-
perimenter for forty days. The result was negative. No fluorescent
effects could be detected by an observer behind the target. A rapid-
fire gun might yield different results.

The same experiment was made with a thousand copper shells
loaded with mercury fulminate. They were exploded in twos, one
being fired electrically, the other being exploded by the concussion.
The first shell was laid upon a wooden block resting on a two-inch
plank. The second shell, to be exploded by it, was laid upon it with
a heavy iron bolt-head just above. No metal was interposed between
the explosive and the photographic film beneath the plank, and it
was necessary to replace the block by a fresh one at each explosion.
These explosions were so violent that a photographic plate of glass
was shattered by the shock at almost every shot, and the windows
thirty feet distant were perforated by bits of copper which occasion-
ally escaped through the surrounding screens. A sensitive film of
gelatine was used, on which the shadow picture was expected, but
none was obtained. There is yet some reason to expect positive
results from experiments of this kind. It may well be that explosives
differ in this respect as in others. An investigation of the products
of such explosions by the electrical means now used in the study of
radioactive bodies is a wide and most inviting field, which is likely
to aid in the explanation of radioactive phenomena.

Some of the products of explosion in the case of radium and
uranium are more nearly elementary in character than other bodies
yield, and some of the products are more elementary than others.

Now there is nothing unusual in finding here and there a substance
which has some property to a very exalted degree. The diamond
is such a case. Iron is vastly more magnetic than any other substance.
All substances are magnetic. A group consisting of iron, cobalt, nickel,
etc., are more magnetic than the great body of substances, and iron
heads the list. There is nothing more remarkable in finding a group
of radioactive substances with one wrhich enormously surpasses all
others than there is in finding an Academy of Science with some
member surpassing all the others in some particular direction.

100 PHYSICS OF MATTER

The relations which have been found to exist between atoms and
molecules are no more disturbed by the behavior of radioactive sub-
stances than by the explosion of nitroglycerine. We have learned that
what we have provisionally called atoms are, at least in some cases, as
has long been believed, very complex in their structure. We should
hardly expect an architect to lose confidence in houses, if he finally
learns that the bricks with which he is familiar are not the final ele-
ments in their structure. That the bricks are made up of molecules,
and the molecules of atoms, and the atoms of electrons, and that
some houses have been observed to fall into pieces and give off energy,
would hardly affect the usefulness of houses which do not fall to
pieces, even if inertia is shown to be an electromagnetic phenomenon.
And I think we should all remember that the proposition that matter
has mass is fundamentally different from the proposition that a mass
of matter has inertia. If inertia can be explained to be an electro-
magnetic quantity, and if it can be measured in new units, we have
not changed the properties of matter. It is still matter, and it still
has both mass and inertia. If inertia is an electromagnetic phe-
nomenon, it may be measured in terms of the fundamental units in
which all electromagnetic quantities are measured, — the units of
length, time, and mass.

Formerly a force was measured in terms of the unit of mass only.
People talked about a force of one pound. Later it was discovered
that a force could also be measured in terms of the pound, the foot,
and the second. At this time we did not hear any intimation that
matter had had its day and was about to be abolished.

In physics we now think we have reached the domain of small
things. But the electron may also be a very complex structure. If
we accept Poynting's view of the nature of electromagnetic in-
duction, the electron in a conductor is acted upon by a distant and
moving electron, through a medium external to the conductor. The
experimental verification of this is very convincing. In addition to
this complex machinery we have to deal with machinery of gravita-
tion.

We may always assume that nature is everywhere complex and
ingenious. A visitor to our solar system, who should begin to study
it from our earth, might begin with physical astronomy. He finally
comes to chemistry, to zoology, and the phenomena of life, to gov-
ernmental organization, to the moral and religious influences which
dominate the lives and actions of men, to the simultaneous juris-
diction of state and federal courts within the same territory. By the
time he had come to know this world as we know it, he would con-
clude that this universe of ours, which he first perceived as a faint
and distant speck of light in the blazing firmament of stars, is, after
all, very wonderful, and very much more complex than was at first

PRESENT PROBLEMS 101

believed. In arriving at our present ideas of the mechanism through
which matter reacts on matter, we have not reached them by finding
that the old ideas must be renounced, in order to explain some new
phenomenon which is apparently out of harmony with the explan-
ation previously made. It is rather that each new development has
confirmed what had gone before, has made it seem more reasonable,
and has filled in some gap in the knowledge of the past. The ether,
which only a few years since was assumed to exist because it seemed
to be necessary, has become more and more centrally important,
and has finally come to monopolize most of the attention of those
who would seek to understand matter. It is no reproach to modern
ideas concerning the physics of matter, that they are complex. The
fact that they are also harmonious and beautiful, and that they
furnish an explanation of why a mass of matter has inertia, and pro-
mise the explanation of other long-standing puzzles, converts the
accusation of complexity into a crowning glory.

SECTION B — PHYSICS OF ETHER

SECTION B — PHYSICS OF ETHER

(Hall 11, September 23, 3 p. m.)

CHAIRMAN: PROFESSOR HENRY CREW, Northwestern University.

SPEAKER: PROFESSOR DEWITT B. BRACE, University of Nebraska.
SECRETARY: PROFESSOR AUGUSTUS TROWBRIDGE, University of Wisconsin.

THE ETHER AND MOVING MATTER

BY DEWITT BRISTOL BRACE

[Dewitt Bristol Brace, Professor of Physics, University of Nebraska, b. Wilson ,
New York, 1859; died, October 2, 1905. A.B. Boston, 1881 ; A.M. Boston, 1882;
Ph.D. Berlin, 1885; post-graduate, Massachusetts Institute of Technology, 1879-
81; Johns Hopkins University, 1881-83; University of Berlin, 1883-85. Acting
Assistant Professor of Physics, University of Michigan, 1886. Vice-President
of American Association for the Advancement of Science; Vice-President of
American Physical Society. Author of Radiation and Absorption.]

THE question whether the luminiferous ether passes freely through
matter or participates in the translation of the same, considered as
a moving system, stands to-day without positive answer, notwith-
standing the numerous experimental attempts and the varied hypo-
theses which have been made since the discovery of aberration by
Bradley in 1726. The simple explanation of this phenomenon on
the corpuscular theory may have caused the century of delay in the
closer examination of the question until it became necessary to
consider it from the standpoint of undulations in an ether. As com-
pared with the many efforts to examine the question in the second
or ether period we have perhaps but two belonging to the first or
corpuscular period. Boscovich, in 1742, reasoning from this theory
on the ground of a difference of velocity in air and water, proposed
to examine the aberration of a star with a telescope whose tube was
filled with water. This experiment was not carried out till long after
by Airy in 1872, who found that the variation in the aberration was
absolutely insensible. Arago, in the second instance, reasoning on
the same theory, concluded that the deviation produced by a prism
would vary with the direction of the earth's motion; but he was
unable to detect any such change, a result verified later by more
delicate means in the hands of Maxwell, Mascart, and others. This
experiment, which demonstrated the absence of any effect of the
earth's movement on refraction is of great historical interest. This
negative result, which to Arago was inconsistent with the corpuscu-
lar theory, suggested to Fresnel the important hypothesis of a qui-
escent ether penetrating the earth freely but undergoing a change

106 PHYSICS OF ETHER

of density within the medium proportional to the square of its index
and being convected in proportion to this excess of density, which
would give an apparent velocity to the ether of (1 —fjc~2}v, instead of
the velocity of the earth. Stokes suggested, as a simpler idea, that we
suppose the ether is not convected but passes freely through the
earth, being condensed as it passes into a body in the ratio of 1 to /*2,
so that its velocity within the refracting medium becomes (1 —fj-~'2)v,
from the law of continuity.1

Babinet in the second-century period attempted to test Fresnel's
theory by examining the interference of two rays traversing a piece
of glass, the one in the direction of the earth's motion and the other
in the opposite direction. Stokes showed that a negative result was
not contrary to the theory of aberration, since the retardation would
be the same as if the earth were at rest.

He showed further, what Fresnel had not proven to be true in
general, that on Fresnel's theory the laws of reflection and refraction
for single refracting media are uninfluenced by the motion of the
earth. In fact, Rayleigh has shown that, in using terrestrial sources,
no optical effect can be produced by any system of reflecting or
refracting optical surfaces moving as a rigidly connected system
relatively to the ether, if we take into account the Doppler "effect,"
and neglect quantities of the second order of the aberration. Since,
as Stokes says, the theory of a quiescent ether may be dispensed with,
and as there is no good evidence that the ether moves quite freely
through the solid mass of the earth, he proposes to explain the
phenomenon of aberration on the undulation theory of light, upon
the supposition that the earth and the planets carry a portion of the
ether along with them, so that the ether close to their surfaces is
at rest relatively to those surfaces and diminishes in velocity till at
no great distance in space there is no motion. Cauchy had previ-
ously discussed the theory of a mobile ether, and had proposed to
explain aberration by a shearing of the wave-fronts due to the trans-
latory motion of the medium, but he did not develop his method
sufficiently to explain how much the aberration would be.

On the other hand Stokes has specifically indicated his assumptions
and formulated his conclusions. He examines the displacements of
a wave-front in its passage from the ether at rest, across the region
of transition to the ether in the neighborhood of the observer, which
is at rest relatively to him. Adopting the same method which is
used in the case of an ether at rest in determining the wave-front at
any future time from that of a given one at any instant, he shows,

1 If x is the velocity of the ether relative to the moving matter, and the dens-
ity of ether within it is M2, the density of free ether being unity, we have from
the law of continuity v — (v — z),u2 and hence,

THE ETHER AND MOVING MATTER 107

on the one condition, viz. that the motion of the ether is differentially
irrotational, that if we neglect the square of the aberration and of
the time, the change in direction of the ray as it travels along is nil,
and therefore the course of a ray is a straight line, notwithstanding
the motion of the ether. Following out the analysis on this sup-
position, a body, a star for example, will appear displaced toward the
direction in which the earth is moving through an angle equal to the
ratiq of the velocity of the earth to that of light, when moving normal
to the star's direction. This rectilinearity of propagation of a ray,
which would likely seem to be interfered with in the motion of the
ether, is the tacit assumption made in explaining aberration. If
the physical causes, in consequence of which the motion of the ether
becomes irrotational, could be adduced, the theory of Stokes would
satisfy completely aberration and the negative results of the many
and various experimental investigations which have thus far been
made and whose validity is unquestioned, whether in refraction,
interference, diffraction, rotary polarization, double refraction,
induction, electric convection, etc. In an ordinary fluid, tangential
forces proportional to the relative .velocities destroy the irrotational
condition in a steady state of motion. If we suppose these forces
to be diminished indefinitely we obtain now a motion totally differ-
ent from that for the steady state when these forces are assumed
to be absent initially; and hence such a motion would be unstable.
When, however, tangential forces depending on relative displace-
ments in the ether are considered, it becomes possible to explain the
irrotational condition. Any deviation from this state, for example
at a surface of slip, would be dissipated away into space with the
velocity of light by means of transverse vibrations. He illustrates
such apparent incompatibilities in physical states by successive
dilutions of gelatine. Such a medium shows elastic tangential forces
for small constraints, and yet apparent fluidity for motions through
it, mending itself as soon as dislocated. He regards these qualities
as consistent and self-sufficient to explain the phenomena in ques-
tion. Against the view of Stokes, Lorentz raises objection to his
assumptions concerning the ether motions in the neighborhood of
the earth, which he considers inconsistent, a difficulty which he is
unable to set aside. Larmor demurs against an appeal to a highly
complex medium, such as pitch, for studying the behavior of a
simple one like the ether. A time-rate much shorter than the time of
relaxation will of course provide approximate rigidity, while a time-
rate much longer will provide approximate fluidity, but this requires
inevitable dissipation. This objection would be valid for a viscous
solid, but such Stokes apparently did not have in mind, since he speci-
fically proves such a case unstable. A solid like pitch is a very differ-
ent type of solid from that of a vesicular solid like jelly. An ether

108 PHYSICS OF ETHER

after the model of a viscous solid would always contain the viscous
terms, so that even for the high time-rates of light-waves there would
be dissipation however small. Such a condition, it can be proven,
would give coloration to the remote members of the stellar system;
a fact inconsistent with observation. On the other hand, a soft vesic-
ular solid like gelatine may not necessarily contain the time-factor,
and yet be so soft that dislocation may occur even with constraints
of the order of aberration, but not of the square of that order. Such
an ether without a time of relaxation factor would fulfill completely
the conditions of a luminiferous ether, if, as Stokes tried to show, it
could be reconciled with the phenomena of aberration and the motions
of the heavenly bodies. The method of double refraction shows that
a solution of gelatine of one part in a thousand is rigid, while at the
same time it appears as mobile as water, and its rate of flow through
small tubes does not vary largely from the same. This experiment
illustrates very markedly Stokes's example. When such a solution
is continuously dislocated between two surfaces in relative motion,
the same double refraction is present, indicating that the stress is
still active during dislocation. Also a metal, like copper, shows a
similar stress while being strained beyond its elastic limit. If this
takes place by slip or dislocation throughout the mass which, though
irregular, may give a mean uniformity for sensible dimensions, such
a medium might serve as our model. Any deviation from perfect
regularity in molecular distribution and activity we might anticipate
would give such minute irregular dislocations at the limit of elas-
ticity. Such a medium would thus transmit completely any dis-
turbance within this strain limit.

It is difficult, however, to conceive of the transmissions of a dis-
turbance across a surface of dislocation. For many ordinary media,
we should expect at such a surface total reflection. If we suppose
such a transmission of disturbance, its mode is not apparent, even if
we suppose a thin lamina in rotational motion which would diffuse
at least a portion, if not all, of the incident disturbance. Similar
difficulties would arise if we assume the ether a solid which becomes
fluid under stress and thus allows bodies to pass through it (as, for
example, through a block of ice, as Fitzgerald suggested). While
such solutions may seem highly artificial and do violence to our
convictions, the consequences of a quiescent ether may, when fully
developed and tested, demonstrate its impossibility and command
a more extended examination into the structural qualities of an all-
sufficient medium than the single case of an essentially vesicular
medium like jelly brought forward by Stokes and in a different form
as a contractile ether by Kelvin. The theory of Fresnel of a quies-
cent ether in space presupposes a change of its density proportional
to /i2 within a ponderable medium, and a convection coefficient

THE ETHER AND MOVING MATTER 109

(1 — /*2)v. This hypothesis satisfies the phenomena of aberration
and the uniformity of the laws of reflection and refraction of a body,
whether in motion or at rest, and, as already mentioned, does not
affect interference, as Stokes showed, so far as the earth's motion is
concerned. That the ether apparently is carried along within moving
matter not with its full velocity, but diminished to the extent indi-
cated by Fresnel's coefficient of convection, Fizeau demonstrated
in his famous interference experiment with streaming water, repeated
later with greater refinement by Michelson and Morley. The signi-
ficance of this experiment in its bearings on the question of the drift
of the ether has perhaps been overestimated. In fact, neglecting the
square of the aberration, it is exactly what we should expect from
the dynamical reaction of a moving material system on a periodic
disturbance, propagated through it without reference to the motion
of translation of the interpenetrating medium, but simply to the fre-
quency of the vibration impressed upon the system by this ether.
Thus if we transform the ordinary differential equations of motion
of the material system from fixed to moving axes, the form of the
solution contains Fresnel's convection coefficient as a factor exactly,
neglecting quantities of the second order of the aberration. This
experiment cannot then be adduced as a positive result in favor of a
quiescent ether. On account of its physical consequences, however,
it should be extended to the case of gases and to absorbing substances,
using light corresponding to the natural frequences of the latter if
possible. Although negative results have heretofore been obtained
with a gas, yet, with high pressures and greater dimensions and
velocities, the test is within present experimental limitations. Results
with solid bodies are still lacking, but a preliminary examination of
the problem encourages us to expect successful results, at least with
double-refracting substances. Reasoning in a similar manner as on
the dynamical reaction of a moving system, we should look for the
acceleration of a circularly polarized ray propagated coaxially within
a rapidly rotating medium. This may possibly be brought within
experimental limits. Again we have the important experiment of
Lodge on the effect of moving masses upon the motion of the ether
near them. This experiment, like that of the preceding one of Fizeau,
is a first order test, i. e. the effect to be observed would arise from
a change in the first power of the aberration factor. Two interfering
beams were sent around several times in opposite directions between
two rotating steel disks, and the effect on the bands noted from rest
to motion or reversal. With a linear velocity not far from one two-
hundredth that of the earth's orbital motion, and a distance of
some ten meters or more, no influence on the interfering rays could
be detected, thus making the effect, calculated from the aberration
factor if the ether were carried around between the disks, something

110 PHYSICS OF ETHER

like twice the limit of observation. Lodge estimates from this experi-
ment that the disks must have communicated less than the eight-
hundredth part of their velocity to the ether. It is to be noted that
the masses of these disks were not great, being only some two or three
centimeters thick and about one meter in diameter. If we suppose
the ether to be set in motion by means of reactions of a viscous
nature, the experiment would be conclusive. To this extent, that the
ether is not viscous, the test seems to be valid, but as there are other
modes conceivable by which such movement might be brought about,
it is not conclusive. If now we have to give up the notion of a quies-
cent ether, it will be necessary to suppose such motions are engen-
dered in some way depending on the mass of the moving system, which
we might imagine to be the fact in the case of the earth and the sur-
rounding ether (possibly as, Des Coudres suggests, through gravita-
tional action). It would be desirable to repeat this experiment, using
great masses, and also testing to a much higher degree of sensibility
(the third order would be possible) by means of double refraction.
Michelson has recently attempted to determine directly whether the
velocity of the ether diminished as we recede from the earth, but
with negative results. He sent two interfering rays in opposite
directions around the four sides of a rectangle of iron piping from
which the air had been exhausted, the same being in a vertical east
and west plane, the horizontal length of which was 200 feet and the
height 50 feet. Assuming an exponential law for the variation in the
velocity of the ether as we recede from the earth, he finds that if
the earth carries the ether with it, this influence must extend to a dis-
tance comparable with the earth's diameter. The negative result in
many of the experiments on refraction and interference which differ-
ent investigators have obtained and which apparently follow on the
assumption of a mobile ether have been usually experiments capable
of giving only second order effects instead of the first order effects
looked for, which, as mentioned above, are quite as consistent with
a quiescent ether, as Stokes and Rayleigh have shown. Among these
may be mentioned the experiments of Hoek, Ketteler, Mascart, and
others on interference in ponderable media, over opposite paths rela-
tively to the earth's motion ; as also those of the two latter with double-
refracting media. All of the experiments were first order tests, and
hence should give negative results on either theory, since, with a ter-
restrial source of light, the phenomena are independent of the orient-
ation of the apparatus neglecting second order effects.

The positive results of Fizeau and of Angstrom have not been
confirmed and should not be seriously considered. In the experiments
of the latter, the variation of the position of the Frauenhofer lines,
as obtained by a grating when observed in directions with and oppo-
site to the earth's orbital motion, has never been noted since, beyond

THE ETHER AND MOVING MATTER 111

the anticipated displacement calculated from the purely kinetical
principle of Doppler. The experiments of the former, as a first order
test, on the rotation of the plane of polarization of a ray after passing
through a pile of plates has perhaps offered the greatest difficulty to
the exponents of both theories in reconciling the observations with
the results which should follow from each theory. In this experi-
ment, performed in 1859, the optical systems was mounted so as to
be rotated about a vertical axis alternately from east to west, or
vice versa. This system consisted of the usual polarizing nicol or sen-
sitive tint-system and analyzing nicol between which were placed
several piles of plates and compensating systems for producing the
rotations and the magnifying of the same, and also for compensating
for the rotary dispersion and elliptic polarization of the transmitted
light which was polarized in an azimuth of 45°. In a series of ob-
servations extending over some time the mean of the rotations of
the plane of polarization showed a maximum excess in the direction
toward the west at noon and at the time of the solstice. It is to be
noted that light from a heliostat was reflected into the system alter-
nately by two fixed mirrors when the system was rotated. This
required an interruption and readjustment of the heliostat during
a single observation, i. e. from east to west and west to east, the dif-
ference in the setting of the analyzer in the two positions to give the
same field of view being, of course, the effect sought for. Fizeau
refers to the irregularities arising from successive settings of the helio-
stat. The calculated effect was much below that which could have
been observed directly with the usual polarizing system. To magnify
any such effect, a second system of plates was used which gave an
amplification as high as eighty times. Thus any residual rotation from
whatever cause would receive the corresponding amplifications. Now,
in experiments with polarizing systems using sunlight as a source of
illumination, it has frequently been noted that any shift in the direc-
tion of the light through the apparatus, either due to a change in the
direction of the beam (arriving, say, from the heliostat) or to a shift
in the optical system itself, produced a change in the field of view,
whether with a half-shade system or otherwise. In the former the
match was destroyed, the change being of an order much greater
than that which Fizeau anticipated from calculation. Further, with
such limited beams of light, a mere shift of the eye may produce
an effect of similar magnitude. Hence, in all polariscopic experi-
ments where sunlight is used, it is absolutely essential that, during
any single observation, the ray of light pass through the system and
into the eye over exactly the same path. This Fizeau failed to carry
out, and this is entirely sufficient to explain the very great discrepan-
cies in his various series of observations, and probably the apparent
constant difference in the results of his settings in the two directions.

112 PHYSICS OF ETHER

In fact, Fizeau himself has stated since that his observations were
not absolutely decisive. While the test is now probably within ex-
perimental limits with the more highly refined half-shade systems,
other modes of experimenting on different optical principles with
greater sensibilities have given negative results, thus disproving the
existence of a phenomenon which Fizeau's experiment apparently
established, and making a repetition of this experiment, which is of
doubtful execution, unnecessary.

The effect of the motion of a natural rotative substance through
the ether on the rotation of the plane of polarization is of considerable
importance in its bearings on certain controverted points in some of
the recent theories of a quiescent ether. Mascart, who first studied
the problem in the case of quartz, was unable to detect any differ-
ence in the rotation when a ray was propagated in and against the
direction of motion of the earth. This variation in the total rotation,
which he could detect, was one part in 20,000, or one part in 40,000 on
reversal. This experiment as thus carried out corresponds to a first
order effect. Rayleigh quite recently has repeated this experiment
with a sensibility five times as great, and obtained negative results,
likewise. The impossibility of obtaining quartz in sufficient quantity
and purity, or natural rotary liquids of sufficient power, to attain the
extreme limit of polariscopic possibilities seems to make even an
approximation to a second order effect entirely improbable, although
the higher frequencies might be used, where the power may be ten
times as great. On the other hand, the effect of the mechanical
rotation of such a medium on the circular components is, however,
probably not beyond experimental possibilities in polariscopic work.

On the electrical side several first order experiments have been
made which likewise have given negative results. Des Coudres has
attempted to determine the difference in the induction on each of
two coils placed symmetrically, with respect to a third coaxial coil
between them. On compensating for the effects of each on the galvan-
ometer when the axis of the system was in the direction of drift, and
then reversing the direction of the system, no influence on the gal-
vanometer could be observed. The effect which should be observed
corresponds to the second order of the aberration. However, without
compensating factors, the theory of induction phenomena shows
that second order effects should be looked for in systems moving
through the ether. The same may be said of other electrical exper-
iments.

The difficulties in formulating a theory which will explain the
results of all experiments involving tests to the first order of sensi-
bility only on the assumption of either a quiescent or a convected
ether, are much easier met than when second and higher orders have
to be taken into consideration. Here we find what, at first sight,

THE ETHER AND MOVING MATTER 113

appear as rather startling assumptions; but it is only in this man-
ner that present observational facts can be reconciled with a quies-
cent ether. With each advance in experimental refinement, theory
has had to adapt itself by the adoption of new hypotheses. This has
now been done up to second order phenomena for a quiescent ether.
Thus far, however, no hypothesis has been brought forward to adapt
specifically the theory of a quiescent ether to observations which have
already been carried up to the third order of the aberration constant.
The first second order experiment was carried out by Michelson
and Morley, and was an optical test in which the method of interfer-
ence of two rays passing over paths mutually at right angles to one
another was used. The apparent intent of the originators of this
experiment was initially to look for a first order change in the aberra-
tion factor by means of a second order interference effect. The diffi-
culty in reconciling the negative results of this test has, however,
given rise to hypotheses involving second order dimensional factors,
so that from this point of view it becomes a second order experiment.
It could not, however, show a first order change in the velocity of the
moving system, which latter, referred to the velocity of light, is taken
as a magnitude of the first order, and hence the former change would
count as a second order magnitude. In this experiment the entire
system was mounted on a float so that the optical system could be
rotated consecutively through all quadrants of the circle while the
interference bands were being continuously observed. If now the
difference in time of passage of one of the rays, say along the line of
drift, and the other at right angles to it, is calculated on the basis
of a moving ether, we find it to be equivalent to the time of passage
over a length corresponding to a diminution of this length, in the
direction of drift, proportional to the square of the aberration. Their
results show that had there been an effect, it must have been probably
sixteen times, certainly eight times, less than that calculated. It is
understood that Morley and Miller will soon report as the result of
a repetition, during the present year, of this experiment on a much
larger scale, that, if there is any effect, it must be one hundred times
less than the calculated value. This result is entirely consistent with
a moving ether, but seemingly contradictory to a quiescent ether,
as proposed by Fresnel. Apparently, then, either some condition in
the fundamental hypothesis of such a medium has been overlooked,
or a supplementary hypothesis must be imagined. Similar hypotheses
were conceived of by both Lorentz and Fitzgerald independently,
shortly after the publication of the experiments of Michelson and
Morley in 1887. They assume that a contraction in the direction of
motion takes place in a system moving through the ether, so that this
dimension is reduced by a fraction of itself equal to one half the
square of the constant of aberration. This of course, as an assump-

114 PHYSICS OF ETHER

tion, merely suggests a compensation to meet an apparent residual
effect, and would be of no significance if it were impossible to incor-
porate such a condition into a consistent theory of ethereal action.
This has been done by Lorentz and by Larmor in their theories of
moving systems. Lorentz, who was the first to develop a satisfactory
theory of a quiescent ether, assumes that, in all electrical and optical
phenomena taking place in ponderable matter, we have to deal with
charged particles, free to move in conductors, but confined in dielec-
trics to definite positions of equilibrium. These particles are perfectly
permeable to the ether, so that they can move while the ether remains
at rest.

If now we apply the ordinary electromagnetic equations of a system
of bodies at rest to a system having a constant velocity of transla-
tion in addition to the velocities of its elements, the ether remaining
at rest, the displacements of the electrons arising from the electric
vibrations in the ether and the electric and magnetic forces are the
same functions of the new system of parameters as for the case of
rest, if we neglect quantities of the second order of the aberration.
This theorem assumes that the distance of molecular action is con-
fined to such excessively small distances that the difference in their
local times would have no effect. An exception to this may be found
in a rotary substance like quartz which, as mentioned above, has
been examined by Mascart and Rayleigh to the first order with
negative results, which seems to warrant the conclusion that the
molecular forces are themselves altered by translation. This theory
of Lorentz seems capable, then, of explaining the uniformly negative
results of all the first order tests which have been described previously,
without, however, necessarily establishing it finally, since we have
not yet studied its adaptability to second and higher orders of the
aberration.

The suggestion of a contraction, as stated above, lends itself in
a similar manner and under like restrictions to that for the first order
transformation. This requires the introduction of a second coefficient
differing from unity by a quantity of the second order as did the
coefficient used in the first transformation, but differing from the
latter in that it is left indeterminate from the fact that there are no
means as yet for giving it a definite value. Introducing these new
parameters we again obtain a set of equations in which the velocity
of translation does not explicitly appear. Such a moving system
has therefore its correlate in a system at rest, the former having
changed into the latter through the assumed contraction the moment
motion begins. The occurrence of these coefficients as factors in the
electric forces and the accelerations arising from the electric vibra-
tions in the ether in the expression for the corresponding system at
rest, necessitates that if the degree of similarity required is to exist

THE ETHER AND MOVING MATTER 115

in the two systems, the electrons must have different masses depend-
ing on whether their vibrations are parallel or perpendicular to the
velocity of translation. This startling conclusion of Lorentz is borne
out by what we now know of the dependence of the effective mass
of an electron upon what is taking place in the ether. Such an hypo-
thesis as this would require that Michelson and Morley's experiment
should always give a negative result.

Of electrical experiments on the drift of the ether we have one
second order test carried out very recently by Trouton at the sugges-
tion of the late Professor Fitzgerald. The latter, reasoning on the con-
dition of a magnetic field produced by a charged condenser moving
edgewise to the drift of the ether, and the consequent additional
supply of energy of such a system on charging, thought that this
might produce a mechanical drag on charging and an opposite impulse
on discharging, just as might occur if the mass of earth were to
become suddenly greater. This experiment was carried out in the
form of a condenser mounted upon an arm carried by a delicate sus-
pension, with negative results. A second and more sensitive test
was made later in a modified form by Trouton and Noble. Since,
edge on to the drift, we have a magnetic field, while at right angles
it vanishes, the energy will vary with the azimuth, and we shall have
a maximum in an azimuth of 45°. A delicate suspension carrying
the armature of a condenser showed no movement, although the
calculated effect was ten times the limit of observation. The negative
results of these experiments may be accounted for on like assump-
tions with that of the Michelson and Morley experiment, namely
a contraction or change in the dimensions of the condenser pro-
ducing corresponding changes in density and potential difference of
the charge.

The assumption of a contraction suggests at once, from what we
know of transparent media, the anisotropic state which such media
are thrown into under dimensional strain. Rayleigh has examined
this question in the case of water, carbon disulphide, and glass with-
out result. In the case of glass his sensibility was several times the
calculated second order effect, and much more in case of liquids.

The degree of refinement to which the polariscopic test lends
itself is perhaps beyond that of any other instance in physical appli-
cation. Here then is an opportunity to examine the question beyond
what theory has anticipated, and the test has been carried so as to
reach safely a third order effect, with negative results. The experi-
ments as performed by the writer consisted in sending a beam of
sunlight plane polarized at 45° to the horizon, through 28.56 meters
of water in a horizontal direction and examining the same by a sen-
sitive elliptic analyzer. On rotating the entire system from the me-
ridian, where the one component of vibration to the drift was parallel

116 PHYSICS OF ETHER

(
and the other perpendicular, into a plane at right angles to the mer-

idian where both components would be at right angles to the drift,
and therefore where no differential effect would be produced, no
change in the field of view could be detected. Had there been a
total difference of 7.8 X 10~13 of the whole velocity between the com-
ponents, the effect would have been manifest. We may, therefore,
conclude that there is no third order effect. How well the various
theories of a quiescent ether will lend themselves to this further
adaptation remains to be seen, but undoubtedly by properly choosing
the coefficients it may be done; however, any theory which does not
contain explicitly the exact and complete adaptation to all orders of
the aberration must certainly impress itself as highly artificial in its
successive auxiliary hypotheses and approximations.

Larmor, in reference to his theory, says, " It is, in fact, found that
the Maxwellian circuital equations of SBthereal activity, in the am-
bient aether referred to axes moving along with the uniform velocity
of convection, v, can be reduced to the same form as for axes at rest

up to and including ( — J but not I - 1 by adopting certain coeffi-

cients." "If, then, matter is for physical purposes a purely aethereal
system, if it is constituted of simple polar singularities or electrons,
positive and negative, in the Maxwellian aether, the nuclei of which
may be either practically points or else small regions of aether with
internal connections of pure constraint, the propositions above stated

v
for the first order are extended to the second order of — with the

single addition of the Fitzgerald-Lorentz shrinkage in the scale of
space and an equal one in the scale of time, which, being isotropic,
is unrecognizable." "On such a theory as this the criticism presents
itself, and was in fact at once made, that one hypothesis is needed
to annul optical effects to the first order; that when these were
found to be actually null to the second order, another hypothesis
had to be added : and that another hypothesis would be required for
the third order, while in fact there was no reason to believe that they
were not exactly null to all orders. Such a train of remarks indicates
that the nature of the hypothesis has been overlooked. And if indeed
it could be proved that the optical effect is null up to the third order,
that circumstance would not demolish the theory, but would rather
point to some finer adjustment than it provides for; needless to say
the attempt would indefinitely transcend existing experimental possi-
bilities." And further, "up to the first order the electron hypo-
thesis, that electricity is atomic, suffices by itself, as Lorentz was
first to show." "Up to the second order, the hypothesis that matter
is constituted electrically — of electrons — is required in addition."
The necessity in view of the present experimental data for leaving

THE ETHER AND MOVING MATTER 117

indeterminate the units of transformation is here illustrated in the
theory of Larmor.

In the most recent discussion by Lorentz, the necessity of a general
treatment is shown for not only the second but also the higher orders.
In a consideration of transparent media, his theory attempts to show
that translation would not alter interference, diffraction, or polar-
ization. He would thus, by means of the assumption of so-called
" Heaviside ellipsoids " as the shape of electrons, explain the negative
results of optical experiments, as well as the observations of Kauf-
mann on Becquerel rays.

Attention should also be called to the recent theory of Abraham,

4 / v\3
who gives as the ratio of the axes of the moving electron 1 — — ( ~ ] : 1 ,

omitting fourth and higher orders. This would give a residual in

1/vV
double refraction of -( =z J =2X 10 ~9 for transparent media, which

he acknowledges is difficult to reconcile with the experimental results
which show no double refraction to the first order beyond this.

SECTION C — PHYSICS OF THE ELECTRON

SECTION C — PHYSICS OF THE ELECTRON

(Hall 5, September 22, 3 p. m.)

CHAIRMAN: PROFESSOR A. G. WEBSTER, Clark University.
SPEAKERS: PROFESSOR PAUL LANGEVIN, College de France.

PROFESSOR ERNEST RUTHERFORD, McGill University, Montreal.
SECRETARY: PROFESSOR W. J. HUMPHREYS, Mount Weather, Va.

THE RELATIONS OF PHYSICS OF ELECTRONS TO OTHER
BRANCHES OF SCIENCE

BY PAUL LANGEVIN
(Translated from the French by Bergen Davis, Ph.D., Columbia University)

[Paul Langevin, Assistant Professor of Physics, College de France, Paris, since
1903. b. Paris, France/January 23, 1872. Licenci6 in Physical and Mathematical
Science; Fellow of the University; Ph.S.D.; Instructor at Sorbonne, 1899-
1903.]

THE remarkable fertility shown by the new idea, based on the
experimental fact of the discontinuous corpuscular structure of
electrical charges, appears to be the most striking characteristic of
the recent progress in electricity.

The consequences extend through all parts of the old physics;
especially in electromagnetism, in optics, in radiant heat; they
throw a new light even on the fundamental ideas of the Newtonian
mechanics, and have revived the old atomistic ideas and caused
them to be lifted from the rank of hypotheses to that of principles,
owing to the proper relation which the laws of electrolysis have
established between the discontinuous structure of matter and that
of electricity.

Without seeking here to run through the whole field of their appli-
cations, I hope to indicate upon what solid foundations, both experi-
mental and theoretical, rests at present the notion of the electron so
fundamental to the new physics; to indicate the points which seem
to require more complete light, and to show how vast is the synthesis
which we can hope to attain, a synthesis whose main lines only are
fixed to-day.

Under actual and provisional form, this synthesis constitutes
an admirable instrument of research, and owing to it the questions
extend in all directions. There is there a kind of New America, full of
wealth yet unknown, where one can breathe freely, which invites
all our activities, and which can teach many things to the Old World.

122 PHYSICS OF THE ELECTRON

I. The Electromagnetic Ether

(1) Fields and Charges. One can say that the combined efforts of
Faraday, Maxwell, and Hertz have resulted in giving us a precise
knowledge of the properties of the electromagnetic ether, and of
light; of a medium, homogeneous and void of matter, whose state
is completely defined, with the exception of gravitation, when we
know at any point the direction and magnitude of the electric and
magnetic fields.

I insist, for the present, on the possibility of arriving at a concep-
tion of fields of force, as well as the related idea of electric charges,
independently of all dynamics; I wish by this to imply only a know-
ledge of the laws of motion and of matter.

The two fields possess this property, that their divergence is zero in
all parts of the ether; that is to say, the flux of electric and mag-
netic force is rigorously zero across a closed surface which does not
contain any matter in its interior. It is in fact always matter in the
ordinary sense of the word which contains and can furnish the electric
charges around which the divergence of field exists whose direction
varies with the sign of the charges.

In extreme cases where the electric charges appear to be most
completely separated from their material support, as in the case
of the cathode rays for example, the experimental fact of the
granular structure of these rays and the complete indestructibility
of their charge, the fact finally that cathodic particles are charges
possessing the fundamental property of matter, inertia, and expe-
riencing acceleration in the electromagnetic field, these facts do not
allow us to distinguish their charge from the so-called free charge
of ordinary electrified matter.

Furthermore, we shall come to the idea not only that there can be
no electric charge without matter, but that, in fact, there can be no
matter without electricity, an aggregation of electrical centres of the
two kinds. Electrons, analogous to the cathode particles, possess
almost all the known properties of matter by the fact alone that
these centres are electrified. We shall see within what limits this con-
ception can be considered sufficiently known, and if it is necessary to
superimpose other properties on those which result from electrically
charged centres in order to obtain a satisfactory representation of
matter; the ether alone, on the contrary, never contains any electricity.

If experiment obliges us to admit the existence of electric charges,
positive and negative, from the flux of electric force different from
zero across a closed surface drawn entirely in the ether and con-
taining matter, it is otherwise for the magnetic field. Experiment
has never furnished an instance where a closed surface drawn in the
ether was traversed by a magnetic field different from zero. One

RELATIONS TO OTHER SCIENCES 123

interesting phenomenon observed recently by P. Villard in $he
effect of an intense magnetic field on the production of the cathode
rays, appears to receive a simple explanation in the hypothesis of
free magnetic charges; but it is not certain that this hypothesis is
necessary.

(2) The Equations of Hertz. The two fields, electric and magnetic,
of which the ether can be the seat, are related to one another in such
a manner that one of them can exist only on the condition that the
other varies; all variations of an electric field produce a magnetic
field; it is the displacement current of Maxwell: and all variations
of the magnetic field produce an electric field ; this is the phenome-
non of induction discovered by Faraday. These two relations are
expressed by Hertz's equations; they sum up completely our know-
ledge of the electromagnetic medium, and from these it results that
all disturbances of this medium are propagated with the velocity
of light. Hertz had the glory of proving this fact experimentally.

(3) Energy. We can now say that the ether is the seat of two
distinct forms of energy, the electric and the magnetic, capable of
transformation from the one into the other, but only through matter
as an intermediary, that is to say, by means of the electrified centres
which it contains.

In the ether alone, in fact, in the free radiation which it propagates,
the electric and magnetic fields, transverse with respect to the direc-
tion of propagation, represent always equal energies in each element
of volume, without oscillation of the energy from one form to the
other. In the presence of matter, on the other hand, the electric
energy can exist alone, and it is the motion of electrified centres
which allows the transformation into magnetic energy, and vice
versa. Matter only can be the source of radiation.

It is necessary, to the two preceding forms of energy, to add
gravitation, which corresponds probably to a third mode of activity
of the ether, whose connection with the two others is still obscure.

I insist here on the point that the principle of equivalence of vari-
ous forms of energy, as far as the process allows of measurement,
can be attained independently of all dynamical notions, by the
process of using solely material systems in equilibrium.

One can find some information on this subject in a recent exposi-
tion by M. Perrin.1

(4) The Theory of Lorentz. The ether being thus completely
known to us from the electromagnetic and optical point of view, the
problem which follows as a continuation of the work of Maxwell and
of Hertz is that of the connection between ether and matter, inert
matter, the source and recipient of the radiations which the ether

1 I. Perrin, Traite de chimie Physique. Les Principes. Gauthier-Villars, Paris.

124 PHYSICS OF THE ELECTRON

transmits. The connection sought for is furnished us by the electron
or corpuscle, an electrical centre movable with respect to the ether,
and carrying with it its divergent electric field.

This was the fundamental idea which caused Lorentz to conceive
of the possibility of a relative displacement of electrified centres of
divergence of the electric field, and of the ether considered as im-
movable. This displacement takes place without any change in the
amount of the charge, that is to say, that the surface which is dis-
placed in the ether with the electron is crossed by an electric flux
which is completely invariable. It is the fundamental principle
of the conservation of electricity, which will perhaps absorb the
principle of the conservation of matter, as we cannot have matter
without electricity. It is, however, probable that electricity alone
is not sufficient to constitute matter.

We have actually no very precise information of the relative dis-
placement of charges and of the ether, of electrified centres in an
immovable medium, no tangible form under which we can conceive
it. The attempts which have thus far been made to obtain a concrete
representation, in order to give a material structure to the ether, have
all been sterile of results. Perhaps there is a difficulty which belongs
to the actual constitution of our minds, habituated by our secular
evolution to think through matter, unable to form a concrete repre-
sentation which is not material; also it seems scarcely reasonable to
seek to construct a simple medium such as the ether by consider-
ing it to spring from a complex and various medium like matter.
I believe it will be necessary to think ether, to conceive of it inde-
pendently of all material representations, by means of those electro-
magnetic properties which put us in contact with it. I will return
to this point later in reference to the mechanical theories of the ether.

If the electric charge is assumed to have a volume distribution
in a portion of the medium, the principle of the conservation of elec-
tricity, and also the possibility of relative displacement of electricity
and ether, makes it necessary for us, in this portion of space, to
modify the equations of Hertz relative to the displacement current
by the addition of a convection current, a necessary consequence of
the existence of a displacement current connected with a motion
of charges, and implying the production of a magnetic field by the
motion of electrified bodies across the medium. This consequence of
Hertz's equations has now received complete experimental con-
firmation.

Moreover, the experimental facts impose on these movable
charges a discontinuous, granular structure, and lead to the idea
of the electron as a singular region of the ether, carrying a charge
equal to that of the hydrogen atom in electrolysis, but of different
sign, and distributed on the surface or in the volume of the electron

RELATIONS TO OTHER SCIENCES 125

according as the intensity of the electric field is supposed to present,
or not, a discontinuity when it crosses the surface which limits the
volume occupied by the electron. Inertia, of electromagnetic origin,
which we are about to refer to a similar centre, is opposed also, under
the difficulty of its becoming infinite, to the hypothesis of a finite
electric charge condensed in a point without extension.

The various considerations, more and more precise, all converging
toward this notion of the atomic structure of charges, form the start-
ing-point of all recent works on electricity.

II. The Atom of Electricity

(6) The Electron. The remarkable laws of electrolysis discovered
by Faraday establish an intimate and necessary connection between
the atomic structure of matter and that of electricity. They were
sufficient to lead Helmholtz to conceive the latter as constituted of
distinct, indivisible portions, elements of charge, all identical from
the point of view of the quantity of electricity which they carry, and
differing only in the sign. This elementary charge is equal to that
carried by a monovalent atom or radical in electrolysis; a polyvalent
atom or radical carries an equivalent number of such charges.

It was Johnstone Stoney who first used the word electron to desig-
nate atoms of electricity as distinct from matter, with which they
combine to furnish the electrolytic ions. The presence of similar
electrons combined with material atoms allows us to represent certain
peculiarities of the spectrum, the existence of doublets of like fre-
quencies; the electron, in motion, is thus considered as the origin
of the emission of all luminous rays.

(7) Gaseous Conductors. But there are the researches on the
electrical conductivity of gases, which have presented to us in a
forcible manner the idea of electrical atoms, which have made this
notion more tangible by allowing us to count these electric centres,
to lay hold of them individually, and to measure for the first time
the charge of each of them in absolute value.

As early as 1882, Giese, in observing the peculiarities of the con-
ductivity of gases escaping from flames, the departure from Ohm's
law, the impossibility of drawing from the gas, whatever might be
the electric field employed, more than a limited amount of electricity
of each kind, the progressive recombination of the free charges in the
gas, had expressed in a precise manner the idea, that as in electrolytes
the free electric charges in a gas are carried by distinct positive
and negative centres in limited numbers, capable of moving in oppo-
site directions under the action of an external electric field in order
to discharge the electrified body which produces the field.

It is difficult, in fact, to conceive how, on the hypothesis that the

126 PHYSICS OF THE ELECTRON

charges are distributed in a continuous manner in space, a mass of
gas electrically neutral could furnish a limited quantity of electricity
of each kind, decreasing with the time by progressive recombination
if one delays the establishment of the electric field in the gas.

It is indeed necessary to admit, for the two electricities, a discon-
tinuous structure in order to allow their coexistence without com-
pletely neutralizing one another. The progressive recombination of
the charged particles or ions of two kinds would produce this neutral-
ization at the moment of their mutual collisions.

The phenomena of the saturation current, of the limited quantity
of free electricity in a gas, were obtained under conditions most favor-
able to experimental study, when, immediately after the discovery
of Roentgen rays and like radiations, one had recognized their property
of making the gas they traversed a conductor of electricity. The
limited charge which we can extract from a gas thus modified, the
velocity, finite and easily measured, with which they move under
the action of an electric field, their progressive recombination, are
interpreted in an admirable manner on the hypothesis that the radi-
ations, as well as the intense heat agitations in a flame, dissociate
a certain number of the molecules of the gas into electrified parts
carrying charges of opposite kinds.

(8) The Phenomena of Condensation. We know how the phe-
nomena of condensation of supersaturated water vapor in the pre-
sence of a conducting gas, already referred by R. von Helmholtz to
the presence of ions, has given the preceding hypothesis a brilliant
confirmation. As a result of the researches of J. J. Thomson, Town-
send, C. T. R. Wilson, and H. A. Wilson, these droplets of visible
water, each formed by condensation around an electrified centre,
bring forward a tangible witness to the existence of these centres, and
furnish a means of measuring the individual charge, present on each
drop of water formed, and equal to about 3.4X10~10 electrostatic
units of electricity according to the recent measurements of J. J.
Thomson and H. A. Wilson.

The fundamental idea in these kinds of measurements, applied
for the first time by Townsend to the charged drops which are pro-
duced in the presence of saturated water vapor in recently prepared
gases, consists in deducing the mass of each drop from its velocity
of fall under the action of gravity by means of Stokes's formula, which
gives the frictional resistance of a sphere moving through a viscous
medium, and which expresses the velocity of fall in terms of the
radius of the drop and consequently of its mass. We can obtain
from this the electric charge carried by each drop if we know the
ratio of this charge to the mass.

This ratio can be obtained, as was done by Townsend and J. J.
Thomson, by measuring or calculating the total mass of water carried

RELATIONS TO OTHER SCIENCES 127

by the droplets, considered as uniform, as well as the total quantity
of electricity carried by the ions which have served as centres for
the formation of the drops. The charge thus obtained by Townsend
was found to be 3X 10~10 electrostatic units for each centre in the
case of gases of electrolysis, and to 6.5 X 10~10 by J. J. Thomson
from the first series of measurement on gases ionized by Roentgen
rays.

H. A. Wilson obtained the ratio of charge to the mass of a drop
more simply by comparing the velocity of fall under the action of
gravity alone with the velocity of fall in a vertical electric field. He
obtained thus directly the ratio sought for. This method has the
advantage of showing that the electric charges are really carried by
the drops, and of separating those drops which carry a single ele-
mentary charge from those which, by diffusion of the ions toward
one another, carry a double or triple charge.

Wilson gives as the mean result of his measurements 3. IX 10~10,
a value very near to that of Townsend.

A second series of experiments by Professor J. J. Thomson, in
which he used radioactive substances as sources of ionization more
constant than the Crookes tube, and in which he took care to cause
the drops to form on all the ions present in the gas, by producing a
supersaturation of the water vapor by a rapid expansion of sufficient
magnitude to cause the condensation on the ions of both kinds, gave
as a mean result 3.4 X 10~10, a value in complete agreement with
the other two experimenters. The principles of thermodynamics
account perfectly for the influence of electrified centres on the con-
densation of water vapor : the electric charge of a drop in fact dimin-
ishes the pressure of water vapor in equilibrium with it. Moreover,
the least supersaturation found necessary, by C. T. R. Wilson, for
the formation of drops of water on the ions, which are the same what-
ever may be the means of producing them (Roentgen rays, Becquerel
rays, brush discharge, action of ultra-violet light on metal negatively
charged), allows us by purely thermodynamical reasoning to calcu-
late approximately the charge carried by each of the ions, and this
calculation, entirely distinct from direct measurement, gives in the
case of the positive centres a value of 4X 10~ 10 E. S. units.

(9) The Radiation Integral. More surprising still is the result
recently obtained by H. A. Lorentz, who succeeded in basing a pre-
cise measurement of the elementary charges carried by the electrified
centres present in metals on the experimental study of the radia-
tion integral or black body radiation.

We will see how the emission and absorption of heat- and light-
waves by matter are dependent on the presence in it of electrons
in motion. The ratio, for a radiation of given wave-length, between
the emissive and absorptive power, a ratio independent of the nature

128 PHYSICS OF THE ELECTRON

of the substance, represents the emissive power of the radiation
integral, which bolometric measurements give directly.

Now this ratio can be calculated, as Lorentz has shown, for wave-
lengths which are long in comparison with the mean path of free
electrons in the metal, as a function of the charge carried by each
of them. The comparison of these results with those of Kurlbaum
furnishes an entirely new method of obtaining this charge, and gives
3.7X1(T10E. S. units.

(10) The Kinetic Theory. Finally, the last confirmation, which
states more precisely still our knowledge of the electric atom, and
our confidence in this fundamental idea, Townsend, through com-
paring by the simple reasoning of the kinetic theory the velocities of
ions in a gas under the action of an electric field with their coefficient
of diffusion through the interior of the gas, two quantities directly
measurable by experiment, has been able to demonstrate the identity
of the charge of one of these gaseous ions with the electric atom of
Helmholtz, the charge of a monovalent atom in electrolysis.

From this comes directly a new confirmation of the values pre-
viously obtained, for it allows us to know, owing to Townsend's
results, the charge on an atom in electrolysis, and from it to deduce
immediately the constant of Avogadro, the number of molecules
contained in a given volume of a gas. The results are well in agree-
ment with the values of this constant (in general a little greater),
which we can directly deduce from the kinetic theory of gases.

Here is an important group of concordant indications, all of abso-
lutely distinct origin, which show without doubt the granular struc-
ture of electric charges, and consequently the atomic structure of
matter itself. The measurements which I have just enumerated
allow us to establish, in great security, the hypothesis of the exist-
ence of molecular masses.

I seek to point out here this extremely remarkable result, which
belongs without doubt to some fundamental property of the ether
and of the electrons, that all these electrified centres, whatever may
be their origin, are now identical from the point of view of the
charge which they carry.

It is necessary for us to penetrate further into their properties,
into their relations with material atoms, to determine their relative
sizes, in order to add among others to the more exact ideas which we
possess in this field, that the electrons, or negative cathode cor-
puscles, are all identical not only from the point of view of their
charge, but also from the point of view of their dynamic properties
and of their masses. We are unhappily not so well informed in
regard to the positive centres.

RELATIONS TO OTHER SCIENCES 129

III. Inertia and Radiation

(11) The Electromagnetic Wake.1 Before going farther it is im-
portant to point out what we can draw from the point of view to
which we have now come. Electrified centres, whose existence is
experimentally proven, whose charge we know in absolute units,
are movable with respect to a fixed ether defined according to the
equations of Hertz, without its having been necessary for us to have
recourse to dynamic principles to arrive at this point of view.

To what extent can the known properties of matter be deduced
from these two ideas of the electron and the ether, and is it necessary
to add something to them in order to build up a synthesis? We are
going to see rapidly and definitely from our idea of the electron,
how it is sufficient to represent at the same time the inertia of matter,
its dynamic properties, also how it can emit and absorb the radi-
ations which the ether transmits.

The possibility of conceiving of inertia, mass, not as a funda-
mental idea, but as a consequence of the laws of electromagnetism,
is a conception which owes its origin to an important memoir pub-
lished in 1881 by Professor J. J. Thomson.2 He studies there, basing
his assumptions on the existence of the displacement currents of
Maxwell, the electromagnetic field accompanying an electrified sphere
in motion. This motion implies a change in the electric field at a
point fixed with respect to the medium, and this displacement current
immediately produces a magnetic field according to the ideas of
Maxwell. The necessity of a convection current is pointed out later.
The magnetic field thus produced, identical with that of an element
of current parallel to the velocity of the moving charge, is propor-
tional at each point to that velocity, at least, if it does not approach
too nearly to that of light.

The creation of a magnetic field at the time of setting the charged
centre in motion implies an expenditure of energy, energy of self-
induction of the convection current, proportional to a first approxi-
mation to the square of the velocity, for those velocities which are
small compared to the velocity of light. It is thus an expression of
the same form as that of ordinary kinetic energy. A part, at least,
of the inertia of an electrified body, of its capacity for kinetic energy,
is thus a consequence of its electric charge.

Moreover, the magnetic field thus produced, and the electric field
as well, modified by the velocity as it approaches more nearly to that
of light, constitute around the electrified centre in translation a wake
which accompanies it in its translation through the ether without
change so long as the velocity remains constant. It is besides neces-

1 Le Sillage Electro-magnitique.

2 J. J. Thomson, Phil. Mag. t. 11, p. 229. 1881.

130 PHYSICS OF THE ELECTRON

sary that an external action should intervene in order to modify the
energy of this wake and consequently to increase or diminish the
velocity. This implies, in the absence of all other kinetic energy than
this of electromagnetic origin, corresponding to the production of
the wake, by the law of Galileo on the conservation of the velocity
acquired, in the absence of action of all external fields of force,
that an electrified centre possesses inertia by the fact alone that it is
electrified.

It is the immovable ether, the electromagnetic medium, which
serves as a fixed support for the axes with respect to which the prin-
ciple of inertia is applicable, and of which the ordinary mechanics
limits itself in affirming the existence by saying: there exists a sys-
tem of axes, determined by a nearly uniform translation with respect
to which the principle of Galileo is exactly verified.

(12) The Absolute Motion. If we are able, from the actual point
of view, to conceive of the ether as supporting these Galilean axes,
it does not necessarily follow that the electromagnetic phenomena
enable us to arrive at this absolute motion. It seems, on the contrary,
so far, that static experiments, carried on in a material system by an
observer carried along with it with a uniform motion of translation,
do not allow, whatever may be the degree of accuracy of observa-
tion, the detection of a relative motion of the ether with respect to
matter.

Larmor, and more completely Lorentz, have shown that there
exist in the system actions of electromagnetic origin; it is possible
to establish in a complete manner a static correspondence (relating to
the positions of equilibrium or to the black fringes in optics) between
the system in motion and a system fixed with respect to the ether, by
means of a change of variables which preserves for the equations of
the medium for a moving system the exact form which they possess
for a system at rest.

The two systems differ from one another in that the moving system
is slightly contracted compared with the fixed system in the direction
of the resultant motion by an amount always very small, propor-
tional to the square of the ratio of the velocity of motion to the veloc-
ity of light. This contraction affects equally all the elements of the
moving system, i. e. the electrons themselves, if we admit with Lorentz
that the interior actions of these electrons are solely electromagnetic
actions or are modified in the same manner by the translation, — with
the result that observation cannot prove this contraction any more
than it can prove the general dragging of the ether. These elements
behave as though they belonged to a corresponding fixed system.
Thus is found an explanation of the negative results of experiments
undertaken to show the absolute motion of the earth, by Michelson
and Morley, Lord Rayleigh, Brace, Trouton, and Noble, if one admits

RELATIONS TO OTHER SCIENCES 131

that all the internal forces of matter are of electromagnetic origin,
and that the energy is entirely divided between the two fields, elec-
tric and magnetic.

We shall see, however, farther on that it Is difficult to eliminate
in this way all other forms of energy, all other forces, such as grav-
itation; and it would then be necessary to admit with Lorentz, in
order that the correspondence between the two systems should
actually subsist, that in the moved system the forces and masses of
different origins are modified exactly as the electromagnetic forces
and masses, an hypothesis too complicated and arbitrary in the
actual state of the question.

But this does not seem to be a necessary consequence: it appears
probable that these actions, foreign to electromagnetism, and necessary
at the interior of the electron in order to give stability and in order
to represent gravitation, and which are probably connected with
one another, do not intervene in a sensible manner in the negative
experiments referred to above, and that everything transpires as if
the electromagnetic forces alone played a role, alone existed.

We shall see farther on that perhaps experiments of another kind
than those referred to here, for example, some dynamic measure-
ments bringing in a relative motion of the system moved, or some
static experiments bringing in gravitation, would enable us to under-
stand the absolute motion, the axes bound to the ether, instead of
conceiving simply of their existence.

(13) Electromagnetic Inertia. The problem of the electromagnetic
wake accompanying an electrified sphere or ellipsoid in the ether
has been taken up since J. J. Thomson by Heaviside and Searle.

Max Abraham has shown their results to consist approximately
of a numerical factor when, instead of supposing the body to be a
conductor having a surface charge, we suppose its charge to have
a uniform volume distribution.

Among the more important results contained in this solution of
J. J. Thomson's problem, I will point out these: that in the case of a
conducting sphere, the charge remains uniformly distributed on the
surface whatever may be the velocity, and that in all cases the electric
field at a distance tends to become more and more concentrated in the
equatorial plane with respect to the direction of the velocity in pro-
portion as this velocity approaches that of light.

Moreover the kinetic energy which it is necessary to expend at the
moment of putting it in motion in order to create the electromagnetic
wake ceases to be proportional to the square of the velocity, and
increases indefinitely as the velocity approaches the velocity of light-
waves; the law of the increase of this kinetic energy with the velocity,
the energy of self-induction of the current to which the charged body
in motion is equal, may be easily deduced by Searle's solution.

132 PHYSICS OF THE ELECTRON

Without any other hypothesis than that of its electric charge,
the electron is found to have inertia defined as capacity for kinetic
energy, but with a particular law of variation of this as a function of
the velocity, and this inertia appears to approach infinity as the
velocity approaches that of light.

The behavior of this law depends very little on the hypothesis
made as to the form of the electron and the distribution of the electric
charge which it carries. In all cases it is found to be impossible to
give the electron a velocity equal to that of light, at least permanently.

Instead of considering with Max Abraham the electron to be spher-
ical at all velocities, Lorentz admits it to be spherical when at rest
and to have a uniform distribution of charge; but if all internal forces
are solely electromagnetic or act as such, we have the view that the
electron is flattened in the direction of motion by a quantity propor-

/ v\

tional to the square of the ratio I /?=. of its velocity to that of

light, becoming an ellipsoid of revolution, the equatorial diameter
remaining equal to that of the original. This leads, as we shall see, to
a law of inertia different from that of an invariable sphere.

We shall likewise see that it does not appear to be necessary to
assign to the electrons, the negative ones at least, any other inertia
than this in order to account for the dynamic properties of the cathode
rays; however, experiments are not yet sufficiently exact to allow us
to infer the form of the electron itself, which depends on the law of
the variation of the kinetic energy with the velocity.

(14) Two Problems. We have examined, so far, only the case of
an electron in uniform motion in the absence of any external electro-
magnetic field capable of modifying the motion of the electron by
giving it an acceleration.

The general problem of the connection between the ether and the
electron, which probably represents the most important of the con-
nections between ether and matter, is double.

In the first place, what is the electromagnetic disturbance in the
ether accompanying any given motion of the electrons whatsoever?

In the second place, what motions would free electrons have if dis-
placed in an external magnetic field superimposed on that which
constitutes their wake?

(15) The Velocity Wave — The Acceleration Wave. We actually
possess all the elements necessary for the solution of the first pro-
blem, in which the motion is uniform in a particular case. Lorentz
has given in a very simple form the general solution by the use of
a delayed potential.

Each element of the charge in motion is determined by its position,
its velocity, and its acceleration at the time T, the electric and mag-
netic fields at the time T + t, on a sphere having for its centre the

RELATIONS TO OTHER SCIENCES 133

position at the time T and for radius the path passed over by light
during the time t.

Lorentz has given in this way the expressions for the two electric
and vector potentials from which the fields can be deduced by the
well-known formula. The complete expressions for these fields have
been given for the first time, I believe, by Lenard ; I obtained them
independently at the same time as Schwartzschild by putting them
in the following form. ,

The expressions for the two fields consist of two parts: the first
depends solely on the velocity of the element at the time T and
contributes to form the wake (sillage) which accompanies the elec-
tron in its motion; I shall call this the velocity wave. This velocity
wave, which exists only in the case of uniform motion, has its elec-
tric field always directed toward the position which the element of
charge will occupy at the time T + t, if it had retained from the
time T the velocity which it had at that moment. Schwartzschild
calls this position the point of aberration. It coincides with the
true position of the moving element at time T if the motion has
been uniform. The other part of the two fields is proportional to
the acceleration projected on the direction of propagation, and the
directions of the two fields are there perpendicular to one another,
and perpendicular to the radius, at the same time the two electric and
magnetic fields represent equal energies per unit volume; they have
all the characteristics of a radiation which is freely propagated in
the ether. I shall call this part the acceleration wave. Moreover, the
intensities of the fields in this case vary inversely as the distance
from the centre of emission, the energy represented by this wave
does not tend toward zero as the time T increases indefinitely ;
there is thus energy radiated to infinity by the acceleration wave.

The velocity wave, on the contrary, in which the fields vary
inversely as the square of the radius Vt, does not carry any energy
to infinity: the energy of the velocity wave accompanies the electron
in its motion and corresponds to its kinetic energy.

(16) Radiation implies Acceleration. We can conclude from this
that when an electrified centre experiences an acceleration, and only
then, it radiates to infinity in the form of a transverse wave, electro-
magnetic radiation, a definite quantity of energy, proportional per
unit of time to the square of the acceleration.

The origin of electromagnetic radiation, of all radiation, is, then,
in the electron undergoing acceleration. It is through the electron
that matter acts as the source of Hertzian or light waves. All
acceleration, all change which takes place in the state of motion of
electrons, result in the emission of waves. The character of the emitted
waves changes naturally according as the acceleration is abrupt, dis-
continuous, or periodic.

134 PHYSICS OF THE ELECTRON

In the first case, realized, for example, in the sudden stopping of the
negative electrons, or corpuscles, by the anti-cathode, the radiation
consists of an abrupt pulse whose thickness is equal to the product
of the velocity of light into the time taken to stop them, and which
gives us a good representation of the Roentgen rays or of the rays
from radioactive substances.

If the acceleration is periodic, on the contrary, as in the case when
the electron revolves around an electrified centre of opposite sign
to itself, the acceleration is periodic, and the radiation emitted con-
stitutes a light-wave whose length is determined by the period of
revolution of the electron.

The solution of the first of the two fundamental problems thus
appears complete and raises no difficulty.

IV. Dynamics of the Electron

(17) Maxwell's Idea. The inverse problem is less simple. It
consists in finding the motion, the acceleration which a movable
electron experiences in electric or magnetic fields of given intens-
ities; it is, properly so to speak, the problem of the dynamics of the
electron.

The equations which solve this problem ought to consist, like
the equations of ordinary dynamics, of two kinds of terms: one of
these dependent on the external fields, which produce their actions
on the electron, and are analogous to the external forces in dynam-
ics; the other, representing forces dependent on the motion itself,
and producing a resistance to motion, similar to the forces of in-
ertia.

The terms corresponding to external actions, the forces, have been
obtained by Lorentz following a method which was the natural con-
tinuation of Maxwell's idea as to the possibility of a mechanical
explanation, otherwise indeterminate, by the facts of electromag-
netism. The analogy to the equations of electrodynamic induction,
and to the equations of Lagrange, appeared to justify such an ex-
planation, and it was natural to continue to look upon the ether-
electron system as a mechanical system, and to apply to the motions
of electrified centres Lagrange's equations, deducing thus the forces
exerted on the electrons by its electric and magnetic energies con-
sidered as corresponding to the potential and kinetic energies of a
mechanical system, substituted in the ether. We are thus led to
apply to the medium, ether, in consideration of the fundamental
notions of force and mass, which they imply, the equations of ma-
terial dynamics, deduced from principles founded on observations of
matter only, always taken in mass and without an appreciable amount
of radiation.

RELATIONS TO OTHER SCIENCES 135

(18) Ether in Matter. We extend thus, by a bold deduction, these
principles to a region for which they have not been designed, and
thus admit implicitly the possibility of a material representation of
the ether. However, as I have already pointed out, an attempt at
such a representation raises many difficulties, and the efforts so far
made to extend these principles in a more precise manner have not
been successful. The most profound attempt, that of Lord Kelvin,
the gyrostatic ether, lends itself rigorously only to the represent-
ation of the propagation of periodic disturbances in the ether, but
makes impossible the existence of a permanent deformation, neces-
sary, however, for the representation of a constant electrostatic
field. The gyrostats would turn back again at the end of a finite
time, and the system would cease to react against a deformation
which has been imposed. Moreover, it would appear impossible to
include in this conception the permanent existence of electrons,
centres of deformation in the medium.

To get around this difficulty, Larmor had occasion, in the material
image which he proposed for the ether, to superimpose on the gyro-
static system of Lord Kelvin the properties of a perfect fluid, of
which the displacements representing the magnetic field should be at
each instant irrotational in order not to produce an electric field by
the rotation of the gyrostats present in the medium. But a great
difficulty is added to the preceding : if the motion of a fluid satisfies
at every moment the condition of being irrotational for infinitely
small displacements, it is not so for finite displacements, and a
magnetic field could not continue to exist without giving rise to an
electric field.

I believe it impossible to overcome these difficulties and to give
a material image of the ether, whose properties are entirely distinct,
and probably much more simple than those of matter.

(19) Action and Reaction. Let us, however, retain this view in order
that we may meet new difficulties. By means of Lagrange's equations
Lorentz obtains two external forces acting on each electron in motion,
two terms representing the action of the electromagnetic field.

One force is parallel to the electrostatic field; it is the ordinary
electric force, due to the superposition of the electric field produced
by the electron on the external electric field: the other is perpendicu-
lar to the direction of the velocity of the electron and of the external
magnetic field; it is the electromagnetic force analogous to the force
of Laplace exerted by a magnetic field on an element of current, and
due to the superposition on the external magnetic field of the magnetic
field produced by the electron during its motion. This double result
includes all the elementary laws of electromagnetism and of electro-
dynamics, if we consider the current in ordinary conductors as due to
the displacement of electrified particles.

136 PHYSICS OF THE ELECTRON

We easily see that the forces thus obtained, exerted on the electrons
by the ether, i. e. on the matter which contains them, do not satisfy
the principle of the equality of action and reaction, if we consider all
the forces which act at the same moment on all the electrons con-
stituting matter. In the case of a body which radiates in an unsym-
metrical manner, a recoil, an acceleration, is produced which is not
compensated at the same moment by an acceleration set up in
another portion of the matter. Later, at the time that the emitted
radiation meets an obstacle, the compensation is made (but only in
a partial manner if all the radiation is not absorbed) by means of
the pressure which the radiation exerts on the body which receives it;
a pressure whose existence is shown by experiment.

The equality of action and reaction has never been verified in
similar cases, and it adds no difficulty to this subject if we do not
seek to extend the principle beyond the facts which suggested it.

(20) Quantity of Electromagnetic Motion. If we could nevertheless
realize this extension of the principle, an extension somewhat arbi-
trary, we should be led not only to apply this principle to matter,
but to suppose the ether to have a quantity of motion which would
be that of a material system to which we compare it.

Poincare has shown that this quantity of electromagnetic motion
ought to be, at every point in the ether, in direction and in magni-
tude, proportional to Poynting's vector, which gives at the same
time a definition of the energy transmitted through the medium.

By starting with this idea of the quantity of electromagnetic
motion, Max Abraham has been able to calculate the terms, put to
one side by Lorentz, which depend on the motion of the electron
itself, its force of inertia, by the variation of the quantity of electro-
magnetic motion contained in its train. He was led for the first time,
by the form of the terms which represent this force of inertia, to the
notion of an unsymmetrical mass as a function of the velocity.

(21) Quasi-Stationary Motion. The calculation can be completely
made only in the case, always realizable from the experimental point
of view, where the acceleration of the electron is so small that its
train can be considered at each instant as identical with that of an
electron having the actual velocity, but whose motion has been
uniform for a long time. This is what Abraham calls a quasi-station-
ary motion. In this case, the train is entirely determined at each
moment by the actual velocity of the electron, also the quantity of
electromagnetic motion which it contains, and consequently the
variation of this quantity which represents the force of inertia. The
condition of quasi-stationary motion is simply that in the neighbor-
hood of the electron, where the quantity of electromagnetic motion
is localized, the wave of acceleration may be neglected in compari-
son with the velocity wave.

RELATIONS TO OTHER SCIENCES 137

(22) Longitudinal Mass and Transverse Mass. We find under these
conditions that the force of inertia is proportional to the acceleration
with a coefficient of proportionality analogous to mass, but which is
here a function of the velocity, and increases indefinitely, like the
kinetic energy, as the velocity tends to approach that of light.
Moreover, this electromagnetic mass differs for the same velocity,
according as the acceleration is parallel or perpendicular to the
direction of the velocity. There is, corresponding to the direction,
a longitudinal and a transverse mass. Mass is then no longer a
scalar quantity, but has the symmetry of a tensor parallel to the
velocity. No experimental fact yet allows us to verify this dis-
symmetry of the mass of the electrons, which becomes evident only
when the velocity is of the same order as that of light, but the vari-
ation of the transverse mass with the velocity has been proven by
Kaufmann for the ft rays of radium, which consist of particles
identical with the cathode rays. It is sufficient to compare the
deviations of these rays in the electric and magnetic fields perpen-
dicular to their direction in order to deduce, by application of
the equations of the dynamics of the electron, their velocity and the
ratio of the charge to the transverse mass of the particles which
compose them. This ratio decreases as the velocity increases, and,
if we consider as fundamental the principle of the conservation of
electricity, we conclude from it an actual increase of the transverse
mass according to a law easy to compare with that which the theory
gives for the electromagnetic mass.

(23) Matter of the Philosophers. But before discussing the result of
this comparison, I wish to point out a logical difficulty raised by the
course which we have followed: we are accustomed to consider as
fundamental the ideas of mass and force, built up in order to repre-
sent the laws of motion of matter; we, a priori, conceive of mass as
a perfectly invariable scalar quantity.

Now, let us suppose the possibility of a material representation of
the ether: we apply to it the equations of material dynamics, and
we are led to admit for the electrons, which form a part of matter,
and consequently for matter itself, a dissymmetrical mass, tensorial
and variable.

To what, then, should the equations of ordinary dynamics apply,
and what are the ideas considered as fundamental which they imply?
To an abstract matter, the matter of the philosophers, which could not
be ordinary matter, since it is inseparable from electric charges, and
which is probably made up of an agglomeration of electrons in periodic
motion, stable under their mutual actions? Or to the ether? But
we have no idea of what can be its mass or motion.

It is, indeed, rather the ether which it is necessary to consider as fun-
damental, and it is then natural to define it initially by those proper-

138 PHYSICS OF THE ELECTRON

ties of it which we know, that is to say, by the electric and magnetic
fields, which it is possible to arrive at, as I have already remarked,
without admitting at any time the laws of dynamics, the ideas of
mass and force under their ordinary form. We will find this last to be
a derived and secondary idea.

V. Electromagnetic Dynamics

(24) Change of Point of View. It seems thus much more natural to
reverse the conception of Maxwell and to consider the analogy which
he has pointed out between the equations of electromagnetism and
those of dynamics under Lagrange's form as justifying much more
the possibility of an electromagnetic representation of the principles
and ideas of ordinary, material mechanics, than the inverse possi-
bility.

It is necessary then for us to solve our second problem, that of
the dynamics of the electron, of its motion in a given external field,
without having recourse to the principles of mechanics, by purely
electromagnetic considerations.

Hertz's equations, which permit a solution of the first problem,
are here not sufficient, and we have need of a more general principle,
which assumes not the motion of the electrons given, but that
determines it.

(25) The Law of Stationary Energy. We will use this principle
under a form indicated by Larmor, and which we can look upon as
a generalization of the known laws of electrostatics and of electro-
dynamics. We know that the distribution of electric charges and
electric fields in a system of electrified bodies is always such that the
electrostatic energy We, contained in the medium modified by the
field, is a minimum. The analogous principle holds for the magnetic
field produced by currents of given intensities. The energy Wm local-
ized in the magnetic field is less for the real distribution of it than for
all other distributions satisfying the condition that the integral around
a closed line is equal to 4^ times the intensities of the currents in-
closed by the line.

If displacements are possible, the conductors maintained at con-
stant potential are in stable equilibrium if the electrostatic energy
is a maximum, and the currents of given intensities are likewise in
stable equilibrium if the energy of their magnetic field is a max-
imum. In all cases of maxima and minima, an infinitely small mod-
ification of the system from the configuration of equilibrium produces
a zero variation in the energy: it is stationary.

(26) General Principle. When, instead of remaining permanent,
the state of the system is variable, and if there are represented neces-
sarily at the same time the two kinds of fields, we seek to find how,

RELATIONS TO OTHER SCIENCES 139

as in the permanent case, an expression which remains stationary.
that is to say, the variation of which is zero when supposed slightly
modified, can start from its real state. We are thus led to replace the
energies We, Wm) which play this role in the permanent case, by an
integral taken with respect to the time, and which represents not the
sum of the energies, since this quantity, equal to the total energy,
ought to remain constant if only electromagnetic action come in,
but their difference :

an integral which remains stationary for all virtual modifications
of the system, such modifications being subject to the condition of
disappearing at the limits t0 and ^ of the integral, exactly as in the
analogous principle of Hamilton in mechanics. The principle of zero
variation just announced, and which we will consider as the result of
an induction based entirely on electromagnetic principles, allows
us in fact to find three of Hertz' s equations, if we admit the three
others as an imposed interconnection of the system, and furnishes in
the most simple manner the solution which we have obtained for the
first problem by means of these equations. Moreover, the motion of
the electrons supposed given only at the times t0 ^ comes into the
integral, and the condition that this must be stationary allows us
to find the law of the motion during the interval, by starting from
a principle whose signification is purely electromagnetic. We obtain
thus exactly the results of Max Abraham; the equations of motion
contain terms which depend first on the motion of the electron, and
are proportional, in the hypothesis of quasi-stationary motion, to
its acceleration, having coefficients that are functions of the velocity
which we will call the longitudinal and transverse masses of the
electron; also some terms depending on the charge, and on the ex-
ternal fields, which we will call the forces, and we find that they coin-
cide with those given by Lorentz. The external motion of the electron
is thus determined by the actual electromagnetic state of the system.

(27) The Process in the Electron. In order to simplify the analysis
and to avoid considering the motion of rotation of the electron, I will
consider it as a cavity in the ether; the volume integrals which express
the energies We, Wm of the electric and magnetic fields extend only
over the space external to the surface which bounds the cavity. We
can suppose as a special condition outside of the electric charge that
the form of this surface is fixed, spherical for example, due to an
unknown action of nature, and we find the equations of Abraham
for the longitudinal and transverse masses of a spherical electron.

But we can suppose a more simple condition, implying only a fixed
volume of the cavity on account of the incompressibility of the ex-

140 PHYSICS OF THE ELECTRON

ternal ether; if we seek, then, what is, in the case of uniform trans-
lation, the form that the electron would spontaneously take in order
to satisfy the condition of zero variation, we find precisely the oblate
ellipsoidal form assumed by Lorentz, with this difference, that the
equatorial diameter increases with the velocity instead of remaining
constant, as Lorentz considers it; this constancy implies a diminution
of the volume as the velocity increases. The equations which express
in this case the variation of the longitudinal and transverse mass
with the velocity are different from those of Abraham and Lorentz,
although giving always an indefinite increase of the two masses as
the velocity approaches that of light.

1YL

The equations thus obtained for the ratio — of the transverse mass

ra0

m, the only one so far accessible to experiment, to the mass m0 for

v
very small velocities, as a function of the ratio fi—-y of the velocity

of the electron to that of light are :
(1) Invariable spherical electron,

m 3 3

?7i L i/

C Equatorial diameter constant — = (1 — /92)

m0

(2) Variable Electron -j

Volume constant — = (1 — /?2)

W^ * /

v 0

(28) Comparison. The researches of Kaufmann are not yet exact
enough to determine which of these equations represents most nearly
the experimental variation of the ratio — with the velocity. In
order to make the comparison, I have used a process similar to that
of Kaufmann, who eliminated the two electric and magnetic fields used
to deviate the /? rays, seeking to obtain the best concordance pos-
sible between the experimental variation of ^ and the theoretical
variation calculated on the hypothesis that the mass is entirely
electromagnetic.

In order to make this elimination, I draw the two experimental and
theoretical curves representing ^ as a function of /?, on logarithmic
coordinates, and seek for what relative positions of the curves we ob-
tain the best correspondence. The results are given for the three
theoretical equations and the same series of experimental values.
The experimental points corresponding to four different series are
given by Kaufmann, and we see that they correspond equally well
with the three theoretical curves.

OSf

0.9
0.32

0.3S
018

log + 1 ?!

1*" * .o

s.

142 PHYSICS OF THE ELECTRON

The more important values from the point of view of choice of
equations are those corresponding to values of the velocity very near
to that of light, and which amounted to ninety-five per cent of it in
Kaufmann's experiments. But the /? rays are then very little devi-
ated, and exact measurements are extremely difficult.

It would be extremely important to determine the longitudinal
mass by the use of an intense electric field parallel to the velocity of
the electron, furnishing to it a known energy and producing a variation
of the velocity, which if measured would give the longitudinal mass.

(29) Matter and Electrons. But if the accuracy of experiment is
not sufficient to determine completely the law, the agreement with
the equations, obtained by supposing the mass to be entirely electro-
magnetic, is so good that we can reasonably conclude that cathode
particles constituting the /? rays have no mass other than that due
to their electric charges or the train which they carry with them in
their motion through the ether.

It is interesting to extend the same result to ordinary matter by
conceiving it as made up of an aggregation of electrons of both
signs; it is unreasonable on the other hand to apply to two phe-
nomena so nearly identical as inertia of ordinary matter and that
of the cathode particles, two entirely distinct explanations, of which
the one, the electromagnetic explanation, is definite and confirmed
by experiment, while the other remains entirely unknown.

The inertia of a similar aggregation of electrons should be equal to
the sum of the partial inertias because of the great distance of the
electrified centres from one another compared to their radii, which
one can calculate by supposing all their inertia electromagnetic.

In these conditions, the trains of the different electrons do not
interfere appreciably, and we find thus the law of the conservation
of inertia as a consequence of the conservation of the electrons in the
transformations to which matter is subject. But the theory is not
incompatible, on account of the interference of trains, with a slight
disagreement between the inertia of an assemblage and the sum of
the partial inertias.

The complexity of the atomic system to which we are led, each
atom of the molecule containing probably a very great number of
electrons, seems also to be a necessary consequence of the com-
plexity of the luminous spectrum sent out from the atoms, by the
electrons which they contain, when an external disturbance displaces
the system from its state of stable periodic motion. In such a state
the radiations emitted by the various electrons on .account of the
acceleration which keeps them in their intermolecular orbits com-
pensate one another almost completely from the point of view of
energy radiated ; so that there is in general no decay of the periodic
intermolecular motion.

RELATIONS TO OTHER SCIENCES 143

This conception, this electronic theory of matter in which matter
becomes, at least partially, synonymous with electricity in motion,
appears to account for an enormous number of facts, which increase
constantly under the efforts of physicists impatient to contemplate
in a less primitive form the synthesis which it promises to bring
forward.

(30) Stability of the Electron. The fundamental conception, that
of the electron, does not go without raising difficulties still further,
besides the impossibility already pointed out of representing to our-
selves by material images its displacement with respect to the ether.
It seems necessary to admit something else in its structure than its
electric charge, an action which maintains the unity of the electron
and prevents its charge from being dissipated by the mutual repul-
sions of the elements which constitute it. The form of the electron
is determined by some relation which insures its stability, the con-
dition of incompressibility of the medium being insufficient, since
the spherical form corresponds only to unstable equilibrium for an
electrified body of given volume in which no force opposes the deform-
ation.

This condition, which belongs to some fundamental property of
the medium, determining the charge carried by the electrons, all
identical from this point of view, is perhaps closely connected with
the third mode of activity of the ether, a third form of energy, the
gravitational form, of which our principle of stationary energy ought
to take account by the addition of terms to those expressing the
electrostatic energy, but of infinitely smaller magnitude.

(31) Gravitation. Gravitation remains obstinately outside of our
electromagnetic synthesis; the Newtonian forces not only do not
appear to be propagated with the velocity of light, but also it seems
difficult to found them on electromagnetism without modifying
profoundly our fundamental ideas in regard to field and quantity
of electricity and the possibility of an attraction of one aggregation of
neutral electrons for another aggregation of the same nature.

It appears probable that gravitation results from a mode of activity
of the ether and a property of electrons entirely different from the
electromagnetic mode, and we must admit besides electric and mag-
netic energies, a third distinct form, that of gravitation.

It remains to understand how it is possible, and what is the sig-
nificance of the equivalence, the passage of this third form into one
of the first two. Also we are no more capable of understanding, out-
side of the formal equations which express it, the connection
between the electric and magnetic energies themselves and their
transformations, the one into the other, by means of the electrons.

(32) An Experiment Necessary. It does not seem impossible to
connect the forces of cohesion with electromagnetism, especially

144 PHYSICS OF THE ELECTRON

from the point of view of the mutual attractions which orientation
causes in the constitution of crystalline media, on account of the
complex electric and magnetic fields which surround a system of
electrons in its immediate vicinity.

Gravitational forces alone remain distinct, superimposed on the
electromagnetic forces, and no difficulty comes from this on account
of the negative results of the experiments undertaken to show the
absolute motion of the earth.

The negative results can be explained, as we shall see, if all the
internal forces of matter are of electromagnetic origin; but gravi-
tational force, alone different, can be superimposed on them without
introducing an appreciable modification of this result, for its in-
tensity is extraordinarily small compared to electromagnetic actions,
even if there is no mutual compensation between them, and in all
the experiments in question, interference of light or equilibrium of
an elastic system, the gravitational forces play no appreciable role.

It would be, indeed, important to obtain a condition in a case of
equilibrium where the forces of gravity would play an important
part, and if the equilibrium remains independent of the total motion
to nearly the second order, if we could only observe the mutual
motion to this order of precision, it would be necessary to conclude
that the forces of gravitation also are modified by motion of trans-
lation in the same manner as the electromagnetic forces, since the
equilibrium between the two kinds of forces is not disturbed, and this
would be an important indication of the necessity of an electro-
magnetic representation of gravitation. We would be able, for
example, if the sensibility allowed it, to perform the experiment of
Trouton and Noble by suspending the condenser with a bifilar to
the pan of a balance instead of by an elastic fibre.

Since this test has not been made, since experiments designed to
show the absolute motion have not involved weight, it would be
more reasonable to consider gravitation as a force distinct from
electromagnetic action, which acts at the interior of the electrons
in order to insure their stability, without its being possible actually to
imagine in what manner we can seek a more profound knowledge of
the ether and of the electrons which it incloses.

It does not seem, in any manner and for many reasons, that this
can be of the nature of a material and mechanical representation of
the ether.

VI. Cathode Rays

(33) The Ratio e/m. Before examining the consequences involved in
the electronic conception of matter, I should like to examine a few
points relative to the electrons of two kinds. Those which we know
the better, the more intimately, are the negative electrons, which

RELATIONS TO OTHER SCIENCES 145

are always identical with one another in all their properties, what-
ever may be the matter which has furnished them. We have already
seen how the direct measurement of the charge leads always to the
same result, The mass, both the longitudinal and transverse mass,
having the same value for small velocities, can be determined by the
measurement of the ratio of the charge to the mass.

The results obtained for this ratio in the case of cathode rays
show some quite marked divergences when different methods of
measurement are employed. The first values were given by J. J.
Thomson by combining the magnetic deviation of the rays with a
. measurement of the energy which they possess by means of the heat
produced in a thermoelectric couple which receives them, or by
combining this magnetic deviation with the deviation in an electro-
static field. The ratio ^ furnished by this second method, the more
accurate of the two, is approximately 107 electromagnetic units
C.G.S.

Another method first pointed out by Schuster was used successively
by Kaufmann and Simon. It consists in combining the magnetic
deviation with the measurement of the difference of potential under
which the rays are produced, considering that this difference of po-
tential is that which exists between the cathode and anode. This
hypothesis admitted, the method is capable of great accuracy, and
the results which it gives appear to agree with the limiting values, for
small velocities of the ratio ^ for the /? rays, although the method
employed by Kaufmann in this last measurement is different from
that of Schuster. The number obtained by Simon is 1.865X107,
nearly double that of J. J. Thomson. The explanation proposed by
the latter for this disagreement, according to which the cathode
rays are not produced by the total difference of potential between the
cathode and the anode, but originate in a region situated in front of
the cathode, does not, however, appear satisfactory, since it does not
account for the constancy of the results of Kaufmann and Simon
when the conditions of the experiment, the difference of potential
in particular, were varied between large limits.

A means of deciding the question would consist in performing a
type of experiment already used by Lenard, by subjecting the cathode
rays, after their production, to a supplementary and known fall of
potential, and determining by the modification which would result
in their magnetic deviation the initial fall of potential under which
they had been produced.

(34) The Cathode Corpuscle. However it may be, we can, owing
to the results of Kaufmann, affirm the identity of the cathode rays
already found independent of the gas and the electrode contained
in the Crookes tube, with the /? rays of radium. The measurements

146 PHYSICS OF THE ELECTRON

by J. J. Thomson and Lenard of the negative charges emitted by
a negatively charged metallic surface under the action of light and of
those spontaneously emitted by incandescent bodies also show an
identity with the cathode rays. Wehnelt has recently shown that the
oxides of the alkaline earths possess in an extraordinary degree this
property of spontaneously emitting cathode rays at high tempera-
tures, and furnishes a means of performing, on this particular kind
of rays, simple and exact measurements.

Finally, we know that the magnitude of the Zeeman effect, in the
case where the spectrum lines considered present the appearance of
a normal triplet, leads to the conclusion that the light corresponding
to these lines is emitted by negatively electrified centres, present in
matter and having the same ratio ^ as the cathode rays.

Moreover, the magnitude of this ratio, one thousand to two thou-
sand times greater than for the hydrogen atom in electrolysis, leads
us, as a consequence of the identity of charges established by Town-
send, to consider the mass of the cathode corpuscle as one thousand
times smaller at least than an atom of hydrogen; a result in perfect
agreement with the conception which makes material atoms an agglom-
eration of electrons of two kinds. On the hypothesis that the mass
is entirely of electromagnetic origin, the knowledge of the ratio ^
gives for the electron a sufficiently small radius (10~13 centimeters
about) in order to be, conformably to our conception also, negligible
in comparison with atomic dimensions.

(35) Flames, The small mass of the cathode corpuscle, and the
possibility of separating from matter electrified centres a thousand
times smaller than the smallest atom, is confirmed by the mobility
of the negative ions in flames. We obtain enormous mobility com-
pared to that observed in gases at ordinary temperatures, and the
methods of the kinetic theory of gases permits us to calculate, by
means of this experimental mobility, that the movable negative
centres in flames have a mass about a thousand times smaller than
the hydrogen atom, and should consequently be identical with the
cathode corpuscles. At ordinary temperatures the negative ions are
less mobile because the cathode corpuscles surround themselves with
neutral molecules by simple electrostatic attraction, and form an
agglomeration which the feeble agitation allows to remain stable.

VII. Positive Electrons — a Rays

(36) Goldstein Rays, a Rays. Our knowledge of the structure of
positive charges is much less advanced than for the negative. Two
important cases show us the existence of positively charged particles,
besides the positive ions in conducting gases, which at ordinary tem-
peratures consist of an agglomeration of neutral molecules around

RELATIONS TO OTHER SCIENCES 147

a charged centre: these are the Kanalstrahlen of Goldstein, an efflux
of positive charges toward the cathode, the electric and magnetic
deviations of which lead to values for the ratio of ^ varying between
wide limits, but always several thousand times smaller than for the
cathode rays. The mass of these positive centres is of the order of that
of the atoms. The a rays of radioactive bodies, easily absorbed, and
particularly easy to observe in the case of polonium and the active
bismuth of Marckwald, appear to be, in fact, Kanalstrahlen. The
mass of the positively charged particles which constitute these rays is
of the same order as that of the hydrogen atom, and their velocity
does not exceed 20,000 to 25,000 kilometers per second, so that it is
impossible to verify whether their mass is entirely electromagnetic
or not. Can we consider them as electrons as simple as the nega-
tive corpuscle itself, or are they of much more complex structure;
are they, for example, atoms or molecules which have lost a cathode
corpuscle?

(37) Electrons or Atoms. On the first hypothesis, the great mass
of the positive centres would lead us to assign them dimensions much
smaller than the cathode corpuscles themselves, the electromagnetic
mass of an electrified sphere being inversely proportional to its radius.
One is thus led to the result that an electron possesses inertia, I will
not say weight, inversely proportional to its radius. H. A. Wilson
thinks to find an argument in favor of this conception of a very small
and consequently very inert positive electron in the observation
that the a rays are much less easily absorbed than the /? rays of the
same velocity.

Many other reasons lead us to adopt the contrary hypothesis that
an a particle is very complex and little different from an atom.
Rutherford has given serious reasons for identifying the a particle
with the helium atom deprived of a cathode corpuscle; also Stark
gives experimental reasons referring to the emission spectra of posi-
tive centres in vacuum tubes, which imply a complex structure.
Finally the theory of the disruptive discharge attributes the produc-
tion of cathode rays in part at least to the impact against the cathode
of particles which constitute the Goldstein rays; an electron smaller
than the cathode particle itself seems scarcely able to produce a sur-
face disturbance sufficiently intense, while on the other hand, an
atom, unable to penetrate another atomic structure, and projected
with a high velocity, would produce by its impact a considerable
local disturbance.

(38) The Positive charge of the a Rays. It is perhaps by this con-
siderable disturbance produced by the a or canal rays in matter
which they meet that one can explain the interesting fact that the
positive charge of the a rays has not been directly shown so far by
the negative charge which a polonium salt should spontaneously

148 PHYSICS OF THE ELECTRON

acquire if it emits only a rays. However high may be the vacuum
around a piece of radioactive bismuth, or polonium, it does not
acquire any charge, and loses rapidly, on the contrary, its positive or
negative charge. Possibly one might explain this discharge by the
ionizing action of the a rays on the gas, however rare. The passage
of a particles, projectiles of large dimensions, through the surface of
radioactive bodies from which they come, can play the same part
as the impact of Kanalstrahlen on the surface of the cathode, and
cause the emission of cathode rays of very little penetrating power,
whose presence would suffice, added to that of the a rays, to prevent
any permanent charge of the radioactive body, whatever may be its
sign.

(39) The Positive Electrons. If the positive centres, as we know,
ought not to be represented as free electrons, it seems, however,
necessary to admit the presence of probable electrons which cause the
neutralization of the negative charges in the atomic structure, but
which for some reason come out of this structure with extreme diffi-
culty, contrary to what is the case for the negative centres. More-
over, it would appear necessary in order that the theory of metals,
which ascribes their conductivity to the presence of free electrified
centres moving under the action of a field can take account of all the
facts, the Hall effect in particular, of variable sign in different metals,
that the centres of two kinds coexist in the metal, free to move about
in all directions. These positive centres do not appear to be the
metallic atoms themselves, necessarily immovable in order to main-
tain the solid framework of the metal. It is possible that the positive
electron, which no known action in a gas can maintain separate
from the atomic material, may be free in large numbers in the en-
tirely different medium which constitutes the metal. Many problems
present themselves here on the subject of the nature of the positive
charges.

VIII. Theory of Matter. Radioactivity

(40) Atomic Instability. Let us examine now a little more closely
the consequences to which we are led by the conception of matter
as made up of electrons of two signs, of atoms formed of electrified
bodies in motion under their mutual actions. From the first, — outside
of gravitation, whose intensity is infinitely small compared to the
electromagnetic forces in the interior of atoms which determine all
the physical and chemical changes of state, — the elementary laws of
action reduce to the forces of Lorentz, which allow us, as we have seen,
to calculate the acceleration to which an electron is subjected as
function of the electric and magnetic fields produced by the other
electrons at the point where the first electron is situated. In the case

RELATIONS TO OTHER SCIENCES 149

where the acceleration is sufficient for it to radiate an appreciable
energy to a distance by means of the acceleration wave, it is probably
necessary to bring in, by other terms in the equation of motion of the
electron, some forces by which it can receive again the energy which
it radiates, and which disappear in the case of quasi-stationary
motion. It does not seem, however, in any experimental case that
these corrective terms can become appreciable.

From the same point of view, the electrons in periodic motion in the
material atom are necessarily subject throughout their closed orbits
to accelerations which are accompanied by a radiation of energy
borrowed from the internal electric and magnetic energy of the atom.
This radiation must be extremely small, as in the simple case of sev-
eral cathode corpuscles circulating at equal distances in the same
orbit, and can be compensated for by energy obtained from external
radiation. We can suppose that this continual radiation, much more
important naturally when the atom, as the result of external shock,
is displaced from its most stable equilibrium, is a cause of decay to the
atomic structure and which at the end of a certain length of time
ought necessarily to give the structure a fundamental rearrange-
ment, as a top falls when its rotation has sufficiently diminished in
velocity. A condition of instability is thus reached, the consecutive
rearrangement being accompanied by a violent projection of certain
electrified centres from the atom. This conception furnishes at least
an image of radioactive phenomena, and the successive transforma-
tions in the life of an atom, an hypothesis of which has been advanced
by Rutherford. It seems, however, that it is not necessary to admit
a probable decay of atomic structures, sensible only for radioactive
substances. The fact that the dispersion takes place as a function of
the time according to a rigorous exponential law, the quantity which
is destroyed in a given time being exactly proportional to the quantity
present, seems to indicate that the substance not destroyed remains
identical with itself. Perhaps the reorganization of the atomic struc-
ture might result from its accidental passage through a particularly
unstable configuration, the probability that a like configuration
should be reproduced being independent, in the mean, of the previous
history of the atom, and the mean life of the latter would be short in
proportion as this probability is great.

(41) Internal Energy and Heat set Free. A very simple calculation
shows also that the stock of energy represented by the electric and
magnetic fields surrounding the electrons contained in an atom is
sufficiently great to supply for ten million years the evolution of heat
discovered by Curie in the radium salts. As it appears now well
established that the mean life of a radium atom is of the order of a
thousand years, it results that the ten-thousandth part only of this
reserve of energy is utilized during this especially active period in

150 PHYSICS OF THE ELECTRON

the life of the atom. There is then no difficulty in conceiving how the
enormous evolution of heat by radium can be ascribed to its internal
energy.

No atom being free from this loss of energy due to the radiation
of the electrons, one ought to expect on this hypothesis of decay a
universality of radioactive phenomena, the atoms which we con-
sider as actually stable suffering only an extraordinarily slow waste.

IX. Electric Properties

(42) Polarization. It remains now to show in a few words how
the preceding conceptions lend themselves easily to a representation
of the principal electric and magnetic properties of matter and make
possible for the first time a theory of the disruptive discharge and
of metallic conduction.

A common property of all forms of matter is electrostatic polariz-
ation arising from the variation of the specific inductive power with
the nature of the substance.

This polarization results in a manner quite natural by the modi-
fication which an external electric field produces in the motions of the
electron which constitute the atom. This modification is caused in
the mean by an excess of positive centres on the side where the field
tends to displace them and by an excess of negative centres on the
opposite side. The system takes then on the average an electrostatic
polarization.

(43) Corpuscular Dissociations. If the electric field becomes suf-
ficiently intense, as, for example, during the passage of one of those
brief pulsations which constitute the Roentgen rays, or during the
passage through the atomic structure of an a or /? particle of very
great velocity, the modification produced may be very great, a cathode
corpuscle may be separated from the structure which remains posi-
tively charged; there is produced thus a corpuscular dissociation
which explains the conductivity acquired by insulating mediums
under the action of Roentgen or Becquerel rays, and which manifests
itself especially in gases, where the electrified centres thus freed can
move more easily, although by electrostatic attraction on the neutral
molecules, electrically polarizable, they surround themselves with
a group of molecules which accompany them during their motion.

It seems well established that the negative ions in particular, also
produced in a gas, have a cathode corpuscle for centre, since the pene-
tration of cathode rays into a gas produces in it negative ions identical
with those of Roentgen rays, at least from the point of view of their
mobility or of their power of condensing supersaturated water vapor.
It seems, nevertheless, important to make sure, by measuring the
mobility of ions produced by different causes in the interior of gases,

RELATIONS TO OTHER SCIENCES 151

whether the differences which appear to exist are real and are
caused by the difference in the molecules which adhere to them, or
are due to the electrified centres which serve as the nuclei for them.

(44) Mobility and Recombination. It is equally important to be
able, by measurement of mobility, to follow the modification which
a change of temperature produces in the size of the agglomeration
and to connect the ions observed at ordinary temperatures with
the incomparably more mobile ions which we observe in flames, and
which appear to be made up of single electrical centres, cathode cor-
puscles and perhaps a particles.

The rate of recombination of ions is as yet not well known in
respect to the variations with pressure and temperature, although
it certainly plays an essential part in the phenomena of disruptive
discharge through gases at low pressures; it would be desirable if
this point were better fixed.

(45) lonization by Impact. Every actual theory of the disrup-
tive discharge rests on the conception that the impact of an electri-
fied particle in sufficiently rapid motion against a molecule can cause
corpuscular dissociation.

This idea was a natural consequence of the known fact that cathode
and Becquerel rays, made up of similar particles, make a gas through
which they pass a conductor. If the corpuscular dissociation pro-
duces in the gas, separated from the molecule, a cathode corpuscle
and a positive residue, these fragments can, if a sufficiently intense
electric field exists in the gas, acquire a velocity great enough to act
as /? or a rays and cause from point to point a rapid increase in con-
ductivity.

Townsend has shown how this consequence is capable of exact
experimental verification, and he has found that between certain
limits of velocity, each impact between the cathode corpuscle and
a molecule results in a corpuscular dissociation of the same kind.
The velocity acquired ought not, however, to exceed a certain limit
beyond which the negative corpuscle or /? particle passes through
the atomic edifice without producing a sensible disturbance in it.

In order that a disruptive discharge may exist without an external
cause to maintain the production of the first electrified centres, it is
necessary that the positive centres should be able, like the negative,
although with more difficulty, to produce corpuscular dissociation
at the moment of their impact with the molecules, as this latter
causes the conductivity produced in gases by the a rays.

Townsend has been able, in support of this hypothesis, to deter-
mine the exact moment when the disruptive phenomenon is pro-
duced, and to analyze the mechanism of it.

In addition to this fundamental conception of ionization by impact,
the theory of the disruptive discharge has yet much progress to make.

152 PHYSICS OF THE ELECTRON

The extremely varied aspects which this discharge takes, the pro-
duction of striations, an explanation of which was first given by
J. J. Thomson, the influence of a magnetic field on the conditions of
the discharge, the phenomena that are produced when the electrodes
are only of the order of a micron apart, where the molecules do not
appear to take part in the production of the spark, are many of the
essential points which to-day attract attention.

(46) The Electric Arc. By the side of the ordinary disruptive
discharge, by brush or spark, the electric arc, with an entirely differ-
ent aspect, brings in the new phenomenon of the emission of cathode
corpuscles by the surface of incandescent bodies. This incandescence
of the electrode, of the cathode especially, is, in fact, characteristic of
the arc discharge; the cathode is raised to a sufficiently high tem-
perature by the impact of the positive ions which flow toward it,
so that the corpuscles present in the electrode, and which give it its
conductivity, experience a true evaporation and carry the greater
part of the current. In fact, a filament of incandescent carbon is able
to emit, at a much lower temperature than that of the voltaic arc,
cathode corpuscles representing a current density of two amperes
per square centimeter.

(47) Evaporation of the Cathode. This phenomenon, known under the
name of the Edison effect, is very general and has been connected in
a quantitative manner by Richardson on the fundamental hypothesis
of the kinetic theory with the presence of freely moving cathode
particles in the interior of conductors.

At ordinary temperatures this emission of corpuscles is diminished
to such an extent that electrostatics is possible and a metal can
keep a permanent charge. Every corpuscle present in the metal is
immersed in a medium of high specific inductive capacity, and a
finite amount of work is necessary to make them pass from this
medium to a region where the specific inductive capacity is equal
to unity. Only the corpuscles having a sufficient velocity would be
able to supply this work on leaving the conductor, and their num-
ber, absolutely negligible at ordinary temperatures, increases with
extreme rapidity with the rise in temperature. Richardson has
shown that the variation obtained by experiment agrees very well
with that predicted by theory.

(48) Metals. The spontaneous dissociation of atoms which the
kinetic theory implies, the separation of electrified centres free to
move in the interior of the metal, is a consequence of the high specific
inductive capacity of the medium, of the ease of electrostatic polar-
ization of metals, owing to the ease with which the metallic atoms
lose corpuscles in order to remain positively charged. The potential
energy of an electrified particle in such a medium is much smaller than
anywhere else, and conformably with the laws of the distribution of

RELATIONS TO OTHER SCIENCES 153

energy given by the kinetic theory, the free particles ought to be
more numerous in it.

(49) Chemical Phenomena. It is by an action of the same kind that
water, of great specific inductive capacity (smaller, however, than
that of metals) causes the electrolytic dissociation of salts that are
dissolved in it; it would be of great interest to determine the relation
between this electrolytic dissociation, especially of liquid conductors,
and the corpuscular dissociation common probably to gases and
metals.

In electrolytic dissociation, the cathode corpuscles lost by the
metallic atoms, instead of remaining free as in corpuscular dissocia-
tion, remain united to an atom or to a radical to form the negative
ion in electrolytes. This question touches the relations between our
actual ideas and chemistry, relations still very obscure, and which it
would be very important to clear up. The electric dissociation pro-
duced in gases by Roentgen rays does not appear connected with
any chemical modification; however, in air all intense ionization is
accompanied by the formation of ozone. Here is a domain almost
entirely unexplored.

X. Magnetic Properties

(50) Ampere and Weber. However, the complex phenomena of
magnetism and diamagnetism have seemed so far to lead us to ex-
pect more difficulties, although the electrons gravitating in the atom
in closed orbits furnish at first sight a simple representation of the
molecular currents of Ampere, capable of turning under the action
of an external magnetic field in order to give birth to induced
magnetism, or of reacting by induction, according to the idea of
Weber, against the external field so as to make the substance dia-
magnetic.

Those who have tried to follow out this idea have found it so far
sterile; independently, different physicists have come to the conclu-
sion that the hypothesis of electrons in undiminished motion cannot
furnish a representation of the permanent phenomena of magnetism
or diamagnetism.

I am enough of a parvenu to attempt to show, contrary to the
preceding opinion, that it is possible to give, by means of the electrons,
an exact signification to the ideas of Ampere and Weber, to find
for para- and diamagnetism completely distinct interpretations,
conforming to the laws experimentally established by Curie: weak
magnetism, an attenuated form of ferromagnetism, varies inversely as
the absolute temperature; on the other hand diamagnetism is shown
to be, in all observed cases with the exception of bismuth, rigorously
independent of the temperature. The theory which I propose takes

154 PHYSICS OF THE ELECTRON

entire account of these facts and clears up at the same time the com-
plex question of magnetic energy.

I shall give here only the principal results of this work which will,
be published in full elsewhere.

(51) Molecular Currents. An electrified particle of charge e mov-
ing with a velocity v is equivalent to a current of moment ev. One
easily deduces from this that a molecular current made up of an
electron which describes in the periodic time t an orbit inclosed by
the surface S is equivalent from the point of view of the magnetic
field produced to a magnet of magnetic moment M—^j- normal to
the plane of the orbit.

There would be a corresponding current for each of the electrons
present in a molecule, and the magnetic moment resulting from these
would be zero or different from zero, according to the degree of
symmetry of the molecular structure.

(52) Diamagnetism. If on a group of such molecules we superim-
pose an external magnetic field, all the molecular currents experience
a modification independent of the manner in which the superposition
is obtained, whether by the establishment of the field or by motion
of the molecule in a preexisting field. The direction of this modi-
fication, due to the induction experienced by the molecular currents,
corresponds always to diamagnetism, the increase of the magnetic
moment being AM" = — -^^-S in the case of a circular orbit. H is the
component of the magnetic field normal to the plane of the orbit and
m the mass of the electron which describes the orbit.

(53) The Magnetic Energy. When the molecule is supposed im-
movable, the work necessary for the modification of the molecular
currents is furnished by the electric field produced, according to the
equations of Hertz, during the establishment of the magnetic field.

In the opposite case, where the modification is due to the motion
of the molecules, the work is furnished to the molecular currents by
the kinetic energy of the molecule or by the action of neighboring
molecules. The diamagnetic modification produced at the moment
of the establishment of the field continues in spite of the molecular
agitation.

This modification is manifested in three distinct ways :

1. If the resulting motion of the molecules is zero, the substance
is diamagnetic in the ordinary sense of the word, and the order of
magnitude of the experimental diamagnetic constants is in good
agreement with the hypothesis of molecular currents circulating
in intra-molecular paths.

This conception leads to the law of independence established by
Curie between the diamagnetic constants and the temperature or
the physical state.

RELATIONS TO OTHER SCIENCES 156

2. If the resulting motion of the molecules is not zero, the initial
diamagnetic modification is followed by an orientation of the mole-
cules under the action of the external field, which cause a para-
magnetism to appear that masks the underlying diamagnetism, the
new phenomenon being considerable compared to the first, when the
symmetry permits it to appear.

In slightly paramagnetic bodies, such as gases, the heat agitation
is opposed to the complete orientation of the molecular magnets, to
saturation, and one finds, in seeking what permanent condition is
established, the law of Curie, that the variation of paramagnetic
constants is in inverse ratio to the absolute temperature.

3. Finally, the change of period of revolution in consequence of
the diamagnetic modification corresponds to the Zeeman effect, as
general as diamagnetism itself; iron, certain rays of which show the
Zeeman effect, is diamagnetic before the orientation of the molecular
magnets under the action of the external field makes it appear para-
magnetic.

The orbits considered, which represent the molecular currents of
Ampere, are also the circuits of zero resistance of the diamagnetism
of Weber, with this remarkable peculiarity that the flux which passes
through them is not constant, as Weber supposed, if the inertia of the
electrons is entirely of electromagnetic origin.

I have shown, on the other hand, that the orbits of the electrons
supposed circular, and described under the action of central forces,
experience no deformation during the diamagnetic modification, this
latter consisting only in a change of velocity of the electrons in their
orbits. We can thus form an exact and simple conception of the facts
of magnetism and diamagnetism by considering the molecular cur-
rents as non-deformable but movable currents, of zero resistance and
of enormous self-induction, to which all the ordinary laws of induc-
tion are applicable.

XI. Conclusion

The rapid perspective which I have just sketched is full of pro-
mises, and I believe that rarely in the history of physics has one had
the opportunity of looking either so far into the past or so far into
the future. The relative importance of parts of this immense and
scarcely explored domain appears different to-day from what it did
in the preceding century: from the new point of view the various
plans arrange themselves in a new order. The electrical idea, the
last discovered, appears to-day to dominate the whole, as the place
of choice where the explorer feels that he can found a city before
advancing into new territories.

The mechanical facts, the most evident of all those of which matter
is possessed, from the first attracted the attention of our ancestors,

156 PHYSICS OF THE ELECTRON

and led them to conceive of the notions of mass and force which
appeared a long while the most fundamental, those from which all
the others ought to raminate. As the means of investigation have
increased, as the more hidden facts have been discovered, we have
thought for a long while to be able to reduce them to the old laws,
to be able in fact to find an explanation of mechanical origin.

The actual tendency, of making the electromagnetic ideas to
occupy the preponderating place, is justified, as I have sought to
show, by the solidity of the double base on which rests the idea of
the electron; on the one hand by the exact knowledge of the electro-
magnetic ether which we owe to Faraday, Maxwell, and Hertz, and
on the other hand by the experimental evidence brought forward by
the recent investigations into the granular structure of electricity.
Moreover, this assurance which we express when considering the
past is increased, if it is possible, when we consider the future.

Already all views, not only of the ether, but of matter, source and
receiver of luminous waves, obtain an immediate interpretation
which mechanics is powerless to give, and this mechanics itself
appears to-day as a first approximation, largely sufficing in all cases
of motion of matter taken in mass, but for which a more complete
expression must be sought in the dynamics of the electron.

Although still very recent, the conceptions of which I have sought
to give a collected idea are about to penetrate to the very heart of the
entire physics, and to act as a fertile germ in order to crystallize
around it, in a new order, facts very far removed from one another.

Falling in ground well prepared to receive it, in the ether of Fara-
day, Maxwell, and Hertz, the idea of the electron, an electrified
movable centre which experiment to-day allows us to lay hold of
individually, constitutes the tie between the ether and matter formed
of a group of electrons.

This idea has taken an immense development in the last few years,
which causes it to break the framework of the old physics to pieces,
and to overturn the established order of ideas and laws in order to
branch out again in an organization which one foresees to be simple,
harmonious, and fruitful.

PRESENT PROBLEMS OF RADIOACTIVITY

BY ERNEST RUTHERFORD

[Ernest Rutherford, Macdonald Professor of Physics, McGill University, Montreal,
b. August 30, 1871, Nelson, New Zealand. M.A. and D.Sc. University of New
Zealand; B.A. Cambridge, England; F.R.S. 1903; F.R.S.C. 1899; post-
graduate, Cambridge, England, 1895-98; Professor of Physics, McGill Univer-
sity, Montreal, 1898. Member of American Physical Society, and others;
awarded Rumford Medal of the Royal Society, 1904. Author of books and
articles.]

SINCE the initial discovery by Becquerel of the spontaneous emis-
sion of new types of radiation from uranium, our knowledge of the
phenomena exhibited by uranium and the other radioactive bodies
has grown with great and ever increasing rapidity, and a very large
mass of experimental facts has now been accumulated. It would be
impossible within the limits of this article even to review briefly the
more important experimental facts connected with the subject, and,
in addition, such a review is rendered unnecessary by the recent pub-
lication of several treatises J in which the main facts of radioactivity
have been dealt with in a fairly complete manner.

In the present article, an attempt will be made to discuss the more
important problems that have arisen during the development of the
subject and to indicate what, in the opinion of the writer, are the
subjects which will call for further investigation in the immediate
future.

II. Nature of the radiations

The characteristic radiations from the radioactive bodies are very
complex, and a large amount of investigation has been necessary to
isolate the different kinds of rays and to determine their specific
character. The rays from the three most studied radio-elements,
uranium, thorium, and radium, can be separated into three distinct
types, known as the a, /?, and f rays.

The nature of the a and /? rays has been deduced from observations
of the deflection of the path of the rays by a magnetic and electric
field. According to the electromagnetic theory, a radiation which
is deflectable by a magnetic or electric field must consist of a flight of
charged particles. If the amount of deflection of the rays from their
path is measured when both a magnetic and an electric field of known

1 Mme. Curie, Thbses presentees 6 la Faculty des Sciences. Paris, 1903.
H. Becquerel, Recherches sur une propriete nouvelle de la Matiere. Typographic
de Firmin, Didot et Cie. Paris, 1903.

E. Rutherford, Radioactivity. Cambridge, University Press, 1904.

F. Soddy, Radioactivity. Electrician Co., London, 1904.

158 PHYSICS OF THE ELECTRON

strength is applied, the value V of the velocity of the particles and
the ratio ^ of the charge carried by the particle to its apparent mass
m can be determined. From the direction of the deviation, the sign
of the electric charge carried by the particle can be deduced.

Examined in this way, the /? rays have been shown to consist of
negatively charged particles projected with a velocity approaching
that of light. The experiments of Becquerel and Kaufmann have
shown that the /? rays are identical with the cathode rays produced
in a vacuum tube. This relationship has been established by show-
ing that the value of — is the same for the two kinds of rays. In
both cases the value of ^ has been found to be about 107 electro-
magnetic units, while the corresponding value of ^ for hydrogen
atoms set free in the electrolysis of water is 104. If the charge on the
/? particles — or electrons as they may be termed — is the same as
that carried by the hydrogen atom, this result shows that the appar-
ent mass of the electrons at slow speeds is about ^ of that of the
hydrogen atom. The /? particles from the radio-elements are expelled
with a much greater speed than the cathode ray particles in a vacuum
tube. The velocity of the /? particles from radium is not the same
for all particles, but varies between about 1010 and 3X 1010 cms. per
second. The swifter particles move with a velocity of at least 95 per
cent of that of light. The emission by radium of electrons with high
but different velocities has been utilized by Kaufmann to determine
the variation of ^ with speed. He found that the value of ^
decreased with increase of velocity, showing that the apparent mass
increased with the speed. By comparison of the experimental results
with the mathematical theory of a moving charge, he deduced that the
mass of the electrons was in all probability electromagnetic in origin,
i. e., the apparent mass could be explained purely in terms of elec-
tricity in motion without the necessity of a material nucleus on which
the charge was distributed. J. J. Thomson, Heaviside, and others,
have shown that a moving charged sphere increases in apparent
mass with the speed, and that, for speeds small compared with the
velocity of light, the increase of mass m = | ^ where e is the charge
carried by the body and a the radius of the conducting sphere over
which the electricity is distributed. Kaufmann deduced that the
value of ^ = 1.86X107 for electrons of slow velocity. If the mass
of the electrons is electrical in origin, it is seen that a = 10~13 cms.,
since the value of e = 3.4XlO~10 electrostatic units. The results of
various methods of determination agree in fixing the diameter of
an atom as about 10~8 cms. The apparent diameter of an electron
is thus minute compared with that of the atom itself.

The highest velocity of the radium electrons measured by Kauf-

159

mann was 95 per cent of the velocity of light. The power of electrons
of penetrating solid matter increases rapidly with the velocity, and
some of those expelled from radium are able to penetrate through
more than 3 mms. of lead. It is probable that a few of the electrons
from radium move with a velocity still greater than the highest value
observed by Kaufmann, and it is important to determine the value
of ^ and the velocity of such electrons. According to the mathemat-
ical theory, the mass of the electron increases rapidly as the speed of
light is approached, and should be infinitely great when the velocity
of light is reached. This leads to the conclusion that no charged body
can be made to move with a velocity greater than that of light. This
result is of great importance, and requires further experimental veri-
fication. A close study of the high speed electrons from radium may
throw further light on this question.

Only a brief and imperfect statement of our knowledge of electrons
has been given in this paper. A more complete and detailed account
of both the theory and experiment will be given by my colleague,
Dr. Langevin.

III. The a rays

The /? rays are readily deflected by a magnetic field, but a very
intense magnetic field is required to deflect appreciably the a rays.
The writer showed by the electric method that the a rays of radium
were deflected both by a magnetic and electric field, and deduced
the velocity of projection of the particles and the ratio ^ of the
charge to the mass. The direction of deflection of the a rays is opposite
in sense to the /? rays. Since the /? rays carry a negative charge, the a
particles thus behave as if they carried a positive charge. The mag-
netic deflection of these rays was confirmed by Becquerel and Des
Coudres, using the photographic method, while the latter, in addition,
showed their deflection in an electric field and deduced the value of
the velocity and ^. The values obtained by Rutherford and Des
Coudres were in very good agreement, considering the difficulty of
obtaining a measurable deviation.

o

Observer Value of Velocity Value of —

Rutherford 2.5 X 108 cms. per sec. 6 X 10s electromagnetic units

Des Coudres 1.6 X 10 cms. per sec. 6 X 10s electromagnetic units

Since the value of ^ for the hydrogen atom is 104, on the assump-
tion that the a particle carries the same charge as the hydrogen atom,
this result shows that the apparent mass of the a particle is about
twice that of the hydrogen atom. If the a particle consists of any
known kind of matter, this result indicates that it is either the atom

160 PHYSICS OF THE ELECTRON

of hydrogen or of helium. The a particles thus consist of heavy
bodies projected with great velocity, whose mass is of the same order
of magnitude as the helium atom and at least 2000 times as great as
the apparent mass of the /? particle.

If the a particles carry a positive charge, it is to be expected that the
particles, falling on a body of sufficient thickness to absorb them,
should under suitable conditions give it a positive charge, while the
substance from which they are projected should acquire a negative
charge. The corresponding effect has been observed for the /? rays.
The /? particles from radium communicate a negative charge to the
body on which they fall, while the radium from which they are emit-
ted acquires a positive charge. This effect has been very strikingly
shown by a simple experiment of Strutt. The radium compound , sealed
in a small glass tube, the outer surface of which is made conducting,
is insulated by a quartz rod. A simple gold-leaf electroscope is
attached to the bottom of the glass tube, in order to indicate the
presence of a charge. The whole apparatus is inclosed in a glass
vessel, which is exhausted to a high vacuum, in order to reduce the
loss of charge in consequence of the ionization of the gas by the rays.
Using a few milligrams of radium bromide, the gold leaf diverges to
its full extent in a few minutes and shows a positive charge. The
explanation is simple. A large proportion of the negatively charged
particles are projected through the glass tube containing the radium,
and a positive charge is left behind. By allowing the gold leaf, when
extended, to touch a conductor connected to earth, the gradual
divergence of the leaves and their collapse becomes automatic, and
will continue, if not indefinitely, at any rate for as long a time as the
radium lasts.

When the radium is exposed under similar conditions, but un-
screened in order to allow the a particles to escape, no such charging
action is observed. This is not due to the equality between the num-
ber of positively and negatively charged particles expelled from the
radium, for no effect is observed when the radium is temporarily
freed from its power of emitting /? rays by driving off the emanation
by heat. The writer recently attempted to detect the charge carried
by the a rays from radium by allowing them to fall on an insulated
plate in a vacuum, but no appreciable charging was observed. The
/? rays were temporarily got rid of by heating the radium in order to
drive off its emanation. There was found to be a strong surface ion-
ization set up at the surface from which the rays emerged and the
surface on which they impinged. The presence of this ionization causes
the upper plate to lose rapidly a charge communicated to it. Although
this action would mask to some extent the effect to be looked for, a
measurable effect should have been obtained under the experimental
conditions, if the a rays were expelled with a positive charge; but not

PRESENT PROBLEMS OF RADIOACTIVITY 161

the slightest evidence of a charge was observed. I understand that
similar negative results have been obtained by other observers.

This apparent absence of charge carried by the a rays is very
remarkable and difficult to account for. There is no doubt that the
a particles behave as if they carried a positive charge, for several ob-
servers have shown that the a rays are deflected by a magnetic field.
It is interesting, in this connection, that Wien was unable to detect
that the "canal rays" carried a charge. These rays, discovered by
Goldstein, are analogous in many respects to the a rays. They are
slightly deflected by a magnetic and electric field, and behave like
positively charged bodies atomic in size. The value of ^ is not a
constant, but depends upon the nature of the gas in the tube through
which the discharge is passed. The apparent absence of charge on
the a particles may possibly be explained on the supposition that
a negatively charged particle (an electron) is always projected at the
same time as the positively charged particle. Such electrons if they
are present should be readily bent back to the surface from which
they came by the action of a strong magnetic field. It will be of in-
terest to examine whether the charge carried by the a rays can be
detected under such conditions. Another hypothesis, which has some
points in its favor, is that the a particles are uncharged at the mo-
ment of their expulsion, but, in consequence of their collision with
the molecules of matter, lose a negative electron, and consequently
acquire a positive charge. This point is at present under examin-
ation. The question is in a very unsatisfactory state, and requires
further investigation.

It is remarkable that positive electricity is always associated with
matter atomic in size, for no evidence has been obtained of the exist-
ence of a positive electron corresponding to the negative electron.
This difference between positive and negative electricity is apparently
fundamental, and no explanation of it has, as yet, been forthcoming.

The evidence that the a particles are atomic in size mainly rests on
the deflection of the path of the rays in a strong magnetic and electric
field. It has, however, been suggested by H. A. Wilson that the a
particle may in reality be a "positive" electron, whose magnitude
is minute compared with that of the negative. The electric mass of
an electron for slow speeds is equal to ^ £. Since there is every reason
to believe that the charge carried by the a particle and the electron
is the same, in order that the mass of the positive electron should
be about 2000 times that of the negative, it would be necessary to
suppose that the radius of the sphere over which the charge is dis-
tributed is only ^ of that of the electron, i. e., about 10~16 cms.
The magnetic and electric deflection would be equally well explained
on this view. This hypothesis, while interesting, is too far-reaching

162 PHYSICS OF THE ELECTRON .

in its consequences to accept before some definite experimental
evidence is forthcoming to support it. The evidence at present ob-
tained strongly supports the view that the a particles are in reality
projected matter, atomic in size. The probability that the a particle
is an atom of helium is discussed later, in section vm.

Becquerel showed that the a rays of polonium were deflected by a
magnetic field to about the same extent as the a rays of radium. On
account of the feeble activity of thorium and uranium, compared
with radium and polonium, it has not been found possible to examine
whether the a rays emitted by them are deflectable. There is little
doubt, however, that the a particles of all the radio-elements are pro-
jected matter of the same kind (probably helium atoms). The a rays
from the different radioactive products differ in their power of
penetration of matter in the proportion of about three to one, being
greatest for the a rays from the imparted or "induced" activity of
radium and thorium, and least for uranium. This difference is prob-
ably mainly due to a variation of the velocity of projection of the
a particles in the various cases. The interpretation of results is
rendered difficult by our ignorance of the mechanism of absorption
of the a rays by matter. Further experiment on this point is very
much required.

It is of importance to settle whether the a particles of radium
and polonium have the same ratio of ~. Becquerel states that the
amount of curvature of the a rays from polonium in a field of constant
strength was the same as for the a rays from radium. This would
show that the product of the mass and velocity is the same for the
a particles from the two substances. The a rays of polonium, how-
ever, certainly have less penetrating power than those of radium, and
presumably a smaller velocity of projection. This result would indi-
cate that ^ is different for the a particles of polonium and radium.
It is of importance to determine accurately the ratio of ^ and the
velocity for the rays from these two substances in order to settle this
important point.

IV. The f Rays

In addition to the a and /? rays, uranium, thorium, and radium all
emit very penetrating rays known as f rays. These rays are about
100 times as penetrating as the /? rays, and their presence can be
detected after passing through several centimeters of lead. Villard,
who originally discovered these rays in radium, stated that they were
not deflected in a magnetic field, and this result has been confirmed
by other observers. Quite recently, Paschen has described some
experiments which led him to believe that the f rays are corpuscular

PRESENT PROBLEMS OF RADIOACTIVITY 163

in character, consisting of negatively charged particles (electrons)
projected with a velocity very nearly equal to that of light. This
conclusion is based on the following evidence. Some pure radium
bromide was completely inclosed in a lead envelope 1 cm. thick,- — a
thickness sufficient to absorb completely the ordinary /? rays emitted
by radium, but which allows about half of the f rays to escape. The
lead envelope was insulated in an exhausted vessel, and was found
to gain a positive charge. In another experiment, the rays escaping
from the lead envelope fell on an insulated metal ring, surrounding
the lead envelope. When the air was exhausted, this outer ring was
found to gain a negative charge. These experiments, at first sight,
indicate that the 7- rays carry with them a negative charge like the
/? rays. In order to account for the absence of deflection of the path
of the f rays in very strong magnetic or electric fields, it is necessary
to suppose that the particles have a very large apparent mass. Pas-
chen supposes that the -/- particles negative are electrons like the /?
particles, but are projected with a velocity so nearly equal to that
of light that their apparent mass is very great.

Some experiments recently made by Mr. Eve, of McGill Univers-
ity, are of great interest in this connection. He found by the electric
method that the f rays set up secondary rays, in all directions, at the
surface of which they emerge and also on the surface of which they
impinge. These rays are of much less penetrating power than the
primary rays, and are readily deflected by a magnetic field. The
direction of deflection indicated that these secondary rays consisted,
for the most part, of negatively charged particles (electrons) pro-
jected with sufficient velocity to penetrate through about 1 mm. of
lead. In the light of these results, the experiments of Paschen receive
a simple explanation without the necessity of assuming that the f rays
of radium themselves carry a negative charge. The lead envelope
in his experiment acquired a positive charge in consequence of the
emission of a secondary radiation consisting of negatively charged
particles, projected with great velocity from the surface of the lead.
The electric charge acquired by the metal ring was due to the absorp-
tion of these secondary rays by it, and the diminution of this charge
in a magnetic field was due to the ease with which these secondary
rays are deflected. It is thus to be expected that the envelope sur-
rounding the radium, whether made of lead or other metal, would
always acquire a positive charge, provided the metal is not of suffi-
cient thickness to absorb all the /- rays in their passage through it.

No conclusive evidence has yet been brought forward to show
that the ?- rays can be deflected either in a magnetic or electric field.
In this, as in other respects, the rays are very analogous to the Roent-
gen X rays.

According to the theory of Stokes, J. J. Thomson, and Weichert,

164 PHYSICS OF THE ELECTRON

of rays are transverse pulses set up in the ether by the sudden arrest
X the motion of the cathode particles on striking an obstacle. The
more sudden the stoppage, the shorter is the pulse, and the rays, in
consequence, have greater power of penetrating matter. In some
recent experiments Barkla found that the secondary rays set up
by the X rays, on striking an obstacle, vary in intensity with the
orientation of the X-ray tube, showing that the X rays exhibit the
property of one-sidedness or polarization. This is the only evi-
dence so far obtained in direct support of the wave-nature of the
X rays.

If X rays are not set up when the cathode particles are
stopped, conversely, it is to be expected that X rays should be set
up when they are suddenly set in motion. Now this effect is not
observable in an X-ray tube, since the cathode particles acquire
most of their velocity, not at the cathode itself, but in passing through
the electric field between the cathode and anti-cathode. It is, how-
ever, to be expected theoretically that a type of X rays should
be set up at the sudden expulsion of the /? particles from the radio-
atoms. The rays, too, should be of a very penetrating kind, since
not only is the charged particle projected with a speed approaching
that of light, but the change of motion must occur in a distance
comparable with the diameter of an atom.

On this view, the f rays are a very penetrating type of X rays,
having their origin at the moment of the expulsion of the /? particle
from the atom. If the /? particle is the parent of the 7- rays, the
intensity of the /? and j- rays should, under all conditions, be propor-
tional to one another. I have found this to be the case, for the f rays
always accompany the/? rays and, in whatever way the /?-ray activity
varies, the activity measured by the f rays always varies in the same
proportion. Active matter which does not emit /? rays does not give
rise to 7- rays. For example, the radio-tellurium of Marckwald, which
does not emit /? rays, does not give off f rays.

Certain differences are observed, however, in the ionizing action of
f and X rays. For example, gases and vapors like chlorine, sulphur-
etted hydrogen, methyl-iodine, and chloroform, when exposed to
ordinary X rays, show a much greater ionization, compared with air,
than is to be expected, according to the density law. On the other
hand, the relative ionization of these substances by 7- rays follows the
density law very closely. It seemed likely that this apparent differ-
ence was due mainly to the greater penetrating power of the 7- rays.
This was confirmed by some recent experiments of Eve, who found
that the relative conductivity of gases exposed to very penetrating
X rays from a hard tube approximated in most cases closely to that
observed for the 7- rays. The vapor of methyl-iodine was an excep-
tion, but the difference in this case would probably disappear if

.PRESENT PROBLEMS OF RADIOACTIVITY 165

X rays could be generated of the same penetrating power as that of
the f rays.

The results so far obtained thus generally support the view that
the f rays are a type of penetrating X rays. This view is in agreement,
too, with theory, for it is to be expected that very penetrating /? rays
should always appear with the /? rays.

No evidence of the emission of a type of X rays is observed from
active bodies which emit only a rays. If the a particles are initially
projected with a positive charge, such rays are to be expected. Their
absence supplies another piece of evidence in support of the view
that the a particle is projected without a charge, but acquires a
positive charge in its passage through matter.

V. Emission of Energy by the Radioactive Bodies

It was early recognized that a very active substance like radium
emitted energy at a rapid rate, but the amount of this energy was
very strikingly shown by the direct measurements of its heating
effect made by Curie and Laborde. They found that one gram of
radium in radioactive equilibrium emitted about 100 gram calories
of heat per hour. A gram of radium would thus emit 896,000 gram
calories per year, or over 200 times as much heat as is liberated by
the explosion of hydrogen and oxygen to form one gram of water.
They showed that the rate of heat emission was the same in solution
as in the solid state, and remained constant when once the radium
had reached a stage of radioactive equilibrium. Curie and Dewar
showed that the rate of evolution of heat from radium was unaltered
by plunging the radium into liquid air, or liquid hydrogen.

It seemed probable that the evolution of heat by radium was
directly connected with its radioactivity, and the experiments of
Rutherford and Barnes proved this to be the case. The heating effect
of a quantity of radium bromide was first determined. The emana-
tion was then completely driven off by heating the radium, and con-
densed in a small glass tube by means of liquid air. After removal
of the emanation, the heat evolution of the radium in the course of
about three hours fell to a minimum corresponding to one quarter
of its original value, and then slowly increased again, reaching its
original value after an interval of about one month. The heat emis-
sion from the emanation tube at first increased with the time, rising
to a maximum value about three hours after its introduction. It then
slowly decreased according to an exponential law with the time,
falling to half value in about four days. If Qmax is the maximum
heating effect of the emanation tube, the heat emission Q at any
time t, after the maximum is reached, is given by

Q = Qmaxe^f
where A is the radioactive constant of the emanation.

166 PHYSICS OF THE ELECTRON

The curve expressing the recovery with time of the heating effect
of radium from its minimum is complementary to the curve express-
ing the diminution of the heating effect of the emanation tube with-
time. The curves of decay and recovery agree within the limit of ex-
perimental error with the corresponding curves of decay and recovery
of the activity of radium when measured by the a rays. Since the
minimum, or non-separable activity of radium, measured by the a
rays, after the emanation has been removed, is only one quarter of
the maximum activity, these results indicate that the heating effect
of radium is proportional to its activity measured by the a rays. It
is not proportional to the activity measured by the /? or f rays, since
the /? or f ray activity of radium almost completely disappears
some hours after removal of the emanation.

These results have been confirmed by further observations of the
distribution of the heat emission between the emanation and the
successive products which arise from it. If the emanation is left for
several hours in a closed tube, its activity measured by the electric
method increases to about twice its initial value. This is due to the
"excited activity," or in other words to the radiations from the active
matter deposited on the walls of the tube by the emanation. The
activity of this deposit has been very carefully analyzed, and the
results show that the matter deposited by the emanation breaks up in
three successive and well-marked stages. For convenience, these suc-
cessive products of the emanation will be termed radium A , radium B,
and radium C. The time T taken for each of these products to be half
transformed, and the radiations from each product, are shown in the
following table:

Product T Radiations

Radium a rays

I
Emanation 4 days

I
Radium A 3 mins.

1
Radium B 21 mins.

i
Radium C 28 mins.

When the emanation has been left in a closed vessel for several
hours, the emanation and its successive products reach a stage of
approximate radioactive equilibrium, and the heating effect is then
a maximum. If the emanation is suddenly removed from the tube by
a current of air, the heating effect is then due to radium A, B, and C
together. On account, however, of the rapidity of the change of
radium A (half value in three minutes), it is experimentally very diffi-

PRESENT PROBLEMS OF RADIOACTIVITY 167

cult to distinguish between the heating effect of the emanation and
that of radium A. The curve of variation with time of the heating
effect. of the emanation tube after removal of the emanation is very
nearly the same as the corresponding curve for the activity measured
by the a rays. These results show that each of the products of radium
supplies an amount of heat roughly proportional to its activity meas-
ured by the a rays. Each product loses its heating effect at the same
rate as it loses its activity, showing that the heating effect is directly
connected with the radioactive changes. The results indicated that
the product, radium B, which does not emit rays does not supply an
amount of heat comparable with the other products. This point is
important, and requires more direct verification.

Since the heat emission is in all cases nearly proportional to the
number of a particles expelled, the question arises whether the bom-
bardment of these particles is sufficient to account for the heating
effects observed. The kinetic energy of the a particle % mv2 can be at
once determined since ^ and V are known. The following table
shows the kinetic energy of the a particle deduced from the measure-
ments of Rutherford and Des Coudres. The third column shows the
number of a particles expelled from 1 gram of radium per second on
the assumption that the heating effect of radium (100 gram-calories
per gram per hour) is entirely due to the energy given out by the
expelled a particles.

Number of a particles

Observer Kinetic energy expelled per second

from 1 gram of radium.

Rutherford 5.9 X lO"8 ergs. 2 X lO"11

Des Coudres 2.5 X 10~6 ergs. 5 X 10~u

This hypothesis that the heating effect of radium is due to bombard-
ment of the a particle can be indirectly put to the test in the follow-
ing way. It seems probable that each atom of radium in breaking up
emits one a particle. On the disintegration theory, the residue of the
atom, after the a particle is expelled, is the atom of the emanation, so
that each atom of radium gives rise to one atom of the emanation.
Let q be the number of atoms in each gram of radium breaking up
per second. When a state of radioactive equilibrium is reached, the
number N of emanation particles present is given by N — %, where
^ is the constant of change of the emanation. Now Ramsay and
Soddy deduced from experiment that the volume of the emanation
released from 1 gram of radium was about one cubic millimeter at
atmospheric pressure and temperature. It has been experimentally
deduced that there are 3.6X 1019 molecules in one cubic centimeter
of gas at ordinary pressure and temperature. The emanation obeys
Boyle's law and behaves, in all respects, like a heavy gas, and we may

168 PHYSICS OF THE ELECTRON

in consequence deduce that N =3.6 X 1016. Now ^ =2.0X 1(T6. Thus
<7 = 7.2X1010. Now the particles expelled from radium in a state
of radioactive equilibrium are about equally divided between four
substances, viz., the radium itself, the emanation, radium A and C.
We may thus conclude that the number of a particles expelled per
second from 1 gram of radium in radioactive equilibrium is 2.9X 1011.
The value deduced by this method is intermediate between the values
previously obtained (see previous table) on the assumption that the
heating effect is entirely due to the a particles.

I think we may conclude from the agreement of these two methods
of calculation that the greater portion of the heating effect of radium
is a direct result of the bombardment of the expelled a particles, and
that, in all probability, about 5X 1010 atoms of radium break up per
second.

The energy carried off in the form of (3 and f rays is small compared
with that emitted in the form of a rays. By calculation it can be
shown that the average kinetic energy of the $ particle is small in com-
parison with that of the a particle. This result is confirmed by com-
parative measurements of the total ionization produced by the a and
/? rays, when the energy of the rays is all used up in ionizing the gas,
for the total ionization produced by the /? rays is small compared with
that due to the a rays. The total ionization produced by the f rays
is about the same as that produced by the /? rays, showing that, in
all probability, the energy emitted in the form of these two types of
radiation is about the same. From the point of view of the energy
radiated, and of the changes which occur in the radioactive bodies,
the a rays thus play a far more important role in radioactivity than
the /? or f rays. Most of the products which arise from radium and
thorium emit only a rays, while the /? and f rays appear only in the
last of the series of rapid changes which take place in these bodies.

Since most of the heating effect of radium is due to the a rays, it
is to be expected that all radioactive substances, which emit a rays,
should also emit heat at a rate proportional to their a ray activity.
On this view, both uranium and thorium should emit heat at about
one millionth the rate of radium. It is of importance to determine
directly the heating effect for these substances, and also for actinium
radio-tellurium.

According to the disintegration theory, the a particle is expelled
as a result of the disintegration of the atom of radioactive matter.
While it is to be expected that a greater portion of the energy emitted
should be carried off in the form of kinetic energy by the expelled
particles, it is also to be expected that some energy would be radiated
in consequence of the rearrangement of the components of the system
after the violent ejection of one of its parts. No direct measure-
ments have yet been made of the heating effect of the a particles,

PRESENT PROBLEMS OF RADIOACTIVITY 169

independently of the substance in which they are produced. Experi-
ments of this character would be difficult, but would throw light on
the important question of the division of the energy radiated between
the expelled a ray particle and the system from which it arises.

The enormous evolution of energy by the radioactive substances
is very well illustrated by the case of the radium emanation. The
emanation released from 1 gram of radium in radioactive equilibrium
emits during its changes an amount of energy corresponding to about
10,000 gram-calories. Now Ramsay and Soddy have shown that the
volume of this emanation is about 1 cubic millimeter at standard
pressure and temperature. One cubic millimeter of the emanation
and its product thus emits about 107 gram-calories. Since 1 centimeter
of hydrogen, in uniting with the proportion of oxygen required to form
water, emits 3.1 gram-calories, it is seen that the emanation emits
about 3 million times as much energy as an equal volume of hydrogen.

It can readily be calculated, on the assumption that the atom of
the emanation has a mass 100 times that of hydrogen, that 1 pound
of the emanation some time after removal could emit energy at the
rate of about 8000 horse-power. This would fall off in a geometrical
progression with the time, but, on an average, the amount of energy
emitted during its life corresponds to 50,000 horse-power days. Since
the radium is being continuously transformed into emanation, and
three quarters of the total heat emission is due to the emanation and
its products, a simple calculation shows that 1 gram of radium must
emit during its life about 109 gram-calories. As we have seen, the
heat emission of radium is about equally divided between the radium
itself and the three other a ray products which come from it.

The heat emitted from each of the other radioactive substances
while their activity lasts, should be of the same order of magnitude,
but in the case of uranium and thorium the present rate of heat emis-
sion would probably continue, on an average, for a period of about
1000 million years.

VI. Source of the Energy emitted by the Radioactive Bodies

There has been considerable difference of opinion in regard to the
fundamental question of the origin of the energy spontaneously emit-
ted from the radioactive bodies. Some have considered that the
atoms of the radio-elements act as transformers of borrowed energy.
The atoms are supposed to be able, in some way, to abstract energy
from the surrounding medium and to emit it again in the form of the
characteristic radiations observed. Another theory, which has found
favor with a number of physicists, supposes that the energy is de-
rived from the radio-atoms themselves and is released in consequence
of their disintegration. The latter theory involves the conception

170 PHYSICS OF THE ELECTRON

that the atoms of the radio-elements contain a great store of latent
energy, which only manifests itself when the atom breaks up. There
is no direct evidence in support of the view that the energy of the
radio-elements is derived from external sources, while there is much
indirect evidence against it. Some of this evidence will now be con-
sidered. There is now no doubt that the a and /? rays consist of parti-
cles projected with great speed. In order for the a particle to acquire
the velocity with wThich it is expelled, it can be calculated that it
would be necessary for it to move freely between two points differ-
ing in potential by about five million volts. It is very difficult to
imagine any mechanism which could suddenly impress such an
enormous velocity on one of the parts of an atom. It seems much
more reasonable to suppose that the a and /? particles were originally
in rapid motion in the atom, and, for some reason, escaped from the
atomic system with the velocity they possessed at the instant of their
release. There is now undeniable evidence that radioactivity is
always accompanied by the production of new kinds of active mat-
ter. Some sort of chemical theory is thus required to explain the
facts, whether the view is taken that the energy is derived from the
atom itself or from external sources. The "external" theory of the
origin of the energy was initially advanced to explain only the heat
emission of radium. We have seen that this is undoubtedly con-
nected with the expulsion of a particles from the different disintegra-
tion products of radium, and that the radium itself only supplies one
quarter of the total heat emission, the rest being derived from the
emanation and its further products. On such a theory it is neces-
sary to suppose that in radium there are a number of different active
substances, whose power of absorbing external energy dies away
with the time, at different but definite rates. This still leaves the
fundamental difficulty of the origin of these radioactive products
unexplained. Unless there is some unknown source of energy in the
medium which the radioactive bodies are capable of absorbing, it is
difficult to imagine whence the energy demanded by the external
theory can be derived. It certainly cannot be from the air itself,
for radium gives out heat inside an ice calometer. It cannot be any
type of rays such as the radioactive bodies emit, for the radioactivity
of radium, and consequently its heating effect, are unaltered by her-
metically sealing it in a vessel of lead several inches thick. The evi-
dence, as a whole, is strongly against the theory that the energy is
borrowed from external sources, and, unless a number of improbable
assumptions are made, such a theory is quite inadequate to explain
the experimental facts. On the other hand, the disintegration theory,
advanced by Rutherford and Soddy, not only offers a satisfactory
explanation of the origin of the energy emitted by the radio-elements,
but also accounts for the succession of radioactive bodies. On this

PRESENT PROBLEMS OF RADIOACTIVITY 171

theory, a definite, small proportion of the atoms of radioactive
matter every second becomes unstable and breaks up with explosive
violence. In most cases, the explosion is accompanied by the ex-
pulsion of an a particle, in a few cases, by only a /? particle, and in
others by a and /? particles together. On this view, there is at any
time present in a radioactive body a proportion of the original
matter which is unchanged and the products of the part which has
undergone change. In the case of a slowly changing substance like
radium, this point of view is in agreement with the observed fact that
the spectrum of radium remains unchanged with its age.

The expulsion of an a or /? particle or both from the atom leaves
behind an atom which is lighter than before and which has different
chemical and physical properties. This atom in turn becomes un-
stable and breaks up, and the process, once started, proceeds from
stage to stage with a definite and measurable velocity in each case.

The energy radiated is, on this view, obtained at the expense of
the internal energy of the radio-atoms themselves. It does not con-
tradict the principle of the conservation of energy, for the internal
energy of the products of the changes, when the process has come
to an end, is supposed to be diminished by the amount of energy
emitted during the changes. This theory supposes that there is a
great store of internal energy in the radio-atoms themselves. This
is not in disagreement with the modern views of the electronic con-
stitution of matter, which have been so ably developed by J. J.
Thomson, Larmor, and Lorentz. A simple calculation shows that the
mere concentration of the electric charges, which on the electronic
theory are supposed to be contained in an atom, implies a store
of energy in the atom so enormous that, in comparison, the large
evolution of energy from the radio-elements is quite insignificant.

Since the energy emitted from the radio-elements is for the most
part kinetic in form, it is necessary to suppose that the a and /? parti-
cles were originally in rapid motion in the atoms from which they are
projected. The disintegration theory supposes that it is the atoms
and not the molecules which break up. Such a view is necessary to
explain the independence of the rate of disintegration of radioactive
matter, of wide variations of temperature, and of the action of chemi-
cal and physical agents at our command. This must be conceded if the
term atom is used in the ordinary chemical sense. It is, however,
probable that the atoms of the radio-elements are in reality complex
aggregates of known or unknown kinds of matter, which break up
spontaneously. This aggregate behaves like an atom and cannot be
resolved into simpler forms by external chemical or physical agencies.
It breaks up, however, spontaneously with an evolution of energy
enormous compared with that released in ordinary chemical changes.
This question is further considered in section viu of this paper.

172 PHYSICS OF THE ELECTRON

The disintegration theory assumes that a small fraction of the
atoms break up in unit time, but no definite explanation is, as yet,
forthcoming of the causes which lead to this explosive disruption
of the atom. The experimental results are equally in agreement with
the view that each atom contains within itself the potentiality of
its final disruption, or with the view that the disintegration is pre-
cipitated by the action of some external cause that may lead to the
disintegration of the atom in the same way that a detonator is neces-
sary to start certain explosions. The energy set free is, however, not
derived from the detonator, but from the substance on which it acts.
There is another general view which .may possibly lead to an explana-
tion of atomic disruption. If the atom is supposed to consist of elec-
trons or charged bodies in rapid motion, it tends to radiate energy
in the form of electromagnetic waves. If an atom is to be permanently
stable, the parts of the atom must be so arranged that there is no
loss of energy by electromagnetic radiation. J. J. Thomson has in-
vestigated certain possible arrangements of electrons in an atom
which radiate energy extremely slowly, but which ultimately must
break up in consequence of the loss of internal energy. According to
present views, it is not such a matter of surprise that atoms do break
up as that atoms are so stable as they appear to be. This question
of the causes of disintegration is fundamental, and no adequate
explanation has yet been put forward.

VII. Radioactive Products

Rutherford and Soddy showed that the radioactivity was always
accompanied by the appearance of new types of active matter
which possessed physical and chemical properties distinct from the
parent radio-element. The radioactivity of these products is not
permanent, but decays according to an exponential law with the
time. The activity It and at any time t is given by It =/0e~A * where
I0 is the initial activity and ^ a constant. Each radioactive pro-
duct has a definite change-constant which distinguishes it from all
other products. These products do not arise simultaneously, but in
consequence of a succession of changes in the radio-elements; for
example, thorium in breaking up gives rise to Thorium X, which
behaves as a solid substance soluble in ammonia. This in turn breaks
up and gives rise to a gaseous product, the thorium emanation. The
emanation is again unstable and gives rise to another type of matter
which behaves as a solid and is deposited on the surface of the vessel
containing the emanation. It was found that the results would be
quantitatively explained on the assumption that the activity of any
product at any time is the measure of the rate of production of the
next product. This is to be expected, since the activity of any sub-

PRESENT PROBLEMS OF RADIOACTIVITY 173

stance is proportional to the number of atoms which break up per
second ; and since each atom in breaking up gives rise to one atom
of the next product together with a or /? particles or both, the activity
of the parent is a measure of the rate of production of the succeeding
product.

Of these radioactive products, the radium emanation has been very
closely studied on account of its existence in the gaseous state. It has
been shown to be produced by radium at a constant rate. The amount
of emanation stored up in a given mass of radium reaches a maximum
value when the rate of supply of fresh emanation balances the rate of
change of the emanation present.

If q be the number of atoms of emanation produced per second by
the radium and N the maximum number present when radioactive
equilibrium is reached, then N = *, where X is the constant of
change of the emanation. This relation has been verified experi-
mentally. The emanation is found to diffuse through air like a gas of
heavy molecular weight. It is unattacked by chemical reagents, and
in that respect resembles the inert gases of the argon family. It con-
denses at a definite temperature — 150°C. Its constant of change is
unaffected between the limits of temperature of 450°C and — 180°C.
Since the emanation changes into a non-volatile type of matter which
is deposited on the surface of vessels, it was to be expected that the
volume of the emanation should decrease according to the same law,
as it lost its activity. These deductions, based on the theory, have
been confirmed in a striking manner by the experiments of Ramsay
and Soddy. The radium emanation was chemically isolated and found
to be a gas which obeyed Boyle's law. The volume of the emanation
observed was of the same order as had been predicted before its sepa-
ration. The volume was found to decrease with the time according to
the same law as the emanation lost its activity. Ramsay and Collie
found that the emanation had a new and definite spectrum similar in
some respects to that of the argon group of gases.

There can thus be no doubt that the emanation is a transition sub-
stance with remarkable properties. Chemically it behaves like an
inert gas, and has a definite spectrum, and is condensed by cold. But,
on the other hand, the gas is not permanent, but disappears, and is
changed into other types of matter. It emits during its changes about
one million times as much energy as is emitted during any known
chemical change.

From the similarity of the behavior of the emanation of thorium
and actinium to that of radium, we may safely conclude that these
also are new gases which have only a limited life and change into other
substances.

The non-volatile products of the radioactive bodies can be dis-
solved in strong acids and show definite chemical behavior in solution.

174 PHYSICS OF THE ELECTRON

They can be partially separated by electrolysis and by suitable chem-
ical methods. They can be volatilized by the action of high tempera-
ture, and their differences in this respect can be utilized to effect in
many cases a partial separation of successive products. There can
be little doubt that each of these radioactive products is a transition
substance, possessing, while it lasts, some definite chemical and phys-
ical properties which serve to distinguish it from other products and
from the parent element.

The radioactive products derived from each radio-element, together
with the type of radiation emitted during their disintegration, are
shown graphically in Fig. 1.

riu-rn . TTior. X £ma noTior, 77>or- /} Ttior. B 7%or C

O

Ui~a n i </ rn X Final fr-oolitcf

Actirt X. £ ~rna n a h'on /Icfin.A Acffn-B Ac.fm C

2

The radiations from actinium have not so far been examined suf-
ficiently closely to determine the character of the radiation emitted
by each product. There is some evidence that a product, actinium
X, exists in actinium corresponding to Th X in thorium. It has not,
however, been very closely examined.

The question of nomenclature for the successive products is impor-
tant. The names Ur X, Th X have been retained, and also the term
emanation. The emanation from the three radio-elements in each case
gives rise to a non-volatile type of matter which is deposited on the
surface of the bodies. The matter initially deposited from the radium
emanation is called radium A, radium A changes into B, and B into
C, and so on. A similar nomenclature is applied to the further pro-
ducts of the emanation of thorium and actinium. This notation is

PRESENT PROBLEMS OF RADIOACTIVITY

175

simple and elastic, and is very useful in mathematical discussion of
the theory of successive changes. In the following table a list of the
products is given, together with the nature of the radiation and the
most marked chemical and physical properties of each product. The
time T for each of the products to be half transformed is also added.

Radioactive
products.

T

Rays.

Some chemical and physical properties.

URANIUM

2.5 X 10" years.

a.

Soluble in excess of ammonium car-

4

bonate.

Uranium X

22 days.

0, 7

Insoluble in excess of ammonium

4

carbonate.

Final product.

THORIUM

109 years.

a

Insoluble in ammonia.

4

Thorium X

4 days.

a

Soluble in ammonia.

4

Emanation

1 min.

a

Inert gas condenses about -120°C.

4

Thorium A

11 hours.

no rays.

Attaches itself to negative electrode,

4

soluble in strong acids.

Thorium B

55 mins.

"i/3, 7

Separable from A by electrolysis.

4

Final product.

ACTINIUM

4

Actinium X ?

4

Emanation

3.9 sees.

a

Gaseous product.

4

Actinium A

41 mins.

no rays.

Attaches itself to negative electrode,

. 4

soluble in strong acids.

Actinium B

1.5 mins.

a

Separable from A by electrolysis.

4

Final product.

RADIUM

1000 years.

a

4

Emanation

4 days.

a

Inert gas, condenses -150° C.

4 -

Radium A

3 mins.

a

Attaches itself to negative electrode

4

soluble in strong acids.

Radium B

21 mins.

no rays.

Volatile at 500°C.

4

Radium C

28 mins.

«, 0, 7

Volatile about 1100°C.

4

Radium D

about 40 years.

3,7

Soluble in sulphuric acid.

4

Radium E

about 1 year.

a

Attaches itself to bismuth plate in

4

solution, volatilizes at 1000°C.

The changes which occur in the active deposits from the emanation
of radium, thorium, and actinium have Keen difficult to determine on
account of their complexity. For example, in the case of radium, the
active deposit, obtained as a result of a long exposure to the emana-
tion, contains quantities of radium A, B, and C. The changes occur-
ring in the active deposit of radium have been determined by P. Curie,
Danne, and the writer. The value of T for the three successive changes

176 PHYSICS OF THE ELECTRON

is 3, 21, and 28 minutes respectively. Radium A gives only a rays, B
gives out no rays at all, while C gives out a, /?, and j rays. These
results have been deduced by the comparison of the change of activity
with time, with the mathematical theory of successive changes. The
variation of the activity with time depends upon whether the activity
is measured by the a, /?, or 7- rays. The complicated curves are very
completely explained on the hypothesis of three successive changes
of 'the character already mentioned.

The activity of a vessel in which the radium emanation has been
stored for some time rapidly falls to a very small fraction after the
emanation is withdrawn. There, however, always remains a slight
residual activity. The writer has recently examined the activity in
detail. The residual activity at first mainly consists of /? rays, and the
activity measured by them does not change appreciably during the
period of one year. The a ray activity is at first small, but increases
uniformly with the time for the first few months that the activity has
been examined. These results receive an explanation on the hypothe-
sis that radium C changes into a product D which emits only /? rays.
D changes into a product E, which emits only a rays. This view has
been confirmed by separating the a ray product by dipping a bismuth
plate into the solution containing radium D and E. The probable
period of these changes can be deduced from observations of the mag-
nitude of the a and {3 ray activity at any time. It has been deduced
that radium D is probably half transformed in forty years, and
radium E is half transformed in about one year. The evidence at
present obtained points to the conclusion that radium E is the active
constituent present in Marckwald's radio-tellurium, and probably
also in the polonium of Mme. Curie.

The changes in the active deposit of thorium have been analyzed
by the writer, and the corresponding changes in actinium by Miss
Brooks.

The occurrence of a " rayless change " in the active deposits from the
emanation of radium, thorium, and actinium is of great interest and
importance. As these products do not emit either a or /? or f rays,
their presence can only be detected by their effect on the amount of
the succeeding products. The action of the rayless change is most
clearly brought out in the examination of the variation of activity
with time of a body exposed for a very short interval in the presence
of the emanations of thorium and actinium. Let us consider, for
simplicity, the variation of activity with time for thorium. The activ-
ity (measured by the a rays) observed at first is very small, but gradu-
ally increases with the time, passes through a maximum, and finally
decays according to an exponential law with the time falling to half
value in 11 hours. The shape of this curve can be completely ex-
plained on the assumption of the two successive changes, the second

PRESENT PROBLEMS OF RADIOACTIVITY 177

of which alone gives out rays. The matter deposited on the body
during the short exposure consists almost entirely of thorium A.
Thorium A changes into B and the breaking up of B gives rise to
the activity measured.

Let n0= number of particles of thorium A deposited on the body
during the time of exposure to the emanation.

Let P and Q be the number of particles of thorium A and B re-
spectively at any time after removal.

Let ^i, ^2 be the constants of the two changes.

The number of particles of P existing at any time t is given by
P = n0e~Al*. If each atom of A in breaking up gives rise to one atom
of B, the increase dQ in the number of Q in the time dt is given by
the difference between the number of atoms of B supplied by the
change in A and the number of B which break up.

dQ

Thus, — = ^P - 12Q = ,U0e-M - Jl,Q.

at

The solution of this equation is of the form Q = oe~Al* + be~^.
Since for a very short exposure Q = 0

3 J

AI AI

and (/ == "i r~ ( c 6 ) ,

/!-X2

If thorium A does not give out rays, the activity of the body at any
time after removal is proportional , to Q. Thus the activity at any
time t is proportional to e~^i — e~^t. Now the experimental curve
of variation of activity is found to be accurately expressed by an equa-
tion of this form. A very interesting point arises in settling the values
of ^! and ^ corresponding to the two changes. It is seen that the equa-
tion is symmetrical in ^ and X2 and in consequence is unaltered if the
values of At and ^z are interchanged. Now the constant of the change
is determined by the observation that the activity finally decays to
half value in 1 1 hours. The theoretical and experimental curves are
found to coincide if one of the two products is half transformed in 1 1
hours and the other in 55 minutes. The comparison of the theoretical
and experimental curves does not, however, allow us to settle whether
the period of change of thorium A is 55 minutes or 1 1 hours. In order
to settle the point, it is necessary to find some means of separating
the products thorium A and B from each other. In the case of tho-
rium, this is done by electrolysing a solution of thorium. Pegram
obtained an active product which decayed according to an exponen-
tial law with the time falling to half value in a little less than 1 hour.
This result shows that the radiating product thorium B has the
shorter period. In a similar way, by recourse to electrolysis, it has
been found that the change of actinium B has a period of 1.5 minutes.

178 PHYSICS OF THE ELECTRON

In the case of radium, P. Curie and Danne utilized the difference in
volatility of radium B and C in order to fix the period of the changes.

It is very remarkable that the third successive product of radium,
thorium, and actinium should not give out rays. It seems probable
that these ray less changes are not of so violent a character as the
other changes, and consist either of a rearrangement of the compon-
ents of the atom or of an expulsion of an a or /? particle with so slow
a velocity that it fails to ionize the gas. The appearance of such
changes in radioactive matter suggests the possibility that ordinary
matter may also be undergoing slow "rayless changes/7 for such
changes could not have been detected in the radio-elements, unless
its succeeding products emitted rays.

It is seen that the changes occurring in radium, thorium, and
actinium are of a very analogous character and indicate that each
of these bodies has a very similar atomic constitution.

While there can be no doubt that numerous kinds of radioactive
matter with distinct chemical and physical properties are produced
in the radio-elements, it is very difficult to obtain direct evidence in
some cases that the products are successive and not simultaneous.
This is the case for products which have either a very slow or very rapid
rate of change compared with the other product. For example, it is
difficult to show directly that radium B is the product of radium A
and not the direct product of the emanation. In the same way,
there is no direct evidence that radium C is the parent of radium
D. At the same time the successive nature of these products is in-
dicated by indirect evidence.

There can be little doubt that each of the radioactive products
is a distinct chemical substance and possesses some distinguishing
physical or chemical properties. There still remains a large amount
of chemical work to be done to compare and arrange the chemical
properties of these products and to see if the successive products
follow any definite law of variation. The electrolytic method can in
many cases be used to find the position of the product in the electro-
chemical series. The products which change most rapidly are present
in the least quantity in radium and pitchblende. Only the slower
changing products like the radium emanation and radium D and E
exist in sufficient quantities to be examined by the balance. It is
possible that the products radium A, B, and C may be obtained in
sufficient quantity to obtain their spectrum.

VIII. Connection between the a Particles and Helium

The discovery of Ramsay and Soddy that helium was produced by
the radium emanation was one of the greatest interest and importance,
and confirmed in a striking manner the disintegration theory of radio-

PRESENT PROBLEMS OF RADIOACTIVITY 179

activity, for the possible production of helium from radioactive
matter had been predicted on this theory before the experimental evi-
dence was forthcoming. Ramsay and Soddy found that the presence
of helium could not be detected in a tube immediately after the intro-
duction of the emanation, but was observed some time afterwards,
showing that the helium arose in consequence of a slow change in the
emanation itself or in its further products.

The question of the origin of the helium produced by the radium
emanation and its connection with the radioactive changes occurring
in the emanation is one of the greatest importance. The experimental
evidence so far obtained does not suffice to give a definite answer to
this question, but suggests the probable explanation. There has been
a tendency to assume that the helium is the final disintegration pro-
duct of the radium emanation, i. e., it is the inactive substance which
remains when the succession of radioactive changes in the emanation
have come to an end. There is no evidence in support of such a con-
clusion, while there is much indirect evidence against it. It has been
shown that the emanation which breaks up undergoes three fairly
rapid transformations; but after these changes have occurred, the
residual matter — radium D — is still radioactive, and breaks up
slowly, being half transformed in probably about forty years. There
then occurs a still further change. Taking into account the minute
quantity of the radium emanation initially present in the emanation
tube, the amount of the final inactive product would be insignificant
after the lapse of a few days or even months. It thus does not seem
probable that the helium can be the final product of the radioactive
changes. In addition, it has been shown that the a particle behaves
like a body of about the same mass as the helium atom. The expulsion
of a few a particles from each of the heavy atoms of radium would not
diminish the atomic weight of the residue very greatly. The atomic
weight of the atoms of radium D and E is in all probability of the
order of two hundred, since the evidence supports the conclusion that
each atom expels one a particle at each transformation. In order
to explain the presence of helium, it is necessary to look to the other
inactive products produced during the radioactive changes. The a
particles expelled from the radioactive product are themselves non-
radioactive. The measurement of the ratio ^ shows that they have
an apparent mass intermediate between that of the hydrogen and
helium atoms. If the a particles consist of any known kind of matter
they must be either atoms of hydrogen or of helium. The actual value
of ~ has not yet been determined with an accuracy sufficient to
give a definite answer to the question. On account of the very slight
curvature of the path of the a particles in a strong magnetic or electric
field, an accurate determination of ^ is beset with great difficulties.

180 PHYSICS OF THE ELECTRON

The experimental problem is still further complicated by the fact that
the a particles escaping from a mass of radium have not all the same
velocity, and in consequence it is difficult to draw a definite conclusion
from the observed deviation of the complex pencil of rays.

The results so far obtained are not inconsistent with the view that
the a particles are helium atoms, and indeed it is difficult to escape
from such a conclusion. On such a view, the helium, which is grad-
ually produced in the emanation tube, is due to the collection of a
particles expelled during the disintegration of the emanation and its
further products. This conclusion is supported by evidence of another
character. It is known that thorium minerals like monazite sand con-
tain a large quantity of helium. In this respect, they do not differ
from uranium minerals which are rich in radium. The only common
product of the different radioactive substances is the a particle, and
the occurrence of helium in all radioactive minerals is most simply
explained on the supposition that the a particle is a projected helium
atom. This conclusion could be indirectly tested by examining
whether helium is produced in other substances besides radium, for
example, in actinium and polonium.

The experimental determination of the origin of helium is beset
with difficulty on all sides. If the a particle is a helium atom, the total
volume of helium produced in an emanation tube should be three
times the initial volume of the emanation present, since the eman-
ation in its rapid changes gives rise to three products each of which
emits a particles. This is based on the assumption, which seems to be
borne out by the experiments, that each atom of each product in
breaking up expels one a particle. This at first sight offers a simple
experimental means of settling the question, but a difficulty arises in
accurately determining the volume of helium produced by a known
quantity of the radium emanation. It would be expected that, if the
emanation were isolated in a tube and left to stand, the volume of gas
in the tube should increase with time in consequence of the liberation
of helium. In one case, however, Ramsay and Soddy observed an
exactly opposite result. The volume diminished with time to a small
fraction of its original value. This diminution of volume was due to
the decomposition of the emanation into a non-gaseous type of matter
deposited on the walls of the tube, and followed the law of decrease to
be expected in such a case, namely, the volume decreased according
to an exponential law with the time, falling to half value in four days.
The helium produced by the emanation must have been absorbed by
the walls of the tube. Such a result is to be expected if the particle is
a helium atom, for the a particle is projected with a velocity sufficient
to bury itself in the glass to a depth of about ^ mm. This buried
helium would probably be in part released by the heating of the tube,

PRESENT PROBLEMS OF RADIOACTIVITY 181

such as occurs with the strong electric discharge employed in the
spectroscopic detection of helium. Ramsay and Soddy have examined
the glass tubes in which the emanation had been confined for some
time, to see if the buried helium was released by heat. In some cases,
traces of helium were observed.

Accurate measurements of the value of ^ for the a particle, and
also an accurate determination of the relative volume of the emana-
tion and the helium produced by it, would probably definitely settle
this fundamental question.

Certain very important consequences follow on the assumption
that the a particle is, in all cases, an atom of helium. It has already
been shown that the radio-elements are transformed into a succession
of new substances, most of which in breaking up emit an a particle.
On such a view, the atom of radium, thorium, uranium, and actinium
must be supposed to be built up in part of helium atoms. In radium,
at least five products of the change emit a particles, so that the
radium atom must contain at least five atoms of helium. In a similar
way, the atoms of actinium and thorium (or if thorium itself be not
radioactive, the atom of the active substance present in it) must be
compounds of helium. These compounds of helium are not stable, but
spontaneously break up into a succession of substances, with an
evolution of helium, the disintegration taking place at a definite but
different rate at each stage. Such compounds are sharply distin-
guished in their behavior from the molecular compounds known to
chemistry. In the first place, the radioactive compounds disintegrate
spontaneously and at a rate that is independent of the physical or
chemical forces at our control. Changes of temperature, which exert
such a marked influence in altering the rate of molecular reactions are
here almost entirely without influence. But the most striking feature
of the disintegration is the expulsion, in most cases, of a product of
the change with very great velocity — a result never observed in
ordinary chemical reactions. This entails an enormous liberation of
energy during the change, the amount, in most cases, being about one
million times as great as that observed in any known chemical reac-
tion. In order to account for the expulsion of an a or /? particle with
the observed velocities, it is necessary to suppose that their particles
exist in a state of rapid motion in the system from which they escape.
Variation of temperature, in most cases, does not seem to affect the
stability of the system.

It is well established that the property of radioactivity is inherent
in the radio-atoms, since the activity of any radioactive compound
depends only on the amount of the element present and is not affected
by chemical treatment. As far as observation has gone, both uranium
and radium behave as elements in the usual accepted chemical sense.

182 PHYSICS OF THE ELECTRON

They spontaneously break up, but the rate of their disintegration
seems to be, in most cases, quite independent of chemical control.
In this respect, the radioactive bodies occupy a unique position. It
seems reasonable to suppose that while the radioactive substances
behave chemically as elements, they are, in reality, compounds of
simpler kinds of matter, held together by much stronger forces than
those which exist between the components of ordinary molecular
compounds. Apart from the property of radioactivity, the radio-
elements do not show any chemical properties to distinguish them
from the non-radioactive elements, except their very high atomic
weight. The above considerations thus evidently suggest that the
heavier inactive elements may also prove to be composite.

IX. Origin of the Radio-Elements

We have seen that the radio-elements are continuously breaking
up and giving rise to a succession of new substances. In the case of
uranium and thorium, the disintegration proceeds at such a slow rate
that in all probability a period of about 1000 million years would be
required before half the matter present is transformed. In the case of
radium, however, where the process of disintegration proceeds at over
one million times the rate of uranium and thorium, it is to be expected
that a measurable proportion of the radium should be transformed
in a single year. A quantity of radium left to itself must gradually
disappear as such in consequence of its gradual transformation into
other substances. This conclusion necessarily follows from the known
experimental facts. The radium is continuously being transformed
into the emanation which in turn is changed into other types of
matter. Since there is no evidence that the process is reversible,
all the radium present must, in the course of time, be transformed
into emanation. The rate at which radium is being transformed can
be approximately calculated either from the number of a particles
expelled per second or from the observed volume of the emanation
produced per second. Both methods of calculation agree in fixing
that in a gram of radium about one milligram is transformed per
year. From analogy with other radioactive changes, it is to be
expected that the rate of change of radium would be always propor-
tional to the amount present. The amount of radium would thus
decrease exponentially with the time, falling to half value in about
1000 years. On this point of view, radium behaves in a similar way
to the other known products, the only difference being that its rate
of change is slower. We have already seen that, in all probability,
the product radium D is half transformed in about 40 years and ra-
dium E in about one year. In regard to their rate of change, the two
substances radium D and E, which are half transformed in about 40

PRESENT PROBLEMS OF RADIOACTIVITY 183

years and 1 year respectively, occupy an intermediate position be-
tween the rapidly changing substances like radium A, B, and C and
the slowly changing parent substance radium.

If the earth were supposed to have been initially composed of pure
radium, the activity 20,000 years later would not be greater than the
activity observed in pitchblende to-day. Since there is no doubt that
the earth is much older than this, in order to account for the existence
of radium at all in the earth, it is necessary to suppose that radium
is continuously produced from some other substance or substances.
On this view, the present supply of radium represents a condition of
approximate equilibrium where the rate of production of fresh radium
balances the rate of transformation of the radium already present. In
looking for a possible source of radium, it is natural to look to the sub-
stances which are always found associated with radium in pitchblende.
Uranium and thorium both fulfill the conditions necessary to be a
source of radium, for both are found associated with radium and both
have a rate of change slow compared with radium. At the present
time, uranium seems the most probable source of radium. The activ-
ity observed in a good specimen of pitchblende is about what is to
be expected, if uranium breaks up into radium. If uranium is the
parent of radium, it is to be expected that the amount of radium
present in different varieties of pitchblende obtained from different
sources should always be proportional to the amount of uranium con-
tained in the minerals. The recent experiments of Boltwood, Strutt,
and McCoy indicate that this is very approximately the case. It is
not to be expected that the relation should, in all cases, be very exact,
since it is not improbable, in some cases, that a portion of the active
material may be removed from the mineral, by the action of perco-
lating water or other chemical agencies. The results so far obtained
strongly support the view that radium is a product of the disinte-
gration of uranium. It should be possible to obtain direct evidence
on this question by examining whether radium appears in uranium
compounds which have been initially freed from radium. On account
of the delicacy of the electric test of radium by means of its emanation,
the question can very readily be put to experimental trial. This has
been done for uranium by Soddy and for thorium by the writer, but
the results, so far obtained, are negative in character, although, if
radium were produced at the rate to be expected from theory, it
should very readily have been detected. Such experiments, however,
taken over a period of a few months, are not decisive, for it is by no
means improbable that the parent element may pass through several
slow changes, possibly of a "rayless" character, before it is trans-
formed into radium. In such a case, if these intermediate products
are removed by the same chemical process from the parent element,
there may be a long period of apparent retardation before the radium

184 PHYSICS OF THE ELECTRON

appears. The considerations advanced to account for radium apply
equally well to actinium, which in all probability, when isolated, will
prove to be an element of the same order of activity as radium. The
most important problem at present in the study of radioactive min-
erals is not the attempt to discover and isolate new radioactive sub-
stances, but to correlate those already discovered. Some progress
has already been made in reducing the number of different radio-
active substances and in indicating the origin of some of them. For
example, there is no doubt that the " emanating substance" of Giesel
contains the same radioactive substance as the actinium of Debierne.
In a similar way, there is very strong evidence that the active con-
stituent in the polonium of Mme. Curie is identical with that in the
radio-tellurium of Marckwald. The writer has recently shown that
the active constituent in radio-tellurium or polonium is, in all prob-
ability, a disintegration product of radium (radium E). The same
considerations apply to the radio-lead of Hofmann, which is probably
identical with the product radium D. It still remains to be shown
whether or no there is any direct family connection between the
radioactive substances uranium, thorium, radium, and actinium.
It seems probable that some at least of these substances will prove to
be lineal descendants of a single parent element, in the same way
that the radium products are lineal descendants of radium. The
subject is capable of direct attack by a combination of physical and
chemical methods, and there is every probability that a fairly definite
answer will soon be forthcoming.

X. Radioactivity of the Earth and Atmosphere

It is now well established, notably by the work of Elster and Geitel,
that radioactive matter is widely distributed both in the earth's
crust and atmosphere. There is undoubted evidence of the presence
of the radium emanation in the atmosphere, in spring water, and in
air sucked up through the soil. It still remains to be settled whether
the observed radioactivity of the earth's crust is due entirely to
slight traces of the known radioactive elements or to new kinds of
radioactive matter. It is not improbable that a close examination
of the radioactivity of the different soils may lead to the discovery of
radioactive substances which are not found in pitchblende or other
radioactive minerals. The extraordinary delicacy of the electro-
scopic test of radioactivity renders it possible not only to detect the
presence in inactive matter of extremely minute traces of a radio-
active substance, but also in many cases to settle quickly whether
the radioactivity is due to one of the known radio-elements.

The observations of Elster and Geitel render it probable that the
radioactivity observed in the atmosphere is due to the presence of

PRESENT PROBLEMS OF RADIOACTIVITY 185

radioactive emanations or gases, which are carried to the surface
by the escape of underground water and by diffusion through the
soil. Indeed, it is difficult to avoid such a conclusion, since there is
no evidence that any of the known constituents of the atmosphere
are radioactive. Concurrently with observations of the radioactivity
of the atmosphere, experiments have been made on the amount of
ionization in the atmosphere itself. It is important to settle what
part of this ionization is due to the presence of radioactive matter in
the atmosphere. Comparisons of the relative amount of active matter
and of the ionization in the atmosphere over land and sea will prob-
ably throw light on this important problem.

The wide distribution of radioactive matter in the soils which have
so far been examined has raised the question whether the presence
of radium and other radioactive matter in the earth may not, in
part at least, be responsible for the internal heat of the earth. It
can readily be calculated that the presence of radium (or equivalent
amounts of other kinds of radioactive matter) to the extent of about
five parts in one hundred million million by mass would supply as
much heat to the earth as is lost at present by conduction to its sur-
face. It is certainly significant that, as far as observation has gone,
the amount of radioactive matter present in the soil is of this order
of magnitude.

The production of helium from radium indirectly suggests a method
of calculation of the age of the deposits of radioactive minerals.
It seems reasonable to suppose that the helium always found asso-
ciated with radioactive minerals is a product of the decomposition
of the radioactive matter present. About half of the helium is re-
moved by heating the mineral and the other half by solution. It
thus does not seem likely that much of the helium found in the
mineral escapes from it, so that the amount present represents the
quantity produced since its formation. If the rate of the production
of helium by radium (or other radioactive substance) is known, the
age of the mineral can at once be estimated from the observed
volume of helium stored in the mineral and the amount of radium
present. All these factors have, however, not yet been determined
with sufficient accuracy to make at present more than a rough esti-
mate of the age of any particular mineral. An estimate of the rate of
production of helium by radium has been made by Ramsay and
Soddy by an indirect method. It can be deduced from their result
that 1 gram of radium produces per year a volume of helium of about
25 cubic mms. at standard pressure and temperature. They, how-
ever, consider this to be an underestimate. On the other hand, if the
a particle is a helium atom, it can readily be calculated that 1 gram
of radium produces per year about 200 cubic mms. of helium.

Let us consider for example the mineral fergusonite. Ramsay and

186 PHYSICS OF THE ELECTRON

Travershave shown that it yields about 1.8 cc. of helium per gram and
contains about 7 per cent of uranium. It can readily be deduced
from known data that each gram of the mineral contains about one
four-millionth of a gram of radium. Supposing that one gram of
radium produces ^ cc. of helium per year, the age of the mineral is
readily seen to be about 40 million years. If the above rate of pro-
duction of helium by radium is an overestimate, the time will be
correspondingly longer. I think there is little doubt that when the
data required are accurately known this method can be applied,
with considerable confidence, to determine the age of the radioactive
minerals.

XI. Radioactivity of Ordinary Matter

The property of radioactivity is exhibited to the most marked
extent by the radioactive substances found in pitchblende, but it is
natural to ask the question whether ordinary matter possesses this
property to an appreciable degree. The experiments that have so
far been made show conclusively that ordinary matter, if it possesses
this property at all, does so to a minute extent compared with
uranium. It has been found that all the matter that has so far been
examined shows undoubted traces of radioactivity, but it is very
difficult to show that the radioactivity observed is not due to a
minute trace of known radioactive matter. Even with the extra-
ordinarily delicate methods of detection of radioactivity, the effects
observed are so minute that a definite settlement of the question
is experimentally very difficult. J. J. Thomson has recently given an
account at the Meeting of the British Association of the work done on
this subject in the Cavendish Laboratory, and has brought forward
experimental evidence that strongly supports the view that ordinary
matter does show specific radioactivity. Different substances were
found to give out radiations that differed in quality as well as in
quantity. A promising beginning has already been made, but a great
deal of work still remains to be done before such an important con-
clusion can be considered to be definitely established.

SHORT PAPER

PROFESSOR R. A. MILLIKAN, of the University of Chicago, presented a paper to
this Section on " The Relation between the Radioactivity and the Uranium Con-
tent of Certain Minerals," of which the following is an abstract:

In March, 1904, the author, assisted by Mr. H. A. Nichols, Assistant Curator of
Geology at the Field Columbian Museum (Chicago) , began an investigation of the
relation between the radioactivity and the uranium content of uranium-bearing
minerals with a view to ascertaining whether the radioactive substances found
in pitchblende are not all decomposition products of uranium. If such be the
case the ratio between the uranium content and the radioactivity of uranium
ought obviously to be constant, in case the assumption may be made that the
active products of the decomposition are not washed out of the mineral by per-
colating water or other agencies.

Since the beginning of this investigation some preliminary results have been
published in Nature by Boltwood which indicate a constancy in this ratio in the
case of a few American ores which he has examined. McCoy (cf. Ber. d. Chem. Ges.
36, 3043) has also found a similar indication of constancy in the case of the six
different kinds of uranium minerals which he has studied.

The present investigation is not yet complete, but so far as it has gone it fur-
nishes additional evidence in support of the view that uranium is the parent of
radium, for it extends somewhat the number of minerals for which the ratio
between the activity and the uranium content is approximately constant. The
following table gives the results thus far obtained.

Per cent

Activity

% ura-

% of de-

of ura-

in terms

nium di-

parture

Name •

nium con- of ura-

vided by

from

of mineral

Locality

tained

nium oxide

activity

mean

Pitchblende

Colorado

59.1

3.24

18.2

4.2

Clevite

Norway

69.3

4.03

17.2

9.4

Gummite

North Carolina

55.4

2.56

21.6

13.6

Pitchblende

Cornwall,

Eng.

9.23

.55

16.9

11.6

Autunite

Cornwall,

Eng.

6.9

.33

20.8

9.5

Autunite

Saxony

4.0

.205

19.5

3.6

Mean

19To

It will be seen that the departures from the mean value of the ratio amount in
some cases to as much as 13 %, but this was found to be no more than the dif-
ferences which might be obtained by "resurfacing" the same specimen of a given
substance.

The measurements on activity were all made as follows: three hundred mg. of
the very carefully powdered mineral were spread as uniformly as possible over
three square inches of a metal sheet. This sheet was then placed upon the lower
plate of an air-condenser which was connected with one pair of quadrants of an
electrometer, the other pair being earthed. The condenser-plates were ten cm. on
a side and 3 cm. apart. A potential of one hundred and thirty volts was applied to
the upper condenser-plate, and the rate of charging of the electrometer noted.
The potential to which the needle of the electrometer was charged was one hundred
and twenty-five volts. The chemical analyses were all made by Mr. Nichols.

BIBLIOGRAPHY: DEPARTMENT OF PHYSICS

(Prepared for the Department by the courtesy of Professor Henry Crew)

GENERAL PHYSICS

ARRHENIUS, Lehrbuch der kosmischen Physik.

GriLLAUME and POINCARE, Rapports pre'sente's au Congres International de

physique. (Paris, 1900.)

POYNTING and THOMSON, Text- Book of Physics.
WATSON, Text-Book of Physics (Longmans).
WINKELMANN, Handbuch der Physik, 6 vols. 2d ed.
VIOLLE, Cours de Physique.

DYNAMICS

CLIFFORD, Elements of Dynamic.

HERTZ, Die Principien der Mechanik.

LAGRANGE, Me"canique Analytique.

LAMB, Hydrodynamics.

LOVE, Treatise on the Mathematical Theory of Elasticity.

MINCHEN, Treatise on Statics.

NEWTON, Principia.

THOMSON and TAIT, Treatise on Natural Philosophy.

WEBSTER, Dynamics.

WIEN, Lehrbuch der Hydrodynamik.

SOUND

BLASERNA, Theory of Sound.

DONKIN, Acoustics.

HELMHOLTZ, Sensations of Tone. (Trans, by Ellis.)

RAYLEIGH, Theory of Sound.

HEAT

BOLTZMANN, Vorlesungen iiber Gastheorie.

EWING, Steam Engine and other Heat Engines.

FOURIER, Analytical Theory of Heat. (Trans, by Freeman.)

GUILLAUME, Thermome'trie.

JEANS, Dynamical Theory of Gases.

MAXWELL, Theory of Heat.

MEYER, Kinetic Theory of Gases. (Trans, by Baynes.)

PLANCK, Thermodynamics. (Trans, by Ogg.)

PRESTON, Theory of Heat.

LIGHT

CZAPSKI, Theorie der Optischen Instrumente.

DRUDE, Theory of Optics. (Trans, by Mann and Millikan.)

EDSER, Light for Students.

KAYSER, Handbuch der Spectroscopie.

KELVIN, Baltimore Lectures.

LARMOR, Ether and Matter.

BIBLIOGRAPHY : DEPARTMENT OF PHYSICS 189

NEWTON, Opticks.
SCHUSTER, Theory of Optics.

ELECTRICITY AND MAGNETISM

BOLTZMANN, Vorlesungen tiber Maxwells Theorie der Electricitat und des

Lichtes.

Du Bois, Magnetische Kreise.

EWING, Magnetic Induction in Iron and other Metals.
FARADAY, Experimental Researches.
HERTZ, Electric Waves. (Trans, by Jones.)
MAXWELL, Treatise on Electricity and Magnetism.
RUTHERFORD, Radioactivity.
STARK, Die Electricitat in Gasen.
THOMSON, Conduction of Electricity through Gases.
THOMSON, J. J., Recent Researches in Electricity and Magnetism.
WEBSTER, Theory of Electricity and Magnetism.

PHYSICAL PAPERS

Scientific Papers of Abbe, Heaviside, Helmholtz, Hertz, Hopkinson, Joule,
Kirchhoff, Maxwell, Rankine, Rayleigh, Reynolds, Rowland, Tait.

SIR WILLIAM RAMSAY, Ph.D., LL.D.
Professor of Chemistry, University College, London

DEPARTMENT X— CHEMISTRY

DEPARTMENT X — CHEMISTRY

(Hall 5, September 20, 4.15 p. m.)
CHAIRMAN: PROFESSOR JAMES M. CRAFTS, Massachusetts Institute of Techno-

SPEAKERS: PROFESSOR JOHN U. NEF, University of Chicago.

PROFESSOR FRANK W. CLARKE, Chief Chemist, U. S. Geological
Survey.

THE Chairman of the Department of Chemistry was Professor
James M. Crafts, of the Massachusetts Institute of Technology, who
in opening the work of the Department spoke of the great stimulus
which American chemists owed to European laboratories and the
lively remembrance of the freedom of these laboratories and priceless
instruction given. The application which Americans make of the
scientific methods acquired abroad are characteristic of our nation-
ality, but at the same time strongly reminiscent of other sources.

The decade within which this Congress meets has been one of
extraordinary interest in the history of chemistry. I say a decade,
although perhaps I should say a half-decade, since we are told by the
British Premier, addressing the meeting for the advancement of
science at Cambridge, that " until five years ago our race has without
exception lived and died in a world of illusions." His admirably
turned periods appear to signalize our old conceptions of the consti-
tution of matter as the chief among illusions, and he seems to look
forward to the immediate replacement of the false doctrine by a
more idealistic conception of the universe. The atomic theory is
naturally dismissed with censure, and thus we have taken away from
us those blocks with which we built so happily our toy houses in the
days of our innocent, childish faith. The last Faraday lecturer has
been less cruel, for although he has no faith in the indivisibility of
atoms, from which we can knock off electrons, nor in the individu-
ality of the elements, his criticism is not merely negative, but, like
a truly scientific engineer, he offers us a new model for our construc-
tions. Professor Ostwald invites us to enter a beautiful stalactite
cavern, groping, indeed, in some obscurity, but with the vision of
a brighter light beyond.

The observations of the Rontgen and Becquerel rays have led in
Germany, France, England, and Canada to a study of emanations,
which has been distinguished by extreme skill in the invention of new
methods and by the minute study of phenomena, which seemed even
a year ago beyond the reach of human ingenuity.

194 CHEMISTRY

The simplest statement of facts is sufficiently wonderful and mys-
terious. Although not more than two or three grams of radium have
been gathered from the earth's crust, its natural history is already
well developed, and at latest news we are told that one gram of
radium bromide will evolve 0.0022 milligrams of helium in one year;
that the life of a radium atom is 1050 years, or, in another experiment,
1250 years.

It may be said that within this decade the knowledge of the struc-
ture of carbon compounds has become so complete that the way to the
production of the most useful bodies has become evident in theory,
and I need not remind you of the consequent achievements by that
happy combination of pure and industrial science in Germany.

Also within this decade, the somewhat neglected study of mineral
chemistry has acquired unexpected interest by the discovery in
France of metallic carbides and nitrides, formed at temperatures
comparable with those of the sun, and these discoveries, besides giving
rise to most unexpected industrial applications, show entirely new
possibilities for the geology of the primitive rocks.

The active pursuit of physical chemistry has extended over some
thirty years. Great dates are the publication, just two decades ago,
of van 't Hoff 's Etudes de dynamique chimique, and one year after-
wards of Ostwald's Allgemeine Chemie; and, again, ten years ago Dix
Annees d'une Theorie.

Suffice it to say that the title, General Chemistry, has been amply
justified, The attractive presentation of bold theories, their rapid
confirmation by experiment, and the completeness of treatment by
the founders of the new science have led to the immediate acceptance
of their views, until the mathematical analysis of chemical pheno-
mena has become the dominating feature of our science, and has
transformed our methods of thought, as Kepler and Newton's
theories transformed the study of astronomy.

ON THE FUNDAMENTAL CONCEPTIONS UNDERLYING
THE CHEMISTRY OF THE ELEMENT CARBON

BY JOHN ULRIC NEF

[John Ulric Nef, Professor of Chemistry, and head of the Chemical Department,
University of Chicago, b. Herisau, Canton Appenzell, Switzerland, June 14,
1862. A.B. Harvard, 1884; Ph.D. Munich, 1886; Kirkland Fellow, Harvard,
1884-87. Professor, Purdue University, 1887-89; Assistant Professor, Clark
University, 1889-92; Professor,University of Chicago, since 1892. Member of
American Academy of Arts and Science, National Academy of Sciences, Royal
Society of Science, Upsala, Sweden.]

Two fundamental conceptions underlie our present system of
carbon chemistry. First, the idea of the constant quadri valence
of carbon, which explains most adequately the existence of the vast
array of carbon compounds. Second, the conception of substitution
or metalepsis, which gives us a basis for interpreting many of the
reactions shown by these substances.

These ideas are, however, in the light of investigations of the past
twenty years, inadequate; the}* must be replaced by the conception
of a variable valence of carbon and by the conception of dissociation
in its broadest sense. A rigid application of the latter conception
gives a far simpler basis for interpreting all the reactions of carbon
chemistry; they are • naturally also applicable to the chemistry of
all the other elements.

I. On the Valence of the Carbon Atom

The progress of organic chemistry since 1858 is due chiefly to the
development of a few very simple ideas concerning the valence of
the elements, ideas which were first clearly and fully presented at
that time by Kekule.

Hydrogen, oxygen, and nitrogen are the elements which most
frequently combine with carbon to form the so-called organic com-
pounds. Since the compounds of one atom of oxygen, nitrogen, or
carbon with hydrogen possess the empyrical formulae, 0 =H2, N = H3,
C=H4, the conception naturally presents itself that the capacity
of the various elements for holding hydrogen atoms varies. Oxygen
is capable of holding two such atoms, nitrogen holds three, and car-
bon four atoms of hydrogen.

Therefore we assume, taking hydrogen as our unit, that the valence

I
of the element oxygen is two, — O — , of nitrogen, three, — N — , and

I
of carbon, four, — C — .

196 CHEMISTRY

Without going into much detail concerning the nature of valence,
or, what is the same thing, concerning the nature of the forces in-
herent in our atoms, we assume briefly that every atom of an ele-
ment possesses one, two, three, four, or more such units of force, and
we call the element univalent, bivalent, trivalent, quadrivalent, etc.,
according to the number of such units it possesses. It is by virtue
of the existence of these units of force that the compounds made up
of the same or of various elementary atoms exist. We assume that
in such a molecular compound the atoms are bound one to another
in a definite way by means of their affinity units.

Since the development of these ideas concerning the valence of the
elements there has been a great deal of work carried on with the ob-
ject of determining whether the valence of an element is constant or
whether it may vary; the majority of chemists are now convinced
that it may vary. The valence of nitrogen may be three or five. The
valences of hydrogen, oxygen, and carbon, on the other hand, have,
until recently, been assumed always to remain constant, i. e., one, two,
and four, respectively.

Since the complexity, the very great variety and number of exist-
ing compounds containing carbon are unquestionably to be attrib-
uted to the peculiar nature of the forces inherent in the carbon atom,
let us consider a little more in detail what hypotheses we make in our
present system of carbon chemistry concerning this element. We
assume, first, that the valence of the carbon atom is always four;
second, that the four valences or affinity units of the carbon atom
are equivalent; third, that they are distributed in space in three
dimensions and act in tetrahedral directions; fourth, that the carbon
atoms can unite with one another by means of one, two, or three
affinity units to form what we usually call chains.

These chains may be open, or closed rings or cycles. The number of
carbon atoms thus bound to one another may be exceedingly large.
The closed chains usually contain three, four, five, six, or seven carbon
atoms in the ring. We may have in these chains, whether open or
closed, some of the carbon atoms replaced by oxygen, nitrogen,
sulphur, or other elements. If now we unite the extra valences of each
carbon or other atom — i. e. , those affinity units which are not neces-
sary for binding the atoms together in chains — with other atoms
or radicals, it is at once evident that we can represent theoretically,
by so-called graphical formulae, molecules of great complexity. It is
also at once obvious that with a small number of atoms it must be
possible to construct a relatively large number of aggregates which
differ from one another simply in the way the atoms are bound to-
gether. In 1884, for instance, fifty-five totally different substances of
the empyrical-formula C9H10O3 were actually knowyn. We call them
isomers. One of the chief problems of organic chemistry since 1858

FUNDAMENTAL CONCEPTIONS 197

has been to determine on the basis of these ideas of valence the " con-
stitution" of the carbon compounds; we determine by methods
which are called synthetic, as well as by an exhaustive study of the
reactions of a given compound, what may be called the "architec-
ture" of its molecule, i. e., we determine how the various atoms of
carbon, nitrogen, oxygen, and hydrogen, etc., of which the substance
may be composed are joined together by virtue of their affinity units.
How much has been accomplished on the basis of these ideas during
the past forty-six years, and how beautifully and simply all the facts
known with regard to the almost countless carbon compounds are
thus explained, only those can fully appreciate who have a detailed
knowledge of the subject. Notwithstanding the large number of
workers in the field, it has often required more than a decade of
work to determine the molecular architecture of one single carbon
compound, and the question at times seriously presents itself whether
we must not reach our limitations in this respect. In any case one
point is deserving of especial emphasis: this idea of structure which
has been applied chiefly to molecules containing the element carbon
attributes to them a rigidity which is improbable from a purely
dynamic standpoint.

The present system of organic chemistry is thus founded upon the
assumption that the valence of all the atoms of carbon, wherever
found, remains invariably four. In the earlier part of the last century
many attempts were made to isolate the hydrocarbon methylene,
C =H2, which must contain bivalent carbon. Dumas and Peligot tried

H

to obtain this substance from methylalcohol, H2C, by loss of

OH

H

water. Perrot tried to isolate it from methylchloride, H2C

Cl

by dissociation into methylene and hydrogen chloride at high tem-
perature. Berthelot, Butlerow, Wurtz, and Kolbe also made many
fruitless attempts in this direction. As a final result of these repeated
and negative efforts, chemists finally became convinced that com-
pounds containing bivalent carbon could not be isolated, and the con-
clusion, therefore, that carbon was one of the few elements possessing
a constant valence became very general.

There has, however, long existed one very simple compound of
carbon which does not adjust itself to this system, — namely, the
inactive and poisonous carbon monoxide. If we assume the valence
of oxygen as two, then we have here simply a derivative of methylene
in which the two hydrogen atoms are substituted by oxygen, C=O.

198 CHEMISTRY

To be sure there were many chemists who preferred to consider the
valence of carbon in carbon monoxide as four, thus making the valence
of oxygen four, C = 0 ; and when we bear in mind that the other mem-
bers of the oxygen group, sulphur selenium and tellurium, exist as
di-, tetra-, and hexavalent atoms, there is some justification for this
interpretation. To me personally, however, it seems in the highest
degree improbable that two atoms should be thus bound to each other
by four affinity units.

About fourteen years ago a series of systematic experiments was
undertaken with the object of ascertaining whether carbon can exist
in a bivalent condition. The experiments have established this pornt
in a most decisive manner; we have now quite an array of substances
which contain bivalent carbon. Furthermore it has been possible
to prove, from the experience gained in their study, that methylene
chemistry plays an important role in many of the simplest reactions
of organic chemistry, reactions which have hitherto been explained
on the basis of substitution. At the time when these experiments
were undertaken there existed besides carbon monoxide several sub-
stances which might contain bivalent carbon — namely, prussic acid
and its salts the cyanides, HN=C and MN=C. Also the so-called
carbylamines, RN=C, discovered in 1866 by Gautier.

These substances were, therefore, exhaustively studied in order to
establish rigidly by experiment whether bivalent carbon was present
or absent. The presence of dyad carbon having been established and
its properties thus being known, the problem then presenting itself
was the isolation of methylene and its homologues.

You are probably all aware that Gay Lussac established in 1815
the existence of a radical, composed of one atom of carbon and one
of nitrogen, in prussic acid and the cyanides. This radical, cyanogen,
plays in its compounds a role similar to that of the elements of the
halogen group.

In 1832 Pelouse discovered the alkylcj^anides, R — C = N, by treating
cyanide of potash with alkyliodides or with alkylpotassic sulphates,
KCN + RI or ROSO2OK^R-C = N + KI or KOS02OK, an appar-
ent double decomposition reaction by which we obtain a compound
in which the radical R(=Cn H2n+1) is joined to the cyanogen group
by means of carbon. The alkylcyanides thus obtained are neutral,
pleasant-smelling, harmless liquids, resembling ether, chloroform,
and the alkylhalides, RC1, RBr, and RI.

In 1866 Gautier discovered by treating cyanide of silver with alkyl-
iodides, RI + AgNC^RN=C + AgI, also an apparent double decom-
position reaction, a new class of organic compounds; they are iso-
meric, not identical, with the alkylcyanides of Pelouse. He called
them the carbylamines or isonitriles, and proved that the alkyl group
is bound to the cyanogen radical by means of nitrogen RN=C or

FUNDAMENTAL CONCEPTIONS 199

RN = C. It thus became evident that we must distinguish between
two cyanogen radicals, viz., one which in its compounds is bound
to alkyl, hydrogen, or metal by means of carbon, R — C = N, H — C == N,
MC = N, and another which is joined to these elements or groups by
means of nitrogen, RN = C, HN =C, MN =C. We may call the former
radical cyanogen, — C = N, and the latter isocyanogen,

-N=Cor -N = C;

these radicals may obviously combine with each other to form three
isomers of the empyrical formula C2N2. The substances discovered
by Gautier, the alkylisocyanides, R — N=C or R — N = C, have
properties strikingly different from those of their isomers, the
alkylcyanides, R — C = N, of Pelouse. They are poisonous, nauseat-
ing compounds which affect the throat like prussic acid and color
the blood intensely red; they produce violent headaches and vomit-
ing. Their odor is most pronounced and persistent. Hofmann, who,
in 1868, discovered another method for making them from primary
amines, chloroform, and caustic potash,

RNH2 + 3KOH + CHCU^RN=C + 3KC1 + 3H2O,
found it impossible to work with them except for very short periods.

An exhaustive study of the reactions of these alkylisocyanides,
carried out in 1891-92, led to the definite conclusion that they contain
a dyad carbon atom, i.e., they possess the constitution represented
by the formula RN: C; the other possible formula with quadrivalent
carbon and quinquivalent nitrogen, RN = C, is excluded by the facts.

The alkylisocyanides belong to the vast category of unsaturated
compounds whose chemistry will be briefly discussed from a perfectly
general standpoint below; they manifest especially their great chem-
ical activity by absorbing other substances forming new molecules
in which the valence of carbon has changed from two to four.
Such reactions we call additive. Two molecules simply unite to form
one new molecule — the addition product. A molecule containing an
unsaturated carbon atom, i.e., one with two of its valences latent or
polarized, RN=C or RN=CJ, cannot per se show any chemical
activity whatever. This is also true of a system containing a pair of
doubly or triply bound carbon atoms, ethylene,CH2 = CH2,and acety-
lene, CH = CH; and finally of a saturated system which we may
represent by a paraffine, CnH2n + 2, for instance, marsh gas,

H H

\c/

/°\
H H

All these substances manifest chemical activity simply because they
are to a greater or less degree in a dissociated or what may be called
an active condition. A given quantity of alkylisocyanide contains
an extremely small per cent of molecules with two free affinity units,

200 CHEMISTRY

RN=C ; these are in dynamic equilibrium with the absolutely

inert molecules RN = C or better RN=C]. That this percentage
varies with the nature and mass of R is shown by the fact that
various alkylated and arylated isocyanides manifest different degrees
of chemical activity. Carbon monoxide possesses relatively a smaller

number of such active particles, O =C , and consequently is a com-

paratively inert substance, since the speed of addition reactions
shown by unsaturated compounds must naturally be directly in pro-
portion to the per cent of active molecules present. A similar concep-
tion obviously explains the relative differences in reactivity shown
by the various members of the olefine and acetylene series. Marsh
gas, a saturated system, reacts with other substances because it is par-

tially dissociated as follows, CH, ?± CH3 - + H - and ^. CH2 + 2H - .

From this point of view chemical action depends entirely upon dis-
sociation processes. The reactions often proceed with very great
slowness because the percentage of dissociation is extremely low,
possibly one tenth to one thousandth of one per cent, or even less.

Turning now to a consideration of the reactions of alkylisocyan-
ides; the substances which are absorbed by the unsaturated carbon
atom present in the isonitriles are the following.

(1) Halogens [(chlorine, bromine, iodine) ; speed of reaction in the
order named].

X X

\ \

X X

The reactions, especially those with chlorine and bromine, take place
with great evolution of heat at —20°.

(2) Acidchlorides, such as RCO-C1, C1-OC2H5, C1-CO-C1,
Cl-CN, Cl-COOR, to form the addition products:

Cl Cl

/ I I

RN=C/ RN = C-CO-C:NR, etc.

COR,

A hyphen denotes the point where the compounds are partially dis-
sociated and consequently absorbed. These reactions, especially those
with phosgene and ethylhypochlorite, take place with great violence
at -20°.

(3) Oxygen and sulphur, to form isocyanates and mustard oils,
RN=C=O and RN=C=S. Methylisocyanide unites directly at
its boiling-point, 58°, with the oxygen of the air. The dry oxides

FUNDAMENTAL CONCEPTIONS 201

of silver and mercury are reduced to metals with violence at 40°,
alkylisocyanates being first formed. This shows the great affinity of
bivalent carbon for oxygen.

(4) Primary amines and hydroxylamine,

RN = C/+H-NHR
H
or H-NHOH-*RN = C^

NHR
H

or RN=C/

NHOH

giving amidines or oxyamidines.

(5) Alcohols in the presence of an alkali are absorbed, giving

H

imido ethers, RN=C

ORt

(6) Hydrogen sulphide and mercaptans give readily at 100° the

H

H

/

addition products RNH — C=S and RN=C

SR,

(7) Adds. Aqueous mineral acids act with great violence on the
isonitriles giving primary amines and formic acid :

H
RN=C/+2H20->RNH2 + ^C:O.

In the absence of water and on diluting the alkylisocyahides with
absolute ether, perfectly dry halogen hydride causes the separation
of white hygroscopic salt-like substances of the empyrical formula
2RNC, 3HX[X=C1 Br or I]. For this reason Gautier as well as
Hofmann supposed the isonitriles to be basic compounds, i. e., sub-
stances behaving like ammonia — hence the name carbylamine was
given them by Gautier. Further study has shown, however, that
this conclusion was erroneous. The isonitriles are entirely devoid of
basic properties; the great violence with which they act with halogen
hydrides is due to the presence of unsaturated carbon. The reaction
probably takes place as follows:

C (1)

X
X

RN=C/+X-CH=NR->RN:C-CH=NR (2)

X X2

I II

RN=C-CH=NR + 2H-X->RNH-C-CHX-NHR (3)

Reversibility of the reactions. The most striking property of the

X

addition products of the isonitriles, RN=C^ is their low point of

Y

dissociation, i. e., the carbon atom which has absorbed the X — Y,
thus becoming quadrivalent, is unable to hold XY above certain tem-
perature limits. There is consequently in every case a temperature,
varying with the nature an4 mass of X and Y as well as with the
nature and mass of the groups bound to the other two affinity units
of carbon, at which the carbon atom becomes spontaneously dyad
and is unable to remain in a quadrivalent condition; it was subse-
quently possible to prove that this is a perfectly general property of
this atom. All the addition products under discussion are partially

X

dissociated, the dissociation RN =C <=^ RN : C< + XY, increasing

Y

as the temperature is raised, — in other words the valence of carbon
at temperatures below the dissociation-point is an equilibrium phe-
nomenon; dynamic equilibrium exists between bivalent and quadri-
valent carbon.

The point of complete dissociation of the various addition products
of the isonitriles has not been accurately determined in every case.
The following data with reference to the dissociation-points of carbon
monoxide addition products are of interest and therefore used for
illustration in this connection:

Dissociation-Point

Formaldehyde, O : C =H2 600°

H

Formamide, 0 = C ^ about 250°

NH3

FUNDAMENTAL CONCEPTIONS 203

H

Formic acid, 0=C^ 169°

OH

H

Formhydroxamic acid, 0=C^ 85°

NHOH
H

Formylchloride, 0=C^ below —20°

Cl

Since these substances containing quadrivalent carbon decom-
pose spontaneously into carbon monoxide, i. e., cannot exist in the
quadrivalent condition at temperatures above those indicated, it is
self-evident that at lower temperatures the addition products must
be partially dissociated and that in the future we must be able to
determine in each case with absolute accuracy the per cent of dis-
sociation at any temperature. A striking experiment with formhy-
droxamic acid, dissociation-point 85°, proves the correctness of this
conclusion; on allowing this crystalline substance to stand at 20° in
acetone solution the following reaction takes place quantitatively:

HONH

H

HO

NOK + H-OH+ ^C: O-^(CH3)2: C=NOH+ ^C: 0.

H

In a similar manner we can prove that the isonitrile addition pro-
ducts, many of which have definite boiling-points and are quite stable,
are partially dissociated at ordinary temperatures. Thus the addition

X

products with halogen RN=C^ are all converted back quantita-

X

tively into the alkylisocyanides by treatment with finely divided
metals, zinc-dust or sodium, which simply abstract the free halogen.
Many of the acylhalide addition products dissociate spontaneously
into the components on distillation; these phenomena are perfectly
analogous to the dissociation of dry ammonium chloride :

204 CHEMISTRY

H

Cl

For this reason the majority of the addition products of the isoni-
triles can be kept only for a short time; this property rendered
futile many attempts to isolate definite addition products. The
continual dissociation of such products sets free active or disso-
ciated alkylisocyanide particles, and these slowly condense with one

another, xRN=C^ — >(RN=C)x, giving rise to the so-called al-

kylisocyanide resins (non-reversible), — products whose molecular
weight has not yet been determined and which are perfectly analogous
to azulmic or polymerized prussic acid. Consequently in carrying
out an addition reaction with an isonitrile, especially if it requires
much time or a temperature above 20°, large quantities of these resin-
ous polymers are formed from which it is possible to isolate the addi-
tion product only with great difficulty.

Many of the isonitriles themselves even when perfectly pure undergo
rapid polymerization to resins so that they can be kept only for a very
short time. Phenylisocyanide, C8H5N=C], is the most striking in-
stance, as it changes in a few minutes from a colorless to a dark blue
liquid, and in a few days condenses to a dark brown resin. Have we not
here a possible explanation of the fact that it is impossible to isolate
methylene and a large number of its derivatives, although marsh gas,

H H H

methyl alcohol, and chloride of methyl, H2C ^ H2C ^ H2C ^

H OH Cl,

all contain a relatively small per cent of active methylene particles
at ordinary temperatures?

The presence of bivalent carbon in the alkylisocyanides having
been established, the next question presenting itself was whether
prussic acid and its salts contain the cyanogen or the isocyanogen
radical. In the latter case, H — N=C, M — N=C, these substances
must be analogous to Gautier's isonitriles. It had hitherto been con-
sidered as established, but without sufficient evidence, that prussic
acid and the cyanides were cyanogen compounds analogous, to the
nitriles of Pelouse.

When one considers the physical and physiological properties of
prussic acid [boiling-point 25°, specific gravity 0.7, a violent poison]
and contrasts these with the corresponding properties of methyl-
cyanide [boiling-point 81°, specific gravity 0.81, sweet-smelling harm-
less oil] and of methylisocyanide [boiling-point 58°, specific gravity
0.75, a poison], one at once comes to the conclusion that prussic acid

FUNDAMENTAL CONCEPTIONS 205

as well as its salts must belong to the isocyanogen compounds and
consequently must contain bivalent carbon. An exhaustive study
of prussic acid and the cyanides establishes this sharply, especially in
the case of the salts, from a chemical standpoint. The relation of
fulminic acid to prussic acid corroborates the evidence.

You are all familiar with fulminate of mercury — a substance which
is made on a commercial scale and used for explosives. It was dis-
covered in 1800 by Howard, and analyzed in 1824 by Liebig in Gay
Lussac's laboratory. We obtain it by dissolving mercury in concen-
trated nitric acid and adding the resulting solution to ordinary
alcohol. It has the empyrical formula HgC2N2O2, and being obtained
from ethylalcohol, CH3CH2— OH, fulminic acid was supposed to
have two carbon atoms in its molecule, H2C2N202. The constitution
of this substance was for a long time a great puzzle to chemists. That
we have here a substance very closely related to prussic acid was dis-
covered by accident. In working with the mercury salt of isonitro-
methane it was found that this compound is spontaneously converted
at 0° into fulminate of mercury according to the equation,

O H

H C = NOh -H / C = NO*1^11'0 + C : NOhS-

HO

This synthesis led directly to the conclusion that fulminate of
mercury possesses a constitution entirely analogous to cyanide of
mercury, C = Nhg, i. e., that it contains the isocyanogen radical with
bivalent carbon. A further study of the fulminates established this
point with precision. Especially striking is the behavior of fulminates
towards dilute acids. Liebig and Gay Lussac state in 1824, judging
from the odor, that fulminate of silver gives prussic acid with dilute
hydrochloric acid. A more careful study of this reaction in 1894
proved that not a trace of prussic acid but a substance formylchloride

H

oxime, C = NOH, is formed which possesses the following re-

Cl

markable properties. Long needles, clear as glass, which decompose
and explode at 20° ; extremely volatile even at 0° and having an odor
similar to prussic acid which is obviously due to a partial dissociation
into fulminic acid. Aqueous silver nitrate converts it quantitatively
into chloride and fulminate of silver,

H

^ C = NOH + 2AgNO3->AgON = C + AgCl + 2HNO3.
Cl

206 CHEMISTRY

Up to 1897 the presence of bivalent carbon had been established
in the following compounds, (1) carbon monoxide, 0 = C; (2) the
alkyl and aryl isocyanides, RN = C; (3) prussic acid and the cyanides,
HN = C, MN = C; (4) fulminic acid and the fulminates, HO - N = C,
MO — N = C. (2) , (3) and (4) are all compounds containing the iso-
cyanogen radical.

In 1897 the presence of bivalent carbon was established in a series
of nitrogen free carbon compounds obtained from acetylene. They

H

are the mono-and dihalogen substituted acetylidenes, ^ C = C and

X
X

C = C [X = Cl Br or I]. The corresponding members of the acetyl-

X

ene series, XC^CH and XC=CX, do not exist, although we have
substances like CH3C = CI, C6H5C==C — X, whose properties are in
marked contrast to those of the acetylidene derivatives.

Diiodacetylidene, which possesses an odor deceptively like that of
the isonitriles, dissociates at 100° with violence into iodine and dia-
tomic carbon, I2 = C = C-^I2 + C = C; the latter cannot be isolated as
such, but polymerizes explosively to graphite and amorphous carbon.
The mono- and dihalogen substituted acetylidenes are all poisonous
and spontaneously combustible compounds, possessing, therefore, like
methylisocyanide a marked affinity for oxygen Up to the present
time it has not been possible to isolate compounds containing
bivalent carbon other than those mentioned above. We are, how-
ever, now in a position to explain clearly why we cannot hope by
methods now known to isolate methylene and its homologues as
such, although these substances play a great role in many of the
fundamental reactions of organic chemistry. In order to approach
this point more intelligently, let us consider briefly the properties of
unsaturated compounds in general, their possibility of existence, etc.

.II. On the Unsaturated Compounds

The unsaturated compounds may, first of all, be divided into three
categories, namely; (1) those in which two atoms, which may be the
same or different, are bound doubly or triply to each other by two
or three affinity units, such as defines, acetylenes, chlorine, C1 = C1,

R

oxygen, O = 0, aldelhydes, C = 0, alkylcyanides, RC = N, nitric

H

FUNDAMENTAL CONCEPTIONS 207

i

acid, HON = O, sulphur trioxide, O = S^ etc.; (2), those in which

Xo,

an atom itself is unsaturated, i.e., does not exert its maximum valence
capacity, as, for instance, amines, R3N, thioethers, R, =S], methylene
derivatives, etc. We must assume that the remaining affinity units are
latent, or, what is far more probable, especially where two or four
affinity units are available, that they mutually polarize each other in
a manner entirely similar to unsaturated compounds containing
doubly or triply linked atoms.

Finally we have a third class of unsaturated compounds, (3) those
containing closed atomic chains such as trimethylene,
CH2 O

/ \ / \

CH2 — CH2,propyleneoxide, CH3CH — CH2 etc., which show

apparently a saturated molecular system like the paraffines, and yet
react in a manner perfectly analogous to defines and methylene de-
rivatives. Fundamentally considered, these three classes of unsatur-
ated compounds manifest their chemical activity in the same way;
they absorb a great variety of other molecules and thus form com-
binations, called addition products. How does this union take place?
An unsaturated compound with its affinities polarized represents in
reality a saturated system; it cannot per se show chemical activity.
This is also true of molecular systems in which the atoms are bound
to one another by single affinity units. The sole basis for reactivity
in either case is the presence of a relatively greater or smaller number
of dissociated particles. The reactivity of any unsaturated, as well as
of a saturated compound, must in fact be directly proportional to the
ratio of such active particles present. If that ratio is very small, the
substance may be entirely inert; if it is greater, absorption of reagents
proceeds with regularly increasing speed.

Experience has shown, furthermore, that many unsaturated
compounds cannot be isolated, but polymerize spontaneously. It is
clear that when the per cent of active particles present in an unsatur-
ated compound becomes relatively great, the possibility of their
uniting with each other to form condensed molecules increases —
in fact, we may imagine a condition in which the active molecules
simply cannot be prevented from combining with each other. This
shows us why we cannot isolate and keep substances like formalde-
hyde, H2C = 0, or alkylcyanates, R — O — C = N, in the monomo-
lecular form. Similarly in many cases where attempts were made
to isolate methylene derivatives like mono- and diphenyl methyl-
ene, benzoyl and acetyl methylene, cyanmethylene-carboxylate,

208 CHEMISTRY

CN

^ C], a spontaneous polymerization to the di- or tri-molecular
COOR

X XX C/'

systems, ,C=CV or Cv

/ \ / \

Y Y Y C

X
\

„ took place.

\
Y

One further point with reference to unsaturated compounds must
now be presented.

Intramolecular Rearrangements shown by Unsaturated Systems

From the discussion presented above it is obvious that trimethyl-
ene and propyleneoxide, belonging to class 3, must contain a small
percentage of active particles; the dissociation of the triatomic ring
in the former case can lead to only one form of active molecule, namely,
— CH2 — CH2 — CH2 — ; whereas propyleneoxide may give the follow-
ing three active molecules:
0-

I
CHS-CH-CH2- (A);

I I

CH3 CH-CH2-O (B);

I I

and CHSCH-0-CH2 (C).

Since propyleneoxide absorbs dry ammonia or hydrogen chloride,
as was proved by especially careful and exhaustive experiments, giv-
ing addition products of the general formula

CH3 CHOH-CH2 X [X = C1 or NH2],

the only possible conclusion that can be reached is that propylene-
oxide contains relatively more active A than active B or C mole-
cules; consequently the absorption reactions proceed by preference
in only one of three theoretically possible directions.

When trimethylene or propyleneoxide is heated or placed in con-
tact with various catalytic agents, the percent of active particles must
naturally increase, and when a definite limit has been reached a spon-
taneous transformation of trimethylene into propylene and of propy-
leneoxide into propylaldehyde (f) and acetone (£) takes place; both
reactions are non-reversible. These results can only be explained in
the following manner: aside from the increase in active particles

FUNDAMENTAL CONCEPTIONS 209

dissociation in other parts of the molecule and especially of hydrogen
from carbon must also take place. Consequently the following intra-
molecular addition reactions finally occur spontaneously:
H

active trimethylene propylene

particles

0- O- 0

I I II

CH, - C - CH, - -> CH, - C - CH3 -> CH3 - C - CHS[£].

I I

H

active propyleneoxide acetone

particles [A]

H

I I I

CH3-CH-CH-0-^CH3CH2-CH-O-<=>CH3CH2-CH:0[§].

active propyleneoxide

particles [B] propionaldehyde.

It is interesting to note that the active B propyleneoxide molecules
which are present in smaller ratio suffer rearrangement more readily
than the active A molecules. The active C molecules, on the other
hand, must be present in far smaller amount and certainly no transfor-
mation of propyleneoxide to vinylmethyloxide, CH2=CH — 0— CHS,
takes place. It is important to realize that propyleneoxide, acetone
and propionaldehyde are isomers but do not stand in a tautomeric
relation to one another. This is also true of trimethylene and propy-
lene as well as of a and /? amylene and isoamylene, etc.

Similarly it can be rigidly shown by experiment that a and /?

propylidene, CH3CH2 — CH= and (CH3)2=C(^ , which are sponta-

neously combustible substances not capable of isolation as such,
transform themselves by intramolecular addition,
H

I / I I

CH,-CH-CH(->CH3-CH-CH2;f±CH3-CH = CH2,

II I I

and CH3 C - CH, -H->CH3CH - CH2^CH3-CH =CH2,

into propylene [non-reversible].

There is not the slightest doubt that such intramolecular addition
reactions are the basis of the majority of our synthetic methods for
making cyclic compounds. The cycloparaffines in Russian petroleum
are probably formed from ordinary paraffines by dissociation into

210 CHEMISTRY

hydrogen and methylene derivatives, and the latter then sponta-
neously transform themselves, by intramolecular addition, into penta-
and hexamethylene rings.

On the Reactions of the Paraffines and the Benzene Derivatives

The reactions of the paraffines and the benzene derivatives towards
halogens, nitric and sulphuric acids, whereby substitution products
are formed, are still interpreted in the text-books from the standpoint
of metalepsis or substitution, although a vast amount of evidence
has accumulated which makes this axiomatic assumption improbable.
The fact that ethane and benzene, for instance, decompose into hy-
drogen and into ethylene and diphenyl at 800° and 600° respectively
proves that an extremely small per cent of these molecules must
exist at ordinary temperatures in an active or dissociated condition,

CH3CH3 <=t CH3CH2 — h H —

and CH3CH = +2H-;orC6H6<=»C6H5-+H-.

The same is true of ammonia, H3N <=± H2N — |-H — and HN = + 2H -
and N= +3H, and of a great variety of other non-ionizable hydrogen
compounds.

Consequently when chlorine or nitric acid acts with benzene or
ethane to give the monochlor or mononitro substitution products,
we have these reagents, in the active molecular condition, simply uniting
by addition with the dissociated ethane or benzene particles,

H C2H5

o o

or HO-N-0 + H-C6H5->HON-OH.

The resulting addition products then lose hydrogen chloride and water
respectively and thus give the monochlor or nitro substitution pro-
duct of the mother substance. From this point of view all so-called
substitution reactions belong to the category of addition reactions.
What is now especially needed in order to place the reactions of organic
chemistry on an exact mathematical basis is a precise method of
determining the ratio of active particles present at various tempera-
tures in the case of the unsaturated as well as of saturated compounds.
As the substances under discussion are almost exclusively non-electro-
lytes, the sole methods that suggest themselves for this purpose are
determinations of the speed of decomposition as well as of addition
reactions.

The above discussion makes it evident that all unsaturated com-

FUNDAMENTAL CONCEPTIONS 211

pounds belonging to classes 1 and 3 contain a small and relatively
varying per cent of active particles with one or more carbon atoms
temporarily in an active or trivalent condition; the same is true
of compounds containing hydrogen bound to carbon-paraffines,
CnHJn+1 — H, benzene derivatives, etc. The isolation of compounds
containing trivalent carbon as such, I believe, however, to be an impos-
sibility. Gomberg's triphenylmethyl, for instance, has recently been
proved by him and others to be a bimolecular aggregate C38H3o, —
identical with hexaphenylethane — which, however, like the above-
mentioned compounds, contains a very small percentage of active
triphenylmethyl, (C6H5)3=C — , particles in dynamic equilibrium
with the bimolecular aggregate.

We are now in a position to consider the evidence showing that
methylene and its homologues play a great role in many of the
fundamental reactions of organic chemistry which have hitherto
been explained on the basis of substitution.

III. On the Reactions of the Monatomic Alcohols and the Alkylhaloids

The experiments which first suggest themselves as a means of
isolating methylene and its homologues are, (1) dissociation of de-
fines as ethylene :

CH2<=»2H2C = ,/? butylene,

CHSCH: CH-CH3<=±2CH3CH etc.

Since ethylene gives hydrogen and acetylene by heat and the higher
olefines also decompose with evolution of hydrogen, there was little
prospect of success by experiments in this direction. (2) Dehydration
of the monatomic alcohols, CnH2n+1OH, or removal of halogen
hydride from the alkylhalides, CnH2n+1X; naturally only primary and

R

secondary derivatives, RCH2X and ^ CHX [X = OH Cl Br or I],

R'

and not tertiary compounds, R3 = C — X, can yield methylene and
its homologues. Furthermore since many of the alcohols and alkyl-
halides containing more than one carbon atom in the molecule are
known to give olefines by dissociation, dehydration, or treatment
with alcoholic potash respectively, the conclusion might naturally
at first be drawn that only a direct olefine dissociation existed in
these cases. From a purely theoretical standpoint, however, it is
clear that a primary or secondary alkylhalide or a corresponding
alcohol with more than one carbon atom in the molecule may disso-

212 CHEMISTRY

ciate with loss of halogen hydride or water in two possible ways:
it may undergo (1) methylene dissociation, as

X

R — CH2 — CH 4^R'CH.
\
H

R H R

and /^\ ^ /^

R' X R'

or (2) olefine dissociation, as R-CH2-CH2X<=»R-CH-CH2+HX,

, , ' '

and CH3CH2-CHX-CH3<=»CH3CH-CH-CH3

and CH3CH2-CH-CH2+HX;

I I

or both kinds of dissociation may take place simultaneously. A
third kind of dissociation, where the hydrogen atom does not come
from the atom containing the X or from a carbon atom adjacent to it,
is also possible, and at times important, but it need not be considered
in this connection.

An exhaustive study of the primary and secondary alcohols and
alkylhalides covering a period of nine years has proved very con-
clusively that these substances undergo methylene dissociation
only. Preliminary experiments with alcohols and alkylhalides where

H

no olefine dissociation is possible, i. e., in the methane, H2C^

X

H H

toluene, C6H5CH , diphenylmethane (C6H5)2 : C , acetone

X X

H H

and acetophenone, CH3CO — CH and C8H5COCH. , malonic

X X

X CN H

and cyanacetic ether series (COOR)2C and s®\ > nave

H COOR X

proved that all these compounds have very low dissociation-points
— never over 300° in the aromatic nor with few exceptions in the
aliphatic series. Nevertheless, it was found impossible to isolate in

FUNDAMENTAL CONCEPTIONS 213

any case the methylene derivative as such; there was either a spon-
taneous conversion to a di- or trimolecular polymer, an olefine or a
trimethylene derivative, or a conversion to resinous polymers ana-
logous to azulmic acid and the alkylisocyanide resins. Most import-
ant was the discovery that these nascent or active methylene residues,

Z

^C, are always spontaneously combustible, burning often with

Y

Z

marvelous evolution of heat to the corresponding oxides, ^C=O ;

Y

this was not surprising in view of the properties of the methylene
derivatives described above. Furthermore, the affinity of unsaturated
carbon for oxygen is strikingly shown by the fact that these residues
have the power of decomposing water,
Z Z

^C + 0=H2^ ")C:0+2H-,

Y Y

with evolution of hydrogen.

A subsequent investigation of the primary and secondary alcohols
and alkylhalides containing more than one carbon atom proved,
first of all, that all these substances have comparatively low points
of dissociation. In no case was the decomposition-point found to be
higher than 700°; it was often as low as 160° to 300°. The products
of dissociation are water or halogen hydride and CnH2n respectively;
and the latter, as emphasized above, is invariably methylene or a
homologue and never an olefine. This naturally means that all these
compounds are partially dissociated in this way at ordinary tempera-
R H R'

tures, /C\ <=* ^C + HX, — relatively more the lower the
R' X R'

actual decomposition-point. It is, therefore, possible that in all the
interactions of the primary and secondary alkylhalides with other
substances, such as salts, ammonia, metals, benzene, etc., they do
not act as such, but by virtue of being partially dissociated. An
enormous amount of evidence has accumulated in favor of this con-
clusion. Let us consider chiefly the results obtained in the ethyl series
including ethyl alcohol and its derivatives. The dissociation or
decomposition-point of the following compounds containing ethyl
has been determined with a fair degree of accuracy.

214 CHEMISTRY

Decomposition-Point

H
Ethane, CH3CH/ 800°

H

H

Ethylalcohol, CH3CH/ 650°

OH

OM

Sodium and potassium ethylate, CH3CH (^ 250°

" H

/HH\

Ethylether, CH3CH 0- CH'CH3 550°

H

Ethylchloride, CH3CH/ 600°

Cl
H

Ethylbromide, CH3CH / 500°

Br
H

Ethyliodide, CH3CH/ 400°(?)

V11 H\

Diethylsulphate;CH3CH-O-SO2-0 • CHCH3 200°

H

Monoethylsulphate, CH3CH ^ 160°

OSO2OH
H

Ethylpotassium sulphate, CH3CH^ 250°

OS02OK
H

Ethylnitrate, CH3CH^ 200°(?)

ON02

Ethane, ethylchloride and bromide, when heated to the temperatures
named, give ethylene and hydrogen or halogen hydride respectively,
and on cooling these products do not again recombine. We can there-

FUNDAMENTAL CONCEPTIONS 215

fore obtain ethylene quantitatively from chloride or bromide of ethyl
by simply passing their vapors through tubes heated to the decompo-
sition-point. Nevertheless, it is impossible to obtain more than very
small amounts of ethylene from the ethylhalides by means of alcoholic
potash, caustic potash, or quicklime; in these cases ethylether or
ethylalcohol is the chief product even when the ethylhalide is passed
over quicklime in tubes heated from 300° to 500°. Furthermore,
the per cent of ethylene obtained varies remarkably with the tem-
perature, the concentration, and with the nature of the halogen in the
alkylhalide used. The conclusions finally reached from these data
and also from an exhaustive study of the behavior of the various
alkylhalides, nitrates, sulphates, alkylpotassium-sulphates towards
heat, sodium ethylate, caustic potash, quicklime, and other salts, are
that ethylene cannot possibly be a primary product of dissociation
of the ethylhalides, sulphates and nitrates and of free ethylalcohol.
The ethylene, when obtained, is formed from ethylidene by an intra-
molecular addition reaction,
H

I / I I

CH2-CH^->CH2-CH2^CH,=CH2,

which is not reversible.

A similar intramolecular change always, in fact, takes place when-
ever an olefine is formed, whether from a primary or secondary alcohol
or from a corresponding alkylhalide sulphate or nitrate. This trans-
formation is perfectly analogous to the conversion, discussed above, of
trimethylene and of propylene oxide into propylene, propionaldehyde
and acetone.

When ethylalcohol or ethylether is heated to its dissociation-point
the ethylidene interacts at once in great part with the other dissocia-
tion-product, water, to give hydrogen and acetaldehyde,

CHSCH/+O=H2->CHSCH:O+2H.

In the case of ether, since there are two ethylidene molecules to
one of water, the atomic hydrogen is in part absorbed by ethylidene
to give ethane. Finally, a portion of the ethylidene, 20 and 37 per
cent respectively, is transformed, by intramolecular addition, into
ethylene. The most striking proof that ether is dissociated into water
and two C2H4 particles is the following : on passing ether vapor over
phosphorous pentoxide at temperatures varying from 200° to 400°
ethylene is formed quantitatively.

The primary and secondary alcohols and their corresponding ethers
being in a state of very slight dissociation at ordinary temperatures,
we are able to understand perfectly their behavior towards oxidizing
agents. The alkylidenes are all spontaneously combustible substances

216 CHEMISTRY

possessing a great affinity for oxygen. Absolutely pure, dry ethylether,
dissociation-point 550°, contains a sufficient per cent of ethylidene
particles at ordinary temperatures to burn very slowly in dry oxygen ;
sodium ethylate, dissociation-point 250°, on the other hand, being
dissociated to a far greater extent, burns with great violence in dry
air. Ethylalcohol, dissociation-point 650°, is not capable of burning
in the air; if, however, we increase the per cent of ethylidene parti-
cles by means of catalytic agents, enzymes, platinum sponge, etc.,
it, too, oxidizes readily, with incandescence with platinum sponge,
giving acetic acid. The aldehydes, RCH =0, as has long been known,
reduce Fehling's solution and silver solutions with great ease. This

R

is due to the presence of oxyalkylidene particles, /^\ which

HO

burn at the expense of the oxygen of the water. The discovery that
all primary and secondary alcohols reduce silver oxide to metallic
silver in aqueous solution in the presence of caustic alkalies has only
very recently been made. The function of the alkali is obviously to
form first the metallic alcoholate,

R H R H

^C/ + MOH^± \C/ +H20,

R' OH R7 OM

which, having a far lower dissociation-point than the free alcohol,
causes a great increase in the per cent of alkylidene particles present ;
consequently the following reaction takes place,
R R

, etc.,

R' R'

giving as the end result a fatty acid in the case of primary alcohols.
The most striking proof that ethylalcohol is dissociated only into

H

ethylidene and water, CH3CH <=± CH3CH = + H20, i. e., contains

OH

no ethylene particles, is the following. Ethylalcohol, containing one
molecule of aqueous sodic hydrate, gives in the cold with potassium
permanganate solution practically acetic acid only. If any active

I I

ethylene particles were present, CH3CH2OH<=»CH2 — CH3 + H2O,

these must necessarily, in view of the work of Wagner with olefines
and permanganate, be first converted by oxidation to ethyleneglycol,

FUNDAMENTAL CONCEPTIONS 217

! I

3CH2 - CH2 + 6H - OH + 2KMn04 -» OHOH
3CH2 - CH2 + 2MnO, + 2KOH + 2H20.'

Analogous results would naturally be expected in the case of all
the homologous primary and secondary alcohols. Now a primary
alcohol invariably first gives by oxidation with potassium permangan-
ate or other oxidizing agents the corresponding fatty acid; glycols or
their oxidation products have never been observed in such cases. The
fact that ethylalcohol gives glyoxal, glyoxylic and oxalic acids with
nitric acid, is no exception to this rule because these substances result
from the hydrolysis and oxidation of isonitrosoacetaldehyde which
is formed by the action of nitrous acid on acetaldehyde as follows:

I I
H-CH2CHO + O-NOH-»HO-N-OH-»HON=CHCH:O + H2O.

I
CH2-CH:O

The behavior of aldehydes and of primary alcohols towards aqueous
or solid caustic potash also leads to the conclusion that only alkylidene
dissociation occurs. Ethylalcohol gives at 250°, with an excess of
caustic potash, hydrogen and potassium acetate quantitatively,
H

CH3CH / + 2KO - H -> CH3CH / + 3KOH -»

OK

CH3

CH3CH(OK)2 + 2H-+KOH-* \C/

KO
CH3

KO

If any of the potassium ethylate, which is first formed, were disso-
ciated into ethylene and caustic potash,

CH3.CH2OK<=±CH2-CH2

the define must naturally give, besides hydrogen, ethylene glycol,

! I

CH2.CH2 + 2HOK-»KOCH2-CH2OK + H2, or its decomposition pro-
ducts; these are, however, not formed. The reaction with potash
lime and primary alcohols is so delicate and accurate that it has
been suggested by Hell as a means of determining the molecular
weight of an unknown primary alcohol.

218 CHEMISTRY

As mentioned above, ethylether is the chief product when ethyl-
halides are treated with alcoholic potash or with dry sodium ethyl-
ate; this is also true when dry silver oxide and ethylhalides are used.
These reactions, which have been interpreted by Williamson and
others on the basis of double decomposition or of minute ionization,
must obviously be attributed to the absorption by the ethylidene of
alcohol or of water which is set free by the action of the halogen
hydride particles on the sodium ethylate or silver oxide respectively,

H
CH3CH / + H - OC2H5-»CH,CH /

OC2H5,
H H

or 2CH3CH^+0(/ -^CHsCH/ .CHCH,.

\ \ \ / I

H OH

We are now able to consider an entirely new explanation of the
function of sulphuric acid, or of phenylsulphonic acid, in converting
ethylalcohol into ether. Sulphuric acid acts first of all with alcohol
at ordinary temperatures to give both mono- and diethylsulphate;
the first stage in the reaction cannot be ascribed to the union of
ethylidene, formed by dissociation of alcohol, with free sulphuric
acid, since ethylether, which is relatively more dissociated than alcohol,
acts only very slowly with concentrated sulphuric acid at ordinary
temperatures to give primary ethylsulphate. Furthermore since
sulphuric acid itself is completely dissociated into its components,
sulphurtrioxide and water, at 400° it .is extremely probable that
monoethylsulphate is formed by the union of sulphurtrioxide, pre-
sent by dissociation, H2S04<=iSOs+H2O, with alcohol,

2

OCaHs

Now it is well known that ether formation in a mixture of sulphuric
acid and alcohol begins perceptibly only at 95° and proceeds very
slowly at that temperature. The favorable temperature for ether
manufacture is 140°. This is self-evident in view of the following
considerations. Primary and secondary ethylsulphate possess the
dissociation-points 160° and 200°, respectively; consequently these
substances must be dissociated at 140° to a very great extent into
sulphuric acid and one or two molecules of ethylidene respectively.
Addition of alcohol at 140°, therefore, simply necessitates a com-
bination with the ethylidene particles,

FUNDAMENTAL CONCEPTIONS 219

H
CH.CH/ +H-OC2H4->CH,CH/

OC,HS

to give ether, and this process can naturally go on indefinitely.

When ethyl alcohol is mixed with an excess of concentrated sul-
phuric acid and heated to 160° no ether but some ethylene is formed;
in fact this method is still suggested and used as the best means of pre-
paring ethylene. The yield of olefine, however, can never be raised
above 20 per cent of the theory, and the operation is extremely tedious
because carbonization and formation of sulphurdioxide takes place
to a very marked extent. These results are now easily understood.
The ethylidene molecules, formed by dissociation of ethylated sul-
phuric acid, burn chiefly at the expense of the oxygen present in

sulphuric acid, CH3CH(^ +0 =SO2^CH3CH:O+SO, and the result-
ing acetaldehyde is then at once charred by the vitriol present. Only
twenty per cent at the utmost of the ethylidene particles escape this
oxidation by intramolecular conversion to ethylene.

Finally we may summarize the conclusions reached in the above
discussion as follows. The valence of carbon is not a constant. At
definite temperatures, which vary remarkably with the nature of the
groups bound to it, a carbon atom becomes spontaneously dyad. Be-
low these limits there is dynamic equilibrium between bivalent and
quadrivalent carbon. The existence of carbon compounds containing
bivalent carbon has been definitely established ; methylene chemistry
plays a great r61e in many of the fundamental reactions of organic
chemistry. The conception of substitution or metalepsis, which has
been our guide in interpreting the reactions of carbon chemistry since
1833 is no longer tenable. It must be replaced by the conception of
dissociation in its broadest sense. Fundamentally speaking, there
are but two classes of carbon compounds, — the saturated and the
unsaturated compounds. Excluding reactions called ionic, a chemical
reaction between two substances always first takes place by their
union to form an addition product. The one molecule being unsatur-
ated and partially in an active molecular condition absorbs the
second molecule because it is partially split or dissociated into two
active portions. The resulting addition product then often dissociates
spontaneously, giving two new molecules.

The similarity of such reactions to those called ionic is at once ap-
parent, but their relationship cannot in the present state of our know-
ledge be clearly understood.

SPECIAL WORKS OF REFERENCE

LIEBIG, Annalen der Chemie, 1892-1904.

Vol. 270, pp. 267-335; Vol. 280, pp. 263-342;

Vol. 287, pp. 265-359; Vol. 298, pp. 202-374;

Vol. 308, pp. 264-333; Vol. 309, pp. 126-189;

Vol. 310, pp. 316-335; Vol. 318, pp. 1-57 and 137-230;

Vol. 335, pp. 191-333.

THE PROGRESS AND DEVELOPMENT OF CHEMISTRY
DURING THE NINETEENTH CENTURY

BY FRANK WIGGLESWORTH CLARKE

[Frank Wigglesworth Clarke, Chief Chemist, U. S. Geological Survey, b. March 19,
1847, Boston, Mass. S.B. Harvard, 1867; D.Sc. Columbian, 1899; D.Sc. Victoria
University, England, 1903; Chevalier de la Legion d'Honneur, 1900. Professor
in Howard University, 1873-74; Professor of Chemistry and Physics, University
of Cincinnati, 1874—83; Honorary Curator of Minerals, U. S. National Museum,
and Professor of Mineral Chemistry, George Washington University. Fellow of
American Association for the Advancement of Science; Corresponding member
of British Association for the Advancement of Science; Corresponding member
of the Edinburgh Geological Society; member of the American Philosophical
Society, and Washington Academy of Sciences: Honorary member of Man-
chester Literary and Philosophical Society, and London Chemical Society;
Past-President of the American Chemical Society (1901). Author of Weights,
Measures, and Money of all Nations; Elementary Chemistry, and many bulletins
of the Geological Survey. About one hundred articles in scientific journals. Wilde
lecturer and medalist, Manchester, England, Literary and Phil. Society, 1903].

THE history of any science is a record of progress from empiricism
to philosophy; from isolated details to systematic knowledge. At
the outset, certain facts impress themselves upon the minds of men,
either because the observed phenomena are beneficial, or for the
opposite reason. Between the facts, the simpler and more obvious re-
lations of cause and effect are first noted, but only in the most super-
ficial way and without deliberate intention. By degrees, after many
wanderings along paths that lead nowhere, and in spite of countless
misinterpretations, mankind slowly accumulates a mass of data in
which something like unity begins to appear, and through which it
is seen that the universe is not a creature of caprice, but an existence
organized and orderly. This conception lies at the foundation of all
science; it is the one article of faith which the student dares not
doubt; for rational investigation would be impossible without it.
The belief in order, and the hope that we may discover its laws, inspire
all scientific researches.

Speaking broadly, the development of science takes place in three
stages, which merge one into another and often overlap. First, there
is the collection of data; classification follows; and attempts at inter-
pretation come last of all. This is the logical course, which, however,
is not always followed. Premature speculations, efforts to determine
what the universe should be, are not unknown in the history of
human thought, nor have they been altogether futile. Hypotheses,
framed in advance of positive knowledge, help to stimulate investi-
gation, and so, despite their errors, lead us ultimately to the truth.
In reality, the three stages of growth coexist; experiment and specu-
lation go on side by side; and each one reinforces the others.

At the beginning of the nineteenth century, chemistry was in a

222 CHEMISTRY

transitional, I might almost say a formative, period of its existence.
It was just emerging from the morasses of a philosophy unchecked by
experiment, and from the vagaries of the alchemists, and was assum-
ing something like its present form. A goodly mass of data had been
gathered ; they were partly classified, and the work of interpretation
was successfully begun. The analyses of air and water, the discrimin-
ation between elements and compounds, and a recognition of the
constancy of mass, had laid the foundations of the new science. This
word "new" I use advisedly. In its earlier days chemistry was only
an empirical art, in which discoveries were made by chance, and
remembered because of their utility. Chemical facts were secrets in
the hands of artisans, or held by initiated priesthoods; and when
they were recorded at all it was only in the form of useful recipes
or as medical prescriptions. As a science, as an organized body of
knowledge with a philosophy of its own, chemistry hardly existed
before the time of Boyle. Alchemy, groping in the darkness, had
made useful discoveries; but their successful correlation was an
affair of a much later period. To Lavoisier, more than to any other
one man, the transformation of chemistry from an art into a science
must be ascribed. There were greater discoverers than Lavoisier,
perhaps, but he was the organizing spirit, and his proof that matter
was indestructible made quantitative chemistry a possibility. With-
out such a basis a rational science would be almost inconceivable.
It is a necessary complement to the older philosophical maxim that
from nothing nothing can be made. Creation and destruction are
equally beyond our powers — a truism which the ancients may have
apprehended, but which before the time of Lavoisier rested on spec-
ulation alone. Indeed, the conception was defective until the middle
of the nineteenth century, when the doctrine of the conservation of
energy raised it to completion.

Let us now return, to the opening of the century and see how mat-
ters stood. The simpler gases, acids, and bases, and the commoner
metals were known, and many compounds had been more or less
completely examined. Richter and Fischer had shown that reactions
took place in proportions which exhibited simple relations to one
another; the doctrine of phlogiston had ceased to dominate chemical
opinion, and the law of definite proportions, despite the opposition
of Berthollet, was generally received. That chemical changes should
be governed by fixed quantitative laws was a natural condition to
expect, but it needed both proof and explanation. So many reason-
able theories had already broken down that a healthy skepticism
prevailed, and chemists demanded concrete evidence in favor of
every proposition that philosophy might offer for their edification.
Rubbish had been cleared away, — what structure should rise in its
place? An answer to this question was speedily forthcoming.

PROGRESS IN NINETEENTH CENTURY 223

It was in October, 1803, that John Dalton published the beginning
of his famous atomic theory, but it was not until five years later that
he gave it completely to the world. Merely as a speculation, the idea
of atoms was as old as philosophy; but in its scientific form it was
something entirely new. Under it, the law of definite proportions
became necessary and significant; the law of multiple proportions,
which had been partially anticipated by others, was made complete ;
and these considerations alone would have justified the provisional
acceptance of the doctrine. It unified the known or suspected laws of
chemical combination and gave them philosophic validity. It incited
chemists to verify the evidence in its favor, and so led to new dis-
coveries; in short, it fulfilled all the conditions of a good scientific
theory. Its chief peculiarity, however, its prime difference from all
preceding atomism, remains to be stated. Dalton discovered that
to every element a single definite number could be assigned, and that
these numbers or their multiples governed the formation of all com-
pounds. Oxygen, for instance, unites with other elements in the
proportion of eight parts by weight or some multiple thereof; never
in other ratios. These combining numbers, under Dalton's theory,
became the relative weights of the atoms; and atomism, hitherto a
qualitative notion only, received a quantitative expression. With the
help of these atomic weights, or combining numbers, as some anti-
theorists preferred to call them, the composition of any substance
could be represented by a simple formula; and chemical calculations,
which had been empirical and arbitrary, became systematic and easy.
In short, Dalton had discovered a new class of constants, the funda-
mental numbers of quantitative chemistry, whose significance has
steadily increased and is probably not even yet completely appre-
ciated. To this point I shall recur later.

The decade following Dalton's unique discovery was chiefly
characterized by two lines of research, the study of inorganic com-
pounds, and the investigation of their physical relations. Davy,
by decomposing the alkalies and earths, gave precision and definite-
ness to the conception of a chemical element, while Gay Lussac and
Avogadro discovered the laws which connected the volume relations
of gases with their chemical composition. To Avogadro we owe the
discrimination between atoms and molecules — a distinction which
physics, unaided by chemical evidence, could probably not have
reached, and which even now is often overlooked by physicists. Max-
well, for example, in his article upon atoms in the Encyclopaedia
Britannica, deals with molecules throughout, and fails to mention
Dalton's work at all. To Maxwell the physical arguments were clear,
the chemical relations were not adequately appreciated.

In 1819 Dulong and Petit discovered the law connecting the
specific heat of a solid element with its atomic weight, but apart

224 CHEMISTRY

from that investigation chemical research became for thirty years
largely a matter of detail. Discoveries were many, successful general-
izations were few. During this epoch, Wohler, by his synthesis of
urea, broke down the barrier between organic and inorganic com-
pounds; Liebig and others proved that groups of atoms, the so-
called compound radicles, could play the part of pseudo-elements;
Dumas established the principle of substitution, and Faraday con-
nected the phenomena of electrolysis with the atomic constants.
Inorganic chemistry, however, received the lion's share of attention,
and the commanding figure of the period was that of Berzelius. To
him we owe the development of chemical formulae and equations,
the thorough determination of many atomic weights, the discovery
of new elements, and the investigation of innumerable compounds.
And yet his gigantic labors were performed in a laboratory which a
modern high school would despise, in which the chemist of to-day
would be able to accomplish next to nothing. It was, in fact, a kitchen,
wherein cookery and research were carried on almost side by side.
Had Berzelius possessed our wealth of resources, could he have
achieved a greater success? Perhaps not, for we must remember that
he had a virgin field to cultivate, and the implements of the pioneer
are less elaborate than those which his successors require. A great
part of the work done by Berzelius was necessarily crude, and much
of it is still awaiting revision, for the man who clears the ground is not
the one to give it the highest cultivation. As knowledge grows, the
demands for facilities increase, and we could not return to primitive
methods even if we wished to do so. Imagine a modern astronomer
with Galileo's telescope, and no more mathematics than Kepler
could command! Berzelius labored in the days of small things, and
being great he overcame the obstacles that confronted him; we
to-day are the slaves of a complexity such as the earlier chemists
could never have imagined. I refer now to the material side of science;
in its theoretical aspects simplicity has been gained and our range
of vision has widened correspondingly. We work in clearer air and
with much more powerful appliances than the investigators of earlier
times, but to say that we do better would be rash indeed. There are
giants in all days, and no age has a monopoly of greatness.

During the Berzelian period, as I have said, inorganic chemistry
was the main subject of chemical research. But it was not the only
theme, for chemical physics also received a good deal of attention,
and organic compounds were by no means neglected. Inorganic
substances were apparently simple, the organic were complex; and
so the former were naturally considered first, the more obvious pro-
blems taking precedence over the less evident. By degrees, however,
opinion changed, and the great discoveries of Wohler, of Liebig, and
of Dumas, the theoretical discussions of Laurent and Gerhardt, and

PROGRESS IN NINETEENTH CENTURY 225

perhaps also the physical regularities pointed out by Kopp, turned
the current of research into a new channel. Substances that could be
arranged in series, with progressive differences in composition and
properties, were evidently worth examining; a compound radicle
was, in its way, as fascinating an object of study as a new element;
the possibilities of substitution and the marvelous chemical plasticity
of organic matter were noted, and all of these considerations worked
together in effecting a transformation of chemical thought. Organic
chemistry became the fashion, and for nearly fifty years it was the
central subject of research.

Before entering upon this new period, let us go back and examine
the conditions under which progress had previously been made.
How was the work done, and what impulses urged it forward? What
purposes, what demands, what encouragement, led chemists to pursue
their labors? At first, chemistry was a branch of the older natural
philosophy, and the discovery of natural laws, the reaching after
truth for its own sake, was the chief aim of investigators. These, as
a rule, were individuals, working independently, each on his own
resources, and without thought of practical results. Science and
industry were as yet unallied; chemistry had but a small part in
schemes of education; institutions for the aid of research were few,
and those which did exist were scantily endowed. Davy, to be sure,
had the Royal Institution behind him, and in it he discovered
Faraday; Berzelius was secretary of the Academy at Stockholm;
but these were exceptional cases, and not by any means the rule.
Personal initiative and voluntary effort were almost the sole agencies
at work. The great discoveries were made by amateurs, by men who
among other labors found some leisure in which to study; and only
the occasional man like Cavendish, with ample means, could give his
whole time to research. Priestley was a clergyman; Scheele an
apothecary; Lavoisier a public official, and these are typical examples.
The impulse to investigate came from within, uninfluenced by thought
of profit or by any manner of external compulsion. An inspiration,
not the pressure of a duty, drove our predecessors forward.

By degrees, however, chemistry was found to be useful, and the
commercial demand for chemical services began. Manufacturers
discovered that processes and products could be improved, and that
waste material had value; metallurgy developed along chemical
lines, medicine gained new remedies, and agriculture was turned from
its traditional empiricism into scientific courses. A new set of im-
pulses was given to chemistry, and many of its practitioners became
professional in expectation of material profit and reward. The field
of research was widened, and civilization was thereby advanced.
Chemistry was not merely a philosophical amusement, but an agent for
"the betterment of man's estate;" and so a double motive existed

226 CHEMISTRY

for its further development. This combination of intellectual interest
with utility gave the science a higher place in educational affairs; and
when Liebig opened the first university laboratory for students at
Giessen, a new era for chemistry began. Before that time the chemist
was either self-taught or trained in private laboratories; now he
could aspire to scholastic honors and assume his proper position as a
learned man. As a discoverer, Liebig was great, but his chief services
to chemistry were in his educational work and in the application of
science to agriculture. To those achievements his wide reputation
is mainly due.

For chemistry, then, the second half of the century opened aus-
piciously. Chemists were needed for technical purposes and as teach-
ers, and resources were placed at their disposal almost without stint.
Discovery was stimulated, investigation became more systematic,
theory and practice developed side by side. Practical applications
followed the most abstract researches; new industries sprang into
existence, and in education mere bookishness gave way to experi-
mental methods. A great but silent revolution had taken place, whose
magnitude will be better appreciated by posterity than by ourselves.
Had science done no more than to replace supposition by experiment,
and chance discovery by orderly research, the revolution would still
have been one of the greatest in the history of mankind. Chemistry
was not the sole agent in effecting the transformation, but it surely
played one of the leading parts.

All of the agencies which I have mentioned helped to encourage
the study of organic chemistry. It was systematic, and therefore
easily taught, and it was full of suggestiveness both for teacher and
pupil. Its practical applications were many, and gave the investi-
gator hope of material rewards; the revelations of coal-tar alone were
enough to stimulate chemists to the greatest activity. So it happened
that inorganic chemistry fell into neglect, and the majority of chem-
ists followed the leaders into the new field. The conceptions of chem-
ical structure, which had been slowly evolving during many years,
were given definiteness by the discovery of valence, and of this the
benzene theory was perhaps the most brilliant application. Frank-
land, Williamson, and Perkin in England; Dumas and Wurtz in
France, Kekul6 and Hofmann in Germany, and the Russian Butlerow,
are the conspicuous names connected with the modern movement.
Organic chemistry became an imposing structure, and yet it rested
upon the foundations which the older chemists had laid. The con-
stitutional formulae were built upon atomic conceptions, valence
itself was a property of the atom, and complete acceptance of the
new ideas was impossible until after Cannizzaro had revised the
atomic weights and brought them into harmony with Avogadro's
law. Up to that point there was a chaos of rival doctrines, after-

PROGRESS IN NINETEENTH CENTURY 227

ward order reigned. The full significance of valence could not appear
until the old system of chemical equivalents had been set aside.

Naturally, as the mass of chemical data increased, specialism
became necessary. No man could expect to know the whole of chem-
istry; a small part of it was all that any one could handle, and the
inevitable results followed. A specialist may be broad, but the direct
tendency of specialism is to narrow one's field of view, and to con-
centrate the attention upon details rather than generalities. The
theories which fit immediate conditions then become satisfactory, and
the chance that they may be only partial glimpses of greater laws is
disregarded. Only the stronger and more philosophical minds can
escape these limitations and see things in their larger aspects.

To the organic specialist, at least in most cases, the doctrine of
valence was adequate ; for it explained the combinations with which
he had to deal. Relatively few of the chemical elements were seri-
ously considered by him, and they offered no insuperable difficulties.
Carbon was the typical element, the key to all organic matter; its
quadrivalency in terms of the hydrogen unit was assured; its ability
to unite with itself in chains or rings was established; with these
data constitutional formulae became truly significant, and useful for
the correlation of existing knowledge. Even more can be said in
their favor, for they had a certain prophetic ability which guided
research and foretold discovery. But, after all, carbon was only one
among many elements, and nobody was justified in assuming that
its modes of combination represented general laws, or that ideas
drawn from the study of organic matter alone were applicable else-
where. The theory of valence must be tested with regard to all
the elements before its full validity could be recognized, and that
test implied a renewal of interest in inorganic problems. It was neces-
sary to discriminate between special cases and fundamental principles,
and so a much larger field than organic chemistry could offer had to
be surveyed. Clues had been found in the study of carbon compounds,
but where were they to lead?

So far as actual knowledge went, the chemical elements were dis-
tinct entities, and speculation as to their nature had been looked
upon generally with disfavor. And yet they had points in common
which rendered their classification possible, and it was perfectly
evident that they could be arranged in a small number of natural
groups. Certain elements were obviously types of others; some were
isomorphous, as shown by Mitscherlich, and some exhibited serial
relations as in Dobereiner's triads; but no one scheme of classifica-
tion covered the entire ground. Analogies were numerous enough,
but their meaning was not clear. A process of evolution was at work,
however, and in due time it culminated in Mendelejeff's develop-
ment of the periodic system. All partial classifications, all the dim

228 CHEMISTRY

foreshadowings of law, now fell into place together, and one simple
generalization occupied the field. The atomic weights became more
than ever the fundamental constants of chemistry, and all the pro-
perties of the elements were seen to be periodic functions of these
quantities. In Mendelejeff's table stress was laid upon valence and
the form of compounds which each element could yield; in Lothar
Meyer's curves the physical relations were emphasized, and so each
statement reinforced the other. Newlands, it is true, had partially
anticipated Mendelejeff, but his law of octaves fell just short of com-
pleteness.

At first, the periodic classification attracted comparatively little
attention, and its general acceptance might have been slow had it
not been for certain prophecies. In Mendelejeff's table there were
many gaps; these were attributed to the existence of elements as
yet unknown, and for three of them the author ventured upon pre-
dictions. Each element must have a certain atomic weight, a pre-
scribed density and melting-point, and should form compounds of
a stated character. In due time the three unknown elements were
actually found, and gallium, scandium, and germanium confirmed
all of Mendelejeff's anticipations. The importance of the classifica-
tion was thus established, and the periodic law became one of the
foundation stones of modern chemistry. The conception of valence
as a property of the atom acquired a broader significance; in cases
that had been doubtful its magnitude could be determined, and with
its aid the chaos of inorganic chemistry began to exhibit signs of
something like order. The deficiencies of the periodic system I need
not mention here, for this is no time for details; neither shall I dis-
cuss the obvious difficulties which arise when we seek to apply the
doctrine of valence to inorganic compounds; only the larger verities
concern us now. In the broadest sense the periodic classification is
sound; the principle of valence is general, and the obstacles which
now appear will doubtless be overcome by future investigation.
That the greatest generalization has been reached, we cannot assume;
but so far as we have gone we stand on solid ground, and can continue
our explorations in safety.

Up to a certain point organic compounds had been successfully
interpreted in terms of valence. Isomerism was explained, and the
existence of unknown isomers could be predicted; different atoms
were assignable to different positions within the molecule, as in the
case of the four hydrogen atoms of acetic acid, one fixed and three
replaceable; but after all this had been done there were still some
difficulties outstanding. Isomers existed whose chemical structure
seemed to be the same, and for their interpretation an extension of
chemical theory was needed. This want was supplied by van't Hoff
and Lebel, who almost simultaneously pointed out the consequences

PROGRESS IN NINETEENTH CENTURY 229

of assigning a tetrahedral form to the atom of carbon. From the pro-
perties of such an atom a new class of structural formulae could be
deduced, by means of which the so-called cases of "physical isomer-
ism" were simply interpreted. The molecules of tartaric and racemic
acids, for example, resemble each other as an object resembles its
reflection in a mirror, the one being a reversal of the other. Our
science accfuired a new province, that of stereochemistry, which in less
than thirty years has grown to impressive dimensions. The theory of
van't Hoff and Lebel did more than to interpret the troublesome
known phenomena, it encouraged additional research and led to many
discoveries. At first, the asymmetric carbon atom alone was consid-
ered, but its peculiar properties are now shared by other elements,
and physical or stereochemical isomers are found even among in-
organic compounds. When one atom is combined with four other
atoms or groups of as many different kinds, optical asymmetry
appears, and physical isomerism becomes possible.

During the ninth decade of the century the dominating interest in
organic chemistry began to wane, for the reason that other subjects
were demanding their share of attention. I do not mean by this that
the activity of organic chemists diminished, for their output of dis-
covery was never greater than now; but the centre of the stage was
slowly being filled by other groups of actors. Inorganic chemistry was
reviving from its long neglect, and physical chemistry loomed large
upon the horizon. In each of these branches journals were started,
and no difficulty was found in filling their pages with the records of
successful investigations. In theory, physical chemistry has made
the greatest advances, inorganic research has been more a matter of
detail. Let us briefly consider the two themes separately.

To the inorganic chemist several duties were apparent. Old work
needed revision, the compounds of many elements were almost unde-
scribed, there was a lack of system to remedy, and the theories de-
rived from organic chemistry were to be tested and applied. A very
large part of the work was necessarily descriptive, a preparation for
the future, but back of it all lay a fundamental question with which
all physical science is connected, for the nature of matter itself was
to be determined. In its broadest sense this question demands the
cooperation of all science and all philosophy, but to inorganic chem-
istry one phase of it may be assigned. What is the nature of the
chemical elements? Are they one or many? And how shall an element
be defined? To these questions there is as yet no final answer, but
clues to follow are many, and some of them are offered by the periodic
law. To remedy its imperfections is an obvious duty for inorganic
chemists to perform.

Near the middle of the Mendelejeff table and of the Lothar Meyer
curve there is an area which is partly blank and partly filled with the

230 CHEMISTRY

symbols of uncertain elements. That some of them were tri- and
others quadrivalent was well established; but their number was
undetermined, and the places which they should occupy were even
more doubtful. Some of the uncertainties still remain, and some
have been cleared away; but the main problem is as yet unanswered,
and therefore the metals of the rare earths are still actively studied.
Supposedly definite earths have proved to be mixtures; Others, like
cerium, lanthanum, yttrium, and scandium, seem to be definitely
placed; but what shall we say of the rest? Didymium was thought
to be a distinct element, and yet it has been split in two; samarium,
gadolinium, erbium, and ytterbium are probably definite; but sev-
eral other metals are claiming recognition; and so, notwithstand-
ing the progress which has been made, a large part of the field is still
obscure. Through the study of the rare earths, one side of our pro-
blem, the nature of the elements, is open to attack; but only the
outworks have been carried so far.

According to modern ideas, the integrity of an element is deter-
mined by two conditions; it must have a distinct spectrum and a
definite atomic weight. In the study of the rare earths these criteria
have been systematically applied, and to great advantage; but what
has been done elsewhere? To answer this question we must go back
more than forty years in time and make a new beginning.

It was near the middle of the century that August Comte, seeking
to find some limits to positive knowledge, argued that it would be
impossible for us ever to determine the nature of the heavenly
bodies. Are they composed of matter like that which forms the earth,
or are they different in kind? — on that theme we might speculate,
but we could never know. The prophecy was futile; Kirchhoff and
Bunsen, with the spectroscope, swept the limitations away, and all
the universe, as far as eye could reach, was found to contain familiar
elements, but under conditions not always like our own. Astronomy,
physics, and chemistry had gained a new weapon, and discovery
followed discovery along widely different lines.

In the chemical laboratories the value of the new instrument was
immediately proved. Two metals, caesium and rubidium, were pre-
sently discovered by its aid, thallium and indium were found a little
later, and their analogies to other elements made them comparatively
easy to classify. The periodic system, which was developed later still,
gave them their proper positions among the metals, and they in turn
made the classification more complete, and therefore easier to esta-
blish. In each case the double criterion was applicable, and definite
spectrum was connected with definite atomic weight. I speak now of
emission spectra; but they are not the only kind. Certain solutions
give absorption spectra, and they have been of great assistance in the
study of the rare earths. In the identification of the elements, then,

PROGRESS IN NINETEENTH CENTURY 231

the spectroscope has rendered service of inestimable value, and dis-
covery would have been very slow without it. Quite recently, at the
very end of the last century and during the few years of the new, re-
lations have appeared between the wave-lengths of the spectral lines
and the atomic weights of the elements; but the general expression
which shall connect them all is yet to be revealed.

Another discovery in the realm of inorganic chemistry is deserving
of mention now on account of its peculiar significance. The atmo-
sphere was thought to be well known, and yet in 1895 a new element,
argon, was discovered in it. This find was quickly followed by others
of like kind, and now five gases previously unknown have been
extracted from the air. Each gas is identifiable by its spectrum and
its density, and from the latter datum the atomic weight can be
inferred.

Now the interesting fact concerning these atmospheric gases -
helium, neon, argon, krypton, and xenon — is that they represent
matter of a new kind. So far as evidence goes, they are monatomic,
absolutely inert, and incapable of union with other elements. Their
valence is zero, and when the periodicity of the elements is repre-
sented by a vibratory curve, they occupy the points of rest, — the
nodes. They are matter having physical, but no chemical, properties,
and therefore they can be investigated only upon the physical side.
This conclusion, perhaps, should be stated provisionally, for it may^
be reversed by future discovery; but of this possibility we have only
one suggestion. Helium was first extracted from the mineral uraninite
in which it is firmly held, and we cannot say with certainty that it
is not chemically combined. Altered or massive uraninite contains
little or no helium; the crystallized varieties yield more, and the
most brilliant and perfect crystals are the richest of all. The gas
may be merely occluded, but the bare chance of combination should
not be overlooked. Either supposition is legitimate; but there is
still one more possibility, namely, that helium may be generated by
the decay of another substance, and not be an original constituent
of uraninite at all. Here we touch the mystery of radium — a body
which challenges our former conceptions of an element, for seemingly
it can be decomposed.

The discovery of radium by Mme. Curie belongs to the nineteenth
century, and therefore it falls within the scope of this essay. How
it was found, how laboriously the phenomena of radioactivity were
observed in order to isolate traces of the new metal, we all know, and
the details need no repetition here. At last pure salts of radium were
obtained, and the two criteria of spectrum and atomic weight were
satisfied. Radium is clearly a metal of the barium group, it fills a
definite place in the periodic table, its claims to elementary rank are
on a level with those of other elements, and yet it exhibits an apparent

232 CHEMISTRY

instability which is difficult to explain. Radium gives off material
emanations that are different from itself; they are gaseous and inert;
in them the spectrum of helium has been observed. From one element
another seems to be derived, and all our notions of what an element
should be are thrown into temporary confusion. I say "temporary,"
for I believe that order will be restored, and that a deeper insight
into the constitution of matter is close at hand.

Pardon me now if I seem to wander from one part of my subject to
another. Between the various departments of knowledge there are
no sharp boundaries, and the solution of a problem often depends
upon the convergence of testimony from many different directions.
The nature of the elements is primarily a question for the inorganic
chemist; but physics has much to say upon the subject, and even the
serial relations of organic compounds offer suggestive analogies which
are entitled to some consideration. The periodic system, with its
fulfilled prophecies, tells us that the elements are related one to
another by some distinct law; the spectroscope gives us evidence of
a different order; electrical phenomena have their share in the story,
and the modern phenomena of radioactivity offer the latest testi-
mony of all. What conclusions seem to be foreshadowed by the data
now in hand?

One of the earliest achievements of the spectroscope was the
rehabilitation of the nebular hypothesis. The resolution of some
nebulae into clusters of stars had shaken faith in Laplace's specu-
lation; but when it was proved that others were really clouds of
incandescent gas, belief in the hypothesis was restored. One point,
however, was of peculiar interest: in the nebulae only one or two
elements, low in the scale of atomic weights, could be seen; in the
whiter and hotter stars a few more substances appeared; colored
stars were of still greater complexity, and so on progressively from
the simplest constitution to the material heterogeneity of our globe.
If suns and planets were evolved from nebulae, it seemed as if the
chemical elements had been successively generated at the same time
— a supposition which was certainly legitimate, although it was at
first denied by some chemists as unworthy to be heard. At all events,
here was testimony bearing upon the problem of the elements,
although its full significance was not so clear. It could be pigeon-
holed, but not thrown away.

Recently, and in great part through the researches of J. J. Thom-
son, evidence has been obtained of the reverse order. On one side an
evolution of the elements is apparently indicated; Thomson's experi-
ments suggest a breaking-down. By studying the ionization of gases,
phenomena were observed which point to the existence of particles
smaller than the Daltonian atoms, and a beginning has been made
toward the identification of matter with electricity. The negative

PROGRESS IN NINETEENTH CENTURY 233

particles, corpuscles or electrons, have been split off from ordinary
matter, and they are always the same, regardless of the element from
which they separate. Even their mass can be estimated, and it ap-
pears to be about the thousandth part of that which represents an
atom of hydrogen. These conclusions are perhaps not final, but they
are emphasized by the results obtained in the study of radioactivity.
The investigations of Rutherford and Soddy, of Ramsay, Dewar,
and others, all tend in the same direction, and lead to the suspicion
that the atoms are complex and subject to decay. The three most
radioactive elements are radium, thorium, and uranium, and these
have the highest atomic weights of any substances known. If the
elements are complex, these are the most so, and therefore presum-
ably the least stable. If we take this testimony in connection with
that given by Thomson, the evidence offered by the spectra of the
heavenly bodies, and the regularities of the periodic law, we have
a strong argument in favor of the supposition that the so-called ele-
ments are not the simplest forms of matter, and that they may be
ultimately one. The doctrines of unity of matter and the unity of
force are thus philosophically allied, and only negative evidence can
be adduced to support a belief in the actual diversity of the elements.

Speaking broadly, organic and inorganic chemistry, at least as
they are commonly studied, are essentially descriptive in their
character, and they deal with statical phenomena. Physical chemis-
try, on the other hand, is more concerned with dynamics, and seeks
to determine the conditions of chemical equilibrium, and the nature
of chemical change. What substances are and what substances do
are of course only two phases of the same fundamental problem,
which are separable ideally, but not otherwise. Descriptive chemistry
lays stress upon one side of the science, physical chemistry emphasizes
the other; but they blend together by imperceptible degrees, and no
clear line of demarcation can be drawn between them.

Every science, when viewed historically, is seen to have a central
line of growth, to which its various branches are naturally related.
In chemistry this line is marked by physical phenomena, and from
their study the greater generalizations have been derived. Avogadro's
law, the law of Dulong and Petit, Faraday's theory of electrolysis,
and the periodic classification of the elements are good illustrations of
this principle. The atomic theory itself, which connects all of the
other relations, is fully as much physical as chemical; valence is best
explained in electrical terms, and stereochemistry arose from optical
and crystallographic considerations. Physical chemistry is the main
stem of our science, and statical conditions are merely the results
of dynamical equilibrium. The description of a product is incom-
plete unless we have noted the physical phenomena, the trans-
formations of energy, which took place during its formation, and to

234 CHEMISTRY

studies of this kind the chemists of the future must devote a large
part of their time.

During the last twenty years the importance of physical chemistry,
or rather the recognition of its importance, has steadily increased, and
to-day it seems to dominate the entire field of chemical research.
Laboratories are equipped for its purposes alone, journals are de-
voted to it, and the activity of investigators has become so great
that subdivision has already begun, and men are known as thermo-
chemists, electro-chemists, and so on. Electro-chemical societies have
been formed and are prosperous; specialism is passing into subspe-
cialism; in short, chemistry is swiftly assuming an entirely new form.

In the evolution of any science successes and disappointments
are almost equally influential; the former stimulating, the latter
tending to arrest research. The fruitful line is followed, and attracts
workers; the barren field is deserted or nearly so. Barrenness, how-
ever, may be due not to lack of fertility, but to premature effort;
and the truth which is beyond our reach to-day may drop into our
hands to-morrow. Thermo-chemistry, for example, has so far failed
to repay the labor spent upon it, and has fallen into disfavor; but
the future may tell a different story. Its importance is obvious,
and its general laws cannot elude discovery forever. The thermal
changes which accompany all chemical reactions must sometime be
interpreted.

On the other hand, success has followed the physical study of
solutions, and thereby chemical theory has been enriched. First,
it was found that substances in solution exerted pressure — a phe-
nomenon attended by depression of the melting-point and increased
temperature for boiling. This pressure resembled that observed in
gases, and a relation between the two was apparent. It was van 't
Hoff's privilege to trace the connection, and to develop a kinetic
theory of solutions. Avogadro's law was completely paralleled,
and equal volumes of solutions at equal osmotic pressures were shown
to contain equal numbers of molecules. For both laws, the liquid
and the gaseous, however, there were certain apparent exceptions,
which, for gases, were easily explained as the result of dissociation.
Arrhenius applied this explanation to the exceptional solutions,
taking into consideration also the ionic conceptions developed in the
study of electrolysis, and the abnormalities vanished. A salt in dilute
solution is electrically dissociated into its ions, which remain in
equilibrium although separate. From these generalizations several
important consequences followed. First, it became a simple matter
to determine the molecular weights of soluble substances — a class
of measurements that had previously been possible for gases alone.
Secondly, much light was thrown upon the subject of reactions
between dissolved salts, especially such as involve precipitation or

PROGRESS IN NINETEENTH CENTURY 235

double decomposition. In most cases, although not invariably, the
phenomena are ionic, and the molecules are first broken down. In
the third place, the uniform heat of neutralization between acids
and bases was explained by showing that in all cases it represented
one and the same change, namely, the union of hydrogen and hy-
droxyl ions to form water — a conclusion which gave a significant
datum to thermo-chemistry. In brief, many distinct lines of physico-
chemical research converge in the kinetic theory of solutions — a
theory whose development has hardly more than begun. Like most
successful theories, its importance may at first be exaggerated; we
have not yet the perspective which shall enable us to judge it truly;
in all probability, it is but one phase of some larger law; but, not-
withstanding all difficulties and all objections, it is a stride forward,
and will bring us to new truth.

We now reach a point where it is difficult to disentangle the many
threads of investigation, and to determine their relations to one
another and to the past. Current work is more or less confusing, for
it is too near our eyes, and its ultimate significance is not easily
apprehended. The theory of solutions, the law of mass-action enun-
ciated by Guldberg and Waage, and the phase rule of Willard Gibbs
interact in so many ways, and are so rapidly developing, that I for
one dare not attempt to predict what the outcome shall be. Chem-
istry is becoming more and more a mathematical science, and so is
gaining in precision; but mathematical reasoning leads to correct
conclusions only when its premises are secure. The data must be
verified and reverified before we can certainly determine their mean-
ing, and in the enthusiasm of new investigation this necessary duty
is often deferred. The pioneer leaves much undone behind him, and
patient laborers are needed to follow in his lead. The first glimpse
of truth is rarely the whole truth, for that is best gained by what
we may call the method of successive approximations.

If prophecy is difficult, retrospection is easy; we may therefore
retrace our steps and see what road we have followed. Boyle, Priest-
ley, Scheele, and Lavoisier prepared the way for Dalton, and his
atomic theory, the first quantitative theory of its kind, has been for
a century the key to all chemistry. All of the great advances in our
science have hinged directly upon Dalton's conception, and his atomic
weights, as developed by Berzelius and Cannizzaro, are now seen to
be fundamental constants, with whose aid the physical relations of
different substances are easiest interpreted. The periodic law is based
upon the atomic weights, valence is an atomic function, in stereochem-
istry we have a hint of atomic form, isomerism is intelligible only
upon the assumption of variable atomic position, and the structure
of a molecule depends upon atomic groupings. The ions of physical
chemistry and the molecules of thermodynamics are either atoms

236 CHEMISTRY

or groups of atoms; and, in short, whichever way we turn in physical
science we find ourselves, consciously or unconsciously, thinking in
atomic terms. And yet we are sometimes told that the atomic theory
is outworn, and that some other conception should replace it. We
may well ask, therefore, whether atomism has any basis in reality.
Is it the truth or only an illusion — a concrete fact, or misinterpret-
ation of testimony?

That the atomic theory has rendered great service to chemistry,
and that it correlates our positive data, is clear; but after all it is
hypothetical, for no atom has been isolated and seen. The molecule
and the atom are inferred from the properties of matter in mass;
and if we need a theory at all, there is none other at hand. The
attempts to evade it are agnostic in character, and are based upon
the tacit assumption that it is unscientific to speculate upon ultimate
questions, which, in the nature of things, can never be absolutely
solved. We can observe and classify relations, but it is useless to ask
what they mean. The phase rule has been suggested as a basis for
our classification, and under it the different kinds of matter become
different phases of something which we may or may not be able
to comprehend. Perhaps I misrepresent the position of the anti-
ato mists; but if so it is because their statements are to my mind far
from clear. If we object to the atom, we must object to the ether, for
that is equally unknowable ; we cannot divorce matter and motion,
for they are never observed apart; in short, we must reconstruct all
physical science and keep within the limits of things known. But is
the agnostic position sound? Is not the imagination as truly an instru-
ment of science as is the reason? May we never look forward and
anticipate what is to come, shall we always observe and experiment
without the help of ideals? To do so we must assume limitations
where no limits can be seen, and the human mind refuses to work in
that way. Speculation is the guide of science ; an indispensable assist-
ant in our exploration of the unknown; a good servant, but the
worst of masters. Scientific methods differ from unscientific methods
partly by their use of system, and partly in their employment of
disciplined as against unrestrained speculation.

That the atomic theory has been a useful tool no one can deny;
but can we, in the light of present knowledge, imagine a universe
without it? We see that matter differs in its properties from point
to point, and all of our experiments end in records of these differences.
But is not difference a proof of discontinuity? How could a plenum
vary? Even the ether itself, that mysterious medium which is thought
to pervade all space, is now believed to have a granular structure,
or, in other words, to be atomic. Several mathematicians have worked
upon this phase of the problem with curious results; but their con-
clusions lie outside of my theme. The chemical atom alone concerns

PROGRESS IN NINETEENTH CENTURY 237

us now, regardless of its ultimate or physical nature, which may be
exceedingly complex. The conception is so bound up with all modern
chemical ideas that we cannot abandon it if we would, so long as
nothing better is offered us in its place.

The chemist, then, may legitimately claim that matter, as we know
it, is made up of small, distinct particles, which, so far as they have
been chemically defined, are of few kinds. These particles gather
into clusters, through some form of attraction whose nature is still
unknown, and in which differences of position probably represent
differences of chemical structure. Allotropy and isomerism are thus
explainable, two phenomena that are perhaps the same, and for
which the atomic theory alone has offered any reasonable interpret-
ation. But this is not all. Certain numerical constants, commonly
known as the atomic weights, have been discovered, one for each
element, which are fundamental for all quantitative chemistry and
for an important part of physics. These constants are real ; they repre-
sent definite, measurable relations; and in one form or another they
will remain in use, apart from all changes in theory. Whether they
are independent of one another is yet to be determined; there are
indications that they may be connected by some mathematical law;
and should such an expression, a quantitative periodicity, be dis-
covered, it would go far towards enlightening us as to the real nature
of the elements themselves. The exact determination of the atomic
weights is therefore a matter of supreme importance and one bearing
directly upon the profounder problems of chemistry. If the atoms
are separable into electrons, the masses of the latter should bear some
relation to the atomic weights and give us clues to their mathematical
interpretation. Future investigations along this line are certain to
be made, and we may fairly hope that they will prove successful.

The nineteenth century is often called the age of steam, and its
latter half the age of electricity. May we not, with equal propriety,
name it the age of chemistry? During the passage of its years chem-
istry has developed from an art into a science, with a clear philo-
sophy of its own, and with useful applications which affect all other
sciences and many industries. A great university may now employ
twenty chemists as teachers where fifty years ago there was barely
work for one. Training in chemical research has become a recognized
feature in higher education; the student is taught to think and
investigate; the production of new knowledge is seen to be a distinct
function of the teacher. Scholarship is now rated according to its
fertility ; and the man who merely knows, no matter how thoroughly,
the work of his forerunners, is given a low rank in the thinking world.
In the industries, chemical thought is translated into action, and so
becomes doubly creative, yielding at the same time new knowledge
and material wealth. Governments maintain public laboratories; it

238 CHEMISTRY

may be in preparation for warfare, for sanitary purposes, as aids in
the enforcement of revenue laws, or for their own protection as pur-
chasers of supplies; and so the usefulness of chemistry is felt along
innumerable lines. The science advances with ever-increasing rapidity
and there are as yet no signs of slackening. What shall the future be?
We can distinguish necessities and express our hopes, even if we can-
not prophesy. An essay of this kind would have small value if it
failed to offer any helpful suggestions for the work that is to be done.

In the realm of descriptive chemistry certain work is obviously
needed and is therefore likely to be done. Part of this, and the least
attractive part, is revisionary — a verification of the older data with
the correction of venerable errors. On the inorganic side we may
predict many advances, and some of the possible lines of research
we have already considered. In order to complete the periodic table
the rare earths must be exhaustively studied, and the irregularities
shown by iodine and tellurium, or by potassium and argon, ought to
be explained. The problems of chemical structure which are offered
by complex bases and acids and by double salts require elucidation,
and here physico-chemical methods are likely to be most applicable.
The correlation of chemical structure with crystalline form is sure to
receive much attention; but what direction researches of this kind
may take is not easy to foresee.

For organic chemistry I am hardly qualified to speak, at least not
with regard to the more immediate urgencies. It is plain, however,
to every one that there are large and important groups of compounds
which await constitutional interpretation, the alkaloids and albumin-
oids being among thein. Organo-metallic bodies also deserve a good
deal of attention, for in them the two departments of descriptive
chemistry meet, and each one, organic or inorganic, can be made to
shed light upon the other. Finally, the relations between physical
properties and chemical composition are most easily investigated upon
the organic side, and here are problems enough to keep men busy for
a good part of the present century. All the properties of a substance
should be calculable from its composition; but the adequate data and
the conclusive theory are far beyond our reach. We have a few begin-
nings, nothing more.

In physical chemistry, it seems to me, we find the unifying prin-
ciples which are to bind all the subdivisions of our science into one.
Some of the problems mentioned under the heading of descriptive
chemistry are almost wholly physical in their nature; only they are
statical, and leave dynamics untouched. They deal with equilibria
established by transformations of energy — a statement which holds
true whether we connect it with the atomic theory or base it upon
the phase rule. The laws of chemical equilibrium are fundamental,
beyond question; but antecedent to their application there was an

PROGRESS IN NINETEENTH CENTURY 239

interplay of active forces whose statutes are more general still. What
is the nature of chemical change, and what laws govern its trans-
formations of energy? These, to my mind, are the most general ques-
tions of dynamical chemistry. They are raised by every reaction,
and they involve the consideration of all the physical forces. The
problems of thermo-chemistry, of electro-chemistry, of optical chem-
istry, are mere special cases arising under the more universal gen-
eral laws, and they will cease to exist when the latter have been
discovered. So ideal a condition may never be reached, but we can
approach it.

How, now let me ask, shall the work of the future be done? Hitherto
individual initiative has been the chief agency in effecting progress,
and each man has handled his own problems in his own way. By
individual geniuses the greatest discoveries are made, but they are
tried and tested by the collective intelligence of many laborers, more
humble, perhaps, but also more patient and thorough. The genius
is fortunate, but science has use for plodders as well, who furnish the
commonplace facts that are the raw material from which laws and
generalizations are developed. The great thinker needs only oppor-
tunity and encouragement; the rank and file of investigators, it
seems to me, require something more. We need not fear that personal
effort will cease; and still we may fairly ask whether it is sufficient
for the tasks which are now waiting to be done.

One result of individualism in scientific research is evident. Our
knowledge increases irregularly, unsymmetrically — with one phase
over-developed and another neglected. In every group of data there
are gaps to be filled, side by side with needless duplications. One man
finishes a research only to find himself anticipated by some more
fortunate worker, and he feels that his labor has been thrown away.
Competition is a good thing, but cooperation is better, for it insures
that economy of effort which is as important in intellectual affairs as it
is in the factory or in commerce. Can we, without stifling enthusiasm,
without harming the individual, encourage the organization of re-
search, and so give to science a swifter growth and a more perfect
symmetry? That vague but potent agency, "the spirit of the time,"
has taken "organization" for one of its watchwords, and we cannot
escape from its spell. Collectivism and individualism, however, are
not necessarily antagonistic; they are two forces acting side by side,
and each helping the other. A man best develops himself when he
works in harmony with his fellows.

Chemical societies are an invention of the nineteenth century, and
they stand for one step in the right direction. In their meetings,
by conference and discussion, and in their publications, by making
research effective, they have done much to encourage investigation,
and to avert, in some measure, useless duplications of effort. Through

240 CHEMISTRY

committees, they sometimes direct the growth of science, not by the
exercise of compulsion, but by classifying work that has been done
and showing where work is needed. An extension of this process
might easily be devised, in such manner that a definite field of study
should be divided among a number of scholars, each doing his own
share and earning whatever independent credit he deserved. In
astronomy we already have an example to follow, for observatories
have divided a part of their work in exactly this manner, each insti-
tution mapping a zone of stars assigned to it by mutual agreement.
Cooperative research upon a well-considered plan ought to be possible
among chemists. Some overlapping, some duplication, cannot be
avoided, but the waste can at least be diminished.

There is one other step which needs to be taken, and one which I
have repeatedly urged on other occasions. There should be laborato-
ries organized, equipped, and manned for systematic chemical research
upon those problems which are too large for individuals to handle.
The exhaustive determination of constants, for example, must pre-
cede the development of laws, and few chemists laboring singly care
to attempt work of so tedious a nature. Each one often feels the need
of data which do not exist, wants that he is unable personally to
supply, and such a laboratory as I have in mind could render invalu-
able service. Astronomy has its observatories, biology is provided
with experimental stations, physics is represented by institutions like
the Reichsanstalt, while chemistry is almost unaided. Chemistry, the
creator of wealth, receives few endowments, and those which have
fallen to its share have been in aid, not of research, but of teaching.
Great things have been and will yet be achieved in the universities,
but their laboratories can cover no more than a small portion of the
field. A laboratory for research would not compete with them; it
would, on the other hand, reinforce their efforts. When, a .hundred
years hence, the progress and development of chemistry during the
twentieth century is summed up, investigations carried on under
endowments will fill a conspicuous portion of the stage. I have faith
in the future; I believe it will be better than the past; and to my
mind the great advances in science which we celebrate are only a
beginning.

SECTION A — INORGANIC CHEMISTRY

SECTION A — INORGANIC CHEMISTRY

(Hall 16, September 21, 10 a. m.)

CHAIRMAN: PROFESSOR JOHN W. MALLET, University of Virginia.

SPEAKERS: PROFESSOR HENRI MOISSAN, The Sorbonne; Member of the Institute

of France.

SIR WILLIAM RAMSAY, K.C.B., Royal Institution, London.
SECRETARY: PROFESSOR WILLIAM L. DUDLEY, Vanderbilt University.

INORGANIC CHEMISTRY: ITS RELATIONS WITH THE
OTHER SCIENCES

BY HENRI MOISSAN
(Translated from the French by Professor R. S- Woodworth, Columbia University)

[Henri Moissan, Professor of General Chemistry at The Sorbonne, University of
Paris, since 1900. b. September 28, 1852. D.S., LL.D., Universities of Princeton,
Glasgow, and Oxford. Professor at School of Pharmacy of Paris, 1887-1900.
Member of Institute of France; Academy of Medicine of Paris; Academies of
London, Berlin, Vienna, St. Petersburg, Washington, Brussels, Amsterdam,
Munich, Denmark, Turin, etc., etc. Author of The Electric Oven; Fluorspar
and its Component Parts ; Treatise on Mineral Chemistry.]

CHEMISTRY, though young as a science, traces its first applica-
tions back to the very cradle of the human race. As soon as man
in his struggle with nature had come into possession of his individu-
ality, his observing intelligence enabled him to take cognizance
of some of the phenomena occurring about him, and to engage in
the study of them. He saw the importance of fire, and soon recog-
nized that certain metallic substances could take the place of flint
in the manufacture of weapons. Thereupon he devoted himself to
that primitive metallurgy of copper, of which we still find so many
examples, more or less transformed, in the earliest foundations of
Babylon; these remain as witnesses, not dumb, though far from
explicit, of the most remote of our civilizations.

The importance of metal in the different stages of human develop-
ment is so well recognized that a single name is used to cover all the
centuries that have made use of the same metal.

To the age of copper succeeds the age of bronze. At the same time
gold, being found in the native state, becomes known, and is wrought
with the hammer. Iron, since its preparation is much more difficult,
cannot be utilized till later.

In these distant times, the epoch most fertile in chemical applica-
tions was that of the Egyptian civilization. After many industrial
experiments, this people succeeded in dyeing fabrics with purple, in

244 INORGANIC CHEMISTRY

working the rare metals, in making enamels by fusion, in producing
and fashioning glass, and in preparing fermented liquors.

On the other hand, a little people, whose part in everything was to
throw the most brilliant light on it without inventing new applica-
tions, sought to explain, philosophically, these transformations of
matter. The Greek philosophers discussed this subject at length.
Empedocles reduced all the bodies that nature can present to four
elements: fire, air, water, and earth. For him, these elements were
composed of a multitude of minute particles, indivisible and insecable.
Such a theory leads easily to the atoms of Democritus. Whether
these elements were considered as symbols or as a veritable classi-
fication of matter, the idea of Empedocles, adopted by Aristotle, and
taught by all the schools, was destined to maintain for centuries
the position of a doctrine that could not be disputed.

Later, Epicurus revives the theory of atoms, and Lucretius, in a
poetic divination, can write:

Principio, quoniam terra corpus, et humor
Aurarumque leves animae, calidique vapores,
E quibus haec rerum consistere summa videtur,
Omnia native ac mortal! corpore constant,
Debet eodem omnis mundi natura putari.1

The idea of the four elements reappears unchanged among the
Arabian chemists, and among the alchemists of the Middle Ages;
though it undergoes various transformations at the hands of Paracel-
sus who recognized five elements : spirit; mercury; phlegm or water;
salt, sulphur, or oil; and earth, — and later at the hands of Beecher
who admits the existence of three essences in earth, — vitrifiable,
inflammable, and mercurial earth.

The theory of four elements reigns without contest up to the
moment when Stahl, professor at the University of Halle, develops
his important conception of phlogiston. For Beecher, combustible
bodies and metals contained his three sorts of earth combined.
For Stahl, "inflammable earth" becomes phlogiston. Carbon, by its
combustion, gives heat and light; it therefore contains phlogiston.
When a calx, that is to say, a metallic oxide, is heated with carbon,
it extracts and fixes the phlogiston, and becomes a metal. These
were important conceptions, because they made it possible to unite in
one body of doctrine the phenomena of oxidation and of reduction.

Such was the state of the science when Lavoisier followed up his
memorable experiments by developing the notion of simple sub-
stances. This great savant showed that the same body could change
its state, and he separated clearly, among the phenomena of chemistry,
on the one side the weight of the reacting bodies, and on the other

1 " First, since earth, and water, and the light breath of air, and glowing fire —
of which our world seems to be composed — all consist of matter that is subject
to birth and to death, we must think that the whole universe is made up of this
same sort of matter."

RELATIONS TO OTHER SCIENCES 245

side the heat set free. By weighing the simple substances that were
combined and also the compound produced, he definitely established
the weight equation of the chemical reaction. By measuring with the
calorimeter the amounts of heat set free, -he separated ponderable
matter from the imponderable agents. All these views were, in addi-
tion, logically bound up with each other, and it would have been
impossible to study properly the phenomena of combustion if he
had not been forming an exact idea of the passage of a body from
the solid to the liquid and gaseous states.

We need not here recall his memorable experiments on the com-
position of air and of water, on the increase of the weight of metals
during their oxidation, on the phenomena of combustion, respiration,
and the production of animal heat, and on fermentation, or finally
his creation of the nomenclature. These new ideas overthrew the
theory of phlogiston. They brought light into the midst of the labori-
ous researches of the alchemists, they prepared the way for organic
and physiological chemistry, they gave rigor and exactitude to
chemical reactions. In a word, they established chemistry in the
position of a science.

Starting from this epoch, we can divide into three great periods
the numerous researches that were pursued in different countries.
In the first period, the modern idea of elements takes shape; in the
second, the chemical laws are established; and in the third, the
atomic weights of the elements are determined.

The first period includes the studies of a great number of investiga-
tors, but among them four names emerge above all others, — Scheele,
whose chemical genius enriched our science; Priestley, a mind at once
original and conservative ; Cavendish, whose analyses have never been
surpassed; and finally Humphry Davy, who, by the discovery of
the metals of the alkalies and alkaline earths, explained the compo-
sition of the earths and won definitive acceptance for the conception
of elements.

The second period presents to us the legislators of our science.
Wenzel, following up the work of Rouelle, gives precision to the
knowledge of salts and of double decompositions. Richter pub-
lishes the first tables of neutralization for acids and bases. Proust
formulates the law of the constancy of proportions (1806); and
Dal ton, at the same time, gives a complete exposition of the law of
multiple proportions, a first sketch of which he had presented, in
1803, to the literary and scientific society of Manchester. As we
shall see further on, the importance of Dalton's law was not appre-
ciated at its full value till much later. Finally, in 1808, Gay Lussac
indicated the laws, so simple, of the combination of gases. By their
promulgation, Gay Lussac gave to the concept of combination a
truly mathematical exactitude.

246 INORGANIC CHEMISTRY

After that, the study of the weight of the different elements which
enter into combination could be pursued with success, especially
when Mitscherlich's law of isomorphism (1819) and Dulong and
Petit's law of specific heat (1819) became known. In this third period,
in which experimental precision was carried to its furthest limits,
alongside of the researches of Victor Regnault, Faraday, Marignac, and
many others, the most important works published on the subject
that we are considering are those of Berzelius, Dumas, and Stas.

The magnificent effort of Berzelius provided a study, as complete
as possible, of most of our simple substances. This line of experiment
was taken up with the greatest care by Dumas, who first determined
the composition, by weight, of water and of air; and then by means
of simple and elegant experiments gave us a certain number of atomic
weights, among them that of carbon, the pivot of all organic chem-
istry. Stas next took up the study of these questions, and, d pro-
pos of William Prout's hypothesis of the unity of matter, showed
clearly that the atomic weights are not simple multiples of unity.
Stas's experiments will remain in our science as models of precision.

During this splendid period, which requires about a century, the
theories by which we bind together the innumerable details of our
science had time to change more than once.

We have already seen how Lavoisier's ideas replaced the theory
of phlogiston. Later, Humphry Davy, after his splendid discoveries,
assigned a predominating role to electricity and created the electro-
chemical theory, which was adopted and modified by Berzelius.
Then came the investigations of vapor density, and, after prolonged
discussions, many chemists abandoned the numbers of Berzelius, and
followed the so-called notation of equivalents, proposed by Wollas-
ton and adopted by Gmelin, Liebig, and Dumas. But soon Gerhardt,
considering as equivalents the quantities of hydrochloric acid, water,
and ammonia, which correspond to equal volumes, proposed a sys-
tem of atomic weights, which won as adherents: in France, Laurent,
Wurtz, Friedel; in England, Williamson, Frankland; in Germany,
Hofmann, Kekule", Baeyer; in Italy, Cannizaro. The hypothesis of
Avogadro and Ampere took on new life, and a sharp distinction
between atoms and molecules made possible a reconstitution of the
atomic theory on the basis of the great law of Dalton.

Considerably before this time chemistry was divided into two
great chapters: inorganic and organic.

The study of organic chemistry had begun with the investigations
of Lavoisier. During the succeeding century and more, chemists
tried first of all to isolate the proximate principles of vegetables and
animals. These studies, pursued on all sides with varying success,
endowed chemistry with a great number of clearly defined com-
pounds, some of which possessed important therapeutic properties.

RELATIONS TO OTHER SCIENCES 247

The analysis of all these substances was a rather delicate task, and,
as is the rule in the sciences, no definitive results could be established
'until the methods of analysis were carried to a sufficient point of
exactness. Only after this preliminary work was it possible to
classify these innumerable compounds. Various theories then fol-
lowed, till at last synthesis came to complete the work that had been
begun. We recall the great researches of Berthelot on this subject:
synthesis of the proximate principles of the animal fats, of the
alcohols, acids, carbides (acetylene in particular), camphor, different
essences. The vital force accepted by Berzelius, Liebig, and Gerhardt
no longer existed. Though man's power was limited in so many
things, he could make by synthesis inert organic matter.

Soon appears Kekule's schema giving a new orientation to organic
chemistry; and the synthesis of the most complicated compounds
is successfully attempted. Graebe and Liebermann accomplish the
synthesis of alizarin; and later, in a magnificent study of indigo,
Baeyer is able to state that the position of every atom in the mole-
cule of this dye has been experimentally determined. From these
researches issue the different syntheses of indigo. Finally Emil
Fischer achieves the syntheses of the sugars and so opens new hori-
zons to biology.

For fifty years, the chemistry of carbon has formed a separate
chapter, and has presented a marvelous spectacle in its develop-
ment and in its important industrial applications. From the stand-
point of research, organic chemistry — the fruitful theories of which
have been slowly transformed — no longer finds any difficulty in
determining the composition of the innumerable derivatives that it
studies. Inorganic chemistry, on the contrary, though it has aroused
so many efforts to establish the qualitative and quantitative analysis
of its various compounds, is still far from completion. It is still in a
stage of evolution, in spite of the recent work of Gooch, Clarke, and
so many others. The reason for this is that some of the elements are
as yet very incompletely studied. The large number of simple bodies
included in inorganic chemistry increases the difficulty.

When a good part of the atomic weights had been established, the
amount of effort that had to be devoted to organic chemistry caused
the number of researches in inorganic chemistry to decrease. To-day,
however, when the main lines of organic chemistry have been traced,
and when in place of the virgin forest, as Hofman called it, there
appears a complete city, beautifully laid out, the study of inorganic
chemistry has come again into honor.

However, inorganic chemistry has been continuing its discoveries
in the mean time. A certain number of new, and for the most part
rare, elements have been isolated in the last thirty years. Lecoq de
Boisbaudran, in 1875, obtained from Asturian blende a new and

248 INORGANIC CHEMISTRY

curious metal melting at 50°, gallium. Winkler, after very delicate
analysis, obtained germanium from Freiberg argyrodite. Also, in
1886, the author of this lecture succeeded in isolating fluorin, which,
though having a fairly wide distribution in nature, had previously
resisted the efforts of Humphry Davy, Louyet, the Knox brothers,
Fremy, and Gore.

Within the last few years, another series of discoveries has aroused
the keen interest of the scientific world. As the result of delicate
experiments for determining the density of nitrogen when prepared
by chemical reaction, and when obtained from the air, Lord Rayleigh
declared that the difference, which affected only the third decimal
of his figures, was to be attributed to the existence of a gaseous ele-
ment heavier than nitrogen, present in the atmosphere. Following up
this physical determination, Lord Rayleigh and Sir William Ramsay
isolated argon; and Sir William Ramsay obtained the satellites of
argon, such as krypton, xenon, and neon. These studies led him also,
turning his attention to the surface of the earth, to observe and study
helium, the spectrum of which had been simultaneously discovered
in the sun's rays by Sir Norman Lockyer and by Janssen.

These are splendid results, and they are the more curious since
they deal with a series of gaseous bodies which, because of their chem-
ical inertness, are a great embarrassment to the scientist and the
philosopher.

But there is a group of metals which, in spite of the continued
efforts of chemists, has never yet been fully studied. I refer to the
rare earths, divided into the two series of cerium and yttrium.

In 1751, Cronstedt discovered cerite in a mine at Bastnaes. In 1794,
Gadolin pointed out a rare earth, yttria, in a heavy black mineral
which was found abundantly in the neighborhood of Ytterby, and
which was afterwards named gadolinite. Cerium wras characterized
as an element, in 1804, by Berzelius and Hisinger in Sweden, and by
Klaproth in Germany.

This first work was followed by numerous rather confused investi-
gations, until Mosander, in 1839 and 1842, separated lanthanum and
didymium from the true cerium. The study of cerium and its com-
pounds was completed by the masterly researches of Cleve, and by
Marignac, Brauner, Wyrouboff, and Verneuil. Still later, Mosander's
didymium was separated by Auer von Welsbach into two elements,
praseodymium and neodymium.

Samarium was studied by Cleve, Lecoq de Boisbaudran, Demargay,
Brauner, and Bettendorf. While examining the action of samarium,
Demargay proved-the existence of a new element, europium.

Terminating his work on cerium, Mosander at once took up the
study of yttria and from it separated erbia and terbia. This study was
continued by Cleve, Marignac, Crookes, Delafontaine. In 1879, Cleve

RELATIONS TO OTHER SCIENCES 249

made it certain that erbia was a mixture of several earths, and since
that epoch, many researches have followed up this subject. Four
elements, yttrium, ytterbium, erbium, and Nilson's scandium, seem
proved beyond question. The splendid work of Cleve has shown that
there are yet other elements belonging to this group, in particular
holmiurn. From the yttria group, too, Marignac has isolated an earth
which has been named by Lecoq de Boisbaudran, oxide of gadolin-
ium.

In spite of the continued efforts of the Swedish school, in spite of
the researches of so many authorities, Berzelius, Mosander, Cleve,
Nilson, Crookes, Marignac, Lecoq de Boisbaudran, Demargay, Brau-
ner, Wyrouboff, and Verneuil, this great problem of the rare earths is
far from being finished. The separation of these different oxides re-
mains one of the most difficult operations of chemistry, and yet when
one compares elements so closely akin as these, one feels what inter-
est for science would attach to a complete study of them.

In short, inorganic chemistry has never ceased progressing; and it
has taken advantage of all the discoveries achieved in the other
sciences.

The most striking example of this is spectral analysis. We may
recall that Wollaston had in 1802 indicated the discontinuity of the
solar spectrum. Later, in 1815, Frauenhofer studied the darkened
rays of the solar radiation, and the luminous rays of certain spectra.
Though numerous studies of this subject were made by Brewster,
Wheatstone, Alter, Angstrom, Masson, and Pliicker, it was not till
KirchhorFs great discovery, in 1860, that the perfect correspondence
between the luminous rays of different spectra and the black por-
tions of the solar and stellar radiations became known.

Spectral analysis was thereupon inaugurated by Kirchhoff and
Bunsen, and its value was immediately demonstrated by their dis-
covery of the new elements, rubidium and caesium. Inorganic chem-
istry appropriated the new method. Sir William Crookes indicated
the existence of thallium, which was isolated soon after by Lamy.
Reich and Richter discovered indium. Next came the discovery of
gallium. Finally, hi the hands of many authorities, Bunsen, ThaleX
Cleve, Nilson, Crookes, Lecoq de Boisbaudran, Demargay, Becquerel,
Benedicks, this method was applied to that difficult problem of the
rare earths.

The simple phenomenon of reversed lines was to extend the field
of analytical chemistry to the limits of the furthest visible stars. It
was destined to demonstrate that the same matter was distributed
throughout the whole universe. In fact, Kirchhoff detected in the
atmosphere of the sun the presence of sodium, calcium, and barium ;
of manganese, iron, chromium, copper, and zinc. Subsequently,
Angstrom and Thalen proved the existence in the sun of hydrogen.

250 INORGANIC CHEMISTRY

magnesium, and aluminium. Sir Norman Lockyer, in his beautiful
spectral studies on the analysis of the heavenly bodies, showed that
the sun contained also cadmium, strontium, cerium, lead, and potas-
sium. Higgins followed this up by examining the spectra of stars
and nebulae, in which he met with the same elements. Le P. Secchi
showed that the spectrum of comets gave the rays of the hydro-
carbons.

This whole great question was reviewed and put into shape by use of
new methods, by Rowland, professor in the university at Baltimore.
He has given us the most important results that we possess on the
composition of the sun as determined by the study of its spectrum.
He has distinguished 20,000 rays, only a third of which are surely
coincident with our terrestrial rays. It is true, however, that among
the coincidences occur the most powerful rays of elementary bodies.
Rowland concludes from this that if the earth were raised to the tem-
perature of the sun, it would give almost the same spectrum.

Inorganic chemistry has further utilized spectral analysis for the
study of band spectra, which serve the chemist as a means of ana-
lysis.

Were there any need of another example to show the fusion of inor-
ganic chemistry and physics, we could recall the many applications
of electrolysis which are utilized by chemists. Scarcely had Volta
published his great discovery of the electric pile when Carlisle and
Nicholson put it into use for the decomposition of water, and only
a few years afterwards Humphry Davy prepared by this process the
metals of the alkalies and alkaline earths. These metals in their turn
served for the isolation of boron, silicon, magnesium, and aluminium.

Since that time, not a year passes without calling in electrolysis to
enlarge the field of our discoveries. Many metalloids and metals are
obtained to-day by this means, and the most active agent in inorganic
chemistry, fluorin, could be isolated by no other method. But we
ought also to recall that the study of electrochemistry, and the splen-
did researches of Faraday on electric conductivity, completed and
extended as they were by Kohlrausch, have started chemistry in a
new direction which has led to most valuable results. So true is this
that Lord Rayleigh could say, at the Montreal meeting of the British
Association, "It is by the study of electrolysis that we can hope to
increase our knowledge of chemical reactions and of the forces that
produce them; in my opinion, the next advance of the science will be
by that road."

This penetration of physics into chemistry became more complete
as the result of the masterly studies of Henri Sainte Claire Deville
on dissociation. By systematically examining the incomplete decom-
positions of a certain number of substances and by showing the close
connection between this dissociation and evaporation, Deville broke

RELATIONS TO OTHER SCIENCES 251

down the barriers that separated physical and chemical phenomena.
He explained many little-understood reactions, and showed how in-
verse reactions were produced and how the minerals were formed in
metallic veins.

Henri Debray soon demonstrated the value of Deville's ideas by his
elegant experiments on the decomposition of carbonate of lime and
of hydrated salts.

This matter of dissociation had also certain points of contact with
the phenomena of equilibrium of which mention was made in the im-
portant memoirs of Berthelot and Pean de Saint Gills, on speeds of
etherification. But I do not wish to enter upon the history of this
question, for my colleague Mr. van't Hoff will speak to you about it at
the Congress of Physical Chemistry, much better than I could. I will
only say that at all times the two sciences of physics and chemistry
have been of mutual aid and support. Victor Regnault began this
great movement of physical chemistry, illumined by the brilliant
discoveries of Deville, enlarged by the work of Joule, and continued
with such success by Gibbs, van der Waals, van 't Hoff, Arrhenius,
and Ostwald.

Passing to a different order of ideas, we may recall the splendid
work of Pasteur on molecular dyssymmetry, from which started the
very original investigations of Le Bel and van't Hoff on the isomerism
of substances possessing rotatory power.

At every turn, inorganic chemistry depends on the data of physics.
The determination of physical constants is an everyday performance
in our laboratories, and often is the only guarantee of the purity of
our preparations. In doubtful cases, when it becomes difficult to estab-
lish an atomic weight, the law of Dulong and Petit gives us valuable
information. The whole of thermo-chemistry, indeed, founded with
such success by Berthelot and by Thomson, makes use only of the
methods of calorimetry.

There is another branch of physics which is called upon to render
service to inorganic chemistry, and which has had a great develop-
ment in the last few years; I refer to the easy production of high and
low temperatures.

Metallurgy and ceramics have for thousands of years made use,
industrially, of high temperatures for obtaining metals, glass, and
terra-cotta. These high temperatures were secured by the combus-
tion of wood or coal. Later, savants concentrated the solar heat by
means of mirrors or burning-glasses for accomplishing some interest-
ing experiments. Two centuries ago, the importance of the action
of heat in the different reactions was so well appreciated that it served
as the basis for Stahl's theory of phlogiston. And when chemistry
established itself as a science, the ideas of Lavoisier on combustion
were the starting-point of this profound transformation.

252 INORGANIC CHEMISTRY

The employment of the oxyhydrogen blowpipe enabled Robert
Hare, professor in Philadelphia, to obtain, in 1802, temperatures
higher than those of the most powerful industrial furnaces, and to
carry out on a small scale several very curious experiments, such as
the fusion of platinum and the volatilization of silica. You know
how happily Deville and Debray later applied the oxyhydrogen jet
for studying the metallurgy of the platinum group. The question
of the heating of ordinary furnaces was after long discussion an-
swered both practically and theoretically by the work of Ebelmen
and the important researches of Siemens.

Each of these advances was followed by a set of chemical discov-
eries consisting either in the more profound study of certain reactions
or in the appearance of new compounds which enriched first science,
and finally industry.

But the oxyhydrogen jet does not permit the attainment of a
higher temperature than 1800°. The melting-point of platinum,
as determined by M. Violle, is 1775°. It would be useful to study
our chemical reactions above that temperature.

When we wished to reproduce the diamond, we soon saw that our
study must be extended to include the various forms of carbon. So
generalized, the question included the interesting topic of the solu-
bility of carbon in melted metals. Now, as some of the metals had
very high melting-points, we tried experiments with the aid of the
oxyhydrogen blowpipe. Under these conditions, the fusion of the
metal, in presence of an excess of carbon, occurred in an atmosphere
rich in watery vapor, and therefore oxidizing. On the other hand, the
combustion of the coal, and the vapor of carbon, furnished a reduc-
ing medium. The consequence was that unless a constant tempera-
ture was maintained, it was impossible to get a definite equilibrium
between these opposite reactions. Besides, in these conditions com-
plete reactions could not be obtained, and the results were variable
from one experiment to another.

Already different investigators, among both scientific and indus-
trial workers, had tried to utilize the high temperature of the electric
arc, discovered a century ago by Humphry Davy. But these attempts
could not be successful until the perfection of the dynamo-electric
machine. Gramme's discovery and the gradual improvement of
the dynamo finally placed in the hands of chemists a powerful
source of electric current which was easily transformed into heat.

By a curious coincidence, our science has been able, within a few
years, to thrust back the known frontiers of both heat and cold.
After the important experiments of M. Cailletet, which served as the
starting-point of these new studies, and after the original investiga-
tions of Raoul Pictet, Olszewski and Wroblewski, Sir James Dewar
was able to obtain liquid hydrogen in the static condition, and by

RELATIONS TO OTHER SCIENCES 253

vaporizing this to reach the lowest temperature yet attained, that of
the solidification of hydrogen, — 252. 5°, or 20.5° above absolute zero.
Thus the usable scale of temperature has been considerably lengthened.

Less fortunate than Sir James Dewar, we have not succeeded, in
a long series of experiments made by use of the electric furnace, in
determining exactly the extreme limit of temperature reached. As
the outcome of some delicate experiments, M. Violle has assigned as
the boiling-point of carbon the temperature of 3500°. But as we shall
prove further on, the temperature of the arc increases with the in-
tensity of the current, and the measurement of these high tempera-
tures requires further investigation. In order to fix the conditions
of our experiments, we carefully stated the voltage and amperage of
the current and the duration of each experiment. The diameter
of the electrodes and the capacity of the furnace had been deter-
mined beforehand, and remained constant.

At the very start, we found that at the temperature of our electric
furnace, the metallic oxides, hitherto regarded as irreducible, were
easily decomposed. Also reactions which were only partial at the
highest temperatures of ordinary furnaces became total here. A large
number of compound substances were dissociated at these high
temperatures, and on the other hand, new series of combinations,
definite and crystallized, were obtained. We thus prepared unknown
compounds of great stability, such as the carbides, borides, and sili-
cides. Most of these new binary compounds can also be partly or
wholly broken up by still further increasing the intensity of the
current and with it the temperature.

Some of these carbides will furnish us a very definite scale of dis-
sociation. We also meet here, in the neighborhood of 3000°, the
same general laws which govern the decomposition of substances
by heat at lower temperatures. Moreover, the boiling of a mixture
of copper and lead, of tin and lead, or of copper and tin, presents the
same peculiarities between 2000° and 3000° as does a mixture of
water and ether, of water and alcohol, or of water and formic acid, at
much lower temperatures. The laws of the fractional distillation of
two liquids apply therefore to the boiling of metals at very high
temperatures.

In using our electric furnace, we operate in a reducing atmosphere,
and if a strong enough current is employed we get very quickly a
constant temperature, which is the boiling-point of quicklime. If,
on the contrary, the substance to be studied is put very close to the
arc, that is to say, very close to the gaseous conductor composed of
carbon vapor which unites the electrodes, the temperature rises
with the intensity of the current. A chemical reaction proves this.
With a current of 100 amperes and 50 volts, the reduction of titanic
acid by the carbon gives an oxide of an indigo blue color. With

254 INORGANIC CHEMISTRY

500 amperes and 70 volts, a fused mass of yellow nitride is obtained;
while the high temperature of an arc of 1200 amperes and 70 volts
gives a carbide of titanium free from nitrogen. With so intense a
current as this last the nitride of titanium can no longer be formed ;
its dissociation by heat is complete and only the carbide can remain.

In pursuing this study, we have found still other examples of
combination and then decomposition under the action of an electric
arc of greater and greater intensity.

Organic chemistry comes into contact with biology; whence its
greatness and also its difficulties.

Biological chemistry could not be developed till after a systematic
study of the chemistry of carbon had been made. For a century it
was thought that physiological chemistry needed in its manifold
transformations only the four elements, carbon, hydrogen, nitrogen,
and oxygen. But in recent years our ideas on this point have changed
considerably. It has long been known that iron was indispensable in
both the animal and vegetable kingdoms. Further, Raulin had proved
by some curious experiments the important influence of traces of
foreign metals on the culture of aspergillus niger. These experiments
had been forgotten; they came too early.

But to-day discoveries relating to this point appear in constantly
greater numbers. For example, the fine work of Frederick and of
Henze has shown that copper is a constituent of hemocyanin in the
blood of cuttlefishes and Crustacea. We know now that iodin and
bromin should be found in the thyroid gland; these elements are
seen to be indispensable to the regular course of normal life. The
existence of arsenic in animal tissues was a thing unheard of a few
years ago. Professor Armand Gautier has now established by very
delicate .experiments that arsenic is always present in the horny
tissues and in the thyroid gland; and M. Gabriel Bertrand has
demonstrated the normal existence of arsenic in the living cells of
fishes taken from the sea-bottom at a depth of 3000 meters.

In the same way, a trace of another element, such as manganese,
may intervene in the form of a soluble ferment, in the oxydases. One
understands then the importance of the different elements and sees
that sometimes, in traces, they may play a physiological role of
capital importance.. We have long known that sulphur forms part
of the proteid molecule, although we are still quite ignorant of the
transformations which bring this element into complex compounds.

It is quite evident that on this point great discoveries still await
their realization. We are only beginning to-day the study of the dif-
ferent elements in their combinations with carbon, from the physio-
logical point of view; it may be said that the physiology of the cell
remains wholly to be made. We are happy to know that a start in
this direction is being made by means of microchemical reactions.

RELATIONS TO OTHER SCIENCES 255

Biology therefore unites again inorganic and organic chemistry.
The truth is, there is but one chemical science; every separation is
artificial. Just as energy is one, chemistry is one. The splendid
researches of Curtius on nitrohydric acid, and our own investiga-
tions of the metallic carbides and of the hydrides, of the alkalies
and alkaline earths, show how the two chemistries constantly inter-
penetrate, and demonstrate the unity of the science.

It is true, however, that inorganic chemistry has a technique of
its own. To make discoveries there, the precision of physics must
be applied. A few examples will make my thought clearer.

Lavoisier only overthrew the theory of Stahl as the result of rigor-
ous experiments prepared with the greatest care and exactitude.
We may refer in this connection to his experiments on combustion,
on respiration, and on fermentation.

Cavendish, when studying the action of the electric spark on a
mixture of oxygen and nitrogen, pushed the experiment till there
remained but a very small quantity of gas incapable of uniting with
oxygen. He mentions its existence. Since his time, over a century
ago, in how many universities, lycea, and gymnasia has this experi-
ment of Cavendish been repeated? And yet no one, during the cen-
tury, completed the experiment. It was always begun, but never
finished. Any one who had carried it on patiently till the nitrogen
was entirely absorbed would have discovered argon. It was needful
that Lord Rayleigh should determine the densities of the gas, vouch-
ing for the third decimal place, in order that the discovery should
be realized. The method is elegant, but the path of discovery is
rather circuitous.

Shall I cite you another example? When Gay Lussac, in 1815,
discovered cyanogen, that first example of a compound playing the
part of an element, that first radical formed of nitrogen and carbon,
he prepared it by moderately heating pure, dry cyanide of mercury.
The cyanide in these conditions split into cyanogen gas and mercury.
The experiment is of the simplest. Only a few years before, Proust
also had heated cyanide of mercury in a retort. He. had obtained
ammonia, an apparently oily compound, nitrogen, carbon dioxide and
monoxide. The reason was that Proust used damp cyanide. This
difference in manner of conducting the same experiment between
two men of the ability of Gay Lussac and Proust seemed to me very
interesting.

To return to Gay Lussac's preparation of cyanogen. He had left
in the bottom of his retort a small quantity of a black powder. After
establishing the formula of cyanogen, the existence of hydrocyanic
acid and of the cyanides and cyanates, he made an analysis of this
powder. It had exactly the same composition as cyanogen. Gay
Lussac notes this fact, but he takes care not to go farther, and

256 INORGANIC CHEMISTRY

chemistry had to wait till the researches of Troost and Hautefeuille,
published in 1873, before knowing the laws of the transformation of
cyanogen into its polymere paracyanogen.

I might further cite for you on this point Humphry Davy's method
of work; I might recall to your minds the fact that Wohler was a
master of chemical analysis, and outline for you the excellent studies
of Berzelius or of Stas. If I have lingered on this topic, it is because
I regard it as most important. Many great investigations remain to
be made in inorganic chemistry, but to get them done, the methods
must be refined and attain great precision. In a word, experimental
research in chemistry should have the rigor of physical experiments.

But to return to the relations of chemistry with the other sciences.
We have already spoken of physics and biology. I do not wish to
enlarge beyond measure on this point. I will remind you that astro-
nomy, thanks to spectrum analysis, the joint product of physics and
chemistry, has been able to extend and develop certain of its theories
to include the remotest star visible in our horizon. Moreover, the
spectroscopic method of Doppler and Fizeau has been of great serv-
ice in determining the speed of the heavenly bodies.

Our chemistry also comes into contact with mathematics at two
important points. It comes into contact with statics in stereochem-
istry and the special grouping of atoms, in questions of symmetry
and in combinational analysis which studies the combinations of
objects associated in different conditions. It comes into contact
with mathematics also on the dynamic side, in that it involves the
principles of molecular mechanics in connection with the conservation
of energy and the mechanical theory of heat.

Chemical analysis is one of the foundations of mineralogy. It is of
the greatest service to geology, which could not do without it. The
majority of the sciences have need of its assistance, and even the
historian comes to it to inquire the age of the successive foundations
of the ruins of Babylon, bringing to it the bronze or copper objects
which the latest excavations have put into his hands.

When it comes to industrial applications of the various sciences,
very few of them are not in debt to chemistry. The engineer has con-
stant need of it. Studies of the metals and their alloys have given
all their efficiency to machines, ships, and firearms. Two chapters,
however, in the applications of science will depend absolutely on the
progress of chemistry; we refer to the chemical industry and to
rural economy, — so important that they change the destinies of
nations, mingle the stocks of peoples, and modify the conditions of
their existence.

It is not our part to enlarge upon this side of the question; it is
enough to have mentioned it, and to recall, in closing, the vast sum
of effort which these researches have demanded. In the midst of

RELATIONS TO OTHER SCIENCES 257

these incessant transformations, this continued progress, we see
that scientific research has never had but one method: experiment.
Faraday's dictum is always true: "Chemistry is essentially an ex-
perimental science."

THE PRESENT PROBLEMS OF INORGANIC CHEMISTRY

BY SIR WILLIAM RAMSAY

[Sir William Ramsay, K.C.B., Professor of Chemistry, University College, Lon-
don, since June, 1887. b. October 2, 1852, Glasgow, Scotland. Ph.D. Tubingen,
Wurtemberg, 1872; LL.D. Glasgow; Sc.D. Dublin, Cambridge, England, and
New York; Ph.D. Cracow; M. D. Heidelberg; Officier de la L4gion d'Honneur,
France; winner of Davy Medal, 1897; Nobel Prize, 1904; Grande Me"daille of
H. I. M. the German Emperor, 1904, and many others. Professor of Chemistry,
University College, Bristol, 1880-87; Principal, ibid. 1881-87. FeUow of Royal,
Chemical, and Physical Societies, and of Society of Chemical Industry, London;
Honorary Member of Academies of Ireland, America, Berlin, Vienna, St. Peters-
burg, Stockholm, Copenhagen, Paris (Institute), and of Societies of Vienna,
Turin, Mexico, Philadelphia, Bohemia, Hungary, Frankfort, Geneva, Glasgow,
Manchester, Bristol, etc. Author of many books on Chemistry; editor of text-
books on Physical Chemistry.]

To discuss the "present problems of inorganic chemistry" is by no
means an easy task. The expression might be taken to mean an
account of what is being actually done at present by those engaged
in inorganic research; or it might be taken to relate to what needs
doing — to the direction in which research is required. To summarize
what is being done in an intelligible manner in the time at my disposal
would be an almost impossible task; hence I will choose the latter
interpretation of the title of my address. Now, a considerable expe-
rience in attempting to unveil the secrets of nature has convinced
me that a deliberate effort to discover some new law or fact seldom
succeeds. The investigator generally begins unmethodically, by ran-
dom and chance experiments; or perhaps he is guided by some indi-
cation which has struck his attention during some previous research;
and he is often the plaything of circumstances in his choice. Expe-
rience leads him to choose problems which most readily admit of
solution, or which appear likely to lead to the most interesting re-
sults. If I may be excused the egotism of referring to my own work,
I may illustrate what I mean by relating the following curious coin-
cidence: After Lord Rayleigh had announced his discovery that
"atmospheric nitrogen" was denser than "chemical nitrogen," I
referred to Cavendish's celebrated paper on the combination of the
nitrogen and the oxygen of the air by means of electric sparks.
Fortified by what I read, and by the knowledge gained during the
performance of lecture-experiments that red-hot magnesium is a
good and fairly rapid absorbent of nitrogen, it was not long before
a considerable quantity of nearly pure argon had been separated
from atmospheric nitrogen. Now it happens that I possess two
copies of Cavendish's works; and some months afterwards I con-
sulted the other copy and found penciled on the margin the words

PRESENT PROBLEMS 259

"look into this." I remembered the circumstance which led to the
annotation. About ten years before, one of my students had investi-
gated the direct combination of nitrogen and hydrogen, and I had
read Cavendish's memoir on that occasion. I -mention this fact to
show that, for some reason which I forget, a line of work was not
followed up, which would have been attended by most interesting
results; one does not always follow the clue which yields results
of the greatest interest. I regard it therefore as an impossible task
to indicate the lines on which research should be carried out. All
that I can do is to call attention to certain problems awaiting solution ;
but their relative importance must necessarily be a matter of personal
bias, and others might with perhaps greater right suggest wholly
different problems.

The fundamental task of inorganic chemistry is still connected
with the classification of elements and compounds. The investigation
of the classification of carbon compounds forms the field of organic
chemistry; while general or physical chemistry deals with the laws
of reaction, and the influence of various forms of energy in furthering
or hindering chemical change. And classification centres at present
in the periodic arrangement of the elements, according to the order
of their atomic weights. Whatever changes in our views may be con-
cealed in the lap of the future, this great generalization, due to New-
lands, Lothar Meyer, and Mendeleeff, will always retain a place,
perhaps the prominent place, in chemical science.

Now it is certain that no attempt to reduce the irregular regu-
larity of the atomic weights to a mathematical expression has suc-
ceeded; and it is, in my opinion, very unlikely that any such expres-
sion, of not insuperable complexity, and having a basis of physical
meaning, will ever be found. I have already, in an address to the
German Association at Cassel, given an outline of the grand problem
which awaits solution. It can be shortly stated then: While the fac-
tors of kinetic and of gravitational energy, velocity, and momentum,
on the one hand, and force and distance, on the other, are simply
related to each other, the capacity factors of other forms of energy ;
— surface, in the case of surface-energy; volume, in the case of
volume-energy; entropy for heat; electric capacity when electric
charges are being conveyed by means of ions; atomic weight, when
chemical energy is being gained or lost ; — all these are simply con-
nected with the fundamental chemical capacity, atomic weight, or
mass. The periodic arrangement is an attempt to bring the two sets
of capacity factors into a simple relation to each other; and while the
attempt is in so far a success, inasmuch as it is evident that some
law is indicated, the divergences are such as to show that finality
has not been attained. The central problem in inorganic chemistry is
to answer the question, why this incomplete concordance? Having

260 INORGANIC CHEMISTRY

stated the general question, it may conduce to clearness if some
details are given.

(1) The variation of molecular surface-energy with temperature is
such that the surface-energy, for equal numbers of molecules distrib-
uted over a surface, is equal for equal intervals of temperature below
the temperature at which surface-energy is zero — that is, the critical
point. This gives a means of determining the molecular weights of
liquids, and we assume that the molecular weight of a compound is
accurately the sum of the atomic weights of the constituent elements.

(2) The volume-energy of gases is equal at equal temperature from
that at which volume-energy is zero — i. e., absolute zero. And it fol-
lows that those volumes of gases which possess equal volume-energy
contain equal numbers of molecules — again, a close connection with
atomic weights.

(3) The specific heats of elements are approximately inversely pro-
portional to their atomic weights; and of compounds to the quotient
of their molecular weights divided by the number of atoms in the
molecule. Specific heat and entropy are closely related ; hence one of
the factors of thermal energy is proportional (nearly) to the recipro-
cal of the atomic weights.

(4) The ion carries in its migration through a solution one or more
electrons. Now, the ion is an atom carrying one or more charges —
one for each equivalent. Here we have the capacity for electric charge
proportional to the equivalent.

(5) The factors of chemical energy are atomic weight and chemical
potential; and as the former is identical numerically, or after multi-
plication by a simple factor with equivalent, electric potential is pro-
portional to chemical potential.

We see, therefore, that surface, volume, thermal, electrical, and, no
doubt, other forms of energy have as capacity factors magnitudes,
either identical with, or closely related to, units of chemical capacity;
while kinetic and linear energy are not so related, except through the
periodic arrangement of the elements.

It appears, therefore, to be a fundamental problem for the chemist
to ascertain, first, accurate atomic weights, and, second, to investigate
some anomalies which still present difficulties. In America, you have
excellent workers in the former branch. Mallet, Morley, Richards, and
many others have devoted their time and skill to perhaps the best
work of this kind which has been done; and F. W. Clarke has col-
lated all results and afforded incalculable help to all who work at or
are interested in the subject. Valuable criticisms, too, have been
made by Hinrichs; but it must be confessed that in spite of these,
which are perhaps the best determinations which have been made,
the problem becomes more, and not less, formidable.

There are lines of work, however, which suggest themselves as pos-

PRESENT PROBLEMS 261

sibly likely to throw light on the question. First, there is a striking
anomaly in the atomic weight of nitrogen, determined by analysis
and determined by density. Stas obtained the number 14.04 (O = 16),
and Richards has recently confirmed his results; while Rayleigh
and Leduc consistently obtained densities which, even when corrected
so" as to equalize the numbers of molecules in equal volumes, give
the lower figure 14.002. The difference is 1 in 350; far beyond any pos-
sible experimental error. Recently, an attempt to combine the two
methods has led to a mean number; but that result can hardly be
taken as final. What is the reason of the discrepancy? Its discovery
will surely advance knowledge materially. I would suggest the pre-
paration of pure compounds of. nitrogen, such as salts of hydrazine,
methylanine, etc., and their careful analysis; and also the accurate
determination of the density and analysis of such gaseous compounds
of nitrogen as nitric oxide and peroxide. I have just heard from my
former student, R. W. Gray, that he has recovered Stas's number
by combining 2NO with O2 ; while the density of NO leads to the
lower value for the atomic weight of nitrogen.1

The question of the atomic weight of tellurium appears to be set-
tled, at least so far as its position with regard to the generally accepted
atomic weight of iodin is concerned; recent determinations give the
figures 127.5 (Gutbier), 127.6 (Pellini), and 127.9 (Kothner). But
is that of iodin as accurately known? It would appear advisable to
revise the determination of Stas, preparing the iodin preferably from
an organic compound, such as iodoform, which can be produced in a
high state of purity. The heteromorphism of selenates and tellurates,
too, has recently been demonstrated; and it may be questioned
whether these elements should both belong to the same group.

The rare earths still remain a puzzle. Their number is increas-
ing yearly, and their claim to individuality admits of less and less
dispute. What is to be done with them? Are they to be grouped by
themselves as Brauner and Steele propose? If so, how is their con-
nection with other elements to be explained? Recent experiments
in my laboratory have convinced me that in the case of thorium, at
least, ordinary tests of purity such as fine crystals, constant subliming
point, etc., do not always indicate homogeneity; or else that we are
sadly in want of some analytical method of sufficient accuracy. The
change of thorium into thorium X is perhaps hardly an explanation of
the divergences; yet it must be considered; but of this, anon.

To turn next to another problem closely related to the orderly
arrangement of the elements, — that of valency, — but little progress
can be chronicled. The suggestions which have been made are specu-

1 Note added February, 1906: Researches by Gray and by Guye have since
shown that Stas's results are in error; and determinations by Richards allow
the same conclusion to be drawn. The actual atomic weight cannot differ much
from 14.007.

262 INORGANIC CHEMISTRY

lative, rather than based on experiment. The existence of many
peroxidized substances, such as percarbonates, perborates, persul-
phates,and of crystalline compounds of salts with hydrogen peroxide,
makes it difficult to draw any indisputable conclusions as regards
valency from a consideration of oxygen compounds. Moissan's bril-
liant work on fluorides, however, has shown that SF6 is capable
of stable existence, and this forms a strong argument in support of
the hexad character of sulphur. The tetravalency of oxygen, under
befitting conditions, too, is being acknowledged, and this may be
reconciled with the existence of water of crystallization, as well as of
the per-salts already mentioned. The adherence of ammonia to many
chlorides, nitrates, etc., points to the connecting link being ascribable
to the pentavalency of nitrogen ; and it might be worth while investi-
gating similar compounds with phosphoretted and arseniuretted
hydrogen, especially at low temperatures.

The progress of chemical discovery, indeed, is closely connected
with the invention of new methods of research, or the submitting of
matter to new conditions. While Moissan led the way by elaborating
the electric furnace, and thus obtained a potent agent in temperatures
formerly unattainable, Spring has tried the effect of enormous pres-
sure, and has recently found chemical action between cuprous oxide
and sulphur at ordinary temperature, provided the pressure be raised
to 8000 atmospheres. Increase of pressure appears to lower the tem-
perature of reaction. It has been known for long that explosions will
not propagate in rarefied gases, and that they become more violent
when the reacting gases are compressed: but we are met with diffi-
culties, such as the non-combination of hydrogen and nitrogen, even
at high temperature and great pressure ; yet it is possible to measure
the electromotive force (0.59 volt) in a couple consisting of gaseous
nitrogen and gaseous hydrogen, the electrolyte being a solution of
ammonium nitrate saturated with ammonia. Chemical action be-
tween dissolved hydrogen and nitrogen undoubtedly occurs; but it is
not continuous. Again we may ask, Why? The heat evolution should
be great; the gain of entropy should also be high were direct combina-
tion to occur. Why does it not occur to any measurable extent? Is
it because for the initial stages of any chemical reaction, the reacting
molecules must be already dissociated, and those of nitrogen are not?
Is that in any way connected with the abnormally low density of gas-
eous nitrogen? Or is it that, in order that combination shall occur,
the atoms must fit each other; and that, in order that nitrogen and
hydrogen atoms may fit, they must be greatly distorted? But these
are speculative questions, and it is not obvious how experiments can
be devised to answer them.

Many compounds are stable at low temperatures which dissociate
when temperature is raised. Experiments are being made, now that

PRESENT PROBLEMS 263

liquid air is to be purchased or cheaply made, on the combinations of
substances which are indifferent to each other at ordinary tempera-
tures. Yet the research must be a restricted one, for most substances
are solid at — 185°, and refuse to act on each other. It is probable,
however, that at low temperatures compounds could be formed in
which one of the elements would possess a greater valency than that
usually ascribed to it; and also that double compounds of greater
complexity would prove stable. Valency, indeed, appears to be in
many cases a function of temperature; exothermic compounds, as is
well known, are less stable, the higher the temperature. The sudden
cooling of compounds produced at a high temperature may possibly
result in forms being preserved which are unstable at ordinary tem-
peratures. Experiments have been made in the hope of obtaining
compounds of argon and helium by exposing various elements to the
influence of sparks from a powerful induction coil, keeping the walls
of the containing- vessel at the temperature of liquid air, in the hope
that any endothermic compound which might be formed would be
rapidly cooled, and would survive the interval of temperature at
which decomposition would take place naturally. But these experi-
ments have so far yielded only negative results. There is some indi-
cation, however, that such compounds are stable at 1500°. It might
be hoped that a study of the behavior of the non-valent elements
would have led to some conception of the nature of valency ; but so
far, no results bearing on the question have transpired. The condi-
tion of helium in the minerals from which it is obtainable by heat is
not explained; and experiments in this direction have not furnished
any positive information. It is always doubtful whether it is advis-
able to publish the results of negative experiments; for it is always
possible that some more skilled or more fortunate investigator may
succeed, where one has failed. But it may be chronicled that attempts
to cause combination between the inactive gases and lithium, potas-
sium, rubidium, and caesium have yielded no positive results; nor
do they appear to react with fluorin. Yet conditions of experiment
play a leading part in causing combination, as has been well shown
by Moissan with the hydrides of the alkali-metals, and by Guntz,
with those of the metals of the alkaline earths. The proof that sodium
hydride possesses the formula NaH, instead of the formerly accepted
one, removes one difficulty in the problem of valency; and SrH3
falls into its natural position among hydrides.

A fertile field of inorganic research lies in the investigation of struc-
ture. While the structure of organic compounds has been elucidated
almost completely, that of inorganic compounds is practically un-
developed. Yet efforts have been made in this direction which appear
to point a way. The nature of the silicates has been the subject of
research for many years by F. W. Clarke; and the way has been

264 INORGANIC CHEMISTRY

opened. Much may be done by treating silicates with appropriate sol-
vents, acid or alkaline, which differentiate between uncombined and
combined silica, and this in some cases, by replacement of one metal
by another, gives a clue to constitution. The complexity of the mole-
cules of inorganic compounds, which are usually solid, forms another
bar to investigation. It is clear that sulphuric acid, to choose a com-
mon instance, possesses a very complicated molecule; and the fused
nitrates of sodium and potassium are not correctly represented by the
simple formulae NaNO3 and KNO3. Any theory of the structure of
their derivatives must take such facts into consideration; but we ap-
pear to be getting nearer the elucidation of the molecular weights of
solids. Again, the complexity of solutions of the most common salts is
maintained by many investigators; for example, a solution of cobalt
chloride, while it undoubtedly contains among other constituents
simple molecules of CoCl2, also consists of ions of a complex character,
such as(CoCl4)". And what holds for cobalt chloride also undoubtedly
holds for many similar compounds.

In determining the constitution of the compounds of carbon, stereo-
chemistry has played a great part. The ordinary structural formulae
are now universally acknowledged to be only pictorial, if, indeed, that
word is legitimate; perhaps it would be better to say that they are
distorted attempts at pictures, the drawing of which is entirely free
from all rules of perspective. But these formulae may in almost every
case be made nearly true pictures of the configuration of the mole-
cules. The benzene formula, to choose an instance which is by no
means the simplest, has been shown by Collie to be imitated by a
model which represents in an unstrained manner the behavior of that
body on treatment with reagents. But in the domain of inorganic
chemistry, little progress has been made. Some ingenious ideas of the
geologist Sollas on this problem have hardly received the attention
which they deserve; perhaps they may have been regarded as too
speculative. On the other hand, Le Bel's and Pope's proof of the
stereo-isomerism of certain compounds of nitrogen; Pope's demon-
stration of the tetrahedral structure of the alkyl derivatives of tin;
and Smiles's syntheses of stereo-isomeric sulphur compounds give us
the hope that further investigation will lead to the classification of
many other elements from this point of view. Indeed, the field is
almost virgin soil ; but it is well worth while cultivating. There is no
doubt that the investigation of other organo-metallic compounds will
result in the discovery of stereo-isomerides; yet the methods of inves-
tigation capable of separating such constituents have in most cases
still to be discovered.

The number of chemical isomerides among inorganic compounds is
a restricted one. Werner has done much to elucidate this subject in
the case of complex ammonia derivatives of metals and their salts;

PRESENT PROBLEMS 265

but there appears to be little doubt that if looked for, the same or
similar phenomena would be discoverable in compounds with much
simpler formulae. The two forms of SO,, sulphuric anhydride, are
an instance in point. No doubt formation under different conditions
of temperature and pressure might result in the greater stability of
some forms which under our ordinary conditions are changeable and
unstable. The fact that under higher pressures than are generally at
our disposal different forms of ice have been proved to exist, and the
application of the phase rule to such cases will greatly enlarge our
knowledge of molecular isomerism.

The phenomena of catalysis have been extensively studied of recent
years, and have obviously an important bearing on such problems.
A catalytic agent is one which accelerates or retards the velocity of
reaction. Without inquiring into the mechanism of catalysis, its ex-
istence may be made to influence the rate of chemical change, and to
render bodies stable which under ordinary conditions are unstable.
For if it is possible to accelerate a chemical change in such a way that
the usually slow and possibly unrecognizable rate of isomeric change
may be made apparent and measurable, a substance the existence of
which could not be recognized under ordinary circumstances, owing
to its infinitesimal amount, may be induced to exist in weighable quan-
tity, if the velocity of its formation from an isomeride can be greatly
accelerated by the presence of an appropriate catalytic agent. I am
not aware that attempts have been made in this direction. The dis-
covery of catalytic agents is, as a rule, the result of accident. I do not
think that any guide exists which would enable us to predict that any
particular substance would cause an acceleration or a retardation of
any particular reaction. But catalytic agents are generally those
which themselves, by their power of combining with or parting with
oxygen, or some other element, cause the transfer of that element to
other compounds to take place with increased or diminished velocity.
It is possible, therefore, to cause ordinary reactions to take place in
presence of a third body, choosing the third body with a view to its
catalytic action, and to examine carefully the products of the main
reaction as regards their nature and their quantity. Attempts have
been made in this direction with marked success; the rate of change
of hydrogen dioxide, for example, has been fairly well studied. But
what has been done for that compound may be extended indefinitely
to others, and doubtless with analogous results. Indications of the
existence of as yet undiscovered compounds may be derived from
a study of physical, and particularly of electrical, changes. There
appears to be sufficient evidence of an oxide of hydrogen containing
more oxygen than hydrogen dioxide, from a study of the electro-
motive force of a cell containing hydrogen dioxide; yet the higher
oxide still awaits discovery.

266 INORGANIC CHEMISTRY

The interpretation of chemical change in the light of the ionic
theory may now be taken as an integral part of inorganic chemistry.
The ordinary reactions of qualitative and quantitative analysis are
now almost universally ascribed to the ions, not to the molecules. And
the study of the properties of most ions falls into the province of the
inorganic chemist. To take a familiar example : The precipitation of
hydroxides by means of ammonia-solution has long led to the hypo-
thesis that the solution contained ammonium hydroxide; and indeed,
the teaching of the text-books and the labels on the bottles supported
this view. But we know now that a solution of ammonia in water is a
complex mixture of liquid ammonia and liquid water; of ammonium
hydroxide, NH4OH; and of ions of ammonium (NH4)', and hydroxyl
(OH)'. Its reactions, therefore, are those of such a complex mixture.
If brought into contact with a solution of some substance which will
withdraw the hydroxyl ions, converting them into water, or into some
non-ionized substance, they are replaced at the expense of the mole-
cules of non-ionized ammonium hydroxide; and these, when dimin-
ished in amount, draw on the store of molecules of ammonia and
water, which combine, so as to maintain equilibrium. Now the investi-
gation of such changes must belong to the domain of inorganic chem-
istry. It is true that the methods of investigation are borrowed from
the physical chemist; but the products lie in the province of the
inorganic chemist. Indeed, the different departments of chemistry
are so interlaced that it is impossible to pursue investigations in any
one branch without borrowing methods from the others; and the
inorganic chemist must be familiar with all chemistry, if he is to
make notable progress in his own branch of the subject. And if the
substances and processes investigated by the inorganic chemist are
destined to become commercially important, it is impossible to place
the manufacture on a sound commercial basis without ample know-
ledge of physical methods, and their application to the most econom-
ical methods of accelerating certain reactions and retarding others, so
as to obtain the largest yield of the required product at the smallest
cost of time, labor, and money.

I have endeavored to sketch some of the aspects of inorganic chem-
istry with a view to suggesting problems for solution, or at least the
directions in which such problems are to be sought. But the develop-
ments of recent years have been so astonishing and so unexpected,
that I should fail in my duty were I not to allude to the phenomena of
radioactivity, and their bearing on the subject of my address. It is
difficult to gauge the relative importance of investigations in this
field; but I may be pardoned if I give a short account of what has
already been done, and point out lines of investigation which appear
to me likely to yield useful results.

The wonderful discovery of radium by Madame Curie, the prepara-

PRESENT PROBLEMS 267

tion of practically pure compounds of it, and the determination of its
atomic weight, are familiar to all of you. Her discovery of polonium,
and Debierne's of actinium, have also attracted much attention.
The recognition of the radioactivity of uranium by Becquerel, which
gave the first impulse to these discoveries, and of that of thorium by
Schmidt, is also well known.

These substances, however, presented at first more interest for the
physicist than the chemist, on account of the extraordinary power
which they all possess of emitting "rays." At first, these rays were sup-
posed to constitute ethereal vibrations; but all the phenomena were
not explicable on that supposition. Schmidt first, and Rutherford
and Soddy later, found that certain so-called " rays " really consist of
gases; and that while thorium emits one kind, radium emits another;
and no doubt Debierne's actinium emits a third. The name " emana-
tions " was applied by Rutherford to such radioactive bodies; he and
Soddy found that those of radium and thorium could be condensed
and frozen by exposure to the temperature of liquid air, and that they
were not destroyed or altered in any way by treatment with agents
which are able to separate all known gases from those of the argon
group, namely, red-hot magnesium-lime, and it was later found that
sparking with oxygen in presence of caustic potash did not affect the
gaseous emanation from radium. The conclusion therefore followed
that in all probability these bodies are gases of the argon group, the
atomic weight of which, and consequently the density, is very high ;
indeed, several observers, by means of experiments on the rate of
diffusion of the gas from radium, believe it to have a density of approx-
imately 100, referred to the hydrogen standard. This conclusion has
been confirmed by the mapping of the spectrum of the radium emana-
tion, which is similar in general character to the spectra of the inactive
gases, consisting of a number of well-defined, clearly cut brilliant
lines, standing out from a black background. The volume of the gas
produced spontaneously from a given weight of radium bromide in
a given time has been measured ; and it was incidentally shown that
this gas obeys Boyle's law of pressures. The amount of gas thus col-
lected and measured, however, was very minute; the total quantity
was about the forty-thousandth of a cubic centimeter.

Having noticed that those minerals which consist of compounds
of uranium and thorium contain helium, Rutherford and Soddy made
the suggestion that it might not be impossible that helium is the pro-
duct of the spontaneous change of the emanation; and Soddy and
I were able to show that this is actually the case. For, first, when
a quantity of radium salt which has been prepared for some time is
dissolved in water, the occluded helium is expelled, and can be recog-
nized by means of its spectrum; further, the fresh emanation shows
no helium spectrum, but after a few days the spectrum of helium

268 INORGANIC CHEMISTRY

begins to appear, proving that a spontaneous change is in progress;
and last, as the emanation disappears its volume decreases to zero;
and on heating the capillary glass tube which contained it, helium is
driven out from the glass walls, into which its molecules had been
imbedded in volume equal to three and a half times that of the eman-
ation. The a rays, as foreshadowed by Rutherford and Soddy, con-
sist of helium particles.

All these facts substantiate the theory, devised by Rutherford and
Soddy, that the radium atom is capable of disintegration, one of the
products being a gas, which itself undergoes "further disintegration,
forming helium as one of its products. Up till now, the sheet-anchor
of the chemists has been the atom. But the atom itself appears to
be complex, and to be capable of decomposition. It is true that only
in the case of a very few elements, and these of high atomic weight,
has this been proved. But even radium, the element which has by far
the most rapid rate of disintegration, has a comparatively long life;
the period of half-change of any given mass of radium is approximately
1 100 years. The rate of change of the other elements is incomparably
slower. This change, too, at least in the case of radium and its eman-
ation, and presumably also in the case of other elements, is attended
with an enormous loss of energy. It is easy to calculate from heat
measurements (and independent and concordant measurements have
been made) that one pound of emanation is capable of parting with
as much energy as several hundred tons of nitroglycerine. The order
of the quantity of energy evolved during the disintegration of the
atom is as astonishing as the nature of the change. But the nature of
the change is parallel to what would take place if an extremely compli-
cated hydrocarbon were to disintegrate; its disruption into simpler
paraffins and defines would also be attended with loss of energy. We
may therefore take it, I think, that the disintegration hypothesis of
Rutherford and Soddy is the only one which will meet the case.

If radium is continually disappearing, and would totally disappear
in a very few thousand years, it follows that it must be reproduced
from other substances, at an equal rate. The most evident conjecture,
that it is formed from uranium, has not been substantiated. Soddy
has shown that salts of uranium, freed from radium, and left for a
year, do not contain one ten-thousandth part of the radium that one
would expect to be formed in the time. It is evident, therefore, that
radium must owe its existence to the presence of some other substances,
but what they are is still unascertained.

During the investigation of Rutherford and Soddy of the thorium
emanation, a most interesting fact was observed, namely, that precipi-
tation of the thorium as hydroxide by ammonia left unprecipitated a
substance, which they termed thorium X, and which was itself highly
radioactive. Its radioactive life, however, was a short one; and as

PRESENT PROBLEMS 269

it decayed, it was reproduced from its parent thorium at an equal rate.
Here is a case analogous to what was sought for with radium and
uranium; but evidently uranium is not the only parent of radium;
the operation is not one of parthenogenesis. Similar facts have been
elicited for uranium by Crookes.

The a rays, caused by the disintegration of radium and of its eman-
ation, are accompanied by rays of quite a different character; these
are the ft rays, identical with electrons, the mass of which has been
measured by J. J. Thomson and others. These particles are projected
with enormous velocity, and are capable of penetrating glass and metal
screens. The power of penetration appears to be proportional to the
amount of matter in the screen, estimated by its density. These elec-
trons are not matter; but, as I shall relate, they are capable of causing
profound changes in matter.

For the past year, a solution of radium bromide has been kept in
three glass bulbs, each connected to a Topler pump by means of capil-
lary tubing. To insure these bulbs against accident, each was sur-
rounded by a small beaker; it happened that one of these beakers con-
sisted mainly of potash glass; the other two were of soda glass. The
potash-glass beaker became brown, while the two soda-glass beakers
became purple. I think there is every probability that the colors are
due to liberation of the metals potassium and sodium in the glass.
They are contained in that very viscous liquid, glass, in the colorless
ionic state; but these ions are discharged by the ft rays, or negative
electrons, and each metal imparts its own peculiar color to the glass,
as has been shown by Maxwell Garnett. This phenomenon, however
interesting, is not the one to which I desire to draw special attention.
It must be remembered that the beakers have been exposed only to
ft rays; a rays have never been in contact with them; they have
never been bombarded by what is usually called matter, except by the
molecules of the surrounding air. Now these colored beakers are
radioactive, and the radioactive film dissolves in water. After careful
washing, the glass was no longer radioactive. The solution contains
an emanation, for on bubbling air through it, and cooling the issuing
air with liquid air, part of the radioactive matter was retained in the
cooled tube. This substance can be carried into an electroscope by
a current of air, after the liquid air has been withdrawn, and as long
as the air-current passes, the electroscope is discharged; the period of
decay of this emanation, however, is very rapid, and on ceasing the
current of air, the leaves of the electroscope cease to be discharged.
In having such a short period of existence, this emanation resembles
the one from actinium.

Owing to the recess, only a commencement has been made with the
investigation of the residue left on evaporation of the aqueous solu-
tion. On evaporation, the residue is strongly active. Some mercur-

270 INORGANIC CHEMISTRY

ous nitrate was then added to the dissolved residue, and it was treated
with hydrochloric acid in excess, to precipitate mercurous chloride.
The greater part of the active matter was thrown down with the mer-
curous chloride, hence it appears to form an insoluble chloride. The
mercurous chloride retained its activity unchanged in amount for ten
days. The filtrate from the mercurous chloride, on evaporation,
turned out to be active; and on precipitating mercuric sulphide in it,
the sulphide precipitate was also active; but its activity decayed in
one day. The filtrate from the mercuric sulphide gave inactive pre-
cipitates with ferric salts and ammonia, with zinc salts and ammo-
nium sulphide, with calcium salts and ammonium carbonate; and on
final evaporation, the residue was not radioactive. Hence the active
matter forms an insoluble chloride and sulphide. The precipitated
mercurous chloride and mercuric sulphide were dissolved in aqua
regia, and the solution was evaporated. The residue was dissolved in
water, and left the dish inactive. But the solution gave an insoluble
sulphate, when barium chloride and sulphuric acid were added to it;
hence the radioactive element forms an insoluble sulphate, as well as
an insoluble chloride and sulphide.

This is a sample of the •experiments which have been made. It
may be remarked that the above results were obtained from a mix-
ture of the potash and soda glass; somewhat different results were
obtained from the potash glass alone. These changes appear to be
due to the conversion of one or more of the constituents of the glass
into other bodies. Needless to say, neither of the samples of glass
contained lead.

I have mentioned these experiments in detail, because I think they
suggest wholly new lines of investigation. It would appear that if
energy can be poured into a definite chemical matter, such as glass,
it undergoes some change, and gives rise to bodies capable of being
tested for; I imagine that radioactive forms of matter are produced,
either identical with or allied to those at present known. And just
as radium and other radioactive elements suffer degradation sponta-
neously, evolving energy, so I venture to think that if energy be con-
centrated in the molecules of ordinary forms of matter, a sort of poly-
merization is the result, and radioactive elements, probably elements
with high atomic weight, and themselves unstable, are formed. Of
course further research may greatly modify these views; but some
guide is necessary, and Mr. Ternent Cook, who has helped me in these
experiments, and I, suggest this hypothesis (in the words of Dr. John-
stone Stoney, an hypothesis is " a supposition which we hope may be
useful ") to serve as a guide for future endeavor.

In the light of such facts, speculation on the periodic arrangement
of the elements is surely premature. It is open to any one to make
suggestions; they are self-evident. Most of you will agree with the

PRESENT PROBLEMS 271

saying, "It is easy to prophesy after the event."- I prefer to wait
until prophecy becomes easy.

I must ask your indulgence for having merely selected a few out
of the many possible views as regards the Problems of Inorganic
Chemistry. I can only plead in excuse that my task is not an easy
one; and I venture to express the hope that some light has been thrown
on the shady paths which penetrate that dark region which we term
the future.

SECTION B — ORGANIC CHEMISTRY

SECTION B — ORGANIC CHEMISTRY

(Hall 16, September 21, 3 p. TO.)

CHAIRMAN: PROFESSOR ALBERT B. PRESCOTT, University of Michigan.
SPEAKERS: PROFESSOR JULIUS STIEGLITZ, University of Chicago.

PROFESSOR WILLIAM A. NOYES, National Bureau of Standards.

THE Chairman of the Section of Organic Chemistry was Professor
Albert B. Prescott, of the University of Michigan, who opened the
proceedings by saying that "we are indebted to every one of the
speakers so far heard in the Department of Chemistry for important
studies of the element carbon, whether studies of the history, the
present problems, or the co-relations of chemical science. We are
under special obligations to the departmental address, yesterday, on
the ' Fundamental Conceptions of Chemical Change/ by a devotee
and a master of the investigation of carbon compounds. We found
the keenest interest, this morning, in the utterances of authority and
mature judgment upon questions touching the nature and relation-
ship of chemical elements in general, all bearing upon the character
of this element, whose unlimited synthetic powers have enlisted so
large a share of the labor of the chemical world. It but exemplifies
the unity of scientific truth, that all the divisions of chemical science
interweave with each other, so that each is strengthened and directed
by the growth of all the others. And in the addresses in Sections
to-morrow, in Physical Chemistry at ten and in Physiological Chem-
istry at three, I confidently expect that organic chemists will find
no less direct an interest bearing upon their own labors."

BY JULIUS STIEGLITZ

[Julius Stieglitz, Professor of Chemistry, University of Chicago, since 1905. A.M.
and Ph.D., University of Berlin, 1889; University Scholar, Clark University,
1890; Chemical Laboratory, Detroit, 1890-92; Decent in Chemistry, Univers-
ity of Chicago, 1892-93; Assistant, ibid. 1893-94; Instructor, ibid. 1894-97;
Assistant Professor, ibid. 1897-1902; Associate Professor, ibid. 1902-05.]

THE very name of the branch of chemistry on whose relations to
other sciences I have the privilege of addressing you to-day tells
us with what sciences in particular, other than sister branches of
chemistry itself, organic chemistry must stand in closest relation-
ship. Since Wohler in 1828 by the synthesis of urea showed that
there is no fundamental difference between compounds prepared in
the laboratory and the same compounds formed in Irving organisms
under the influence of what until then was known as "vital force,"
organic chemistry has become knitted more and more closely with
all branches of the great science of organic life. Its achievements in
the past culminated, we may say, in Fischer's synthesis of the impor-
tant hexoses and his magnificent development, with the aid of van ;t
Hoff and Le Bel's great theory, of the fact that there is an intimate
connection between the stereochemical configurations of organic
compounds and their production and assimilation in living organisms.
Great as these and similar achievements have been, they can be but
an earnest of what must still be done and is being done to have
organic chemistry do its full duty in the study of life's development,
its maintenance, its decay. The very fact that every stage of life
in the animal and the vegetable kingdom, in the lowest and the high-
est orders, is indissolubly connected with the formation or trans-
formation of very complex organic compounds shows us where the
path of the organic chemist must lead to, difficult as the way may
be. The plant physiologists, physiological chemists, physiologists,
anatomists, bacteriologists have piled up questions for us at a far
greater rate than we have been able to answer them.

Before this host of questions there is one to which I wish to call
your attention in particular this afternoon in the time at our disposal,
and to whose answer I wish to bring a small contribution based on
work done with Messrs. Derby, McCrackon, and Schlesinger. The
composition, structure, and configuration of the innumerable com-
pounds connected with vital phenomena are problems of the highest
importance. But the questions as to how and why such molecules
are formed and transformed seem equally important, for if the trans-

RELATIONS TO OTHER SCIENCES 277

formation ceases, life also ceases, even in the presence of abundance
of valuable molecules that build up organisms. Now we are all aware
of the fact that there is almost no vital transformation of matter that
is not regulated by so-called "catalytic " agents, — enzymes, acids,
bases, salts, — in fact, as not long ago was pointed out, an organism
seems an almost perfectly regulated machine for the transformation
of matter, and the regulators seem to be the catalytic agents! They
determine the speed of chemical changes, and, as Euler states, it seems
almost certain that without the catalyzer there would be no trans-
formation, no chemical action at all. When, a short time ago, Jacques
Loeb startled the world by the artificial fertilization of eggs of sea-
urchins and other marine animals by salt solutions of definite com-
position and concentrations of their ions, he suggested in one of his
addresses that the key to his results would most likely be found in
the fact that in all eggs there is a tendency to develop, but that if the
development were not hastened so as to reach a certain stage within
a given time-limit, death would follow without the production of the
young animal. But if the development were accelerated sufficiently,
a normal development of life would follow. According to Loeb, then,
even this fundamental life-fact, the fertilization of eggs, involves
probably to a large extent a question of an accelerated reaction, or, as
we may say, a "catalytic" problem. In Loeb's experiments and
hundreds of others we know what the ultimate results of the cata-
lytic reactions are, but we are just beginning to have any experi-
mental answers to the question why and how catalyzers exert their
marvelous accelerating influences. It may be there is no general
answer possible to this question which would cover all cases — we
can only know after the study of a large number of individual cases.
In this semi-darkness we may distinguish for the present two classes
of catalytic reactions, first those produced by so-called heterogeneous
or physical agents, like platinum black, and secondly those produced
by what may be called homogeneous chemical catalyzers such as
acids, alkalies, salts. In an endeavor to ascertain in some individual
case the exact manner in which a catalytic agent acts, a reaction of
the second simpler class was chosen, namely, the thoroughly studied
catalysis of an ester like methyl acetate under the influence of acids
and water. As we all know, the facts are the following: the saponi-
fication of methyl acetate by water according to the equation

CH3C02CH, + H20 <=» CH3CO2H + CH,OH

proceeds exceedingly slowly. Acids greatly accelerate the saponi-
fication proportionately to the concentration of the hydrogen ions
used. It has been established by Knoblauch and Kistiakonsky that
the ultimate condition of equilibrium of the reversible reaction is
not sensibly altered by the catalyzer, in other words, that the acid

278 ORGANIC CHEMISTRY

accelerates the reaction velocity in either direction to the same
extent. In the third place, the catalytic agent appears to act by its
presence simply; it appears, at least, to remain unchanged throughout
the course of the reaction. These three properties have been assumed
by some to be necessary and typical characteristics of catalytic action.
But in this investigation, in order not to overlook possibly the real
answer to our problem, the vital fact of acceleration alone was con-
sidered as characteristic and it was left to the rigorous application of
well-known fundamental laws of chemistry to develop why, inci-
dentally, in this and similar cases the equilibrium is not disturbed
sensibly and why the catalyzing acid appears to have no share in the
reaction. From this point of view, in endeavoring to imagine just
how an acid could affect the speed of the above reaction, the most
fundamental fact concerning acids was recalled, the fact that they
have the power to form salts with bases and oxides. Here we have the
acid and the oxide, and the idea was at once suggested that methyl
acetate has basic properties and that salt formation with the acid
is the cause of its catalysis. It was clear, however, that the basic
functions of a substance like methyl acetate must be far too weak for
quantitative measurements of its constants and for a rigorous quan-
titative test of the idea just developed. Under these circumstances it
was thought best to study all these conditions in a class of closely
related bodies, the imido-esters, with which quantitative measure-
ments of all important factors could readily be carried out. As the
name implies, these are esters in which we have an imide group
(: NH), replacing the oxygen atom of the ester, as in imidomethyl
acetate, CH3C( : NH)OCH3. They are markedly basic substances
which form well-defined salts. The free bases, for instance benzimido-
esters, are very slowly decomposed by water, chiefly according to the
reaction

C6H5C(: NH)OCH3 + H20->C6H5CONH2 + CH3OH (1)
and yet more slowly according to

C6H5C(: NH)OCH3 + H2O-»C6H6C02CH3-f NH3 (2)

Both reactions are practically non-reversible. The addition of
hydrochloric acid enormously increases the velocity of the second
reaction, and it becomes almost the exclusive one. Again the question
arises, how does the acid accelerate the action. Of course the acid
forms the hydrochloride, but as imido-esters are weak bases we have
in the aqueous solution partial hydrolysis and a condition of equilib-
rium according to

C6H5C (NH2C1)OCH3 + H20 <=± C6H5C (NH2OH)OCH3 + HC1 (3)

The reaction presents, therefore, at least three possibilities, — the
velocity may be proportionate to the concentration of the salt

RELATIONS TO OTHER SCIENCES 279

present at any moment, or to that of the free base, or to either indiffer-
ently, to the total substance, as expressed in:

dr

(i) £=M(Salt)

at

(n) — =kbx (Base)

dt

dx

(in) —=kgux (Substance)

dt

In order to decide between these three possibilities and thus answer
our question, it was necessary to determine two things experimentally,
first the actual change, x in time t, and second the proportions of
salt, free base, and acid present at any moment t. The latter may be
determined on the basis of the well-known equation for the solution
of a hydroiyzed salt, namely, according to Arrhenius:
(Positive Ion) fcbaae

_

(Base)z(H) fcwater

The constant k was determined experimentally by conductivity
measurements, and with its aid the concentrations of salt, base, and
acid for the above differential equations calculated. The experi-
mental results show unmistakably that the true course of the reac-
tion is given by equation (i), which alone leads to a true constant.

For instance, we have among our many results for methyl imido
benzoate :

43430 fcsalt = 246; 246; 256; 238; 236; 234.

257; 239; 259; 249; 237; 242; 252; 248.
43430 fcsubgt = 202; 184; 183; 172.

231; 201; 188; 175; 168; 163; 158; 138.
100fcbase=72; 67; 59; 51; 46; 42.
58; 49; 44; 41; 35; 33.

For the corresponding nitrobenzoate we have:
10,000*^ = 256; 252; 246; 255; 252; 248; 263; 261; 257.
4343 fcsubst = 102; 98; 96; 93; 90; 87;

febaae = 1.17; 1.23; 1.01; 0.78; 0.69.

It is therefore certain that hydrochloric acid, which enormously
increases the velocity of saponification of the imido-ester according
to equation (n), does so simply and quantitatively through salt
formation, as was expected. As the experiments were carried out
in dilute solutions in which the salts are practically completely
ionized, it is obvious that it is the positive ion which is decomposing
in the direction given and the acceleration is exclusively due to the
formation of more such active or unstable ions.

The accelerating, or let us call it the catalytic, action of the acid
is here surely due then to salt or ion formation, or in other words,

280 ORGANIC CHEMISTRY

to the formation of a different, less stable, or more reactive molecule.
Now if this is the correct explanation of the catalytic action of acids,
it is clear that by this same salt or ion forming power, they should
in certain cases retard action instead of accelerating it, provided the
ion or salt is more stable than the free base. That must be an inevit-
able consequence of this theory, and we have brought its complete
experimental confirmation in work recently published on the molecu-
lar rearrangement of certain organic bases according to

NH2 NHCO2C2H5

OC02C2H5 OH

by a shifting of a carbethoxy group. The bases are exceedingly weak
ones, and their salts, again, are hydrolyzed according to

NH3C1 NH3OH

X + H20<=>X +HC1

OC02C2H5 OC02C2H5

Hydrochloric acid retards but does not prevent the rearrangement,
and it was proved that it retards it quantitatively by salt formation
and that the velocity of the rearrangement remains rigorously pro-
portionate to the concentration of the free base present at any
moment. For instance the velocity constant was found to be &base =
.0566 at the beginning of the reaction, and .0567 at its end ten hours
later.

Now these great changes in speed of reaction are the main charac-
teristics of catalytic action; and we have in these cases a very simple
explanation of it. It remained, however, to ascertain whether the
two other important characteristics for many catalytic reactions are
also in agreement with our conception of salt formation when rig-
orously applied — first, as explained for the catalysis of methyl
acetate, the fact that the catalyzing acid need not appear to combine
with any of the reacting substances, and second the fact that in a
reversible reaction it need not measurably change the final condition
of equilibrium. These points were tested by the application of our
fundamental conception to the catalysis of methyl acetate. The
intimate connection with the work on the imido-ethers is recognized
as follows: we found above the velocity of saponification of imido-
esters to be

doc

— = kX(Salt)

ctt

But according to Walker and Arrhenius
(Salt) =KX (Base) X (H)

RELATIONS TO OTHER SCIENCES 281

and therefore

dx

But this is exactly the velocity equation for the ester catalysis, (Ester)
being substituted for (Base) :

dx

37 = fc X (Ester) X (H)

And so if the ester could form salts with acids its saponification could
undoubtedly be due to the saponification only of its salt or positive
ion. This important link in the chain of argument has been supplied
by the discovery of Baeyer and others that the esters do form well-
defined salts with acids, very unstable ones, but still salts. They are
almost certainly salts of quadrivalent oxygen bases or oxonium salts.
Coehn has proved that they are true electrolytes by showing that the
positive ion of dimethyl pyronium hydrochloride moves to the nega-
tive pole when the solution is electrolyzed.

Now if we start from the idea that it is only the positive ion of
methyl acetate which is saponified by water, we can put for the reac-
tion:

CH3C02CH, +H20 -> CH3C02H +CH3OH

dx

fa = fcsap X (Posit. Ester Ion) X (H20)

as was proved experimentally for the imido-esters. For the combin-
ation of methyl acetate with water to form an oxonium base and for
the ionization of this base, we have

OH

I
CH3CO-O-CH3+H20<=>CH3CO-O-CH3 and

I
H

OH

I +

CHsCO-O-CH3«r±CH3CO-0-CH,+OH

I I

H H

and consequently

(Posit. Ester Ion) = ^? X (Ester) X (H)
K

By the substitution of this value for the concentration of the posi-
tive ester ion in the above equation, we obtain for the velocity of
saponification of methyl acetate by water alone

i^base

?MP = ksap X -- X (Ester) X (H) X (H,0)

282 ORGANIC CHEMISTRY

If we add hydrochloric acid some salt must be formed, which will
be almost completely hydrolyzed according to
Cl OH

I I

CH3CO-O-CHS + H2O ^±CH3CO-O-CHS + HC1

I I

H H

Then according to Arrhenius's equation for hydrolyzed solutions
of salts of weak bases with strong acids we have

IT"

(Posit.Ester Ion) = -^X (Ester-?/) X (H')
K.

For an almost completely hydrolyzed salt the change in the con-
centration of the ester will not be perceptible, y will be entirely
negligible, and we obtain then for the velocity of saponification of
methyl acetate in the presence of hydrochloric acid:

Tf-

^sap (HOD = fcsap X -jp X (Ester) X (HO X (II20)

By a comparison of the two velocity equations, for the reaction in
the presence of water alone and in the presence of added acid, we find
that the velocity must in fact increase directly proportionate to the
concentration of the hydrogen ions, since all other factors remain
unchanged. This consequence of our theory is evidently in perfect
agreement with the well-known experimental results.

And now we come to the last important fact, namely, that the
reaction is a reversible one, viz.:

CH3CO-O-H +CH3OH -»CH3COOCH3 +H2O

and that the velocity of this reaction is also accelerated by the addi-
tion of hydrochloric acid. Following out our idea rigorously, this
increased velocity under the influence of an acid must be due to min-
imal basic properties of acetic acid or methyl alcohol. It could easily
be shown, if time permitted, that it is to the basic properties of acetic
acid that we must look in this instance. This conclusion is not so sur-
prising as it may appear at first glance, we have so many substances
that are both basic and acid, and

Cl Cl

I I

CH3CO-0-H is not more different from CH3CO'0'CH3

I I

H H

Cl Cl

than CH3.O.H is from CHS.O.CH

H H

RELATIONS TO OTHER SCIENCES 283

It may be added that Rosenheim from a comparison of compounds
of acetic acid and its esters with metal chlorides arrived by a different
way at the conclusion that acetic acid must form oxonium salts.
Euler has also decided that acetic acid must have some basic func-
tions. Applying again this conception and the laws of equilibrium to
the study of the velocity of esterification, we find the velocity, in the
absence of any acid, in the same way as was used,above,

yest = fce8tX (Posit. Acetate Ion) X (CH3OH)

= fcegt X *^-e X (Acet. Acid) X (H) X (CH.OH)
K.

In the presence of acid:

= *est X X (Acet. Acid) X (H') X (CH3OH)

We find again that the change in the velocity of esterification de-
manded by the application of this theory is simply proportionate to
the change in concentration of the hydrogen ions — which agrees
with experience.

When equilibrium is established between the two reversible reac-
tions in the absence of hydrochloric acid we have

V = V or
' sap ' est> u

*W X ^ X (Ester) X (H,O) X (H) -

fcest X ?-^ X (Acet. Acid) X (CH3OH) X (H)
K

An inspection of the equations shows that we also have then :

*sap(HCl) = ^est(HCl)

since according to the equation just given

*sap X ~^ X (Ester) X (HO X (H,O)

jrf

must equal fcestX —^X (Acet. Acid) X (CH3OH) X (HO
K.

In other words the addition of the catalyzing acid will not affect the
ultimate condition of equilibrium between ester, water, acid, and
alcohol.

We find thus the theory that acids may act as catalytic agents
simply through salt or ion formation, as proved by the experiments
with the imido-esters and the rearranging amino carbonates, leads by
the rigorous application of our laws of dynamics also directly to the
very facts in regard to the catalysis of methyl acetate which have long
been known as vital results of experimental observation. The three
important characteristic features of the catalysis — a velocity pro-

284 ORGANIC CHEMISTRY

portionate to the concentration of the hydrogen ions, the catalyzer
apparently acting only by its presence and not changing the final
condition of equilibrium, all are in perfect agreement with this simple
conception of the manner in which the catalyzer produces its appar-
ently marvelous result. The future must determine how many of the
catalytic actions which are of such fundamental importance in the
economy of organic life will be capable of explanations equally simple
and quantitative in their ultimate terms, however much these terms
may vary in details.

PRESENT PROBLEMS OF ORGANIC CHEMISTRY

BY WILLIAM ALBERT NOYES

[William Albert Noyes, Chief Chemist of the National Bureau of Standards, Wash-
ington, D. C. b. November 6, 1857, near Independence, Iowa. A.B. Iowa Col-
lege, 1879; S.B. ibid. 1879; Ph.D. Johns Hopkins University, 1882; Graduate
scholarship, Johns Hopkins University, 1881-82; student in Munich, 1889.
Professor of Chemistry, University of Tennessee, 1883-86; Professor of Chemis-
try, Rose Polytechnic Institute, 1886-1903. Member of Indiana Academy of
Sciences (President, 1904); American Chemical Society (Editor since 1902, Sec-
retary since 1903); Society of Chemical Industry; Deutschen Chemischen Gesell-
schaft. Author of Organic Chemistnj for the Laboratory; Elements of Qualitative
Analysis; Text-book of Organic Chemistry, and many scientific papers.]

THERE is a strong tendency on the part of some chemists, at the
present time, to claim that chemical science in the true sense includes
only such portions of our knowledge as can be stated in accurate
mathematical terms. One distinguished representative of this school
of chemistry has said, " It is not in the province of science to explain
phenomena," and another has written, " It is not a part of its ultimate
object [i. e., of natural science] to acquire knowledge in regard to
mentally conceived existences, such as the atoms of matter, or the
particles of luminiferous ether, which are of such a magnitude and
character as to lie far beyond the limits of human conception." I think
that nearly all of those now actively engaged in working over the
problems of organic chemistry would dissent strongly from these
statements. Long experience in dealing with the cumulative, non-
mathematical evidence upon which our knowledge of chemical struc-
ture is founded has led to a very firm conviction that human know-
ledge is not bounded by the limits of sense-perception. We are in-
clined rather to the view that, while there are, undoubtedly, many
things which will always remain beyond any direct cognizance of
our senses, yet, so far as these have a real existence, we may in the
end secure, regarding them, very practical and positive knowledge.
It is impossible to conceive that those theories with regard to struc-
ture which have guided the work of thousands of chemists for the
last fifty years do not in some measure express the actual truth
with regard to atoms and their relation to each other in organic
compounds.

Let us follow, for a few moments, in very brief outline, the steps
which have led to the present standpoint. So far as the matters which
interest us most are concerned, there was practically no knowledge of
organic chemistry before the nineteenth century. The first steps were,
of course, the preparation of pure substances and the development
of accurate methods of analysis. In both of these fields Liebig was the
great master. The formulae which were calculated were, at first, of

286 ORGANIC CHEMISTRY

little value except to check the accuracy of the analyses and as a sim-
ple expression for empirical composition. I need not dwell on the
confusion which existed throughout the first half of the century be-
cause there was no agreement as to the basis for molecular weights
or atomic weights, nor upon the large part played by the study of or-
ganic compounds in finally clarifying the view of chemists upon these
matters. Yet, in spite of this confusion, two discoveries of funda-
mental importance date from this period: (1) That the empirical
composition alone does not fix the nature of a compound, i. e., the
fact of isomerism; (2) that certain groups of atoms may remain
together in passing from one compound to another through a whole
series. The first fact furnishes one of the strongest reasons why an
empirical formula for an organic compound is not enough; and the
second fact furnishes the most important experimental basis at the
foundation of our structural formulae.

The studies of this period furnished a knowledge of the empirical
composition of many natural products and of the products obtained
from these by oxidation, reduction, and the action of various agents.
But while some might, perhaps, be inclined to look upon this mass of
empirical knowledge as the most valuable acquisition of that time
and to think that the theories in vogue were so imperfect or erroneous
as to be of no value, such a view is certainly superficial. There were
plenty of chemists in that day, too, who were ready to decry theories
which seemed to them worthless, and it is interesting to read to-day
what the great Laurent said upon this matter. He wrote in 1837 : x "If
I could believe that the purpose of my work was only to find a few
new compounds or that it would end in my being able to say that
there is an atom more or less in this compound or that, I would give it
up on the spot. Only the desire of finding an explanation for some
phenomena and of proposing some more or less general theories can
give me the courage to follow a course in which I have found so little
encouragement and where I have met with so many obstacles to over-
come." Any one who has followed the story of how the older theories
of radicals paved the way for the theory of types and of how the typi-
cal formula? were so easily transformed into structural formulae when
the fact of valence was once grasped, cannot fail to see that the
larger and fuller view is an outgrowth from the earlier theories. And
we must acknowledge that Laurent was right and that the theories
upon which he was working were of vastly more importance than
the mass of empirical facts which furnished him with their scaffold-
ing.

Do not misunderstand me. There were two theories of radicals at
that time — one which devised radicals in the study which should
accord with the electro-chemical theories held at the time and which
1 Ann. d. Chem. (Liebig), 22, 143.

PRESENT PROBLEMS 287

did not attempt to secure evidence of their existence from the conduct
of the compounds containing them, another which kept in much
closer touch with the facts discovered in the laboratory. It was only
the latter theory which contributed much to the growth of our know-
ledge. A theory which cannot secure for itself a sound experimental
basis is, of course, of only ephemeral value.

These, then, are the steps which have led to our present standpoint
in organic chemistry: The discovery of isomerism, the discovery of
radicals, the older radical theory, the theory of types, the establish-
ment of true molecular weights, the discovery of the fact of valence,
the determination of structure.

I think that all workers in organic chemistry will accept the follow-
ing as a conservative statement of our present knowledge: (1) That
in organic compounds, at least, each atom is attached directly to only
a limited, small number of other atoms; (2) that in the sense of the
order of the successive direct attachments the structure of a very
large number of compounds is known with a degree of probability
that amounts to practical certainty.

This brings me to the task which has been set, an attempt to out-
line the problems which lie before us in the further development of
our science.

In the first place, there is still much to be done to extend our know-
ledge of compounds found in nature. This field is much less cultivated ,
relatively, than was the case sixty years ago. There has been good
reason for this because of the problems of absorbing interest which
have arisen in the preparation and study of new compounds and in
the extension of our knowledge of old ones. But there must still re-
main many compounds to discover among both animal and vegetable
products. On this side organic chemistry resembles the descriptive
sciences of botany, zoology, and mineralogy. And just as botanists
think it worth their while to secure as complete a description as possi-
ble of the plants to be found on the earth, so it lies in our province to
isolate and identify the carbon compounds of the animal and vege-
table worlds — with the difference that in our case each compound,
new or old, may be the starting-point for the preparation of an almost
endless number of others. But here most of us recognize that unless
a compound has some further interest than that it is new it is not
worth the time taken in its preparation. I am afraid, however, as we
look over the pages of our journals, there is too much evidence that
not every one lives up to this view. Our ever-increasing army of
nascent doctors must needs have something to do, and it is so easy
to make new compounds, and so difficult to find something new of
larger scope and really worth the doing.

There still remains much to do in the determination of the struc-
ture of compounds which have long been known. The study of a

288 ORGANIC CHEMISTRY

single compound often involves an incredible amount of work. Baeyer
worked with indigo for fifteen years before his labors were crowned
with a successful synthesis, and twenty years more and the work of
very many chemists were needed before the scientific achievement
could become a commercial success.

It was nearly twenty-five years after the first structural formula
was proposed for camphor before Bredt was fortunate enough to
suggest the true arrangement of its atoms, and it was ten years longer,
and required in all the work of more than fifty chemists, before Bredt 's
suggestion was confirmed by Komppa's beautiful synthesis.

More than thirty formulae were proposed for camphor, and those
who think little of organic chemistry have some reason if they say
that we jump at conclusions too hastily and propose too many formulae
that are mere guesses. Some might even say that the last formula
is not worth much, but those who have followed the matter know
that step by step we have arrived at an almost positive certainty
even in this complex problem.

The final solution of a problem with regard to the structure of a
compound of natural origin is not usually considered to have been
satisfactorily attained until its synthesis has been effected. Those who
have attempted work of this character know that months or even
years of work are frequently spent to obtain the synthesis of a single
compound. In spite of the wealth of methods at our command, — a
wealth so great that it is often very difficult to select between several
which are equally unpromising, — it is evident that these methods of
synthesis need improvement at many points. Not only do we need
new and better methods, but many old methods require further study
to disclose why they succeed in some cases and fail in others, and to
secure a fuller knowledge of secondary reactions which often occur.
As recent remarkable achievements in this field of synthetic methods
may be mentioned the brilliant results obtained by Grignard with
magnesium compounds, Bouveault's elegant new solution of the old
problem of transforming an acid into the corresponding alcohol, and
Scheuble's reduction of the amides of bibasic acids to the corre-
sponding glycols.

Work along the lines suggested needs to be done in order to fill out
and complete our knowledge in a systematic way, and occasionally
work along such lines is rewarded by results of epoch-making signifi-
cance, as when Gomberg discovered triphenylmethyl in his endeavor
to prepare hexaphenylethane. Such work is not likely, however, to
greatly advance our insight into the real nature of carbon compounds,
and we all feel that there are far more fundamental problems which
demand attention.

As outlined above, the theories of valence and of structure now
universally accepted imply a certain amount of knowledge of the

PRESENT PROBLEMS 289

arrangement of atoms in space. So far as the original and funda-
mental conceptions are concerned, however, this knowledge is quite
vague. The much more definite conception proposed by van't Hoff,
and in a somewhat different manner by Le Bel, is, of course, familiar
to you all. In discussing any hypothesis it is always important to
have clearly before us the facts upon which it is based. As I have
already hinted, I believe that the theory of valence and the theory
of structure in the sense of a sequence of atoms within the molecule
are supported by our knowledge of such a vast accumulation of con-
sistently interrelated phenomena that we are justified in believing
that we have positive knowledge with regard to the structure of the
molecules of organic compounds. I am as ready as any one to de-
mand that every theory, no matter how old or how universally
accepted, shall be continually brought back to the test of agreement
with experimental facts, but I am not willing to admit that we may
not, in the end, acquire positive knowledge by the process of inductive
reasoning.

Assuming, then, the fact of a knowledge of the sequence of atoms
in organic compounds, we have this basis for van't Hoff s hypothesis:
(1) When four unlike atoms or groups are combined with a single
carbon atom, optical activity results in such a manner that there may
always be found two compounds having identical sequence of the
atoms within the molecule, and exactly equal rotary power, but of
opposite signs. (2) That when two adjacent carbon atoms are com-
bined each with three unlike groups, two compounds may result
which, while optically inactive and having the same sequence of
atoms, still differ in physical properties. An illustration of this is
found in racemic and mesotartaric acids. (3) Rings containing five
and six atoms are formed with especial ease, those containing three,
four, and seven atoms less readily, and rings containing more than
seven atoms are scarcely known. (4) Derivatives of cyclopropane,
cyclobutane, cyclopentane, and cyclohexane having two substituents
combined with different carbon atoms often exist in two isomeric
forms in which the sequence of the atoms is the same. (5) Deriva-
tives of ethylene often exhibit a similar isomerism.

Assuming as true that we have acquired a knowledge of the sequence
of atoms in carbon compounds, the facts which I have enumerated
lead almost inevitably to the corollary that the four atoms attached
to a given carbon atom are arranged in approximate symmetry
around the centre of that atom for their position of most stable
equilibrium. The -relation between this conclusion and the theory of
the sequence of atoms in carbon compounds, or what is ordinarily
understood as structure, is very similar to the relation between the
atomic theory and Avogadro's law. If we accept the atomic theory,
there seems to be no rational escape from the acceptance of Avogadro's

290 ORGANIC CHEMISTRY

law. In a similar manner, if we accept the theory of the sequence of
atoms in carbon compounds, there seems no reasonable possibility
other than that van't Hoff's hypothesis is true in its broad out-
lines.

I hope I may be pardoned here for a brief digression. I am aware
that Franz Wald 1 believes that he can give a satisfactory explanation
of the laws of fixed and multiple proportion and of combining weights
without the aid of the atomic theory, and that Professor Ostwald in
his recent Faraday lecture 2 has accepted and expanded the same
thought. I will say frankly that their reasoning does not appear to
me conclusive. Ostwald defines a chemical individual as "a body
which can form hylotropic phases within a finite range of tempera-
ture and pressure," 3 and deduces from this the fact that a given
hylotropic phase must have a fixed composition. He appears to forget
that the existence of these hylotropic phases implies that the pro-
perties of matter are discontinuous, or, in other words, that there is
a finite number of hylotropic bodies, one of the facts for which the
atomic theory gives an explanation.

There is another characteristic, too, of a chemical compound which
all chemists will agree is at least as important as that it shall consist
of a "hylotropic phase. " This is that the compound must not only
have a fixed composition, but this composition must bear a definite
relation to those numerical quantities which represent the proportion
in which each element of which it is composed always combines
with other elements. I need hardly add that these numerical quan-
tities are so deeply seated in the properties of matter that, having
adopted a unit, all chemists are absolutely agreed in selecting one
and only one such quantity for each of the well-known elements.

In attempting to deduce this law of combining weights Ostwald
assumes that three elements form the compounds AB, AC, BC, and
ABC, and adds, "There shall be but one compound of every [each]
kind." With this assumption, his reasoning may be sound, but I
fail to see how it applies when we find ten thousand compounds
ABC instead of one. The case which he supposes is so far theoretical
that I have been unable to find an actual case where the compound
ABC can be formed, by the union both of AB with C and of AC with
B.4 But I have taken too much time with a matter which is aside

1 Ztschr. Phys. Chem., 24, 633, 1897.

2 J. Chem. Soc. (London), 35, 506.

3 Ibid., p. 515.

4 It is quite possible that such an illustration may be found, but, in any case,
Professor Ostwald's deduction cannot be made to apply to those cases in which
the compound ABC does not exist, nor to those cases where the compound ABC
cannot, even theoretically, be supposed to consist in turn of a known compound
AB combined with C and of another known compound AC combined with B.
Such cases are common because of the fact of valence. In its simplest form the
law of combining weights is quite independent of the existence of the compound
ABC and may be stated thus: If the composition of two compounds A B and BC

PRESENT PROBLEMS 291

from my main purpose. Before leaving this topic I must add, how-
ever, that I have used the phrase "Avogadro's law" advisedly, in
spite of the fashion set by some chemists of calling it Avogadro's
hypothesis.1

I remarked, a few moments ago, that the facts which have been out-
lined almost compel us to the acceptance of van't Hoff 's hypothesis
in some form. It is of the utmost importance for us to recognize,
however, that we are here at the very confines of our present know-
ledge, and that we must, at every step, bring ourselves back to the
rigorous test of experimental fact. In accepting the hypothesis we
are not compelled to consider molecules as set pieces of mechanism;
on the contrary, there is strong reason for thinking that the positions
assumed by the atoms are positions of dynamic and not of static
equilibrium. While there have been many speculations in the matter,
we have no strong reason for assuming, as yet, any definite shape
for the carbon atom, nor even that there are within it definite points
of attraction for other atoms. All that seems to be thoroughly estab-
lished is that for their position of most stable equilibrium the four
atoms or groups attached to a given carbon atom are arranged in
approximate symmetry around its centre. I say approximate sym-
metry because the existence of compounds containing rings of three
and four carbon atoms demonstrates that the symmetry is not
always absolute, and makes it probable that in cases where the four
atoms or groups are unlike the symmetry is also imperfect. So far as
I am aware, no fact inconsistent with this fundamental conception
is known, while very many facts about optically active and cyclic
compounds find in this conception the only satisfactory explanation
which has thus far been given. It is true, also, that many facts with
regard to optically active compounds indicate that when one group
is exchanged for another the exact configuration is often retained,
or, in other words, the entering group takes the same position with

has been determined, the composition of a series of compounds between A and C
can be predicted and a compound which does not belong to this series has never
been discovered. A still more general statement of the law, and one which includes,
by implication, all of those facts which are used in the selection of atomic weights,
is given above. In that form it is more properly called the law of atomic weights.
1 Two reasons may be given for this usage. My own view is that we have, by a
process of inductive reasoning, acquired such positive knowledge of the existence
of atoms and molecules that the expression " Avogadro's law " is fully justified.
But even if we admit the contention of those who think that the atomic theory
must always remain an unproved hypothesis, it is possible to frame a definition
of the word molecule which would be merely a generalized statement of those
empirical facts which lie at the basis of our atomic and molecular theories. Such
a generalized, empirical definition must, of course, be very complex, but it would
not include the concept of discrete particles. Yet it will be still true of these
empirically defined molecules that equal volumes of gases contain equal numbers
under the same conditions of temperature and pressure. For instance, the term
gram-molecule may be considered as a purely empirical generalization, and it is
true that a gram-molecule of one gas occupies the same volume as a gram-mole-
cule of any other. But this is, in essence, Avogadro's law.

292 ORGANIC CHEMISTRY

regard to the other three atoms or groups as was held by the group
which was displaced. The manner in which it has been possible to
work out, consistently, the complex relations between a considerable
number of sugars, gives a very strong experimental basis for this
statement. On the other hand, it is well known that such reactions
often give racemic mixtures, which indicates that a shifting of groups
with regard to a central carbon atom takes place much more easily
than the shifting of a group from one carbon atom to another, at
least in saturated compounds. There are also a number of extremely
interesting cases where a reaction gives rise to the optical antipode.
Thus Walden has shown1 that 1-chlorsuccinnic acid is converted by
silver oxide into 1-malic acid, while potassium hydroxide converts
it into the dextro-rotatory acid. It is evident that in one case or the
other there has been a shifting of the groups. Again Ascham 2 has
shown that when d-camphoric acid is heated with hydrochloric and
acetic acids it may be about half converted into 1-isocamphoric acid,
and that the latter suffers a similar transformation. This case is
more complicated, as a "cis" and "trans" isomerism of cyclic com-
pounds is involved as well as the optical difference. Not many cases
of this character are known, at present, but such cases certainly
deserve further study and must be reckoned with in considering the
question we have before us. Le Bel 3 has already pointed out the the-
oretical significance of Walden's work.

While we may feel that we have comparatively sure ground in the
application of the theory of van 't Hoff and Le Bel to optically active
and to cyclic compounds, the case is quite different when we come
to the consideration of what are commonly known as "double" and
"triple" unions. Professor Michael has done a very great service to
chemistry in showing that the supposition of a more or less definite
tetrahedral shape for the carbon atom and of "favored" configura-
tions often leads to conclusions which are at variance with the facts.
Philips 4 and Blanchard 5 and myself have found a case in which the
addition of hydrobromic acid to an unsaturated compound produces
an optically active body which evidently has the same configuration
as the amino and hydroxy acids from which the unsaturated body is
formed by the loss of ammonia or of water. We have here, apparently,
a potential asymmetry occasioned by the double union which it is
difficult to reconcile with the prevailing conception of such unions.
This case is complicated by the presence of a second asymmetric
carbon atom in the molecule and is worthy of further study. Rabe and

1 Ber. d. Chem. Ges. 32, 1855 (1899).

2 Ibid. 27, 2004.

8 J. Chim. Phys. 2, 344 (1904).
* Am. Chem. J. 24, 428.
8 Ibid. 26, 281; 27, 428.

PRESENT PROBLEMS 293

Billmann l have recently described a similar case, but very few in-
stances of this kind are known.

Pfeiffer 2 has recently suggested a new interpretation of van 't
Hoff's hypothesis as applied to unsaturated compounds. Pfeiffer
assumes that unsaturated compounds retain essentially the same
configuration as the saturated compounds, from which they are
derived. On this side his interpretation is closely related to the old
theory of free valences, which, if I understand him correctly, is favored
by Professor Michael. Pfeiffer also brings his interpretation into a
close relationship to Werner's theory of inorganic metallic compounds.
The most serious objection to the theory is that it supposes the
existence either of trivalent carbon atoms or of free valences, in ethyl-
ene and its derivatives, an objection which has appeared to most
chemists very strong in the past. Pfeiffer points out, it is true, that
since the discovery of triphenylmethyl we can no longer deny the
possible existence of a trivalent carbon atom.3 It would seem, how-
ever, that the great difference between the intense chemical activity
of triphenylmethyl and the comparative inactivity of ethylene
demonstrates that, if the latter does in reality have free valences,
the fact that there are two such valences reduces the activity of each
enormously. The inactivity of carbon monoxide may be significant
in this connection.

A more serious objection to Pfeiffer's hypothesis lies in the fact
that he supposes so slight a difference in the configuration of fumaric
and of racemic acids that it is difficult to see why the former as well
as the latter might not be split into a pair of optically active bodies.

We must admit, then, that we have, at present, no satisfactory
theory of double and triple unions, and that we have here a problem
which demands a large amount of further work before it is solved.
When the solution is reached we shall probably gain a new insight into
the perennial question of the structure of benzene, and our knowledge
of tautomerism will cease to be, as it is at present, almost purely
empirical. It is possible, perhaps probable, that Thiele's "conjugated
double unions" will contribute toward the solution.

While I have no comprehensive theory with regard to double unions
to advance, I will, with a good deal of hesitation, venture to express
some thoughts with regard to the combination of atoms in general

1 Ann. d. Chem. (Liebig), 332, 25.

2 Ztschr. Phys. Chem. 43, 40.

3 The fact that triphenylmethyl exists as a doubled molecule in solution should
not, I think, lead us to discard the monomolecular formula for it any more than
we consider that acetic acid has, in the ordinary sense of structure, a doubled
molecule because it exists as a doubled molecule in solution in benzene or in the
state of vapor just above its boiling-point, nor because it forms acid salts. In these
cases the chemical evidence appears to be more important and more conclusive
than the physical. It is probable that the doubled physical molecule is the result
of forces which do not produce a stable structure in the ordinary sense.

294 ORGANIC CHEMISTRY

which have some bearing on this question. We are all familiar with
Faraday's law, that, if a current of electricity is passed through
a number of cells filled with solutions of different electrolytes and
arranged in series, exactly equivalent amounts of the various com-
ponents will be liberated at the electrodes in the successive cells.
The beautiful experiments of Professor T. W. Richards have demon-
strated that we are dealing here with a law which is true for different
solvents and over a wide range of temperature ; and also that the law
is true with a degree of absolute accuracy which is of the same order
as the laws of the combination of elements by weight. We are com-
pelled, then, to believe that there is associated with each valence of an
ion as it is transported through a solution, or at least as it separates
at an electrode, a quantity of electricity which is invariable and
independent of the nature of the ion. In other words, we have here
a natural electrical unit which can be denned in its relation to atomic
weights with a degree of accuracy which seems to be limited only by
the refinement of our manipulations.

It is not always recognized as clearly as it should be that this unit
quantity of electricity which is associated with one valence of any
ion is not a unit of electrical energy. If it were, the same energy
would be required to decompose the equivalent quantity of one
electrolyte as of every other, which is manifestly not true. While the
same current causes the separation of equivalent quantities in the
different cells, the differences of potential, and so the amounts of
energy required for the separation, vary greatly. It is evident then
that when we say that a unit quantity of electricity is associated
with each valence of every ion, we do not use the term quantity in
the sense of quantity of electrical energy. Instead of this, when this
conception of a unit quantity of electricity is examined, it will be
seen that it is a conception of something whose properties are those
of matter rather than those of energy. The facts appear to be con-
sistent with the idea that the unit quantity of electricity of which
we are speaking is of a material nature, and you have doubtless
already perceived that I have the theory of electrons in mind. The
ingenious experiments of J. J. Thomson have given us considerable
reason for thinking that the negative electrons are capable of an in-
dependent existence and have also given a probable estimate of their
mass, which is small in comparison with the mass of the hydrogen
atom.

It has been customary to think of the unit charge of electricity as
being involved only in those reactions which occur in solution. If,
however, we accept the theory of electrons, it is evident that the
electrons must be present in the molecule of an electrolyte, no matter
in what manner it is formed. It is but a step further to the conclusion
that the electrons are involved in every combination or separation

PRESENT PROBLEMS

295

of atoms, and, indeed, may be the chief factor in chemical combin-
ation.

Professor Kahlenberg 1 has shown that a practically instantaneous
reaction takes place between hydrochloric acid and copper oleate in
a solution in dry benzene, although the solution does not conduct an
electric current and there is no evidence of the dissociation of either
the copper oleate or of the hydrochloric acid. Professor Kahlenberg
points out very justly that there is no apparent difference between
these reactions and those which take place in aqueous solutions, where
we have much independent evidence of the existence of ions. He
draws the conclusion that no ions exist in either case. It would seem
that we are equally justified in supposing that a substance not already
in the form of ions may separate into them under the influence of
a second substance with which it can react.

Some time ago Mr. Lyon and myself 2 showed that the primary
reaction between chlorine and ammonia gives nitrogen tri-chloride,
nitrogen, and hydrochloric acid, and that these products are formed
in such proportion as to lead to the conclusion that three molecules
of ammonia react simultaneously with six molecules of chlorine. It
was pointed out at the time that the simplest explanation of this
result is to be found in supposing that chlorine atoms separate
during the reaction into positive and negative ions, while the ammonia
separates partly into positive nitrogen and negative hydrogen and
partly into negative nitrogen and positive hydrogen.3 This hypothesis
has met with some approval,4 but has also received the criticism
that such a dissociation as is supposed would result in the spon-
taneous decomposition of ammonia into nitrogen and hydrogen.5

1 J. Phys. Chem. 6, 1.

2 J. Am. Chem. Soc. 23, 460.

3 This was represented graphically thus:

N

N

4 Stieglitz, J. Chem. Soc. 23, 707.
1 Ztschr. Phys. Chem. 41, 378.

H

Cl

Cl

H

Cl

Cl

N

H

Cl

Cl

H

Cl Cl

H

H

Cl Cl

H

H

Cl Cl

H

296 ORGANIC CHEMISTRY

This criticism loses its force if we suppose that the separation into
ions takes place only under the immediate influence of the chlorine
with which the ammonia reacts. It has been pointed out by many
different authors * that a separation of atoms from each other must
occur either before or at the same time that they enter into com-
bination with other atoms. The only part essentially new in the
hypothesis proposed is that this separation is into positive and
negative parts and that the same atom may be sometimes positive
and sometimes negative. The idea of a dissociation which occurs under
the influence of a reacting substance appears to be implied in a part
of Professor Nef's discussion of methylene dissociation, but it is not
always clear whether he has in mind chiefly a dissociation of this
sort or one which is independent of the interaction of different com-
pounds.

The thought that the same atom may be at one time positive and
at another time negative is related to the older electrochemical theory
which supposed water to be positive in acids and negative in bases.

We assume, then, that in every combination of atoms each union
involves an attraction between the positive and negative electrons
which are associated with the two atoms that unite. In saying this
I do not lose sight of the fact that such a thing as attraction per se,
in the sense that one body can influence another at a distance with-
out an intervening medium, is apparently inconceivable. I think
of the attraction as probably caused by some motion of the electrons
which enables them to act on each other through the aid of the
ether. It is convenient, however, to speak of this effect as an attrac-
tion, since our conception of its real nature is, of necessity, very
vague. One advantage of the idea that the attraction of the electrons
is of a kinetic nature is that we may conceive of the same electron
as becoming positive or negative, according to the nature of its
motion.

The common conception, at present, is that an atom which has lost
an electron becomes positive, while either the electron in its inde-
pendent existence or the atom to which it is attached becomes nega-
tive. So far as I am aware, it has not been pointed out that this
view leads to the conclusion that the same atom must, under different
conditions, have a different weight. Thus a bivalent copper atom
which has lost two electrons must weigh less than a univalent copper
atom, which has lost only a single electron. It is true that our meth-
ods of determining atomic weights are scarcely accurate enough to
detect differences of this order. The suggestion which is made is that
the electrons of two atoms which are united have motions which
correspond to positive and negative charges, respectively, and that
when the atoms separate these motions may be retained, or lost as
1 See Erlenmeyer, Jr., Ann. Chem. (Liebig), 316, 50.

PRESENT PROBLEMS 297

in the case of a mercury atom which is uncombined, or that the mo-
tions may be reversed. In accordance with the hypothesis outlined
above, we must assume that when two atoms separate either one
may become positive; dependent partly on their nature, partly on
the nature of the reacting substance. The conception here proposed
is that of something very similar to the action of the pole of a magnet,
which may attract another pole of the opposite kind, or induce the
formation of a pole of the opposite kind, or it may reverse the polarity
of another magnet.1 This is, perhaps, simpler than to suppose the
transfer of an electron from one atom to another in those cases where
the electrical charges of the atoms are reversed in the ionization.
A very accurate determination of the atomic weight of cupric copper
as compared with that of cuprous copper might possibly decide be-
tween the two hypotheses.

It should be noted that the hypothesis that the electrical charges
associated with the atoms are of a kinetic nature, and that these
charges may be transferred without gain or loss of matter, is quite
independent of the first hypothesis, which is that the atoms are
ionized when they separate from each other and that the same atom
may become either positive or negative.

In following farther the thought of the attraction between elec-
trons as the cause of chemical combination, we must suppose that
in addition to the effect of this attraction in holding together the
atoms which are immediately attached, there is a residual effect
upon other atoms within the molecule. This gives a rational explana-
tion of the very great difference in the stability of the union between
carbon atoms in different compounds as, for instance, the instability
of acetic acid in comparison with butyric acid, occasioned by the sub-
stitution of an oxygen atom for two hydrogen atoms of the latter.
The study of organic compounds has given us a knowledge of a large
number of cases of .this sort, and our text-books contain many em-
pirical rules about them, but there have been few, if any, attempts
to give for such facts any rational explanation.

In considering double unions three explanations suggest themselves :
(1) We may suppose with Pfeifferthat such unions are in reality single
unions and free valences. In this case the presence in adjacent car-
bon atoms of positive and negative electrons which are uncombined
would reduce the attraction of each for the electrons of another
molecule, thus explaining why two free valences are so much less
active than a single one. (2) We may suppose that the carbon atoms
are in reality doubly united, but that, owing to the localization of

1 This is, of course, only an analogy and must not be pressed too far; just as
the electrical charges of atoms or ions conduct themselves very differently from
those of masses. The latter divide themselves between two bodies in contact; the
former may be transferred completely from one ion to another.

298 ORGANIC CHEMISTRY

the electrons in definite parts of the carbon atoms, the four electrons
involved cannot approach as near to each other as is the case in a
single union. This is Baeyer's theory of strain, and is much better
in accord than is the theory of free valences with the fact that cyclo-
propane and propylene appear to be about equally unsaturated, as
evidenced by their heats of combustion and by their conduct toward
bromine. On the other hand, it seems to lead logically to conclusions
with regard to the addition of bromine to triple unions, which Pro-
fessor Michael has shown are contrary to the facts. (3) Without
a condition of strain, we may suppose that the presence of both a
positive and a negative electron in each of the atoms united by the
double union causes a lessening of the attraction of the electrons.
This would result in such a union being less stable than a single union.
The second and third views appear, at present, most in accord with
the facts — possibly the truth lies in some combination of the two.

Whatever view we may take, it is noteworthy that double unions
are usually formed by the loss of a positive and negative atom or
group from adjacent carbon atoms, as hydrogen and hydroxyl or
hydrogen and bromine. It is also true that in many double unions
one of the carbon atoms is more positive than the other, causing the
addition of halogen acids in a definite manner which may be pre-
dicted in accordance with Michael's "positive negative law." Apply-
ing this thought to conjugated double unions, we see that of the four
atoms involved the two central ones are likely to be positive and
negative respectively and neutralize each other's attraction for out-
side atoms, while an intensified attraction for outside atoms would
be found in the exterior atoms. The effect may be analogous to that
of the attractive forces of a magnet which exhibit themselves chiefly
at the ends.

But I have permitted myself to wander much farther in the field of
speculation than was my first intention — farther than is at all
profitable, I fear, for these questions furnish, at present, few points
for experimental study, and speculations divorced from experiment
have usually been profitless. I should be very sorry if what has been
said should give encouragement to such speculations. On the other
hand, I have a very firm conviction that we should not be content
with rounding out organic chemistry as a descriptive science, nor even
with adding to the number of empirical rules which enable us to predict
certain classes of phenomena. We must, instead, place before our-
selves the much higher ideal of gaining a clear insight into the nature
of atoms and molecules and of the forces or motions which are the
real reason for the phenomena which we study. When we consider the
progress which has been made and the knowledge of structure we
now possess, which would have appeared sixty years ago to lie be-
yond the limits of possible acquirement, it is not presumptuous to

PRESENT PROBLEMS 299

think that a more complete knowledge of these questions will at some
time be gained. This fuller knowledge will take account, too, of many
lines of work upon which I have no time to dwell, such as the question
of changing atomic volume to which Professors Richards and Traube
have directed our attention, and the knowledge of heats of combus-
tion, of molecular refraction and dispersion, of color, viscosity,
dielectric constants, and other physical properties. The future must
give to us a new theory, or a development of old ones, which shall
include all of these phenomena in one comprehensive view.

SHORT PAPER

PROFESSOR OSWALD SCHREINER, of the United States Department of Agricul-
ture, read a short paper on " A Study of the Sesquiterpene Class of Hydrocar-
bons."

SECTION C — PHYSICAL CHEMISTRY

SECTION C — PHYSICAL CHEMISTRY

(Hall 16, September 22, 10 a. TO.)

CHAIRMAN: PROFESSOR WILDER D. BANCROFT, Cornell University.
SPEAKERS: PROFESSOR J. H. VAN 'T HOFF, University of Berlin.

PROFESSOR ARTHUR A. NOTES, Massachusetts Institute of Tech-
nology.
SECRETARY: MR. W. R. WHITNEY, Schenectady, N. Y.

THE Chairman of the Section of Physical Chemistry was Professor
Wilder D. Bancroft, of Cornell University, who opened the work of
the Section with the following remarks :

"Twenty years ago physical chemistry was not recognized as a
subdivision of chemistry. To-day nearly every larger university in
this country has a chair of physical chemistry, and we have our regu-
lar place on the Programme along with inorganic and organic chem-
istry. In fact, Professor Clarke in his address rather implied that
physical chemistry now dominates the whole of chemistry. Certain
it is that physical chemists are much in demand at this Congress and
that to hear them all you must go to many Sections. Van 't Hoff is
to speak to you here this morning, Arrhenius delivers an address the
next hour before the Section of Geophysics, Ostwald is to speak this
afternoon as a philosopher, while Sir William Ramsay was one of the
chief speakers yesterday before the Section of Inorganic Chemistry.

"In addition to the two longer addresses, our Programme includes
shorter papers on the chemical affinity between solvent and solute,
on the chemistry of liquid ammonia, on transference in acetic acid
solutions, and on the application of physical chemistry to agriculture.
The first address is on the history of physical chemistry by the man
who made that history possible, Professor van 't Hoff of Berlin."

THE RELATIONS OF PHYSICAL CHEMISTRY TO PHYSICS
AND CHEMISTRY

BY JACOBUS HENRICUS VAN *T HOFF

[Jacobus Henricus van't Hoff, Member of the Academy of Sciences, Berlin; Ordi-
nary Honorary Professor, University of Berlin, Germany, b. August 30, 1852,
Rotterdam. Ph.D. Polytechnic School, Delft; M.D. University of Utrecht;
LL.D. University of Chicago; ibid. Harvard University. Honorary course,
Griefswald and Utrecht. Tutor in Clinics, Veterinary School, Utrecht, 1876-
78; Professor in Chemistry, Mineralogy and Geology, University of Amsterdam,
1878-96. Member of various societies in Amsterdam, Bologna, Christiania, Delft,
Erlangen, Frankfurt. Gottingen, Batavia, Copenhagen, Lund, Mexico, New
York, Philadelphia, Rotterdam, St. Petersburg, Turin, Utrecht, Vienna, Venice,
Washington.]

ACCORDING to the Programme, I have to consider the "General
Principles and Fundamental Conceptions which connect Physical
Chemistry with the Related Sciences, reviewing in this way the
development of the science in question itself."

Let me begin by denning physical chemistry as the science devoted
to the introduction of physical knowledge into chemistry, with the
aim of being useful to the latter. On this basis I can limit my task to
the relations of physical chemistry to the two sciences it unites,
chemistry and physics.

But even if I limit myself to these relations, which are not the only
two,1 1 wish to restrict myself yet more, in order, in the spirit of this
Congress, to call your attention to broad views. So I shall follow up
only two lines, in answering two questions regarding two funda-
mental problems in chemistry: (1) What has physical chemistry
done for our ideas concerning matter? (2) What has it done for our
ideas concerning affinity?

The small table which I have the honor to put before you will
enable us to answer these questions by appeal to the scientific develop-
ment of our science, which also I have to review:

I. Ideas concerning Matter

(1) Lavoisier, Dalton (1808).

(2) Gay-Lussac, Avogadro (1811).

(3) Dulong, Petit, Mitscherlich (1820).

(4) Faraday (1832).

(5) Bunsen, Kirchhoff (1861).

(6) Periodic System (1869).

1 In Chicago I devoted to this subject eight lectures, which have since appeared
in'the Decennial Publications under the title ' Physical Chemistry in the Service
of the^Sciences, ' Chicago, 1903.

RELATIONS TO PHYSICS AND CHEMISTRY 305

(7) Pasteur (1853), Stereochemistry (1874).

(8) Raoult, Arrhenius (1886-87).

(9) Radioactivity (Becquerel, Curies).

II. Ideas concerning Affinity

(1) Berthollet, Guldberg, Waage (1867).

(2) Berzelius, Helmholtz (1887).

(3) Mitscherlich, Spring (1904).

(4) Deville, Debray, Berthelot.

(5) Thomson, Berthelot (1865).

(6) Horstmann, Gibbs, Helmholtz.

I. Physical Chemistry and our Ideas concerning Matter

The Concepts of Atoms and Molecules. Regarded as a whole, we
may say that the initial application of physical knowledge for the
purpose of developing our ideas of matter consisted chiefly in the
employment of physical methods and instruments in the study of the
properties of matter. This stood foremost in physical chemistry in
the first period of its existence.

Reviewing the history of chemistry, we must acknowledge that one
of the first fundamental steps was made by the study of the physical
property of weight, and the introduction of a physical instrument,
the balance, for this purpose. It was, in large part, on this basis that
Lavoisier was the great innovator of chemistry; and it was due solely
to the following of chemical change with the balance that chemistry
got its fundamental laws of constant weight and of constant and
multiple proportions. These were summarized by Dalton in the fruit-
ful though hypothetical conception of atoms, which, as is well
known to you all, asserts that every element exists in the form of
small unchangeable particles, identical for a given element, but
differing with the latter.

As the study of weight led to the idea of atoms, so the study of
another physical property, that of volume and density, led to our
idea of molecules. These molecules, which might be described as con-
stellations of atoms, were a necessity with Dal ton's conception; but,
in a binary compound, for instance, they might consist of two atoms
or of twenty. Now, it hardly needs to be recalled that Gay-Lussac,
and especially Avogadro, in following the volume relations of gases
in chemical action, drew the conclusion that the molecules of gases
occupy equal volumes under identical conditions. Thenceforward
we had a reliable method for determining the relative weights of such
molecules.

As the study of the physical properties weight and volume led to
the concepts of atoms and molecules, so sharply defined that the

306 PHYSICAL CHEMISTRY

relative weights of these entities form the fundamental constants of
chemistry, so a further study of physical properties has led to broad
generalizations concerning the nature of atoms and molecules, which
we shall now outline.

Properties of Atoms. As to atoms, I would call your attention to
four peculiarities which seem to me of fundamental importance. First,
Dulong and Petit found that the physical property called heat ca-
pacity is nearly the same for different atoms, i. e., that the quantity
of heat requisite to produce a given rise of temperature does not vary
greatly for atomic quantities, for 7 parts of lithium and for 240 parts
of uranium.

Second, Faraday, in studying the electrical conductivity of electro-
lytes, e. g., of aqueous solutions of salts, found that the quantity of
electricity which atoms can transport varies as the whole numbers,
— from one in potassium to two in zinc. This fundamental property,
which gives the sharpest expression to our notion of valency, was
brought by Helmholtz into a very clear form by the assumption that
electricity as well as matter consists of atoms, either negative or posi-
tive, and that material atoms are able.to combine with them, — potas-
sium with one of the positive kind, zinc with two, chlorine with a
negative one, — and so transport them in electrolysis.

The third great step was made by the study of light, a physical
property again. Bunsen and Kirchhoff found that, heated in the gas-
eous state, every atom emits a definite set of light-waves, producing a
characteristic line-spectrum which is yet the sharpest test of the kind
of atoms one is dealing with, and which so became the most fruitful
guide in the detection of new kinds.

The last generalization that I have to mention, and which we owe
to Newlands, Mendele"eff, and Lothar Meyer, includes physical pro-
perties in general, and asserts that they vary with increasing atomic
weight in a periodic way. This shows itself most sharply in the atomic
volume, which passes through maximum values in lithium (7), sodium
(23), potassium (39) , rubidium (85), and cesium (133). A correspond-
ing periodicity is observed in other properties, as, for example, that
of combining with electrical atoms, or valency, which in the said ele-
ments passes through unity. Analogous behavior is exhibited by the
melting-points and boiling-points, which for these metals are excep-
tionally low.

If my programme did not to a certain extent exclude quite recent
investigations, confining me to a view of past history, I should like to
consider one more physical property, that of radioactivity, which also
seems to be a property of atoms. I can only insist on the fact that it
was physical properties again, the making the air conductive for elec-
tricity, and the spectrum, which revealed radium.

Properties of Molecules. Turning to molecules, I have three pre-

RELATIONS TO PHYSICS AND CHEMISTRY 307

dominant generalizations to outline. The first is Mitscherlich's dis-
covery of the fact that analogous molecular constitution corresponds
to analogous outer crystalline form, to so-called isomorphism. Let me
add that there is hardly any more satisfactory proof of the soundness
of our concept of the internal structure of matter than, e. g., the
identity of the crystalline forms of the alums, which we consiaer to
have corresponding internal structure.

A second step, to a certain extent a similar one, was made by Pas-
teur when he deduced disymmetry of molecular constitution from
disymmetry in behavior, optically as well as crystallographically. For
instance, the dextrorotatory ordinary tartaric acid and its Isevorota-
tory antipode showed this disymmetry both in optical rotation and
in the particular so-called enantiomorphous crystalline form. The
molecules were supposed to have analogous structures differing from
each other as the right hand from the left. As is well known, it was
only later that the probable molecular structure was sharply defined,
and stereochemistry was founded.

The third great step was the opening of a way to determine the
molecular weights of dissolved substances. It was chiefly the appli-
cation of Avogadro's law to osmotic pressures, in connection with
Raoult's measurements of freezing-points and vapor-pressures, that
opened the way. . We may now assert that the liquid state is not char-
acterized by high molecular complexity. But the great innovation,
introduced by Arrhenius and immediately brought into relation with
the achievement in question, was the admission of the existence of
ions in electrolytes — for example, the presence of negatively charged
chlorine atoms and positively charged sodium atoms in an ordinary
salt solution. Once more it was a physical property, the electrical
conductivity, that led to this extremely fruitful supposition.

Conclusion. If, after this short summary of its properties, we try
to look into the nature of matter, we conclude that matter is not con-
tinuous, but that there are centres of action which seem to have an
eternal existence, changing only in the place that they occupy -
these are the atoms. They keep together in some way and form the
molecule; how, it is pretty hard to say. The planetary constellation,
with ordinary attraction and centrifugal force in equilibrium, is ex-
cluded by the consideration that at the absolute zero there is no
movement at all. The repulsive force that we want might be of elec-
trical nature: and so we come to our combination of material and
electrical atoms. There is indeed something fascinating here, and
when we admit for carbon that it may unite to four equally charged
electrical atoms and hold them by a force of the nature of elasticity,
we have at once a possible equilibrium and the tetrahedral grouping.
My only difficulty is that an uncharged atom of carbon, coming into
contact with the ions just described, would take away half the electric

308 PHYSICAL CHEMISTRY

charge, and so the valency of any element might be reduced to unity.
The latest supposition, that matter is built up of electricity alone, lies
again beyond the scope of this address.

Let me now turn to the second part of my subject, and touch upon
the problem of affinity; indeed, the action that keeps atoms together
must be closely related to affinity.

II. Physical Chemistry and our Notions concerning Affinity

While physical chemistry, in the first period of its development,
was chiefly devoted to the study of the physical properties of matter,
the second and present period is characterized by the predominant
place of the problem of affinity.

This change in the general aspect of our science goes hand in hand
with a different way of working : in the development of our ideas of
matter, physical chemistry introduced physical methods and instru-
ments for the study of physical properties; in the development of our
ideas of affinity, physical chemistry has introduced physical prin-
ciples.

Affinity considered as Force. The first line of thought considered
affinity as a force, and in this direction it was natural to think of the
Newtonian attraction as the chemical agent. So it was that Berthollet ,
and with far more success Guldberg and Waage, applied the laws of
mass-action to problems of affinity, formulating a relation still known
as the mass-law, according to which affinity is proportional to the
weight in the unit of volume.

Now, as we all know, affinity is of a specific nature, and does not
depend on weight merely; on the contrary, the least heavy elements
are generally the most active. So Berzelius built up his system
founded on the notion that elements have a specific electrical charac-
ter, either positive or negative, and, in combining, act by electrical
attraction. In this direction Helmholtz made a further step in taking
into account the quantitative side. Considering the electrical charges
involved in Faraday's law, he pointed out as very important that the
attraction due, for instance, to the negative charge in chlorine and
the positive one in hydrogen far exceeds the gravitational attraction
of the masses. Yet a satisfying notion of affinity was not obtained in
this way.

Affinity measured as Work. A second line of thought took into
consideration not the force but the work that affinity represents; and
it seemed a decisive step when Thomson and Berthelot declared that
the heat developed in chemical change corresponds to the work that
affinity can produce. Indeed, it was in this way that in many cases an
a priori calculation of the heat development of a reaction permitted
prediction of the direction in which the process would proceed, the

RELATIONS TO PHYSICS AND CHEMISTRY 309

direction being that of the evolution of heat. Yet this principle,
however weighty, is not absolutely reliable. The chemical actions
that produce cold, as that of hydrochloric acid on sodium sulphate,
are objections not to be overcome.

The step really leading to a clear and unobjectionable notion of
affinity was made in the study of the so-called reversible chemical
changes. This reversible character perhaps needs some explanation,
easily to be provided by an illustration. Kill a chicken and prepare
chicken soup; it would then be very difficult to get your chicken
again. This is because preparing chicken soup is not reversible. On
the contrary, let water evaporate or freeze; it will be easy to repro-
duce the water.

Now, at first sight, chemical change does not seem reversible; and
indeed it often is not, as in the explosion of gunpowder. But investi-
gations of Berthelot and P£an de St. Gilles on the mutual action of
acids and alcohols, and those of Deville and Debray on high tempera-
ture action, which even splits up water, have shown that many chem-
ical changes can be reversed. Indeed, we have types corresponding
absolutely to evaporation, as the loss of water-vapor from hydrates;
and others corresponding as well to freezing and melting, as the split-
ting of double salts into their components at definite temperatures.
e. g., copper calcium acetate at 77° C. Also in analogy with physical
phenomena, we have in these reversible chemical changes the possi-
bility of equilibrium, the two chemically different forms of matter
coexisting, as do water and its vapor at a maximum pressure.

Such a reversal of chemical change can take place under the influ-
ence of temperature, of electricity, of light, of pressure. And the eas-
iest way to arrive at a measure of affinity is presented in the last case,
as was foreseen by Mitscherlich. Let us take gypsum as an example.
Burnt commercial gypsum, mixed with water, will combine with the
water. We know that this chemical change can produce pressure, and
that it may be prevented by sufficient pressure and be reversed by it,
as Spring succeeded in pressing out sulphuric acid from sodium bi-
sulphate. And it is possible in such cases exactly to determine the
limiting pressure, such that a higher one presses out the sulphuric
acid while a lower one is overpowered by the affinity action. If the
chemical change takes place under a pressure only slightly less than
that which would prevent it, thus practically taking place under the
limiting pressure, we get out of affinity the greatest quantity of work
that it can possibly produce; and this quantity is the same whatever
the nature of the opposing action, be it electricity, light, or anything
else. Therefore, in this maximum work we have a sound measure of
affinity.

It was a very happy coincidence indeed, that this conception of affin-
ity made possible the application of a physical principle known as the

310 PHYSICAL CHEMISTRY

second law of thermodynamics. This principle may be formulated
in different ways. For my purpose let me say that it limits the pos-
sibility of natural processes to the occurrence of those in which a dif-
ference of intensity is diminished. If there is a difference of pressure
in two parts of a gas, a movement will occur producing equality; if
there is a difference of temperature, heat will be transported so as to
produce equality once more. It is curious that such simple necessities,
which we all feel as such, can be converted into far-reaching sharply
formulated equations, as was done by Carnot and Clausius. These
principles were first applied in chemistry by Horstmann. Then, by
successive application to chemical problems by Massieu, Gibbs, Helm-
holtz, and others, was won a system of relations touching the problem
of affinity, to which I can give only brief attention :

(1) Affinity may be defined as the maximum quantity of work that
a chemical change can produce. Equilibrium ensues when this quan-
tity is zero.

(2) The mass-lawT can be obtained in a well-founded and somewhat
modified form, restricted to dilute gases and solutions.

(3) The Thomson-Berthelot principle assumes a modified form in
the rule that a fall of temperature induces the formation of that which
develops heat. It is, for instance, in accordance with this rule that
at ordinary temperatures water is stable in comparison with detonat-
ing gas, and that at high temperatures this relation is reversed, as it
was found by Deville to be.

(4) Lastly, we have the phase rule, indicating, for example, in what
cases chemical phenomena will be comparable with melting and freez-
ing, and in what cases they will be comparable with evaporation and
condensation.

Most curious of all, we can treat problems of affinity in an abso-
lutely trustworthy way, so that our calculations furnish a check upon
experiment, without admitting anything concerning the nature of
affinity or of the matter wherein the affinity is supposed to reside.

THE PHYSICAL PROPERTIES OF AQUEOUS SALT SOLU-
TIONS IN RELATION TO THE IONIC THEORY.

BY ARTHUR A. NOTES

[Arthur A. Noyes, Professor of Theoretical Chemistry and Director of the Research
Laboratory of Physical Chemistry of Massachusetts Institute of Technology.
b. September 13, 1866, Newburyport, Mass. S.B. Massachusetts Institute of Tech-
nology, 1886; S.M. Massachusetts Institute of Technology, 1887; Ph.D. Leipzig,
1890; Assistant in Analytical Chemistry, Massachusetts Institute of Technology,
1887-88; student, University of Leipzig, 1888-90. Instructor in Analytical or
Organic Chemistry, Massachusetts Institute of Technology, 1890-94; Assistant
or Associate Professor of Organic and Theoretical Chemistry, 1894-99; Professor
of Theoretical Chemistry to date. President, American Chemical Society, 1904;
Member of the National Academy of Sciences; also of German Bunsen Society
for Applied Physical Chemistry, German Chemical Society, American Academy
of Arts and Sciences, etc. Written many books and articles on Chemistry.]

IT is generally recognized that the further progress of physical
science will be greatly facilitated by a better systematization of the
knowledge already accumulated, and this is true in an especially high
degree of the newly developed branch of science in which this Section
is directly interested. It has therefore seemed to me that the most
valuable contribution that I could make toward the solution of the
present problems of physical chemistry in correspondence with the
aims of this Congress would be a formulation of the present status of
some of our knowledge relating to important classes of phenomena
which are being actively investigated, but which have not yet received
a final interpretation. It was my original hope to discuss several such
classes of phenomena; but the effort involved in the collation and
criticism of the available data connected with the problem which was
first studied forced me to confine my attention to that alone. This
problem concerns the physical properties of aqueous salt solutions in
relation to the ionic theory. This is the subject which I shall attempt
to present to you : I hope that its importance and the greater definite-
ness that can be given to its treatment may compensate for the some-
what limited scope of this paper.

Permit me to say in advance that I have studied this subject
primarily from an empirical standpoint, and that it will be my aim to
present to you a series of generalized statements of the experimental
results, formulated in such a way as to show their relation to the im-
portant hypotheses connected with the ionic theory. Unfortunately,
it will not be possible in this address to reproduce, or even fully refer
to, the data upon which these conclusions are based — a defect serious
in a work of this kind, which will be remedied in a subsequent pub-
lication. I shall, however, try to show the general character of the
evidence for each conclusion and the degree of accuracy within which
it has been confirmed. I wish to add that I have been most ably

312 PHYSICAL CHEMISTRY

assisted in the preparation of the material upon which this paper is
based by Dr. J. W. Brown and Dr. M. S. Sherrill, of the Massachusetts
Institute of Technology.

The principles to be first presented have reference to two of the
main hypotheses which are commonly employed in quantitative
applications of the ionic theory. One of these hypotheses is that the
migration-velocities of the ions of a salt do not vary appreciably with
its concentration, at least up to a moderate concentration; and conse-
quently, that the degree of ionization is equal to the ratio of the equi-
valent conductivity at the concentration in question to the limiting value
of the equivalent conductivity at zero concentration — a ratio which
I will hereafter call simply the conductivity-ratio. The other hypothe-
sis is that ions, and also the un-ionized molecules accompanying them,
produce an osmotic pressure substantially equal to the pressure exerted
by the same number of gaseous molecules at the same temperature, at
least up to a moderate concentration; an hypothesis which may be more
briefly expressed by the statement that the osmotic pressure-constant
for dissolved electrolytes is identical with the gas-constant. It is evi-
dent that with the help of this hypothesis we can calculate, either from
measurements of osmotic pressure or from those of any other property
which is thermodynamically related to osmotic pressure, the number
of mols in the solution resulting from one formula weight of salt, that
is, the quantity which van't Hoff has represented by the letter i.
From the latter, provided the ionization is not complicated by the
formation of complex molecules or ions, the degree of ionization is
readily derived.

The first of these hypotheses cannot be independently tested,
because no direct method of determining the change of migration-
velocity with the concentration is known. But the following princi-
ple, which has an important significance with reference to the relative
influence of concentration on the velocities of different ions, has been
established by measurements of the concentration-changes at the
electrodes attending the electrolysis of salt solutions.

The transference number, or ratio of the conductivity of one ion to
the sum of the conductivities of both ions, is constant within one per
cent, between the concentrations of ~-^ and -^ normal, for all salts
thus far accurately investigated, except lithium chloride, the halides of
bivalent metals, and cadmium sulphate.

This principle holds true, according to the results of various inves-
tigators, in the case of potassium and sodium chlorides, hydrochloric
and nitric acids, silver nitrate, barium nitrate, potassium sulphate,
and copper sulphate — thus in the case of salts of the three different
ionic types, which I will speak of as the uni-univalent, the uni-bi-
valent, and the bi-bivalent types, in correspondence with the valences
of the two ions composing the salt.

PROPERTIES OF AQUEOUS SALT SOLUTIONS 313

Two conclusions are to be drawn from this result. The first is, that
complex ions are not present in important quantity in the solutions
of these salts. And the second is, that the migration-velocities of the
two ions of a salt vary by the same percentage amount, if they vary
at all, with changes in its concentration. It is scarcely admissible,
however, to regard this last fact even as an indication that the hypo-
thesis of constant migration-velocities is correct; for any change in
the character of the liquid medium might well affect the velocities of
different ions not far from equally.

Important evidence in regard to this hypothesis and that stating
that ions and the un-ionized molecules associated with them have
a normal osmotic pressure is, however, furnished by the agreement of
the ionization values derived, on the one hand, from the conductivity-
ratio, and, on the other, from the properties thermodynamically re-
lated to osmotic pressure. Three of these properties have been meas-
ured with sufficient accuracy with certain electrolytes to make the
results of significance, namely, the freezing-point lowering, the electro-
motive force of concentration-cells, and the heat of solution in relation
to change of solubility with the temperature. Under the assumption
that osmotic pressure and gaseous pressure are equal under identical
conditions, a relation between each of these properties and the de-
gree of ionization of an electrolyte can be derived with the help of
the second law of energetics. Then, either this ionization value may
be directly compared with the conductivity-ratio, or, assuming pro-
visionally that the latter is a correct measure of ionization, the mag-
nitude of the property in question may be calculated, and the result
compared with that obtained by direct measurement. In the case of
the freezing-point lowering, I have adopted the first of these methods.
For the five salts for which both reliable freezing-point determina-
tions and accurate conductivity-measurements at 0° exist, the ion-
ization values corresponding to both of these properties have been
computed. Especial attention was given to the selection of the best
value of the freezing-point lowering constant and to the extrapolation
of the conductivity for zero concentration, the details of which can-
not be here described. The results may be summarized as follows:

In case of the two uni-univalent salts and the three uni-bivalent
salts hitherto carefully investigated, the ionization values derived from
freezing-point lowering do not differ from those derived from conduc-
tivity, between the concentrations of -%^-Q and ^ normal, by more than 2
or 3 per cent.

The five salts referred to are potassium and sodium chlorides,
potassium and sodium sulphates, and barium chloride. The two sets
of values for potassium chloride, for which an abundant experimental
material exists, exhibit no pronounced or systematic differences; but
for the other four salts the freezing-point leads to values which are in

314 PHYSICAL CHEMISTRY

general from two to three per cent higher at all concentrations than
the conductivity-ratio. The fact that these differences do not, as a
rule, increase with increasing concentration indicates that they may
be due to some constant experimental error, or to an error in the
extrapolated conductivity value.

Accurate measurements have been made by Jahn of the electro-
motive force of concentration-cells consisting of two silver or mercury
electrodes covered with silver chloride or mercurous chloride, one of
which is immersed in a weak solution and the other in a strong
solution of sodium or potassium chloride. These measured values
were compared by him with those calculated from the thermodynamic
relation between electromotive force and the concentrations and
degrees of ionization of the salt in the cell. Unfortunately, however,
the thermodynamic relation employed involved the assumption that
the ionization varies with the concentration in accordance with the
Mass- Action Law — an assumption which is known not to be true of
the ionization values derived from conductivity. The assumption is,
therefore, an irrational one — one by which the question at issue is
prejudged. What should be done in calculating the electromotive
force so as to determine whether the conductivity-ratio gives ioniza-
tion values consistent with the measured electromotive forces is evi-
dently to assume that the ionization changes with the concentration
in the way that the conductivity indicates that it does. Arrhenius
recognized this error and partially corrected for it by a method of
approximation. I have repeated the calculations by an exact thermo-
dynamic formula based on an empirical law expressing the change of
the conductivity-ratio with the concentration, to which I will refer
later. The results are summed up in the statement that, when the con-
ductivity-ratio is assumed to represent the degree of ionization of the salt,
the calculated values of the electromotive force of concentration-cells exceed
the measured ones by only about one per cent in the case of potassium
and sodium chloride between the concentrations of -g-^ and -£-$ normal.
The measured electromotive force corresponds to an ionization value
at the latter concentration about one per cent less than the conduc-
tivity-ratio.

The thermodynamic relation involving heat of solution has been
accurately tested with only one salt' — potassium perchlorate; but
since it is a different salt from those used in the other experiments,
and since its concentration was fairly high — J normal — the result
is of interest. It was found that the measured heat of solution was less
by only 1.1 per cent than that calculated under the assumption that the
conductivity-ratio is equal to the degree of ionization. The measured
heat of solution corresponds to an ionization value 2£ per cent lower
than the conductivity-ratio.

With respect to these small deviations of the results obtained by

PROPERTIES OF AQUEOUS SALT SOLUTIONS 315

the three methods of comparison, it is important to note that they lie
in opposite directions, the freezing-point lowering corresponding to
larger values of the ionization, and the measured electromotive forces
and heat of solution to smaller ones than the conductivity-ratio. This
fact makes it almost certain that they are due to experimental errors.
Nevertheless, further exact measurements of all these properties are
highly desirable.

From a theoretical standpoint these three methods are based on
the same hypotheses — namely, that the osmotic pressure-constant
for ions and un-ionized molecules is identical with the gas-constant;
that the conductivity-ratio is a correct measure of ionization, and
that complex molecules or ions are not present in the solution. The
concordance of the results furnishes, therefore, a strong confirmation
of the correctness of these fundamental hypotheses. The only alter-
native conclusion is that an error in one of these hypotheses is com-
pensated by an error of opposite effect in one of the others; but it
seems very improbable that such a compensation could occur in the
case of so many salts of different chemical nature and different types
through the range of concentration (^-g- to £ normal) for which the
agreement of the experimental results has been shown to hold true.
It is certainly more consistent with the modern methods of science
to adopt these simpler hypotheses, which are in full accord with the
considerable number of facts thus far known, than deliberately to
introduce more complicated assumptions for which there is at present
no experimental warrant.

The combination of these hypotheses with the experimental values
of the quantities involved at varying concentrations makes necessary
the further conclusion that the degree of ionization of salts, whether
derived from the conductivity-ratio or from thermodynamic relations
involving the equality of the osmotic pressure-constant and the gas-
constant, does not vary with the concentration even approximately in
accordance with the Law of Chemical Mass-Action.

This empirical consequence of the fundamental hypotheses of the
ionic theory has led several investigators to raise a theoretical objec-
tion to them, it being contended that the laws of thermodynamics
require that the validity of these hypotheses involves that of the Mass-
Action Law itself. This apparent inconsistency between the inductive
and deductive conclusions makes it probable that some unproved,
erroneous assumption is tacitly involved in the theoretical derivation.
That there is, in fact, a possible alternative, which has, I believe, been
previously overlooked in the thermodynamic discussions, will be evi-
dent from the following considerations. The thermodynamic relations
between ionization and freezing-point, electromotive force, or heat of
solution, involve only the assumption that the work done in reversibly
separating water from a solution at constant concentration is equal to

316 PHYSICAL CHEMISTRY

that done in producing the same volume-change in a gas, which implies,
of course, that the ions and un-ionized molecules have in the presence
of each other normal osmotic pressures. On the other hand, the deri-
vation of the Mass- Action Law equation is based on cyclical processes
which necessarily involve the separate introduction and removal of
the un-ionized molecules and of the ions into or from solutions of dif-
ferent concentrations, and it further involves the assumption that this
introduction or removal of molecules or ions can be effected by the
application of an external pressure equal to that osmotic pressure
which each of them possesses in the mixture; that is, the possibility
is ignored that the separation of the molecules from the ions may
itself give rise to some new force, and may involve, consequently,
another quantity of work than that corresponding to the osmotic
pressure. The ionic theory would evidently predict a result of this
kind if an attempt were made to separate the positive ions from the
negative, even though their osmotic pressures when present together
were perfectly normal; and it is quite conceivable, even though the
reason for it be not apparent, that the separation of the un-ionized
molecules from the ions, with which they may be in electrical as well
as chemical equilibrium, should involve an abnormal quantity of work.
The assertion that the validity of the osmotic-pressure principle nec-
essarily implies that of the Mass-Action Law is therefore unwarranted
from a deductive standpoint; while the inductive evidence, pointing
strongly as it does to the substantial correctness of the former principle
and the complete inadequacy of the latter one, makes it highly pro-
bable that the separation of un-ionized molecules from ions does in-
volve the expenditure of other work than that corresponding to their
osmotic pressures.

Since the ionization does not change with the concentration in
accordance with the Mass- Action Law, it is natural to inquire what
the law of its change is. This matter has been investigated from
an empirical standpoint by several investigators with the help of the
conductivity data. The results justify the statement of the following
principles :

The un-ionized fraction of a salt as determined from the conductivity-
ratio is proportional to the cube root of its total concentration, or to
that of its ion-concentration, between -^~ ^ and -—$ normal, in the case of
both uni-univalent and uni-multivalent salts. That is, l — f = Kc^ or
1 — 7' = Jf£(cj-)J, where fis the degree of ionization, c the concentration
and K a constant. The first of these functions was proposed by Kohl-
rausch, the second by Barmwater. Owing to the relatively small
variation of the ionization, these two functions cannot differ much as
to their constancy, but on the whole the experimental data indicate
that the second function is somewhat more constant. The average
deviations of the actual measurements from the value s corresponding

PROPERTIES OF AQUEOUS SALT SOLUTIONS 317

to this function are £ per cent in the case of ten uni-univalent salts,
$ per cent in the case of nine uni-bivalent salts, and also J per cent
in the case of three uni-tri- and uni-quadrivalent salts. The maximum
deviations are two or three times as great. It is of interest to note
that the strong mineral acids, hydrochloric and nitric, behave like
salts in this respect. These functions have been shown to apply to
potassium and sodium chlorides through a range of temperature
extending from 18° to 306°. They do not apply at all closely to
such salts of the bi-bivalent type as magnesium and copper sulphates,
perhaps owing to appreciable hydrolysis. Nor do they represent
satisfactorily the experimental data for any kind of salts at the very
low concentrations lying between J-Q^-Q-Q and ^^g-"5" normal, nor at
concentrations higher than ^ normal.

The experimental results are also well expressed by the statement
that in the case both of uni-univalent and uni-bivalent salts, between the
concentrations of y^ J-Q^ and 4- normal, the concentration of the un-
ionized molecules is proportional to the concentration of the ions raised to
a constant power, varying somewhat with the salt and the temperature,
but as a rule only between the limits of 1.43 and 1.56. That is, c (1 — -jr) —
K(cr)n, where n >1.43 and < 1.56.

This general function was first applied by Storch and was afterward
further discussed by Euler and Bancroft. It has the advantage over
the previous ones that it represents the data with accuracy even up
to the highest dilutions, and therefore can be used for obtaining the
limiting conductivity at zero concentration.

The applicability to the salts of different types of either of these
principles governing the change of ionization with the concentration
leads to the important conclusion that the form of the concentration
function is independent of the number of ions into which the mole-
cules of the salt dissociate. This remarkable fact, though previously
recognized, has not been sufficiently emphasized, and it has been
often ignored in discussions of the cause of the deviation of the ioniza-
tion of salts from the requirements of the Mass- Action Law. It seems
to me to show almost conclusively that chemical mass-action has no
appreciable influence in determining the equilibrium between ions and
un-ionized molecules. How complete the contradiction with the Mass-
Action Law is may be illustrated by citing the specific facts that for
di-ionic, tri-ionic, and tetra-ionic salts this law requires that the con-
centration of the un-ionized molecules be proportional to the square,
the cube, and the fourth power, respectively, of the concentration of
the ions; while the experimental data show that it is approximately
proportional to the -| power of that concentration, whatever may be
the type of salt.

Having seen in what manner the degree of ionization varies when
the concentrations of both ions of the salt are simultaneouslv varied

318 PHYSICAL CHEMISTRY

by dilution, it is of interest to determine the effect of changing the
concentration of either ion separately. A study of the conductivity
and the freezing-point of mixtures of two salts having one ion in
common throws much light upon this question, for the following
simple principle has been found to represent this phenomenon : The
conductivity and the freezing-point lowering of a mixture of salts having
one ion in common are those calculated under the assumption that the
degree of ionization of each salt is that which it would have if present
alone at such an equivalent concentration that the concentration of either
of its ions were equal to the sum of the equivalent concentrations of all
the positive or negative ions present in the mixture.

This somewhat complicated statement may be illustrated by the
following example : Suppose that a mixed solution is 0. 1 normal with
respect to sodium chloride and 0.2 normal with respect to sodium
sulphate, and that it is 0.18 normal with reference to the positive or
negative ions of these salts. The principle then requires that the
ionization of either of these salts in the mixture be the same as it is
in water alone when its ion-concentration is 0.18 normal.

This principle in regard to the conductivity of mixtures, which has
been definitely stated by Arrhenius, is shown by the existing data to
hold true, almost, if not quite, within the small experimental error of
the determinations both for mixtures of salts of the same type and
for those of salts of different types up to a concentration of at least
£ normal. Experiments confirming this principle have been made upon
eight pairs of uni-univalent salts by Arrhenius, Manson, and Barm-
water. In addition, the principle has been shown by several Canadian
investigators, Archibald, McKay, and Barnes, to hold true for mix-
tures of potassium and sodium sulphates, potassium and copper or
magnesium sulphates (up to 0.1 normal), potassium sulphate and
chloride, barium and sodium chlorides, and zinc and copper sulphates
— thus for almost every possible typical combination of uni-uni-,
imi-bi-, and bi-bivalent salts. That the same principle is true of the
freezing-point lowering is shown by the measurements of Archibald
with mixtures of potassium and sodium sulphate. This proves that
the phenomenon really has reference to the degree of ionization and
that it does not arise from a possible variation in the migration- veloc-
ities of the ions.

Of especial interest is the relation of this principle to the validity
of the Mass-Action Law. Almost all investigators of the conductiv-
ity of mixtures have concluded, from the fact that upon mixing solu-
tions of equal ion-concentration there is no change in ionization, that
the results do conform to this law. Yet it is scarcely conceivable that
this law can apply to mixtures of salts in which the concentration of
one ion is varied while maintaining that of the other constant, in view
of the fact that it is known not to hold true for the variations of the

PROPERTIES OF AQUEOUS SALT SOLUTIONS 319

concentrations of both ions produced by dilution. And in reality this
conclusion, if regarded as a general expression of the facts, is entirely
unwarranted. It is true that for certain typical combinations of salts
— those for which from one molecule of each salt results by ionization
not more than one ion of the kind not common to the salts — the
principle here stated does coincide with the requirement of the Mass-
Action Law. But for combinations not so characterized the Mass-Ac-
tion Law predicts, as is readily seen upon formulating the equations,
a conductivity of the mixture widely divergent from that actually
found, and, therefore, from that expressed by the principle under
consideration. This last statement applies, for example, to the mix-
tures before referred to of potassium sulphate with sodium sulphate,
and of potassium sulphate with copper or magnesium sulphate, the
first of which have been studied both with respect to their conductiv-
ity and freezing-point. The Law of Chemical Mass- Action here again
shows itself entirely inapplicable to the phenomena connected with
the ionization of salts. The opinion of some investigators that the
deviations from this law indicated by the conductivity were only
apparent, and that they were attributable to variations in the migra-
tion-velocity, has arisen, no doubt, from the fact that they have con-
fined their attention to di-ionic salts, and have failed to recognize, on
the one hand, the striking divergences from it exhibited by tri-ionic
salts, and, on the other, the substantial correspondence of the con-
ductivity and freezing-point results.

Combining this principle in regard to the ionization of mixed salts
in solution with the empirical concentration law of Storch for single
salts, we are led to the conclusion that the ratio of the concentration
of the un-ionized part to the product of the concentrations of the two
ions (but in the case of tri-ionic salts not raised to a power correspond-
ing to the requirements of the Mass-Action Law) is a function of the
sum of the equivalent concentrations of all the ions in the solution and
of that alone.1 This ratio is, moreover, roughly inversely proportional
to the square root of the total ion-concentration.

The correctness of this principle is further demonstrated by the
fact that with its aid the conductivity of a mixture of two salts with-
out a common ion can be computed from their separate conductivities.
This is shown by the conductivity measurements, made by Archibald
and more recently by Sherrill, upon mixtures of potassium chloride
and sodium sulphate, or of sodium chloride and potassium sulphate.
Up to at least 0.2 normal concentration, the agreement between the
observed and calculated values is within 0.5 per cent. On the other

1 This is expressed mathematically by the following equation in which c, and ca
represent the equivalent concentrations of the two salts, and yi and y2 their de-
grees of ionization in the presence of each other:

320 PHYSICAL CHEMISTRY

hand, the divergence of the observed values from the requirement of
the Mass- Action Law amounts to many per cent.

It seems appropriate at once to supplement these principles in
regard to the form of the concentration function by a statement of two
general rules which have been found to express the magnitude of the
ionization of salts of different types. These rules, unlike the preced-
ing principles, are only crude approximations; but, nevertheless,
they prove of some assistance in rough applications of the ionic the-
ory, and undoubtedly possess an important theoretical significance
not yet recognized. They may be stated as follows: (1) the decrease
of ionization with increasing concentration is roughly constant in the
case of different salts of the same type; and (2) the un-ionized fraction
at any definite molal concentration is roughly proportional to the pro-
duct of the valences of the two ions in the case of salts of different types.
Thus, at 0.1 normal concentration the mean value of the degree of
ionization for 17 uni-univalent salts measured at 18° is 83.3 per cent,
the average deviation of the separate values from this mean is 2.1
per cent, and the maximum deviation of any of them is 5.4 per cent,
of the mean value; while for fourteen uni-bivalent salts the mean
value is 69.8 per cent, the average deviation 5 per cent of this, and
the maximum deviation about 10 per cent of it. The un-ionized
fraction in -^ molal solution is 13^ per cent for these univalent salts;
30 per cent, or about twice as great, for the uni-bivalent salts; and
60 per cent, or about four times as great, for the three bi-bivalent salts
investigated (zinc, magnesium, and copper sulphates). The salts of
mercury and cadmium are pronounced exceptions to the rule.

Far more extensive material for testing these rules is furnished by
the measurements made at 25° between the concentrations of ^ and
T oVf normal- In the case of the uni-univalent salts, data exist at this
temperature and these concentrations for thirty-six inorganic salts,
about sixty-five sodium salts of organic acids, and about an equal
number of hydrochlorates of organic bases. A consideration of all
these data shows that, with only three or four exceptions not of
a pronounced character, the values of the degree of ionization of all
these salts in ^~ normal solution lie between the limits of 84 and 90 per
cent and are fairly uniformly distributed throughout this range of
6 percent. For sixty-seven uni-bivalent salts the corresponding limits
of the ionization values are 72 and 81 per cent, while for only four
such salts do the values lie beyond these limits. For the six uni-tri-
valent salts investigated the range is from 67 to 76 per cent; for the
three uni-quadrivalent salts from 59 to 63 per cent, and for twelve
bi-bivalent salts from 49 to 63 per cent, while three such salts show
more considerable variations. The values of the un-ionized fraction
corresponding to the mean of these two limits for the different types
of salts at the same equivalent concentration increase somewhat

PROPERTIES OF AQUEOUS SALT SOLUTIONS 321

more slowly than the product of the valences of the ions. The pro-
portionality becomes a fairly close one, however, when the salts are
compared at the same molal instead of the same equivalent concen-
tration. Thus, with the help of the Kohlrausch concentration func-
tion, it is calculated from the preceding values that the un-ionized
fractions in -— molal solution are as follows:

13 per cent for the uni-univalent salts,
29£ per cent for the uni-bivalent salts,
41 per cent for the uni-trivalent salts,
62 per cent for the uni-quadrivalent salts,
55 per cent for the bi-bivalent salts, —

which are seen to be approximately the required multiples of the
constant factor 14.

Before leaving this subject it should be stated that the results con-
form, on the whole, about equally well to the rule that the decrease
of equivalent conductivity (instead of ionization) is roughly constant for
salts of the same type ; and when the comparison is made at the same
equivalent concentration, distinctly better to the rule that the de-
crease of equivalent conductivity is proportional to the product of the
valences of the ions for salts of different types. When compared at the
same molal concentration, however, this rule does not apply. These
rules were originally stated by Ostwald. They differ not inconsider-
ably from those expressing the change in ionization — namely, to an
extent corresponding to the variations of the conductivities at ex-
treme dilution. The deviations are so irregular, however, that, from
an empirical standpoint, the choice between the two pairs of rules is
arbitrary. In either form these rules seem to justify the inference that
the degree of ionization of salts, unlike that of the organic acids and
bases, is not primarily a specific chemical property determined by
chemical affinity, but that it is determined, at least in the main,
by the magnitude of the electric charges on the ions.

The establishment of the principle in regard to the ionization of a
mixture of salts has a direct bearing on the phenomenon of the effect
of one salt on the solubility of another with a common ion. It has
been usually assumed that in a (not too concentrated) saturated solu-
tion the un-ionized molecules of the salt always have the same concen-
tration; and, secondly, that the product of the ton-concentrations
(each raised to a power corresponding to the number resulting from one
molecule) also retains the same value. And the experimental results
in several cases have been shown to accord fairly well with these two
hypotheses. Yet their simultaneous validity is quite inconsistent with
the principle in regard to the ionization in mixtures. In fact, when
considered in the light of this principle, the existing data lead to the
conclusion that the former hypothesis is not even approximately true,
and that the latter one, at any rate in cases where the ionization is far

322 PHYSICAL CHEMISTRY

from complete, is affected by a considerable error. One example may
be cited : when thallous chloride and bromate, each of which alone
has a solubility of about ^ normal in water at 40°, are simultaneously
present as solid phases, the solubility of each is reduced by the other
to an extent which shows that the concentration of the un-ionized
molecules is diminished by about 15 per cent and that the product
of the ion-concentrations is increased by about 5 per cent. This case
is a typical one; but what the quantitative law of the influence in
question is, can be determined only by a further study of the phe-
nomenon. In the case of tri-ionic salts, the ion-concentration product
is even approximately constant, only when the square — not when
the first power — of the concentration of the univalent ion is em-
ployed. This has been shown by experiments with lead iodide in the
presence of potassium iodide, with lead chloride in that of other chlo-
rides, and with calcium hydroxide in that of ammonium chloride.

I will close by calling your attention to a remarkable principle in
regard to the properties of salt solutions, of a character quite distinct
from those thus far considered. That many properties of dilute salt
solutions can be expressed as the sum of values assigned once for all
to the constituent radicals or ions was long ago recognized, and has
often been cited as a corollary from the ionic theory. That this
additivity of properties persists up to fairly high concentrations is
a fact, however, that has received scant consideration, owing to its
apparent lack of relationship to that theory. This fact is shown strik-
ingly in the case of certain highly specific optical properties which are
ordinarily found to be dependent in a high degree on molecular struc-
ture. Thus, the experimental data fully warrant the statement of the
principle that the optical activity and the color of salts in solution, when
referred to equivalent quantities, are independent of the concentration and
therefore of the degree of ionization of the salts, and are additive with re-
spect to the properties of the constituent ions even up to concentrations
where a large proportion of the salt is in the un-ionized state. Abundant
data might be cited in support of this principle, especially with re-
ference to optical activity. But I can only illustrate the character
of the evidence by presenting a few of the results obtained by Wai-
den with the salts of a-brom-camphor-sulphonic acid. In -^ normal
solution he found the following values of the molal rotatory power:

Lithium salt . . .275 Acid itself . •- . . 273

Sodium salt . . . 272 Beryllium salt . . . 274

Potassium salt . . . 273 Zinc salt . . . 272

Thallium salt . . .273 Barium salt . . .272

The values are seen to be substantially identical, although the con-
ductivity shows the acid to have an un-ionized fraction of 7 per cent,
the salts of the univalent metals one of 16 per cent, and those of the

PROPERTIES OF AQUEOUS SALT SOLUTIONS 323

bivalent metals one of 30 per cent, and although the un-ionized
molecules present contain in some cases the elements hydrogen, lith-
ium, and beryllium of very small atomic weights, and in two others
the elements thallium and barium of large atomic weights.

If there were not other evidence to the contrary, the existence of
this general principle, which is also applicable to many other proper-
ties, would almost warrant the conclusion that the salts are completely
ionized up to the concentration in question, and that the decrease in
conductivity is due merely to a change in migration- velocity. But,
in view of the apparently conclusive evidence against such an hypo-
thesis, we can only conclude that the form of union represented by the
un-ionized molecules of salts differs essentially from ordinary chemical
combination, it being so much less intimate that the ions still exhibit
their characteristic properties, in so far as these are not dependent
upon their existence as separate aggregates.

These, then, are the empirical principles to which a critical analysis
of the experimental data leads. Upon these principles must be based
the rational, theoretical explanation of the phenomena in question.
The discovery of that explanation constitutes one of the most import-
ant of the present problems of physical chemistry.

SHORT PAPERS

DR. FRANK K. CAMERON, of the United States Department of Agriculture,
presented a paper on "The Application of Physical Chemistry to Agricultural
Chemistry," in which he stated that there was some difficulty in approaching this
subject, for the reason that what constitutes agricultural chemistry cannot be
clearly defined, and the boundary lines between it and other branches of applied
chemistry are not always evident. Much of biological chemistry and much of geo-
logical chemistry along lines on which notable achievements have been made by
the applications of the principles and methods which in recent years have come
to be called physical chemistry, could with propriety be claimed also for agricul-
tural chemistry. One finds agricultural chemists engaged in the examination of
drugs, fertilizers, leathers, and tannins, etc., as well as in the examination of foods
or soils. Important applications of physical chemistry are to be found along many
of these lines which might be claimed for the agricultural chemist, but disputed as
belonging to the field of the industrial chemist or others. The manufacture of
nitric acid by electrochemical methods, while a problem of industrial chemistry, is
important mainly because of the use of nitrates in agriculture. But confining one's
self strictly to the work professedly done in the immediate interest of agriculture
or farm practices, there is much evidence to be found of the increasing influence
of physical chemistry.

A valuable and interesting paper followed on the close relation and applications
of physical chemistry to the science of agriculture, and the speaker concluded by
saying that " The problems presented by agricultural chemistry do not commend
themselves to the investigator who is interested in chemistry alone for its own
sake. They are generally complex and not well suited to the elucidation or illus-
tration of hypotheses in pure chemistry. The pecuniary rewards which agricul-
tural chemistry offers are not sufficient in comparison with other fields to tempt
the man trained in physical chemistry who wishes to use his equipment to this
end. But to the man who has the training and who cares not so much that his
problems be pure science as that they may be undertaken in a scientific spirit and
with scientific methods, the application of physical chemistry to agriculture offers
many opportunities. He can have the satisfaction of not only doing good scientific
work but directly helping an industry of ultimate importance to all his race and of
immediate importance to the numerically largest class of the race."

PROFESSOR HENRY SNYDER, of the University of Minnesota, read a paper on
"The Digestibility of Bread."

PROFESSOR Louis KAHLENBERG, of the University of Wisconsin, read a paper
"On the Relation between the Processes of Solution, Chemical Action, and
Osmosis."

GOLDEN AGE OF THE MEDICI

Photogravure of a Painting by Andreas Mtiller

While the Medici family were distinguished for cruelty, some of them,
notably the queen of Henry IV, were no less famous lor their patronage of
learning. The artist has admiraoly illustrated the spirit of that age by
grouping the most famous literati of Europe in the Park Monceaux, each
being a portrait, while the assemblage is presided over by Marie de Medici,
who is awarding tributes to those who have contributed to the glory of
literature. The conception is grand, and the execution most excellent.

SECTION D — PHYSIOLOGICAL CHEMISTRY

SECTION D — PHYSIOLOGICAL CHEMISTRY

(Hall 16, September 22, 3 p. m.)

CHAIRMAN: PROFESSOR WILBUR O. ATWATER, Wesley an University.
SPEAKERS: PROFESSOR O. COHNHEIM, University of Heidelberg.

PROFESSOR RUSSELL H. CHITTENDEN, Yale University.
SECRETARY: DR. C. L. ALSBERG, Harvard University.

PROBLEMS IN NUTRITION

BY OTTO COHNHEIM

(Translated from the German by Prof. J. L. R. Morgan, Columbia University)

[Otto Cohnheim, Special Professor of Physiology, University of Heidelberg;
Assistant Physiological Institute, b. May 30, 1873, Breslau, Germany. Grad-
uate Physician, Heidelberg, 1896; M.D. ibid. 1896; Privat-Docent, ibid. 1898;
Zoological Station, Naples, 1900-02; Pawlow's Institute, St. Petersburg, 1902.
Author of Chemistry of Albuminous Substances; Physiology of Alpinism ; and
many articles on biology and physiological chemistry.]

THE object of the papers read here is not so much the consideration
of any one restricted branch of science as it is the discussion of those
broader fields which lie between and are intimately connected with
several branches of science. In accord with this I propose to speak
on a subject belonging primarily to the physiology of nutrition, but
one which at the same time has very great politico-economic import-
ance. To-day, as the result of the great progress which has been
made in the physiology of nutrition, we can in general give a definite
answer to the question as to the extent of the agreement between
the actually observed dietary of an individual or group of individuals,
and the conclusions obtained theoretically. At any rate to-day we can
account physiologically for, and regard as physiologically necessary,
a whole series of phenomena which in the past could only be accepted
as empirical facts. The physiological consideration of race-dietary,
on the other hand, will show how it happened that social considera-
tions for decades have directed and restricted physiological progress.

The food of man, as is well known, is composed of proteids, fats,
and carbohydrates. In most food-stuffs we have all three classes of
substances; only sugar and butter belong solely to one class, the
former being a carbohydrate, the latter a fat. The proteids assume
a particularly important position owing to the fact that our bodies
themselves are composed to a very large extent of proteinaceous
material and hence can only be built up by proteids. The major

328 PHYSIOLOGICAL CHEMISTRY

portion of the proteids we absorb are derived from bread and meat.
The foods richest in proteids are meat, fish, eggs, cheese, milk, etc.;
in short, those foods having an animal origin. The older physiology
considered the material composition of the food as the essential char-
acteristic, although even Liebig recognized metabolism as a process
of combustion, and it is the work of the Voit school which has caused
the calorimetrical value of food to attain its present central position.
By its combustion, the nutriment absorbed supplies the energy which
is required by the human body for its various purposes. The value
of a food, then, can be expressed by the amount of energy it can pro-
duce, and this value can be stated clearly and accurately in the ordi-
nary terms of energy, i. e., in units of heat, or calories. As the result
of years of work by various investigators it has been found that the
individual foods can be almost completely represented by their
calorimetrical values. Rubner, Zuntz, and At water, by differing
methods, have all come to the same conclusion, viz. that for purposes
of heat and muscular action, i. e., for its principal requirements,
proteids, fats, and carbohydrates, the organism can employ vegetable
and animal foods equally well. That the civilized nations of Europe
and America employ bread and meat as the principal source, while
the Indians and Chinese use rice exclusively, and the Esquimos fat,
is not due to any difference in physiological organization, or to differ-
ing needs of the body, but simply to the more or less easy attainment
of the substances, fruitfulness of the soil, and other secondary cir-
cumstances.

The law of the calorimetrical equivalency of all food-stuffs has but
one notable exception. So far as investigation has been carried out it
has been found that the dietary of any man or race always contains
a certain and apparently similar amount of proteinaceous material.
The kind of material seemingly has little influence, but about 100
gr. of protein is found with great constancy in the daily food of
the individual. In the food of a powerful man, who exerts a fairly
large amount of muscular effort, Voit found 118 gr. of proteids per
day, and he assumes this as a basis for the dietary of a soldier.
Weaker men, doing less muscular work, require, according to Voit, a
smaller quantity of proteids. For the poorly nourished, and also for
those who are incapable of any intense effort, the hand-loom weavers
of Zittau, the poor of Naples, and the poorest negroes of Alabama,
von Rechenberg, Manfredi, and Atwater have found much lower
amounts. During comparatively short laboratory experiments,
Munk, Hirschfeld, Kumagawa, and especially Sive"n have found con-
siderably smaller quantities. For well-nourished men, during long
periods, Chittenden, only, found less proteids; otherwise, physio-
logical investigation, as well as the experience of daily life, has shown
that it is not well to consume less than 100 gr. of proteids per day.

PROBLEMS IN NUTRITION 329

This amount, indeed, is rarely exceeded, for Chittenden has shown
that even the diet of well-to-do Americans, which appears to us as
the richest in proteids, scarcely ever exceeds 100 gr. of proteids a day,
and the investigation of the freely-chosen fare of the most various
individuals leads to the same result.

The question as to the need of the human body for 100 gr. of pro-
teinaceous material per day has often been raised ; and even to-day
cannot be answered with certainty. During the last years, however,
we have learned of a series of reasons which may serve to throw some
light upon the subject.

That the growing organism requires proteids is self-evident, for in
this way only can it obtain the materials of which it is composed.
We know further, however, that the adult organism continually
repairs and increases its organs and consequently also requires pro-
teids. According to Zuntz a man increases his muscles when he does
unaccustomed work (for example, when he learns a new sport) or even
by increased exertion upon his usual work. Bunge attributes a con-
siderable requirement of proteids in adults to the loss of organ-
proteids in the sperm of man, and to menstruation, pregnancy, and
lactation in woman. And later years have disclosed the genetic
relations of many decomposition-products of the proteids with carbo-
hydrates, with substances of the bile, and others, which are necessary,
at any rate for a time, to neutralize poisons, or which are essential
for the intermediate metabolism; and these relations appear to render
desirable at least the presence of a copious supply of proteinaceous
material.

A second reason is more difficult to grasp. Even the first metabolic
experiments of Voit showed that although the proteids possess no
higher nutritive (fuel) value than the carbohydrates, and a very much
smaller nutritive (fuel) value than the fats, they burn very much
more rapidly; and this has since been repeatedly confirmed. When
the supply of proteids in the food is increased above the actual need
of the body, the fats and carbohydrates are stored up, and the pro-
teids are burned to a very much greater extent. The relations between
the cells of our bodies and the substances absorbed as nutriment can
best be illustrated by an analogy. For the neutralization of an alkali
any acid may be employed; but when several acids of differing
strength are present together, the strongest one will be partially
saturated before the others even begin to react. In the same way
protoplasm can supply its need with all three nutritive substances;
but when all three are present at once the proteids burn first. With
a large excess of the other two, however, the action of their masses
becomes evident, exactly as in the illustration with the acids, and
they protect the proteids from combustion. In the absence of fats
and carbohydrates, the body readily goes into such a state that its

330 PHYSIOLOGICAL CHEMISTRY

own proteids are attacked, i. e., it consumes itself. This is probably
the most important reason why a definite minimum amount of pro-
teids is essential in a small total amount of nutriment. That further
differences exist among the individual organs themselves is still to
be proven, but at present it appears quite probable.

A third ground has been disclosed during the past few years by
the work of the great Russian investigator, Pawlow. We know from
this that the nervous connection of the digestive system with the sense
organs of the head determines the enjoyment of the food, and hence
regulates the choice of that. We know further from Pawlow, Wein-
land, and Starling that this connection is not fixed once for all, but
varies according to the needs of the time. When any such relation
is observed we must always conclude that it is adapted to an end, for
otherwise it would have disappeared within a short time. Pure pro-
teids are tasteless and odorless, and also fail to act upon the sensi-
tive nerves of the stomach and intestine; in all natural foods, on the
other hand, the proteids are always associated with the pleasant-
tasting constituents of nutriment, and those which stimulate diges-
tion. For us, just as for the carnivorous animals investigated by Paw-
low, the substances richest in proteids are always the most pleasant to
the taste, and those which arouse the appetite the most. The foods
which are poorer in proteids, as rice and potatoes, stimulate the diges-
tion less and consequently are more difficultly digestible, A food-stuff
free from proteids has already been shown in animal experiments to
be impossible as a diet, and even in experiments with substances which
are poor in proteids Siven and Rohl encountered insurmountable dif-
ficulties owing to the tastelessness of the material.

Even though we do not as yet know all the reasons, it is at any rate
obvious that, for long periods of time and for normal nutrition, Voit
has discovered the correct condition, viz., that an amount of proteids
equal to 100 gr. per day is essential, or at any rate can be designated
as desirable.

Since in consequence of the special internal organization of the
human body, and because it is the minimum amount used by all men,
this amount of proteids is independent of the form of nourishment
absorbed, and independent of the habits of life. Even as early as
1860 and 1866 Voit showed that the protein consumption of those
doing hard work is not greater than of those who do none; and this
result has been confirmed many times. The American physiologist,
Atwater, has made an especial study of this question, using his
respiration calorimeter. As the average of numerous experiments,
carried out with the greatest exactness, he found that the subject
of experiment, whether resting or working, decomposed the same
amount of proteids, even when the production of calories by the
work rose to double or more. Indeed, the decomposition of proteids

PROBLEMS IN NUTRITION 331

can even be decreased by muscular activity, for the larger total of
nutriment consumed prevents the decomposition of the proteids of
the body.

The total amount of nutriment of a man is almost exclusively
determined by the muscular work he performs. The mental work
has nothing to do with nutrition; whether the brain is used in-
tensely, or whether it is retained as inactive as is possible, as far as
we know to-day, does not seem to affect the requirement of energy
by the body, nor its requirement of food. The amount of energy
required by the individual to sustain his bodily temperature differs
but slightly, for the differences in external temperature are nearly
compensated by the wonderfully acting heat regulation of our bodies,
and the artificial heat regulation by our clothes and dwellings. The
influence of muscular activity is very much greater. A man resting
quietly in a warm room requires from 1500-1700 calories per day;
while one working in the laboratory, or sitting, produces from 2100-
2400 calories. For light hand labor this is increased to 2800, while
for laborers, Liebig and others have observed from 4000-6000 calories,
and Atwater and Wood found up to 8000 calories for the lumbermen
of Maine. As the average of all his experiments, Atwater found 2270
calories for quiescent, and 4550 calories for hard-working people, i. e.,
exactly double the value.

Although the total number of calories varies according to the work,
the amount of proteids for all men remains approximately equal,
and from this we can draw an important conclusion. The food of
those not doing physical work must be relatively richer in proteids,
for an equal absolute amount of proteids must be contained in a
smaller total amount of food. The foods richest in proteids are meat
and the other products of the animal kingdom, and it is evident that
the diet must be the richer in meat, the less physical work done
by the person. An illustrative example will make this quite clear.
A laborer does hard physical work, and consequently requires a diet
which produces 5000 calories per day. Consuming only bread, pota-
toes, and other vegetable products, he would obtain 100 gr. of pro-
teids and even more without trouble. Let us assume that he moves to
a city and changes his occupation, living a sedentary life. For this he
would require but 2500 calories, and retaining the quality of his diet-
ary he would have to do one of two things. Either he must eat the
previous quantity, which would be impossible for any length of time,
for the body could not use such an excessive amount, or he must
decrease it to one half, whereby he would obtain the requisite number
of calories, but with them only 50 gr. of proteids. To nourish himself
properly, then, he will have to decrease his allowance of food to one
half, and add to it 50 gr. of proteids, i. e., about 250 gr. of meat. This
example, of course, is extreme, and will not often be observed with

332 PHYSIOLOGICAL CHEMISTRY

such distinctness. The principle, however, is always to be observed.
The food of those belonging to the well-to-do classes, i. e.,oi those who
do no hard physical work, in all countries contains the most meat.
This, however, is no luxury, but is based upon physiological grounds.
Comparing different countries, or different classes in the same country,
we always obtain the following result. To the degree that pure hand
labor is replaced by the work of the head, and that of overseeing
machines, to that same degree is the consumption of meat increased.
This is shown most obviously, however, by the comparison of the
country population with that of the city. The modern mill-hand lives,
it is true, by the work of his hands, but that work is quite different
from that done by the farm laborer. The overseeing and directing of
the complicated machines, as every other form of skilled labor, re-
quires attention, intelligence and dexterity, but does not require the
muscular exertion necessary for mowing, threshing, and the felling of
trees. With the difference in activity there must also be a difference
in the quantity and kind of food. The people in a city in general eat
less in total amount, but this food is qualitatively different, i. e.,
must consist of substances relatively rich in proteids, as meat and
other animal products.

From the politico-economical, as well as from the medical, point of
view the smaller amount of food consumed by the mill-hand, as com-
pared to that of the farm laborer, is regarded as a sign of degeneration.
This is obviously untrue, for there is no general standard of nutrition
which is applicable, or even desirable, for all men. The nutriment,
with respect to quantity, is dependent solely upon the amount of
muscular work done. On the other hand, the increased consumption
of meat, eggs, and other foods, agreeable to the taste because rich in
proteids, has been attributed to the greediness of the urban popula-
tion. Nothing could be more false. It is just for this large class that
the enjoyment of meat and other foods rich in proteids is a physio-
logical postulate; and for the other large class making up the urban
population, merchants, officials, clerks, etc., this is true in even a
more striking degree, for the physical work necessary in such occu-
pations is still smaller in amount, and their food must consequently
be even richer in proteids.

It is not for me to draw further conclusions from the physio-
logical principle that the food of the urban population should contain
less vegetable and more animal substance. I must rather consider
the influence of these relations upon physiology itself. The classes not
doing severe physical work are the higher and better-to-do, and, since
they are great meat-eaters, it is but too easy to conclude that in gen-
eral meat-eaters are the most valuable and best. This opinion is very
rife in lay circles, and even physiology has not long been free from it.
The great Liebig, the founder of the doctrine of scientific nutrition,

PROBLEMS IN NUTRITION 333

held that meat is the only active form of food (for muscular activity^,
and ascribed to it a very high nutritive value. Liebig's theory was
disproved 44 years ago by Voit and soon afterward by Fick and Wis-
licenius. But even to-day there are physiologists who hold fast to the
Liebig doctrine, and indeed the relics of it are still to be found every-
where in physiology and medicine. The tenacity of life of this old
error would be difficult to explain, were it not apparently supported
by the daily experience that the well-to-do eat meat, eggs, etc., while
the day laborers satisfy themselves with bread and potatoes.

From the difference in the diet of those who do severe muscular
work and those who do none, there is a further conclusion to be
drawn. The only indigestible constituent of human food is cellu-
lose. Cellulose, being contained only in vegetable food, forms but
a small constituent of the diet of the man doing little physical work.
According to von Knieriem cellulose is of great importance in the pro-
cess of digestion, for as indigestible, solid substance it stimulates the
activity of the intestine. While carnivorous animals, with their short
muscular intestine, do not require it, graminivorous animals, with
their long, coiled, weak intestine, cannot do without it for any length
of time. Man, in the organization of his digestive apparatus, stands
midway between these two extremes, and while cellulose is not abso-
lutely essential to him, its absence sometimes causes a motoric atrophy
of the intestine which results in chronic constipation and its conse-
quences. The connection between constipation and sedentary occu-
pations has long been recognized, but people have been too prone to
attempt to explain it mechanically, whereas the connecting link in
reality is the dietary of the man leading the sedentary life. In his
daily life such a man does but little physical work, and consequently
in general eats little, and especially little of the vegetable food poor
in proteids but rich in cellulose. We hear nothing of digestive troubles
of the people living in the country, while city people, especially the
well-to-do, suffer severely from them. In England and America,
judging from the wide advertisement of purgatives, the trouble is
much more common than in Germany; but in both lands the rye
bread, which is comparatively rich in cellulose, is replaced by fine
wheat bread, which is much poorer in cellulose, and the substitution
of animal products for bread is also more common than with us.

The vegetarians have been agreed on this point for a long time.
They observed how many digestive and other troubles are common
to the dwellers in cities (i. e., where fewr live like the vegetarian pea-
sant), and all without knowledge of the physiological grounds held up
the peasant as the ideal for the citizen. But what is correct for the
peasant, who must produce 4000-5000 calories, is not correct for those
who require but 2300 calories or less. He obtains, then, as explained
above, too little of proteids, or aids himself by his fondness for the

334 PHYSIOLOGICAL CHEMISTRY

vegetables rich in proteids, but at the same time poor in cellulose,
and hence fails utterly to attain his end.

It would be more correct if the transition to the vegetable diet is
combined with a treatment which will increase the need of substance,
as has been done from non-scientific sides and without knowledge of
the physiological principle.

The only rational cure for the disturbances which can ultimately be
traced to the lack of muscular activity is to devote one's self to this
muscular work outside of one's daily occupation, as is possible by aid
of the various sports. It is no accident, but rather a necessary physio-
logical phenomenon, that the need of active sports has always devel-
oped wherever there is a class of society made up of those doing no
intense physical work. Indeed, we can readily follow this in history;
when the citizens of the Greek cities devoted themselves to athletic
sports, when the knights of the Middle Ages jousted, there was always
an aristocracy who did no manual labor. The home of our modern
sports is England, the oldest industrial country. In Germany the first
steps in the direction of sports were made at the universities, where
thousands of young men did mental work. The scope of the sport of
to-day is very much broader, however, for its followers include mer-
chants and the workers in the various industries. Sports lead directly
to a change in the food requirements of the individual; every bicy-
clist, every mountain climber knows that on his trips he can eat things
which do not appeal to him at all when at home. I must be content
here to restrict myself to these indications disclosing the scientific
principles of this subject, which is 'apparently so far removed from our
point of departure. By unwearying work the physiology of nutrition
has established a scientific experimental foundation upon which other
sciences may now build.

THE PRESENT PROBLEMS OF PHYSIOLOGICAL
CHEMISTRY

BY RUSSELL HENRY CHITTENDEN

[Russell Henry Chittenden, LL.D. Sc.D., Director and Treasurer of the Sheffield
Scientific School, Yale University, and Professor of Physiological Chemistry.
b. February 18, 1856, New Haven, Connecticut. Ph.B. and Ph.D. Yale Uni-
versity; Special course, Heidelberg University. President of the American
Physiological Society, 1895-1904; President of the American Society of Natur-
alists, 1893. Member of the Connecticut Academy of Arts and Sciences; National
Academy of Sciences; American Philosophical Society. Author of many papers
on physiological subjects published in American and foreign journals.]

IN considering a proper presentation of the subject assigned me,
I am impressed with the influence which a man's own field of work
and his own line of thought will naturally exercise upon his point of
view. It may be questioned whether his judgment can be wholly
trusted, whether he will not in fact, unconsciously it may be, give a
dwarfed or one-sided presentation of the subject from a natural habit
of looking at things in their bearing upon the line of work and thought
in which he himself is personally most interested. While this may
not be wholly undesirable, of still greater advantage will be a brief but
judicious presentation of all the more important problems that con-
front the physiological chemist of the present day; but whether this
can be done satisfactorily in the time allotted is very questionable.
However, the effort will be made to emphasize, so far as the time will
allow, what to the writer seem the more significant and far-reaching
problems in physiological chemistry that call for speedy solution.

Of fundamental importance is the question, what is the exact
chemical constitution of proteid matter? The basis of all cell-life, the
most complex molecule that enters into the structure of the living
organism, proteid or albuminous material holds a peculiar position. A
labile molecule, it is easily prone to change, and its many decomposi-
tion-products confront us on all sides in our study of life's processes.
Yet to-day, in spite of all that has been accomplished, even with the
brilliant work of Kossel and Emil Fischer, we still lack adequate
knowledge of all the groups and radicles that are combined in this
atomic complex.

In the study of metabolism and nutrition, both in health and in
disease, in our conception of the anabolic processes of life, in our
theories regarding the chemical relationships of the varied katabolites
floating about through the organism, and in many other connections,
we need for our guidance a full knowledge of the chemical nature of
this most important class of substances. Thanks to the work of many

336 PHYSIOLOGICAL CHEMISTRY

brilliant investigators, our knowledge is progressing and broadening,
but we still lack that comprehensive understanding of the inner struc-
ture of the molecule that would serve to illuminate our field of vision
and give us a clear conception of the chemical constitution of this
group of physiologically important ground substances in living pro-
toplasm.

As is well known, the proteid bodies constitute a group of widely
divergent substances. Of these, the basic protamines are undoubtedly
the simplest and lowest in the scale, and it is quite probable, as sug-
gested by Kossel, that these substances constitute the nuclei of all
proteids. The protamines differ somewhat among themselves, but as
a group they are characterized by their high content of diamino-acids,
especially arginin. Thus, salmin yields on decomposition 84 per cent
of arginin, clupein 82 per cent, cyclopterin 62 per cent, and sturin 58
per cent.1 Sturin also contains 13 per cent of histidin and 12 per
cent of lysin, while the other protamines appear to contain no dia-
mino-acids aside from arginin. Further, the protamines contain dia-
mido-valerianic acid, monoamido-valerianic acid, tyrosin or p-oxy-
phenyl-amidopropionic acid, skatolaminoacetic acid, a-pyrrolidin-
carbonic acid and serin.2 Salmin 3 has also been shown to contain
alanin, leucin, probably also phenylalanin and aspartic acid.

If we pass from the simplest of the proteid bodies to the most
complex, as the nucleins, we find present in the latter not only arginin,
lysin, and histidin, but, in addition, such bodies as thymin, the purin
bases, leucin, aspartic, and glutamic acids, two sulphur-containing
groups, furfurol-forming groups, pyrrolidincarbonic acid, a skatol-
forming group, phosphoric acid, amidovalerianic acid, a levulinic
acid-forming group, glycosamine, pentose, uracil, and probably
phenylamido-propionic acid.4 In the histon from the nucleohiston
of the thymus, we find in addition to the hexone bases and the
monoamido-acids characteristic of the ordinary albuminous bodies
such substances as glycocoll, cystin, and alanin.

These statements, brief and incomplete though they are, will serve
to illustrate the complexity of the proteid molecule, and at the same
time they indicate the close genetic relationship which unquestionably
exists between the varied members of this large group of substances.
There is no doubt that Kossel and his co-workers, in their efforts to un-
ravel the constitution of the protamines, are pursuing a wise course in
paving the way for a comprehension of the exact nature of the more

1 Kossel and his students. See Kossel and Dakin, Ueber Salmin und Clupein,
Zeitschrift fur physiologische Chemie, Band 41, p. 407.

2 Kossel und Dakin, Beitrdge zum System der einfachsten Eiweisskorper, Zeit-
schrift fur physiologische Chemie, Band 40, p. 565.

3 Abderhalden, Die M onoaminosauren des Salmins, Zeitschrift fur physiologische
Chemie, Band 41, p. 55.

4 See Kossel, Uber den gegenwartigen Stand der Eiweiss Chemie, Berichte der
Deutschen Chem. Gesellschaft, Jahrgang 34, p. 3214.

PRESENT PROBLEMS 337

complicated proteids. There is no doubt that the protamines of one
type or another are integral parts of every proteid molecule, and when
their chemical constitution is made quite clear, much will have been
accomplished toward a fuller understanding of the more complicated
forms.

It needs no imagination to foresee what a full knowledge of the
chemical constitution of all types of proteid matter will mean for the
physiologist and physiological chemist. Much that is now cloudy and
uncertain in our understanding of cell and tissue metabolism, in our
comprehension of nutritive changes in general, of digestive proteo-
lysis and of intracellular autolysis, will become clear as crystal. The
problem, however, is not a simple one, but is exceedingly complex, for
it is to be remembered that just as the individual proteids differ from
each other in superficial reactions and characteristics, so do they
undoubtedly differ in their inner structure. Hence, we must expect
to find variations in the make-up of the individual molecules, and it is
one of the most important problems of to-day to ascertain the nature
of these chemical variations, to recognize the individual groups that
give character to the molecules, and to learn how these groups are
bound together to make the typical proteid of this and that tissue or
organ. The solution of this problem promises much for the advance-
ment of physiological chemistry, but it holds out the promise of even
more for the good of physiology in general, since there is bound up in
the chemical structure of the proteid molecules a full and complete
explanation of tissue changes, and of many metabolic phenomena
which to-day are as sealed volumes.

The development of our knowledge regarding the cell as a physio-
logical unit has led to a fuller recognition of the importance of dis-
criminating between the primary and secondary cell constituents. As
a result, the physiological chemist has come to realize the necessity of
more exact knowledge as to the nature and distribution of the pri-
mary components of cells, because of the bearing this knowledge may
have upon the general question of how far the lines of chemical decom-
position characteristic of each group of cells are dependent upon the
character of the anabolic processes by which that particular cell pro-
toplasm is formed, and how far the peculiar katabolic or retrogressive
changes of that group of cells are due to outside influences, exerted
by specific nerve fibres, or by the character of the blood and lymph
stream. The physiological chemist would know whether the secret of
glandular secretion, of tissue changes, of metabolic activity, is to be
found in the particular forms of protoplasm that enter into the struc-
ture of the component cells, whether it is associated in any way with
some inherent quality of the primary cell constituents.

There is something marvelous in the unerring certainty with which
a given group of cells performs its work, never deviating a hair's

338 PHYSIOLOGICAL CHEMISTRY

breadth from the beaten course, and turning out year after year a
definite line of products for the specific purpose in view. Why is it
that the epithelial cells of the salivary glands always manufacture
mucinogen and ptyalin; the gastric gland cells pepsinogen, rennino-
gen, and hydrochloric acid; the cells of the pancreas trypsinogen and
steapsin; the hepatic cells bilirubin, biliverdin, and the specific bile
acids; the cells of the thyroid iodothyrin, and the cells of the adrenals
epinephrin? Essentially the same blood and lymph bathe all these
cells with a like nutritive pabulum, and yet each group of cells per-
forms its own line of work, never going astray, in health, and never
even temporarily producing a product wrhich rightfully belongs to the
other class of cells. Are we to suppose that all these varied products
are manufactured from the same cell protoplasm, from a common
stock, that each one owes its origin to some particular force controlled
by extracellular influences, each group of cells being made to manu-
facture a given product out of the same mother substance? Or, on
the other hand, are we to assume that each group of cells, as it is
developed, has as a birthright the quality of producing from its par-
ticular protoplasm a certain line of products, simply because of the
peculiar chemical nature or constitution of that protoplasm?

In other words, do all the intricacies of cellular activity depend pri-
marily upon the character of the anabolic processes by which that
protoplasm is built up out of the food-materials by which the cells are
nourished? It may be just as difficult to explain why and how the
cells are able to manufacture a specific protoplasm out of a common
pabulum, but the main problem which confronts us is surely capable
of being solved. We need to know how far the primary cell constitu-
ents of different groups of cells, of the different organs and tissues, are
similar to or unlike each other. If it is shown that the primary cell con-
stituents differ for each glandular organ and tissue, that each group
of individualized cells has a protoplasm characterized by some specific
feature, then we shall have reason to believe that the anabolic pro-
cesses are as much, if not more, responsible for individuality of func-
tion than the katabolic processes. We may conceive of all protoplasm
being built, so to speak, on a certain general plan of structure, but
with side-chains of varying nature, and that these side-chains deter-
mine in a measure the character of the katabolic or alteration pro-
ducts that result from the natural activity of the cell protoplasm. In
other words, if this conception be true, it is the chemical constitution
of the cell protoplasm that is primarily responsible for the character
of the changes that take place in all active tissues and organs. The
extent of oxygenation as influenced by the circulating blood, the
direct and indirect influence of various nerve fibres, etc., may all act
as modifying agents, but only to the degree of accelerating or inhibit-
ing the rhythmical process which travels along a certain definite chan-

PRESENT PROBLEMS 339

nel because of the peculiar chemical nature of the cell protoplasm.
Once started, the process of katabolism takes a definite course, with
formation invariably of the same products, because that particular
cell protoplasm, owing to its peculiar make-up, tends to break down
along certain definite lines of cleavage, as it were, and so the products
split off are always the same.

We already have considerable knowledge which tends to indicate
that the cells of individual organs and tissues have a certain individ-
uality as regards their primary components, notably in the nucleo-
proteids present, but our knowledge is by no means complete enough
to permit of broad generalization. The problem is an interesting one,
and permits of a definite answer by the application of thorough and
persistent investigation.

As an allied question, more or less in harmony with what has just
been said, reference may be made to the part which ferments and
enzymes possibly play in initiating and carrying forward tissue
changes, as well as the metabolic changes that occur in glandular
organs. Ferments have come into such prominence of late years as
responsible agents for so many transformations that we may well
query whether their influence does not extend far beyond the limits
originally assigned to their field of activity. The discovery of oxidases
and the part which these agents may play in tissue changes, the un-
doubted existence of ferments in such glands as the thymus, supra-
renal, spleen, etc., by which the recently studied autolytic changes in
these glands are produced, raise the question whether ferments or
enzymes are not far more largely responsible for the many trans-
formations that take place in active tissues than has been hitherto
supposed. Consider for a moment the peculiar products which result
from the self-digestion (autolysis) of many of the glands so far
studied. Note how the nucleo-proteid of the thymus, for example,
breaks down, yielding xanthin and a little hypoxanthin, together with
uracil, but no guanin, adenin, or thymin.1 How the adrenal nucleo-
proteid likewise yields by autolysis considerable xanthin, but only
traces at the most of the. other alloxuric bases (Jones). By the self-
digestion of the spleen, guanin as well as hypoxanthin is conspicuous,
but it is a noticeable fact that in the autolysis of the thymus, for ex-
ample, there is no appreciable amount of leucin to be detected, thus
indicating that the above autolytic changes are not due to any or-
dinary proteolytic enzyme, but to some peculiar enzyme which acts
directly and solely upon the nucleo-proteids, splitting off certain of the
contained alloxuric groups. In harmony with this view, Jones has just
announced the presence in the pancreas, thymus, and adrenals, of an
enzyme to which he gives the name of guanase, which has the power of

1 Jones, Ueber die Selbstverdqnung von Nucleoproteiden, Zeitschrift fur physiolo-
gische Chemie, Band 42, p. 35.

340 PHYSIOLOGICAL CHEMISTRY

transforming guanin into xanthin. The same investigator also claims
the presence in the spleen of a related enzyme, called adenase, which
transforms adenin into hypoxanthin. The inference is that in many
glands and tissues there are specific enzymes, as yet undiscovered,
which may be responsible for at least some of the transformations
known to occur there.

That autolysis may be a possible explanation of the process of ani-
mal metabolism has been suggested by Levene 1 and also by Wells.2
It has been clearly indicated by such able workers as Salkowski, Ja-
coby, and others, that practically all animal cells contain within them-
selves ferments or enzymes that are capable, under suitable conditions,
of digesting or breaking down the cell-contents by a process similar to
ordinary proteolysis, and it may perhaps be assumed that all active
cells carry forward their ordinary metabolic processes by the agency
of these intracellular ferments. Moreover, it is not inconceivable
that ferments or enzymes of several kinds may exist side by side in
a given group of cells, just as they are known to exist in the pancreas,
by which we might infer the possibility of a series of transformations
taking place at essentially the same time, through the harmonious
action of a row of enzymes physiologically quite distinct.

Further, the recently discovered reversible action of enzymes, on
which we have at command so much valuable work, suggests the pos-
sibility of a maintenance of cell-equilibrium through this peculiarity
of action, thus affording a tangible explanation of the means by which
intracellular nitrogenous or proteid equilibrium is maintained, the
various cells of the body building up or breaking down the proteid
matter of their own tissues as circumstances require. If these ideas
are true, then our conception of ferment action must be considerably
broadened, and we have before us the possibility of explaining many
of the phenomena of tissue metabolism by the action and interaction
of intracellular enzymes. This is a problem well worthy of broader
study, with a view to the elucidation of the general laws that govern
tissue changes in general. In this connection we also have suggested
the possibility of interaction of another kind, viz., that interdepend-
ence of one tissue or gland upon another for the full development of its
functional activity, as illustrated by the part played by the entero-
kinase of the intestinal glands in the development of an active tryp-
sin from the zymogen of the pancreatic cells, and by the action of the
internal secretion of the pancreas upon the inert constituents of the
muscle to develop in the latter an active glycolytic enzyme. How far
this general principle extends in the metabolic phenomena of the body
is entirely problematical, but merits careful study. Here, then, we

1 Die Endprodukte der Selbsiverdauung tierischer Organe, Zeitschrift fur physio-
logische Chemie, Band 41, p. 393.

2 On the Relation of Autolysis to Proteid Metabolism, Amer. Journal of Physi-
ology, vol. 11, p. 351.

PRESENT PROBLEMS 341

have an added field of inquiry, worthy of careful consideration, if we
are to possess a clear understanding of nature's processes.

Between the animal and the vegetable cell certain sharp lines of dis-
tinction are frequently drawn. Physiologists are wont to believe that
the processes characteristic of the cells of animal tissues and organs are
essentially destructive, i. e., that they are principally katabolic, while
in vegetable tissues, on the other hand, constructive processes are
very conspicuous. In no way is this better illustrated than in the pre-
valent opinions regarding the parts played by the two classes of cells
in the metabolism of proteid matter. We are accustomed to think
that all proteid matter has its primary origin in the synthetical power
of the vegetable cell, aided by its contained chlorophyll and the bene-
ficent action of the sun's rays. The animal cell, on the other hand, can
merely transform and reconstruct the various proteids furnished by
the vegetable world, being without power to manufacture proteid
matter de novo out of the simple groups and radicles which the vege-
table cell utilizes so rapidly. In ordinary proteid katabolism, the
various nitrogenous decomposition-products are presumably all con-
verted into urea and allied substances adapted for excretion. If, how-
ever, there is reversible ferment or enzyme action in the animal body,
why may there not also be power to utilize, in some measure at least,
the crystalline nitrogenous bases and amido-acids so abundantly
formed in trypsin proteolysis, for the construction of fresh proteid
matter? One may well query, considering the vigor of the proteolytic
action of the enzymes poured into the alimentary tract, whether all
these nitrogenous waste products represent just so much lost energy
in their production and a further loss of energy in their immediate
excretion from the body. In harmony with the ' ' luxus consumption"
theory we may assume wisdom and ultimate gain in this speedy de-
composition of excessive proteid foods in the alimentary tract, but
the argument is not very convincing. Why may not animal cells, or
the animal body as a whole, build up proteid matter out of simple
nitrogenous compounds analogous to the action of plant cells? Loew l
has indeed experimented in this direction and states that the biuret-
free end-products resulting from the proteolysis of ordinary food
albumin can be utilized by the animal body for the maintenance of
nitrogenous equilibrium, etc., equally well with the common proteid
food-stuffs. His conclusions, however, have been called in question
by other investigators, notably by Lesser,2 whose experimental data
failed to confirm the above conclusion.

The problem, however, is an exceedingly important one. If the
animal body has no power of utilizing the varied nitrogenous com-

1 Ueber Eiweissynthese im Thierkorper, Archiv furexper. Pharmakol. u. Pat hoi. ,
Band 48, p. 303.

2 Ueber Stoffwechselversuche mil den Endprodukten peptischer und tryptischer
Eiweissverdauung, Zeitschr. fur Biologic, Band 45, p. 497.

342 PHYSIOLOGICAL CHEMISTRY

pounds of simple constitution formed in the gastro-intestinal tract by
the digestive enzymes; if there is a complete lack of ability to con-
struct new proteid matter out of these simple decomposition-products,
then surely we must inquire what is the real purpose of their forma-
tion. It is true that, with the limitations of our present knowledge, it
is difficult to see why, if digestive proteolysis has for its sole object the
conversion of the proteid foods into forms suitable for absorption,
there should be any considerable breaking-down of proteid beyond
the proteose or peptone stage, since the latter bodies would seem to
be most easily adaptable for transformation into the proteids of blood
lymph and tissue. On the other hand, it is well known that the pro-
teid of the food is possessed of a physiological and chemical nature
quite different from that of the proteid in the blood and tissues of the
feeding animal, and it is quite conceivable that a synthetical process
might be essential — in some degree — for the manufacture of the
specific proteids called for by the blood and tissues of that particular
species or individual. The question is one that demands careful con-
sideration and thorough investigation, for it touches upon a chapter
in nutrition on which we have at present very little satisfactory or
convincing knowledge.

In this connection we may call attention to another problem, some-
what far-reaching, but suggested by one of the preceding paragraphs,
viz., the possible physiological action of the many katabolites, or
decomposition-products resulting from tissue-changes throughout the
animal body. In vegetable tissues, many of the nitrogenous products
common to these structures are endowed with marked physiological
power, as witness the vegetable alkaloids and the non-nitrogenous
bodies like salicin, digitalin, picrotoxin, etc. Years ago, physiologists
recognized that some of these nitrogenous bodies present in animal
tissues did have a distinctly toxic action when introduced directly
into the circulation, and hence they were frequently called animal
alkaloids, but our knowledge upon these points is exceedingly obscure
and indefinite. When we take into consideration the large number of
nitrogenous products formed and present in the various tissues and
organs of the body, products of proteolysis and of tissue-changes;
when we consider how these products circulate through the organism,
in blood and lymph; how they come in more or less immediate con-
tact with the different cells of the body prior to their decomposition
or elimination, we cannot avoid being impressed with the part they
may play in stimulating and modifying tissue or other changes.

The significance of this suggestion is made all the more potent by
the knowledge recently acquired concerning several of the internal
secretions of the body and the powerful physiological influence ex-
erted by their components. Where can be found a more active phy-
siological agent than the blood-pressure-raising constituent of the

PRESENT PROBLEMS 343

adrenals, the epinephrin? Where is there a more active agent in mod-
ifying the nutritional processes of the body than the iodine-containing
constituent of the thyroid, the iodothyrin? These may truly be
counted as representing a type of substances manufactured or se-
creted primarily for the physiological effect they are capable of exert-
ing; but what about the host of other substances present in the body,
many of them simple products of katabolism? May they not have
some marked physiological property that if known would serve as a
sufficient excuse for their formation? Or, may they not possess some
hidden or obscure property which if once understood would make
clear a secondary or subsidiary function of no small import for the
maintenance of physiological equilibrium, or for the wrelfare of the
body? Many suggestions and some facts present themselves illustrat-
ing how direct and indirect influences may be exerted, all pointing
toward the harmonious action and interdependence in function of
many of the substances formed in the body. Some, however, un-
doubtedly have more or less of a toxic action, especially when formed
in excessive or undue amounts. Thus, the alloxuric bases seemingly
cause fever when injected into the circulation or taken per os,1 and
according to the recent observations of Mandel 2 there is a very
striking relationship between the quantity of alloxuric bases elimin-
ated in the urine and the temperature of the body in cases of aseptic
fevers, indicating that these substances, with possibly other incom-
plete products of tissue-metabolism, are important factors in the pro-
duction of febrile temperature. We may confidently expect that a
thorough study of the physiological action of all the varied katabolic
products formed in the body will result in a decided expansion of our
knowledge regarding the part these substances may play in normal
and abnormal metabolism, and in nutrition in general.

Just here, reference may be made to the many problems in the
broad field of nutrition that confront the physiological chemist of the
present day. The maintenance of life on a sound physiological basis is
one of the practical problems in physiological chemistry, and its solu-
tion is not yet attained. We need fuller knowledge regarding the part
played by the different nitrogenous food-stuffs, the relative physio-
logical value of animal and vegetable proteid, the relative value of
fats and carbohydrates as nutrients aside from their different calorific
power, and, by no means least, a fuller and more accurate knowledge of
the true physiological needs of the body for proteid foods. Our pre-
sent dietetic standards are absolutely false and valueless. Our present
conception of the physiological needs of the body is altogether faulty
and distorted. Our ideas of the rate and extent of proteid metabol-

1 See Burian and Schur, Archiv fur die gesammte Physiologic, Band 87, p. 239.
1 The Alloxuric Bases in Aseptic Fevers, Amer. Journal of Physiology, vol. 10,
p. 452.

344 PHYSIOLOGICAL CHEMISTRY

ism necessary for the maintenance of health and strength are crude
and inexact. We place the nitrogen requirement of the healthy man
at an absurdly high level, apparently because observation has shown
that man is disposed to consume an equivalent in proteid food per
day. We need to ascertain by scientific experiment how far such stand-
ards are justified; to determine by definite analysis the amounts of
nitrogen actually required to maintain nitrogen equilibrium and keep
up bodily and mental vigor. Upon the physiological chemist of the
present day rests the responsibility for the establishment of nutritive
standards that will endure the test of scientific criticism, that will
harmonize with daily experience, and that will prove to be physio-
logically correct.

Further, we need to know more concerning the relative decomposi-
tion within the body of the truly organized proteid matter of the tis-
sues, and of the albuminous food-stuffs which, having been digested
and absorbed, are in a sense a part of the tissues, but not thoroughly
or completely incorporated as an integral part of the living cells. Does
the urea of the daily excretion come primarily from the breaking-
down of the organized proteid, or does it come preferably from the
disintegration of the circulating proteid? We recall the famous
experiments of Schondorff, in which blood was made to circulate
through the muscles and liver of well-nourished and fasting dogs,
with the result that the urea of the blood was increased only when
the blood circulated through the tissues of a well-nourished animal.
It made no difference with the result whether the blood employed
was from a well-fed or a fasting animal ; the essential factor was the
condition of the muscle tissue through which the blood was made to
flow. Schondorff drew the natural conclusion that the extent of pro-
teid metabolism was dependent upon the nutritive condition of the
cells of the tissue, upon the mass of the living cell-material, i. e., upon
the amount of morphotic proteid present, and that the proteid con-
tent of the intermediary fluids, as blood or lymph, was of no moment
in determining the rate of urea formation.

We may well doubt, however, if all the urea formed daily under
ordinary conditions of life comes solely from the breaking-down of the
truly organized or morphotic proteid. It is more than probable that
the urea has at least a twofold origin, and, if so, it is an important
matter to be able to discriminate between that which comes from the
breaking-down of the unorganized albumen, and that which is derived
from the organized tissues. Unquestionably, the decomposition of
organized proteid, the morphotic part of the living protoplasm, is
quite different from that of the unorganized pabulum of the cell and
surrounding media. Quite possibly, the influences controlling the two
lines of metabolism are different; perhaps, there are even different
kinds of nerve control.

PRESENT PROBLEMS 345

Equally important is it for the physiologist to know more fully
regarding the sources of the carbonic acid resulting from oxidation in
the body. What proportion of the ever-varying output of this gaseous
product of metabolism comes from the oxidation of organized tissue-
material, and what from the oxidation of circulating carbohydrate and
fat and unorganized material in general? We have learned, for ex-
ample, that the excretion of carbonic acid runs more or less closely
parallel with the degree of muscular activity, and we should possess the
means of discriminating between the output from true tissue-oxida-
tion and that which is derived from extracellular sources. A study of
the excretion of carbonic acid by fasting individuals, under different
conditions of life and activity, would be helpful in throwing light upon
this question, and also in giving us a clearer idea of the minimal re-
quirements of the body for non-nitrogenous foods to make good the
loss of energy in heat-liberation, muscular work, etc. By such a study
we might hope for added light upon that much-discussed problem, the
source of the energy of muscular contraction. While most physiologists
are certainly agreed that this energy comes preferably from the oxida-
tion of non-nitrogenous matter, there remain many obscure points
upon which we need enlightenment.

We likewise need fuller and more exact knowledge of the ways in
which uric acid originates in the body, especially regarding its rela-
tionship to intracellular decomposition. Our present understanding
of the twofold origin of this substance — endogenous and exogenous
— is most helpful in making clear many formerly obscure points con-
nected with the formation of this substance from the different classes
of food-stuffs. To-day, however, we understand quite clearly the gene-
tic relationship between the free and combined purin bases and uric
acid, but we are still uncertain whether this substance is formed to
some extent synthetically and whether when once formed it is all elim-
inated unchanged or undergoes oxidation, in part, into less harmful
substances. In other words, we do not yet know how far the uric acid
which is contained in the daily urine is a measure of the production of
uric acid for the twenty-four hours. Uric acid and the alloxuric bases
are such important substances, in their influence upon health and the
general nutritive condition of the body, that it is extremely important
for us to know more concerning their origin and their ultimate fate
in the body. We may likewise inquire where uric acid is formed. Does
it originate entirely in the liver, or are there other depots where it is
produced and collected?

Turning our attention now in another direction, we may revert to
the relationship between stereochemical configuration and physiolog-
ical action as a fruitful subject for investigation. Many interesting
facts have already been gleaned, and certain general rules or laws
have been formulated, connecting given lines of physiological action

346 PHYSIOLOGICAL CHEMISTRY

with a definjte chemical structure. Thus, it is well understood to-day,
for example, that all substances which contain a nitro or nitroso
group united with or bound to oxygen have the effect of dilating
blood-vessels, while, on the other hand, substances which contain the
same nitro or nitroso group joined to carbon have a quite different
physiological action, being mostly blood-poisons. Further, nitrils,
R. CN, tend to produce coma, while isonitrils, R. N = C, are much
more toxic and tend to produce paralysis of the respiratory centre.1
In other words, it is clearly manifest that certain definite group-
ings within the molecule are the cause of the physiological action of
the molecule. At the same time, it is also known that in order to
have the physiological action of a substance manifest, not only must
it contain the necessary group or groupings, but there must likewise
be present a second group which has the power of combining with
and holding fast to the tissue upon which the physiological action
manifests itself. Slight chemical alteration of a substance may, there-
fore, interfere with or nullify its ordinary physiological action without
necessarily altering the physiologically active groups; but by simply
changing these other groups through which the molecule ordinarily
attaches itself, so that the latter can no longer adhere to the cell-
substance or tissue-protoplasm, there occurs a consequent loss of
physiological action.

Another fact clearly understood is that two substances having
the same nucleus and like side-chains, with an entirely similar group-
ing, may still be physiologically unlike, owing to a different arrange-
ment in space. This is well illustrated by the dextro- and laevo-rotary
tartaric acids, one of which is readily utilized by Penicillium glaucum
as nutriment, while the other cannot be so consumed. Many other
illustrations might be cited, especially with various types of organic
poisons, all tending to show that physiological action is dependent
upon the arrangement of the atoms or radicles in space, as well as
upon the nature of the atoms or radicles. With these facts before us,
we see many lines of inquiry presenting themselves, many problems
demanding solution, with reference both to pharmacology and physio-
logy.

Confining our attention more especially to physiological matters, we
are certainly justified in considering the application of these principles
to many of the substances conspicuous in the processes of the body.
The work and suggestions of Pasteur and Emil Fischer have indicated
certain possibilities regarding the nature and action of enzymes, not
to be overlooked. Stereochemical configuration may be just as much
responsible for enzyme action, for proteolysis, amylolysis, etc., as any
other feature of the active molecule, and how far other lines of physio-
logical action may be due to chemical structure and the configuration
1 See Frankel Ergebnisse der Physiologic, Drifter Jahrgang. Biochemie, p. 291.

PRESENT PROBLEMS 347

of the molecule, who can say? One's thoughts naturally turn to the
living muscle plasma and the chemical changes that follow or accom-
pany the advent of rigor mortis; to the circulating blood and lymph,
and the transformations that occur when these fluids are withdrawn
from the protecting influence of the endothelial lining of the living
vessels ; to the axis cylinder of the nerve-fibres and the changes that
occur when the fibres are severed from their connection with the
ganglionic cells. These and many other suggestions arise, all calling
for a further study of the chemical constitution and stereochemical
configuration of the molecules involved, since in the knowledge thus
gained may be found the solution of many physiological processes
now shrouded in mystery.

The reference just made to nerve-fibres and ganglionic cells sug-
gests another problem in physiological chemistry, solution of which
has long been deferred, viz., the exact chemical nature of nerve-tissue,
and the character of the changes involved in the passage of a stimulus
or nervous impulse through a nerve to its ending in the muscle or
secreting cell. Further, what is the real purpose of the complex
myelin surrounding the axis cylinder of medullated nerves, and the
corresponding substance imbedded in the gray matter of the brain
and cord? These are problems that have long waited solution, and
yet they are vital to any clear understanding of the nutritive or other
changes that take place in nerve-tissue, either in rest or in activity.
Nerve-tissue is strikingly peculiar in its large content of phosphorized
bodies of the lecithin type, cerebrosides and cholesterins. These sub-
stances, complex in nature and of large molecular structure, are all
alike in having the physical properties of fats. Further, lecithin and
the cerebrosides all contain fatty acid radicles in large amount, and
in addition lecithin contains the radicle of glycero-phosphoric acid.
Moreover, the cerebrosides contain a carbohydrate group yielding
galactose on decomposition, so it is plain to see that the bodies which
give character to the myelin material are highly nutritive substances
with high calorific power. These facts might readily be taken as indi-
cating that the function of the myelin is to nourish the more import-
ant axis cylinder, to furnish the necessary pabulum for growth and
repair as well as to meet the daily demand for energy-yielding material.
While we may speculate, however, as to the part these peculiar
substances play in the life of nerve-tissue, we really possess very little
positive knowledge of their true purpose. Indeed, we do not know
how these bodies actually exist in the living tissue, as is well evidenced
by the utter lack of agreement among physiological chemists as to
the entity of the so-called protagon. Whether this phosphorized sub-
stance, studied by so many investigators, exists as such in the living
tissue, or whether it is simply an intimate mixture of lecithin, cerebrin,
and one or more other substances, is not yet settled to the satisfac-

348 PHYSIOLOGICAL CHEMISTRY

tion of all concerned. Further, 'it is not at all impossible that the cere-
brosides, as well as lecithin and possibly cholesterin, may exist in the
living tissue combined with some one or more of the proteids present
there. Our lack of knowledge is deplorable, and yet, in the words of
Sir Michael Foster, this is one of the "master tissues" of the body.
Surely, considering the preeminent position and controlling influence
of this tissue, we may look for a speedy clearing away of the darkness
that enshrouds our understanding of the exact chemical composition
of nerve-tissue, and especially of the way these peculiar substances
of the myelin material exist in the living tissue.

Again, we may ask ourselves what is the nature of the chemical
changes that take place in nerve-tissue; in the ganglionic cells of the
gray matter and in the axis cylinder of the nerve-fibres. When a
muscle contracts there is a measurable chemical decomposition. The
energy of muscular contraction comes from the breaking-down of
non-nitrogenous components of the muscle, and perhaps in some
measure from the decomposition of nitrogenous constituents. Fur-
ther, there is a liberation of heat, a development of lactic acids, etc.
When a stimulus is applied to a nerve, on the other hand, no such
manifestations of chemical action are apparent. The muscle to which
the nerve is attached contracts, the secreting cell pours forth the
product of its activity, etc., but there is no noticeable change in the
nerve itself, no recognizable liberation of heat, no change of reaction,
no output of carbonic acid, that can be detected. Are we to conclude,
then, that the axis cylinder of the nerve-fibre acts simply as a con-
ducting agent without itself undergoing any change? Is it to be com-
pared to an electric wire, with the surrounding myelin material, the
substance of Schwan, serving as a convenient insulating or protective
medium? If we are to accept this view, what are we to say regarding
the non-medullated fibres? Do not they need an insulating material
likewise? We can argue that the myelin substance is especially
adapted for the nourishment of the nerve, that its high potential
value renders it peculiarly suitable as a concentrated nutriment, and
that its intimate contact with the neuraxis and with the ganglionic
cells of gray matter proclaims its probable use in this direction.
Moreover, if we follow this line of argument still further, we may be
led to believe that the stimulation of a nerve, its power of conductiv-
ity, etc., are associated with chemical decompositions along its axis
as marked in their way as those that occur in a contracting muscle-
fibre. Truly, we have here a multitude of questions, for which at
present no satisfactory answers are to be found. The problems are
on the surface awaiting solution.

Finally, emphasis must be laid upon a series of problems in physio-
logical chemistry, true solution of which will do much to explain
natural and artificial immunity, the action of toxins and antitoxins,

PRESENT PROBLEMS 349

the bactericidal action of blood-sera, the effect of oxidizing enzymes,
of animal and vegetable origin, upon toxins of various kinds, etc.
Ehrlich's theories regarding the protection furnished by antitoxic
and bactericidal sera, so elaborately devised, constitute a working
hypothesis of great value, but we need much additional knowledge
concerning the nature and action of the so-called complements and
anticomplements, of amboceptors, of haptophor groups, of agglu-
tinins, of precipitins, and of hemolysis. The physiological chemist
studies with care the important and suggestive work being carried
forward by the many brilliant investigators in pathology and bacteri-
ology, with the feeling, however, that the true explanations for most
of the phenomena in question are chemical, and that the actions and
interactions involved are chemical ones, to be eventually made clear
by a fuller chemical knowledge of the toxic and antitoxic substances
themselves and of their alteration and combination under different
physiological conditions.

The well-known natural immunity possessed by some animals
toward certain diseases, together with the difficulty experienced by
most micro-organisms in developing in the healthy body, — a difficulty
which at once disappears when from any cause the tissues of the body
lose their original vitality and vigor, — all point to the presence in the
healthy body of certain general or specific substances which are directly
deleterious to the micro-organisms. Such substances are obviously
bactericidal, and it is equally plain that in the bodies of many species
of animals there are specific antisubstances present which are lacking in
other species, thereby explaining the natural immunity of the former
towards certain diseases. As is well known, blood-serum possesses, as
a rule, a bactericidal power upon most micro-organisms, and we have
every reason to believe in the existence of specific substances in the
serum which exert some influence upon the growth and development
of micro-organisms, and also upon the toxic products they tend to
elaborate. These protective substances — the alexins of Buchner —
appear to be proteid in nature, resembling globulins, since they are
precipitated from serum by the action of certain strong solutions of
alkali salts, as sodium sulphate. We know, however, very little re-
garding their chemical nature aside from the fact that they are ob-
viously very complex, although perhaps even this point is not quite
certain. These protective substances are presumably elaborated by
the leucocytes of the blood and lymph, cells rich in nuclein andnucleo-
proteid material. Doubtless, also, some of the gland-cells in the body
have a corresponding action; statements which, if true, tend to
emphasize the possible proteid nature of the protective substances.

While in a general way we may say that the natural immunity to
certain bacteria possessed by some animals is due in large measure to
an inhibition of the growth of the micro-organism, it must also be

350 PHYSIOLOGICAL CHEMISTRY

remembered that there is in many species a distinct immunity to the
action of the poison which the specific micro-organism produces. This
immunity depends either upon a destruction of the poison as by oxida-
tion, upon a combination between the poison and some constituents
of the active protoplasmic cells of the body, thereby rendering the
poison inactive, or, lastly, upon some action of the specific proto-
plasmic cells of the body usually affected by the poison, by which the
latter is unable to combine with the cells upon which it ordinarily
acts. All these suggestions, however, imply chemical reactions of
some kind, and obviously should be understood for a betterment of
our knowledge upon this important matter.

Again, the specific immunity which shows itself after exposure to a
given disease, so that a second infection becomes practically impossi-
ble, can be explained satisfactorily only on chemical grounds, viz., by
the presence in the blood and lymph of certain protective or immuniz-
ing substances which presumably originate through chemical changes
in the blood-serum, under the influence of the bacteria causing the
disease. These are chemical substances, formed through chemical
decompositions or alterations of normal constituents of the blood,
and obviously we need to know more of their exact nature.

Following Ehrlich's views, specific antitoxins, bactericidal sera, etc.,
result from the overproduction of molecules in cells which are sensi-
tive to the action of toxins and other bacterial products. Antitoxins
so formed unite with toxins, and the so-called complementary bodies
and the bactericidal anti-bodies combine with the bacterial cells, thus
affording protection. These processes of alteration and combination,
however, are presumably all chemical, involving either alteration of
chemical structure, or direct combination of bodies chemically the
opposite of each other. Further, the so-called haptophor groups of the
toxin molecule are probably represented in fact by chemical groups
or radicles, which owe their power of combination with corresponding
groups of other cells to chemical affinity. Again, the complementary
body, normally present in all healthy blood-sera and which is needed
along with the specific anti-body for the destruction of bacterial cells,
must owe its activity to the power of chemical combination. Hence,
we have presented to us at every turn the question of the chemical
nature of these various substances, toxin and antitoxin, complement,
receptor, haptophor, etc., which are of such vital importance in the
production and maintenance of immunity and protection. Surely
this is one of the most important problems of the present day in the
domain of physiological chemistry, and calls for both patience and
skill of the highest order in its solution.

SHORT PAPER

MR. EDWARD MALLINCKRODT, JR., of St. Louis, Missouri, read a paper to this
Section on "The Diet, Physical Condition, and Mental Performance of Certain
Students in Harvard University."

BIBLIOGRAPHY: DEPARTMENT OF CHEMISTRY

(Prepared for the Department by the courtesy of Professor James M. Crafts)

ANALYTICAL CHEMISTRY

CARNOT, Trait6 d'analyse des substances minerales, 2 vols., 1898-1904.
CLASSEN, Ausgewahlte Methoden der analytischen Chemie, 2 vols., 1901-1903.
CROOKES, Select Methods in Chemical Analysis, 1894.
FRESENIUS, Qualitative Chemical Analysis, 1897.

Quantitative Chemical Analysis, 1904, 2 vols.

PHYSICAL CHEMISTRY

DONNAN, F. G., Thermodynamics.

FINDLAY, A., The Phase Rule.

KOHLRATJSCH and HOLBORN, Leitvermogen der Elektrolyte, 1898.

LANDOLT, BORNSTEIN, and MEYERHOFFER, Physikalisch-chemische Tabellen, 1905.

LEHFELDT, R. A., Electrochemistry.

MELLER, J. W., Chemical Statics and Dynamics.

NERNST, W., Theoretische Chemie, 1904 (English translation by Lehfeldt).

OSTWALD, W., Lehrbuch der allgemeinen Chemie, 3 vols., 1891-1902.

OSTWALD, W., and LUTHER, R., Hand-und-Hiilfsbuch physiko-chemischer Mes-

' sugen, 1902.

RAMSAY, W., Text- books of Physical Chemistry, 1904-05.

ROOSEBOOM, B., Die heterogene Gleichgewichte, 1901, 1904.

VAN 'T HOFF, J. H., Vorlesungen tiber theoretische und physikalische Chemie,

1898-99 (English translation by Lehfeldt).
YOUNG, S., Stochiometry.

TECHNICAL CHEMISTRY

AHRENS, Sammlung chemischer und chemisch-technische Vortrage, 9 vols., 1896-

1905.

DAMMER, Handbuch der Chemischen Technologic, 5 vols., 1895-98.
DAVIS, Handbook of Chemical Engineering, 2 vols., 1901-02.
GROVES and THORP, Chemical Technology, 4 vols., 1889-1903.
OSTWALD, Electrochemie, 1896.
STILLMAN, Engineering Chemistry, 1897.
THORPE, Dictionary of Applied Chemistry, 3 vols., 1890-93.
WAGNER, Jahresbericht iiber der Leistungen der Chemischen Technologic, 1855-

1905.
Jahrbuch der Electrochemie, 1895-1903.

GENERAL INORGANIC CHEMISTRY

DAMMER, Handbuch der anorganischen Chemie, 4 vols., 1892-1903.
GRAHAM-OTTO, Ausfiihrliches Lehrbuch der anorganischen Chemie, 8 vols

1885-89.

MENDELEJEFF, Principles of Chemistry, 2 vols., 1897.
OSTWALD, Grundriss der Allgemeinen Chemie, 1899.
ROSCOE and SCHORLEMMER, Treatise on Chemistry, 2 vols., 1895-98.

BIBLIOGRAPHY: DEPARTMENT OF CHEMISTRY 353

WATTS, Dictionary of Chemistry, 4 vols., 1888-94.
WURTZ, Dictionnaire de Chimie pure et applique1, 3 vols.

1st Supplement, 2 vols.

2d Supplement, 5 vols., 1869-1905.

ORGANIC CHEMISTRY

ALLEN, A. H., Commercial Organic Analysis. 3d ed., 4 vols. in 8, 1898-1903. '
BEILSTEIN, F. F., Handbuch der organischen Chemie. 3d ed., 4 vols., und 2 Er-

ganzungsbande, 1893-1903.
GATTERMANN, L., Practical Methods of Chemical Analysis. Trans, by Shober.

6th ed., 1904.
LASSAR-COHN, Arbeitsmethoden fur organisch-chemische Laboratorien. 3d ed.,

1903.
MEYER, V., and JACOBSON, P., Lehrbuch der organischen Chemie, 2 vols. in 3,

1893-1903.
RICHTER, M. M., Lexikon der Kohlenstoff-Verbindungen. 2 vols. und 2 suppl.

1900-05.

SADTLER, S. P., Handbook of Industrial Organic Chemistry. 3d ed., 1900.
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STOHMANN and KERL (Eos.), Muspratt-Handbuch der Technischen Chemie. 4th

ed., 6 vols.

SPECIAL WORKS OF REFERENCE

(Prepared for the paper on "History of Chemistry" by its author, Professor Frank W.

Clarke)

BOLTON, H. CAHEINGTON, Select Bibliography of Chemistry, no. 850, Smithson-
ian Miscellaneous Collections, pp. 85-170; no. 1170, pp. 22-44; no. 1440,
pp. 12-22.

KOPP, H., Geschichte der Chemie, 4 vols. Braunschwig, 1843-47.

LADENBURG, A., Lectures on the History of the Development of Chemistry since
the Time of Lavoisier. Trans, from the German by Leonard Dobbin. Edinburgh
and London.

MEYER, E. VON, A History of Chemistry from the Earliest Times to the Present
Day, etc. Trans, from the German by George McGowan. London, 1898.

THOMSON, T., The History of Chemistry, 2 vols. London, 1830.

TILDEN, W. A., A Short History of the Progress of Chemistry in Our Own Times.
London, 1899.

WURTZ, A., Histoire des Doctrines Chimiques depuis Lavoisier, jusqu'a nos jours,

Paris, 1869.
La Theorie Atomique, Paris, 1886.

SPECIAL WORKS OF REFERENCE

(Prepared for the Section of Physiological Chemistry by Professor 0. Cohnheim)

ATWATER, W. A., Ergebnisse der Physiologic, in; Biochemie, 1904.

ATWATER, W. A., and BENEDIKT, F. G., U. S. Dept. of Agriculture, Office of

Experiment Stations, Bull. no. 136, 1903.
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1898.
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Handbuch der Ernahrungstherapie, Bd. i, p. 20, 1898.
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Zeitschr. f. Biologic, 25, 232, 1889.
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541, 1894, and p. 267, 1898.

ZUNTZ, N., und SCHUMBURG, Physiologie des Marsches, Berlin, 1901.
ZUNTZ, N. (mit HEINEMANN, FRENTZEL, REACH, CASPAR:, BORNSTEIN), Pfluger's

Arch. 83, 1901.

ADDENDA PAGES

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