PHYSICAL SCIENCES IN THE NINETEENTH CENTURY
During the nineteenth century, the path of scientific discovery might
almost be represented by a vertical line. Never before was such
rapid and marvelous progress made. The releasing of the mind from
the oppressive restrictions of earlier conservative ages liberated
the intellectual energies of mankind. A new idealistic philosophy
supplanted that of an earlier period and universal attention was given
to science and material things. Amidst these changes social science was
devolved, and, with it, the study of psychology.
But it was the physical sciences which most felt the stimulus of the
new rationalistic spirit.
The relationships between physical magnitudes are established by
measurements. When these are accurately ascertained, questions
regarding their variable functions can be solved by mathematical
principles. Physics is thus linked with mathematics through
measurements. The more science advances, the greater is the accuracy
needed in physical measurements. The strictness and clearness of
experimentation which has been attained in physics has given birth to a
science of measurement, which has its own instruments, rules, methods,
and formulæ.
Measurement of length is one of the bases of physics. It is a relative
operation carried out by comparing the length of one body with that of
another. Standards of length are preserved by a Bureau of Weights and
Measures in most countries. Delambre, a French authority on the decimal
system of measures, taught at the beginning of the nineteenth century
that magnitudes as small as the hundredth of a millimeter are incapable
of observation. The International Bureau of Weights and Measures now
guarantees to determine two or three ten-thousandths of a millimeter.
So much has the science of measurement progressed in a century.
The undulations of light rays are used for determining standard
lengths. Michelson and Benoit measured a standard length of ten
centimeters, in 1894, in terms of the wave lengths of the red, green,
and blue radiations of cadmium, and then in terms of the French
standard meter. These experiments yielded very accurate results.
The measurement of mass is another important base of physics. Mass is
the quantity of matter in a body and the action which gravity exerts
on mass is called weight. Weight does not depend entirely upon mass,
but also upon the position of the body weighed, because when the
body is weighed in one place and reweighed in another, there will be
a difference in the force of gravity due to change of latitude and
of altitude. National standards of mass have been made of alloys of
iridium and platinum.
Many remarkable measurements of time, temperature, and physical
constants were carried out during the century.
High and low temperature charts were completed, showing temperatures in
the air, the earth, and the sea. Instruments and methods were devised
for measuring any temperature whether of high furnace gases or low
freezing mixtures.
The measuring units of mass, length, time, and temperature are
fundamental, others like velocity, acceleration, power, and area are
referred to them. For that reason the latter are called derived units.
Many of these are important and call for accurate determinations.
One of the first achievements of the century was the establishment of
the doctrine of the conservation of energy.
Francis Bacon had suggested that motion is a phenomenon of heat, and
Newton had divined the principle of the conservation of energy, but
it was Benjamin Thompson, Count Rumford, who discovered the nature of
friction and made the first estimate of the mechanical equivalent of
heat. Sir Humphry Davy showed that two pieces of ice could be melted by
simply rubbing them together, in a vacuum. But he failed to draw the
great inference that this experiment warranted.
If he had observed that the heat could not have been supplied by the
ice because ice is an absorber of heat, he would have anticipated the
great work done by James P. Joule, an English physicist, who published
the results of many experiments carried out by him prior to 1843. His
task was to find the exact mechanical equivalent of heat.
His best results were secured by dropping a mass of lead from a
measured height and using the energy generated during the descent to
operate a revolving paddle in a dish of measured water. Delicate
thermometers recorded the increase of temperature in the water and
showed that the descent of 424 grams of lead through a distance of one
meter, or one gram of lead through 424 meters, generated sufficient
heat to raise one gram of water one degree centigrade (1° C.).
Otherwise expressed, a fall of 772 lbs. of lead through a distance of
1 foot, or 1 lb. of lead through 772 feet, raises the temperature of 1
lb. of water one degree Fahrenheit (1° F.). These 772 foot-pounds, or
424 gram-meters, represent the mechanical equivalent of heat upon which
so many important theories have been based. But Joule's equivalent
was determined for common air temperatures whereas the specific heat
of water increases with the temperature so that the value of the
equivalent rises with increased temperatures. Osborne Reynolds, in
1897, found the mean equivalent for temperatures between the freezing
and boiling points to be 777 foot-pounds.
The discovery of Joule's equivalent established a relationship between
motion or mechanical work performed and the amount of heat generated
when work is completely expanded in friction. The same relationships
continue good when the work is transformed by indirect means as by
generating electric currents or expanding gases. The multitude of
elegant experiments used to confirm the truth of Joule's law showed
that heat is not a substance, or calorie, but a purely mechanical
effect. This great discovery of the relation of friction and heat
lies at the basis of electricity, molecular physics, and chemistry,
and is the source of the formulæ used by engineers in designing power
machinery. The internal combustion engine is largely a result of the
discovery of Joule's equivalent and the physical theories derived from
it.
This great discovery caused a new theory of matter to be developed.
Dalton had suggested, when applying the atomic theory to chemistry,
that when two elements combine to form a third substance, it is
probable that one atom of one element joins itself to one atom of the
other, unless some exceptional condition exists. When water is formed
by bringing oxygen and hydrogen together, he supposed that one atom
of oxygen combined with one atom of hydrogen. Gay-Lussac subsequently
proved that not only does one volume of oxygen combine with two volumes
of hydrogen (not one as Dalton believed) in the production of water,
but that nitric and carbonic acid gases combine with ammonia gas in the
ratio of 1:1 or 1:2. He also demonstrated that one volume of nitrogen
united with three of hydrogen form ammonia, and that carbonic oxide
burning in a mass of oxygen consumes half its volume of oxygen. He
concluded from these and other facts that gases always combine together
in simple proportions by volume and that the apparent contraction of
volume they show on combining bears a similar simple relationship to
the volume of one or more of the gases.
Avogadro, working on Gay-Lussac's experimental data, suggested that
the number of integral molecules in any gas is always the same for
equal volumes, or is always proportional to the volumes. He also
suggested that equal volumes of different gases at the same pressure
and temperature contain the same number of molecules. Experiments
on alcohol made by Williamson raised doubts as to the validity of
Avogadro's hypotheses when applied to chemical combinations. These
doubts were cleared in 1860, when the new chemical atomic weights and
formulæ were introduced into English textbooks.
The molecular theory of matter derived from these experiments supposes
that all visible forms of matter are aggregations of simpler and
smaller chemical elements. Mendeléeff and Newlands showed that the
physical and chemical properties of the elements are functions of their
atomic weights.
Investigations of radioactivity and the observations based upon the
passage of electric currents through gases have recently modified our
views with respect to the atomic theory, but these points will be dealt
with in the chapter dealing with radiation.
Questions regarding the eventual loss of energy in matter are best
studied in gases. A considerable number of important investigations
are now being carried on in Europe with the view of tracing the
interchanges of molecular energies in gas molecules. Maxwell and other
investigators found long ago that the motion of molecules cannot go on
perpetually. The energy of motion will in time be frittered away by
friction, air resistance, collisions with other molecules, vibrations
set up by collisions, and other molecular movements. It has been found
that the energy which is dissipated by air resistance is transformed
into energy in the air. That which is lost by collisions is converted
into internal vibrations within each molecule. The question now arises
as to what effects are exerted on a gas. It involves the effects of the
communicated internal molecular vibrations and their transference of
energy to the surrounding medium. What is known as the Quantum dynamic
theory has been proposed to account for this phenomena. Quantum
dynamics appear to be distinct from the Newtonian.
Carnot and Clausius discovered that the motive power of heat is
independent of the agents brought into play for its realization. The
motive power of a waterfall depends, for example, on its height and
on the quantity of water falling within a given time. Clausius stated
the Carnot idea in mechanical terms by saying: That in a series of
transformations, in which the final is identical with the initial
stage, it is impossible for heat to pass from a colder to a warmer body
unless some other accessory phenomenon occurs at the same time. A heat
motor, which, after a series of transformations, returns to its initial
state, can only supply work, or power, if there exist two sources of
heat, and if a certain quantity of heat is given to one of the sources
which can never be the hotter of the two. The output of a reversible
machine working between two given temperatures is greater than that
of any nonreversible engine, and it is the same for all reversible
machines working between these two temperatures.
Clausius showed that this principle conduces to the definition of an
absolute scale of temperature and there is another factor assisting
in restoring physical equilibrium which he termed entropy. It is a
variable which, like pressure or volume, serves concurrently with
another variable to define the state of a body.
These discoveries of Carnot and Clausius showed the impossibility of
finding a source of perpetual motion and helped to solve many of the
difficulties in securing efficiency from internal combustion engines.
Industrial, as well as scientific results of immense importance have
developed from these principles.
Theories on the compressible fluids and elastic equilibrium were
developed as the result of work done between 1875 and 1896 by J.
W. Gibbs, Helmholtz, Duhem, and others on internal thermodynamic
potentials. These theories have proved of incalculable value in
elucidating electrical and radiation phenomena.
Another discovery of Gibbs, made in 1876, has also had brilliant
results. It is known as the Phase Law. The homogeneous substances
into which a material system is divided is called a phase. Carbonate
of lime, lime, and carbonic acid gas are the three phases of a system
which comprises Iceland spar partially dissociated into lime and
carbonic acid gas. The number of phases, combined with the number of
independent bodies entering into the reactions, fixes the general form
of the law of equilibrium of the system. This discovery of Gibbs has
resulted in greatly extending the field of physics. It is of importance
in molecular and atomic investigations, in osmosis, electrolysis, and
in most questions dealing with thermodynamics.
Light is generally defined as the sense impression received by the eye.
It was formerly believed that it was caused by streams of corpuscles
emitted by the source of light. This was known as the emission theory.
Early in the nineteenth century, the undulatory displaced the emission
theory. According to this, light is a transverse vibratory motion
extended longitudinally through the ether.
The experiments of Faraday, Maxwell, Fresnel, Hamilton, Green, and
others suggested that the undulatory theory required for its validity a
new medium different from the atmospheric air and from every substance
known to man. Just as the results of investigations into reflection,
refraction, diffraction, and polarization showed that the old
corpuscular theory of light was untenable, so these experiments seemed
to cast doubt upon both the undulatory and emission theories.
Fresnel, when studying problems in polarization, noticed that a theory
of light proposed by Hooke appeared to be true. Hooke asserted that
light vibrations are not longitudinal but transverse.
Fresnel found by his experiments that the idea of longitudinal
vibrations acting along the line of propagation in the direction of
the rays would not explain the polarization changes in light. They
suggested that there was a transverse movement perpendicular to the
ray. When Fresnel's researches were published, physicists realized
that if the transverse direction of luminous vibrations was denied
the undulatory movement of light would also be denied. Now transverse
vibrations cannot exist in any medium resembling a fluid, because it
is characteristic of fluids that, so long as the volume continues
constant, its different parts can be displaced without the appearance
of any reaction. This necessitates the assumption that light needs a
solid body for its transmission and Lord Kelvin asserted that this body
must be a solid more rigid than steel.
When the vibratory theory was accepted, it became necessary to
investigate the nature of the ether and to determine its characteristic
properties. Neumann, MacCullagh, Green, and Stokes then developed an
elastic solid theory of the ether.
The experiments of Lord Rayleigh, Lorentz, Drude, Larmor, and others
suggested that light is identical with electromagnetic disturbances
and, consequently, is an electrical phenomenon.
Some of the finest developments in physics during the nineteenth
century were in the realm of electricity. They resulted in an enormous
extension of the use of electricity in industry and commerce and led to
the investigation of radioactivities of various kinds and these in turn
are developing investigations of a most brilliant character.