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in water, the mass is divided into its molecules, which are still parti-
cles of sugar, though they are far too small to be seen by the highest
powers of the microscope. The physical subdivision of every body
is limited by the dimensions of its molecules ; but the chemist can
carry the process further. He " decomposes," or breaks up, these mole-
cules into " atoms " ; but the parts thus obtained have no longer the
qualities of the original substance. Hence the molecule may be con-
sidered as the smallest particle of a substance in which its qualities
inhere ; and every molecule, though physically indivisible, can be
broken up chemically into atoms, which are themselves the molecules
of other and elementarv bodies.



WHAT IS A MOLECULE j* 689

No one has ever seen or handled a single molecule, and molecular
science therefore deals with things invisible and imperceptible by our
senses. AYe can not magnify a drop of water sufficiently to see its
structure ; and the theory that matter is built up of molecules depends,
like the philosophy of every science, on its competence to explain ob-
served facts. These are of two kinds — namely, physical and chemical.
A physical change in the condition of a body is illustrated by dissolv-
ing a lump of sugar in water. The sugar disappears, but remains
present in the water, from which it may be recovered by evaporation.
But if we burn the lump, we effect a chemical change in its condition.
The sugar again disappears, and in its place we get two other sub-
stances — namely, carbon and water.

Similarly, water is converted by boiling into the invisible vapor,
steam ; but the change in its condition is physical only, for the steam
condenses to water on being cooled. If, however, we pass water
through a red-hot iron tube, it disappears, and is replaced by the two
gases, oxygen and hydrogen. In the latter case, the liquid suffers a
chemical change, or, as we say, is " decomposed " into its constituent
elements. Those changes, therefore, which bodies undergo without
alteration of substance are called physical ; while those which are ac-
companied by alteration of substance are called chemical.

Turning our attention first to the physical side of the question, let
us inquire how far some of the fundamental laws of science are illus-
trated by the molecular hypothesis. Among the most important of
these is the law of Boyle, which declares that the pressure of gases is
proportional to their density. The theory under review is based at
present on the phenomena of gases, and considers these as aggrega-
tions of molecules in constant motion. Their movements are supposed
to take place in straight lines, the molecules hurrying to and fro across
the containing vessel, striking its sides, or coming into contact with
their neighbors, and rebounding after every collision, like a swarm of
bees in a hive flying hither and thither in all directions.

We know that air, or any gas, confined in a vessel, presses against
its sides, and against the surface of any body placed within it. This
pressure is due to the impact of the flying molecules ; and the con-
stant succession of their strokes is, according to this theory, the sole
cause of what is called the pressure of air and other gases. As each
molecule strikes the side of the vessel the same number of times, and
with an impulse of the same magnitude, the pressure in a vessel of
given size must be proportionate to the number of molecules — that is,
to the quantity of gas in it ; and this is a complete explanation of
Boyle's law. Let us next suppose that the velocity of the molecules
is increased. Then each molecule will strike the side of the containing
vessel not only more times per second, but with greater force. Now,
an increase in the velocity of the molecules corresponds in theory to
I rise of temperature ; and in this way we can explain the increase of
^L. XIX.— 44



690 THE POPULAR SCIENCE MONTHLY.

pressure, and the proportions of such increase which result from heat-
ing a gas. Similarly, Charles's important law, that the volume of a
given mass of gas under a constant pressure varies directly as its tem-
perature, follows obviously from the hypothesis,

Priestley was the first to remark that gases diffuse through each
other. This fact is familiarly illustrated by the passage of odorous
gases through the atmosphere. If a bottle of ether is opened in a
room, its vapor diffuses through the air, and its presence is soon recog-
nized by the sense of smell. In this case, the ether-molecules may be
figured as issuing from the bottle with great velocity ; and, if their
course were not interrupted by striking against the molecules of the
air, the room would be instantaneously permeated by their odor. But
the molecular particles of both air and ether are so inconceivably nu-
merous, that they can not avoid striking one another frequently in
their flight. Every time a collision occurs between two molecules, the
paths of both are changed ; and the course of each is so continually
altered that it is a long time in making any great progress from the
point at which it set out, notwithstanding its great velocity.

We must next inquire how these velocities ai-e measured, and what
is their amount. We have seen that the pressure exerted by a gas is
due to what may be appropriately called the molecular bombardment
of the walls of its containing vessel ; and, knowing this pressure, we
can calculate the velocity of the projectiles, if we can ascertain their
weight, just as we can estimate the speed of a bullet when its weight
and mechanical effect are known. Kow, a cubic centimetre of hydro-
gen at a pressure of one atmosphere weighs about one thousandth part
of a gramme ; we have, therefore, to find at what rate this mass must
move — whether altogether or in separate molecules makes no differ-
ence — to produce this pressure on the sides of a cubic centimetre. The
result gives six thousand feet per second as the velocity of the mole-
cule of hydrogen, while in other gases the speed is much less.

The question of molecular weights brings us face to face with the
chemical aspect of the hypothesis ; and we have now to examine the
support which is given to it by chemical phenomena, and show how
wonderfully these are correlated with the physical proofs. Bearing in
mind the distinction between physical and chemical changes, we know
that we can make a mixture of finely divided sulphur and iron, for
example, in any proportion. But these bodies when heated combine
chemically to form a new substance called sulphide of iron ; and the
two classes of products exhibit great differences, which are indicated
by a most remarkable characteristic. Chemical combination, unlike
mechanical mixture, always takes place in certain definite proportions.
Thus, fifty-six grains of iron combine with exactly thirty-two grains
of sulphur ; and, if there is any excess of either substance, it remains
uncombined. This principle is known as the law of definite combin-
ing proportions, and the atomic theory, which, in one shape or an-



WHAT IS A MOLECULE? 691

other, is as old as philosophy, was first applied to its explanation by
the English chemist Dalton in 1807. He suggested that the ultimate
particles of matter, or atoms between which union is assumed to take
place, have a definite weight ; in other words, that they are distinct
masses of matter. In the combination of the two elements in ques-
tion, therefore, an atom of iron unites with an atom of sulphur to
form a molecule of sulphide of iron ; and the union takes place in the
proportion by weight of fifty-six to thirty-two, simply because these
numbers represent the relative weights of the two sorts of atoms.
Now, Dalton may be wrong, and there may be no such things as
atoms ; but every science postulates fundamental princijiles, of which
the only proof that can be offered is a certain harmony with observed
facts ; and the chemist assumes the reality of atoms and molecules
because they enable him to explain what would otherwise be a chaos
of unrelated facts. The combining proportions of substances, then,
indicate their relative molecular weights ; and, bearing this in mind,
we must turn again for a moment to the physical side of the question,
to inquire whether, and in what way, the physicist can determine the
weight of a molecule.

Water, alcohol, and ether expand when heated, like other forms of
matter, but they do so very unequally. Their vapors, on the other
hand, are expanded by heat at exactly the same rate under like con-
ditions. The theory supposes that the molecules which are close to-
gether in the liquids become widely separated when these are converted
into vapors ; and the action of the particles on each other becomes
less and less as they are driven farther apart by heat, until at last it
is inappreciated. When the molecules of the vapors in question are
thus freed from other influences, it is found that heat acts in an exactly
similar manner upon each of them ; and this is found to be true of all
gaseous bodies. The obvious explanation in the case before us is, that
there are the same number of particles within a given space in the
vapors of all three liquids. This is the law of Avogadro, which is
formulated as follows : " Equal volumes of all substances, when in the
form of gas, contain the same number of molecules " ; and we shall see
how simply this conception is applied for the purpose of determining
the molecular weights of all bodies which are capable of being vapor-
ized. It will be understood that we are still dealing, as in the case of
chemical combination, with relative weights only. We have no means
of ascertaining the absolute weight of a molecule of any substance ;
but we can state with perfect accuracy what relation these weights
bear to one another. For this purpose, the molecule of hydrogen,
which is the lightest body known to science, has been selected as the
unit. Calling the weight of a litre of hydrogen one, we find by the
balance that a litre of oxygen weighs sixteen ; and as, by Avogadro's
law, both litres contain the same number of molecules, the molecule
of oxygen is sixteen times heavier than that of hydrogen. The mo-



692 THE POPULAR SCIENCE MONTHLY.

lecular weight of any substance, therefore, which can be brought into
the gaseous condition, is found by simply determining experimentally
the specific gi-avity of its vapor relatively to hydrogen.

In this way the physicist ascertains the molecular weights of all
easily vaporizable bodies, and these are found to be in uniform and
exact agreement with those which the chemist deduces from the law
of combining proportions. The molecular hypothesis is thus brought
to a crucial test ; and two entirely independent lines of inquiry agree
in giving it support of such a character as compels conviction. The
law of gravitation and the undulatory theory of light do not com-
mand more cogent circumstantial evidence than this.

We have now briefly reviewed the fields from which the certain
data of molecular science are gathered. We have weighed the mole-
cules of gases, and measured their velocity with a high degree of pre-
cision. But there are other points, such as the relative size of the
molecules of various substances, and the number of their collisions
per second, about which something is known, though not accurately.

With regard to the absolute diameter of a molecule and their num-
ber in a given space, everything at present is only probable conjecture.
Still, it may be interesting to state the views which are held on these
questions by such investigators as Sir William Thomson and the late
Professor Clerk-Maxwell ; but we give these without attempting to
indicate the character of the speculations on which their conclusions
rest.

Summing up, then, both the known and unknown, Ave may say that
the molecular weights and velocities of many substances are accu-
rately known. It is also conjectured that collisions take place among
the molecules of hydrogen at the rate of seventeen million-million-
million per second ; and in oxygen they are less than half that number.
The diameter of the hydrogen molecule may be such that two million
of them in a row would measure a millimetre. Lastly, it is conject-
ured that a million-million-million-million hydrogen molecules would
weigh about four grammes ; while nineteen million-million-million
would be contained in a cubic centimetre. Figures like these convey
no meaning to the mind, and they ai-e introduced here only to show
the character and present state of the research.

A few concluding words must indicate the tremendous energy re-
siding in the forces by which the molecules of matter are bound to-
gether. The molecules of water, for example, can not be separated
from each other without changing the liquid into a gas, or, in other
words, converting the water into steam ; and this can only be accom-
plished by heat. The force required is enormous ; but, since the deter-
mination, by Joule, of the mechanical equivalent of heat, we are able
not only to measure this force, but also to express it in terms of our
mechanical standard. It has been found that, in order to pull apart
the molecules of one pound of water, it is necessary- to exert a mechan-



SKETCH OF JAMES CRAIG WATSON. 693

ical power which would raise eight tons to the height of one hundred
feet. Such is the energy with which the molecules of bodies grasp
each other ; such is the strength of the solder which binds the uni-
verse together. — Chambers^ s Journal.



SKETCH OF JAMES CEAIG WATSOK

By ALEXANDER WINCIIELL, LL. D.

JAMES CRAIG WATSOX, Professor of Astronomy in the Uni-
versity of Wisconsin, and Director of the Washburne Observa-
tory at Madison, Wisconsin, died on the morning of November 23,
1880, after an illness of one week, at the age of forty-two years and
ten months. Professor Watson was one of the most gifted and dis-
tinguished of modern astronomers, and his life-work is identified with
the name of the University of Michigan.

He was born of American parentage, during a sojourn of his par-
ents in Middlesex (now Elgin) County, Ontario, January 28, 1838.
The mathematical genius revealed by the boy at the early age of nine
determined the father to secure him a liberal education, and the fam-
ily accordingly removed to Ann Arbor in 1850. Here James displayed
equal aptitude for mathematical and linguistic studies, and, being pre-
pared for college, almost without the evidences of effort, he entered the
University of Michigan in the autumn of 1853. He attained equal
scholarly distinction as a student of ancient and modern languages and
of mathematics. It is said that, before the close of his junior year, he
had performed the phenomenal feat of reading from beginning to end
the " Mecanique Celeste " of Laplace. During his senior year, he was
the solitary pupil of Dr. Briinnow, and graduated in 1857. His mechani-
cal tact was such that, in the absence of a mathematical bent, he would
have become an eminent mechanician and inventor. While in college,
some of his spare hours were spent in grinding lenses and the construc-
tion of a telescope. Other portions of his time he was compelled to
devote to the earning of means to defray collegiate expenses.

During the two years succeeding his graduation, he was employed
as assistant in the Observatory, and in the prosecution of studies for
his second degree. In this work he displayed such remarkable apti-
tude as an observer, and such marvelous rapidity in his computations,
that, on the retirement of Dr. Briinnow, in June, 1859, young Watson
succeeded him in the chair of Astronomy. He was already known as
a frequent contributor to the " American Journal of Science," BrUn-
now's " Astronomical Notices," Gould's " Astronomical Journal," and
the " Astronomische Nachrichten," of Altona. Not less than twelve



694 ^^^ POPULAR SCIENCE MONTHLY.

communications, written before he was twenty-one, are recorded in the
Royal Society's " Catalogue of Scientific Papers," which also enumer-
ates twenty-one others between 1859 and 18T4. His wonderful keen-
ness as an observer was signalized, while yet an undergraduate, by the
discovery of a comet on the 29th day of April, 1856, and, four months
after graduation, by the discovery of a planet on the 20th of October,
1857, which, however, proved to have been obsen^ed by Luther a few
days before, and has been named Aglaia. His observations of Dona-
ti's comet, in 1858, possess a standard value, and his computation of
the orbit is recognized as authoritative. The interest awakened by
this comet prompted to the preparation of " A Popular Treatise on
Comets," published early in 1860.

In 1800 Dr. Brunnow resumed the directorship of the Observatory,
and young Watson was assigned to the chair of Physics in the uni-
versity, which he retained for three years, when, on the final retire-
ment of Dr. Brunnow, Watson was made Professor of Astronomy and
Director of the Observatory, a position which he held and honored for
sixteen years. Scarcely had he been clothed with full control of the
instruments, when he resumed his remarkable career of discovery.
There seemed almost a magic in his powers. Unrecognized celestial
objects seemed to crowd spontaneously upon his notice. On Septem-
ber 14, 1863, he made his first independent planetary discovery. This
was Eurynome. On January 9, 1864, he discovered the comet since
known as 1,863, VI, which Respighi, as it proved, had already noted.
On the 9th of October, 1865, he discovered a planet which also proved
to have been announced by Peters, and has since been named lo. He
discovered Minerva, August 24, and Aurora, September 6, 1867.
During 1868 he added no less than six minor planets to the solar sys-
tem, furnishing the only instance in which the list of planetary dis-
coverers presents the same name four times in immediate succession.

Meantime he was engaged upon a work which might well have
engrossed all his powers, and must have quite exceeded the abilities of
any but a gifted mathematical genius. It was no less than a complete
digest of the results and methods of all the great writers on theoreti-
cal astronomy, and an independent development of the great principles
of the science. " Having carefully read the works of the great mas-
ters," he says in his preface, *' my plan was to prepare a complete
work on the subject, commencing with the fundamental principles of
dynamics, and systematically treating, from one point of view, all the
problems presented." This broad plan, conceived by a young man of
twenty-eight, and completed when twenty-nine, was executed with
ability so commanding, that the work, on its appearance in 1869, was
immediately accepted as an authoritative exposition of the higher
principles and processes of dynamical astronomy, and was made a
text-book at Leipsic, at Paris, and at Greenwich. The same year he
was sent by the General Government on an expedition to observe the



SKETCH OF JAMES CRAIG WAT SOX. 695

solar eclipse at Mount Pleasant, Iowa, and, in 1870, to Carlantini, Sici-
ly, for a similar purpose. In 1874 he was appointed .to the charge of
an expedition to Peking, China, to observe the transit of Venus. His
observations were favored by the weather, and conducted with con-
summate skill. The results, though reduced and discussed, are not
yet published. Even at the antipodes, fresh discoveries awaited him.
He had already raised his list of planetary discoveries to seventeen,
and now added Jueica, the eighteenth. In 187G he was one of the
Judges of Awards at the Centennial Exposition, and wrote the cele-
brated " Report on Horological Instruments." In 1878 also appeared
his " Tables for the Calculation of Simple and Compound Interest," a
work which, in spite of the subject, is marked by great originality,
and demanded a vast amount of wearisome labor. The same year he
was sent by the General Government in charge of an expedition to
Wyoming, to observe the total solar eclipse. Professor Watson, hav-
ing long entertained a belief in the existence of an intra-Mercurial
planet, as well as of an extra-Neptunian one, gave special attention at
this time to a search for the former, and was the first astronomer to
note certainly (July 29, 1878) the existence and position of the planet
Vulcan. He also satisfied himself of the existence of a second intra-
Mercurial planet. This brought the number of his original planetary
discoveries to twenty-six (including one lost July 29, 1873, and two
anticipated). He was now animated by an intense desire to control
instruments of suitable power and adjustment to confirm his last ob-
servations, and enable him to detect the outlying planet beyond Nep-
tune. Coincidently came the invitation to assume the charge of the
Washbume Observatory at Madison, Wisconsin, which was to be
improved and newly equipped with instruments far more efficient
than those at Ann Arbor. The temptation was great, but he natu-
rally clung to his alma mater, whose authorities made such efforts
as they thought authorized to content their astronomer. But the
requisite means could only be obtained by a grant from the Legisla-
ture, a measure defeated by an inadequate appreciation of the honor
shed upon the State by such a name as Watson's. Reluctantly, but
sustained by a high and noble aspiration, he removed, in the summer
of 1879, to Madison, and immediately devoted himself with intense
energy to remodeling the observatory structure, and introducing some
original provisions thought to be suited to the special researches on
which he was bent. A cellar twenty feet deep was sunk at the bot-
tom of the first slope of Observatory Hill. Into this, light was to be
thrown through a long tube, from powerful reflectors on the top of the
hill. This, with other accessory work, was actually in progress, when
a severe cold brought on peritonitis, which over-confidence in his physi-
cal powers permitted to reach a fatal stage before medical aid was
summoned. His remains, accompanied by an escort from the Univer-
sity of Wisconsin, were removed to Ann Arbor, where they lay in



696 THE POPULAR SCIENCE MONTHLY.

state, in the university, during the 25th of November, and, on the fol-
lowing day, with due honors and imposing ceremonies conducted by
his late colleagues, were reverently laid beneath the shade of Oakwood
Cemetery.

Professor Watson possessed extraordinary intellectual endowments.
His quickness of perception nothing escaped. His mathematical intu-
itions scorned the ordinary processes of calculation, and gave him a
masterly command of mathematical logic and formulae, which made
so many portions of his work on " Theoretical Astronomy " strictly
original, and all parts virtually his own. Yet he never mentions any
claim to originality, but pursues his majestic intellectual march with
the dignity almost of an inspiration. His memory served him equally
well. It was both circumstantial and philosophical. Every new ob-
servation was immediately illuminated by all which he had previously
observed or known, and he saw instantly the proper conclusions. His
mechanical gifts gave him perfect command of instruments and their
construction, and the AVashburne Observatory would have been
equipped with several of his inventions. His versatility extended to
matters of business. He was for years the actuary of the Michigan
Mutual Life Insurance Company, and performed service pronounced
invaluable. He managed his private means with such success that he
died possessed of a considerable fortune, which his will secures to
the National Academy of Science. Physically, he was vigorous and
healthy, and reached, in the last years of his life, a weight of two
hundred and forty pounds. His religious nature held fast to the fun-
damental religious beliefs. He used to say it is impossible for a math-
ematician to be an atheist, and his works offer frequent recognition of
the being of the supreme Creator and Governor of the universe.

The woi'ld was not slow to recognize his worth. He was elected a
member of the National Academy of Science in 1867, and of the Royal
Academy of Sciences in Italy in 1870. He received the degree of
Doctor of Philosophy from the University of Leipsic in 1870, and the
French Academy of Sciences conferred upon him the Lalande gold
medal for the discovery of six new planets in one year. Yale College
honored him with the degree of Doctor of Philosophy in 1871. In
1875 the Khedive made him Knight Commander of the Imperial Order
of Medjidieh of Turkey and Egypt. He was elected member of the



Online LibraryD. S. (David Samuel) MargoliouthThe Popular science monthly (Volume 19) → online text (page 86 of 110)