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GIVEN TO
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COLLEGE OF LIBERAL ARTS
BY
MRS. JOHN EASTMAN CLARKE
MEMORY OF HER HUSBAND
JULY, 19,4
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MISCELLANEOUS
SCIENTIFIC PAPERS.
WORKS BY PROFESSOR RANKINE.
In crown 8uo, cloth.
I. A MANUAL OF APPLIED MECHANICS. Ninth Edition, L2s. 6d.
II. A MANUAL OF CIVIL ENGINEERING. Thirteenth Edition, 16a.
III. A MANUAL OF MACHINERY AND MILLWORK. Fourth
Edition, L2s. 6d.
IV. A MANUAL oF THE STEAMENGINE AND OTHEE PRIME
MOVERS. Ninth EdMm, 12s. 6d.
Y. USEFUL RULES AND TABLES for Engineers, Surveyors, and
Others. Fifth Edition
VI. A MECHANICAL TEXTBOOK; or, Introduction to the Study of
Mechanics. By Pkop. RanKINE and E. F. Bambeb, C.E. Second
Edition, 9s.
Chables Griffin and Company, London.
College of Liberal Arts
Boston University
MISCELLANEOUS
SCIENTIFIC PAPERS:
W. .1. MACQUORN EANKINE, (J.E.. LL.I).. F.R.S.,V
LATE REGIUS PROFESSOR OF CIVIL ENGINEERING AND MECHANICS IN THE
UNIVERSITY OF GLASGOW.
FROM THE TRANSACTIONS AND PROCEEDINGS OP THE ROYAL AND OTHER SCIENTIFIC
AND PHILOSOPHICAL SOCIETIES, AND THE SCIENTIFIC JOURNALS.
WITH
A. nVCEJMIOXK, OIF 1 THE ATJTHOB
By P. G. TAIT, M.A., **'
PROFESSOR OF NATURAL PHILOSOPHY IN THE UNIVERSITY OF EDINBURGH.
EDITED BY
W. J. MILLAR, C.E.,
SECRETARY TO THE INST. OF ENGINEERS AND SHIPBUILDERS IN SCOTLAND.
titfe 0thwit, f h\Us, attbf iiapams.
LONDON:
CHARLES GRIFFIN AND COMPANY,
stationers' hall court.
MDCCCLXXXI.
Sc\*f\C* ^ ^S AV \S
GLASGOW
PRINTED BY BELL AND BAIN, 41 MITCHELL STKEET
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PREFACE.
The republication of a Selection of the Papers of the late Professor
Macquorn Rankine was originally projected by several of his personal
friends shortly after his decease, the object being to combine, in a
suitable volumeform, papers which were to be found only in the Records
of Scientific Societies, and in the Scientific and Engineering Journals, and
thus to present to the many admirers of the talented author a memorial
of his great worth and ability. \
Introductory to the Selection of Papers now published is an exhaustive
Memoir by Professor Tait, who kindly consented to embody \n this
form the main features of Rankine's life, together with his recollections
of one with whom he had been intimately associated as a fellow worker.
There remains, therefore, for the Editor merely to point out, briefly, the
principle that has guided him in making the selection from the papers
placed in his hands for the purpose of republication.
The object, then, kept in view has been the preservation of such papers
as are most characteristic of their author in his capacity of a scientific
and mathematical inquirer. Professor Rankine was not, nor did he claim
to be, a popular writer; his command of mathematical expression was
such, that he naturally embodied his reasoning and conclusions in
symbols. All his writings, however, are marked by a power of state
ment so clear and logical, that the reader, even should he fail entirely
to follow the demonstrations, cannot but be benefited in the attempt to
master them.
Besides the papers in the present Collection, Professor Rankine con
tributed many others to Scientific and Engineering Societies and Journals.
A number of these, from their nature, (descriptions of works and of
machines,) were but of passing interest. The papers now published are
b
BOSTON UNIVERSITY LIBRARIES
vi PREFACE.
of permanent value, and deal rather with general scientific principles and
their applications to practice.
The leading characteristics of Professor Rankine's writings are too
wellknown to require comment here. One special feature, however, in
his method of treatment, may be pointed out — viz., the carefully arranged
division of the subject into sections. Starting with a general statement of
the object of the paper, he advances by degrees in the argument, giving full
reference to the subordinate parts of the paper, which arc marked by
numerals or letters to distinguish them, thus progressing with logical pre
cision until the conclusion is reached. Another noteworthy feature may
also be referred to — viz., that of the introduction of new and suitable words
or phrases, proposed by the author, to convey more dearly his meaning.
Lastly, we observe throughout all Professor Rankine's writings the most
minute accuracy of statement, and the mosl scrupulously honourable care
to give to all fellowworkers in the same held with himself their just due,
whilst pointing out what he considers to be original on his own part.
The principal papers in the volume are those relating to Thermo
dynamics and to Hydrodynamics, where such subjects as the Action of
ileat in the SteamEngine, and the Forms of Waves and "WaterLines of
Ships, arc discussed at length — the scientific and mathematical investi
gation of these questions being perhaps most eminently characteristic of
sor Rankine.
The papers have been grouped into three Divisions, so as to bring as
nearly as possible kindred subjects together; and in every case the
name of the Society or Journal through which the paper was originally
brought forward, together with the date of its publication, has been
given.
It need hardly be added, that the papers appear without change of any
kind. In their present form they stand precisely as they finally left their
author's pen; and no pains have been spared to ensure perfect accuracy
in the reproduction.
The Editor, like many others, has a grateful remembrance of Professor
Rankine, having enjoyed the great privilege of being one of his students
in the old College of Glasgow, and having afterwards had the advantage
of his friendship in many ways. Rankine's lectures, although simpler than
his textbooks, were marked by the same clearness of arrangement, and
were enforced by his distinct and vigorous enunciation, and admirably
illustrated by carefully prepared diagrams. As chairman of the meetings
of the societies of which he was president, his methodical habits and
business qualifications were of marked service. These qualities were also
evidenced in the drawing up of reports in committee; one of the last
services which he rendered to engineering science being in connection
with an experimental inquiry on safety valves on behalf of the Institu
PREFACE. Vli
tion of Engineers and Shipbuilders in Scotland — his decease, unfortunately,
occurring before the completion of the experiments.
The Papers classed under Part I. relate more or less to Temperature
and Elasticity. The First, originally published in 1849, gives an
approximate equation for the elasticity of vapour in contact with its
liquid, (this equation appears to have been one of the results obtained
by Professor Eankine whilst investigating the molecular constitution of
matter,) and concludes with the statement of a proposition borne out by
experiment, which " may be safely and usefully applied in practice."
In the Second Paper, published in the same year, is given a formula
for calculating the expansion of liquids by heat. This formula, the
author states, he had found useful whilst considering the comparative
volumes of liquids at various temperatures.
The Third Paper, on " The Centrifugal Theoiy of Elasticity, as applied
to Gases and Vapours," published in 1851, shows how the laws of the
pressure and expansion of gases may be deduced from the hypothesis of
molecular vortices. The investigation was begun in 1842, but was laid aside
on account of the want of experimental data ; it was again resumed after the
publication of the results of M. Kegnault's experiments on gases and
vapours, and laid before the Royal Society of Edinburgh in February, 1850.
The hypothesis is defined to be " that which assumes that each atom of
matter consists of a nucleus or central point enveloped by an clastic atmosphere,
which is retained in its position by attractive forces ; and that the elasticity due
to heat arises from the centrifugal force of these atmospheres revolving or
oscillating about their nuclei or central points." After showing that some
what similar ideas had been entertained by philosophers at different
times, there follows a supposition which, he says, " so far as I am aware,
is peculiar to my own researches. It is this, that the vibration which,
according to the, undulatory hypothesis, constitutes radiant light and heat, is a
motion of the atomic nuclei or centres, and is propagated by means of their
mutual attractions and repulsions.'" Tables are given showing the closeness
of agreement between the formula? made use of, and the experimental
results.
In the Fourth Paper we have an extension of the preceding one ; by
means of a fresh investigation the complete applicability of the hypothesis
of molecular vortices to all substances in all conditions is shown, and
this demonstration is followed by the deduction from that hypothesis of
the law of the equivalence of heat and power.
The Fifth Paper deals with the Laws of Elasticity in reference to
the strength of structures — the relations between pressures and strains ;
it shows how the Laws of Elasticity are simplified by adopting the
supposition of atomic centres of force, but also points out that this
nil PREFACE.
supposition requires modification, and by means of the hypothesis of
molecular \ orl ici Bimplifie I he i ion of the
In the Sixth Paper the distinction between Strain and Stress is made
clear, and a nomenclature adopted (from Greek equivalents) di icriptive of
their relations. Cr talline fo] a idered, to pith their
ad ion on li;
The Seventh Paper treats of "The Vibrations of PlanePolari ed Light."
The principles laid down in the paper are shown to be incompatible
with the "idea of a luminiferous ether enveloping ponderable particles;"
much as thai "the Luminiferous medium is a system of atomic nuclei
or centres of force, whose • give form to matter; while the
atmo n by which they are surrounded give of themselves merelj
extension."
In the Eighth Paper an atl mpl is made to diminish the difficul
attending the undulatory theory of light, hy proposing a theory of
oscillations round axes, in tead of the theory of vibratii
In ill" Ninth Paper, we have :i mathematical investigation into the
relations existing between the velocity of sound in tances and
; he elastic] of the m
Pari II. relates principally to Energy and the Mechanical Action of
The first Paper of this division  entitli d " The Etecon
ition of the Mechanical : Universe" a remarkable
speculation, laid before the Briti iation al Belfast, and published
in 1852, alluded to by Professor Jevons hi bis Principles of Science,
and acknowledi I by Sir Wm. Thomson f ;. characteristic of
Rankine. This paper, after referring to all experimental evidenci
being in favour of the doctrine of the mutual convertibility of the
different kind i of I he pi of the universe, ami of the tendency
irds a uniform temj of matte] a to point out how it is
eivable that ultimately the diffused energy may he gathered into
foci, and renewed chemical power produced from the now inert
compounds when passing through Hie intense heat of these foci.
Tie' next Paper deals with the Law of the Transformation of Enei
and the ratio <>f Work done {,, Energy expended in various forms of
engines; and is followed by a comprehensive paper entitled " Outlines of
Science of Energetics," treating of the laws of physical phenomena —
the Sri, 'lice of Mechanics (" the Onlj example yet existing of a complete
physical theory") — the use of hypotheses— definitions of Energy, Work,&c,
and Efficiency of Engines.
The Thirteenth Paper is mainly descriptive of the use of the term Energy,
and gives Professor Rankine's reason for his introduction of the phra c
" I'ui, ntial Energy."
PREFACE. ix
Xo. XIV., with its supplements XV.XIX., on "The Mechanical
A.ction of Heat," is an important contribution of considerable length
to the Royal Society of Edinburgh, extending over the years 18501853.
It is based upon the hypothesis of molecular vortices, and relates chiefly
to the " mutual conversion of heat and mechanical power, by means of
the expansion and contraction of gases and vapours."
In the Twentieth Paper the various conditions existing in a heated
substance are shown geometrically by curves, a method adopted first, it
is stated, by James "Watt in his Steamengine Indicator. The efficiency
of thermodynamic engines is considered, and Stirling and Ericsson's hot
air engines described.
The next Paper is on "Formulae for the Maximum Pressure and Latent
I [eat of Vapours," followed by one on " The Density of Steam," in which
the general equation of thermodynamics, stated in paper Xo. XIV., is
again given, to show the connection existing between it and the law of the
density of steam.
In Paper Xo. XXIII. the Two Laws of Thermodynamics are stated, and
it is shown that the derivation of the Second Law from steady molecular
motion (e.g., in circular streams or in circulating streams of any figure),
as given in previous papers, may lie more simply effected than by the
methods adopted in these papers.
The following Paper, published in The Engineer in 1867, refers to
the want of popular illustrations of The Second Law of Thermodynamics,
and explains the nature of the two laws, with the particular questions to
which they are respectively applicable.
"The First Law informs us that when mechanical work is done by
means of heat, a quantity of heat disappears. ... To calculate this
disappearance of heat, the work done must be sensibly external, and subject
to direct measurement."
" The Second Law informs us how to deduce the whole amount of work
done, internal and external, from the knowledge which we have of the
external work."
An illustration is given by the expansion of a perfect or sensibly perfect
gas ; but it is pointed out that it is different when we have to deal with
fluids in the act of evaporating, instanced by a case showing where the
second law is applicable.
Paper Xo. XXVI. is on "The Working of Steam in Compound
Engines," defines such engines, and states their advantages, with rules for
the construction of indicatordiagrams.
This is followed by Papers on "The Theory of Explosive GasEngines,"
and on " The Explosive Energy of Heated Liquids." In the first of these
it is shown that, in calculations respecting the practical use of heat
engines, it is convenient to use pressures and volumes rather than
X PREFACE.
temperatures. The mixtures of gas and air most suitable for gasengines
are also given.
The Papers in Part III. relate to WaveForms, Propulsion of Vessels,
Stability of Structures, &c.
The first of the series, No. XXIX., which is entitled " On the Exact
Form of Waves near the Surface of Deep Water," shows that the form
of such waves is trochoidal, and states that this form was first pointed
out by Mr. Scott Eussell.
The next Paper, "On Plane WaterLines," investigates the curves
suitable for the waterlines of a ship. Waterline curves are designated
Neoids, Cyclogenous Neoids (or waterline curves generated by circles),
Oogenous Neoids (or those generated by oval bodies), and Lissoneoids, or
waterdines of smoothest gliding. It is noticed that although, from
lengthened practice in the art of shipbuilding, the forms of waterlines
have attained a high degree of excellence, jet that this is due rather
to empirical means, than to a knowledge of general principles. The
system of Chapman is shown to be wholly empirical, consisting of
parabolic forms ; and Mr. Scott Russell's is instanced as the first useful
theory of ships' waterlines, being based on wave figures. The various
forms of waterlines are then considered in reference to their fitness for
different classes of vessels.
Other two Papers follow, the first of which is intended to assist those
who are not familiar with the higher mathematics in understanding the
subject of StreamLines. A streamline is defined as the line traced by a
particle in a current of fluid, and an elementary method is given for
determining circular streamlines, a subject mathematically investigated in
the preceding paper. The other Paper is an investigation " to determine
the relations which must exist between the laws of the elasticity of any
substance, whether gaseous, liquid, or solid, and those of the wavelike
propagation of a finite longitudinal disturbance in that substance." A
Paper on " The Theoretical Limit of the Efficiency of Pro} tellers " follows,
showing the theoretical limit of efficiency which improvements in pro
pellers may attain ; states the formulae for reaction and effective power,
shows at what relative velocities the propeller is most effective, and
compares the advantages of various forms of propeller, with numerical
examples.
Paper No. XXXIV., " On the Design and Construction of Masonry
Dams," originally consisted of a report to the municipality of Bombay,
made in 1870, in reference to proposed extensions of the Water Works
there, and was afterwards published in The Engineer.
This Paper enters into the question of stability of structures, showing
the best and most economical form which a high masonry reservoir wall
PREFACE. Xi
should have, and is of value to the civil engineer when proposing to
adopt a masonry or concrete wall, instead of an ordinary embankment, for
reservoir purposes.
Papers Xos. XXXV. and XXXVI. are extensions of methods adopted
by Professor Eankine in his Manual of Applied Mechanics, in connection
with the stability of structures of various figures, such as Frames and
Arches ; and the last paper of the series, Xo. XXXVII., is a mathematical
demonstration of a property of certain curves, bearing on the forms of the
slopes of wavecrests.
In conclusion, the Editor desires to acknowledge the courtesy shown
by the executive officers of the various Societies and Journals to which
the papers, thus brought together, were originally contributed: and to
express his thanks — not only for the permission, readily accorded, to
republish — but also for the kindness which supplied, in many instances,
copies of the Papers selected.
W. J. MILLAP.
Glasgow,
October, 18S0.
CONTENTS.
PART I.
PAPERS RELATING TO TEMPERATURE, ELASTICITY, AND
EXPANSION OF VAPOURS, LIQUIDS, AND SOLIDS.
I'AGE
I. ON AN EQUATION BETWEEN THE TEMPERATURE AND
THE MAXIMUM ELASTICITY OF STEAM AND OTHEE
VAPOURS.
Table of Vapour of Water— Formulae for calculating Temperature and
Pressure — Values of Constants— Tables of Vapour of Alcohol, Ether,
Turpentine, Petroleum, and Mercury, 1 — I'
ll. ON A FORMULA FOR CALCULATING THE EXPANSION OF
LIQUIDS BY HEAT.
Table of Constants — Tables of Expansion of Water, Mercury, Alcohol, and
Sulphuret of Carbon, ........... 13 — 15
III. ON THE CENTRIFUGAL THEORY OF ELASTICITY, AS
APPLIED TO GASES AND VAPOURS.
Hypothesis of Molecular Vortices— Relations between the Heat and
Elasticity of a Gaseous Body— Temperature— Specific Heat— Coefficients
of Elasticity and Dilatation of Gases — Table of Coefficients of Dilatation
Elasticity of Vapour, and Tables of Constants for the same— Mixtures
of Gases and Vapours, 16— 4S
IV. ON THE CENTRIFUGAL THEORY OF ELASTICITY AND ITS
CONNECTION WITH THE THEORY OF HEAT.
Relations between Heat and Expansive Pressure— Hypothesis of Molecular
Vortices— Elastic Pressures— Temperature and Specific Heat— Heat and
Expansive Power— Latent Heat— Joule's Law— Carnot's Law, . . 49—66
V. LAWS OF THE ELASTICITY OF SOLID BODIES.
Science of Elasticity as applied to Strength of Structures— Strains and
Molecular Pressures— Homogeneous Bodies— Eigidity— Hypothesis of
Atomic Centres— Hypothesis of Molecular Vortices— Coefficients of Pli
ability, Extensibility, and Compressibility— Modulus of Elasticity
Tables of Coefficients of Elasticity— Rigidity, Extensibility, and Com
pressibility—Application of Method of Virtual Velocities to the Theory
of Elasticity— Proof of the Laws of Elasticity by the Method of Virtual
Velocities, 67118
XIV CONTENTS.
VI. ON AXES OF ELASTICITY AND CRYSTALLINE FORMS.
PAGE
Definition of Axes of Elasticity — Strain — Stress Potential Energy —
Coefficients of Elasticity— Symmetry— Polarised J ,ight, .... 110 149
VII. ON THE VIBRATIONS OE PLANEPOLARISED LIGHT.
Theories — Hypothesis of Molecular Vortices Experiments on Light, . 150—155
VIII. GENERAL VIEW OE AN OSCILLATORY THEORY OE
LIGHT.
Various Hypotheses — Polarised Light— Hypothesis of Oscillations — Diffrac
tion — WaveSurface in Crystalline Bodies Reflexion — Refraction —
Dispersion, 150 — 107
IX. ON THE VELO( [TY OF SOUND IX LIQUID AXI) SOLID
BODIES OF LIMITED DIMENSIONS, ESPECIALLY ALONG
PRISMATIC MASSES OF LIQUID.
Application to Elasticity of Materials— Vibratory Movement in Homo
ueous Bodies Transmission of a Definite Musical Tone— Propagation
of Sound, 168—199
PART II.
PAPERS RELATING TO ENERGY AND ITS TRANSFORMATIONS,
THERMODYNAMICS, MECHANICAL ACTION OF HEAT
TN THE STEAMENGINE, .\r.
X. ON THE RECONOENTRATION OF THE MECHANICAL
ENERGY OF THE UNIVERSE.
Convertibility of the different kinds of Physical Energy — Interstellar
Medium— Diffusion, an 1 probable Reconcentration of Radiant Heat, . 200—202
XL ON THE GENERAL LAW OF THE TRANSFORMATION OF
ENERGY.
Actual or Sensible Energy— Potential or Latent Energy— Conservation of
Energy— Transformation of Energy, 203—208
XII. OUTLINES OF THE SCIENCE OF ENERGETICS.
Physical Theory— Science of Mechanics— Mechanical Hypotheses— Science
of Energetics— Substance— Property— Mass— Accident— Effort— Work
Equivalence of Energy and Work— Equality of Action and Reaction
Potential Equilibrium— Transformation of Energy — Rate of Transforma
tion — Metabatic Function — Carnot's Function— Efficiency of Engines
Diffusion of Actual Energy— "Frictional Phenomena "—Measurement of
Time, 209—228
CONTENTS. XV
XIII. ON THE PHRASE " POTENTIAL ENERGY," AND ON THE
DEFINITIONS OF PHYSICAL QUANTITIES.
PAGE
Potential Energy— Sir John HerschePs Views— Newton's Method— Force —
The word "Energy" substituted by Dr. Thomas Young for Visviva—
Extension of Application by Sir William Thomson— Qualification of the
Noun "Energy" by the Adjectives "Actual" and "Potential," pro
posed by Eankine, and adopted by others— Carnot's "Force Vivi
Virtudlc "—Definitions of the Measurement of Time, Force, and Mass, . 229—233
XIV. ON THE MECHANICAL ACTION OF HEAT, ESPECIALLY
IN GASES AND VAPOURS.
Hypothesis of Molecular Vortices— Mutual Conversion of Heat and Expan
sive Power— Joule's Experiments— Latent Heat— Conservation of Vis
viva— Carnot's Theory of Heat— Specific Heat— Thermal Unit— Total
Heat of Evaporation— Power of the SteamEngine— Pambour's Principle
— Numerical Examples from Cornish Engines — Efficiency of Engines
Tables of Pressure and Volume of Steam, 234284
XV. NOTE AS TO THE DYNAMICAL EQUIVALENT OF TEM
PERATURE IN LIQUID WATER, AND THE SPECIFIC
HEAT OF ATMOSPHERIC AIR AND STEAM : Being a Supple
ment to a Paper On the Mechanical Action of Heat.
Joule's Experiments— Specific Heat of Air and Steam, .... 285—287
XVI. ON THE POWER AND ECONOMY OF SINGLEACTING
EXPANSIVE STEAMENGINES: Being a Supplement to the
Fourth Section of a Paper On the Mechanical Action of Heat.
Cornish Engines — Wicksteed's Experiments, 288— 299
XVII. ON THE ECONOMY OF HEAT IN EXPANSIVE MACHINES.
Carnot's Law— Clausius and Thomson's Investigations— Action of Steam in
Engine, 300—306
XVIII. ON THE ABSOLUTE ZERO OF THE PERFECT GAS
THERMOMETER.
Measurement of Temperature— Absolute Zero of Temperature, . . . 307309
XIX. ON THE MECHANICAL ACTION OF HEAT.
Properties of Expansive Heat— Heat Potential— Properties of Temperature
—Hypothesis of Molecular Vortices— Sir Humphry Davy's Researches
Thermic Phenomena of Currents of Elastic Fluids, 310—338
XX. ON THE GEOMETRICAL REPRESENTATION OF THE
EXPANSIVE ACTION OF HEAT, AND THE THEORY
OF THERMODYNAMIC ENGINES.
Introduction of IndicatorDiagram and Diagrams of Energy— Isothermal
Curves— Curves of No Transmission of Heat— Total Actual Heat— Latent
Heat of Expansion— Law of the Transformation of Energy— Thermo
dynamic Functions— Curves of Free Expansion— Thomson and Joule's
Experiments— Velocity of Sound— Efficiency of Thermodynamic Engines
XVI CONTENTS.
PAGB
— Use of Economiser or Regenerator— Stirling and Ericsson's AirEngines
Hypothesis of Molecular "Vortices— Relation between Temperature and
Actual Heat — Efficiency of AirEngines Liquefaction of Vapour by Ex
pansion under Pressure — Efficiency of a VapourEngine — Heat in a
I 'ound of Coal — Wiredrawn Steam — Composite VapourEngines < iurves
of Eree Expansion for Nascent Vapour Total Heat of Evaporation —
Approximate Law for a Vapour which is a Perfect Gas, .... 339—409
XXI. ON FORMULAE FOB THE MAXIMUM PRESSURE AND
LATENT HEAT OF VAPOURS.
Elasticity of Vapours— Regnault's Experiments— Comparison of Formulae
with Experiments, 410—416
XXII. ON THE DENSITY OF STEAM.
General Equation of Thermodynamics, and its Application to the Latent
Heat and Density of Steam Fairbairn and Tate's Experiments, . . 417—426
XXIII. ON THE SECOND LAW OF THERMODYNAMICS.
First Law Defined Second Law Defined Absolute Temperature — Steady
Motion — Metamorphic Function, 427 — 431
XXIV. ON THE WANT OF POPULAB [LLUSTRATIONS OF THE
SECOND LAW OF THERMODYNAMICS.
First and Second Laws— Exl rnaland Internal Wori r ofaPer
fect Heat Engine— Hypothe i of Molecular Vortices, .... 432—438
XXV. EXAMPLES OF THE APPLICATION OF THE SECOND LAW
OF THERMODYNAMICS TO A PERFECT STEAMENGINE
AND A PERFECT AIRENGINE.
Definition of a "Perfect Engin B aerator— Examples from Steam
Engine and AirEngine Economical Action of Regenerator in anAir
— ( iomparison of Economy of Steam and AirEn in< I [eat due to
Combustion of a Pound of Coal, 139 — 153
XXVI. OX THE WORKING OF STEAM IN COMPOUND ENGINES.
Principal Kinds of Compound Engines, and their Advantages— Number of
Cylinders used— Diagrams of the High and Low Pressure Cylinders
Rates of Expansion— Theoretical Expansion Diagram— The Common
Hyperbola a good Approximation to the true Expansion Curve — Calcula
tion of Mean Absolute Pressure and of Indicated Work — Theoretical
Diagrams of Compound Engines with and without Reservoirs, . . ;."J4 — 4G3
XXVII. OX THE THEORY OE EXPLOSIVE GASENGINES.
Thermodynamical Propositions — Rules as to Heat and Expansion— Indicator
Diagram of an Explosive Engine — Total and Available Heat of Explosion —
Mixture of Air and ( Jas— Adiabatic Curves— Indicated Work— Efficiency, 4G4— 470
XXVIII. OX THE EXPLOSIVE EXERGY OF HEATED LIQUIDS.
General Formulas for all Fluids — Efficiency of Projection— Fullpressure
Steam Gun— Expenditure of Water and HeatEfficiency— Full pressure
Dry Steam Gun, • 471480
CONTENTS.
PART III.
PAPERS RELATING TO WAVE FOP MS, PROPULSION OF VESSELS,
STABILITY OF STRUCTURES, ; \r.
XXIX. OX THE EXACT FORM OF WAVES NEAR THE SURFACE
OF DEEP WATER.
PAGE
Trochoidal Form of Waves first stated by Mr. Scott Russell— Demonstra
tion of Trochoidal Form— Height of Crests— Cycloidal Waves— Friction
between a Wave and a Waveshaped Solid— Mr. Stokes' Investigations, . 481—494
XXX. ON PLANE WATERLINES IN TWO DIMENSIONS.
Plane WaterLines Defined— Flow of Liquid past a Solid— ^WaterLine
Functions— Water Line Curves generated by Circles and Ovals— Velocities
of Gliding— Lines of Smoothest Gliding— Orbits of the Particles of Water
—Trajectory of Transverse Displacement— General Problem of the Water
Line of Least Friction— Previous Systems of WaterLines— Chapman's
System— Scott Russell's System— Preferable Figures of WaterLines—
Lissoneo'ids compared with Trochoids— Combination of Bow and Stern
Provisional Formula for Resistance • • 495— 5_1
XXXI. ELEMENTARY DEMONSTRATIONS OF PRINCIPLES
RELATING TO STREAMLINES.
StreamLines Explained— Condition of Perfect Fluidity— Circular Stream
Lines— Straight StreamLines— Compound StreamLines, . . . 522—529
XXXII. ON THE THERMODYNAMIC THEORY OF WAVES OF
FINITE LONGITUDINAL DISTURBANCE.
Mass Velocity— Waves of Sudden Disturbance— Thermodynamic Conditii his
— Adiabatic State— Absolute Temperature— Previous Investigations, . 530  543
XXXIII. ON THE THEORETICAL LIMIT OF THE EFFICIENCY
OF PROPELLERS.
Slip, Reaction, and Efficiency of a Propeller— Energy Expended— Useful and
Lost Work— Forms of Screw Propellers— Examples from Practice, .
54454'.)
XXXIV. REPORT ON THE DESIGN" AND CONSTRUCTION OF
MASONRY DAMS.
Material and Mode of Building— Form and Limits of Pressure— Mathemati
cal Principles of the Profile Curves, 550 501
XXXV. ON THE APPLICATION OF BARYCEXTRIC PERSPEC
TIVE TO THE TRANSFORMATION OF STRUCTURES.
Parallel Projection — Application to Skew Arches, &c, .... 562—563
XXXVI. PRINCIPLE OF THE EQUILIBRIUM OF POLYHEDRAL
FRAMES, ~ lM
XXXVII. ON A PROPERTY OF CURVES FULFILLING THE
CONDITION^ + 5^ = 0, 5G5  n6T
MEMOIE.
The life of a genuine scientific man is, from the common point of view,
almost always uneventful. Engrossed with the paramount claims of
inquiries raised high above the domain of mere human passions, he is
with difficulty tempted to come forward in political discussions, even when
they are of national importance ; and he regards with surprise, if not
with contempt, the petty municipal squabbles in which local notoriety
is so eagerly sought. To him the discovery of a new law of nature, or
even of a new experimental fact, or the invention of a novel mathematical
method, no matter who has been the first to reach it, is an event of an
order altogether different from, and higher than, those which are so
profusely chronicled in the newspapers. It is something true and good
for ever, not a mere temporary outcome of craft or expediency. "With
few exceptions, such men pass through life unnoticed by, almost unknown
to, the mass of even their educated countrymen. Yet it is they who, far
more than any autocrats or statesmen, are really moulding the history of
the times to come. Man has been left entirely to himself in the struggle
for creature comforts, as well as for the higher appliances which advance
civilisation ; and it is to science, and not to socalled statecraft, that he
must look for such things. Science can and does provide the means;
statecraft can but more or less judiciously promote, regulate, or forbid
their use or abuse. One is the lavish and utterly unselfish furnisher of
material good, the other the too often churlish and ignorant dispenser of
it. In the moral world their analogues are charity and the relieving
officer! So much it is necessary to say for the sake of the general
reader; to the world of science no apology need be made. In it
Rankine's was and is a wellknown name.
It is high eulogy, but strictly correct, to say that Eankine holds a
prominent place among the chief scientific men of the last half century.
He was one of the little group of thinkers to whom, after the wondrous
Sadi Carnot, the world is indebted for the pure science of modern thernio
xx MEMOIR.
dynamics. "Were this all, it would be undoubtedly much. But his
services to applied science were relatively even greater. By his admirable
teaching, his excellent text books, and his original memoirs, he has done
more than any other man of recent times for the advancement of British
Scientific Engineering. He did not, indeed, himself design or construct
gigantic structures, but he taught, or was the means of teaching, that
invaluable class of men to whom the projectors of such works entrust
the calculations on which their safety as well as their efficiency mainly
depend. For behind the great architect or engineer, and concealed by
his portentous form, there is the real worker, without whom failure
would be certain. The public knows but little of such men. Not every
von Moltke has his services publicly acknowledged and rewarded by his
imperial employer! But he who makes possible the existence of such
men confers lasting benefit on his country. And it is quite certain that
Rankine achieved the task.
William John Macquorn Rankine was born in Edinburgh on the
nth July, 1820. lie was a Scot of Scots. His father was descended
from the Rankines of Carrick and the Cochranes of Dundonald. His
maternal grandfather was Grahame of Drumquhassle, a descendant of tin;
Grahams of Dougalston. In Rankine's .MSS. there is to be found a
tracing of the various steps of his pedigree from Robert the Bruce. His
father, David Rankine, was in youth Lieutenant in the 21st Regimenl (RiHc
Brigade) ; but, as will be seen by what follows, was also a man of great
general information, especially in practical matters. He was employed in
later life in constructing railways, ami afterwards became Secretary to the
Caledonian Railway Company. Rankine repeatedly notes in his journal
the hints and instruction he had received from his father. He was
profoundly attached to his parents; and one of the most touching notes
in his journal is the brief record of his lasting obligations to them for
early instruction in the fundamental principles of the Christian religion
and the character of its Founder.
It will be convenient to give, in the first place, a brief sketch of
Rankine's career, and to reserve for a time such comments upon his more
important investigations and treatises as would materially interfere with
the continuity of the sketch.
From Rankine's private journal it appears that his first introduction to
arithmetic and elementary mathematics, mechanics, and physics was
obtained mainly from his father. He attended Ayr Academy in 18289,
and Glasgow High School in 1830. After this he seems to have been for
some years privately instructed in Edinburgh, his state of health preventing
his being sent to a public school. In December, 1831:, his uncle, Archibald
:
MEMOIR. XX i
Grahame, presented him with a copy of Newton's Frincijiia, which he read
carefully. He remarks — " This was the foundation of my knowledge of the
higher mathematics, dynamics, and physics. My knowledge of the higher
mathematics was obtained chiefly by private study." About this period
he paid much attention to the theory of music. In 183G he studied
Practical and Theoretical Chemistry under David Boswell Eeid ; and in
November of the same year entered Edinburgh University. He there
attended the Natural Philosophy course under Professor Forbes, and gained
(before completing his seventeenth year) the Gold Medal for an essay on
the "Undulatory Theory of Light." In the summer of 1837, he studied
Natural History under Professor Jameson, and Botany under Professor
Graham. He attended the Natural Philosophy Class a second time in
1S378, and obtained an extra prize for an essay on " Methods in Physical
Investigation." He records in his journal, that in 18368, during
leisure, he read much metaphysics, chiefly Aristotle, Locke, Hume, Stewart,
and Degerando. I have learned from himself that about this period he
" wasted " a great deal of time in the fascinating but too often delusive
pursuit of " Theory of Numbers."
In 18378 he made his first accjuaintance with the practice of engineer
ing, having assisted his father in superintending the works of the Leith
branch of the Edinburgh and Dalkeith Eailway. In the latter year he
became a pupil of the late Sir John Macneill, C.E., having among his
fellowpupils many who have since risen to eminence. His journal records
the names of G. W. Hemans, J. W. Bazalgette, W. E. Le Fanu, Matthew
Blackiston, John Moffat, and Jonas S. Stawell.
During the succeeding four years he was employed by Macneill on
various surveys, and schemes for river improvements, waterworks, and
harbours for Ireland. Also, for some time, on the Dublin and Drogheda
Railway. "While engaged on this railway in 1841, he contrived the method
of setting out curves " by chaining and angles at circumference combined,"
which has since been known as " Eankine's method."
In 1842 appeared his first published work, a pamphlet entitled An
Experimental Inquiry into the advantage of Cylindrical Wheels on Railways.
This was based upon experiments suggested to him by his father, and
carried out by them together.
In the same year Queen Victoria visited Edinburgh for the first time,
and Eankine was charged with the superintendence of the erection of the
huge bonfire which blazed on the top of Arthur's Seat. He constructed
it with radiating air passages under the fuel, and succeeded, as he com
placently records, in partially vitrifying the rock !
During this and the succeeding year he sent several papers to the
Institute of Civil Engineers, for some of which prizes were awarded to
him. He records that most of them were based on suggestions by his
XX11 MEMOIR.
tather, especially that on the " Fracture of Axles." He showed that such
/'iclures arose through gradual deterioration or fatigue, involving the
gradual extension inwards of a crack originating at a squarecut shoulder.
In this paper the importance of continuity of form and fibre was first
shown, and the hypothesis of spontaneous crystallisation Avas disproved.
In 18445 he was employed under Locke and Errington on the Clydes
dale Junction Railway project; and afterwards, till 1S48, on various
schemes promoted by the Caledonian Railway Company. In 18456
he engineered a project for Edinburgh and Leith Waterworks, which was
defeated by the opposition of the Edinburgh AVater Company.
About 1848 he seems to have commenced that extensive series of
researches on molecular physics which occupied him at intervals during
the rest of his life, and which constitutes bis chief claim to distinction in
the domain of pure science. The first paper he published on the subject,
with the title "Elasticity of Steam," appeared in the Edinburgh New
Philosophical Journal in July, 1849; and at the end of that year he sent
to the Royal Society of Edinburgh his great paper on the "Mechanical
Action of Heat," It was not, however, read to the Society till February,
1850. On the contents of this, and his subsequent papers dealing with
similar subjects, some remarks will lie made below. In duly, 1850, he
read to the British Association at Edinburgh another paper on a closely
connected subject, "Elastic Solids."
In 1852 the Loch Katrine Waterworks scheme for the supply of
Glasgow was revived by Rankine and John Thomson. This scheme, now
successfully carried out, was first proposed by Lewis Cordon and Laurence
Hill, Junior.
In 1853, one of Rankine's most characteristic papers in pure science,
•• On the General Law of Transformation of Energy," was read by him to
the Glasgow Philosophical Society. In the same year, along with the late
J. R. Xapier, he projected and patented a new form of airengine. The
patent was afterwards abandoned.
He was now elected to the Fellowship of the Royal Society, and sent
to that body his next great paper on Thermodynamics — viz., " On the
Geometrical Representation of the Expansive Action of Heat," which was
printed in the Philosophical Transactions.
From January to April, 1855, he acted in Glasgow University as substi
tute for Professor Lewis Gordon, on whose resignation he was appointed
to the Chair of Engineering, which he held till his death. His inaugural
discourse, delivered on Dec. 10 of the lastmentioned year, bore the title
" De concordld inter Scientiarum Machinalium Contem/plationem et Usum." In
this year he wrote, among several contributions to Kichol's Cyclopcedia, an
article on " Heat, Mechanical Action of," the earliest formal treatise on
Thermodynamics in the English language. In 1856, the preparation of
memoir. xxiii
his course of lectures led him to the invention of some remarkable
methods connected with Transformation of Structures. These are based
on the discovery of " reciprocal diagrams " of frames and forces, since
greatly extended and simplified by ClerkMaxwell. The remarkable
storm which occurred on February 7 of this year, directed his inquiries to
the " Stability of Chimneys," on which he has published a valuable article.
In 1857, he resigned the associateship of the Institute of Civil
Engineers ; and shortly afterwards, on the establishment of the Institute
of Engineers in Scotland, he delivered the opening address as first
President. At this time he was busily engaged on a Treatise on Ship
building, his Manual of Applied Mechanics, an article on the same
subject for the Encyclopedia Britannica, and an investigation (based on
J. R. Napier's experiments) of the theory of skinresistance of ships. He
also sent to the French Academy of Sciences a memoir, — " Be TE<£uililre
intdrieure d'un corps solide, elastigue, ct homogene."
In July, 1859, an offer of service was sent to the Lieutenancy by the
" Glasgow University Rifle Volunteers." It was accepted in October,
and Rankine received his commission as Captain. He spent the greater
part of November at the Hythe School of Musketry, and, on his return,
instructed the officers and sergeants of his corps. In the same year
appeared his valuable Ma mud of Ihc SteamEngine and other Prime Movers.
In 1860, he was made Senior Major, and commanded the second
battalion of his regiment at the memorable Volunteer Review, held by the
Queen in the Queen's Park, Edinburgh.
In 1861, he finished his Manual of Civil Engineering, which was pub
lished early in the following year. At the International Exhibition in
1862, at London, he was a Juror in Class VIII, " Machinery in General."
In 1864, he resigned his commission in the Volunteers, "finding it
impossible to attend at once to duties as fieldofficer and as professor, to
engineering business, and to literary work, especially Treatise on Ship
building."
In 1865, he was appointed Consulting Engineer to the Highland and
Agricultural Society of Scotland ; and became a regular contributor to the
Engineer, in which many excellent articles of his appeared.
In 1866, was published his Treatise em Shipbuilding, Theoretical and
Practiced. Though four names were announced on the title page as joint
authors, by far the greater part of the work was written by Rankine;
but the proofs were revised by all four.
1869 produced Machinery and Millwork, the fourth of Rankine's great
engineering treatises. The other three had then reached their eighth,
sixth, and fifth editions respectively. In his journal for this year occurs
the following note : —
" Sept. 16. Thomas Graham, Master of the Mint, died [son of a cousin
xxiv MEMOIR.
of my mother's father], I applied for vacancy to Chancellor of Exchequer
(Lowe). Application well supported by friends, and civilly received ; but
the office was virtually abolished, being conjoined with the Chancellorship
of the Exchequer."
Eankine lost his father in 1870, and his mother in the following year.
Both had passed the age of seventy. The loss of his parents, to whom he
was so fondly attached, seems to have accelerated the development of the
illness which had for some j  ears been growing upon him. He was well
enough, in 1871, to contribute most valuable matter to the proceedings
of the t; Committee on Designs for Ships of War," which was appointed
shortly after the loss of the " ( laptain." He investigated for that Committee
the "Stability of Unmasted Ships of Low Freeboard," and the "Stability
of Ships under Canvas."
In February, 1872, Rankine completed his memoir of his friend, John
Elder, and in July reported on the cause of the explosion of the Tradeston
Flour Mill. In May the increase of the endowment of his chair, which
he had in vain sought from Government, was given by Mr. Elder's widow;
and the income of Rankine's post was at last made sufficient to maintain
its occupant. But by this time his energy was fast failing, the simplest
work fatigued him; and he died on December 24, of a general decline
rather than of any special disease. He had been for some years liable to
violent headaches, and towards the close of his life these affected his sight.
They were probably symptoms of heart disease, which ultimately developed
paralysis. The gradual decay of his physical powers is painfully evident
in the last pages of his journal, where, though the substance is correct
and to the point, the handwriting becomes more and more irregular at each
succeeding entry.
Such are the more prominent events in the life of this great and good
man. Even now, after the lapse of eight years, it is difficult to realise the
fact of his death. He was so manysided, and yet so complete in himself,
that the mental image of him formed by each of his friends remains
almost as clear and distinct as if it had been formed but a few days
ago.
Of the man himself it is not easy to speak in terms which, to a stranger,
would appear unexaggerated. His appearance was striking and prepos
sessing in the extreme, and his courtesy resembled almost that of a gentle
man of the old school. His musical taste had been highly cultivated, and
it was always exceedingly pleasant to see him take his seat at the piano
to accompany himself as he sang some humorous or grotesquely plaintive
song — words and music alike being generally of his own composition.
Some of the best of these songs have been collected in a small volume,
Songs and Fables (Second Edition; Glasgow, Maclehose, 1874). We
MEMOIR. XXV
extract one which give, in a very telling form, one point of view of a
muchdebated semiscientific question : —
THE THEEEFOOT EULE.
When I was hound apprentice, and learned to use my hands,
Folk never talked of measures that came from foreign lands :
Xow I 'm a British workman, too old to go to school ;
So whether the chisel or file I hold, I '11 stick to my threefoot rule.
Some talk of millimetres, and some of kilogrammes,
And some of decilitres, to measure beer and drams ;
But I 'm a British workman, too old to go to school ;
So by pounds I '11 eat, and by quarts I '11 drink, and I '11 work by my
threefoot rule.
A party of astronomers went measuring of the earth,
And forty million metres they took to be its girth ;
Five hundred million inches, though, go through from pole to pole ;
So let 's stick to inches, feet, and yards, and the good old threefoot
rule.
The great Egyptian pyramid 's a thousand yards about ;
And when the masons finished it, they raised a joyful shout ;
The chap that planned that building, I 'm bound he was no fool ;
And now 'tis proved beyond a doubt, he used a threefoot rule.
Here 's a health to every learned man that goes by common sense,
And would not plague the workman on any vain pretence ;
But as for those philanthropists, who 'd send us back to school,
Oh, bless their eyes, if ever they tries to put down the threefoot rule.
When the "Red Lions" met during the British Association week of 1871,
in Edinburgh, Eankine was hailed with universal acclaim as the LionKing.
His versatility in that singular post was very much akin to that of Pro
fessor Edward Forbes; though their paths in science were widely different.
His conversation was always interesting, and embraced with equal seeming
ease all topics, however various. He had the still rarer qualification of
being a good listener also. The evident interest which he took in all that
was said to him had a most reassuring effect on the speaker; and he could
turn without apparent mental effort from the prattle of young children to
the most formidable statement of new results in mathematical or physical
science. Then his notebook was at once produced, and in a few lines he
xxvi _ MEMOIR.
jotted down the essence of the statement, to be pondered over at leisure,
provided it did not ut once appear to him how it was to be verified. The
questions which he asked on such occasions were always almost startlingly
to the point, and showed a rapidity of thought not often met with in
minds of such calibre as his, where the mental inertia, which enables them
tn overcome obstacles often prevents their being quickly set in motion.
His kindness, shown in the readiness with which he undertook to read
proof sheets tor a friend, or even to contribute a portion of a chapter
(where the subject was one to which he had paid special attention), was.
for a man so constantly at work, absolutely astonishing. The writer of
this brief notice has several times availed himself of such assistance. It
was given almost as soon as asked, and it was invariably of sterling value.
Nothing is more precious to a writer on scientific subjects (especially when
questions of priority are involved ) than the assistance of a friendly though,
if necessary, seven — critic, such as was Rankine.
AVe must not refrain from pointing out, in connection with his scientific
merits, how very good and how exemplary for scientific writers and
investigators his character was. I Ie was ambitious ; that is obvious from
the number and variety of his books and papers, and the quite unnecessary
display of symbols in several of his less popular writings. But he was tie'
very soul of honour in respect to giving all credit to others, ami in never
attempting in anything, small or great, to go a hairbreadth beyond the
line of right as to his own claims. He showed a particularly good and
generous temper in cases of difference on scientific questions — a temper
which proved the true metal, unalloyed by any mean quality.
Rankine was, in many subject., an almost selftaughl man, and the
direction of his earlier scientific wort seems not to have been a very pro
fitable one. But, once on the right tack, his progress became very rapid.
Every mathematician worthy of the name has made himself: some, as
Rowan Hamilton, by attacking at an early age the grander works of
Lagrange and Laplace; others by attempting original flights without the
assistance of books. Rankine published only one or two papers on sub
jects of pure mathematics; ami even these, though not containing any
direct allusion to physics, were connected somewhat closely with kine
matical or physical investigations, such as the deformation of an elastic
solid.
The number of Rankine's scientific papers seems absolutely enormous,
when we consider the minute and scrupulous care with which he attended
to every point of detail in the writing and printing of them. How he
managed, in addition to these, to find time for the composition of his
many massive (not heavy) and elaborate volumes — all marked with the
most striking stamp of originality — for his memoirs, and his almost weekly
communications to The Engineer and other professional papers, must
memoir. xxvii
always remain matter for conjecture. In the Royal Society's splendid
Catalogue of Scientific Paper* we find that from 1843 to 1872 (both
inclusive) he published, in recognised scientific journals alone, upwards of
a hundred and fifty papers— many of these being exhaustive essays on
mathematical or physical questions, and all, save one or two, contain
ing genuine contributions to the advance of science. Leaving out of
account the more strictly professional of these papers, we find among
the titles of the rest such heads as the following : — Molecular Vortices,
Elasticity of Solids, Isorrhopic Axes, Compressibility of Water at Different
Temperatures, Centrifugal Theory of Elasticity, Oscillatory Theory of Light,
(!. neral Law of Transformation of Energy, Plane WaterLines, Oogenous
Neo'ids. To indicate even briefly the nature and importance of the
varied contents of these papers alone, would require vastly more time
and space than are at present at our disposal. The more important of
them are included in the present volume; others of less importance, or of
less characterised originality, may be consulted by the reader in the
scientific publications where they originally appeared.
Unquestionably the greatest pure scientific work of Rankine'sis contained
in his numerous papers bearing on the Dynamical Theory of Heat, and on
Energy generally. As Sir "William Thomson has remarked, even the mere
title of his earliest paper on this subject, "Molecular Vortices," is an
important contribution to physical science. The mode in which Rankine,
in 1819, attacked the true theory of heat, which had just been recalled to
the attention of scientific men by the admirable experiments and numerical
determinations of Joule, was quite different from that adopted by any
one of his concurrents; and though objections may fairly be raised to
certain parts, even his first paper constituted a remarkable contribu
tion to our physical knowledge. The essential characteristic of his
method is the introduction of a hypothesis as to the nature of the motions
and displacements (of the ultimate parts of bodies) upon which temper
ature depends, and in which heat, whether latent or sensible, consists.
He thought it necessary to defend this mode of investigation, and did so
in a remarkable address to the Philosophical Society of Glasgow, from
which we extract the following passages, which are valuable not alone
from their intention, but also from the insight^they give us into the
character of the man : —
" In order to establish that degree of probability which warrants the
reception of a hypothesis into science, it is not sufficient that there should
be a mere loose and general agreement between its results and those of
experiment. Any ingenious and imaginative person can frame such
hypotheses by the dozen. The agreement should be mathematically
exact to that degree of precision which the uncertainty of experimental
data renders possible, and should be tested in particular cases by numerical
XXVlll MEMOIR.
calculation. The highest degree of probability is attained when a hypo
thesis leads to the prediction of laws, phenomena, and numerical results
which are afterwards verified by experiment" as when the wavetheory
of light led to the prediction of the true velocity of light in refracting
media, of the circular polarisation of light by reflexion, and of the
previously unknown phenomena of conical and cylindrical refraction; and
as when the hypothesis of atoms in chemistry led to the prediction of
the exact proportions of the constituents of innumerable compounds.
I think I am justified in claiming for the hypothesis of mole
cular vortices, as a means of advancing the theory of the mechanical
action of heat, the merit of having fulfilled the proper purposes of a
Mechanical hypothesis in physical science, which are to connect the laws
of molecular phenomena by analogy with the laws of motion, and to
suggest principles such as the second law of thermodynamics and the
laws of the elasticity of imperfect gases, whoso conformity to fact may after
wards be tested by direct experiment. And I make that claim the more
confidently, that 1 conceive the hypothesis in question to be in a great
measure the development, and the reduction to a precise form, of ideas
concerning the molecular condition which constitutes heat, that have been
entertained from a remote period by the leading minds in physical
science . . . I wish it, however, to be clearly understood, that although
I attach great value and importance to sound mechanical hypotheses as
means of advancing physical science, 1 firmly hold that they can never
attain the certainty of observed facts; and accordingly, I have laboured
assiduously to show that the two laws of thermodynamics are demon
strable as facts, independently of any hypothesis; and in treating of the
[aactical application of those laws, I have avoided all reference to
hypothesis whatsoever."
The application of the doctrine, that heat "ml work an ble, to the
discovery of new relations among the properties of bodies, was made
about the same time by three scientific men — W. Thomson, Rankine, and
Clausius.
Of these, Thomson cleared the way for the new theory by his account
of the almost forgotten work of Carnot on the " Motive Power of Heat."
This excessively important investigation was published in 182f, when the
world of science was not prepared for its reception, and had been allowed
to drop out of notice. Thomson gave a very full abstract of its contents
in the Transacts ras of the Royal Society of Edinburgh, 1849, and pointed out
that they would require modification if the new theory were adopted,
as Carnot had throughout assumed that heat is a substance, and therefore
indestructible. He showed, besides, that Carnot's method was capable
of giving an absolute definition of temperature ; independent, that is,
of the properties of any particular substance. He also experimentally
MEMOIR. xxix
verified a deduction made by his brother, James Thomson (from Carnot's
theory), as to the alteration of the freezing point of water by pressure.
Eankine (late in 1849) and Clausius (early in 1850) took the first step
towards the formation of a true theory of the action of heat on bodies, by
showing (by perfectly different modes of attacking the question) the nature
of the modifications which Carnot's theory required. The recent publica
tion of Carnot's MSS. proves that that verj r remarkable man had himself
recognised the necessity for such modifications (and had all but succeeded
in making them) before his premature death. Thomson, in 1851, put the
foundations of the theory in the form they have since retained.
In RanMne's paper of 1849, he applied the theory to the determination
of the relation between the latent heat of steam and its density, and
made a very noteworthy prediction of the true value of the specific heat
of air, at a time when the experimental results which were considered
the best were far from the truth. [Rankine's results were soon after
verified by the experimental researches of Joule and Eegnault.] He also
showed that saturated steam, pressing out a piston in a vessel impervious
to heat, must cool so as to keep constantly at the temperature of satura
tion ; and that, besides, a portion of it licmefies.
A very excellent statement of the claims of Eankine in thermodynamics
is given in the following cmotation from an article by ClerkMaxwell
{Nature, 1878, Vol. XVIL, p. 257) :—
" Of the three founders of theoretical thermodynamics, Eankine availed
himself to the greatest extent of the scientific use of the imagination.
His imagination, however, though amply luxuriant, was strictly scientific.
Whatever he imagined about molecular vortices, with their nuclei and
atmospheres, was so clearly imaged in his mind's eye, that he, as a practical
engineer, could see how it would work.
"However intricate, therefore, the machinery might be which he
imagined to exist in the minute parts of bodies, there was no danger of
his going on to explain natural phenomena by any mode of action of this
machinery which was not consistent with the general laws of mechanism.
Hence, though the construction and distribution of his vortices may seem
to us as complicated and arbitrary as the Cartesian system, his final
deductions are simple, necessary, and consistent with facts.
" Certain phenomena were to be explained. Eankine set himself to
imagine the mechanism by which they might be produced. Being an
accomplished engineer, he succeeded in specifying a particular arrangement
of mechanism competent to do the work, and also in predicting other
properties of the mechanism which were afterwards found to be consistent
with observed facts.
"As long as the training of the naturalist enables him to trace the action
only of particular material systems, without giving him the power of
XXX MEMOIR.
dealing with the general properties of all such systems, he must proceed
by the method so often described in histories of science — he must imagine
model after model of hypothetical apparatus, till he finds one which will
do the required work. If this apparatus should afterwards be found
capable of accounting for many of the known phenomena, and not demon
strably inconsistent with any of them, he is strongly tempted to conclude
that his hypothesis is a fact, at least until an equally good rival hypothesis
has been invented. Thus Elankine,* long after an explanation of the
properties of gases had been founded on the theory of the collisions of
molecules, published what he supposed to be a proof that the phenomena
of heat were invariably due to steady closed streams of continuous fluid
matter.
"The scientific career of Rankine was marked by the gradual develop
ment of a singular power of bringing the most difficult investigations
within the range of elementary methods. In his earlier papers, indeed,
1m appears as if battling with chaos, as he swims, or sinks, or wades, or
creeps, or flies,
' Ami through the palpable obscure finds out
I Ik uncouth way •.'
but he soon begins to pave a broad and beaten way over the dark abyss;
and his Latest writings how such a power of bridging over the diiliculties
oi science, that his premature death must have been almost as great a loss
to the diflusion of Bcience as it was to its advancement.
"The chapter on thermodynamics in his book on the steamengine was
the first published treatise on the subject, and is the only expression of
his views addressed directly to students.
" In (hi boot he has disencumbered himself to a great extent of the
hypothesis of molecular vortices, and builds principally on observed facts,
though he, in common with Clausius, makes several assumptions, some
expressed as axioms, others implied in definitions, which seem to us
anything but selfevident. A an example of Kankine's best style Ave
may .take the following definition : —
"'A PERFECT Gas is a substance in such a condition that the total
pressure exerted by any number oi' portions of it, at a given temperature,
against the sides of a vessel in which they are enclosed, is the sum of the
pressures which each portion would exert if enclosed in the vessel separately
at the same temperature.'
" Here Ave can form a distinct conception of every clause of the definition;
but when we come to Eankine's Second Law of Thermodynamics we find
* "On the Second Law of Thermodynamics," Phil. Mo;/., Oct., 1865, § 12, p. 244 ;
but in his paper "On the Thermal Energy of Molecular Vortices," Trans. U.S. Edin.,
XX \"., p. 557 (1S(50), he admits that the explanation of gaseous pressure by the impacts
of molecules has beeu proved to be possible.
MEMOIE. XXXI
that though, as to literary form, it seems cast in the same mould, its
actual meaning is inscrutable.
" 'The Second Law of Thermodynamics. — If the total actual heat of
a homogeneous and uniformly hot substance be conceived to be divided
into any number of equal parts, the effects of those parts in causing work
to be performed are equal.'
" We find it difficult enough, even in 1878, to attach any distinct meaning
to the total actual heat of a body, and still more to conceive this heat
divided into equal parts, and to study the action of each of these parts;
but as if our powers of deglutition were not yet sufficiently strained,
Eankine follows this up with another statement of the same law, in which
we have to assert our intuitive belief that
" 'If the absolute temperature of any uniform^ hot substance be divided
into any number of equal parts, the effects of those parts in causing work
to be performed are equal.'
" The student who thinks that he can form any idea of the meaning of
this sentence is quite capable of explaining, on thermodynamical principles,
what Mr. Tennyson says of the great Duke —
' Whose eighty wiuters freeze with one rebuke
All great selfseekers trampling on the right.' "
Ivankinc's researches on heat were for the most part connected, as we
have already said, with a theory of the constitution of bodies, and a specula
tion as to the physical nature of a hot body, to which he gave the name of
Theory of Molecular Vortices. In this theory, the invisibly small parts
of bodies apparently at rest are supposed to be in a state of motion, the
rapidity of which may be compared with that of a cannon ball. It was
distinguished from other theories, which attribute motion to bodies
apparently at rest, by the further assumption that this motion is like that of
very small vortices, each whirling about its own axis, and that the centri
fugal force of this motion contributes to the elasticity of the body. A
theory of a similar kind has since been applied by ClerkMaxwell to the
explanation of magnetic phenomena ; and Sir W. Thomson has made the
rigorous investigation of vortices possible by his paper on " Vortex
Motion," and has also contributed to the philosophy of speculation by his
theory of " Vortex Atoms."
Eankine's researches on the general theory of elastic bodies are charac
terised by the fact that while, in laying the foundation of the theory, he
confines himself to the use of rigorous methods, and does not shrink from
any mechanical difficulty in their application, he always prepares the way
for the application of the results to practice, by making the definitions so
clear, the methods so simple, the results so definite, that they can be
XXXU MEMOIR.
mastered by the exercise of a little thought, without special mathematical
training. This quality is prominent also in his researches on fluid motion,
three of which are of special importance.
I. The theory of the propagation of waves, such as those of sound in
elastic media, is generally supposed to belong to the most abstruse depart
ments of mathematical science. Even Newton made some oversights in
liis investigation, and it required more than a century of hard mathematical
development before the theory reached its present state — which is still very
imperfect. Rankine, by the introduction of a few new conceptions in the
elementary part of the investigations, has rendered it possible for any one
acquainted with elementary dynamics to follow the theory up to the point
at which it was left by Laplace ami almost as much further as it has yet
been carried
II. The tlnory of waves on the surface of water, when their height
is not regarded as infinitely small, is still more difficult than that of
sound waves. Stokes has, indeed, in a masterly series of investigations,
arrived at a second, and for some purposes a third, approximation. An
solution, however, but of a particular ease only, was arrived at by
Rankine. He was not aware that it had been given by Gerstner in 1802,
having been deduced from an assumption, generally erroneous, but true for
this special case. Unfortunately, as this theory essentially involves rota
tion of fluid elements, it is not a solution of the usual problem of waves at
the surface of a perfect liquid, for it implies a kind of motion which could
not be produced in such a substance if originally in a state of rest.
III. Rankine's third investigation is that of lines of motion of water
flowing past a ship. lie begins with the mathematical theory of such
lines, but soon applies his results to the determination of good forms of
the "line; of a ship, and the investigation of the principal causes of the
resistance to the motion of the ship, and the means of diminishing that
nice.
Xo other person has done so much as either Rankine or William
Froude to promote naval dynamics, and the application of science to the
shaping of ships, and to the estimation of their performances.
To Rankine, tin' Scientific SubCommittee of the late Admiralty Com
mittee on Designs owed most of its reports, and a very large proportion
of their effectiveness. Even those most disposed to disparage that
Committee and its work, have made exception as regards the Reports of
tlie Scientific SubCommittee, and, in particular, Eankine's contributions.
(It seems to us that only ignorance or unfavourable bias could attempt to
disparage the committee at all ; for it undoubtedly did. though in an
unostentatious manner, very good sendee indeed.)
Eankine's works on Applied Mechanics, on the SteamEngine, and on
Engineering, contain many valuable and original methods ; and while the
MEMOIK. xxxiii
publication of any one of them would have established the fame of one of
our average scientific men, that on the steamengine could not have been
produced by any but an original discoverer of a high order. Some of the
investigations contained in this series of volumes are as remarkable for
the material aid they afford to the man of practice as for the light they
throw upon his work.
The following gives a striking instance of Eankine's tact under a novel
and somewhat puzzling combination of circumstances. In August, 1858,
he wrote to the Philosophical Magazine the annexed short letter, which
was printed in the September number of that journal : —
" In the course of last year there were communicated to me, in con
fidence, the results of a great body of experiments on the engine power
required to propel steamships of various sizes and figures, at various
speeds. From those results I deduced a general formula for the resistance
of ships, having such figures as usually occur in steamers, which, on
the 23rd of December, 1857, I communicated to the owner of the
experimental data, and he has since applied it to practice with complete
success.
" As the experimental data were given to me in confidence, I am for the
present bound in honour not to disclose the formula which I deduced from
them ; but as I am desirous not to delay longer the placing it upon record,
I have recourse to the old fashion of sending it to you in the form of
an anagram, in which the letters that occur in its verbal statement are
arranged in alphabetical order, and the number of times that each letter
occurs is expressed by figures:— 20 A. 4 B. 6 C. 9 D. 33 E. 8 F. 4 G. 1G H.
10 1. 5 L. 3 M. 15 N. 14 0. 4 P. 3 Q. 14 R. 13 S. 25 T. 4 U. 2 V. 2 W. 1 X.
4 Y. (219 letters in all). I hope I may soon be released from my present
obligation to secrecy."
There could be no doubt that this refers to a remarkable investigation
which Eankine carried out for his friend James E. Xapier, who had asked
him to estimate the horsepower necessary to propel at a given rate a
vessel which he was about to construct. Guided by this consideration, I
found, in 1872, the following sentence of Eankine's in The Civil Engineer
and Architects Journal (October 1, 18G1), but without any reference what
ever to the anagram or to the Plulosopldcal Magazine : —
" The resistance of a sharpended ship exceeds the resistance of a
current of water of the same velocity in a channel of the same length and
mean girth, by a quantity proportional to the square of the greatest
breadth, divided by the square of the length of the bow and stern."
Curiously enough, Eankine seems to have made an arithmetical mistake,
or a misspelling, because this sentence exactly fits all of the above
numerical data, with the exception that it contains just one E too much,
and has, therefore, 220 letters in all. Eankine's private MSS., to which I
XXXIV MEMOIR.
have recently had access, show that my guess was correct, but Jo not
enable me to find how the numerical error just noticed arose.
Mr. Napier informed me that in all his business relations with Rankine,
nothing had so much impressed him as the rapid and keen insight with
which he seemed at once to seize upon the most essential points in the
solution of a practical question, though stated to him for the first time;
how he first shook himself free from the petty complications, and gave
almost immediately an approximate estimate embracing all the larger
bearings of the question; and then, much more formally and deliberately,
and with the minute accuracy and system of a man of business, proceeded
to work the question with the desired exactness. Mr. Napier said that
on the occasion of his first consultation with Rankine on the matter
referred to in the anagram above, Rankin e's very first words pointed out
to him what a large proportion of the resistance to a vessel's motion is due
to friction, and how illconsidered was the then growing demand for long
and narrow ships.
Kankine's textbooks on engineering subjects are in many respects the
most satisfactory that have been published in any country. At the time
of their publication they have always been in advance of the professional
knowledge of the day, but they possess much greater merits than that of
mere novelty. Rankine was peculiarly happy in discriminating between
those; branches of engineering knowledge which grow from daily experi
ence, and those which depend on unchangeable scientific principles. In
his books he dealt almost exclusively with the latter, which may, and
certainly will, be greatly extended, but so far as they have been established
can never change. Hence his books are a mine which smaller men may
work for many years, rendering his knowledge more generally available
by giving it a popular setting of their own. By the bulk of the engineer
ing profession the books are considered hard reading, but as engineering
education improves they will more and more be recognised as both
wonderfully complete and essentially simple. Rankine, by his education
as a practical engineer, was eminently qualified to recognise the problems
of which the solution is required in practice; but the large scope of his mind
would not allow him to be content with giving merely the solution of
those particular cases which most frequently occur in engineering as we
now know it. His method invariably is to state the problem in a very
general form, find the solution, and then apply this solution to special
cases. This method does not make the books easy reading for students,
nor does it give the most convenient work of reference for the practical
man ; but it has produced writings the value of which is permanent,
instead of being ephemeral.
In his Applied Mechanics we have the best existing work on the applica
tion of the doctrines of pure mechanics to general engineering problems.
MEMOIR. XXXV
No specious reasoning has been detected in this great work, a fact which
should for ever dispel the old and false antithesis between theory and
practice — a contrast drawn by practical men who never understood fully
any theory, and assented to by scientific men who were not candid
enough to point out where their theories were incomplete. In the
A\'ork named Civil Engineering, Rankine applied the general doctrines of
applied mechanics to the special problems which the civil engineer of
today meets with in his practice, and his volume contains much valuable
statistical information. In his work on Prime Movers we have a most
thoroughly original statement of the thermodynamic theory, so far as it
bears on the design and use of the steamengine. This work especially
shows Rankine's clear discrimination of what is permanent and can be
taught, from that which must vary from day to day, and can only be
acquired by personal experience; the distinction between the science and the
art of the engineer.
His treatise on Machinery and Millvorl gives the mechanical engineer
instruction of a kind analogous to that which the civil engineer may derive
from the book called Civil Engineering. The problems stated generally in
the Applied Mechanics are in it applied to the special cases which arise in the
design of machinery. Several of these .Manuals have been recently
translated ; Prime Movers into French, Civil Engineering into German, &c. ;
and Machinery and Millwork will soon appear in Italian.
Most of the common treatises on engineering subjects are mere rechauffes
or compilations ; and no library becomes sooner worthless than that of the
engineer, the practice of this year being wholly different from that of
five years since. Really original papers and monographs rapidly lose their
interest and importance, except as historical landmarks, but Eankine's
works will retain their value after this generation has passed away.
In concluding the scientific part of this brief notice of a true man, we
need scarcely point out to the reader how much of Rankine's usefulness
was due to steady and honest work. The unscientific are prone to imagine
that talent (especially when, as in Rankine's case, it rises to the level of
genius) is necessarily rapid and offhand in producing its fruits. No greater
mistake could be made. The most powerful intellects work slowly and
patiently at a new subject. Such was the case with Newton, and so it is
still. Rapid they may be, and in general are, in new applications of
processes long since mastered ; but it is only your pseudoscientific man
who forms his opinion at once on a new subject. This truth was pro
minently realised in Rankine, who was prompt in reply when his know
ledge was sufficient, but patient and reticent when he felt that more
knowledge was necessary. With him thought was never divorced from
work — both were good of their kind — the thought profound and thorough,
XXXVI MEMOIR.
the work a workmanlike expression of the thought. Few. if any,
practical engineers have contributed so much to abstract science, and in
no case has scientific study been applied with more effect to practical
engineering. RanMne's name will ever hold a high place in the history
of science, and will worthily be associated with those of the great men we
have recently lost. And, when we think who these were, how strangely
does such a list — including the names of Babbage, Boole, Brewster,
Leslie Ellis, Faraday, Forbes, Herschel, Rowan Hamilton, ClerkMaxwell,
Rankine, and others, though confined to physical or mathematical science
alone— contrast with tfa inning utterance of the Prime Minister of
Cieat Britain and Ireland, to the effect that the present is by no means an
age abounding in minds of the first order ! Tin such men lost by this
little country within the last dozen years or so — any one of whom
would have made himself an enduring name had he lived in any pre
ceding it that of Hooke and Newton, or that of Cavendish and
Watt: Nay more, even such losses as these have not extinguished the
hopes of science amongst i . E ry one of these great men has, by
some mysterious influence of hi . kindled the sacred thirst for
new knowledge in yonngei but kindred spirits, many of whom will
certainly rival, some even may excel, their teachers !
For the dates and statements of fact in this Memoir, I am indebted
mainly to RanMne's private MSS., access to which has been given me by
his relative. Some special details I have had from his own lips. I have
also to acknowledge my obligations to Sir William Thomson, to Professor
Jcnkin, and specially to the late Mr. J. R. Napier, who was one of
Kankine's most enthusiastic admirers, lie furnished me with much of
the more technical part of the materials for a notice of Kankine's scientific
work, which I wrote immediately after his death for the Glasgow U'.niJJ.
December 28, 1872, and of which I have made considerable use in what
precedes.
Of Kankine's purely scientific work I have spoken from actual acquaint
ance with his writings ; but I have found it necessary to apply for
assistance while attempting to discuss the merits of his more practical
investigations.
P. G. TAIT.
College, Edinbetegh,
October, 1880.
PART I.
PAPERS RELATING TO TEMPERATURE, ELASTICITY, AND
EXPANSION OF VAPOURS, LIQUIDS, AND SOLIDS.
PAET I.
PAPERS RELATING TO TEMPERATURE, ELASTICITY, AND
EXPANSION OF VAPOURS, LIQUIDS, AND SOLIDS.
I.— OX AN EQUATION BETWEEN THE TEMPERATURE AND
THE MAXIMUM ELASTICITY OF STEAM AND OTHER
VAPOURS. (See Plate I.)*
In the course of a series of investigations founded on a peculiar hypothesis
respecting the molecular constitution of matter, I have obtained, among
other results, an equation giving a very close approximation to the maxi
mum elasticity of vapour in contact with its liquid at all temperatures
that usually occur.
As this equation is easy and expeditious in calculation, gives accurate
numerical results, and is likely to be practically useful, I proceed at once
to make it known, without waiting until I have reduced the theoretical
researches, of which it is a consequence, to a form fit for publication.
The equation is as follows : —
Log.P = a£J 2 , . . . (1.)
where P represents the maximum pressure of a vapour in contact with
its liquid ;
/, the temperature, measured on the airthermometer, from a point which
may be called the absolute zero, and which is —
274 0 6 of the Centigrade scale below the freezing point of water ;
462°  28 of Fahrenheit's scale below the ordinary zero of that scale,
* Originally published in the Edinburgh New Phibsophical Journal for July, 1849.
A
2
ON THE ELASTICITY OF VAPOURS.
supposing the boiling point to have been adjusted under a pressure
of 29  922 inches of mercury, so that 180° of Fahrenheit may
be exactly equal to 100 Centigrade degrees j
461°  93 below the ordinary zero of Fahrenheit's scale, when the
boiling point has been adjusted under a pressure of 30 inches of
mercury, 180° of Fahrenheit being then equal to 100 o 0735 of the
Centigrade scale.
The form of the equation has been given by theory; but three constants,
represented by a, [3, and y, have to be determined for each fluid by
experiment.
The inverse formula, for finding the temperature from the pressure, is
of course
M
l0£
P + 0*
4 7 
/3
V
(2.)
It is obvious that for the determination of the three constants, it is
sufficient to know accurately the pressures corresponding to three tem
peratures ; and that the calculation will be facilitated if the reciprocals of
those temperatures, as measured from the absolute zero, are in arithmetical
progression.
In order to calculate the values of the three constants for the vapour of
water, the following data have been taken from 3L Regnault's experi
ments : —
Temperatures in Cen
tigrade Degrees.
Common
Logarithms of
the Pressure in
Millimetres of
Mercury.
Remarks.
Above the
Freezing
Point.
Above the
Absolute
Zero.
o
220
100
2686
o
4946
3746
30146
o
42403
2S80S136
1419S
( Measured by M. Eegnault on his curve,
< representing the mean results of his ex
( periments.
Logarithm of 760 millimetres.
1 Calculated by interpolation from M. Eeg
{ nault's general table.
These data give the following results for the vapour of water, the
pressures being expressed in millimetres of mercury, and the temperatures
in Centierade decrees of the airthermometer : —
Log. 7 = 50827176 Log. (5
a=7'831247
31851091
ON THE ELASTICITY OF VAPOURS.
111
<d > a
PiO'B
as o
§ d P
O OO O O O OOOO OOOO _J
Ma «;
BOP
oortTfcowaTfiMHiwoo
00090090090000 —.
6 000 0000 6 6 00
I ++ l + + + + I + + 1 C
Differences be
tween Calcula
tion and Experi
ment iu Loga
rithms.
ONflLmNjdMHHWOil
00000000000000
00000000000000 ~~
oooopopoopoooo ^
b bb b b b bo b b b b bb — '
+ l l + 1 1 l 1 + l l +
rithms of the
Millimetres j
ng to
M. Roguault's
Experiments.
OOMNM«N1QN*NOOO ■— »
QOMNOMOtSNl^NOOttl «!
KOh«^i"CN00OOh«M ii
cicococococococococo*'*"**
Common Loga
Pressures in
accord
the Formula.
CCN^HOttOMOWtCOOM
^h r CO IO O O ~TI co
CO CO 01 O O CO CO O t* 01 1> co O ■ — 
OHWt^nMOTtn^^NOOH •
OCWNOWnONt^NOlOfCl i_
00 O h co * O O r co O O ri Ol co
dcoeocococococococo^^'**
&0a ■
a o g
IP
8 a •
© © £*
Hta a
S o
D = H
01 10 O CO CO — 1 10 101 •■+ CO >< O CO 01 01 01 t< CO O t~ O O CI CO
■* 01 pcpHpoppppppppppppppppppppp ,_,
00666006606000066660666060600 CO
I+ + + + + lllllll 1 1 + I 1 + + + 1 1 1 1 "'
Difference be
tween Calcu
lation and Ex
periment in
Millimetres.
H«J«^e0M^»00C150IM
OOO'irtOOOHeiNK^C'lOrtHOeOia^ODMiS _
ooooooooooooooo^^hocococo— < >n r~» tei
+ 111 11 + + + + + + + ++•++' <7+++ + ""
[illimetres of
cording to
M. Regnault's
Experiments.
«*hcOOOOOOhCOOOtHiOOO
noooHMti 10 00 r © •**< © t— 00000000
OOtl^eiN'JSHiiHOJMTllOOMOOKflNOnWOMOlO 7
HNMOO'l'nLOtlONiK'NHNt'Or'M'OOOH _u
h ci n lo t> ■# n « 3 c. >o ;< co co c^
11 1— 1 01 01 CO * 10 t O — 1 P t^
HHrlCl
Pressures in 1
Mercury, ac
the Formula.
«oi.nw«oi> 10 onanooci
cocpp^pcpq<lippo^7H^pi^pcp7Hpcp>pp01t>>0
6 6 Ol ■* b l b ^h b 01 b co 10 b 6 <* 6 co co co ^1 eo 00 »o 10 •
Hcmoo"f mioci'onoci u>#w«on — p jo
rH Ol CO O C O Tfl t lO Ci l.O * CO co ^
11 1— lOlOlCO^OtO^^rO
i— 1 r— 1 1— 1 01
es in Centi
reos of the
motor from
the Abso
lute Zero.
O
OOOOOO^OOOOOOOOOOOOOOOOOOOO© ^_,
^OOM»C!OOHiMW#OOM»OOHCin)<lOOI>tOOO ^~
oioioio^oic^coeocococococococococoT]HTj<*TjH^iTi(TH^*Triin
Temperatur
grade Deg
AirThormo
the Freez
ing Point.
CO
0000000 b 000000000000000000000 ^
CO 01 ( HCKIM^OOtOOOOi^M^OOIXBOO'JlM •
4 ON THE ELASTICITY OF VAPOURS.
Table I. exhibits a comparison between the results of the formula and
those of M. Eegnault's experiments, for every tenth degree of the Centi
grade airthermometer, from 30° below the freezing point to 230° above
it, being within one or two degrees of the whole range of the experi
ments.
M. Eegnault's values are given, as measured by himself, on the curves
representing the mean results of his experiments, with the exception
of the pressures at 26°*86, one of the data already mentioned, and that
at  30°, which I have calculated by interpolation from his table,
series h.
Each of the three data used in determining the constants is marked
with an asterisk *.
In the columns of differences between the results of the formula and
those of experiment, the sign + indicates that the former exceed the latter,
and the sign — the reverse.
Beside each such column of differences is placed a column of the cor
responding differences of temperature, which would result in calculating
the temperature from the pressure by the inverse formula. These are
found by multiplying each number in the preceding columns by
dt . dt
— z= or by 7i = as the case may require.
dP J d log. P J n
In comparing the results of the formula with those of experiment, as
exhibited in Table I., the following circumstances are to be taken into
consideration : —
First, That the uncertainty of barometric observations amounts in
general to at least onetenth of a millimetre.
Secondly, That the uncertainty of thermometric observations is from
onetwentieth to onetenth of a degree under ordinary circumstances, and
at high temperatures amounts to more.
Thirdly, That, in experiments of the kind referred to in the table, those
two sorts of uncertainty are combined.
The fifth column of the table shows that, from 30° below the freezing
point to 20° above it, where the minuteness of the pressures makes the
barometric errors of most importance, the greatest difference between
experiment and calculation is y 1 ^ of a millimetre, or giro °f an mcn °^
mercury, a very small quantity in itself, although, from the slowness
with which the pressure varies at low temperatures, the corresponding
difference of temperature amounts to ££$ of a degree.
The sixth and tenth columns show that, from 20° to 230° above the
freezing point, the greatest of the discrepancies between experiment and
observation corresponds to a difference of temperature of only jfa of a
degree, and that very few of those discrepancies exceed the amount
corresponding to ^ of a degree.
ON THE ELASTICITY OF VAPOURS. 5
A comparison between the sixth and tenth columns shows that, for
four of the temperatures given— viz., 120°, 150°, 200°, and 210°— the
pressures deduced from M. Eegnault's curve of actual elasticities, and
from his logarithmic curve respectively, differ from the pressures given
by the formula in opposite directions.
If the curves represented by the formula were laid down on M.
Regnault's diagram, they would be almost undistinguishable from those
which he has himself drawn, except near the freezing point, where the
scale of pressures is very large, the heights of the mercurial column being
magnified eightfold on the plate. In the case of the curves of logarithms
of pressures above one atmosphere, the coincidence would be almost
perfect.
The formula may, therefore, be considered as accurately representing
the results of all M. Regnault's experiments throughout a range of
temperatures from 30° of the Centigrade scale below the freezing point to
230° above it, and of pressures from o^ott °f an atmosphere up to 28
atmospheres.
It will be observed that equation (1.) bears some resemblance to the
formula proposed by Professor Roche in 1828, viz. : —
■p
W.P=A
T + C
where T represents the temperature measured from the ordinary zero
point, and A, B, and C, constants, which have to be determined from
three experimental data. It has been shown, however, by M. Regnault,
as well as by others, that though this formula agrees very nearly with
observation throughout a limited range of temperature, it errs widely
when the range is extensive. I have been unable to find Professor
Roche's memoir, and I do not know the reasoning from which he has
deduced his formula.
The use in computation of the equations I have given, whether to
calculate the pressure from the temperature, or the temperature from the
pressure, is rapid and easy. In Table II. they are recapitulated, and the
values of the constants for different measures of pressure and temperature
are stated.
In calculating the values of a, the specific gravity of mercury has been
taken as 13'596.
Temperatures measured by mercurial thermometers are in all cases to
be reduced to the corresponding temperatures on the airthermometer,
which may be done by means of the table given by M. Regnault in his
memoir on that subject.
ON THE ELASTICITY OF VAPOURS.
Table II. — Vapour of Water.
Formula for calculating the Maximum Elasticity of Steam (P), from
the Temperature on the AirThermometer, measured from the Absolute
Zero (0 :—
Inverse Formula for calculating the Temperature from the Maximum
Elasticity of Steam : —
1 /qlog.P ffl" _ g_
t > 7 4 7 2 2y
Values of the Constants depending on the Thermometric Scale :
For the Centigrade scale : —
Absolute zero 274°'6 below the freezing point of water.
Log. /3 = 31851091 Log. 7 = 50827176
£ = 000G3L".) i ft = 000004006
For Fahrenheit's scale; boiling point adjusted at 29  922 inches: —
Absolute zero 462°*28 below ordinary zero.
Log. 3  34403816 Log. y = 55932626
£=00035163 ft= 0000012364
 y 4y
For Fahrenheit's scale ; boiling point adjusted at 30 inches : —
Absolute zero 461°*93 below ordinary zero.
Log. /3 = 34400625 Log. y = 55926244
ft= 00035189 ft = 0000012383
2y 47^
Values of the Constant a depending on the Measure of Elasticity :
For millimetres of mercury, .... a =7831247
English inches of mercury, .... 6426421
ON THE ELASTICITY OF VAPOURS.
Atmospheres of 760 mil. = 29*922 inches^
= 147 lbs. on the square inch=l0333 kil. v .
on the centimetre 2 , . . . . . )
Atmospheres of 30 inches = 761 mil. "99 \
= 14'74 lbs. on the square inch=r T036 kil. V
on the centimetre 2 , . . . . .J
Kilogrammes on the square centimetre,
Pounds avoirdupois on the square inch,
N.B. — All the Constants are for common logarithms.
4950433
4949300
4964658
6117817
I have applied similar formula? to the vapours of alcohol and ether,
making use of the experiments of Dr. Ure.
In order to calculate the constants, the following experimental data
have been taken, assuming that, on Dr. Ure's thermometers, 180° were
equal to 100 Centigrade degrees.
For Alcohol of the
specific gravity O'Slo,
For Ether, boiling at )
105° F., under 30 [
inches of pressure, )
For Ether, boiling at )
104° F., under 30 [
inches of pressure, )
Temperatures on
Fahrenheit's Scale
from
the Ordi the Abso
nary Zero, iuto Zero.
250
173
11102
200
14S8
105
104
G67
34
7123
6353
573 32
6623
6111
5073
5663
5290
4963
Pressures
in Inches
of Mer
cury.
13230
30 00
630
1428
6624
3000
3000
1376
6 20
From Dr. Ure's Table.
Do.
Interpolated in the same
Table.
From Dr. Ure's Table.
Interpolated.
From the Table.
From Dr. Ure's Table.
Interpolated.
From the Table.
The values of the constants in equation (1.), calculated from these data,
are as folloAvs, for inches of mercury and Fahrenheit's scale : —
Alcohol, specific gravity 0813,
Ether, boiling point 105°F.,
Ether, boiling point 104°F.,
616620
533590
544580
Log. £.
33165220
320S4573
32571312
Absolute zero 462° '3 below ordinary zero.
Log. y.
57602709
55119893
53962460
8 ON THE ELASTICITY OF VAPOURS.
The curves represented by the formulae for those three fluids are laid
down on the diagram which accompanies this memoir (Plate I.), and
which has been reduced to onefourth of the original scale. The horizontal
divisions represent the scale of Fahrenheit's thermometer, numbered from
the ordinary zero ; the vertical divisions, pressures of vapour, according
to the scales specified on the respective curves. The points corresponding
to the experimental data are surrounded by small circles.
The curve for alcohol extends from 32° to 264° of Fahrenheit. It is
divided into two portions, having different vertical scales, suitable to
high and low pressures respectively.
The curve for the less volatile ether extends from 105° to 210°; that
for the more volatile ether, from 34° to 104°.
The results of Dr. Ure's experiments are marked by small crosses.
The irregular and sinuous manner in which those crosses are dis
tributed, indicates that the errors of observation, especially at high
temperatures, must have been considerable. This does not appear
surprising, when we recollect how many causes of uncertainty affect all
the measurements required in such experiments, especially the ther
mometric observations, and how little those causes have been understood
until very recently. The data from which the constants have been
calculated arc, of course, affected by the general uncertainty of the
experiments.
When those circumstances are taken into account, it is obvious, from
inspection of the diagram, that the curves representing the formulae agree
with the points representing the experiments as nearly as the irregularity
of the latter and the uncertainty of the data permit ; and that there is
good reason to anticipate that, when experiments shall have been made
on the vapours of alcohol and ether with a degree of precision equal to
that attained by M. Kegnault in the case of the vapour of water, the
equation will be found to give the elasticities of those two vapours as
accurately as it does that of steam.
Although the diagram affords the best means of judging of the
agreement between calculation and experiment, three tables (III., IV.,
and V.) are annexed, in order to show the numerical amount of the
discrepancies at certain temperatures. The data, as before, are marked
with asterisks.
It is worthy of remark, in the case of alcohol, that although the lowest
of the experimental data is at the temperature of lll o 02, the formula
agrees extremely well with the experiments throughout the entire range
of 79 degrees Mow that point.
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BOSTON U
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I
ON THE ELASTICITY OF VAPOURS.
Table III. — Vapour of Alcohol, of the Specific Gravity 0813.
Temperatures in
Pressures in Inches of Mercury
Differences be
Degrees of
Fahrenheit from
the Ordinary
Zero.
accord
the Formula.
ing to
Dr. Ure's
Experiments.
tween Calculation
and Experiment in
Inches.
Corresponding
Differences of
Temperature.
32
41
40
+ 01
05
40
57
56
+ 001
04
50
084
0S6
002
+ 07
60
122
123
001
+ 02
70
174
176
0 02
+ 03
80
2 43
245
0 02
+ 02
90
3 36
3 40
004
+ 04
100
456
4 50
+ 006
05
110
612
6 00
+ 012
07
*lll02
6 30
6 30
00
120
810
810
000
00
130
1061
10 60
+ 01
00
140
1373
13 90
017
+ 05
150
1760
1800
040
+ 09
160
2232
2260
028
+ 05
170
2S06
2830
0 24
+ 04
*173
3000
3000
00
180
34 96
34 73
+023
03
190
4321
4320
+ 01
0
200
5296
5300
0 04
+oo
210
64 47
65 00
0 53
+ 05
220
77 92
7850
058
+ 04
230
9354
9410
056
+ 04
240
11158
11124
+ 034
02
*250
132 30
13230
ooo
260
15598
15520
+ 078
03
264
165 5S
16610
0 52
+02
(1.)
(2.)
(3.)
(4.)
(5.)
Table IV. — Vapour of Ether; Boiling Point, 105°F.
Temperatures in
Pressures in Inches of Mercury
Differences be
Degrees of
according to
tween Calculation
Corresponding
Fahrenheit above
and Experiment
Differences of
the Ordinary
Zero.
the Formula.
Dr. Ure's
Experiments.
in Inches of
Mercury.
Temperature.
*105
30O0
30 00
ooo
00
no
33 OS
32 54
+ 54
09
120
3998
39 47
+ 051
07
125
4383
4324
+ 059
OS
130
4795
4714
+ 0S1
10
140
5710
56 90
+ 020
02
♦1488
66 24
6624
00
150
67 53
67 60
0 07
+ 01
160
79 35
SO 30
0 95
+ 09
170
9268
92 80
012
+ 01
175
9994
9910
+ 084
06
180
10762
10830
0 68
+ 04
190
12429
124 80
051
+ 03
*200
14280
14280
00
oo
205
152 78
15130
+ 148
07
210
16327 166 00
l
1
273
+ 11
10 ON THE ELASTICITY OF VAPOURS.
Table V. — Vapour of Ether; Boiling Point, 104°F.
Temperatures in
Pressures in Inches of Mercury
Differences be
Degrees of
according to
tween Calculation
Corresponding
Fahrenheit above
and Experiment
Differences of
the Ordinary
Zero.
the Formula.
Dr. Dre's
Experiments.
in Inches of
Mercury.
Temperature.
*34
620
6 20
ooo
44
S02
sio
008
+ 04
54
1024
10 30
0 06
+ 02
64
1294
13 00
006
+02
*667
13 76
1376
00
74
1619
1610
+ 09
02
84
20 06
20 00
+ 006
01
94
2464
2470
0 06
+ 01
*104
3000
3000
00
00
(1.).
(2.)
(3.)
(4.)
(5.)
The results of Dr. Ure's experiments on the vapours of turpentine and
petroleum are so irregular (as the diagram shows), and the range of
temperatures and pressures through which they extend so limited, that
the value of the constant y cannot be determined from them with
precision. I have, therefore, endeavoured to represent the elasticities
of those two vapours approximately by the first two terms of the formula
only, calculating the constants from two experimental data for each fluid.
The equation thus obtained
Los. P
a —
is similar in form to that of Professor Roche.
The data and the values of the constants are as follows : —
Temperatures (
Scalo
the Ordinary
Zero.
m Fahrenheit's
from
the Absolute
Zero.
Pressures in
Inches of
Mercury.
Values of the Constants for
Fahrenheit's Scale and
Inches of Mercury.
360
304
370
316
S223
7663
S323
7783
Turpentine.
60 SO
3000
Petroleum.
6070
3000
* = 5981S7
Log. 3 = 3 5380701
*=619451
Log. 3 = 3 5648490
Although the temperatures are much higher than the boiling point of
water, I have not endeavoured to reduce them to the scale of the air
thermometer, as it is impossible to do so correctly without knowing the
nature of the glass of which the mercurial thermometer was made.
ON THE ELASTICITY OF VAPOURS.
11
The diagram shows that the formula agrees with the experiments as
well as their irregularity entitles us to expect.
The followinc; tables crive some of the numerical results : —
Table VI. — Vapour of Turpentine.
Temperatures in
Degrees of
Fahrenheit from
the Ordinary
Zero.
Pressures in In
accord
the Formula
(of two terms).
ehes of Mercury
ing to
Dr. lire's
Experiments.
Differences be
tween Calculation
and Experiment
in Inches of
Mercury.
Corresponding
Differences of
Temperature.
*304
3000
3000
ooo
o
310
32 52
3350
09S
+ 23
320
37 09
3706
+0 03
oo
330
4216
4210
+ 006
oi
340
4778
4730
+ 048
09
350
53 98
5380
+ 018
03
*360
6080
60 SO
ooo
oo
362
6224
6240
016
+oo
Table VII. — Vapour of Petroleum.
Temperatures in
Pressures in Inches of Mercury
Differences be
Degrees of
according to
tween Calculation
Corresponding
Fahrenheit, from
and Experiment
Differences of
the Ordinary
the Formula
Dr. Ure's
in Inches of
Temperature.
Zero.
(of two terms).
Experiments.
Mercury.
*316
30 00
3000
ooo
o
oo
320
31*71
3170
+001
00
330
3635
3640
005
+0T
340
4152
4160
008
+ 02
350
4727
4686
+ 041
07
360
5365
5330
+ 035
05
*370
6070
6070
ooo
375
6450
6400
+ 050
07
(1.)
(2.)
(3.)
(±.)
(5.)
I have also endeavoured, by means of the first two terms of the
formula, to approximate to the elasticity of the vapour of mercury, as
given by the experiments of M. Kegnault. The data and the constants
are as follows : —
Temperatures
Degree
the Freezing
Point.
in Centigrade
s from
the Absolute
Zero.
Pressures in
Millimetres of
Mercury.
Values of the Constants in the
Formula
Log. P = if
358
1779
6326
4525
76000
1072
a. for millimetres = 7 "5305
,, for English inches 6T259
Log. /3 Centigrade scale 3 4685511
,, Fahrenheit's scale, \
*£SF S*ii(»w"
inches, . . . . )
12
ON TOE ELASTICITY OF VArOURS.
The following table exhibits the comparative results of observation and
experiment : —
Tu'.i.r. VIII. — Vapour of Mercury.
TatnjiiTiiturrs in
Centigrade Degreei
i i area In Millimetres or Mercury
ding to
DIfferenoea iiotwoou
Oaloulatlon and
from Hi" l''i I
Point
tin> Formula
(.if two tenna),
M. ltognaull's
Experiments.
i 1 raerlment in
Millimetres.
7274
0115
0183
0 068
10011
0480
((•107
+0073
100 6
049
one,
007
1463
349
346
+ 03
•1779
10*72
1072
ooo
2005
2 1 85
2201
016
»358
760 00
760 00
00
The discrepancies are obviously of the order of errors of observation,
and the formula may be considered correct for all temperatures below
200°C, and for a short range above that point From its wanting the
third term, however, it will probably be found to deviate slightly from
the truth between 200° and 358°; while above the latter point it must
not be relied oil.
I have not carried the comparison below 72°, because in that part
of the scale the whole pressure! becomes of the order of errors of
observation.
In conclusion, it appears to me that the following proposition, to which
1 have been led by the theoretical researches referred to at the commence
ment of this paper, is borne out by all the experiments I have quoted,
especially by those of greatest accuracy, and may be safely and usefully
applied to practice.
If the maximum elasticity of any vapour in contact with its liquid be
tained for three points on the scale of the airthermometer, then the constants
of an equation of the form
may be determined, which equation will give, for that vapour, with an accuracy
limited only by the errors of observation, the relation between the temperature (t),
measured from the absolute zero (27 4'6 Centigrade degrees beloiu the freezing
point of ivatcr), and the maximum elasticity (P), at all temperatures between
those three points, and for a considerable range beyond them.
THE EXP„ ? LIQUIDS EY HI 13
EL— OS A FORMULA FOB CALCULATING THE EXPAJ
: LIQUIDS BY HEAT
Hating been lately much engaged in researches involving the comparative
volumes of liquids at various temperatures, I have found the following
formula very useful :
Log. Y represents the common logarithm of the volar. ..ass
of liquid, as compared with its volume at a certain standard temper;
which, for wa: temperature of its maximum density, or 4**1
i ie, and for other liqui 1 rigrade.
? is the temperature measured from the absolute zero mentioned in
paper on the of Vapours, in *
oi for July. 184 as preat&mg Vapct . ari u found by a/1 
.• " :
B. and C, are three constants, depending on the natur :
liquid, whose values for the Centigrade scale, corresponding to water,
Watg*,
a:../.\ .
Log.B_
:
1 ■
I ■ :7"4
' 
(HB29130
' .
'
. 15033
' 
1289X86
■
' 
12192054
HaHstrom : for mercury, from those of Begnault : and for alcohol and
n.~:.:~; .: :i: .. :r;n :._:. .: ..; :'...' As t:.^ :.~; :.v.\ : :
1L GayLossae give only the apparen t expansion of the liquids in glass, I
have assumed, in order to calrnlate the true expansion, that the dilatatio n
" '. :.. .l: . I   EL. %.:—.': ':■ . •  :' ".:... ':■'
14
THE EXPANSION OF LIQUIDS BY HEAT.
of the glass used by him was 0000258 of its volume for each Centigrade
degree. This is very nearly the mean dilatation of the different kinds of
glass. M. Kegnault has shown that, according to the composition and
treatment of glass, the coefficient varies between the limits "000022 and
•000028.
Annexed are given tables of comparison between the results of the
formula and those of experiment. The data from which the constants
were calculated are marked with asterisks.
The table for water shows, that between 0° and 30° Centigrade, the
formula agrees closely with the experiments of Hallstrom, and that from
30° to 100° its results lie betAveen those of the experiments of GayLussac
and Deluc.
The experiments of GayLussac originally gave the apparent volume of
water in glass as compared with that at 100°. They have been reduced
to the unit of minimum volume by means of Hallstrijm's value of the
expansion between 4 0, 1 and 30°, and the coefficient of expansion of glass
already mentioned.
In the fifth column of the table of comparison for mercury, it is stated
which of the experimental results were taken from M. Eegnault's own
measurements on the curve, representing the mean results of his experi
ments, and which from his tables of actual experiments, distinguishing the
series.
In the experimental results for alcohol and sulphuret of carbon, the
respective units of volume are the volumes of those liquids at their boiling
points, and the volumes given by the formula have been reduced to the
same units.
Expansion of Water.
Temperature on
Volume as compared with that at
Difference between
Authorities for
the Centigrade
4"1 C. ac(
oraing to
Calculation and
the
Scale.
the Formula.
the Experiments.
Experiment.
Experiments.
o
10001120
1 00010S2
+ 000003S
HallstrUm.
*4'1
10000000
10000000
0000000
Do.
10
1 0002234
10002200
+
0000034
Do.
20
1 0015668
10015490
+
00001 7S
Do.
*30
1 0040245
10040245
0000000
Do.
1 0041489

0001244
Deluc.
40
100750
100748
+
00002
GayLussac.
100774
_
00024
Deluc.
60
1 01718
1 01670
+
00048
GayLussac.
...
101773
_
00055
Deluc.
80
103007
102865
+
00142
GayLussac.
103092
_
00085
Deluc.
100
104579
104290
+
00289
GayLussac.
...
104664
 00085
Deluc.
THE EXPANSION OF LIQUIDS BY HEAT.
15
Expansion of Mercury.
Volume as compared with that at
Temperature on
the Centigrade
0° C. ace
ording to
M. Regnault's
Experiments.
Difference between
Calculation and
Remarks.
Scale.
the Formula.
Experiment.
o
*0
1000000
1000000
■oooooo
Curve.
9022
1016333
1016361
 000028
Series I.
100 00
1018134
1018153
 000019 •
Curve.
10052
1018230
1018267
 000037
Series I.
*15000
1027419
1027419
•oooooo
Curve.
19879
1036597
1036468
+ 000129
Series 11.
205 07
1037786
1037805
 000019
Series IV.
20557
1 037905
1037910
 000005
Series III.
*30000
1055973
1 055973
•oooooo
Curve.
Expansion of Alcohol.
Temperature on the
Volume as compared with that at 78°  41 C.
according to
Difference between
Calculation and
Centigrade Scale.
the Formula.
m. wayliussac s
Experiments.
Experiment.
o
3 41
•91795
•91796
 ooooi
*1S41
•93269
•93269
•ooooo
3341
•94803
•94799
+ 00004
*4841
•96449
•96449
•ooooo
6341
•9S183
•98210
 00027
*7S41
100000
100000
•ooooo
Expansion of Sulphuret of Carbon.
Temperature on the
Centigrade Scale.
Volume as compared
accorc
the Formula.
with that at 4G o 60C.
ing to
M. GayLussac's
Experiments.
Difference between
Calculation and
Experiment.
*  1340
+ 160
*1660
3160
*4660
•93224
•9476S
•96417
•98163
100000
•93224
•94776
•96417
•98163
100000
•ooooo
 00008
•ooooo
•ooooo
•ooooo
1G ELASTICITY OF GASES AND VAPOURS.
HI.— ON THE CENTRIFUGAL THEORY OF ELASTICITY,
AS APPLIED TO GASES AND VAPOURS.*
1. The following paper is an attempt to show how the laws of the
pressure and expansion of gaseous substances may be deduced from that
which may be called the hypothesis of molecular vortices, being a peculiar
mode of conceiving that theory which ascribes the elasticity connected
with heat to the centrifugal force of small revolutions of the particles of
bodies.
The fundamental equations of this theory were obtained in the year
1842. After having been laid aside for nearly seven years, from the want
of experimental data, its investigation was resumed in consequence of the
publication of the experiments of M. Regnault on gases and vapours. Its
results having been explained to the Royal Society of Edinburgh in
February, 1850, a summary of them was printed as an introduction to a
paper on the Mechanical Action of Heat in the twentieth volume of the
Transactions of that body. I now publish the investigation in detail in
its original form, with the exception of some intermediate steps of the
analysis in the second and third sections, which have been modified in
order to meet the objections of Professor William Thomson, of Glasgow,
to whom the paper was submitted after it had been read, and to whom I
feel much indebted for his friendly criticism.
This paper treats exclusively of the relations between the density, heat,
temperature, and pressure of gaseous bodies in a statical condition, or
when those quantities are constant. The laws of their variation belong
to the theory of the mechanical action of heat, and are investigated in the
other paper already referred to.
The present paper consists of six sections.
The first section explains the hypothesis.
The second contains the algebraical investigation of the statical relations
"between the heat and the elasticity of a gas.
The third relates to temperature and real specific heat.
The fourth treats of the coefficients of elasticity and dilatation of gases,
and compares the results of the theory with those of M. Regnault's
experiments.
* Eead before the Royal Society of Edinburgh, February 4, 1850, and published in
the Philosophical Magazine for December, 1851.
ELASTICITY OF GASES AND VAPOURS. 17
The fifth treats of the laws of the pressure of vapours at saturation.
The sixth relates to the properties of mixtures of gases of different
kinds.
I have endeavoured throughout this paper to proceed as directly as
possible to results capable of being compared with experiment, and to carry
theoretical researches no further than is necessary in order to obtain such
results with a degree of approximation sufficient for the purpose of that
comparison.
Section I. — On the Hypothesis of Molecular Vortices.
2. The hypothesis of molecular vortices may be defined to be that
which assumes — that each atom of matter consists of a nucleus or central point
enveloped by an elastic atmosphere, which is retained in its position by attractive
forces, and that the elasticity due to heat arises from the centrifugal force of those
atmospheres, revolving or oscillating about tlmr nuclei or central points.
According to this hypothesis, quantity of heat is the vis viva of the
molecular revolutions or oscillations.
Ideas resembling this have been entertained by many natural philoso
phers from a very remote period; but, so far as I know, Sir Humphry
Davy was the first to state the hypothesis I have described in an intelli
gible form. It appears since then to have attracted little attention, until
Mr. Joule, in one of his valuable papers on the Production of Heat by
Friction, published in the London and Edinburgh Philosophical Magazine for
May, 1845, stated it in more distinct terms than Sir Humphry Davy had
done. I am not aware, however, that any one has hitherto applied
mathematical analysis to its development.
3. In the present stage of my researches, there are certain questions
connected with the hypothesis as to which I have not found it necessary
to make any definite supposition, and which I have therefore left indeter
minate. Those questions are the following : —
First, Whether the elastic molecular atmospheres are continuous, or
consist of discrete particles. This may be considered as including the
question, whether elasticity is to a certain extent a primary quality of
matter, or is wholly the result of the repulsions of discrete particles.
Secondly, Whether at the centre of each molecule there is a real nucleus
having a nature distinct from that of the atmosphere, or a portion of the
atmosphere in a highly condensed state, or merely a centre of condensation
of the atmosphere, and of resultant attractive and repulsive forces. There
fore, although the word nucleus properly signifies a small central body, I
shall use it in this paper, for want of a better term, to signify an atomic
centre, whether a real nucleus or a centre of condensation and force. I
B
18 ELASTICITY OF GASES AND VAPOURS.
assume, however, that the volume of the nucleus, if any. is inappree: .
small as compared with that of the atmosphere.
■4. I have now : . supposition, which, so far as I am aware, is
peculiar to my own researches. It is this : — that the vibration ichich,
according to the undulatory hypothesis, t radiant light and heat, is a
motion of the atomic nuclei or centres, and by means of then
mutual attractions and repulsions.
It will be perceived at once, that from the combination of this sup
position with the hypothesis of molecular vortices, it follows that the
absorption of light and of radiant heat consists in the transference of
motion from the nuclei to their atmospheres; and conversely, that the
emission of light and of radiant heat is the b ace of motion from
the atmospheres to the nuclei
It appears to me that the supposition I have stated
advantages over the ordinary hypothesis of a luminiferous ether pervad
ing the spaces between ponderable particles, especially in the following
respects : —
. ue propagation of transverse vibrations requires the operation
of forces, which, if not altogether attractive, are of a very different
nature from those capable of producing gaseous and which it
is difficult to ascribe to such a substance as the ether is suppo;
whii between the atomic centres are perfectly
with their "being kept asunder by the ela~" Leir atm
nd. The immense velocity of light and radiant heat is a natural
consequence of th: ition, according to which the vibrating m
be extremely small as compared with the forces exerted by them.
rding to the most probable view of the theory of dispersion,
the unequal refrangibility of undulations of different lengths is a c
quence of the distances between the pa: the vibrating medium
having an appreciable magnitude as compared with tl _ v.s of the
undulations. This is scarcely conceivable of the ether, but easily con
ceivable of the atomic n
The manner in which the propagation of light and of radiant
heat is affected by the molecular arrangement of crystalline bodi
I much more intelligible if the vibrations are supposed to be t
of the atomic nuclei, on whose mutual forces and positions the form of
.llisation must depend.
— The consequences of this supposition, in the theory of double
:tion and polarisation, are pointed out and shown to be corroborated
by Pi . 5l kest experiments on diffraction, in a paper read to the
of Edinburgh on the 2nd of December, 1850, and publ.
in the 1 fkkai Magazine for June, 1851.]
ELASTICITY OF GASES AND VAPOURS. 19
Section II— Investigation of the General Equations between
the Heat and the Elasticity of a Gas.
5. I now proceed to investigate the statical relations between the heat
and the elasticity of a gaseous body ; that is to say, their relations when
both are invariable. The dynamical relations between those phenomena
which involve the principles of the mutual conversion of heat and
mechanical power by means of elastic fluids, and of the latent heat of
expansion and evaporation, form the subject of another paper.
6. It is obvious that, in the condition of perfect fluidity, the forces
resulting from attractions and repulsions of the atomic centres or nuclei
upon their atmospheres and upon each other, must be considered as being
sensibly functions merely of the general density of the body, and as being
either wholly independent of the relative positions of the particles, or
equal for so many different positions as to be sensibly independent of
them ; for otherwise a certain degree of viscosity would arise, and con
stitute an approach to the solid state. For the same reason, in the state
of perfect fluidity each atomic atmosphere must be considered as being
sensibly of uniform density in each spherical layer described round the
nucleus with a given radius, and the total attractive or repulsive force on
each indefinitely small portion of an atmosphere must be considered as
acting in a line passing through its nucleus ; that force, as well as the
density, being either independent of the direction of that line, or equal for
so many different and symmetrical directions as to be sensibly independent
of the direction.
7. An indefinite number of equal and similar atoms, under such con
ditions, will arrange themselves so that the form of their bounding
surfaces will be the rhombic dodecahedron, that being the nearest to a
sphere of all figures which can be built together in indefinite numbers.
8. I may here explain that by the term bounding surfaces of the atoms,
I understand a series of imaginary surfaces lying between and enveloping
the atomic centres, and so placed that at every point in these surfaces the
resultant of the joint actions of all the atomic centres is null. To secure
the permanent existence of each atom, it must be supposed that the force
acting on each particle of atomic atmosphere is centripetal towards the
nearest nucleus or centre.
The variation of that force in the state of perfect fluidity must be so
extremely small in the neighbourhood of those surfaces, that no appreci
able error can arise, if, for the purpose of facilitating the calculation of
the elasticity of the atmosphere of an atom at its bounding surface, the
form of that surface is treated as if it were a sphere, of a capacity equal
to that of the rhombic dodecahedron.
9. If the several atoms exercised no mutual attractions nor repulsions,
20 ELASTICITY OF GASES AND VAPOURS.
the total elasticity of a body would be equal to the elasticity of the atomic
atmospheres at their bounding surfaces. Supposing such attractions and
repulsions to exist, they will produce an effect, which, in the state of
perfect fluidity, will be a function of the mean density of the body ; and
which, for the gaseous state, will be very small as compared with the
total elasticity. Therefore, if p be taken to represent the superficial
elasticity of the atomic atmospheres, P the actual or total elasticity of the
fluid, and D its general density,
PP+/(D), .... (1.)
where /(D) is a function of the density, which may be positive or
negative according to the nature of the forces operating between distinct
atoms.
10. The following relations must subsist between the masses of the
atmosphere and nucleus, and the density and volume of each atom :
Let R represent the radius of the sphere already mentioned, whose
capacity is equal to the volume of an atom, that volume being equal to
t es 
Let /i denote the mass of the atmosphere of an atom, m that of the
nucleus, and M = fi + m the whole mass of the atom (so that if there is no
real nucleus, but merely a centre of condensation, m = 0, and M = /u).
Then D being the general density of the body, ^D is the mean
density of the atomic atmosphere, and M=— E 3 D.
o
If ull be taken to denote the distance of any spherical layer of the
atmosphere from the nucleus, the density of the layer may be repre
sented by
and the function ifm will be subject to this equation of condition,
ix=\ du\
J u0 V
M
which is equivalent to
iTrW^Du^u),
f du(u*rpu) (2.)
11. So far as our experimental knowledge goes, the more substances
are rarefied — that is to say, the more the forces which interfere with the
operation of the elasticity of the atomic atmospheres are weakened — the
more nearly do they approach to a condition called that of perfect gas, in
ELASTICITY OF GASES AND VAPOURS. 21
which the elasticity is simply proportional to the density. I therefore
assume the elasticity of the atomic atmosphere at any given point to be
represented by multiplying its density at that point by a constant
coefficient b, which may vary for different substances, but, as I have
already stated, without deciding whether that elasticity is a primary
quality or the result of the repulsion of particles. Consequently, the
superficial atomic elasticity
P ^bp)m, .... (3.)
^(1) being the value of \pu, which corresponds to the bounding surface of
the atom, where u = 1.
12. Let an oscillatory movement have been propagated from the
nuclei to every part of their atmospheres, the size of the orbits of
oscillation being everywhere very small as compared with the radii of
the atoms, and let this movement have attained a permanent state, which
will be the case when every part of each atmosphere, as well as each
nucleus, moves with the same mean velocity, v — mean velocity signifying
that part of the velocity which is independent of periodic changes. It is
necessary to suppose that the propagation of this movement to all parts
of a molecular atmosphere is so rapid as to be practically instantaneous.
We shall conceive all the masses and densities referred to, to be
measured by weight. Then taking g to represent the velocity generated
by the force of gravity at the earth's surface in unit of time, the whole
mechanical power to which the oscillatory movement in question is
equivalent in one atom will be represented in terms of gravity by
V= 2; {L)
that is to say, the weight of the atom, M, falling through the height —
due to the velocity v ; and this is the mechanical measure of the quantity
of heat in one atom in terms of gravity.
13. Any such motion of the particles of a portion of matter confined
in a limited space, will in general give rise to a centrifugal tendency with
respect to that space. In order to obtain definite results with respect to
that centrifugal tendency in the case now under consideration, it is
necessary to define, to a certain extent, the general character of the
supposed movement.
In the first place, it is periodical ; secondly, it is similar with respect to
so large a number of radii drawn in symmetrical directions from the
atomic centre, as to be sensibly similar in its effects with respect to all
directions round that centre. Thi3 symmetry exists in the densities of
the different particles of the atomic atmosphere in a gas, and in the
22 ELASTICITY OF GASES AND VAPOURS.
forces which act upon them ; and we are therefore justified in assuming
it to exist in their motions.
Two kinds of motion possess these characteristics :
First, Radial oscillation, by which a portion of a spherical stratum of
atmosphere surrounding an atomic centre, being in equilibrio at a certain
distance from that centre, oscillates periodically to a greater and a less
distance. This forms part of the vis viva of the molecular movements ;
but it can only affect the superficial atomic elasticity by periodic small
variations, having no perceptible effect on the external elasticity.
Second, Small rotations and revolutions of particles of the atomic
atmosphere round axes in the direction of radii from the atomic centre,
by which each spherical layer is made to contain a great number of equal
and similar vortices, or equal and similar groups of vortices having their
axes at right angles to the layer, and similarly situated with respect to a
great many symmetrical directions round the atomic centre.
Let us now consider the condition, as to elasticity, of a small vortex
of an atmosphere whose elasticity is proportional to its density, inclosed
within a cylindrical space of finite length, and not affected by any force
at right angles to the axis except its own elasticity. Let Z denote the
external radius of the cylinder, p x its external density, p its mean density,
p the density at any distance z from the axis (all the densities being
measured by weight), w the uniform velocity of motion of its parts. The
condition of equilibrium of any cylindrical layer is, that the difference of
the pressures on its two sides shall balance the centrifugal force ; con
sequently (b being the coefficient of elasticity)
(jz dz '
The integral of this equation is
,>■'■
p' = az b 'S
The coefficient a is determined by the following relation, analogous to
that of equation (2), between the densities :
Z 2
p =J dz(pz)= ^—Z»
whence
V+ 2
■ti +I
And the general value of the density is
,,2
£+i)®* ' (5 °
ELASTICITY OF GASES AND VAPOURS. 23
Making z = Z, and multiplying by the coefficient of elasticity b, we
obtain for the elasticity of the atmosphere, at the cylindrical surface of
the vortex,
b Pi = b P + l£> • • • W
which exceeds the mean elasticity bp by a quantity equivalent to the
weight of a column of the mean density p, and of the height due to the
velocity u; and independent of the radius of the vortex.
Supposing a spherical layer, therefore, to contain any number of
vortices of any diameter, in which the mean density is equal, it is .
necessary to a permanent condition of that layer that the velocities in
all these vortices should be equal, in order that their lateral elasticities
may be equal.
Although the mean elasticity at the plane end, or any plane section at
right angles to the axis of a vortex, is simply = bp, being the same as if
there were no motion, yet the elasticity is variable from point to point,
and the law of variation depends on the velocity. Therefore, if two
vortices are placed end to end, it is necessary to a stable condition of
the fluid, not only that their terminal planes should coincide, and that
their mean elasticities should be in cquilibrio, but also that their velocities
should be equal, or subject only to periodical deviations from a state of
equality.
Therefore, the mean velocity of vortical motion, independent of small
periodic variations, is the same throughout the whole atomic atmosphere;
and the mean total velocity, independent of small periodic variations,
being uniformly distributed also, the vis viva of the former may be
expressed as a constant fraction of that of the latter, so that
v? = ^, . . . . (5b.)
 being the mean value of a coefficient which is subject to small periodical
k
variations only *
* As it has been represented to me that I have, without stating sufficient grounds,
assumed the velocity of revolution w to be constant throughout each individual
vortex, I add this note to assign reasons for that supposition.
First, Unless w, the velocity of revolution of a particle, is independent of z, its
radius vector, the atomic atmosphere cannot be in a permanent condition.
For if w is a function of z, the external elasticity of a vortex will be a function of
its diameter. If the whole atmosphere is in motion, vortices of different diameters
must exist in the same spherical layer ; and if their external elasticities are different,
their condition cannot be permanent.
,Second, Whatsoever may be the nature of the forces by which velocity is communi
cated throughout the atmosphere, the tendency of those forces must be to equalise
that velocity, and thus to bring about a permanent condition.
24 ELASTICITY OF GASES AND VAPOURS.
This coefficient, being the ratio of the vis viva of motion of a peculiar
kind to the whole vis viva impressed on the atomic atmospheres by the
action of their nuclei, may be conjectured to have a specific value for each
substance, depending, in a manner "as yet unknown, on some circumstance
in the constitution of its atoms. It will afterwards be seen that this
circumstance is the chemical constitution.
Let the entire atmosphere of an atom be conceived to be divided into
a great number of very acute pyramids meeting at the centre, and having
even numbers of faces, equal and opposite in pairs ; and let one of these
pyramids, intersecting a spherical layer whose distance from the nucleus
is R?i and thickness Tulu, cut out a frustum, containing and surrounded
by vortices. Consider one pair of the faces of that frustum ; their length
being TMu, let their breadth be h, and their distance asunder/. Then
they make with each other the angle at the apex of the pyramid
o s ; n i ; J .
their common area is KRdu ; and the sum of the volumes of the two
triangular frusta of the spherical layer, included by diagonal planes drawn
between their radial edges, is
fKRflll
2 '
the sum of all such triangular frusta being the whole volume of the
spherical layer.
The additional pressure due to the centrifugal force of vortices, viz. —
2gV
acts on the two lateral faces, its total amount for each being
2gk
The transverse components of this pair of forces balance each other.
Their radial components, amounting to
/ v 2 p 1  rt7 iPpfhdu
Hu 2gk 2gku
constitute a centrifugal force relatively to the atomic centre, acting on the
pah* of triangular frusta whose mass is
pfJiRdu
2 '
The condition of permanent, or periodical, equilibrium of this pair of
frusta, requires that this centrifugal force shall be balanced by the varia
tion of the mean elasticity of the atmosphere at the two surfaces of the
ELASTICITY OF GASES AND VAPOURS. 25
spherical layer, combined with the attraction of the nucleus. The action
of the former of these forces is represented by
7 dp y ill
— b^au X — .
cm 2
Let the accelerating force of attraction towards the nucleus be repre
sented by
<f>(Rn)
E, '
^ being a function, which, by the definition of an atomic bounding
surface in article 8, is null at that surface, or when u = 1. Then the
attraction on the pair of frusta is
pfhdu(p(Ru)
2 "
Add these three forces together; let the sum be divided by
■^pfhdw,
and let the density p be denoted, as in article 10, by
then the folloAving differential equation is obtained as the condition of a
permanent state of the atomic atmosphere :
T77T r^0 = O. . . . (5c.)
gku \p(u) du r
This equation will be realised for each layer at its mean position, on
each side of which its radial oscillations are performed.
The variation of this expression being of opposite sign to the variation
of , shows that any small disturbance of the density produces a force
tending to restore that distribution to the state corresponding to the
position of equilibrium of the layers, and therefore that the state indicated
by equation (5 c) is stable.
1 4. The integral of equation (5c) is
xPu=u^e a ~"f U 1 du <P. . . . (6.)
The arbitrary constant a is determined from the equation of condition
(2) in the following manner : —
2G ELASTICITY OF GASES AND VAPOURS.
Substituting for $11 in equation (2) its value as given above, we obtain
or
r r w
•=s/*(« 3B+ V*/ 1 * , *) J . . . (7.)
which integration having been effected, we shall obtain for the value of
the superficial elasticity of the atomic atmospheres,
*=OWl) = 6£D*
• (8.)
M rv J M
To obtain an infinite series for approximating to the value of the
integral in equation (7), let the following substitutions be made : —
Lo" u=X
1
gkb
+ 3 = 30
J l — (O,
(9.)
and let the values of the successive differential co
efficients of 10 with respect to A, when A = 0, $ = 0,
and w=r 1, be denoted by
(a/), (to"), (w'"), &c.
Then
e a =3J d\.e sex u>.
 so
The value of which (when the function (p is such as to admit of its having
a finite value) is
6 0V 36 + 90 270 s + &C '/'
whence i
1 + W + ^90^
+
27 3
+ &c.)
Now, because (a/) = — t0 ( m = d = 0,
^(l) = e« = e(l^]+&c.) j '
which may be represented by
Gi +l )( l  r < D ^)' '
(10.)
(10a.)
ELASTICITY OF GASES AND VAPOURS. 27
F(D,0) being a quantity which becomes continually less as the density
becomes less and the heat greater. The complete expression for the
elasticity of a gas is therefore, according to equations (1), (8), and (10a),
P^+/(D)^D(^ + j)(lF(D,e))+/(D);. (11.)
when each atom contains a quantity of heat measured by the mechanical
power corresponding to the velocity v in the weight M, or
Mr 2
according to equation (4).
Section III. — Of Temperature, and of Real Specific Heat.
15. The definition of temperature consists of two parts: — First, the
definition of that condition of two portions of matter when they are said
to be at the same temperature ; and" second, the definition of the measure of
differences of temperature.
Two bodies are said to be at the same temperature when there is no
tendency for one to become hotter by abstracting heat from the other ;
that is to say (calling the two bodies A and B), when there is either no
tendency to transmission of heat between them, or when A transmits as
much heat to B as B does to A. Now it is known by experiment, that
any surface or other thing which affects the transmission of heat being
placed between B and A, has exactly the same influence upon the
same quantity of heat passing in either direction; therefore, to produce
equilibrium of temperature between A and B, the powers of their atoms
to communicate heat must be equal.
15a. If we apply to vortices at the surface of contact of the atmo
spheres of two atoms of the same or different kinds, the conditions of
permanency laid down in article 1 3 for vortices in the same atmosphere,
these conditions take the folloAving form : —
First, The superficial atomic mean elasticities must be the same ; in
other words, the superficial atomic mean densities must be inversely as
the coefficients of elasticity of the atmospheres. This is the condition of
equilibrium of pressure.
Second, The law of variation of the elasticity from the centre to the
circumference of a vortex, as expressed in equation (5), must be the same
.2 2
for both atoms; and this law depends on the quantity ~ =y h ; therefore
the condition of equilibrium of heat is, that the square of the velocity of
vortical motion, divided by the coefficient of atmospheric elasticity, shall
28 ELASTICITY OF GASES AND VAPOURS.
1)0 the same for each atom. Of this quantity, therefore, and of constants
common to all substances, temperature must be a function.
Taking the characteristics (A) and (B) to distinguish the quantities
proper to the two atoms, we have the following equation : —
jgD*(l)(A>=8D*(l)(B)'
>=>),[ • • (12.)
temperature = f — , universal constants ) .
16. In sx perfect gas, equation (11) is reduced to
the pressure being simply proportional to the mean elasticity of the atmo
spheric part of the gas multiplied by a function of the heat, which, as
equation (12) shows, is a function of the temperature, from its involving
only — and universal constants.
kb
Therefore, in two perfect gases at the same pressure and temperature,
the mean elasticities of the atmospheric parts arc the same, and
consequently —
The mean specific gravities (if the atmospheric parts of all perfect gases are
inversely proportional to the coefficients of atmospheric elasticity.
Let n therefore represent the number of atoms of a perfect gas which
fill unity of volume under unity of pressure at the temperature of melting
ice, so that nM is the total specific gravity of the gas, and njx that of its
atmospheric part ; then
bn/n = constant for all gases, . . (12b.)
and consequently
kb k { ' }
Therefore,
Temtperatwre is a function of universal constants, and of the vortical vis viva
of the atomic atmospheres of so much of the substance as would, in the condition
of perfect gas, fill unity of volume under unity of pressure at some standard
temperature.
The equation (12a) further shows, that in any two perfect gases the
respective values of the quotient of the pressure by the density corresponding to
the same temperature, bear to each other a constant ratio for all temperatures,
being that of the values of the coefficient b ~.
° M
ELASTICITY OF GASES AND VAPOURS. 29
Therefore the pressure of a perfect gas at a given density, or its volume
under a given pressure, is the most convenient measure of temperature.
Let P represent the elasticity of a perfect gas of the density D at the
temperature of melting ice, P that of the same gas at the same density, at
a temperature distant T degrees of the thermometric scale from that of
melting ice, and C a constant coefficient depending on the scale employed;
then the value of T is given by the equation
PPol
T=C
Po
(13.)
or r •
TfC=C~j
The value of the constant C is found experimentally as follows: — Let
Pj represent the elasticity of the gas at the temperature of water boiling
under the mean atmospheric pressure, T 1 the number of degrees, on the
scale adopted, between the freezing and boiling points of water; then
P — P 1
rp p r l X
1 1 5
and 1 (14.)
c = t 'pFpJ
C is in fact the reciprocal of the coefficient of increase of elasticity with
temperature, or the reciprocal of the coefficient of dilatation, of a perfect
gas at the temperature of melting ice.
17. As it is impossible in practice to obtain gases in the theoretical
condition referred to, the value of C can only be obtained by approxima
tion. From a comparison of all M. Eegnault's best experiments, I have
arrived at the following values, which apply to all gaseous bodies.
For the Centigrade scale, C = 274 0, 6, being the reciprocal of
000364166.
For Fahrenheit's scale, if adjusted so that 180° are equal to 100°
Centigrade, —
C for temperatures measured from the freezingpoint of water
= 494°28.
C for temperatures measured from the ordinary zero
= 494°2832° = 462°28.
The point C degrees below the ordinary zero of thermometric scales
may be called the absolute zero of temperature ; for temperatures measured
from that point are proportional to the elasticities of a theoretically
perfect gas of constant density.
30
ELASTICITY OF GASES AND VAPOURS.
Temperatures so measured may be called absolute temperatures. Through
out this paper I shall represent them by the Greek letter t, so that
T + C.
(15.)
It is to be observed, that the absolute zero of temperature is not the
absolute zero of heat.
18. If we now substitute for P in equation (13) its value according
to equation (12a), we obtain the following result: —
'= T + C = c £i(£*+ J )
o V: V/''
Let n represent, as before, the theoretical number of atoms in unity of
volume under unity of pressure, at the temperature of melting ice, of the
gas in question, supposing the disturbing forces, represented by  F(D,0)
and /(D), to be inappreciable; then nM is the weight of unity of volume
under those circumstances, and it is evident that
Consequently
D AT
1
*{jf k + h
(16.)
being the complete expression for that function of heat called temperature.
It follows that the function 0, which enters into the expressions for the
elasticity of gases, is given in terms of temperature by the equation
oglcb
Cnjub'
(16a.)
If, according to the expression 4, for the quantity of heat in one atom
we substitute ^ for v 2 in equation (16), we obtain the following equa
tions :
Mv 2 3£M/ r A
(17.)
and if Q represent the quantity of heat in unity
of weight,
19. The real specific heat of a given substance is found by taking the
differential coefficient of the quantity of heat with respect to the tempera
ELASTICITY OF GASES AND VAPOURS. 31
ture. Hence it is expressed in various forms by the following equations,
in which the coefficient j is supposed not to vary sensibly with the tem
perature.
Eeal specific heat of one atom,
dq _ 3&M
dr ~2CV
real specific heat of unity of weight,
dQ__U_
dT~2CV < 18 >
real specific heat of so much of a perfect gas as
occupies unity of volume under unity of pressure
at the temperature of melting ice,
dq _ 3&M
n dr~~2Cjx
The coefficient  — , representing the ratio of the total vis viva of the
motions of the molecular atmospheres to the portion of vis viva which pro
duces elasticity, multiplied by the ratio of the total mass of the atom to
that of its atmospheric part, is the specific factor in the capacity of an
atom for heat. The view which I have stated as probable in article 1 3
— that the first factor of this coefficient is, like the second, a function of
some permanent peculiarity in the nature of the atom — is confirmed by
the laws discovered by Dulong : that the specific heats of all simple atoms
bear to each other very simple ratios, and generally that of equality ; that
the same property is possessed by the specific heats of certain groups of
similarly constituted compound atoms ; and that the specific heats of equal
volumes of all simple gases, at the same temperature and pressure, are
equal.
The coefficient — varies in many instances to a great extent for the
A*
same substance in the solid, liquid, and gaseous states. So far as experi
ment has as yet shown, it appears not to vary, or not sensibly to vary,
with the temperature j and this I consider probable a priori, except at or
near the points of fusion of solid substances.
Apparent specific heat differs from real in consequence of the con
sumption and production of certain quantities of heat by change of
volume and of molecular arrangement, which accompany changes of
temperature.
This subject belongs to the theory of the mechanical action of
heat.
32 ELASTICITY OF GASES AND VAPOURS.
Section IV. — Of the Coefficients of Elasticity and Dilatation
of Gases.
1) T
20. If in equation (11) we substitute f or — ? + b its value ~— we
obtain the following value for the elasticity of a gas,
D
^1_F(D,0))+/(D); . . (19.)
P ~~ nM C
in which  denotes the ratio of the actual weight of unity of volume to
wM
the weight of unity of volume under unity of pressure, at the absolute
temperature C, in the theoretical state of perfect gas ;
t is the absolute temperature ;
 F(D, 6) is a function of the temperature and density, representing the
effect of the attraction of the atomic nucleus or centre in diminishing the
superficial elasticity of its atmosphere ;
And /(D) is a function of the density only, representing the effect of the
mutual attractions and repulsions of the atoms upon the whole elasticity
of the body.
From this equation are now to be determined, so far as the experiments
of M. Eegnault furnish the requisite data, the laws of the deviation of
gases from that theoretical state in which the elasticity is proportional to
the density multiplied by the absolute temperature.
21. The value of F(D, 0) is given by the infinite series of equations
T
(10), (10fl), substituting in which for its value » — = we obtain the
following result : —
A., A 2 , A 3 , &c, being a series of functions, the value of which is given by
the following equation : —
~K= 31^ ®l + mi ' ■ ( 20 )
35 1+7n being the coefficient of z 1+m in the development of the reciprocal of
the series
1  (w')x + (a/> 2  (a/ ")a? + &c,
when (d)') &c. have the values given in equation (9).
Equation (19) is thus transformed into
p =a(ev? &c W< D > • ^
The series in terms of the negative powers of the absolute temperature
ELASTICITY OF GASES AND VAPOURS. S3
converges so rapidly, that I have found it sufficient, in all the calculations
I have hitherto made respecting the elasticity of gaseous bodies, to use the
first term only, .
T
22. Instead of making any assumption respecting the laws of the
attractions and repulsions which determine the functions A and /(D), I
have endeavoured to represent those functions by empirical formulae,
deduced respectively from the experiments of M. Kegnault on what he
terms the coefficient of dilatation of gases at constant volume, which ought
rather to be called the coefficient of increase of elasticity with temperature, and
from his experiments on the compressibility of elastic fluids at constant
temperature.
From the data thus obtained I have calculated, by means of the theory,
the coefficients of dilatation of gases under constant pressure, which, as a test of
the accuracy of the theory, I have compared with those deduced by
M. Kegnault from experiment.
23. The mean coefficient of increase of elasticity with temperature at
constant volume between 0° and 100° of the Centigrade thermometer is
found by dividing the difference of the elasticities at those two tempera
tures by the elasticity at 0°, and by 100°, the difference of temperature.
It is therefore represented by
P — P
E — ^ — ° (22/)
ioo°p ' • K }
where E represents the coefficient in question, and P and P x the elasti
cities at 0° and 100° Centigrade respectively.
Now by equation (21), neglecting powers of  higher than the first, we
have
^=30 £)+'<«
D_ /100° + C _ A \
1_ »M\ C 100° + C/ +M h
whence
D /l A \ , .
_ 7iMP VC + C(C + 100 o )/• ' ' ' t ■'
Supposing the value of ... ■ to be known, this equation affords the
11 ° wMP ' *
means of calculating the values of the function A corresponding to various
densities, from those of the coefficient E as given by experiment.
C
34,
ELASTICITY OF GASES AND VAPOURS.
As a gas is rarefied _ approximates to unity, A diminishes without
limit, and the value of E consequently approximates to ~, the reciprocal
of the absolute temperature at 0° Centigrade. This conclusion is verified
by experiment ; and by means of it I have determined the values already
given— viz., C = 274°6 Centigrade, and ^ = 003641 6G for the Centigrade
scale.
C
24. In order to calculate the values of
D
//Ml',,
1 have made use of
empirical formulae, deduced from those given by M. Regnault in his
memoir on the Compressibility of Elastic Fluids. In M. Regnault's
formulae, the unit of pressure is one metre of mercury, and the unit of
density the actual density corresponding to that pressure. In the formula?
which I am about to state, the unit of pressure is an atmosphere of 760
millimetres of mercury, or 29  922 inches; and the unity of density, the
theoretical density in the perfectly gaseous state at 0° Centigrade, under a
pressure of one atmosphere, which has been found from M. Regnault's
M?iP
formula' by making the pressure = in the value of . — °. M. Regnault's
experiments were made at temperatures slightly above the freezing point,
but not sufficiently so to render the formula; inaccurate for the purpose of
, , . , . . . D
calculating the ratio in question, ^ p .
The formula; are as follows : —
Supposing — given,
(Vdi*
rD
L> a
D\ 2
M/ :
which, when T is small, or  nearly = C, gives an
approximate value of  .
Supposing P given,
I)
CwMP
= l+ 7 P f £ P 2 ;
which, when T is small, gives an approximate
value of
wMP '
,
4.
The values of the constants a, /3, 7, e, and of their logarithms, are giverj,
together with the mean temperatures above the freezing point at which
ELASTICITY OF GASES AND VAPOUES.
35
M. Eegnault's experiments were made, for atmospheric air, carbonic acid
gas, and hydrogen.
ft
Atmospheric Air.
Constants.
 7 =  000860978
+ 000011182
 000009700
4°75.
Logarithms.
49349920
50485140
69867717
Carbonic Acid Gas. T = 3°27.
a =  y =  00641836 38074242
/3 = _ 0000041727 66204126
£ = + 0000865535 59372846
Hydrogen. T = 4°75.
=
+ 000403324
+ 0000048634
_ 0000044981
46056546
66869401
66530291
The three substances abovementioned are the only gases on which
experiments have yet been made, under circumstances sufficiently varied
to enable me to put the theory to the test I have described in article
22.
25. M. Regnault has determined the values of the coefficient of
elasticity E for carbonic acid at four different densities, and for atmospheric
air at ten. By applying equations (23) and (24) to those data, I have
ascertained that the function A for these two gases may be represented
empirically, for densities not exceeding that corresponding to five
atmospheres, by the formula? given below, which lead to formulae for the
coefficient E.
For Carbonic Acid,
A = a
D
where log. a = 03344538, and consequently
«MP CV " r C+100° mM
a
lo
C + 100 1
= 37608860.
(25.)
3G
ELASTICITY OF GASES AND VAPOURS.
For Atmospheric Air,
A = a[ —T7
r,
10 .
where log. «=03176168, and consequently
E =
»MP 'C
S0+
l0£
"C + 100°
1
c + ioo°
37440490
*)
(26.)
The value of log. ~ is 35012995.
The following table shows that those empirical formula; accurately
represent the experiments, the greatest differences being less than one
half of 00001 3 C, which M. Kegnault, in the seventyfirst page of his
memoir, assigns as the limit of the errors of observation due to barometric
measurements alone.
As the coefficient E for hydrogen has been determined for one density
only, it is impossible to obtain an empirical formula for that gas. The
single ascertained value of E is nevertheless inserted in the table.
Table of Coefficients of Increase of Elasticity with
Temperature at Constant Volu.m i
Pressure at
Density
Coefficient E
Coefficient E
Difference be
Atmospheres
= £„.
D
nM"
according to the
Formula.
according to
Experiment.
Formula; and
Experiment.
CARBONIC ACID.
I.
9980
100448
•003G865
•0036856
+ 0000009
II.
1 1857
119487
•0036951
•0036943
+•0000008
III.
2 2931
2327S8
•0037465
•0037523
 0000058
IV.
47225
487475
•003S647
•0038598
+ 0000049
ATMOSPHERIC AIR.
I.
01444
01444
•0036484
•0036482
+ '0000002
II.
022'J4
02294
•0036507
•0036513
 0000006
III.
03501
03502
•0036535
•0036542
 0000007
IV.
0*4930
04932
•0036564
•0036587
 0000023
V.
04937
04939
•0036564
•0036572
 oooooos
VI.
10000
1000S5
•0036652
•0036650
+ 0000002
VII.
22084
22125
•0036810
•0036760
+ 0000050
VIII.
22270
22312
•0036812
•0036800
+ 0000012
IX.
28213
2S279
•0036880
•0036894
 0000014
X.
48100
48289
•0037081
•0037091
 oooooio
]
iYDROGEN.
10000
09996
No formula.
•0036678
1
ELASTICITY OF GASES AND VAPOURS.
37
26. The empirical formula? (24), representing the experiments of M.
Eegnault on the compressibility of carbonic acid gas, atmospheric air,
and hydrogen at certain temperatures, give for these temperatures the
values of a function which is theoretically expressed by
CraMP CA CwM/(D)
rD T f tD *
(27.)
r ■ 1 • C»MP p . ■ . , .,
It is evident, that supposing the value of — =— tor any given density
TXJ
to be known by experiment, and that of A to be calculated from the
value of the coefficient E, or from the empirical formulae (25) and (26),
nMf(D)
the corresponding value of the function
1)
may be determined by
means of equation (27).
By this method I have obtained the following empirical formulae for
calculating the values of that function : —
For Carbonic Acid,
«M/(D) _ D
D
mM'
where log. A =3*1083932.
Far Atmospheric Air,
"M/(D) _ / D
D
where log. h= 38181545.
nMf(D)
*
»il
(28.)
As only one value of
I)
for hydrogen can at present be ascer
tained, it is impossible to determine a formula for that gas. The single
value in question is —
For P = 1 atmosphere, wM ^ D ) = 01059. . (29.)
27. I now proceed to determine theoretically, from the data which
have already been obtained, the mean coefficients of dilatation at constant
pressure, between 0° and 100° of the Centigrade scale, for the three gases
under consideration, at various pressures.
Let E' represent the coefficient required: S and S x the respective
%M
values of =r for 0° and 100° under the pressure P, that is to say, the
volumes occupied by the weight nM at those temperatures ; A and A v
38
ELASTICITY OF GASES AND VAPOURS.
/ and f v the corresponding values of A and /(D). Then from equation
(21) we deduce the following results: —
So = f(l^ Q +S /o)
s V14 1000  A < +S f V
• ' ~i'V r C C + lOO ' ' lJl )'
and consequently
E' =
s i _ ,s ,>
100 s
1 /l
+
S P\C ' 100C 100(C+100)
100
)• • (30.)
In applying the empirical formuhe (25), (26), and (28), to determine
the values of Aj and S, /, in the above equation, it will produce no
appreciable error to use TTTfJ^o as an approximate value of D x for
that purpose only. By making the necessary substitutions, the following
formula? arc obtained : —
For Carbonic Acid,
wMPVC" 1 " »M/ J
where log. 0= 5*5189349.
For Atmospheric Air,
L ",»Ml' Vc +a V»M/ ~P'\nW )
where log. a =5*4717265
log. /3 = 6*9759738
J
(31.)
28. The following table exhibits a comparison between the results
of the formulae and those of M. Eegnault's experiments. It is not, like
the preceding table (article 25), the verification of empirical formulae,
but is a test of the soundness of the theoretical reasoning from which
equations (30) and (31) have been deduced.
It is impossible, from the want of a sufficient number of experimental
data, to give a formula similar to (31) for hydrogen. I have calculated,
however, the value of the coefficient E' for that gas, corresponding to the
pressure of one atmosphere, on the assumption that at that pressure a
formula similar to that for carbonic acid gas is applicable without sensible
error.
ELASTICITY OF GASES AND VAPOURS.
39
The table shows only one instance in which the difference between the
result of the theory and that of experiment exceeds '0000136; the limit,
according to M. Eegnault, of the errors of observation capable of arising
from one cause alone, — the uncertainty of barometric measurements.
That discrepancy takes place in one of the determinations of the coefficient
E' for carbonic acid gas under the pressure of one atmosphere. In the
other determination the discrepancy is less than the limit.
The agreement between theory and experiment is most close for the
highest pressures; and M. Eegnault has shown that the higher the pressure
the less is the effect of a given error of observation in producing an error
in the value of the coefficient.
The theory is therefore successful in calculating the coefficients of
dilatation of gases, so far as the means at present exist of putting it to
the test.
Table of Coefficients of Dilatation under Constant Pressure,
showing a Comparison between Theory and Experiment.
Pressure in
Atmospheres.
Coefficient E' according
to the Theory.
Coefficient E' according
to M. Kegnault's
Experiments.
Difference between
Theory and
Experiment.
CARBONIC ACID GAS.
1000
3316
•00369S8
■0038430
First Memoir.
•0037099
Second Memoir.
•003719
First Memoir.
•0038450
 0000111
 0000202
 0000020
ATMOSPHERIC AIR.
10000
33224
34474
•0036650
■0036955
•0036969
First Memoir.
•0036706
Second Memoir.
003663
•003667
First Memoir.
•0036944
•0036965
 0000056
+ 0000020
 0000020
+ •0000011
+ 0000004
HYDROGEN.
10000
•0036598
•0036613
 0000015
40 ELASTICITY OF GASES AND VAPOURS.
Section V. — Of the Elasticity of Vapour in Contact with the
same Substance in the Liquid or Solid State.
29. As the most important phenomena of evaporation take place
from the liquid state, I shall generally use the word liquid alone through
out this section in speaking of the condition opposed to the gaseous state ;
but all the reasonings are equally applicable to those cases in which a
substance evaporates from the solid state.
30. In considering the state of a limited space entirely occupied by
a portion of a substance in the liquid form, and by another portion of the
same substance in the form of vapour, both being at rest, the most obvious
condition of equilibrium is, that the total elasticity of the substance in each
of the two states must be the same; that is to say,
P=^4V(D )=i J i+/(D 1 ), • • • (32.)
where p represents the superficial atomic elasticity in the liquid state, p x
that in the gaseous state, and /(D ), /(DJ, the corresponding values of the
pressures, positive or negative, due to mutual actions of distinct atoms.
31. A second condition of equilibrium is, that the superficial elastici
ties of two contiguous atoms must be equal at their surface of contact.
Hence, although there may be an abrupt change of density at the bounding
surface between the liquid and the vapour, there must be no change of
superficial atomic elasticity except by inappreciable degrees; and at that
bounding surface, if there is an abrupt change of density (as the reflexion
of light renders probable), there must be two densities corresponding to
the same superficial atomic elasticity.
32. A third condition of equilibrium is to be deduced from the mutual
attractions and repulsions of the atoms of liquid and of vapour. In a gas
of uniform density, those forces, acting on each individual particle at an
appreciable distance from the bounding surface, balance each other, and
have accordingly been treated as merely affecting the total elasticity of the
body by an amount denoted by /(D) ; but near the bounding surface of a
liquid and its vapour, it is obvious that the action of the liquid upon any
atom must be greater than that of the vapour. A force is thus produced
which acts on each particle in a line perpendicular to that bounding
surface, and which is probably attractive towards the liquid, very intense
close to the bounding surface, but inappreciable at all distances from it
perceptible to our senses. Such a force can be balanced only by a gradual
increase of superficial atomic elasticity in a direction towards the liquid.
Hence, although at perceptible distances from the surface of the liquid the
density of vapour is sensibly uniform, the layers close to that surface are
probably in a state of condensation by attraction, analogous to that of the
earth's atmosphere under the influence of gravity.
ELASTICITY OF GASES AND VAPOURS. 41
Professor Faraday has expressed an opinion, founded on his own experi
ments and those of MM. Dulong and Thenard, that a state of condensation
exactly resembling that which I have described is produced in gases by
the superficial attraction of various substances, especially platinum, and
gives rise to chemical actions which have been called catalytic.
To express this third condition algebraically, let the boundary between
the liquid and the vapour be conceived to be a plane of indefinite extent,
perpendicular to the axis of x ; and let positive distances be measured in
a direction from the liquid towards the vapour.
Let x, x + dx represent the positions of two planes, perpendicular to the
axis of x, bounding a layer whose thickness dx is very great as compared
with the distance between two atomic centres, but very small as compared
with any perceptible distance, and let a portion of the layer be considered
whose transverse area is unity.
Let p represent the mean density of the layer. Then it is acted upon
by a force
pXrfx,
the resultant of the actions of all the neighbouring atoms, which has the
negative sign, because it is attractive towards the liquid, X being a func
tion of the position of the layer in question, and of the densities and
positions of all the neighbouring layers.
The superficial atomic elasticity behind the layer being p, and in front
of it p + j dx, it is also acted on by the force
to 6 *'*
hence its condition of equilibrium is
!+» x =° ( 33 >
In order to integrate this equation, so as to give a relation applicable at
perceptible distances from the surface, let x , x x represent the positions of
two planes perpendicular to the axis of x, the former situated in the
liquid, the latter in the vapour, and so far asunder that the densities
beyond them are sensibly uniform, and equal respectively to D for the
liquid and D 1 for the vapour, the corresponding superficial atomic elastici
ties being p and p v Then dividing equation (33) by p, and integrating
between the limits x Q and x v the result obtained is
o l J o
Had we a complete knowledge of the laws of molecular forces in the
solid, liquid, and gaseous states, this equation, taken in conjunction with the
42 ELASTICITY OF GASES AND VAPOURS.
two conditions previously stated, would be sufficient to determine formulae
for calculating the total elasticity and the respective densities of a liquid
and its vapour when in contact in a limited space, at all temperatures.
33. In the absence of that knowledge, I have used equation (34),
so as to indicate the form of an approximate equation suitable Re
calculating the elasticity of vapour in contact with its liquid at all
ordinary temperatures, the coefficients of which I have determined
empirically, — for water and mercury, from the experiments of M. licgnault,
and for alcohol, ether, turpentine, and petroleum, from those of Dr. Ure.
It has been shown (equation 19) that the superficial atomic elasticity
is expressible approximately in terms of the density and temperature
for gases by f , / r \\
where the function F is a very rapidly converging series, in terms of the
negative powers of the absolute temperature, the coefficients being
functions of the density. It is probable that a similar formula is
applicable to liquids, the series being less convergent.
It follows that the density is expressible approximately in terms of
the superficial atomic elasticity by
P=i ,^(i+*( P ,^)),
the function 4> being also a converging scries in terms of the negative
powers of the absolute temperature, and the coefficients being functions of p.
Making this substitution in the first side of equation (34), and
abbreviating <p (j), T J into <£, we obtain the following result : —
?1 j 1 _r_ », _J_
]p o 1K p CnMJp Q ^'j»(l+$)
= cii( l0  * " lo s^o // *£(ifl))
= (Vx; (35.)
o
from which, making
log tPo+l 1 d P ■ n\*\ = *> and CnM I * ch ■ X = Q '
8c/0 Jj; x p(i+*) )\
the following value results for the hyperbolic logarithm of the super
ficial atomic elasticity of the vapour at sensible distances from the
surface of the liquid : — ~
log., ^ = ¥  . . • • (36.)
ELASTICITY OF GASES AND VAPOURS. 43
In the cases which occur in practice, the density of the vapour is very
small as compared with that of the liquid. Hence it follows that in
such cases the value of ^F depends chiefly on the superficial atomic elasticity
of the liquid, and that of Q on its density. The density is known to
diminish with the temperature, but slowly. The superficial atomic
elasticity, according to equation (32), is expressed by
Po = Pi +f(Pi) /(*U
where p x and f(D x ) are obviously small as compared with /(D ), a function
of the density of the liquid, so that the variations of p and of M* with
the temperature are comparatively slow also.
Therefore, when the density of the vapour is small as compared with
that of the liquid, the principal variable part of the logarithm of its
superficial atomic elasticity, and consequently of its whole pressure,
is negative, and inversely proportional to the absolute temperature; and
ft
a
(a and /3 being constants) may be regarded as the first two terms of an
approximate formula for the logarithm of the pressure.
A formula of two terms, similar to this, was proposed about 1828
by Professor Eoche. I have not been able to find his memoir, and
do not know the nature of the reasoning from which he deduced his
formula. It has since been shown, by M. Eegnault and others, to be
accurate for a limited range of temperature only. The quantity corre
sponding in it to t is reckoned from a point determined empirically,
and very different from the absolute zero.
Thus far the investigation has been theoretical. The next step is to
determine empirically what other terms are requisite in order to approxi
mate to the effect of the function /(D), and of the variation of the
functions ^P and Q.
The analogy of the formula for the dilatation of gases, the obvious
convenience in calculation, and the fact that the deviations of the results
of the first two terms from those of experiment are greatest at low
temperatures, naturally induced me to try, in the first place, the effect
of a third term inversely proportional to the square of the absolute
temperature, making the entire formula for the logarithm of the pressure
of vapour in contact with its liquid
a —
r 7"
and the inverse formula, for calculating the abso
lute temperature from the pressure,
1 _ ja  log. P g 2 _ (3_
y 4y 2 2y
(37.)
44 ELASTICITY OF GASES AND VAPOURS.
the values of the constants a, /3, 7, being determined by the ordinary
methods from three experimental data for each substance.
34. The agreement of those formulae with the results of experiment
proved so remarkable, that, as they are calculated to be practically useful,
I thought it my duty not to delay their publication until I should have
an opportunity of submitting my theoretical researches to the Royal
Society of Edinburgh. I therefore communicated the formulas to the
Edinburgh New Philosophical Journal for July, 1849, together with the
full details of their comparison, graphic and tabular, with the experiments
of M. Regnault upon water and mercury, and with those of Dr. Ure upon
alcohol, ether, turpentine, and petroleum, but without giving any account
of the reasoning by which 1 had been led to them.
Without repeating those details here, I may state that the agreement
between the results of the formula' and those of observation is in every
case as close as the precision of the experiments renders possible. This
is remarkable, especially with respect to the experiments of M. Regnault
on the elasticity of steam, which extend throughout a range of tempera
tures from 30° below zero of the Centigrade scale to 230° above it, and
of pressures from ytgv °f an atmosphere to 28 atmospheres, and which,
from the methods of observation adopted, especially those of measuring
temperature, necessarily surpass by far in precision all other experiments
• if the same kind. From 20° to 230° Cent, the greatest discrepancy
between calculation and experiment corresponds to a difference of jfa of
a Centigrade degree, and very few of the other differences amount to so
much as ^ of a degree. Below 20°, where the pressure varies so slowly
with the temperature that its actual value is the proper test of the
formula, the greatest discrepancy is j 3 ^ of a millimetre of mercury, or
2jjxy of an inch. If the curves representing the formula? were laid down
on M. Regnault's diagram, they would be scarcely distinguishable from
those which he has himself drawn to exhibit the mean results of his
experiments.
Annexed is a table of the values of the constants a, log. /3, log. y,
ly Ay
existing experiments on mercury, turpentine, and petroleum, are not
sufficiently extensive to indicate any precise value for the coefficient y
(which requires a great range of temperatures to evince its effect), I have
used for these fluids, as an approximation, the first two terms of the
formula only, a — — .
T
For different measures of pressure, the contact a evidently varies
equally with the complement of the logarithm of the unit of pressure.
For different thermometric scales /3 varies inversely as the length of a
„ for the fluids for which they have been calculated. As the
41 v' 4.V 2 ' J
ELASTICITY OF GASES AND VAPOURS.
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4(> ELASTICITY OF GASES AND VAPOURS.
8
degree, y inversely as the square of that length, ' directly as the length
of a degree, and  , directly as the square of that length.
For all the fluids except water, it will probably he found necessary to
correct more or less the values of the constants, when more precise and
extensive experiments have been made, especially those for the more
volatile ether, and for turpentine, petroleum, and mercury, which have all
been determined from data embracing but a small range of pressures.
In reducing the constants for the Centigrade scale to those for Fahren
heit's scale, 180° of the latter have been assumed to be equal to 100° of
the former. In order that this may be the case, the boiling point of
Fahrenheit's scale must be adjusted under a barometric pressure of 760
millimetres, or 29"922 inches, of mercury, whose temperature is 0°
Centigrade.
In the ninth and tenth columns of tin table are given the limits on the
scales of temperature and pressure between which the formulae have been
compared with experiment. It is almost certain that the formula for the
pressure of steam may be employed without material error for a consider
able range beyond, and probably also that for the pressure of vapour of
alcohol; but none of the formula' are to be regarded as more than
approximations to the exact physical law of the elasticity of vapours, for
the determination of which many data are still wanting, that can only be
supplied by extensive series of experiments.
The following are some additional values of the constant a for steam,
corresponding to various units of pressure used in practice : —
I'nits of Pressure. Values of a.
Atmospheres of 760 millimetres of mercury —
= 29922 inches of mercury
= 14*7 lbs. on the square inch
= 10333 kilog. on the square centim., 4950433
Atmospheres of 30 inches of mercury —
= 76199 millimetres
= 1474 lbs. on the square inch
= 1036 kilog. on the square centim., 4949300
Kilogrammes on the square centimetre, . . 4964658
Kilogrammes on the circular centimetre, . 4 "859 748
Pounds avoirdupois on the square inch, . . 6117662
Pounds avoirdupois on the circular inch, . 6012752
Pounds avoirdupois on the square foot, . . 8276025
All the numerical values of the constants are for common logarithms.
ELASTICITY OF GASES AND VAPOURS. 47
35. According to the principles which form the basis of calculation
in this section, every substance, in the solid or liquid state, is surrounded
by an atmosphere of vapour, adhering to its surface by molecular attrac
tion ; and even when the presence of vapour is imperceptible at all visible
distances from the body's surface, the elasticity of the strata close to that
surface may be considerable, and sufficient to oppose that resistance to
being brought into absolute contact, which is well known to be very great
in solid bodies, and perceptible even in drops of liquid. It is possible
that this may be the only cause which prevents all solid bodies from
cohering when brought together.
The action of an atmosphere of vapour, so highly dense and elastic as
to operate at visible distances, may assist in producing the spheroidal state
of liquids.
If the particles of clouds are small vesicles or bubbles (which is doubt
ful), the vapour within them may, according to these principles, be
considerably more dense than that which pervades the external air, and
may thus enable them to preserve their shape.
Section VX — Of Mixtures of Gases and Vapours of
Different Kinds.
36. The principle stated in Section II. article 11, that the elasticity of
the atomic atmosphere is proportional to its density, might be otherwise
expressed by saying, that the elasticity of any number of portions of atomic
atmosphere, compressed into a given space, is equal to the sum of the elasticities
which such portions would respectively have, if they occupied the same space
separately.
If the same principle here laid down for portions of atomic atmosphere
of any one kind of substance, be considered as true also of portions of
atomic atmosphere of substances of different kinds mixed, and if it be
supposed that when two or more gases are mixed there is no mutual force
exerted between atoms of different kinds, except the elastic pressure of
the atomic atmospheres, it will then evidently follow. —
First, that the mixed gases will only be in equilibria when the particles
of each of them are diffused throughout the whole space which contains
them.
Secondly, that the particles of each gas taken separately will be in the
same condition as to density, elasticity, arrangement, and mutual action,
and also as to gravitation, or any other action of an external body, as if
that gas occupied the space alone.
Thirdly, that the joint elasticity of the mixed atomic atmospheres at any
given point, will be the sum of the elasticities which they would respec
tively have had at that point if each gas had occupied the space alone.
48 ELASTICITY OF GASES AND VAPOURS.
Fourthly, that the value of the elasticity, positive or negative, resulting
from the attractions and repulsions of separate atoms, will be the sum of
the values it would have had if each gas had occupied the space alone ;
and,
Fifthly, that the total elasticity of the mixed gases will be the sum of
the elasticities which each would have had separately in the same space.
If there are any mutual actions between the particles of different gases
except the elasticity of the molecular atmospheres, these conclusions will
no longer be rigidly true • but they will still be approximately true if the
forces so operating are very small. This is probably the actual condition
of mixed gases.
37. On applying the same principle to the case of a gas mixed with a
vapour in contact with its liquid, it is obvious that if the attractions and
repulsions of the particles of the gas upon those of the vapour are null or
inappreciable, the direct effect of the presence of the gas upon the elasticity
assumed by the vapour at a given temperature will also be null or
inappreciable.
The gas, however, may have a slight indirect influence, by compressing
the liquid, and consequently increasing its superficial atomic elasticity and
its 4 attractive power, on which the functions ¥ and £2 in equation (3G)
depend. The probable effect of this will be to make the elasticity of the
vapour somewhat less than if no gas were present. There appear to be
some indications of such an effect ; but they are not sufficient to form a
basis for calculation.
Supposing the gas, on the contrary, to exercise an appreciable attraction
on the particles of vapour, the elasticity of the latter will be increased.
Traces of an effect of this kind are perceptible in M. Regnault's experi
ments on the vapour of mercury in which air was present.
38. I have already referred to the property ascribed by Professor
Faraday to various substances, of attracting, and retaining at their
surfaces, layers of gas and vapour in a high state of condensation. Sup
posing a solid body to acquire, in this manner, a mixed atmosphere,
consisting partly of its own vapour and partly of foreign substances, the
total elasticity of that atmosphere at any point will be equal, or nearly
equal, to the sum of the elasticities which each ingredient would have had
separately; and thus solid metals, glass, charcoal, earthy matters, and
other substances, may acquire a great power of resisting cohesion, although
producing no perceptible vapours of their own at ordinary temperatures.
THE CENTRIFUGAL THEORY OF ELASTICITY. 49
IV.— ON THE CENTRIFUGAL THEORY OF ELASTICITY AND
ITS CONNECTION WITH THE THEORY OF HEAT.*
Section I. — Relations between Heat and Expansive Pressure.
1. In February, 1850, I laid before the Royal Society of Edinburgh
a paper in which the laws of the pressure and expansion of gases and
vapours were deduced from the supposition that that part of the elasticity
of bodies which depends upon heat arises from the centrifugal force of the
revolutions of the particles of elastic atmospheres surrounding nuclei or
atomic centres. A summary of the results of this supposition, which I
called the Hypothesis of Molecular Vortices, was printed in the Transactions
of this Society, Vol. XX., as an introduction to a series of papers on the
Mechanical Action of Heat; and the original paper has since appeared in
detail in the Philosophical Magazine.
In that paper the bounding surfaces of atoms were defined to be imaginary
surfaces, situated between and enveloping the atomic nuclei, and sym
metrically placed with respect to them, and having this property — that at
these surfaces the attractive and repulsive actions of the atomic nuclei and
atmospheres upon each particle of atomic atmosphere balance each other.
The pressure of the atomic atmospheres at those imaginary boundaries is
the part of the total expansive pressure of the body which varies with
heat, the effect of the centrifugal force of molecular vortices being to
increase it.
In the subsequent investigation it was assumed that, owing to the sym
metrical action of the particles of gases in all directions, and the small
amount of those attractive and repulsive forces which interfere with the
elasticity of their atmospheres, no appreciable error would arise from
treating the boundary of the atmosphere of a single atom, in calculation,
as if it were spherical, an assumption which very much simplified the
analysis.
An effect, however, of this assumption was to make it doubtful whether
the conclusions deduced from the hypothesis were applicable to any sub
stances except those nearly in the state of perfect gas. I have, therefore,
* Read before the Royal Society of Edinburgh on December 15, 1S51, and
published in Vol. XX. , Part iii. , of the Transactions of that Society.
D
50 THE CENTRIFUGAL THEORY OF ELASTICITY.
in the present paper, investigated the subject anew, without making any
assumption as to the arrangement of the atomic centres, or the form of
the boundaries of their atmospheres. The equations deduced from the
hypothesis between expansive pressure and heat, are therefore applicable
to all substances in all conditions; and it will be seen that they are
identical with those in the original paper, showing that the assumption
that the atomic atmospheres might be treated, in calculation, as if spherical,
did not give rise to any error.
By the aid of certain transformations in those equations I have been
enabled, in investigating the principles of the mutual transformation of
heat and expansive power, to deduce Joules law of the equivalence of heat
and mechanical power directly from them, instead of taking it (as I did in
my previous papers) as a consequence of the principle of vis viva. Carnofs
law is also deduced directly from the hypothesis, as in one of the previous
papers.
2. Classification of Elastic Pressures. — The pressures considered in the
present paper are those only which depend on the volume occupied by a
given weight of the substance, not those which resist change of figure ill
solids and viscous liquids. Certain mathematical relations exist between
those two classes of pressures ; but they do not affect the present
investigation.
To illustrate this symbolically, let V represent the volume occupied by
unity of weight of the substance, so that is the mean density; Q, the
V
quantity of heat in unity of weight, that is to say, the vis viva of the mole
cular revolutions, which, according to the hypothesis, give rise to the
expansive pressure depending on heat ; and let P denote the total expan
sive pressure. Then,
P = F(V,Q)+/(V),. . . . (1.)
In this equation, F(V, Q) is the pressure of the atomic atmospheres at
the surfaces called their boundaries, which varies with the centrifugal
force of the molecular vortices as well as with the mean density ; and
/( V) is a portion of pressure due to the mutual attractions and repulsions
of distinct atoms, and varying with the number of atoms in a given
volume only. If the above equation be differentiated with respect to the
hyperbolic logarithm of the density, we obtain the coefficient of elasticity
of volume
V V V
where ft denotes the cubic compressibility.
THE CENTRIFUGAL THEORY OF ELASTICITY. 51
The latter portion of this coefficient, — ==f(V), consists of two parts,
T
one of which is capable of being resolved into forces, acting along the
lines joining the atomic centres, and gives rise to rigidity or elasticity of
figure as well as to elasticity of volume, while the other, which is not capable
of being so resolved, gives rise to elasticity of volume only. The ratio of
each of those parts to their sum must be a function of the heat, the former
part being greater and the latter less, as the atomic atmosphere is more
concentrated round the nucleus ; that is to say, as the heat is less ; but
their sum, so far as elasticity of volume is concerned, is a function of the
density only.
That is to say, as in equation (12) of my paper on the Laws of the
Elasticity of Solids {Cambridge and Dublin Mathematical Journal, February,
1851), let the total coefficient of elasticity of volume be denoted thus —
~ = J + cp(C v C 2 ,C s ), . • (IB.)
C 1? C 2 , C 3 , being the coefficients of rigidity round the three axes of elas
ticity, and J a coefficient of fluid elasticity ; then
J = ^F(V,Q)^(V,Q,)^/(V)
T V
d
y • (ic.)
^(C 1; C 2 ,C 3 ) = (l^V,Q))^/(V)
V j
For the present, we have to take into consideration that portion only
of the expansive pressure which depends on density and heat jointly, and
is the means of mutually converting heat and expansive power ; that is
to say, the pressure at the boundaries of the atomic atmospheres, which
I shall denote by
Pressures throughout this paper are supposed to be measured by units
of weight upon unity of area ; densities by the weight of unity of volume.
3. Determination of the External Pressure of an Atomic Atmosphere. —
Let a body be composed of equal and similar atomic nuclei, arranged in
any symmetrical manner, and enveloped by an atmosphere, the parts of
which are subject to attractive and repulsive forces exercised by each
other and by the nuclei. Let it further be supposed that this atmosphere,
at each point, has an elastic pressure proportional to the density at that
point, multiplied by a specific coefficient depending on the nature of the
BOSTON UNIVERSITY
C.ni i FGF or LIBERAL ARTS
52 THE CENTRIFUGAL THEORY OF ELASTICITY.
substance, which I shall denote by h. (This coefficient was denoted by b
in previous papers.)
Let p and p denote the density and pressure of the atomic atmosphere
at any point ; then
/ = h P
be the accelerative forces operating on a particle of atomic atmosphere in
virtue of the molecular attractions and repulsions which I have made
explicitly negative, attractions being supposed to predominate. The
property of the surfaces called the boundaries of the atoms is this
(^=0,(1)^0,(^=0,
the suffix , being used to distinguish the value of quantities at those
surfaces. Hence $ x is a maximum or minimum. Those surfaces are
symmetrical in form round each nucleus, and equidistant between pairs of
adjacent nuclei. Their equation is
$  $! = 0.
Let M denote the total weight of an atom ; /x that of its atmospheric
part, and M — fi that of its nucleus; tl en
MV is the volume of the atom, —
the mean density of the atmospheric part, measured by weight, the
nucleus being supposed to be of insensible magnitude; —
and we have the following equations
MV = //./( 1 )' / ''' /
M = m v j 1 i Q d x '' y '' "■ = 1 1 1 d) p dxd y (l ■
(2.)
J
The suffix (j) denoting that the integration is to be extended to all
points within the surface
($$ 1 = 0).
According to the hypothesis now under consideration, Meat consists in
a revolving motion of the particles of the atomic atmosphere, com
municated to them by the nuclei. Let v be the common mean velocity
possessed by the nucleus of an atom and the atmospheric particles, when
the distribution of this motion has been equalised. I use the term mean
velocity to denote that the velocity of each particle may undergo small
periodic changes, which it is unnecessary to consider in this investigation.
THE CENTRIFUGAL THEORY OF ELASTICITY.
53
Then the quantity of heat in unity of weight is
2g
being equal to the mechanical power of unity of weight falling through
the height — . The quantity of heat in one atom is of course MQ, and in
the atmospheric part of an atom, pQ.
I shall leave the form of the paths described by the atmospheric par
ticles indeterminate, except that they must be closed curves of permanent
figure, and included within the surface (<!> c&^O). Let the nucleus be
taken as the origin of coordinates, and let a, )3, y, be the direction
cosines of the motion of the particles at any point (x, y, z). Then the
equations of a permanent condition of motion at that point, are
P dx ax \ dx ' ay dz/
1 dp
P
1 dp' d$ 2
p' dy dy
1 dp' d&
p' dz dz
1
di
■f.+pi+^
■ (3.)
rt / d ~. d d\ „
Let r be the radius of curvature of the path of the particles through
(x, y, z), and o' /3' 7', its directioncosines; then the above equations
obviously become
1 dp_
p dx
1 dp'
p dy
1 rf_p'
p dz
d$
dx
d$>
dy
d$
dz
a
r
 2Q
_ 2 q£=C
2Q^
r
(3A.)
If these equations are integrable,
— dx + $ dy +^dz
r r r
must be an exact differential. Let — <p be its primitive function ; the
negative sign being used, because a, /3', y' must be generally negative.
Then the integral of the equations (3) is
lo SvP =
dp' 1 ,
 £  = =(2 Q0 — <P) + constant:
h
54
THE CENTRIFUGAL THEORY OF ELASTICITY.
or taking f> y to denote the pressure at the bounding surface of the
atom : —
P = Pi e
■ (4.)
Our present object is to determine the superficialatomic density, p v
and thence the pressure 2 )= hpv ^ u terms °f tne mean density and
heat Q. For this purpose we must introduce the above value of p into
equation (2), giving
^t^p^ J J f/>fi dxdydz
whence
\ ( f h dxdydz. (5.)
Let the volume of the atom be conceived to be divided into layers, in
each of which <p has a constant value. Then we may make the following
transformations.
p = h Pl =hfi+J j  (
f f Id x d y d z=kMvfe ~ d<J>
— 1 (*_* 1 )
h(<p<Pi)
> (C)
dxdydz=kMYje ^d<p
k being a specific constant, and xp and w functions of cf>, and of the nature
and density of the substance.
The lower limit of integration of $ must be made — cc , that it may
include orbits of indefinitely small magnitude described round the atomic
centre.
The nature of the function \p is limited by the following condition,
l=k\ Vl e Td<h
J — GO Wt
(7.)
20
Let^+1 =
hie
Then these transformations give the following result for the pressure at
the bounding surface of an atom : —
p=h Pl
Jl/UL
MV
+ £
h<p x fl*(00i)
kdcp
hfx
MV
Wl
w \ i w l w l , p
► . (8.)
e ■ e 2
THE CENTRIFUGAL THEORY OF ELASTICITY. 55
to' v &c, being the successive differential coefficients of w with respect to
k([>, when <p = (p v
4. The following transformation will be found useful in the sequel.
Let X be the indefinite value of log. e V, and \ its actual value in the
case under consideration. Let G be the same function of A which w is
of k<p, and let G', G", &c, be its successive differential coefficients with
respect to A.
Let
„ f\ *(**!)„ ,. g x g; , g;' ,
Then
puv^ {J >
The function H has the following properties, which will be afterwards
referred to : —
< ^ + 6li 1 G 1 =0
\ mix= d ^
— oo a v
(10.)
5. Case of a Perfect Gas. — As a substance is rarefied, it gradually
approaches a condition in which the pressure, under like circumstances as
to heat, varies proportionally to the density. This is because the effect
of the molecular attractions and repulsions on the pressure diminishes
with the density, so that <P, w, and G approximate to constant quantities.
In the limiting or perfectly gaseous condition, therefore,
G,
H i=7T
and
hnQ hfi /2Q \
P =MY=MvKhk +1 )' * • (1L
6. Equilibrium of Heat : Nature of Temperature and Real Specific
Heat. — When the atmospheres of atoms of two different substances are
in contact at their common bounding surface, it is necessary to a per
manent condition that the pressure in passing that surface should vary
continuously.
Let (a) and (b) be taken as characteristics, to distinguish the specific
quantities peculiar to the two media respectively. Let dm denote the
volume of an indefinitely thin layer, close to the bounding surface.
Then the following equations must be fulfilled to ensure a permanent
condition : —
i;(a) ^ ;(6); ^ (a)= ^ (J)when/ ^ * * (12 °
56 THE CENTRIFUGAL THEORY OF ELASTICITY.
By making the proper substitutions in equation (4), it appears that
j/=pe 7
Hence
*•?
£p=^=^dM + dm
dm
Now, j) is the same for both media :
& • _ *<••>>
/ = c
is either a maximum or a minimum, so that its differential is null ; and
dm is a continuous function of lc<p, so that
dm v dm v
There remains only the function of heat :
20
Therefore the condition of a permanent state of molecular motion, that
is to say, the condition of equttibrivm of heat, is that this function shall be
the same for the two substances ; or that
?%=a as.)
Hence, temperature depends on the above function only ; for the
definition of temperature is, that bodies at the same temperature are in
a permanent condition as to heat, so far as their mutual action is
concerned.
The ratio of the real specific heat of (a) to that of (b) is obviously
KK ■■ *A • • • ( u >
7. Measure of Temperature ami Specific Heat. — The function 6 is
proportional to the pressure of a perfect gas at a constant density. That
pressure, therefore, is the most convenient measure of temperature.
Let t denote absolute temperature, as measured by the pressure of a
perfect gas at constant density, and reckoned from a certain absolute zero,
274 0 G Centigrade, or 494°  28 Fahrenheit below the temperature of
melting ice. Let k be a constant which depends on the length of a
degree on the thermometric scale, and is the same for all substances in
nature. Then
2^, 1
M
hi
+ K
Q = (r«)
C I
'2« J
THE CENTRIFUGAL THEORY OF ELASTICITY. 57
and the real specific heat of the substance, that is to say, the depth of
fall, under the influence of gravity, which is equivalent to a rise of one
degree of temperature in the body, is represented by
fe=P (1G.)
The pressure of a perfect gas is represented in terms of temperature by
h U. T ,. „ \
It may also be expressed thus : let r denote the absolute temperature
of meltiDg ice in degrees of the scale employed, and V the volume of
unity of weight of the substance in the theoretical state of perfect gas, at
the temperature of melting ice and pressure unity : — then
*%P ■ ■ < 18 >
v T o
On comparing this with equation (17) we see that
kfx _ V _ 1
h flT h JUL K
(19.)
MV 'MV r J
Xow h is the specific elasticity of the atomic atmosphere of the substance ;
=£= is the mean specific gravity of that atmosphere, when the body is in
M V
the theoretical state of perfect gas; and k and r are the same for all
substances in nature. Therefore, for every substance in nature, the mean
specific gravity of the atomic atmosphere in the theoretical state of perfect gas is
inversely proportional to the specific elasticity of that atmosphere.
Real specific heat may also be thus expressed : —
k== y Bi *M (20)
V 1 , ^M, 3£M
in which ° corresponds to t^tv m my former papers, and — — to r— 
1
or =r.
N
The latter factor appears to depend on the chemical constitution of the
substance, being the same for all simple gases.
8. Total Pressure of Substances in general, expressed in terms of Temperature.
In equation (9) let  be put for 9 : then
58 THE CENTRIFUGAL THEORY OF ELASTICITY.
v= P +/( v) =/(V) + % £»,+ {«. K ^+^' :  M
where
A x = — — ; A 2 = — ^ 2 (G/ 2 — G/')
a 3 =  j£ 3 (<V 3  20/6/'+ G/"); &c.
This formula is identical with that which I employed in my former
paper to represent the pressure of an imperfect gas, and which I found to
agree with M. Regnault's experiments, when the coefficients A and the
function /(V) had been calculated empirically.
Section II. — Eelations between Heat and Expansive Power.
9. Variations of Sensible and Latent Heat : Fundamental Equation of the
Theory. — If the forms, positions, and magnitudes of the paths described by
the revolving particles of the atomic atmospheres be changed, whether by
a variation of mean density, or by a variation of temperature, an increase
or diminution of the vis lira of their motion, that is to say, of the heat of
the body, will take place in virtue of that change of the paths of motion —
an increase when they are contracted, and a diminution when they are
dilated.
Let S.Q represent, when positive, the indefinitely small quantity of
heat which must be communicated to unity of weight of a substance, and
when negative, that which must be abstracted from it, in order to produce
the indefinitely small variation of temperature S t simultaneously with the
indefinitely small variation of volume 2V. Let S.Q be divided into two
parts
SQ + SQ'=S.Q,
of which SQ, being directly employed in varying the velocity of the particles,
is the variation of the actual or sensible heat possessed by the body ; while
SQ', being employed in varying their orbits, represents the amount of the
mutual transformation of heat with expansive power and molecular action,
or the variation of what is called the latent heat; that is to say, of a
molecular condition constituting a source of power, out of which heat may
be developed. (§Q' in this paper corresponds to — 2Q' in my former
papers.)
THE CENTRIFUGAL THEORY OF ELASTICITY. 59
The variation of sensible heat has evidently this value,
ZQ = \\St (22.)
Let dx, By, $z, be the displacements of the orbit of the particles of atomic
atmosphere at the point (x, y, z) . A molecule p dxclydz is acted upon
by the accelerative forces (see equation 3A).
parallel to the three axes respectively.
The sum of the actions of those forces on the molecule pdxdydz
during the change of temperature and volume, is
<>+^+^V^<
dy
= — 2Q<$<ppdxd yd :.
The sum of such actions upon all the particles in unity of weight is
equal in amount and opposite in sign to the variation of latent heat ; that
is to say,
W = ^fff {i) p$<pdxdydz. . . (23.)
To determine the value of the variation Bcp, let it be divided into two
parts, thus : —
t(p — ccp l + oAcp,
where A<j> = <p — (p r
First, With respect to $<f> v it is obvious that because, according to
equations (6, 7), •
,0i ks<p j.
MY = IMY e fdd>,
J co \p x r
we must have
BV=:kYB(p 1 and $<Pi = t^
Hence the first part of the integral (23) is
=&4 • • • < 23A >
To determine the second part of the integral we have the condition that
the quantity of atomic atmosphere inclosed within each surface at which
A(f> has some given value is invariable; that is to say,
GO THE CENTRIFUGAL THEORY OF ELASTICITY.
Hence
ftp. M Ve — ,
The value of the second part of the integral (23) is now found to be: —
l e±$
M
[ j f f QP SA<pdxdydz= ^ k Pl M v/* x . ' ^ SA0^
In the double integral, let X = log.,, V be put for fc0, G for w, and H for
the single integral, as in equation (9). Then the double integral becomes
G, dr'
Also,because Pl MV = M ' : by Eq. (9), and ^ =  (t k), the second part
of the integral (23) is found to be
hfx
M
('o(»'c +,v ^)© • • (33R)
Hence, adding together (2 3 A) and (23B) we find for the total variation
of latent heat
so' hfL ( r *\ i £r ^ lo S> H i ■ SV ■ (1 4 10 ' H A 1 (24 )
8Q^(r K )Sr.— ^ +6V ^ y + dTd y)$W
To express this in terms of quantities which may be known directly by
experiment, we have, by equations (10) and (9) —
^ + 0^ = 0, that is to say,
d\og., TL 1 _ Gj _ r M _ t_
dV ~H X V K\~hfx P kY'
and, therefore,
TYT f —
log. e H x = — h?fZ V log v V+/(r) + constant.
Il [A J K
/(r) is easily found to be = — log. e r for a perfect gas, and, being indepen
dent of the density, is the same for all substances in all conditions ; hence
we find (the integrals being so taken that for a perfect gas they shall = 0)
THE CENTRIFUGAL THEOEY OF ELASTICITY. 61
dr J Vi/mclr kVJ t
< PlogH 1= Mf^  1
dr 2 A^c.' c?r 2 t 2 '
d?\og. e n x ■_ M dp _ ±
drdV h/u. dr kV
and, therefore,
W(r.){*.(& + /glv) + «7.4} . (3 ,)
is the variation of latent heat, expressed in terms of the pressure, volume,
and temperature ; to which, if the variation of sensible heat, 3Q = feSr, be
added, the complete variation of heat, SQ + SQ' = S.Q, in unity of weight
of the substance, corresponding to the variations SV and Sr of volume and
temperature, will be ascertained.
It is obvious that equation (25), with its consequences, is applicable to
any mixture of atoms of different substances in equilibria of pressure and
7 72
temperature; for in that case r, J, and — are the same for each substance.
We have only to substitute for ~ the following expression : —
X M X 2 M 2
where n v «.„ &c, are the proportions of the different ingredients in unity
of weight of the mixture, so that n^ + n 2 + &c. = 1.
Equation (25) agrees exactly with equation (6) in the first section of my
original paper on the Theory of the Mechanical Action of Heat. It is the
fundamental equation of that theory ; and I shall now proceed to deduce
the more important consequences from it.
10. Equivalence of Heat and Expansive Power: Joule's Law. — From the
variation of the heat communicated to the body, let us subtract the
variation of the expansive power given out by it, or
PSV={j>+/(V)}SV.
The result is the variation of the total power exercised upon or com
municated to unity of weight of the substance, supposing that there is no
chemical, electrical, magnetic, or other action except heat and pressure ;
and its value is —
* This coefficient corresponds to  E in the notation of my previous paper on the
Mechanical Action of Heat. *
G2 THE CENTRIFUGAL THEORY OF ELASTICITY.
^SQ + SQ'P^Sr.{k+^^)+(rK)/0^v}
+ SV.{(tk)^P/(V)}. ■ . . (2G.)
This expression is obviously an exact differential, and its integral is the
following function of the volume and temperature : —
¥=k(rK)+^(log,T+^) +j{(r K )^ T  P }dYff(Y)dV. (27.)
Accordingly, the total amount of power which must be exercised upon
unity of weight of a substance, to make it pass from the absolute
temperature r and volume V to the absolute temperature r x and volume
Y,, is
<P(V 1 ,r 1 )¥(V fll r ).
This quantity consists partly of expansive or compressive power, and
partly of heat, in proportions depending on the mode in which the
intermediate changes of temperature and volume take place; but the
total amount is independent of these changes.
Hence, if a body be made to pass through a variety of changes of temperature
and volume, and at length be brought back to its primitive volume and tempera
tun', the algebraical sum of the portions of power applied to and evolved from
the body, whether in the form of expansion and compression, or in that of heat,
is equal to zero.
This is one form of the law, proved experimentally by Mr. Joule, of
the equivalence of heat and mechanical power. In my original paper on
the Mechanical Action of Heat, I used this law as an axiom, to assist in
the investigation of the equation of latent heat. I have now deduced
it from the hypothesis on which my researches are based — not in order
to prove the law, but to verify the correctness of the mode of investigation
which I have followed.
Equations (26) and (27), like equation (23), are made applicable to
unity of weight of a mixture, by putting 2«ft for ft, and 2ra ^ for =£.
The train of reasoning in this article is the converse of that followed
by Professor William Thomson of Glasgow, in article 20 of his paper on
the Dynamical Theory of Heat, where he proves from Joule's law that
the quantity corresponding to S^ is an exact differential.
11. Mutual Conversion of Heat and Expansive Poiver : Carnot's Law of
the Action of Expansive Machines. — If a body be made to pass from the
volume V and absolute temperature r to the volume V x and absolute
temperature t v and be then brought back to the original volume and
THE CENTRIFUGAL THEORY OF ELASTICITY. Go
temperature, the total power exerted (¥) will have, in those two
operations, equal arithmetical values, of opposite signs. Each of the
quantities ¥ consists partly of heat and partly of expansive power, the
proportion depending on the mode of intermediate variation of the volume
and temperature, which is arbitrary. If the mode of variation be different
in the two operations, the effect of the double operation will be to
transform a portion of heat into expansive power, or vice versa,
Let (a) denote the first operation, (b) the reverse of the second. Then
The terms of ¥ which involve functions of r only, or of V only, are
not affected by the mode of intermediate variation of those quantities.
The term on which the mutual conversion of heat and expansive power
depends, is therefore
f{(rK)f T p}dY(b) = f{(TK)^p}dY(a),
/(^)"©=/(^')"»
f^dY(a)f^dY(b) =jpdY(a)fpd Y (b),
or,
Hence,
which last quantity is the amount of the heat transformed into expansive
power, or the total latent heat of expansion in the double operation.
Let
J dr J t — k a \
dY=( T  K )dF,
Then because
dV
we have
"V V F F
J jnlY(a)f 1 pdY(b) = j \tk) d F («) f '(i— *)tfF(&)
= f Fl (r a r b )dF= f "LlZH ^ d Y. . (28.)
•'F Jv r a K d\
In which r a and r^ are the pair of absolute temperatures, in the two
operations respectively, corresponding to equal values of F.
This equation gives a relation between the heat transformed into
expansive power by a given pair of operations on a body, the latent heat
of expansion in the first operation, and the mode of variation of tempera
ture in the two operations. It shows that the proportion of the original
latent heat of expansion finally transformed into expansive power, is a
04 THE CENTRIFUGAL THEORY OF ELASTICITY.
function of the temperatures alone, and is therefore independent of the
nature of the body employed.
Equation (28) includes Carnot's law as a particular case. Let the
limits of variation of temperature and volume be made indefinitely small.
Then
, 7A . dr dq 7 ,.
and dividing by drdY
dp = _J_ d<$
dr~ t— k' dV
This differential equation is also an immediate consc luence of
equation (25).
If  be put for , and JM for ( V? T , it becomes identical with the
J i — k d V
equation by which Professor William Thomson expresses Carnot's law,
as deduced by him and by Mr. Clausius from the principle, that it is
impossible to transfer heat from a colder to a hotter body without expenditure of
mechanical pov: if.
The investigation which I have now given is identical in principle
with that in the fifth section of my paper on the Mechanical Action of
Heat ; but the result is expressed in a more comprehensive form.
Equation (28), like (25), (26), and (27), is applicable to a mixture,
composed of any number of different substances, in any proportions,
dp, '//'.
provided the temperature, the pressure, and the coefficients j 5 ._, are
the same throughout the mass.
12. Apparent Specific Heat. — The general value of apparent specific
heat of unity of weight is
„ dQ dq dq dY • ( hfi [cPp ]
y (29.)
,dY dp I
"*" dr'dr\ J
agreeing with equation (13) of my previous paper.
The value in each particular case depends on the mode of variation
of volume with temperature. Specific heat at constant volume is
*,*+('«>(& + /£«).• • w
When the pressure is constant, we must have
dV .dp
w clY+dr = 0,
THE CENTRIFUGAL THEORY OF ELASTICITY.
and, consequently,
dp
dV_ _th L
dr~ dF'
dV
therefore specific heat at constant pressure is
65
K p = K V + (t — k)
(ML
_dY
dY
(31.)
J.
This agrees with equation (10) of Professor Thomson's paper, if — in his
notation — r — k.
If the body be a perfect gas, then
r \2 / x" r r
K. = ^ + 5
►r t—k d Y
r 2 + V ' d
*>
T Q \Zfl
y (32.)
K p = K v +
_ *) = lg* + 1 _
/'
The fact that the specific heats of all simple gases for unity of
weight are inversely proportional to their specific gravities, shows that
is the same for them all.
>
13. Velocity of Sound in Fluids. — Let a denote the velocity of sound
in a fluid, and d . P the total differential of the pressure. Then
« = V(,^)=V{,V<
dj
dY
dV_ d
dr' dY
;)}■ (33.)
If it were possible to maintain the temperature of each particle of the
fluid invariable during the passage of sound, this velocity would be
simply
v(,5>
d. y
But Ave have reason to believe that there is not time, during the
passage of sound, for an appreciable transfer of heat from atom to atom,
so that for each particle
E
G6 THE CENTRIFUGAL THEORY OF ELASTICITY.
dQ + dQ' = 0; or, K = in equation (29).
To fulfil this condition, we must have
dr i — k dp
dV = ~ K v ' It'
Consequently,
or, by equation (31),
,/ d? K P \ , nt .
That is to say, the action of heat increases the velocity of sound in a fluid
beyond what it ivould be, if heat did not act, in the ratio of the square root of
the specific heat at constant pressure, to the square root of the specific heat at
constant volume.
This is Laplace's law of the propagation of sound, which is here shown
to be applicable, not only to perfect gases, but to all fluids whatsoever.
LAWS OF THE ELASTICITY OF SOLID BODIES. 67
Y.— LAWS OF THE ELASTICITY OF SOLID BODIES.*
Introduction.
1. The science of the elasticity of solid bodies, considered with
reference to its most important application, the determination of the
strength of structures, consists of three parts :
First, The investigation of what may be specially termed the Laws of
Elasticity ; that is to say, the mutual relations which must exist between
the elasticities of different kinds possessed by a given solid, and between
the different values of those elasticities in different directions.
Secondly. The integration of the equations of equilibrium and motion
of the particles of an elastic solid. The results of this process enable us
to determine the relative displacements of the particles from their natural
positions in a solid body of a given material and figure, subjected to a
given combination of forces.
Thirdly. The application of the results derived from the first two
branches of the theory to our experimental knowledge of the pressures and
relative displacements to which the particles of known materials may safely
be subjected in practice. This enables us to compute the strength of
actual structures.
2. Notwithstanding the great amount of attention which has been paid
to the strength of materials, and the numerous and elaborate experiments
which have been made respecting it, few examples exist of the sound
application of physical and mathematical principles to practice in con
nection with this subject. This has arisen chiefly from the fact, that the
first and second branches of the inquiry have to a great extent been
carried on without reference to their application to the third, and the
third conducted without regard to the principles of the first and second.
The results of investigation, on correct principles, into the theory of
elasticity have been limited in their applications, with a few exceptions,
to the laws of the propagation of vibratory movements ; and those few
exceptions relate almost exclusively to bodies of equal elasticity in all
directions — a class which excludes many of the most useful materials of
* Eead before the British Association at Edinburgh, on August 1, 1S50, and
published in the Cambridge and Dublin Mathematical Journal, May, 1851.
G8 LAWS OF THE ELASTICITY OF SOLID BODIES.
construction. On the other hand, when it has been found necessary to
adopt theoretical principles, for the purpose of reducing the results of
experiments on the strength and elasticity of materials to a system,
assumptions have often been made, with a view chiefly to simplicity
in calculation, of a kind inconsistent with the real nature of elastic
bodies.
3. The present inquiry relates to the first part of the theory of
elasticity — viz., the laws of the relations which must exist between the
elasticities of different kinds possessed by a given substance, and between
their various values in different directions.
Section I. — Composition and Resolution of Strains and
Molecular Pressures.
4. At the outset of the inquiry two preliminary problems present them
selves : the composition and resolution of relative molecular displacements,
and the composition and resolution of pressures such as the parts of elastic
bodies exert upon each other. The former is a question of pure geometry ;
the latter, of pure statics. They are usually considered simultaneously,
on account of the analogy which exists between their solutions. This
is not the result of the physical connection between the two classes of
phenomena, and it would still exist although there were no such physical
connection ; it is merely a consequence of the analogy between forces in
statics and straight lines in geometry.
Those two problems have been so fully investigated by MM. Cauchy,
Lam£, and Clapeyron, as to leave nothing further to be done. The
theorems and formulae which they have obtained are many and important.
In the present paper I shall state those principles and results only to
which there will be occasion to refer in the sequel.
5. It is desirable that some single word should be assigned to denote
the state of the particles of a body when displaced from their natural
relative positions. Although the word strain is used in ordinary language
indiscriminately to denote relative molecular displacement, and the force
by which it is produced, yet it appears to me that it is well calculated to
supply this want. I shall therefore use it, throughout this paper, in the
restricted sense of relative displacement of particles, whether consisting in
dilatation, condensation, or distortion ; while under the term pressure I
shall include every kind of force which acts between elastic bodies, or the
parts of an elastic body, as the cause or effect of a state of strain, whether
that force is tensile, compressive, or distorting.
The nature and magnitude of a simple and uniform strain are defined
by three things :
LAWS OF THE ELASTICITY OF SOLID BODIES. 69
First. The direction of the lines along which the particles of the body
are displaced from their natural position.
Secondly. The direction along which the rate of variation of the displace
ment from point to point is a maximum. This direction is normal to a
series of planes of equal displacement, and may be called the strainnormal.
Thirdly. The amount of that rate of variation, being the differential
coefficient of the displacement with respect to distance along the strain
normal.
C. A strain may be resolved into three components, in which the
directions of displacement shall be respectively parallel to three rectangular
axes, while the strainnormal remains unchanged, by multiplying its
amount by the directioncosines of the total displacement.
Each of these three components may itself be resolved into three com
ponents, in which, the direction of displacement remaining unchanged, the
strainnormals are respectively parallel to the three axes, by multiplying
its amount by the directioncosines of the original strainnormal.
Thus every strain is reducible to nine components.
These nine components, however, are equivalent to but six distinct
strains. If we consider the strains as thus reduced to three rectangular
axes, we shall find that they are of two kinds : longitudinal, that is to say,
strains of linear extension or condensation, where the displacements are
parallel to the strainnormals ; and transverse, or strains of distortion, when
these directions are at right angles. Thus, if x, y, z, denote the three
rectangular axes, and £, jj, £, small molecular displacements respectively
parallel to them, then
dE, dr\ dZ,
</ .!•' dif d z'
are longitudinal strains, which are dilatations when positive, and condensa
tions when negative. I shall denote them respectively by N^ N 2 , N 3 ;
their sum, when positive, is the cubic dilatation of the particles, and when
negative, the cubic condensation.
Transverse strains, or distortions, are represented by the six differential
coefficients of the displacements with respect to axes at right angles to
them — viz.,
dr\ dZ, , dZ, f?s. d% dr)
dz' dy' dx' dz' dy' dx'
Let the axis of x be perpendicular to the plane of the paper. Let
.ABCD be the section, by the plane yz, of a prism which in its natural
state is square, and has its faces normal to the axes of y and z. A distor
tion in the plane yz, relatively to these axes, is measured by the deviation
from rectangularity of this originally square section, that deviation being
70
HWS OF THE ELASTICITY OF SOLID BODIES.
considered positive which makes the angles B and D acute. Now, so far
as the positions of the particles in this prism relatively to each other are
concerned, it is immaterial whether that devia
tion from rectangularity is produced by keep
ing the sides AD and BC parallel to their
original positions, and giving angular motion
to AB and DC — a change represented by
+
y
A
n
D
y
e
ch)
dz
■ or by keeping AB and DC parallel to
their original positions, and giving angular
motion to AD and BC — a change repre
7 V
sented by —  ; or by combining those two oper
dy
ations: so that the total transverse strain in the plane yz is represented
by the sum of these two coefficients,
dr\ dZ, _ T
Tz + dy~ JiLv
Similar reasoning gives, for the total distortion in the plane zx,
and in the plane x y,
dz dz
dj, dn _ T
dy + dx " a '
The factor 2 is used in these expressions for the sake of convenience
in the employment of certain formula?, to be afterwards quoted.
The halveddifferences of the pairs of differential coefficients,
2\dz dyP 2 \dx dz)'2\dy dxF
represent rotations of the prism as a whole round the axes of x, y, z, respec
tively, which have no connection with the positions of its particles
relatively to each other.
The component strains into which all others can be resolved with
respect to a given set of axes are thus reduced to six, three longitudinal
and three transverse.
7. A pressure, like a strain, is defined by three things :
1st. The direction of the pressure.
2nd. The position of the surface at which the pressure is exerted.
3rd. The amount of the pressure as expressed in units of force pes
unit of area of the surface of action.
A pressure on a plane, in whatsoever direction it may act, may be
resolved into three rectangular components, one normal to the plane
LAWS OF THE ELASTICITY OF SOLID BODIES. 71
and two tangential. The normal pressure may be compressive or tensile:
when compressive, it is considered as positive; when tensile, negative.
In an elastic solid which is in equilibrio, let a cube be conceived to
exist with its faces normal to the axes of coordinates, and let the pressures
throughout its extent be uniform. This cube exerts on the matter round
it, and is reacted on by three pairs of normal pressures, at the faces
respectively normal to the axis of x, y, z, which may be denoted by
Pj, P q , P 3 , the pressures at opposite faces being equal.
Let A B C D represent the section of this cube by the plane yz. On the
faces A B and C D, parallel to x z, let a pair of tangential forces act in the
directions denoted by the order of the letters, tending to produce dis
tortion by making the angles B and D acute and A and C obtuse. Let a
pair of forces of similar tendency act on the faces C B and A D, parallel
to zy. These two pairs of forces are equal and opposite to those which
the cube, in consequence of the transverse displacements of its particles,
exerts on the surrounding portion of the solid. No displacement of the
relative situations of a system of particles can give the system a tendency
to revolve as a whole round an axis. Such a tendency must exist in the
cube unless the tangential forces on the faces A B, CD are equal to those
on the faces C B, A D.
Therefore, the tangential pressure parallel to z, on a plane normal to y, is
equal to the tangential pressure parallel to y, on a plane normal to z: a theorem
first proved by Cauchy.
The common value of those forces may be denoted by Q 1 , as they are
both perpendicular to x.
Similar reasoning shows that the two pairs of tangential forces perpen
dicular to y have one common value, Q 2 .
In like manner, those perpendicular to z may be denoted by Q 3 .
Thus the pressures exerted by and on the cube are reduced to six,
three normal and three tangential.
8. The composition of pressures applied to different planes, and their
reduction to new axes, depends on the following principle : —
Conceive a small triangular pyramid, with its apex at the origin of
rectangular coordinates, its sides being formed by the three coordinate
planes, and its base by a plane in any given direction intersecting them.
Let pressures in one given direction act on the three sides, and be
balanced by a pressure in the same direction on the base. Each of the
three sides is equal to the base multiplied by the cosine of the angle
between the normal to the base and the normal to the side in question.
Therefore, the total pressure on the base is equal to the sum of the
pressures on the sides, each multiplied by the cosine of the angle between
the normal to the side in question and the normal to the base. If the
normal to this base is one of three new axes of rectangular coordinates, the
72
LAWS OF THE ELASTICITY OF SOLID BODIES.
total pressure thus found may be reduced to normal and tangential pressures,
by multiplying it by its directioncosines with respect to the new axes.
9. I annex, for convenience of reference, the general formulae which
have been deduced from this principle.
Let x, y, g, be rectangular axes of coordinates, and P^ P 2 , P 3 , Q x , Q , < v > .
normal and tangential pressures which act as shown in the following
table :
Pressuros parallel to
Normals. Planes. X jf Z
x yz P x Q 8 Q 2
y z* Q 3 P Qi
z xy Q 2 Qj P s
Let Ej, R 2 , R 3 , be the rectangular components of the total pressure at
a plane, the directioncosines of whose normal are a v a 2 , a s .
Then
R x = a y Pj + a 2 Q s + « a Q 2 ,>>
R 2 = a 1 Q 3 + « 2 P 2 + « 3 Q 1 , I (i.)
E 3 = a x Q 2 + a.Xi i + « 3 P 3 , J
Let this normal be taken as the axis of x in a new set of rectangular
axes x, y', z, which make with the original axes the angles whose cosines
are given in the following table : —
Original Axes. New Axes.
x' y z'
h
c 2 VDirectioncosines.
c,J
y 3 L 3
Let P/, P 2 ', P 3 ', Q/, Q 2 ', Q 3 ', be the normal and tangential pressures, un
reduced to the new axes : then
P/^P^ + P^ + p^
+ 2Q 1 a 2 a 3 + 2Q 2 a 3 aj, + 2Q^a 1 a 2 ,
+ 2Q 1 b 2 b 3 + 2Q. 2 b s b 1 + 2Q.J) 1 b 2 ,
P3'=P lCl 2 + P 2 ^ + P 3 c 3 2
+ 2Q lC2 c 3 + 2Q 2 c 3 c 1 + 2Q 3 c 1 r 2 ,
Q/ = P 1 5 1 c 1 + P 2 b 2 c 2 + P 3 6 3 c 3
+ Qi(^ 3 + b 3 c 2 ) + Q 2 (6g Cl + b^) 4 Q 3 (V 2 + 6 2 cj).
Qo — Px^i^x + P 2 C 2 a 2 + P 3 C 3 a 3
+ Qj(c 2 a 3 + c 3 a 2 ) + Qo(c 3 a 1 + c x « 3 ) + Q 3 (c 1 « 2 + c./^),
Q 3 ' = P 1 a 1 6 1 + P 2 «/; 2 + P 3 « 3 6 3
+ Q^ao&a + a z b 2 ) + Q 2 (a 3 6 x + a^ 3 ) + Q^aft., + a.p x ),
y (2.)
LAWS OF THE ELASTICITY OF SOLID BODIES. 73
By the substitution of N for P and T for Q, the formulae given above
are made applicable to the reduction of strains to new axes of coordinates.
I shall not here recapitulate the many elegant and important theorems
which MM. Cauchy and Lame and Clapeyron have deduced from those
equations, as they do not relate to the branch of the theory of elasticity
of which this paper treats.
I may mention that in their memoir in the seventh volume of Crelle's
Journal, MM. Lame and Clapeyron have used X and T to denote pressures,
d ?
and have expressed strains simply by the differential coefficients — , &c.
Section II.— Physical Relations between Pressures and Strains,
SO FAR AS THEY ARE INDEPENDENT OF HYPOTHESES RESPECTING THE
Molecular Constitution of Matter.
10. In almost all investigations which have hitherto been made
respecting the elasticity of bodies which have different degrees of
elasticity in different directions, it has been the practice to take some
hypothesis as to the molecular constitution of solid bodies as the basis of
calculation from the outset of the inquiry. It appears to me, however,
that the more philosophical course is, to ascertain, in the first place, what
conclusions can be attained as to the laws of elasticity without the aid
of any such hypothesis, and afterwards to inquire how far the theory
can be simplified, and what additional results can be gained by introducing
suppositions respecting the ultimate constitution of matter.
For the present, therefore, I shall make no assumption as to the
questions whether bodies are systems of physical points, or of atoms
of definite bulk and figure, or are continuous, or have a constitution
intermediate between those three; and I shall use the word particle in its
literal sense of a small part
11. I shall restrict the present inquiry to homogeneous bodies possessing
a certain degree of symmetry in their molecular actions, which consists
in this : that the actions upon any given particle of the body of any two
equal particles situated at equal distances from it within the sphere of
molecular action, in opposite directions, shall be equal and opposite.
Substances may possess higher degrees of molecular symmetry, but this
is the lowest.
The statement that a body is homogeneous means, when applied to
molecular action, that the mutual actiou of a pair of particles, situated at
a given distance from each other in a given direction, shall be equal to
that of any other pair of particles equal to the first, situated at an equal
distance from each other in a parallel direction.
12. It is known by observation, that strains and pressures are physically
74< LAWS OF THE ELASTICITY OF SOLID BODIES.
connected. It is also known by observation, that the pressure with which
a strain is connected consists in a tendency of the body to recover its
natural state, and is opposite or nearly opposite in direction to the strain ;
thus longitudinal condensation is accompanied with positive normal, or
nearlynormal pressure ; longitudinal dilatation, with negative normal, or
nearlynormal pressure ; and distortion in a given plane, with tangential
pressure in the same plane, of opposite sign.
It is known by experiment, that when a pressure and the strain with
which it is connected are given in direction, and when the strain does not
exceed a certain limit, being in most cases the utmost limit to which a
structure can be strained without danger to its permanency, the pressure
and the strain are sensibly proportional to each other. The quantity by
which a strain is to be multiplied to give the corresponding pressure is a
coefficient of elasticity, and is expressed, like a pressure, by a certain
number of units of force per unit of surface.
I have said that a strain and the corresponding pressure referred to the
same plane are opposite or nearly opposite in direction ; for they are not
of necessity exactly opposite for all directions of strain, except in sub
stances which are possessed of the highest degree of molecular symmetry ;
that is to say, which are equally elastic in all directions. For those
having lower degrees of symmetry, the following proposition is true : —
Theorem I. In an elastic substance which is homogeneous and symmetrical
with respect to molecular action, there are three directions at right angles to each
other in which a longitudinal strain produces an exactly normal pressure on a
plane at right angles to the direction of the drum.
Those three directions are called Axes of Elasticity. The proposition is
equivalent to an assertion, that the lowed degree of symmetry of molecular
action necessarily involves symmetry with respect
to three rectangular coordinate planes.
This theorem has been often demonstrated
for systems of atoms. But it is easily seen
that the truth of these demonstrations de
pends, not on the special hypotheses which
they involve, but on the fundamental con
dition of symmetry.
The following demonstration involves no
hypothesis.
Let a point in the interior of a body
be assumed as the origin of rectangular
coordinates, the axes being considered as fixed, and the body as
movable angularly in all directions about the origin. Space round is
divided by the coordinate planes into eight similar indefinitelyextended
LAWS OF THE ELASTICITY OF SOLID BODIES.
75
rectangular threesided pyramids. Let those pyramids be designated as
follows, according to the signs of the coordinates comprised in them.
Signs of Designation of
x 'i I z Pyramid.
. A
. B
. C
. D
. E
. F
. G
. H
To express the relative situations of these pyramids, as taken in pairs,
let the following terms be used :
Diametrically opposite — when the pyramids touch at the apex only;
comprising the following pairs,
A, G ; B, H ; C, E ; D, F.
Diagonally opposite — when they touch at an edge ; comprising the pairs
A, H ; D, E ; B, G ; C, F ;
A, F ; B, E ; D, G ; C, H ;
A, C ; B, D ; E, G ; F, H.
Contiguous — when they touch in a face ; comprising the pairs
A, B ; D, C ; H, G ; E, F ;
A, D ; B, C ; F, G ; E, H ;
A, E ; B, F ; C, G ; D, H.
Each pair of contiguous pyramids forms a rectangular wedge, which
has an opposite wedge touching it along the edge, and a contiguous wedge
touching it at each of its two faces.
The pairs of opposite wedges are
A B, G H ; C D, E F ;
AD,FG; BC,EH;
A E, C G ; B F, D H.
The pairs of contiguous wedges are
AB, CD; CD, GH; GH, EF; E F, AB;
AD, BC; BC, FG; FG, EH; EH, AD;
AE, BF; BF, CG; CG, DH; DH, AE.
76 LAWS OF THE ELASTICITY OF SOLID BODIES.
According to the condition of symmetry already stated, the portions of
matter comprised in any pair of diametrically opposite pyramids must be
symmetrical in their actions on a particle placed at 0, or on any pair of
equal particles symmetrically placed with respect to 0, whatsoever may
be the angular position of the body with respect to the axes.
Suppose the body to receive a longitudinal strain in the direction of
the axis of z. Let a small circular area w be conceived to exist in the
plane of xy, with its centre at 0; and let this area be the base of a
cylinder extending indefinitely in a negative direction along the axis of z,
and denoted by to z. The pressure on the plane x y is proportional and
parallel to the resultant of the actions of the four pyramids A, B, C, D, on
the cylinder u> ~, divided by the area w. The action of each of those
pyramids consists of a normal component parallel to z, and a tangential
component parallel to the plane xy. In order that the total pressure may
be normal, those tangential actions must balance each other, which can
only be the case when the tangential action of the wedge AB, parallel to
the axis of y, is equal and opposite to that of the contiguous wedge CD,
and the tangential action of the wedge B C, parallel to the axis of x, is
equal and opposite to that of the contiguous wedge A D.
The pair of contiguous wedges A B, C D, touch in the plane of xz, having
the axis of x for their common edge. If the actions of this pair of wedges
on w~, when longitudinally strained along .:, are unsymmetrical, this
cannot arise from the form, position, or strain of these wedges, which are
exactly symmetrical with respect to each layer of particles in w:, but from
the nature of the particles occupying tin wi dges. Now by rotating the
body through a right angle about the axis of x, we can bring the particles
which formerly occupied C D into A B, and the particles which formerly
occupied G H (which consists of two pyramids diametrically opposite, and
therefore molecularly symmetrical to A and B) into C D. In this new
situation of the body with respect to the axes of coordinates, the resultant
of the tangential actions, parallel to y, of the wedges A B and C D, on wz,
though not necessarily eqml, will be opposite in direction to the original
resultant; and this change will have been produced, not abruptly, but
continuously, so that the value of the resultant must have passed through
zero. Therefore, whatsoever may be the situation of the axis of x
amongst the particles of the body, it is possible, by rotating the body
about that axis, to find a position in which the tangential actions of the
wedges AB and CD, parallel to y, on the cylinder wz, shall balance each
other. And by similar reasoning it may be proved, that whatsoever may
be the situation of the axis of y amongst the particles of the body, it is
possible, by rotating the body about that axis, to find a position in which
the tangential actions of the Avedges B C and A D, parallel to x, on the
cylinder w z, shall balance each other.
LAWS OF THE ELASTICITY OF SOLID BODIES. 77
Therefore, by combining rotations about the axes of x and y, it is
possible to find a position of the solid with respect to the axes of
coordinates such that the tangential actions of the four pyramids A, B,
C, D, on the cylinder wz, arising from a longitudinal strain along ,~ shall
be in equilibrio, and that the total pressure on x y shall be normal.
The direction, with respect to the solid, which fulfils this condition, is
called an axis of elasticity.
Let — z O + z, being now an axis of elasticity, be considered as fixed in
the solid.
From the manner in which the two pairs of wedges AB, CD and BC,
AD, are composed of the four pyramids A, B, C, D, it is clear that the
actions of the pair of diagonally opposite pyramids A, C, are symmetrical,
and also those of the diagonally opposite pyramids B, D. From this and
the symmetry of the actions of diametrically opposite pyramids it follows,
that the actions of the four pairs of contiguous pyramids, A, E ; D, H ;
B, F; C, G, are symmetrical, and also those of the two pairs of diagonally
opposite pyramids, E, G ; F, H. This symmetry of action (subject to the
condition of symmetry of strain) is not disturbed by rotation about the
axis of z.
Let the small circular area to be now conceived to exist in the plane y~,
and let the cylinder of which it is the base extend in a negative direction
along the axis of x, and be called the cylinder wx. Let the solid receive
a longitudinal strain along the axis of x. The action of A on iox is sym
' metrical to that of E, and the action of D to that of II ; therefore, the
tangential actions of the wedges A D, E H, parallel to z, balance each other.
It remains only to make the tangential actions of the wedges A E, D H,
parallel to y, on wx, balance each other, which is to be done by rotation
about the axis of :.
The solid is now in such a position that a, as well as z, is an axis of
elasticity.
The pairs of contiguous pyramids are now all molecularly symmetrical
about their common faces. Therefore the pairs of contiguous wedges AB,
E F ; A E, B F, are symmetrical in their actions on a cylinder to y, when
longitudinally strained along y.
Therefore y also is an axis of elasticity, and the theorem is proved.
It is not necessary to the existence of rectangular axes of elasticity that
the body should be homogeneous (in the sense in which I have used the
word) throughout its whole extent, but only round each point throughout
a space which is large as compared with the sphere of appreciable mole
cular action of each particle. Hence the rectangular axes of elasticity
may vaiy in direction at different points of the same body ; and some, or
all of them, may follow the course of a system of curves, as they do in a
rope, a piece of bent timber, or a curved bar of fibrous metal.
LXIS.
Plane.
X
yz
y
zx
z
xy
78 LAWS OF THE ELASTICITY OF SOLID BODIES.
13. The axes of elasticity are evidently those which ought to be
selected as axes of coordinates, for the resolution of all pressures and
strains, in researches on the laws of elasticity. The strains and pressures
being so resolved, we shall have the expression  A x N x for part of
the normal pressure on the plane yz : A x being the coefficient of longitudinal
elasticity for the axis of x. But this is not the whole of that pressure ;
for it is known by observation, that the normal pressure on a given
plane is augmented by condensation, and diminished by dilatation of
the particles, in a direction parallel to the given plane as well as normal
to it. The normal pressure P x on yz, therefore, depends not only on the
longitudinal strain N x along x, but also on the longitudinal strains N 2 and
N alon" y and z. Applying similar reasoning to the other normal
pressures, they are found to be represented as follows :
P x =  A X N X  B 3 N  Bo'Ng "]
p^b^aXbX r • ( 3 )
P 3 =B 2 N 1 B 1 'N 2 A 8 N S J
The tangential pressures are represented, in terms of the distortions, in
the following manner :
Tlane.
yz . . Q 1=  20^
zx . . Q„= 2C.X y . . . (4.)
xy . . Q 3 =2C 3 T 3 J
These six equations are merely the representation of observed facts,
framed with regard to the principle of axes of elasticity.
They contain twelve coefficients of elasticity, which may be thus
classified :
A x , A 2 , A 3 , are the coefficients of longitudinal elasticity for the axes of
x, y, z, respectively ;
B 1? B x ', are the coefficients of lateral elasticity in the plane of yz : the
former expressing the effect of a strain along z in producing normal pres
sure parallel to y : the latter, the effect of a strain along y in producing
normal pressure parallel to z.
B 2 , B 2 ', are the coefficients of lateral elasticity in the plane of zx; and
B 3 , B 3 ', in the plane of x y.
C 1; C 2 , C 3 , are the coefficients of transverse or tangential elasticity, or of
rigidity, in the planes of yz, zx, and xy, respectively. The possession of
this species of elasticity is the property which distinguishes solids from
fluids, and is that upon which the strength and stability of solid structures
entirely depend. When a beam, or any other portion of a solid structure,
takes a set, as it is called (or undergoes permanent alteration of figure), it
LAWS OF THE ELASTICITY OF SOLID BODIES. 79
is the rigidity which has been overstrained and has given way. So far as
I am aware, however, it has not hitherto been directly referred to in
researches on the strength of materials, except in those relative to
torsion.
The principal object of the present inquiry is to determine what mutual
relations must necessarily exist amongst those twelve coefficients of
elasticity in each substance.
14. The three coefficients of rigidity, so far as we have as yet seen,
represent the elasticity called into play by three kinds of distortion,
measured respectively by the alteration of the angles of the three rectan
gular sections of a cube whose faces are normal to the three axes of
elasticity. I shall now, however, prove that the tangential pressures pro
duced by equal distortions are equal, so long as the plane in which the
distortion takes place is unchanged, and are not altered by any change of
the direction, in that plane, of the sides of the figure on which the distor
tion is measured ; that is to say —
Theorem II. The coefficient of rigidity is the same for all directions of dis
tortion in a given plane.
Let ABCD be the section at right angles to the edges of a rhombic
prism having any angles ; and G E and F H two
lines normal respectively to the faces of the prism.
Let this prism undergo a small alteration in the ^
angles of its section ABCD.
Whether we estimate the distortion so produced
as a transverse displacement of the particles in
lines parallel to AB, and varying along the strain
normal G E, or as a transverse displacement of
the particles in lines parallel to A D, and vary
ing along the strainnormal F H, the result, so far
as the relative transverse displacements of the par G
tides are concerned, will be the same.
Also, the tangential pressures are the same at the pair of faces A B and
C D, and at the pair of faces B C and A D ; for otherwise a relative dis
placement among the particles of a body would produce a force tending to
make it revolve as a whole round an axis, which is impossible.
Therefore, the tangential forces produced by equal transverse displace
ments relatively to two strainnormals which make any angle with each
other are equal, provided the displacements are in the same plane with the
normals ; therefore the coefficient of rigidity is the same for all directions
in a given plane.
15. This theorem leads to another, which expresses the relations
between the twelve coefficients of elasticity, as far as it is possible to
<S0 LAWS OF THE ELASTICITY OF SOLID BODIES.
determine them independently of hypotheses respecting the constitution of
matter.
Theorem III. In each of the coordinate planes of elasticity the coefficient of
rigidity is equal to onefourth part of the sum of the two coefficients of longitudinal
elasticity for the axes which lie in that plane, diminished by onefourth part of the.
sum of the two coefficients of lateral elasticity in tin' same plane.
For example, let the plane be that of yz, in which the coefficient of
rigidity is C v those of longitudinal elasticity A 2 and A 3 , and those of
lateral elasticity B x and B/.
Let 2 ly represent a distortion in the plane yz, relative to two new
A
.axes y, ,:', in the same plane, and let the angle yy' = 9. Let this distortion
be resolved, with respect to the original axes, according to equation (2).
Then
N x = 0; N 2 =  2 TV °o s sin 6>; X., = 2 T/ cos 6 sin 0;
2 T, = 2 T/ (cos e  sin' 2 0); T 2 = 0; T 3 = 0.
The corresponding pressure. referred to the original axes arc
T 1 =  2 T/ (II.  IV) cos 6 sin 0,
P 2 =  2T X ' ( A.+ 15,) cos 6 sin 0.
p 3 =  2 t; ( ]y + A3) cos e sin e,
Q 2 =  2 C^T/ (cos 2 B  sin 2 0), Q, = 0, Q, = 0.
Let us now determine, according to equation (2), from the above
pressures, the tangential pressure Q/ as referred to the new axes. Then
Q/ =  2 T/ {C x + cos 2 e sin 2 6 (X, + A 3  B,  B,'  4 CJ}.
But by the preceding theorem we have also
0'= 2TT
for all values of 0, which cannot be true unless the coefficient of
cos 2 sin 2 in the first value of Q/ is = 0. Consequently
C i = \ (A 2 + A 3  Bj  B/).
% applying similar reasoning to the planes of
sx and xy it is also proved that
1
C 2 = I (A 3 +A 1 B 2 B 2 '),
C 3 = (A 1 + A 2 B 3 B 3 ,
being the algebraical statement of the theorem enunciated.
(50
LAWS OF THE ELASTICITY OF SOLID BODIES. 81
Thus the number of independent coefficients of elasticity is reduced to
nine, of which the other three are functions ; and this is the utmost reduc
tion of their number which can be made without the aid of suppositions
as to the constitution of matter.
The determination by experiment of nine constants for each substance
is an undertaking almost hopeless ; it is therefore desirable to ascertain
whether, by the introduction of some probable hypothesis, their number
can be further reduced.
Section III. — Eesults of the Hypothesis of Atomic Centres.
16. Almost all the investigations of the laws of elasticity which have
hitherto appeared, are founded on the hypothesis of Boscovich : that
matter consists of physical points or centres of force, or of atoms acting
as if their masses were concentrated at their centres; which physical
points or atoms occupy space, and produce the phenomena of elasticity,
because the forces which act between them, and which depend on their
relative distances and positions, tend to make them remain in certain
relative positions, and at certain distances apart.
Although the results of this supposition are not verified by all solid
substances, still it seems probable that its errors are to be corrected, not
by rejecting it, but by combining it with another, to which I shall
afterwards refer.
I shall now, therefore, show to what extent the laws of elasticity are
simplified by adopting Boscovich's supposition of atomic centres of force,
acting on each other by attractive and repulsive forces along the lines
joining them. It will be seen, that in consequence of the course adopted,
of determining, in the first place, the necessary relations between the
coefficients of elasticity which must exist independently of all special
hypotheses, this investigation is almost entirely freed from the algebraical
intricacy in which it would otherwise be involved.
17. All the consequences peculiar to this hypothesis flow from the
following single theorem, in which the term 'perfect solid is used to denote
a body whose elasticity is due entirely to the mutual attractions and
repulsions of atomic centres of force.
Theorem IV. In each of the coordinate planes of elasticity of a perfect
solid, the tivo coefficients of lateral elasticity, and the coefficient of rigidity, are
all equal to each other.
Take, for example, the plane of y z. The proposition enunciated is
equivalent to the assertion, that the tangential pressure parallel to y at
F
S2 LAWS OF THE ELASTICITY OF SOLID BODIES.
the plane of xy, produced by a given transverse strain 2T X = 5, which
consists in a displacement of the atomic centres parallel to y and varying
with z, is equal to the normal pressure parallel to : at the same plane xy,
d
produced by a longitudinal strain N 8 = , . which consists in condensing
or dilating the atomic centres in a direction parallel to y. provided that
longitudinal strain is equal in amount to the transverse strain.
The pressure on a given area of the plane xy, is the effect of the joint
actions of the atomic centres on the negative side of that plane upon the
atomic centres on the positive side.
In the natural or unstrained condition of the body, this pressure is
null, showing that those forces neutralise each other. "When the body
is strained, therefore, the pressure is the resultant of the variations of all
those forces, arising from the displacements of the atomic centres from
their natural relative positions.*
Let m and //. denote a pair of atomic centres, m being situated on the
positive side of the plane xy, and ju on the negative side. The force
acting between m and /j. is supposed to act along the line joining them,
and to be a function of its length. When the relative displacement of
the atoms is very small as compared with their distance, the variation of
this force will be sensibly proportional to the variation of distance,
multiplied by some function of the distance. It may therefore be denoted
by <pr.dr, where /• denotes the distance (jim). Let this line make with the
axes the angles a. /3. y.
Let the strain to be considered, in the first place, be transverse, the
displacements being parallel to y and varying with .:, the rate of variation
being
dz ~ * L »
and the force to be estimated being in the direction y. Then the dis
placement of m relatively to /x is
A ?; = 2 T, r cos y.
The variation of their distance apart is
£ r = cos j3 A ij = 2 T 1 r cos /3 cosjy.
The variation of the force acting between them is
(pr .^r = 2 T 1 r (pr . cos ft cos y.
* Small quantities of the second order relatively to the strains Tj, kc, are here
nedeeted.
LAWS OF THE ELASTICITY OF SOLID BODIES. 83
And the component of that variation parallel to y, which forms the part
of the tangential pressure due to the action of li on m, is
cos /3 <pr . S r = 2 T x r <p r . cos 2 /3 cos y. . . (a.)
Next, let the strain be longitudinal, parallel to y, and denoted by
N =^
dy
Then the displacement of m relatively to fx is
A tj = N 2 r cos 3.
The variation of their distance apart is
8r = cosj3ATj = N 2 rcos*j3.
The variation of the force acting between them is
(f>r .Sr = ~N 2 r <pr cos 2 /3.
And the component of that variation parallel to z, which forms the part
of the normal pressure on the plane xy due to the action of fx on m, is
cos y (j) r . S r = N 2 r (j> r cos 2 /3 cos y. . . (b.)
On comparing the expressions (a) and (b) it will be seen that the
quantities by which 2l\ and N 2 are multiplied are identical. Therefore,
the tangential force in the direction y on the plane xy produced by a
distortion in the plane yz, and the normal force in the direction z produced
by a longitudinal strain along y, are equal when the strains are equal, for
each pair of atomic centres. They are therefore equal for a perfect solid,
because its elasticity is wholly due to the mutual actions of atomic centres;
and the theorem is proved for the plane yz, and may in the same manner
be proved for the other coordinate planes of elasticity. It is expressed
algebraically as follows :
Plane.
yz . . . B^B^G^
z x . . . B 2 = B 2 ' = C 2
xy . . . B 3 = B 3 ' = C 3
(6.)
18. The combination of these equations with the equations (5) of
Theorem III. leads immediately to the following results :
84 LAWS OF THE ELASTICITY OF SOLID BODIES.
p A 2 + A 3 "I
L i 6
p A 3 + A l
p A l + A 2
^ — 6~ j
A 2 = 3 (C 2 + C s  <
3^
A 2 = 3 (C 8 + C x 
DJ
A 3 = 3 (Cj + c 2 
D 8 )^
• (7.)
• (8.)
that is to say,
Theorem V. In each of the three coordinate planes of elasticity of a perfect
solid, the coefficient of rigidity is equal to onesixth part of the sum of the two
coefficients of longitudinal elasticity ;
and consequently,
For each axis of elasticity of a perfect solid, the coefficient of longitudinal
elasticity is equal to three times the sum of the two coefficients of rigidity for the
coordinate planes which pass through that axis, diminished by three times the
coefficient of rigidity for the plane normal to that axis.
We have now arrived at the conclusion, that in a body whose elasticity
arises wholly from the mutual actions of atomic centres, all the coefficients
of elasticity are functions of the three coefficients of rigidity. Rigidity
being the distinctive property of solids, a body so constituted is properly
termed a perfect solid.
When the three coefficients of rigidity are equal, the body is a perfect
solid, equally elastic in all directions. The equations 6 and 8 become
A = 3 C ; B = C, agreeing with the results deduced by various mathe
maticians from the hypothesis of Boscovich.
Section IV. — Results of the Hypothesis of Molecular Vortices.
19. The great and obvious deviations from the laws of elasticity, as
deduced from the hypothesis of atomic centres, which many substances
present, render some modification of it essential.
Supposing a body to consist of a continuous fluid, diffused through
space with perfect uniformity as to density and all other properties, such
a body must be totally destitute of rigidity or elasticity of figure, its parts
having no tendency to assume one position as to direction rather than
another. It may, indeed, possess elasticity of volume to any extent, and
LAWS OF THE ELASTICITY OF SOLID BODIES. 85
display the phenomena of cohesion at its surface and between its parts.
Its longitudinal and lateral elasticities will be equal in every direction ;
and they must be equal to each other by equation (5), which becomes
= A  B; C = 0.
If we now suppose this fluid to be partially condensed round a system
of centres, there will be forces acting between those centres greater than
those between other points of the body. The body will now possess a
certain amount of rigidity; but less, in proportion to its longitudinal and
lateral elasticities, than the amount proper to the condition of perfect
solidity. Its elasticity will, in fact, consist of two parts, one of which,
arising from the mutual actions of the centres of condensation, will follow
the laws of perfect solidity ; while the other will be a mere elasticity
of volume, resisting change of bulk equally in all directions.
In a paper on the Mechanical Action of Heat in connection with the
Elasticity of Gases and Vapours (Trans, of the Royal Society of Edinburgh,
Vol. XX., Part I.), I have attempted to develop some of the consequences
of a supposition of this kind, called the hypothesis of molecular vortices.""
It assumes that each atom of matter consists of a nucleus or central
physical point, enveloped by an elastic atmosphere, which is retained in
its position by forces attractive towards the atomic centre, and which,
in the absence of heat, would be so much condensed round that centre
as to produce the condition of perfect solidity in all substances; that the
changes of condition and elasticity due to heat arise from the centrifugal
force of revolutions among the particles of the atmospheres, diffusing them
to a greater distance from their centres, and thus increasing the elasticity
which resists change of volume alone, at the expense of that which resists
change of figure also; and that the medium which transmits light and
radiant heat consists of the nuclei of the atoms, of small mass, but
exerting intense forces, vibrating independently, or almost independently
of their atmospheres; absorption being the communication of that motion
to the atmosphere, so that it is lost by the nuclei.
20. A body so constituted, in which the rigidity is considerable, may
be called, in general, an imperfect solid ; and it is obvious that its various
elasticities may be represented in the following manner :
Theorem VI. In an imperfect solid, according to the hypothesis of
molecular vortices, each of the coefficients of longitudinal and lateral elasticity
is equal to the same function of the coefficients of rigidity which would be its
value in a perfect solid, added to a coefficient of fluid elasticity which is the
same in all directions.
* An abstract of that paper is published in PoggendorfF's Annalen for 1850, No IX.
86
LAWS OF THE ELASTICITY OF SOLID BODIES.
Denoting this fourth coefficient by J, we have the following equations,
giving the values of the coefficients of longitudinal and lateral elasticity
in terms of the coefficient of fluid elasticity, and of the three coefficients
of rigidity.
A x = 3 (C 2 + C,  C x ) + J
A, = 3 (C 3 + Oj  C 2 ) + J
A 3 = 3 (C x + 2  C 3 ) + J
B x = C x + J
B 2 = C, + J
B 3 = C 3 + J
(0.)
The utmost number of independent coefficients is thus increased
to four.
If the coefficients of rigidity be progressively diminished without limit,
as compared with the coefficient of fluid elasticity, the body will pass
through every stage of the gelatinous state; and when the coefficients of
rigidity vanish, its condition will be that of a perfect fluid, in which the
longitudinal and lateral elasticities are all equal, and represented by the
single coefficient J.
It is to be observed, that in this condition the independent actions of
the nuclei or physical points at the atomic centres upon each other, which
are the means of radiation, may be very great; their sensible effect on
the elasticity of the body being neutralised by other forces, exerted by
the parts of the atmospheres.
If two of the coefficients of rigidity are equal (as C 9 = C 3 ), the body is
equally elastic in all directions round an axis, which in this case is that
of x ; and equations (9) become
A x = 6 C, 2  3 C x + J
A 2 = A 3 = 3 C x + J
B 2 = B 3 = C 2 + J j
(9A.)
When the three coefficients of rigidity are all equal, the body is an
imperfect solid equally elastic in all directions. The results of this
condition have been investigated by Professor Stokes, M. Wertheim, and
Mr. Clerk Maxwell.
Equations (9) in this case become
C + J; B = C + J.
(9B.)
LAWS OF THE ELASTICITY OF SOLID BODIES.
87
Note respecting Previous Investigations.
(18 a.) The investigations of Poisson (Mem. de VAcad. des Sciences,
XVIIL), of M. Cauchy (Exercices des MatMmatiques, passim), and of Mr.
Haughton (Trans. Boy. Irish Acad., XXL), respecting the elasticity of
substances unequally elastic in different directions, are all founded on the
hypothesis of atomic centres. So far as they relate to substances possessed
of rectangular axes of elasticity, they agree in expressing the elasticity of
such bodies by means of six coefficients, corresponding respectively to those
which I have denoted by
A v A 2 , A 3 , C 15 C 2 , C 3 .
None of those investigations indicate any relations amongst these six
coefficients.
The researches of Mr. Green on the propagation of vibratory movement
(Camb. Trans., VII.) differ materially from those which preceded them,
inasmuch as they are applicable, not merely to systems of atomic centres
or physical points, but to solid substances constituted in any manner
whatsoever.* So far as they are applicable to bodies possessed of
axes of elasticity, they involve nine coefficients: three of longitudinal,
three of lateral, and three of transverse elasticity. The following table
exhibits a comparison between Mr. Green's notation and that of this paper:
Coefficients of Elasticity.
In the Notation
of
Longitudinal.
Lateral.
Transverse.
Mr. Green, .
This Paper,
G H I
K A 2 A 3
P Q &
B x B 2 B 3
L M N
C\ C 2 C 3
There is nothing, however, in the researches of Mr. Green to indicate
any mutual relations amongst those nine coefficients; and to establish
such relations, indeed, it appears to me that the subject must be in
vestigated, not dynamically, but statically.
It may here be observed, that Mr. Green's equations contain three
additional coefficients, to represent the effect of a strained condition of the
medium on the propagation of vibratory movement; but those three
* A second paper by Mr. Haughton (Trans. Roy. Irish Acad., XXII.) is equally
comprehensive.
88 LAWS OF THE ELASTICITY OF SOLID BODIES.
quantities, being foreign to the subject of this paper, have no expressions
corresponding to them in its notation.
In the equations of the propagation of light, Mr. Green effects an
apparent reduction in the number of coefficients by introducing the
supposition that the vibrations are of necessity wholly tangential to each
wavefront. But this supposition is quite at variance with the nature
of elastic solids, and is obviously intended by the author as merely an
assumption for the purpose of facilitating calculation, and obtaining
approximately true results, in the case of luminiferous undulations.
Mr. M'Cullagh's researches on the propagation of light (Trans. Roy. Irish
Acad., XXI.) involve a similar assumption.
The result peculiar to the investigations contained in the present
paper is the establishment of certain mutual relations amongst the different
elasticities of a given substance, whereby the six coefficients of Poisson
and Cauchy are reduced to functions of three, and the nine coefficients
of Mr. Green to functions of four ; the former representing the condition
of a medium whose elasticity is wholly due to the mutual actions of atomic
centres, the latter that of a su bstance whose condition is intermediate
between those of a system of centres of force, and of a continuous and
uniformly diffused fluid.
General equations of vibratory movement, in the particular case of
uncrystallised media, agreeing with those of Mr. Green, are given by
Professor Stokes in his memoir on Diffraction (Camb. Trans. IX.) His two
coefficients of elasticity have the following values in the notation of
this paper : —
Prof. Stokes. This Paper.
6 2
D"
g denotes the accelerating force of gravity; and D, the weight of unity
of volume of the vibrating medium.
a and b, in Professor Stokes's paper, are the velocities of propagation
of normal and tangential vibrations respectively.
In the researches of Poisson, Navier, Cauchy, Lam6, and others, on
the elasticity of bodies equally elastic in all directions, the coefficients
are often expressed in terms of two quantities, denoted by k and K, in
the following expression for a normal pressure on the plane yz :
P 1 = ^N 1 K(N 1 + N 2 + N 3 ):
k represents a species of longitudinal elasticity, under the condition that
LAWS OF THE ELASTICITY OF SOLID BODIES. 89
the volume remains unchanged; and K, an elasticity resisting change of
volume. Their values in the notation of this paper are as follows : —
& = AB = 2Cj K = B = C + J.
It is evidently impracticable to apply an analogous notation to bodies
unequally elastic in different directions.
M. Wertheim has recently made a most elaborate and valuable series
of experiments on the elasticity of brass, glass, and caoutchouc, according
to a method suggested by M. Eegnault, for the purpose of determining
the laws of elasticity of uncrystallised substances (Ami. de Chun, ct de
Phys., Ser. III., torn. XXIII.) He concludes that for brass and glass, and
for caoutchouc moderately strained, the following equation is nearly if not
exactly true, in the notation to which I have just referred, h = K, which,
in the notation of this paper, is equivalent to the following :
J = C; B = 2C; A = 4 C.
M. "Wertheim has investigated the consequences which must follow in
the solution of several problems connected with elasticity, if this law be
universally true for solid bodies.
This supposition must be regarded as doubtful ; and it is not, indeed,
advanced by M. Wertheim as more than a conjecture. So far as our
present knowledge goes, it seems more probable that the relations between
C and J may be infinitely varied. If the effect of heat is to diminish C
and increase J, there maybe some temperature for each substance at which
M. Wertheim's equation is verified. In the sequel I shall consider more
fully the consequences to be deduced from M. Wertheim's experiments on
this subject.
Section V. — Coefficients of Pliability, and of Extensibility
and Compressibility, Longitudinal, Lateral, and Cubic.
Examples of their Experimental Determination.
21. Coefficients of elasticity serve to determine pressures from the
corresponding strains. We have now to consider the determination of
strains from pressures.
To determine a distortion from the corresponding tangential pressure,
it is sufficient to multiply, using the negative sign, by the reciprocal of the
proper coefficient of rigidity. This reciprocal may be called a coefficient
of pliability.
A similar process, however, cannot be applied to the calculation of
longitudinal strains from normal pressures; because, as each normal
90
LAWS OF THE ELASTICITY OF SOLID BODIES.
pressure is a function of all the three longitudinal strains, so each longi
tudinal strain is a function of all the three normal pressures.
Let the longitudinal strains be represented, in terms of the normal
pressures, by the following equations :
N 1 = ;r 1 P 1 + b ;2 P, + ^P 3 ^
N 2 = h,^ BgPss + JbrjPa L . . (10.)
N 3 = ^Pi + ^PsKgPj
Then the coefficients in these equations are found, by a process of
elimination, to have the following values in terms of the coefficients of
elasticity.
Let
K  24(C 2 2 C 3 + C 2 C 3 2 + C 3 2 C x + C 8 (V + C^C, + (W C,»  C 8 «  C 3 3 )
 52C 1 C 2 C 3 + J{8(C 2 C 3 + 0,0x4 0,0.,)  4(C X 2 + C 2 2 + C 3 2 )}.
Then
1
{80^9(0, 03)2 + 40^}
%=^{8C 2 29(C 3 C 1 ) 2 + 4C 2 Jj
a 3 =g{8C 3 2 9(C 1 C 2 ) 2 + 4C,.l;
1
fr 3 =^{3(c x +c 2 c,)C8c 1 c 2 +2(c 1 +c 2 c,)j}
(ii.)
The above coefficients may be thus classified :
tt v cl 2 , n 3 , are the coefficients of longitudinal extensibility and compressibility
parallel respectively to the three axes of elasticity.
b v l) 2 , I) 3 , are the coefficients of lateral extensibility and compressibility for
the three coordinate planes of elasticity, serving to determine the effect of
a normal pressure on those dimensions of a body which lie at right angles
to its direction.
From the manner in which the coefficient J enters into the common
denominator K, it is obvious that when the coefficients of rigidity diminish
without limit as compared with that of fluid elasticity, the six coefficients
of linear extensibility and compressibility increase ad infinitum.
In a body whose three coefficients of rigidity are different, the coefficient
LAWS OF THE ELASTICITY OF SOLID BODIES.
91
of cubic compressibility, that is to say, the quotient of the sum of the
three longitudinal strains by the mean of the three normal pressures, with
the sign changed, has no fixed value unless some arbitrary relation be
fixed between those pressures. Let them be supposed, then, to be all
equal ; let their common value be P, and let the coefficient of cubic com
pressibility in this case be denoted by & : then
h = _ *Ti+% + 3gT. = Ri + , + , 3 _ 2(Irx + ,b 2 + *,)
= 4" { 8 (° 2 C 3 + C a C i + CxC 2 )  4(<V + C 2 s + C 3 2)}
y a 2 )
Hence i = J + 6(C 1 +C, + C 3 )
lDGC^CgC.
~ 8 (C 2 3 + O^ + C X C^  4(0/ + C 2 2 + C 3 *)
So that this coefficient is the sum of the three longitudinal coefficients
of compressibility, diminished by twice the sum of the three lateral
coefficients. It does not, like them, increase ad infinitum when the
rigidity vanishes ; its ultimate value in that case being
1
J'
the reciprocal of the coefficient of fluid elasticity, as might have been
expected.
If C 2 = C 8 , so that the body is equally elastic in all directions round the
axis of x, equations (11) and (12) take the following forms :
K = 4C 1 {12C 1 C 3 6C 1 2 a 2 + J(4C 2 C 1 )}
»i =g ( 8C i 2 + 4ClJ) = UQ X G,  bCV  C„ 2 + J(4C 2  i\)
R 2 = »,«^{8C 1 «9(0 1 C i y + 40 a J}
b ± = ~ {6CVC, 30^a 2 + J(4C 2  2CJ}
1 ( ( Co + J
& 2 = I>3 = g(2C 1 C 2 + 2C X J) = j MCi0g1 2C 1 *30 8 * + J(SC 2  2C X )
ir = s (16C 1 C 2 4C 1 2 ) = ^C^o  6C X 2  C 2 2 + J(4C 2  C~)
1 490 2
£ = J + 12CO + 6C! jp '
M12A)
4CoC x
92 LAWS OF THE ELASTICITY OF SOLID BODIES.
For bodies equally elastic in all directions the coefficients of com
pressibility and extensibility take the following values :
2C + J . C + J
^^=50^ ••■r J+a
In substances of this kind the coefficient of cubic compressibility is the
same, whether the three normal pressures arc equal or unequal, being
equal to the sum of the three longitudinal strains divided by the mean of
the three normal pressures with the sign changed : that is to say,
*~ " " i\ + p 2 + iv
One of the most frequent errors in investigations respecting the elasti
city and strength of materials, and the propagation of sound, has been to
confound the coefficients of longitudinal elasticity with the reciprocals of
the coefficients of longitudinal compressibility. The equations of this
section show clearly how widely these two classes of quantities may
differ.
The reciprocal of the longitudinal extensibility, , is what is commonly
termed the Weight of the modulus of elasticity.
22. The following formula may be found useful in the determination of
the coefficient J of fluid elasticity from experimental data.
Let us suppose that the three coefficients of rigidity of a substance,
C v C 2 , C 3 , have been determined by experiments on torsion, and that
some one of the coefficients of compressibility and extensibility in equa
tion (11), or those derived from it, has also been determined by experi
ment. Let the actual value of this coefficient be called f, and the value
which it would have had, had J been = 0, f . Also let K denote the
value which the denominator K would have had, had J been = 0, and let n
be the factor by which J is multiplied in the numerator of f, and in, in
the denominator.
Then
J = K .^L C (13.)
u mi — n
When applied to coefficients of longitudinal extensibility, this formula
labours under the disadvantage that a comparatively slight error in the
experimental data may cause a serious error in the determination of J.
Let us take, for example, an uncrystallised substance, and make succes
sively the two following suppositions,
J = 0, J = C:
LAWS OF THE ELASTICITY OF SOLID BODIES. 93
it will be found that the results are respectively,
a =  x 04, n = r  X 0375,
being in the ratio of 1 6 : 1 5 ; so that any uncertainty in the experiments
is in this case increased fifteenfold in computing the value of — . Hence
it appears, that without very great precision in the experiments, the
coefficient of fluid elasticity cannot be satisfactorily determined by a
comparison of the effects of longitudinal tension with those of torsion. It
is especially desirable that the two sets of experiments should be made
on the same piece of the material.
The best data for calculations of this kind would be experiments on
cubic compressibility, in conjunction with experiments on torsion ; for, as
equations (12), (12 A), and (12B) show, in order to determine J we have
simply to subtract a certain symmetrical function of the rigidities from
the reciprocal of the cubic compressibility. In the process of calculation,
the errors in the experiments on rigidity are multiplied, on an average,
by ~ only, while those of the experiments on compressibility sustain no
augmentation whatsoever.
Next after data of this kind may be ranked experiments on longitudinal
extensibility, as compared with the cubic extensibility or compressibility
of the same piece of material. Of this method, suggested by M. Eegnault,
and carried into effect by M. Wertheim, I shall presently speak more
fully.
Were it possible to ascertain the velocity of sound in an unlimited
mass of an elastic material along each of the axes of elasticity, the
coefficients of longitudinal elasticity could be determined with great
precision by the formula
where v is the velocity of sound, D the weight of unity of volume of the
substance, and g the accelerating force of gravity. But it is only practi
cable to determine the velocity of sound along prismatic or cylindrical rods;
and, as I shall show in a subsequent paper, it is impossible, in the present
state of our knowledge of the molecular condition of the superficial
particles of solid bodies, to assign theoretically the ratio in which the
velocity of sound along a rod is less than its velocity in an indefinitely
extended mass. That ratio is only known empirically in a few cases,
having various values lying between 1 and ^/f .
23. The experiments of M. Wertheim, on longitudinal and cubic
extensibility {Ann. de Chim. et de Phys., Ser. III., Tom. XXIII.) were made
94 LAWS OF THE ELASTICITY OF SOLID BODIES.
upon brass and crystal, the results being calculated on the supposition
that those substances are homogeneous and equally elastic in all directions.
There can be no doubt of the correctness of this supposition with respect
to well annealed crystal ; and with respect to brass, it is probably very
near the truth.
In those experiments, a cylindrical tube of the substance to be examined
was strained by longitudinal tension. The increase of length was observed
directly. The increase of bulk was found by observing the depression in
a capillary tube connected with the summit of the strained tube, of a
column of liquid with which they were filled. Let R denote the tensile
force reduced to unity of surface ; let L be the original length of a given
portion of the tube, U its original volume, A L and A U the increase of
those quantities by the tension R ; and let the axis of x be that of the
tube. Then we have
N = AL  X +N +N =^ J 
P l = R; P 2 = 0; P 3 = 0:
and consequently, for uncrystallised substances,
1 AL . 3 AU
a =B,'^r> * = r it • • < 14 >
To determine the coefficients of rigidity and fluid elasticity from these
data, we have the following formulae :
C =
3 a  £ & „AL AU
6 T~ IT
Vn / VL AU /
(14A.)
J
The experiments of M. Wertheim were made on three tubes of
brass and five of crystal. In the following table those tubes are
designated as M. Wertheim has numbered them. The coefficients a and
Jr, transcribed or calculated from his statement of the mean results of
numerous experiments on each tube, express the fraction by which the
material is elongated, or increased in bulk, by tension at the rate of one
kilogramme on the square millimetre; that is to say, 142234 pounds
avoirdupois on the square inch. (The common logarithm of this number
is 3153004.) The reciprocals of those coefficients, and the coefficients of
elasticity, as calculated by equation (14A), are given in kilogrammes on
the square millimetre.
LAWS OF THE ELASTICITY OF SOLID BODIES.
95
H
S3
55
H
X!
H
K
O
H
a
»— t ■
m
m
X
11
5,«j
o
Hi
H
O
P5
■ I
q pq
05
a
H
<
t3
O
iJ
o
H
55
SI
E=4
o
Eh
O
H
a£
<! — 3
lap
O 01 g
•S = W
«J
a a
23
Sl
3 "»
a g"
o
^H O ^H
•5
a a
© o
cS
os
m
>o
o
Tfl
CO
CO
T*
CO
CO
C5
CM
t—
lO
CO
Ci
o
o
in
CO
CO
CM
CM
o
~H
CM
<M
CM
<M
CM
o
o
CO
O
o
o
o
O
o
o
o
o
o
o
Q
o
a
o
o
o
o
o
o
O
o
h1
o
o
o
o
o
©
o
o
H
96
LAWS OF THE ELASTICITY OF SOLID BODIES.
The various degrees of elasticity of the brass tubes are ascribed by M.
Wertheim to the relative frequency with which they were subjected to
wiredrawing, to reduce the thickness of metal. It may be observed, that
this operation seems to increase the rigidity more than the fluid elasticity,
a fact which might naturally have been expected.
The means of the three sets of results for brass are given in the
following table : —
Coefficients of
Rigidity,
Fluid Elasticity, .
Longitudinal Elasticity,
Lateral Elasticity,
Reciprocals of Extensibilities.
Longitudinal (or Weight of the ) 1
Modulus of Elasticity), . ) a
1
Cubic, ..... t
Kilogrammes on
the Sq. Millim.
37453
43890
15G250
81343
10D.V11
10G310
Lbs. Avoird. on the
Square Inch.
5327100
6242700
22224000
11579000
14301000
15121000
Coefficients of Extensibility
Per Kilog. on the
Per lb. on the Square
and Compressibility.
Square Millim.
Inch.
Longitudinal,
;t
000009946
00000000699
Cubic,
b
000009406
00000000661
Latei'al,
b
000003405
00000000239
The following result is calculated from the experiments of M. Savart
on the torsion of brass wire (Ann. dc Chim. et de Phys., August.
1829):—
Coefficient of Rigidity,
The difference beins
Kilog. on the
Square Millim.
C 3682
633
Lbs. on the
Square Inch.
5237100
900U0
Hence we see that the rigidity of wiredrawn brass, as determined
directly by torsion, differs from that calculated from the longitudinal and
cubic extensibilities by only onesixtieth part, being a very small discrepancy
in experiments of this kind.
The following table gives the means of the four sets of results, I., III.,
IV., V., for crystal : —
LAWS OF THE ELASTICITY OF SOLID BODIES.
07
Kilog. on the
Square Millim.
Lbs. on the Square Inch
c .
1518
2159100
J .
1438
2045300
A .
5992
8522600
B .
2956
4204400
1
4039
5746100
l
3968
5643800
Per Kilog. on the
Square Millim.
00002476
00002520
00000818
Per lb. on the Square
Inch.
00000001740
00000001772
00000000575
It is obvious that the above mean values for crystal are not to be
relied upon as equally accurate with those for brass ; for the wide dis
crepancies between the results of the experiments on the five crystal tubes
show that this substance, like every kind of glass, is subject to great
variations in the physical properties of different specimens.
24. So far as I am aware, there is no substance whose elasticity varies
in different directions, for which experimental data as yet exist, adequate
to determine the three coefficients of rigidity, and the coefficient of fluid
elasticity.
Supposing the three coefficients of rigidity of a substance of this kind
to be known by experiments on torsion, the process of MM. Regnault and
AVertheim would readily furnish data for calculating the fluid elasticity.
For example : let a tension E per unit of area be applied to the ends
of a tube whose axis is one of the axes of elasticity, say that of x. Let
—== De the fraction by which its volume is increased, as before. Then
AU_ N 1 + N 2 + N 3 _
"\
= 1{8C;2C 1 (C 2 + C 3 )6(C 2 
 C V 21 r
v 3/ / 1
y (is.)
Let the above equation be abbreviated into
A U _ 0(C)
R U ~ K + m J '
G
98 LAWS OF THE ELASTICITY OF SOLID BODIES,
where K = K + mJ, as in equation (13). Then
The formula? corresponding to equation (15) for tubes whose axes are
parallel to y and z, are easily found by permutations of the indices 1, 2,
3. The sum of the three values of ~ TT thus obtained is obviously = tf.
h U
[It may be remarked, with reference to Sect. 1 7 of the preceding paper,
that the effect of alterations of direction in the lines joining pairs of
particles is not taken into account in the investigation of the elastic forces
arising from the states of strain which are there considered. It appears
to me that this effect, except for particular laws of force, will be of the
same order as that which depends on the alterations of the mutual
distances between the particles ; and that if it be taken into account, the
demonstration of Theorem IV. fails.
This objection occurred to me after the whole of the paper was in type,
and I immediately suggested it to the author ; but, as he was not con
vinced of the correctness of my view, he desired that the paper should be
published as it stands, reserving additional explanations or modifications,
if necessary, for the next number of the journal. — W. T.]
Supplementary Paper to Section III., Article 17.
In the portion above referred to of my paper on the Elasticity of Solids,
published in the Cambridge and Dublin Mathematical Journal for February,
1851, the theorem is laid down, that in a given plane in an elastic solid
consisting entirely of atoms acting on each other by attractions and
repulsions between their centres, the coefficients of rigidity and of lateral
elasticity are equal.
The proof of this proposition depends on the principle that the elastic
force in such a solid, called into play by a strain, in which the relative displace
ments of the atoms arc very small as compared with tlieir distances apart, is
sensibly the resultant of the variations of force due to the variations of distance
only, the variations of relative direction producing no appreciable effect. This
principle being granted, it is easily shown that the portion of that
resultant for each pair of atoms is the same for a given amount of strain
in a given plane, whether lateral or transverse with respect to the plane
on which elastic pressure is estimated.
LAWS OF THE ELASTICITY OF SOLID BODIES. 99
In the paper referred to, I assumed this principle without demonstration.
The editor of this journal, however, has since shown me, that my having
done so may be considered as causing a defect in the chain of reasoning.
I shall now, therefore, proceed to prove it.
Let it be possible for a solid to exist in an unstrained condition,
consisting entirely of atomic centres of force acting on each other along
the lines joining them, with forces which are functions of the lengths
of these lines. Then must the pressure, estimated in any direction, on
any portion of any plane in that solid be null. That pressure is the
resultant, in the direction assumed, of the mutual actions of all the atoms
whose lines of junction pass through the given portion of the given
plane.
Let the given portion be indefinitely small, and let it be called w,
being situated in the arbitrarilyassumed plane
P Wj p, which divides the solid into two por +x
tions, A and B. Let — X o> + X be an arbi
trary axis along which pressure is to be esti
mated. The pressure exerted by the portion
A upon the infinitesimal area 10 of the portion
B, is the resultant, reduced to the direction
— X to + X, of all the forces exerted by the
atoms in A on the atoms in B, in lines passing
through (o; and the body being unstrained,
this resultant must be null.
Assume a new position, Vuyp', for the plane of separation, making an
equal angle P'w + X=+XwPon the opposite side of the axis to the
original position. The same letters applying to the two portions of the
solid, the pressure of A on the area 10 of B along XwX must still
be null.
The two planes divide the solid into two pairs of opposite wedges.
The action of A on B along X through w in the original position of the
plane, may be divided into two parts, viz. —
The resultant of the actions of the atoms in the wedge P top' on those
in the opposite wedge Ywp ;
The resultant of the actions of the atoms in the wedge p u>p on those
in the wedge P w P'.
In the new position of the plane, the pressure on u> is made up as
follows :
The resultant of the actions of the atoms in the wedge Pw/on those
in the wedge P'wp, which is the same as hi the original position of
the plane;
The resultant of the actions of the atoms in the wedge PwP' on those
in the wedge p'wjp, being identical in amount but opposite in direction to
100 LAWS OF THE ELASTICITY OF SOLID BODIES.
that of the atoms in p' up on those in P to P', which formed part of the
pressure in the original position of the plane.
Now the pressures in the two positions of the plane of separation cannot
both be null, unless the resultant of the mutual actions of the atoms in
each pair of opposite wedges is separately null; for we see that the action
of a pair of wedges can be reversed in direction without affecting the
nullity of the total resultant. The position of the pair of opposite
wedges is arbitrary; so also is their angular magnitude, which may be
indefinitely small.
Therefore, no mere change of angular position of a pair of opposite
elementary wedges can produce a pressure.
Every strain in which the relative displacements of the particles are
small as compared with their relative distances, may be reduced to
angular displacements of pairs of opposite elementary wedges, and varia
tions of the mutual distances of the particles contained in them. The
angular displacements can produce no pressure of themselves; the
variations of distance are therefore the sole cause of that portion of the
pressure which is of the same order of small quantities with the strain:
being the principle to be proved.
The combination of the angular displacements with the variations of
distance will give rise to pressures of the second and higher orders of
small quantities as compared with the strain; but for the small strains
to which the present inquiry is limited, those are inappreciable, and may
be nesdected.
Note respecting Mr. Clerk Maxwell's Paper <; On the
Equilibrium of Elastic Solids." (Trans. Hoy. Soc. Edin.,
Vol. XX., Part I.)
I HAVE already referred to the researches of Mr. Clerk Maxwell, of the
general nature of which only I was aware at the time of the publication
of my paper on this subject in the Cambridge and Dublin Mathematical
Journal for February, 1851.
Since then I have had an opportunity of reading Mr. Maxwell's paper,
so as to compare his notation with my own.
Mr. Maxwell's investigations relate to such solids only as are equally
elastic in all directions. He expresses their elasticity by means of two
coefficients, fx and m, having the following properties:
LAWS OF ELASTICITY. 101
M =
1 P x + P 9 + P,
3 d£ dv dV
dx dy d z
m =
P — P P 
d £ dr\ dr\
d x dy dy
 P.,
d z
dZ,
dz~
Pi
_dj
dx
From which it is clear, that those coefficients have the following values
in the notation of the paper which I have published.
ju = ^. = JC + J = reciprocal of the cubic compressibility,
m = 2 C = twice the rigidity ;
consequently
C = ^ m, J = fi — 4 m.
The particular problems solved by Mr. Clerk Maxwell are of a very
interesting character, especially those relative to the optical changes
produced in transparent bodies by straining them.
ON THE LAWS OF ELASTICITY.*
Section VI.— Ox the Application of the Method of Virtual
Velocities to the Theory of Elasticity.
25. Lagrange's method of virtual velocities having been applied to the
problems of the equilibrium and motion of elastic media by Mr. Green
(Camb. Trans., VII.) and by Mr. Haughton (Trans. Royal Irish Acad., XXL,
XXIL), it is my purpose in this and the following section to point out the
mutual correspondence between the coefficients in the formulas arrived at
by these gentlemen, and the coefficients of elasticity which form the sub
ject of the previous portion of this paper, and also to show how far the
laws of relation between the nine coefficients of elasticity of a homogene
ous body, which I originally proved by a method chiefly geometrical, are
capable of being deduced symbolically from equations found according to
Lagrange's method.
26. The principle of virtual velocities, as applied to molecular action, it
* Originally published in the Cambridge and Dublin Mathematical Journal, Nov.,
1S52.
102 LAWS OF ELASTICITY.
as follows • Let X, Y, Z, denote the total accelerativc forces applied to
any particle, whose mass is m, of an elastic medium, through agencies dis
tinct from molecular action (such as the attraction of gravitation); let
u v to be the components of the velocity of m; let dx, By, Bz, denote
indefinitely small virtual variations of x, y, z; let S be the total accelera
tive molecular force applied to m, Bs an indefinitely small virtual variation
of the line along which it acts j then the following equation
»[{(x^.. + (y50«» + (^.}]\ w
+ S(mS8s) = J
(the summation 2 being extended to all the particles of the medium),
expresses at once all the conditions of equilibrium and motion of every
particle of the medium.
27. In applying this principle to the theory of elastic media, both Mr.
Green and Mr. Haughton assume the following postulates :
First. That in calculation Ave may treat each particle m as if it were
a small rectangular space, dx dy dz, filled with matter of a certain
density p : so that for the symbol 2 m we may substitute that of a triple
integration.
fffpdxdydz.
Secondly. That the virtual moment m S B s of the total molecular force
acting on any particle m, is capable of being expressed by the product
of the small rectangular space dx dy dz into the variation, B V, of a
certain function V of the relative position of m and the other particles of
the body*
Equation (16) is thus transformed into the following: —
+ (z  ~^Bz\dxdydz +ff/BYdxdydz =
(17.)
J
28. The term elasticity properly comprehends those molecular forces
only whose variations are produced by, and tend to produce, variations
in the volume and figure of bodies. There are, therefore, conceivable
kinds of molecular force, which are not included in the term elasticity.
For example, let us take the forces which Mr. MacCullagh ascribed to
the particles of the medium which transmits light.
* This amounts, in fact, to the assumption that no part of the power developed by a
variation of the relative positions of the particles is permanently converted into heat,
or any other agency: in other words, that the body is perfectly elastic.
LAWS OF ELASTICITY. 103
Let £, 17, £, denote displacements of a point in the medium parallel
respectively to x, y, z. Then Mr. MacCullagh supposes the molecular
forces to be functions of
dr\ dZ, dZ, d£ d£ dr\
dz dy } d x dz ' dy dx'
which are proportional to the rotations of an element dxdydz from its
position of equilibrium about the three axes respectively. This amounts
to ascribing to the particles of the medium a species of polarity, tending
to place three orthogonal axes in each particle parallel respectively to
the three corresponding axes in each of the other particles : the rotative
force acting between the corresponding axes in each pair of particles
being a function of the projection of the relative angular displacement
of the axes on the plane passing through them, of the position of that
plane, and of the distance between the particles.
A portion of a medium endowed with such molecular forces only
would transmit oscillations; but it would not tend to preserve any
definite bulk or figure, nor would it resist any change of bulk or
figure. It would be a medium or system, but not a body. Molecular
forces of this kind, therefore, are not comprehended under the term
elasticity ; and the limits of the present investigation exclude those forms
of the function V which represent the laws of their action.
29. The inquiry being thus restricted to molecular forces dependent
on the variations of the bulk and figure of bodies, there is to be intro
duced a
Third Poshdate: That supposing the body to be divided mentally
into small parts, which, in the undisturbed state of the body, are
rectangular and of equal size, those parts, in the disturbed state, continue
to be sensibly of equal bulk and similar figure, throughout a distance
round each point at least equal to the greatest extent of appreciable
molecular action.
This assumption has been made in all previous investigations, except
those respecting the dispersion of light; and it seems, indeed, to be
perfectly consistent with the real state of tangible bodies.
Its advantage in calculation is, that it enables us to treat the variations
of the molecular forces acting on a given particle, as functions simply of the
variations of bulk and figure of an originally rectangular element situated
at that particle: seeing that the adjoining elements throughout the
extent of appreciable molecular action continue always to undergo
sensibly the same variations of bulk and figure as the element under
consideration.
Let x , y , z be the coordinates of any physical point in a homogeneous
104 LAWS OF ELASTICITY.
body in equilibrio, and whose particles are not operated upon by any
extraneous forces, X, Y, Z. In this condition it is evident that
SV =
at every point, and that we may also make
V = 0.
In the disturbed condition, let £, ?j, Z,, be the displacements of the
point whose undisturbed position is (x , y , z ), so that
x = o: + £, &c.
Then all the variations of bulk and figure which can be undergone
by an originally rectangular element, consistently with the third postulate,
may be expressed by means of the following six quantities, which I have
elsewhere called strains :
d% _ dt) _ r. ell, _
Tz~ a > Ty pm ' Tz~ 7 '
dj ,dX_ d% &%, _ cZj; djj _ ,.
dz + dy~ ' dx + dz~ fM} dy + dx~ V '
of which a, ft, y, are longitudinal extensions if positive, compressions if
negative, and X, ju, v, are distortions in the planes perpendicular to x, y, z,
respectively.
Hence it appears that
Y = <f>(a,ft, 7> \,fx,v). . . . (18.)
30. The first assumption, that we may treat the body in calculation
as composed of rectangular elements p d x d y d z, involves the consequence
that we may express all the molecular forces which act on each such
element by means of pressures, normal and tangential, exerted on its
six faces. Taking yz, zx, xy, to denote the position of the faces of such
an element, P to denote generally a normal pressure expressed in units
of force per unit of area, and Q a tangential pressure similarly expressed,
let the nine component pressures on unity of area of those faces be
thus denoted :
* This notation is substituted for
Ni, N 2 , N 3 , 2Tn, 2T a 2T 3 ,
as being more convenient.
LAWS OF ELASTICITY. 105
Position of Face.
Direction of Pressure.
X
y
Z
y~ Pi
%
<¥
nx q;
p.
Qi
xy Q 2
Qi'
p 3
By the definition of elasticity all pressures are excluded, except those
whose variations produce and are produced by variations of volume and
figure of the parts of the body. Hence the pressures
QiQ/; Q«Qii Q3Q/;
whose tendency is to make the element dxclydz rotate about its three
axes respectively, without change of form, must be null; and therefore
Qi = Qi'; Q 2 = Q 2 '; Q 3 = Q 3 ' • ■ (19)
Mr. Haughton correctly remarks that this often quoted theorem of
Cauchy is not true for all conceivable media. It is not true, for instance,
for a medium such as that which Mr. MacCullagh assumed to be the
means of transmitting light. It is true, nevertheless, for all molecular
pressures which ' properly fall under the definition of elasticity, if that
term be confined to the forces which preserve the figure and volume of
bodies.
Let us now express the sum of the virtual moments of the molecular
forces acting on the element d x d y d ~, in terms of the pressures P 1? &c. ;
to do which, we must multiply each pressure by the virtual variation of
the effect which it tends to produce in its own direction. Thus we obtain
the following result : —
SV = P 1 Sa + PoS/3 + P 3 £7 + Q 1 gA + Q 2 S y u + Q 3 Si/. . (20.)
Hence the function V bears the following relations to the normal and
tangential pressures at the faces of a rectangular element :
1_ da' 2 ~ d(}' 3_ dy
^~d\> ^df,' ^~Jv)
(21.)
31. A Fourth Postulate, generally assumed in investigations of this kind,
is that the pressures are sensibly proportional simply to the strains with
which they are connected. This assumption must be approximately true
of any law of molecular action, when the pressures and strains are suffi
ciently small. It is known to be sensibly true for almost all bodies, so
106 LAWS OF ELASTICITY.
long as the pressures and strains are not so great as to impair their power
of recovering their original volume and figure.
According to this postulate, the pressures P 1? &c., are algebraic functions
of the first order of the strains a, &c. ; and consequently V is an algebraic
function of the second order of those strains. The constant part of V, as
we have already seen (Art. 29), is null.
Following the notation adopted by Mr. Haughton, let (a) denote the
2
coefficient of a in V, (a 2 ) that of — — , (/3y) that of — (5y, &c. Then
V = (a)a + (/3)/3 + (7)7 + W ^ + (/*)/* + W»
« 2 P2 .,2 \2 ..2 ,,2
 (« 2 ) 2  (0 2 ) f ~ M I  (X 2 ) y " M J  M> •>
— ($y) fiy — (yet) ya — (a/3) a/3 — (fiv) fxv — (vX) v\ — (\/ul) \/jl
— (aX) a\ — (a/u) a/j. — (av) av
(P\)P\(M(5f*((3v)pv
(y\)y\ — (yfi)yfA—(yv)yv. . . (22.)
The six coefficients of the terms of the first order. in this equation
obviously represent the pressures, uniform throughout the whole extent of
the body, to which it is subjected when its particles arc in those positions
from which the displacements are reckoned: that is to say, when
I = 0; r, = Oj Z = 0.
Let P 1>0 &c, denote those pressures. Then
(«) = i\,o; 0)p m j (7) = p,,o
W = Qi,oJ (m)=Q 2 , ; W^Q^J
The twentyone coefficients of the terms of the second order are the
coefficients of elasticity of the body, as referred to the three axes selected.
The negative sign is prefixed to each, because it is essential to the stability
of a body that molecular pressures should be opposite in direction to the
strains producing them.
The transformation of the quantities in equation (22) for any set of
rectangular axes, is effected by means of equation (2) of Sect. I, Art. 9,
by making the following substitutions : —
for PPP20'>O'>O
x 1' "" 2' 3' A H'ij * k'01  V3J
substitute a, (3, y, X, p, v,
and make similar substitutions for the accented symbols. By multiplying
the six equations referred to together by pairs, twentyone equations are
LAWS OF ELASTICITY. 107
obtained, serving to transform the squares and products of a, (5, 7, X, jjl, v.
Formulae similar to those which transform the strains a, &c., and their
halfsquares and products, serve also to transform the respective coefficients
of those quantities in equation (22).*
It is shown by Mr. Haughton, that by properly selecting the axes of
coordinates, the number of independent coefficients of elasticity may
always be reduced to three less than when the axes are indefinite ; and
by Mr. Haughton and Mr. Green, that when the body has orthogonal
axes of elasticity at each point, then, if those axes be taken as the axes
of coordinates, the coefficients of elasticity are reduced to the first nine.
The latter proposition is obvious, because if molecular action be sym
metrical about three orthogonal planes, and those be taken for coordinate
planes, then the value of that part of the function V which is of the
second order cannot be altered by a change in the sign of either of the dis
tortions A, ju, v; so that the coefficients of the last twelve terms of
equation (22) must each be null.
The nine coefficients of elasticity of a body in those circumstances have
the following values, in terms of the notation of the previous sections :
Coefficients of Longitudinal Elasticity.
(a 2 ) = A i; m=A 2 ; ( 7 2 )=A 3
(24.)
Coefficients of Lateral Elasticity.
(/3 7 ) = B i; ( 7 a) = B 2 ; (a/3)=B 3 .
Coefficients of Eigidity.
32. Def. Let the term Perfect Fluid be used to denote the state of a
body, which under a given uniform normal pressure, and at a given temperature,
tends to preserve, and if disturbed to recover, a certain bulk; but offers no
resistance to change of figure.
In such a body, if the element whose original bulk was dxdydz,
becomes of the bulk (1 + 0) dxdydz (o being a small fraction), we shall
have
a = a + /3 + J,
and the function V must be of the form
Y = Y „(o») a ~ . . . (25.)
where P is the uniform normal pressure when the particles are not dis
placed, and (o 2 ) a coefficient of elasticity, whose value, in the notation of
the previous sections, is :
* See the Note at the end of this paper.
108 LAWS 0F ELASTICITY.
Coefficient of Fluid Elasticity.
(«*) = (a 2 ) = (/3 2 ) = ( 7 2 ) = (/3y) = (y«) = («0) = J (26.)
The normal pressure in the disturbed state, which is the same in all
directions, is obviously
The tangential pressures are each = 0.
Section VII.— On the Proof of the Laws of Elasticity by the
Method of Virtual Velocities.
33. Having thus followed very nearly the steps of the researches of
Mr. Haughton and Mr. Green, so as to compare their coefficients with
those used in the previous part of this paper, I shall now investigate how
far the method ■ of Lagrange can be used to establish those relations
between the coefficients of elasticity of different kinds in homogeneous
solid bodies, which I have elsewhere deduced from geometrical and
physical considerations.
The fluid elasticity considered in the last article cannot arise from the
mutual actions of centres of force; for such actions would necessarily
tend to preserve a certain arrangement amongst those centres, and would
therefore resist change of figure. Fluid elasticity must arise either from
the mutual actions of the parts of continuous matter, or from the centri
fugal force of molecular motions, or from both those causes combined.
On the other hand, it is only by the mutual action of centres of force
that resistance to change of figure and molecular arrangement can be
explained, that property being inconceivable of a continuous body. The
elasticity peculiar to solid bodies is, therefore, due to the mutual action on
centres of force. Solid bodies may nevertheless possess, in addition, a
portion of that species of elasticity which belongs to fluids.
The investigation is simplified by considering, in the first place, the
elasticity of a solid body as arising from the mutual action of centres of
force only, and afterwards adding the proper portion of fluid elasticity.
It is known that solid bodies are capable of preserving bulk and figure,
although their surfaces are acted upon by no sensible pressure, normal or
tangential. We may take the positions of the particles in this condition
as points from which to measure their displacements. Thus we cause the
coefficients of all the terms of the first order in equation (22) to vanish.
To investigate the properties of the coefficients of elasticity, the function
SV is to be expressed in a new form, — viz., as the sum of the virtual
moments of the actions exerted upon each of the centres of force in the
LAWS OF ELASTICITY. 10D
particle under consideration, by the centres of force in all the other
particles. Mr. Haughton, in his first memoir, having performed this
process, shows by means of its results, that in a body composed entirely
of centres of force acting along the lines joining them, the number of
independent coefficients of elasticity for any system of orthogonal axes is
reduced to fifteen, which, by properly selecting those axes, may be reduced,
for bodies in general, to twelve, and for those having axes of elasticity,
to six.
I shall now endeavour to prove by the method of virtual velocities,
what I have in the third section proved by other modes of reasoning,
that in a homogeneous body constituted of centres of force only, the
independent coefficients of elasticity are reducible to three, of which, and
of the position of the axes, the twentyone in equation (22) are functions.
A fourth independent coefficient is to be added in solids possessing a
portion of fluid elasticity; that is to say, in all known solids.
34. It is known that a homogeneous solid can exist, with its particles
in an unstrained condition, bounded by plane surfaces in any direction.
In this condition, therefore, the total molecular action upon a particle
situated at any bounding plane must be null. Conceive the bounding
plane still to pass through the same particle, but to have its position
shifted through any angle. The molecular action on the particle will still
be null. Now the effect of the shifting of the bounding plane is to take
away a wedge of matter from one side of the particle, and to substitute an
equal and similarly constituted wedge, lying in a diametrically opposite
direction. Hence, in the unstrained condition of a solid body, the action
exercised upon any particle, by a wedge of matter bounded by any two
planes passing through the particle, is null.
This shows that the action of a wedge of solid matter on a particle
situated at its edge, is not altered by varying the angular position of the
wedge; and consequently, that the molecular actions which produce
elasticity are not directly functions of the relative angular positions of
the centres of force which act on each other, but merely of their distances
apart, so that if the actions of the several equal wedges into which a body
may be conceived to be divided, round a given particle, are different, this
does not arise directly from the angular positions of the wedges, but from
the different distribution of their centres of force as to distance from those
of the particle operated upon.
(I have proved this, in a manner slightly different in form, in a
supplementary paper to Sect. III., Art. 17.) See p. 98.
This further shows, that the mutual action of two centres of force in a
solid must be directed along the line joining them; for otherwise it would
tend to bring that line into some definite angular position, and would be
a function of the direction of the line.
HO LAWS OF ELASTICITY.
It finally results, from what has been stated, that the action of an
indefinitely slender pyramid of a solid body upon a particle at its apex
must be a direct attraction or repulsion along the axis of the pyramid,
which is a function of the several distances of the centres of force in the
pyramid from those in the particle at the apex, and which, in the
unstrained condition of the body, must be null.
The principles stated above have to a greater or less extent been taken
for granted in previous investigations, but have not hitherto been demon
strated. They may all be regarded as the necessary consequences of the
following : —
Def. Let the term Elastic Solid be used to denote the condition of a body,
which, when acted upon by any given system of pressures, or by none, and at a
given temperature, tends to preserve, and if disturbed, to recover, a definite bulk
and figure; and such that, if lohile in an unstrained condition it be cut into
parts of any figure, those parts, when separate, will tend to preserve the same
bulk and figure as they did when they formed one body.
Experience informs us that bodies sensibly agreeing with this definition
exist; its consequences are, therefore, applicable to them in practice.
35. Let r denote the distance apart of two centres of force in an
unstrained solid, and let <pr be proportional to their mutual action.
Then
dxdydzd 2 a . 2i<pr —
may be taken to represent the total action of an indefinitely slender
pyramid which subtends the element of angular space d 2 <o upon a particle
at its apex d x d y d z.
In consequence of a strain, let each of the distances r become
(1 +*)r,
£ being a very small fraction. Then the total action of the pyramid
becomes
d x d y d z d 2 o> . 2 (<p r + tr<p'r) = dx dy dzd 2 w . e 2 r <p' r;
for by the third postulate, e is uniform throughout the extent of appreci
able molecular action.
The quantity which the force acting between two centres of force tends
to vary, is their relative displacement along the line joining them, or e r.
Hence the sum of the virtual moments of the actions of all the slender
pyramids into which the solid is conceived to be divided, that is to
say, the total virtual moment of its molecular action upon the particle
dx dy dz, is
SV dxdy dz = dx dy dzffeSt.S (r 2 <f,' r)~.d 2 u,
LAWS OF ELASTICITY.
Ill
the double integration extending to all angular space. Consequently, we
obtain as a new value of the function V,
V = ff\ £ 2 2 (r 2 <f>' r) .cPw. . . (28.)
Let a, b, c be the directioncosines of the axis of a given slender pyramid.
Then it is easily seen that the strain e along that axis has the following value,
in terms of the six strains as referred to the axes of coordinates :
e = a a 2 + /3 b 2 + 7 c 2 + X b c + fx c a + v a b,
and, consequently, that
9 9 /")f>
£ o 4. , P Tj ,
C 4 + y% 2 C 2 + £*rf + .£rfP
+ /3 7 6 2 c 2 + 7 a c 2 ft 2 + a j3 ft 2 & 2
+ /tv(i 2 J(;+ v\«i 2 c \ jul a be 2
+ a \ or be + a fj. a s c + av a 3 b
+ fi\b 3 c + /3m«^ 2 c+ /3va& 3
+ 7 X k 3 + 7 /a a c 3 + 7 v a 5 c 2 .
If this value of 1 1 2 be substituted in equation (28), and the result
compared with equation (22), it is at once obvious that the twentyone co
efficients of elasticity have the following values (putting S (r 2 <p' r) = — R,
which is negative that equilibrium may be stable) :
(a 2 )=//VRd 2 W
(/3 2 )=//S*E^< u
(7 2 )=yyVRrZ 2 a,
(\ 2 )^(/3 7 ) =//5 2 c 2 Rrf 2 (u
(/M 2 ) = (7 a) =ffc 2 a 2 TLd 2 (o
(„*)= ( a j3) = //a 2 b 2 Ud 2 (o
(fjiv) = (oX) =ffa 2 bc~Rd 2 u>
(vA) = (pV) =//aS 2 C R f Z 2 w
(X/*)= (tv) = ffabc i B,d 2 <a
(0A) =ffV i cnd*to
(yX) =//ic 3 EfZ 2 fc >
iir).= //***&»
(a fx) = f/c a 3 ~Rd 2 to
(av) =ffa s bRd 2 to
C/3v) = //*a& 3 Rtf 2 w
112 LAWS OF ELASTICITY.
In the above equations, which agree with those given by Mr. Haughton,
the number of independent coefficients is fifteen.
36. Their reduction to a smaller number arises from the nature of the
function
E =  2 (r 2 r/>' r).
This quantity is a function of the distances of the centres of force in a
given indefinitely slender pyramid from those in a particle at its apex, and
can vary with the directioncosines a, b, c of the axis of the pyramid, solely
because those distances vary with them. Now in a homogeneous solid,
that is, one composed of a succession of similar and regularly placed
groups of centres of force, those distances depend upon a quantity which
may be called the mean interval between the centres of force jn a given
direction : a quantity of such a nature that the product of its three values
for any three orthogonal directions is a constant quantity; being the space
occupied by a centre of force, or by a definite group of such centres. To
have this property, the mean interval must be a quantity of this form:
• __ ,f+ga" + hh' 2 + Jcc + lbc+mca + nab. (30)
that is to say, its logarithm must be proportional to the reciprocal of the
square of the radius of an ellipsoid, whose axes are those of molecular
arrangement, and therefore of molecular action, and of elasticity.
Let the axes of this ellipsoid be taken as axes of coordinates. Then
I = 0, m = 0, n = 0; and the above equation is reduced to
i = cf+gat + hv + ic^ m t m (3QA.)
and because the quantity
R = F (0 =^(f+rj a? + h & + k c 2 ), . (31.)
does not change its sign or value by any change of the signs of the cosines
a, I), c, it follows that all the coefficients in (29) containing odd powers of
those cosines, that is to say, all except the first six, disappear when the
axes of molecular arrangement are taken for axes of coordinates.
These six, for all known homogeneous substances, are reducible to three,
by the following reasoning :
Let us assume as a Fifth Postulate, what experience shows to be sensibly
true of all known homogeneous substances — viz., that their elasticity varies
very little in different directions. Those substances, such as timber, whose
elasticity in different directions varies much, are not homogeneous, but
composed of fibres, layers, and tubes of different substances.
If this be assumed, it follows that, in the expression (31), for the
quantity E, the variable terms
g a 2 + h 6 2 + k r,
LAWS OF ELASTICITY.
113
are very small compared with the constant term /, and that E may be
developed in the form
R = f (/)+ f. (/). (ga* + h& + Ice") + &c.
If this value of R be introduced into equation (29), and if small quantities
of the second order be neglected, it is easily seen, on performing the inte
grations, that the following relations exist amongst the six coefficients
already specified:
(/3 2 ) + ( 7 2 ) = 6(\2) = 6(/3 7 )
( 7 2) + («2) = 6 ( M 2) = 8 ( r «)
(a 2 )+(/3 2 )=6(v 2 ) = 6(«/3)
or, by transformation, S~ . (32.)
(a 2 ) = 3 {(& + (v 2 )  (X 2 )}
(/3 2 ) = 3 {(v 2 ) + (X 2 )  0. 2 )}
(r) = 3 {(x^) + (tf)  (v 2 )}
These equations reduce the number of independent coefficients of elas
ticity, arising from the actions of centres of force, to three. They are
identical with the equations (7) and (8), embodied in the fifth theorem in
Sect. III., although arrived at by a different process.
37. Let us suppose the solid under consideration to possess a portion
of fluid elasticity, represented by the coefficient J. Then the coefficients
of elasticity have evidently the following relations :
(a 2 ) = 3 (Ox 2 ) + („ 2 )  (X 2 )} + J 1
(/3 2 )=3{(v 2 )+(X 2 )( M 2 )} + J
(7 2 ) = 3{(X 2 )+a t 2 )(v 2 )} + J
(/3y) = (A 2 )+J
( 7 «)=(,x 2 ) + J
(a/3) = (v 2 ) + J
which are identical with the six equations (9) comprehended under the
sixth theorem, in Sect. IV.
38. The laws of elasticity stated in this paper are the necessary con
sequences of the definitions of elasticity and of fluid and solid bodies,
given in Arts. 28, 32, and 34, respectively, when taken in conjunction
with five postulates or assumptions, which, however, may be summed up
in two — viz.
H
y
(33.)
114 LAWS OF ELASTICITY.
First, That the variations of molecular force concerned in producing
elasticity are sufficiently small to be represented by functions of the first
order of the quantities on which they depend ; and,
Secondly, That the integral calculus and the calculus of variations are
applicable to the theory of molecular action. It is thus apparent that
the science of elasticity is, to a great extent, one of deduction a priori.
The functions of perceptive experience in connection with it are two
fold : first, by observation, to inform us of the existence of substances,
agreeing to a greater or less degree of approximation with the definitions
and postulates; and, secondly, by experiment, to ascertain the numerical
value of the coefficients of elasticity of each substance.
Note to Sections VI. and VII. of Preceding Paper.
On the Transformation of Coefficients of Elasticity, hy the aid of a Surface
of the Fourth Order.
(The following note contains no original principle, and is designed
merely to put on record, for the sake of convenient reference, a series of
equations which will be found useful in future investigations.)
It has been pointed out by Mr. Haughton, in his first paper, that if
we take into consideration that part only of the elasticity of a solid
which arises from the mutual actions of centres of force, so that the
function V shall contain at most but fifteen unequal coefficients (viz.,
those whose values are given in equation 29), and if, with those fifteen
coefficients, we construct a surface of the fourth order, whose equation is
the following,
U= (« 2 K + (/3 2 )^+( 7 2 )^
+ 6 (X 2 ) f$ + 6 Ou 2 ) z^ + G (v 2 ) x}if
+ 12 («\) x^yz + 12 (j3 M ) xfz + 12 (yv) xyz*
+ 4 (a i<) :c" y + 4 (a n) x z z
+ i(fi\)fz+ 4:(i3v)y*z
+ 4:(yn)£x+4:(y\)z*y
= l ■ • • •■ (A.)
then will U be the same function of the six quantities
a?, y 2 , z\ 2yz, 2zx, 2xy,
LAWS OF ELASTICITY.
115
that — 2 V is of the six strains
«> /3, 7, A, ix, v,
which are known to be transformed by the same equations "with the
above functions of the second order of x, y, g; and consequently, the
same equations which serve to transform the coefficients of the surface
U = 1, into those suitable for a new set of rectangular coordinates, will
also serve to transform the coefficients of elasticity in the function V.
Now, it is obvious that if the equation
^ (», y, z) = xp (x, y, z)
be true for two sets of rectangular coordinates having the same origin,
then must the equation
<1>
f d d d \
\dx' dv' Tz)
i
? )
(B.)
d d d
dx' dy' dz J ~~ r \ d aj" d y" d
be true also.
It follows that the fifteen coefficients of elasticity (a 2 ), &c, which are
proportional to the differential coefficients of U of the fourth order with
respect to x, y, z, are transformable hj means of the same equations which
serve to transform the fifteen algebraical functions of the fourth order of
x, y, z, by which they are respectively multiplied in the value of U.
The following is the investigation of those fifteen equations of the
fourth order, as well as of the six equations of the second order, from
which they are formed by multiplication.
Let the relative directioncosines of the two sets of rectangular axes be
expressed as follows:
1
Original
Axes.
N
ew Axes.
V
z
X
a i
h
c l
V
tt.y
h
c 2
z
a z
h
c '3
\ cosines.
J
Let the following notation be used for functions of those cosines. (It
is the same which is employed by Mr. Haughton.)
q 1 = c l a 1 ;
p 2 — b 2 c 2 ;
q 2 = c 2 a 2 ;
r„ — a„ k 2 ;
% — H a z>
r* = a, \\
116 LAWS OF ELASTICITY.
l 1 = b 2 c 3 + b 3 c 2 ; l 2 = b 3 c x + \ c 3 ; l s — \ c 2 + \ c x ;
m 1 — c 2 a 3 + c z a 2 ; rn 2 = c z a x + c x a z ; m 3 = c x a 2 + c 2 a x ;
n x = a 2 b 3 + a 3 \; n 2 = a 3 b x + a x b 3 ; n z = a x b 2 + a 2 b x ;
then the following are the six equations of transformation of the second
order for the surface U = 1,
x 2 = x 2 a 2 + y 2 a 2 2 + z 2 a z 2 + 2y z a 2 a z + 2 z x a z a x + 2 x y a x a 2 ,
(for y' 2 , z' 2 , similar equations in b, c, respectively).
y'z' = x 2 p x + fv 2 + z 2 p 3 + yzl x + zxl 2 + xyl 3 ,
(for z x, a similar equation in q and m),
(for x y, a similar equation in r and n), . . . (C.)
Those equations are made applicable to the transformation of strains by
the following substitutions :
for x 2 , y 2 , z 2 , 2yz, 2zx, 2xy,
substitute a, (5, j, A, p, v;
and to that of pressures, by the following :
for o: 2 , f, z 2 , yz, zx, xy,
substitute P 1? P 2 , P 3 , Q v Q 2 , Q 3 ,
and similar substitutions for the accented symbols.
The following are the fifteen equations of transformation of the fourth
order:
a/ 4 = re* a x * + y* a 2 * + z * a 3 * + 0>y 2 z 2 « 2 2 a 3 2 + Qz 2 x 2 a 2 a 2 + 6 x 2 y 2 a x 2 a*
+ 12 x 2 y z a x 2 a 2 a 3 + 12 x y 2 z a x a 2 2 a 3 + 12 xy z 2 a x a 2 a z 2
+ 4 x 3 y a x 3 a 2 + 4 x 3 z a x 3 a 3 + 4 y 3 z a 2 3 a z + Ay 3 x a 2 3 a x + 4 c 3 x a z 3 a l
+ 4 s 3 y a z 3 a 2 ;
(for y ri , z ri , similar equations in b, c, respectively).
y' 2 z' 2 = a*p* + y'Pf + s> 3 2 + y 2 # (h 2 + 2p 2 p z ) + z 2 x 2 {l 2 + 2p sPx )
+ * 2 y 2 (h 2 + 2 PiPi) + 2«?yz{lhh + hh) + %xy 2 z{2hk + h l d
+ 2 xyz 2 (p z l 3 + l x l 2 ) + 2x 3 yp x l z + 2 x 3 zp x L
+ 2y 3 zp 2 l x + 2y 3 xp 2 l 3 + 2z 3 xp z l 2 + 2z 3 yp z ( x ;
(for z' 2 x' 2 , a similar equation in q and m) ;
(for x' 2 y' 2 , a similar equation in r and n) ;
LAWS OF ELASTICITY. 117
x 1 y z' = x* a 2 p x + y* a 2 p 2 + s 4 a 3 2 p 3
+ z 2 x 2 {a 3 2 Pi + a \Vz + 2 a 3 a x ? 2 )
+ x 2 y 2 {a 2 p 2 + a 2 p x + 2a t a 2 l 3 )
+ x 2 y z (a x 2 l x + 2 a 2 a 3 p t + 2 a s a x l 3 + 2 a x a. 2 I.)
+ xy 2 z (a.? 1. 2 + 2a 3 a x p 2 + 2 a x a. 2 l L + 2 a 2 a 3 l 3 )
+ xy z 2 (a 3 2 l 3 + 2 a x a. 2 p 3 + 2 a 2 a 3 l 2 + 2 a 3 a x l x )
+ x 3 y(a l 2 l 3 4 2a 1 a 2 p l ) + x 3 z(a 1 2 l 2 + 2a 1 a 3 p 1 )
4 2/ 3 z (a. 2 2 l x + 2 a 2 a 3 p 2 ) + y 3 x (a. 2 2 l 3 + 2 a 2 a x a. 2 )
+ z 3 x (a 2 l 2 + 2a 3 a x p 3 ) + & y (a. 2 l x + 2 a 3 a. 2 p s ) ;
(for x y' 2 z', a similar equation in b, q, m);
(for x y' z' 2 , a similar equation in c, r, n) ;
Ay'  x^a^r x + y i a 2 2 r 2 + z i a 3 2 r 3
+ 3 y 2 z 2 a 2 a 3 ?i x
+ 3 z 2 x 2 a 3 a x n
+ 3 x 2 y 2 a x a 2 n 3
+ 3 x 2 y z (a x 2 n x + 2 a. 2 a 3 r x )
+ 3xy 2 z (a 2 n 2 + 2 a 3 a x r 2 )
+ 3 x y z 2 (a 3 2 n 3 + 2 a t a 2 r 3 )
+ x 3 y (a x 2 n 3 + 2 a x a. 2 r x ) + x 3 z (a, 2 n 2 + 2 a 3 a x r y )
+ y 3 z (a 2 2 n x + 2a 2 a 3 r 2 ) + y 3 x (cc 2 2 n 3 + 2a x a 2 r 2 )
+ z 3 x (a 3 2 n 2 + 2 a 3 a x r 3 ) + z 3 y (« 3 2 n x + 2a 2 a 3 r 3 ) ;
(for x' 3 z', a similar equation in a, q, and m) ;
(for y' 3 z', a similar equation in b, p, and I) ;
(for y' 3 x', a similar equation in b, r, and n);
(for z' 3 x', a similar equation in c, q, and m) ;
(for z' 3 y', a similar equation in c, p, and I); . (D.)
The above equations are made applicable to the transformation of the
■coefficients of elasticity arising from the mutual actions of centres of force
only, by the following substitutions:
for % A , f, z 4 , y 2 z 2 , z 2 x\ x 2 y 2 ,
substitute (a 2 ), (/3 2 ), ( 7 2 ), (j3 7 ) = (X 2 ), (y a) = (ft), (a/3) = (v 2 );
for x 2 yz, xy 2 z, xyz 2 ,
substitute (a X) = (// v), (/3 jui.) = (v X), (7 v) = (\fi);
118 LAWS OF ELASTICITY.
for a 3 y, x 3 z, y 3 z, f x, z 3 x, z 3 y,
substitute (a v), (aim), (/3 A), (/3 v), (y ft), (7 A);
and similar substitutions for the accented symbols.
Should the substance under consideration be endowed with a portion
of fluid elasticity in addition to that which arises from the mutual action
of centres of force, the coefficient of that fluid elasticity J must be sub
tracted from the coefficients into which it enters, viz. —
(a), (/3 2 ), (r), (/3 7) = (X 2 ) + J > (7 «) = 0" e ) + J > (°/ 3 ) = O 2 ) + J >
before effecting the transformation.
The results of the transformation for those six coefficients, being in
creased by the same, quantity J which was previously subtracted, will give
their entire values for the new axes.
If the original axes of coordinates are those of elasticity, each of the
fifteen equations of transformation is reduced to its first six terms, in
which the following substitutions are to be made for the unaccented
symbols :
for x*, y 4 , z 4 , fz\ z 2 x 2 , x 2 f y
substitute Aj J, A 2 J, A 3 J, BjJ^C^ B 2 J=C 2 , B 3 J = C 3 .
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 119
VI.—ON AXES OF ELASTICITY AND CRYSTALLINE FORMS.*
Section 1. — General Definition of Axes of Elasticity.
As originally understood, the term " axes of elasticity " was applied to
the intersections of three orthogonal planes at a given point of an elastic
medium, with respect to each of which planes the molecular actions causing
elasticity were conceived to be symmetrical.
If the elasticity of solids arose either wholly from the mutual attractions
and repulsions of centres of force, such attractions and repulsions being
functions of the mutual distances of those centres, or partly from such
mutual actions and partly from an elasticity like that of a fluid, resisting
change of volume only, it is easy to prove that there would be three such
orthogonal planes of symmetry of molecular action in every homogeneous
solid.
But there is now no doubt that the elastic forces in solid bodies are not
such as can be analysed into fluid elasticity and mutual attractions between
centres simply; and though there are, as will presently be shown,
orthogonal planes of symmetry for certain kinds of elastic forces, those
planes are not necessarily the same for all kinds of elastic forces in a given
solid.
The term " axes of elasticity," therefore, may now be taken in a more
extended sense, to signify all directions with respect to which certain hinds of
clastic forces are symmetrical ; or speaking algebraically, directions for which
certain functions of the coefficients of elasticity are null or infinite.
The theory of axes and coefficients of elasticity is specially connected
with that branch of the calculus of forms which relates to linear trans
formations, and which has recently been so greatly advanced by
the researches of Mr. Sylvester, Mr. Cayley, and Mr. Boole. In such
applications of that calculus as occur in this paper, the nomenclature of
Mr. Sylvester is followed ; f and by the adoption of the " Umbral
* Read before the Royal Society of London, on June 21, 1855.
t See Cambridge and Dublin Mathematical Journal, Vol. VII. ; and Philosophical
Transactions, 1853.
120 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Notation" of that author immense advantages are gained in conciseness
and simplicity. *
Section 2. — Strains, Stresses, Potential Energy, and Coefficients
of Elasticity.
In this paper, the word " Strain " will be used to denote the change
of volume and figure constituting the deviation of a molecule of a solid
from that condition which it preserves when free from the action of
external forces ; and the word " Stress " will be used to denote the force,
or combination of forces, which such a molecule exerts in tending to
recover its free condition, and which, for a state of equilibrium, is equal
and opposite to the combination of external forces applied to it.
In framing a nomenclature for quantities connected with the theory
of elasticity, OXtytQ i s adopted to denote strain, and raaig to denote stress.
It is well known that the condition of strain at any given point in the
interior of a molecule may be completely expressed by means of the
following six elementary strains, in which £, )j, Z, are the components of the
molecular displacement parallel to three rectangular axes, x, y, :.
PI ,. dl£ dr\ r. dZ,
Elongations, . — = a; j= ft; ^ = y;
dj =
dx
a;
dr\
dy~
ft;
dZ>
dz
dy^
dz
= A;
dl
dz
+ T
dx
Distortions, . ~ + j^ = A; "p  ^ = ft; p + — = v .
dy dz dz dx dx ay
It is also well known that the condition of stress at a given point may
be completely expressed, relatively to the three rectangular coordinate
planes, by means of six elementary stresses, viz. —
Normal Pressures, . . P 1? P 2 , P 3 ,
Tangential Pressures, . . Q lt Q 2 , Q 3 ;
these quantities being estimated in units of force per unit of surface.
Let each elementary stress be integrated with respect to the elementary
strain which it tends directly to diminish, from the actual amount of that
strain, to the condition of freedom; the sum of the integrals is the
potential energy of elasticity of the molecule dxdydz, expressed in units
of work per unit of volume ; viz. —
+ j > Q 1 d\ + jq. 2 d^+ j°Q 3 dv. . . (1.)
* See the Note at the end of the paper.
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 121
The condition that the function U shall have the same value, in what
order soever the variations of the different elementary strains take place,
amounts to supposing that no transformation of energy of the kind well
distinguished by Professor Thomson as frictlonal or irreversible, takes place
during such variations ; in other words, that the substance is perfectly
elastic.
Each of the elementary stresses being sensibly a linear function of the
six elementary strains, the potential energy of elasticity is, as Mr. Green
first showed, a function of those strains of the second degree, having
twentyone constant coefficients, which are the coefficients of elasticity of
the body, and will in this paper be called the Tasinomic Coefficients;
that is to say, adopting Mr. Green's notation for such coefficients, —
' U = („ 2 ) 2 + (/3 2 )f + ( 7 2 ) 2 + (X 2 )^ + (m 2 ) 3 + if)t
+ (/37)/3y+( 7 «)7« + («£W3
+ (fX v) /H V + (v X) V X + (XjUi) XyU
+ (a \)a X + (/3/x) /3/x + (7 v) y v
+ (/3X) /3X + ( yf x) yn + (a v) a v
+ (y\)y\+(a[i)an+(Pv)Pv . . (2.)
From a theorem of Mr. Sylvester it follows, that every such function
as U is reducible by linear transformations to the sum of six positive
squares, each multiplied by a coefficient. The nature and meaning of this
reduction have been discussed by Professor William Thomson.
The following classification of the tasinomic coefficients will be used
in the sequel : —
Designation of Coefficients. Elasticities. Symbols.
C Euthytatic, Direct or Longitudinal, (a 2 ) (8 2 ) (7 2 )
Orthotatic l Platytatic, Lateral, . . (/3 7) (7 a) (a /3)
I Goniotatic, Rigidities, . . . (X 2 ) (;u 2 ) (v 2 )
Plagiotatic, . . . Unsymmetrical, . . (jut v), &c, &c.
The twentyone equations of transformation by which the values of
these coefficients, being known for any one set of orthogonal axes, are
found for any other, are founded on the following principles.
It is well known, that for rectangular transformations, the operations
d d d
dz' dy' dz
122 AXES OF ELASTICITY AND CRYSTALLINE FORMS,
are respectively covariant with
•'•, y, ■">
from which it is easily deduced, that because the displacements
are respectively covariant with
&, y, z,
therefore the elementary strains,
a, /3, y, A, fx, v
the operations,
d d d d d d^
da' dp' d^' IX' ~d~ii' dv
and the strains
P P P ^ O ° 2
■*• 1' 2' 3' " 'I' ~ v *2> »o'
must be respectively covariant with the squares and products,
/, f, #, 2yz, 2zx, 2xy.
Section 3. — Thlipsimetric and Tasimetric Surfaces and
Invariants.
Isotropic functions of the elementary strains and stresses, which may
be called respectively Thlipsimetric and Tasimetric Invariants, are easily
deduced from the principle, that the strains may be represented by the
coefficients of the following Thlipsimetric Surface,
as 5 + fitf + 7^ + \yz + ftzx + vxy=l, . (3.)
and the stresses by the coefficients of the Tasimetric Surface,
T> l3 ? + J> 2 y* + Y 3 z* + 2Cl 1 yz+2Q 2 zx+2%xy=l. (4.)
These surfaces, and others deduced from them, have been fully discussed
by M. Cauchy and M. Lame.
The invariants in question may all be deduced from the following pair
of contrasredient matrices : —
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
For Stresses.
Pi Q 3 Q2
123
For Strains.
V JUL
(5.) <! ; fi . 
yU. X
9, 9 *y
Q 3 P 2 Qj ^ (5 a.)
Q* Qi p 3
J
The following are the primitive thlipsimetric invariants, from which an
indefinite number of others may be deduced by involution, multiplication,
addition, and subtraction : —
a + /3 + 7 = 9 1 (the cubic dilatation) ;
/3 7 + 7 « + a/3i(A 2 + /x 2 + i/ 2 ) = 2 ;
(6.)
The potential energy U is what Mr. Sylvester calls a " Universal
Mixed Concomitant," its value being
U =   (P l0 + P 2 /3 + P 37 + Q X A + Q,fx + Q 3 v). (7.)
Section 4. — Tasinomic Functions, Surfaces, and Umbrae.
If, in any isotropic function of the coordinates and the elementary
strains, there be substituted for each square or product of elementary strains
that tasinomic coefficient which is covariant with it, the result will be
an isotropic function of the coordinates and tasinomic coefficients, called
a Tasinomic Function.
The following Table of Covariants is readily deduced from the prin
ciples stated at the end of Sect. 2 : —
r Squares of ) „
!o J Strains, } a ' P~> ?' A '
Co
variant 1 Tasinomic "i
Coefficients J
'(« 2 )> m, ir), 4 (A 2 ), 4( M 2 ), 4(v 2 );
Co
variant
Products ")
J
Tasinomic ")
y£y, 7 a, a/3, M v, vA, A//,
(/3 y), ( 7 a), (a /3), 4 Gu v), 4 („ A), 4 (A M ),
MS)
. Coefficients j
a A, a /a, a v, /3 A, /3 /*, /3 v, 7 A, 7 /z, y v,
2(aA),2(a / x),2(av),2(/3A),2(/3 A ; 2(/3v),2( 7 A),2( 7it< ),2( 7 r).J
124 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Each tasinomic function being equated to a constant, forms the
equation of a Tasinomic Surface; and on the geometrical properties of
such surfaces depend many of the laws of coefficients and axes of
elasticity.
A convenient and expeditious mode of forming tasinomic functions
is obtained by the aid of an Umbral Notation analogous to that intro
duced by Mr. Sylvester in the calculus of forms.
Let each tasinomic coefficient be regarded as compounded of two
Tasinomic Umbrae, those umbra? being expressed by the following notation :
(a), ((3), (y), (X), W, (v);
then the following equation, deduced from that of the thlipsimetric
surface (3), by substituting uinbraj for elementary strains according
to the following Table of Covariance,
Strains, ... a, /3, y, X, f.t, v,
Umbrae, . . . (a), (/?), ( 7 ), 2 (X;, 2 fa), 2 („),
is the equation of the Tasinomic Umbral Ellipsoid, from which, by elimi
nation, multiplication, involution, addition, subtraction, and differentia
tion, various tasinomic functions may be deduced,
(a) a 2 + (/3) f + (y) ,: 2 + 2 (X) yz + 2 fa) zx + 2 ( v ) xy = (0) = 1. (8a.)
Section 5. — Tasinomic Invariants and Spheres.
Tasinomic invariants are constant isotropic functions of the tasinomic
coefficients, which are deduced, either by substitution from thlipsimetric
invariants, or directly from the Umbral Matrix,
(«) W fa) \
(v) 08) (X) I . (9.)
G») (X) (y) )
The following invariant is umbral of the first order : —
(& + i? + £) • W = W + W> + M = W> (9«0
Invariants of the second order in umbra; are real quantities of the
first order, viz. —
(a 2 ) + (/3 2 ) + (y 2 ) + 2 (/3 y) + 2 (y a) + 2 (a /3) = (6J 2 (the Ctt&ic <««%)
(/3y)+(ya) + (a/3) (X 2 )  ( M 2 )  (* 2 ) =(0 2 )
(a 2 ) + (/3 2 ) + (y 2 ) +2(X 2 ) + 2 fa 2 ) +2(v 2 ) =(0 1 ) 2 ~2(0 2 ). . (10).
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 125
The equation of a Tasinomic Sphere is formed by multiplying a tasinomic
invariant by
a? + f + *\
or any power of that quantity, and equating the result to a constant.
Section 6. — Of Two Tasinomic Ellipsoids, and their Axes,
Orthotatic and Heterotatic.
The equations of two ellipsoids with tasinomic coefficients are derived
from that of the umbral ellipsoid (8a.), in one case by multiplying
each term by the umbral invariant (0), and in the other by substituting
for each umbra in the function (fa), the contravariant component of the
Inverse to the umbral matrix (9). The results are as follows : —
Orthotatic Ellipsoid.
(e i )x(^ = {(a 2 ) + (a^) + (y«)}.r 2 +{(^) + (^) + (y«)}!/ 2
+ {(y 2 )+(y«)+(£y)K
+ 2{(aX) + (/5X) + (yX)}^~+2{(a M ) + (/3 / >c) + (y / ,)}.r C
+ 2{(«v) + (/3v) + (yv)}^=l. . . (11).
Heterotatic Ellipsoid.
{(^)(A 2 )!r+{(ya)W}!/ 2 +[M)(v 2 )}^
+ 2{^v)(a\y j yz+2{(v\)^^}zx+2{(\^(yv)}xy=l. (12.)
The three Orthotatic Axes are three rectangular directions for which
the following sums of plagiotatic coefficients are null : —
(aA) + (/3X) + (yX) = 0; (apt) + (/3 *) + (y /») = 0;
(a v) + (/3 v) + (y v) = 0.
} (13.)
It was proved by Mr. Haughton, in a paper published in the Trans
actions of the Royal Irish Academy, Vol. III., Part 2, that there are three
rectangular directions having this property in a solid whose elasticity
arises solely from the mutual actions of physical points, and which has
but fifteen independent coefficients of elasticity. The present investi
gation shows that there are three such axes at each point of every solid,
126 AXES OF ELASTICITY AXD CRYSTALLINE FORMS.
independently of call hypothesis. The physical meaning of this result
is expressed by the following
Theorem as to Orthotatic Axes.
At each point of an clastic solid there is one position in which a cubical
molecule may be cut out, such that a uniform dilatation or condensation of
that molecule by equal elongations or equal compressions of its three dimensions,
shall produce no tangential stress on the faces of the molecule.
The properties of the Hetcrotatic Axes are expressed by the following
equations : —
(/*v)(aX) = 0; (i/X)(/3ju) = 0; (X^)(yv) = 0; (14.)
or by the following
Theorem as to Heterotatic Axes.
At each point of an elastic solid there is one fiosition in which a cubical
molecule may be cut out, such that if there he a distortion of that molecule
round x (x being any one of its three axes), and an equal distoiiion round y
(y being either of its other two axes), the normal stress on the faces normal
to x arising from the distortion round x shall be equal to the tangential stress
round z arising from the distortion round y.
The six coefficients of the heterotatic ellipsoid may be called the
Heterotatic Differences. For a solid whose elasticity is wholly due to the
mutual attractions and repulsions of physical points, each of those
differences is necessarily null ; therefore, they represent a part of the
elasticity which is necessarily irreducible to such attractions and repul
sions. There is reason to believe that part at least of the elasticity of
every substance is of this kind.
If this part of the elasticity of a solid be, as suggested in a series of
papers in the Cambridge and Dublin Mathematical Journal, for 185152,
a species of fluid elasticity, resisting change of volume only, the solid may
be said to be heterotatically isotropic. The equations (14) will be fulfilled
for all directions of axes, and also the following equations : —
(/3y)(X 2 ) = (ya)( M 2 ) = (a / 8)(v 2 )j . . (15.)
that is to say, the excess of the platytatic above the goniotatic coefficient
will be the same in every plane.
In a substance orthotatically isotropic, the equations (13) are fulfilled
for all directions, and also the following : —
(a 2 ) + (a/3) + (y a) = (/3 2 ) + (/3y) + (a/3) = (y 2 ) + (y a) + (/3y), (1 G.)
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 127
that is to say, a uniform compression in all directions produces a uniform
normal stress in all directions, and no tangential stress.
The equations (16) may be reduced to the following form : —
(a 2 )  (/3 y) = (/3 2 )  (y «) = (y 2 )  (a /3). . (17.)
In a substance which is at once orthotatkaUy and hetcrotatically isotropic,
there may still be eleven independent quantities amongst the tasinomic
coefficients, viz. —
(18.)
Three Euthytatic Coefficients, (a 2 ), (/3 2 ), (y 2 ),
The Isotropic excess, . . (a 2 ) — (/3 y),
The Isotropic excess, . . (/? y) — (X 2 ),
Six Plagiotatic Coefficients, (/3X), (yX), (y/x), (a/ui), (av), (/3v), j
Such a substance may, therefore, be far from being completely isotropic
with respect to elasticity.
Section 7. — Biquadratic Tasinomic Surface; Homotatic
Coefficients; Euthytatic Axes Defined.
If the equation (8a.) of the umbral ellipsoid be squared, there is
obtained the following equation of a Biquadratic Tasinomic Surface.
(#=(« 2 K + (/3 2 )^ + (y 2 ).: !
+ 2{(/3y) + 2(X 2 )}^ 2 + 2{(ya) + 2(^ 2 )}, 2 ^ + 2{(«/3) + 2(v 2 )}^ 2
+ 4{2( / xv) + {a\))xhjz + 4{2(„A) + (fti)}atf» + 4{2(X M ) + (yv)}x^
+ l(PX)fz + 4( y X)//: 3 + 4( 7M )r,,: + 4( a/ /)^ 3 + i(av):>hj + 4((3v)xf =1 (19)
The fifteen coefficients of this surface (which will be called the Homo
tatic Coefficients) are covariant respectively with the fifteen biquadratic
powers and products of the coordinates, with proper numerical factors.
It is obvious, that when the fifteen homotatic coefficients, and the
six heterotatic differences, are known for any set of orthogonal axes,
the twentyone tasinomic coefficients are completely determined.
Mr. Haughton, in the paper previously referred to, discovered the
biquadratic surface for a solid constituted of centres of force. It is here
shown to exist for all solids, independently of hypotheses.
Those diameters of the biquadratic surface which are normal to that
surface, are axes of maximum and minimum direct elasticity, and have also
this property, that a direct elongation along one of them produces, on a
plane perpendicular to it, a normal stress, and no tangential stress; so
that they may be called Euthytatic Axes. Though such axes sometimes
128 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
form orthogonal systems, their complete investigation requires the use
of oblique coordinates, and is therefore deferred till after the eighteenth
section of this paper, which relates to such coordinates.
Section 8. — Orthogonal Axes of the Biquadratic Surface.
Metatatic Axes, Orthogonal and Diagonal.
By rectangular linear transformations, it is always possible to make
three of the terms Avith odd exponents, or three functions of such terms,
vanish from the equation of the biquadratic surface. Thus are ascertained
sets of orthogonal axes having special properties.
To exemplify this, let the rectangular transformation be such as to make
the following functions vanish : —
(G8X)(yX)}(^tf>; {(yriMK*^«i {(«v)(Pv)}(?f)xy.
A cubical molecule having its faces normal to the axes fulfilling this
condition has the following property : — if there be a linear elongation along
y, and an equal linear compression along z (or vice versti), no tangential stress
ivill result round x on planes normal to y and z • and similarly of other
pairs of axes.
This set of axes may be called the Orthogonal or Principal Metatatic
Axes, and their planes, Metatatic Planes.
Let the suffix 1 designate coordinates and coefficients referred to these
axes. Let Oy, 0: be any new pair of orthogonal axes in the plane y x z v
Then since (3 X) — (7 X) is co variant with (y 2 — f?)yz, it follows that
(/3 X)  (y X) = {2 (0 y\ + 4 (X 2 ) x  C/3 8 ),  (y\) • ^~~ (20.)
(where w = < y x y),
a quantity which is = for all values of to which are multiples of 45°*
There are of course similar equations for the other metatatic planes.
Hence it appears that in each of the three Metatatic Planes there is a pair of
Diagonal Metatatic Axes, bisecting the right angles formed by the Principal
Metatatic Axes.
Each pair of diagonal axes is metatatic for that plane only in which it
is situated.
Thus there are in all nine metatatic axes, three orthogonal axes, and
three pairs of diagonal axes. The diagonal axes are normal to the faces
of a regular rhombic dodecahedron.
Let Oy, Oz be a pair of rectangular axes in any plane ivhatsoever ;
Oy', Oz' any other pair of rectangular axes in the same plane ; and let
<yOy = u/ ;
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 129
then
(/3 X)'  (y\y = [2 O y) + 4 (X 2 )  m  (y 2 )} ^^
+ {(/3A)(y\)}cos4 w ', . . (21.)
a quantity which is null for eight values of &/, differing from each other
by multiples of 45°. Hence, in each plane in an elastic solid, there is a
system of two pairs of axes meiatatic for that plane, and forming with each other
eight equal angles of 45°.
In equation (21), make
to' = — O)
(/3\)'(yX)' = (/3X) 1 (yX) 1 = 0;
then from equations (20) and (21), it is easily seen that
2 (By) + 4(X 2 )OT(y 2 )
= { 2 (£ 7)i + ^ (A 2 ) x  (/3 2 ) x  (y 2 X} • cos 4 «. . (22.)
The trigonometrical factor cos 4 w is + 1 for all values of to which are
even multiples of 45°, — 1 for all odd multiples of 45°, and = for all
odd multiples of 22i°. Hence, in every plane in an elastic solid, the
quantity (22), which may be called the Metastatic Difference, is a maximum
for one of the two pairs of metatatic axes, a minimum of equal amount and
negative sign for the other, and null for the eight intermediate directions.
Section 9.— Of Metatatic Isotropy.
A solid is Metatatically Isotropic when, if a cubical molecule, cut out in
any position whatsoever, undergoes simultaneously an elongation along one
axis, and an equal and opposite linear compression along another axis, no
tangential stress will result on the faces of that molecule.
For such a substance, the metatatic differences must be null for all sets
of axes, viz. : —
2(/3y) + 4(X 2 )(/3 2 )(r) = C>n
2(ya) + 40u 2 )(y 2 )(a 2 ) = O; L . (23.)
2(a / 3) + 4(„ 2 )(«s)(/3 2 ) := 0. J
In a paper in the Cambridge and Dublin Mathematical Journal, Vol. VI.,
this theorem was alleged of all homogeneous solids, it having been, in
fact, tacitly taken for granted, that homogeneity involves metatatic
isotropy, as above denned.
I
130 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Section 10. — Of Orthotatic Symmetry.
If it be taken for granted that symmetrical action with respect to a
certain set of axes, between the parts of a body under one kind of strain,
involves symmetrical action with respect to the same axes under all kinds
of strains, then one and the same set of orthogonal axes will be at once
orthotatic, heterotatic, metatatic, and euthytatic, and for them the whole
twelve plagiotatic coefficients will vanish at once, and the independent
tasinomic coefficients be reduced to the nine orthotatic coefficients
enumerated in Sect. 2. As long as the rigidity of solid bodies was
ascribed wholly to mutual attractions and repulsions between centres of
force, it is difficult to sec how, with respect to homogeneous substances,
the above assumption could be avoided. It is probable that there exist
substances for which it is true. Such substances may be said to be
Orthotatically Symmetrical.
Orthotatic symmetry requires that the equation (1 9) of the biquadratic
surface should be reducible by rectangular transformations to its first six
terms, and that the axes so found should also be those of the heterotatic
ellipsoid. The conditions which must be fulfilled in order that a
biquadratic function of three variables may be reducible by rectangular
transformations to its first six terms, have been investigated by Mr. Boole.*'
Section 11. — Of Cybotatic Symmetry.
Let a substance be conceived which is not only orthotatically symme
trical, but for which the three kinds of orthotatic coefficients are equal for
the three orthotatic axes, viz. —
(a 2 ) = (/3 2 ) = (y 2 ); (/3y) = (y«) = (aft; (X 2 ) = ( M 2 ) = (v 2 ). (24.)
Then, for such a substance the metatatic difference may be expressed by
2(/3y) + 4(\ 2 )2(a 2 ); .  • (25.)
and if the body be not metatatically isotropic, this difference will have
equal maxima or minima for the three orthogonal axes, normal to the
faces of a cube, and, conversely, equal minima or maxima for the six
diagonal axes, normal to the faces of a regular rhombic dodecahedron.
Symmetry of this kind may be called Cybotatic, from its analogy to that
of crystals of the tessular system.
* Cambridge and Dublin Mathematical Journal, Vol. VI.
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 131
Section 12. — Of Pantatic Isotropy.
When a body fulfils the conditions of cybotatic symmetry, and at the
same time those of metatatic isotropy, it is completely isotropic with
respect to elasticity, or pantatically isotropic. It has but three tasinomic
coefficients — viz., the euthytatic, platytatic, and goniotatic coefficients,
which are equal for all sets of axes, and are connected by the following
equation, expressing the condition of metatatic isotropy :
(a 2 ) = (/3y) + 2(A 2 ). . . . (26.)
The properties of such bodies have been fully investigated by various
authors.
Section 13. — Of Thlipsinomic Coefficients.
If the six elementary strains, a, &c, at a given point in an elastic solid,
be expressed as linear functions of the six elementary stresses, P p &c,
these expressions will contain twentyone coefficients of compressibility,
extensibility, and pliability, which are the second differential coefficients
of the potential energy of elasticity with respect to the six elementary
stresses ; that energy being represented as follows : —
p2 p2 p2 O 2 O 2 o 2
U = (« 2 )f +(5 2 )^ + (c 2 )f + (Z 2 )f + K)f+(^)f
+ (5 C )P 2 P 3 + (ca)P 3 P 1 +(a&)P 1 P 2 +( W w)Q 2 Q3+(wOQ3Qi + (MQiQ 2
\{(al) P 1+ (5/)P 2 +(d) P 3 }Q 1
+ {(am)P 1 + (6OT)P 2 +(cm)P 3 }Q 2
+ (HP 1 + (k)P 2 +(«)P 3 }Q 3 . . . . (27.)
The twentyone coefficients in the above equation may be comprehended
under the general term Thlijisinomic, and classified as follows : —
Designations of Coefficients. Properties expressed by them. Symbols.
rEuthythliptic, Longitudinal Extensibilities, (a 2 ), (JP), (c 2 )
Orthothliptic<j Platythliptic, Lateral Extensibilities, . . (be), (ca), (ab),
[Goniothliptic, Pliabilities, (Z 2 ), (m 2 ), (re 2 ),
Plagiothliptic, .... Unsymmetrical Pliabilities, (mn), &c, &c.
132 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Section 14. — Of Thlipsinomic Transformations, Umbrae,
Surfaces, and Invariants.
»
The equations of transformation of the thlipsinomic coefficients are
easily deduced from the principle, that the operations
d d d d d d
dP^ d¥ 2 ' dP 3 ' dCfc JQ 2 ' dQ 3
are respectively covariant with
V *2> , 3> * ^1' " ^2> ~" ^v3»
and these with
A f, r, 2yz, 2zz, 2xy.
We may regard the thlipsinomic coefficients, like the tasinomic coeffi
cients, as binary compounds of the following six Umbrcr,
(a), (b), (c), (I), (m), (n),
which being respectively substituted for
"l> * 2» 3' * ^1' * ^2> " tw3»
in the equation of the tasimetric surface (4), produce the following
equation of the Umbral Thlipsinomic Ellipsoid,
(a) :>? + (b) f + (c) r + (I) yz + (m) zx + {n)xy = 1, (28.)
from which, by involution, multiplication, and other operations exactly
analogous to those performed on the umbral tasinomic ellipsoid, there
may be deduced the equations of Thlipsinomic Surfaces exactly correspond
ing to the tasinomic surfaces already described ; while, from the umbral
matrix,
(a) }(») J(m)>
too (») *© y • • • (29.)
may be formed Thlipsinomic Invariants corresponding to the tasinomic
invariants.
Hence it appears, that every function of the tasinomic coefficients is
converted into a function of the thlipsinomic coefficients with analogous
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
133
properties, by the substitution of thlipsinomic for tasinomic umbrae
according to the following table : —
Tasinomic Umbra?,
Thlipsinomic Umbra?, .
(«), 08). (y), (X), W, (v),
(«), (»). (0» K0ki(»)»iW
Amongst the thlipsinomic invariants may be distinguished the Cm6/c
Compressibility, which is formed by squaring the umbral invariant
(a) + (^) + (c), and has the following value :
O 2 ) + (J 2 ) + (r) + 2 (6 e) + 2 (c a) + 2 (a J).
Section 15. — Thlipsinomic and Tasinomic Contragredient
Systems.
Let the following square matrices be formed with the tasinomic and
thlipsinomic coefficients respectively : —
(a 2 ) (a/5) (ya) (a X) (ap) (av)
(a/3) OT (/3y) G8X) (/5m) (/5.)
(ya) (/3y) (y 2 ) (yX) (y/t) (yv)
(aX) (/3X) (yX) (X 2 ) (\ M ) (vX)
(a//) (/3/x) (y/*) (X/x) (m 2 ) 0*v)
(av) Q3v) (yv) (v\) (fiv) (v 2 ) ,
>
(a 2 )
(aft)
(ca)
(a2)
(ab)
(b 2 )
(be)
(bl)
(ca)
(be)
(c 2 )
(cl)
(a I)
(bl)
(cl)
(I 2 )
(a m) (a n)
(b m) (b n)
(c m) (c n)
(Im) (nl)
(30.)
(31.)
(a m) (b m) (c m) (I m) (m 2 ) (m n)
(an) (bn) (en) (nl) (mn) (n 2 )
Then will these matrices be mutually inverse, the two systems of
coefficients arrayed in them, with their respective systems of functions
mutually contragredient, and each coefficient or function belonging to one
system contravariant to the corresponding coefficient or function belonging
to the other system.
The values of the coefficients in either of those matrices are expressed
134
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
in terms of those in the other matrix, in Mr. Sylvester's umbral notation,
by twentyone equations, of which the following are examples : —
(« 2 ) =
(ab) =
03), (y), (X), (a*), (v)
0), (y), (a), 0*), to
(/3), (y), (X), (/*), W
(a), (y), (X), GO, to
(«), (/3), (y), (X), W to
(«), O), (y)> ^)' 0*). to
(«), (/3), (y), (A), W to
(«). (/3), (y), (X), Gk), to
V. (32.)
Section 1G. — Of Thlipsinomic Axes.
If, under given conditions, any symmetrical system or function of
the constituents of one of the above matrices be null, then under the
same conditions will the contravariant system or function of the constituents
of the inverse matrix be null or infinite. Therefore, Systems of Thlipsinomic
Axes coincide with the corresponding systems of Tasinomic Axes.
Section 17. — Platythliptic Coefficients are Negative.
It may be observed, as a matter of fact, that in consequence of the
largeness of the euthytatic coefficients (a 2 ), (/3 2 ), (y 2 ), as compared with
the other tasinomic coefficients, the platythliptic coefficients (be), (ca),
(a b), are generally, if not always, negative.
To illustrate this, the case of pantatic isotropy may be taken, for
which the two matrices have the following forms : —
(a 2 ) (/3y) (/3y)
0y) (« 2 ) (Pv) o o o
03 y) (/3y) (a 2 )
(A 2 )
(A 2 )
(A 2 )
(a 2 ) (be) {be) 0'
(be) (a 2 ) (6c)
(be) (be) (a 2 )
(P)
(I 2 )
(Z 2 ) J
y (33.)
from which it is easily seen that the sole platythliptic coefficient has
the following value :
(fly)
(be)
(33A)
(a 2 ) 2 + (a 2 )(/3y)2(/3y) 2, "
The denominator of this fraction is always positive so long as (a 2 )
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 135
exceeds (/3y); a condition invariably fulfilled by solid bodies, and, in
fact, necessary to their existence.
Section 18. — Of Oblique Coordinates and Contraordinates.
As there are, in the relations between two systems of oblique coordinates,
or between a system of oblique coordinates and a system of rectangular
coordinates, six independent constants of transformation, it is possible,
by referring the equation of the biquadratic surface (19) to oblique
coordinates, to make the six terms vanish which contain the cubes of the
coordinates.
The conception of the physical meaning of such a transformation is
much facilitated by the employment of a system of three auxiliary
variables, which will be designated as Contraordinates.
The relations between coordinates and contraordinates are as follows : —
Through an origin let any three axes pass, right or oblique. Let E
be any point, and let
0~R = r.
Through It draw three planes, parallel respectively to the three coordinate
planes, and intersecting the axes respectively in the points X, Y, Z.
Also, on OE, as a diameter, describe a sphere, intersecting the axes
respectively in U, V, W. Then will
OX = <c, OY = y, OZ = z,
be the coordinates of E, as usual, and
OU = w, OV = v, OW = «e,
its contraordinates, being, in fact, the projections of E on the three axes.
For rectangular axes, coordinates and contraordinates are identical.
Coordinates and contraordinates are connected by the following
equation : —
r 2 = u x + v y + we. . . . (34.)
In the language of Mr. Sylvester, a system of coordinates, and the
concomitant system of contraordinates, are mutually Contragredient ; and
the square of the radius vector is their universal mixed concomitant.
Let the cosines of the angles made by the axes with each other be
denoted as follows:
cos2/Os = q; coszOx = c i ; cos xOy = c 3 ;
136
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
then the contraordinates of a given point are the following functions of
the coordinates:
Also let
u = x + c s y + c 2 z }
v = c z x + y f c 1 z \
I
to = c 2 x + c x y + z J
= lcf4ci+2c 1 c 2 c 8 = C;
(35.)
then the coordinates are the following linear functions of the contra
ordinates : —
Also,
x = h x u — k 3 v — k 2 w ;
y = — k, A u + h 2 v — k x w ;
z = — h 2 u — l\ v + h z w ; J
r = x' f y 2 + z + 2 c x y z + 2c 2 zx + 2c % xy
(3C)
(37.)
= h x u 2 + /; 2 v 2 + h z to 2 ll^vw  2k 2 ivu 2 fc s m v. (37a)
Differentiations with respect to the contraordinates are obviously
covariant with the coordinates, and vice versa; that is to say,
,, ,. d d d d d d
the operations . — , — , — , — , — , — 
dx cly dz du dv dw
are respectively )
covariant with )
y ■
(38.)
*■> y,
By making substitutions according to the above law of covariance in
the equations (34), (37), (37a), three equivalent symbols of operation are
obtained, which, being applied to isotropic functions of the second degree,
produce invariants of the first degree.
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 137
Section 19. — Of Molecular Displacements and Strains as
referred to oblique axes.
If the displacement of a particle from its free position be resolved into
three components g, r\, Z,, parallel respectively to three oblique axes, x,
y, z, those components are evidently covariant respectively with the
coordinates x, y, z.
It is now necessary to find a method of expressing the strain at any
particle in an elastic solid by a system of six elementary strains, which
shall be covariant respectively with the squares and doubledproducts
of these oblique coordinates. This condition is fulfilled by considering
the elementary strains as being constituted by the variations of the
components of the molecular displacement with respect to the distances
of the strained particle from three planes passing through the origin,
and normal respectively to the three axes; that is to say, with respect to
the contraordinates of the particle, as expressed in the following equations: —
Elongations, . a = ^— ; j3 — ^ ) Y — T~ ■>
° du } ' dv 3 * dw'
n tv* i ^ d %, dr l d^.dK. _dri d%
QuasiDistortions, A = ; — h t~ ; ix = 3 — h 3 , v — 5 — (7
dv dw die du du dv
y (39.)
The six elementary strains, as above defined, are obviously covariant
with the squares and doubledproducts of the coordinates, according to
the following table :
a, /3, y, X, /*, v, ")
r . . (40.)
x 2 , y\ z 2 , 2yz, 2zx, 2xy. )
Section 20. — Of Stresses, as referred to Oblique Axes.
It is next required to express the stress at any particle of an elastic
solid by means of a system of six elementary stresses, which shall be
contragredient to the system of six elementary strains defined in the
preceding section. This is accomplished in the following manner.
It is known that the total stress at any point may be resolved into
three normal stresses on the three principal planes of the tasimetric
surface. Let the direction and sign of any one of those three principal
stresses be represented by those of a line K, and its magnitude, as
reduced to unity of area of the plane normal to that direction, by the
square of that line.
OR 2 = r 2 .
138
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Let u, v, iv be the contraordinates of E, as referred to the oblique axes
X, Y, Z. Then will the stresses on unity of area of planes normal
to those axes, in the direction E, be represented respectively by
u r, v r, to r.
Let the Elementary Stresses be defined to be, the projections on the three
axes of coordinates of the total stresses on unity of area of the three pairs of
faces of a parallelepiped, normal to the three axes respectively : then, if we
take S to denote the summation of three terms arising from the three
principal stresses, the elementary stresses will be expressed as follows : —
Normal stresses on the faces normal to
x, y, z,
P 1 = S.tt 2 ; P 2 = S.* 2 ; P 3 = S.ir;
Oblique stresses on the faces normal to
(41.)
Iu the directions
"V
These expressions fulfil the condition of making the elementary stresses
P lf P 2 , P 3 , Q lf Q 2 , Q 3
contravariant respectively to the elementary strains
a, ft, y> \ /"> v >
so that for oblique axes, as for rectangular axes, the potential energy of
elasticity is represented by
♦
U =  1 (P ia + Po/3 + P 3 y + Q X X + Q,fx + Q 3 v),
the universal concomitant; and may be expressed either by a homo
geneous quadratic function of the six elementary strains (as iu equation 2)
with twentyone tasinomic coefficients, or by a homogeneous quadratic
function of the six elementary stresses, as in equation (27), with twenty
one thlipsinomic coefficients, forming a system contragredient to that of
the tasinomic coefficients.
axes of elasticity and crystalline forms. 139
Section 21*. — Of Tasinomic and Thlipsinomic Umbrae for
Oblique Axes.
The tasinomic coefficients for oblique axes may be regarded as com
pounded of umbra?
(a), 08), (y), (A), (/*), (v),
contravariant respectively to the elementary strains
«, (3, 7> I A, I ft, hv,
and consequently covariant with the squares and products of the contra
ordinates
v?, v 2 , w 2 , 1)10, wu, uv;
and the thlipsinomic coefficients for oblique axes may be regarded as
compounded of umbra?
(a), (b), (c), (/), (m), («),
contravariant respectively to the stresses
P lf P 2 , P s , 2Q X , 2Q 2 , 2Q 3 ,
and consequently covariant with the squares and products of the coordinaks
ar, ij , (. , *></~, &%j>, *jxy.
Section 22. — Of the Biquadratic Surface, and of Principal
euthytatic axes.
For oblique as well as for rectangular axes of coordinates, the char
acteristic function of the biquadratic tasinomic surface is represented
by equation (19); and the fifteen homotatic coefficients are covariant
respectively with suitable multiples of the fifteen biquadratic powers and
products of the contraordinates.
If by linear transformations a system of three axes, oblique or rectan
gular, be found which reduces the characteristic function of the biquad
ratic surface to the canonical form, consisting of not more than nine
terms, viz. —
(0) 2 = (a 2 K+(W+(rK
+ 2{(/3y) + 2(\ 2 )}^ 2 + 2{(ya) + 2( y a 2 )}^ 2 +2{(a/3) + 2(v 2 )}a ; Y
+ 4{2( M v) + (aA)}ry: + 4{2(^) + (3,i)}^ 2 2 + 4{2(\ M )
H( 7 v)}^ 2 = l; .... (42.)
140 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
then, for that system of axes, the following six plagiotatic coefficients
are null,
(3X) = 0; (yX) = 0; (yfi) = 0; (afx) = 0; (av) = 0; ((Sv) = 0; (43.)
and each of those axes is Euthytatic, according to the definition in Sect.
7, that is to say, is a direction of maximum or minimum direct elasticity
(absolute or relative), and also a direction in which a direct elongation
or compression produces a simply normal stress.
There are necessarily three euthytatic axes at least in every solid —
viz., the three Principal Euthytatic Axes, as above described, which are
normal to the faces of a hexahedron, right or oblique, as the case may
be; but in special cases of symmetry there are additional or secondary
euthytatic axes, of which examples will now be given.
Section 23. — Of Rhombic and Hexagonal Symmetry.
When a solid has three oblique principal euthytatic axes making equal
angles with each other round an axis of symmetry, and having equal
systems of homotatic coefficients corresponding to them, viz. —
(« 2 ) = Q3 2 ) = (y 2 ); (3y) + 2(^)=:( 7 a) + 2(^) = (a/3) + 2(v 2 )1
r ( 43a
2( MV ) + (aA) = 2( v A) + (/3/0 = 2(X/0 + (7v) J
it may be said to possess rhombic symmetry, because the three oblique
axes are normal to the faces of one rhombohedron, and to the edges
of another belonging to the same series, crystallographically speaking.
It is evident in this case, that the axis of symmetry must be a fourth
Euthytatic Axis.
In the limiting case, when the three oblique axes make with each
other equal angles of 120°, they lie in the same plane, normal to the
axis of symmetry, and are normal to the faces of one hexagonal prism
and the edges of another.
Let y x denote the longitudinal axis of symmetry of the prism ; z x
any one of the three transverse axes perpendicular to y v The equation
of a section of the biquadratic surface by the Plane of Hexagonal Symmetry
y x z v is as follows : —
(i3 2 ) 1 ^ + (y 2 ) 1 ^ + 2{(/3y) 1 +2(X 2 ) 1 } 2 /i,1=.l. . (44.)
The equation of the same section, referred to any other pair of
orthogonal axes y, Oz, in the plane of y x z v is as follows : —
(j3 2 ) . y* + (y 2 ) . **+ 2 {(/3y) + 2(\ 2 )} ! /V+ 4{(/3 A) / + (yA)*»Jy*= 1. (44a.)
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 141
From considerations of symmetry, it is evident that the coefficient
(/3»/) must be null for every direction of the axis y, in the plane of y^z y ;
consequently, every direction Oy in that plane, for which (/3\) = 0, is
an euthytatic axis.
To ascertain whether, and under what conditions, there are other
euthytatic axes in the planes of hexagonal symmetry besides the
longitudinal and transverse axes, it is to be considered, that for rectangular
coordinates (/3\) is covariant with yh; hence, let
Zy l Oy = w,
then
(£A) = sin_2<o m j { , (/ , y ^ + 4 (x% _ ^^ _ (y2)i} cog 2 ^
 (P\ + (r)i] • (4r>.)
The first factor of the above expression is null for the longitudinal
and transverse axes only. The condition of there being additional
euthytatic axes in the plane y x z x is, that the second factor shall vanish ;
that is to say, that
cos o„_ (ff)l  (V% (i6)
and that the value of w which makes it vanish shall neither be 0° nor 90°;
that is to say, that the second member of the above equation (46) shall
lie between + 1 and — 1 ; in which case the equation is satisfied by equal
values of to with opposite signs. Hence are deduced the following
theorems, which are stated in such a form as to be applicable to planes
of symmetry, whether hexagonal or otherwise :
If, in any plane of tasinomic symmetry containing a pair of orthogonal
euthytatic axes, the difference of the euthytatic coefficients for these axes be
equal to or greater than the metatatic difference, there are no additional
euthytatic axes in that plane.
If, on the other hand, the difference of such euthytatic coefficients be less
than the metatatic difference, there are, in such plane of symmetry, a pair of
additional euthytatic axes making with each other a pair of angles bisected
by the orthogonal euthytatic axes.
2 to is the angle bisected by the axis y v
In the case of hexagonal symmetry, the additional axes thus found
are normal to the faces of one pyramidal dodecahedron and the edges of
another.
142 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Section 24. — Of Orthorhombic Symmetry.
Let a solid have one of the three principal euthytatic axes, z v normal
to the other two, Ox v 0^; let the last two be oblique to each other, and
have equal sets of homotatic coefficients, viz. —
(« 2 )> = (/3 2 )i ; O y)i + 2 (A 2 )! = (y «)x + ^ (S\ ;
2frv) + (a\) = 2(v\) + (Pfi), • ( i7 )
then, that solid may be said to have Orthorhombic Symmetry, its principal
euthytatic axes being normal to the faces of a right rhombic prism.
The existence or nonexistence, and the position, of a pair of additional
euthytatic axes in the longitudinal planes of y x r. v s t o\, is to be determined
as in the preceding section. "When such axes exist, they are normal to
the faces of an octahedron with a rhombic base.
Section 25. — Of Orthogonal Symmetry.
If the three principal euthytatic axes be orthogonal, they are normal
to the faces of a right rectangular or square prism, and to the edges of a
rigid rhombic or square prism. The existence or nonexistence, and
position, of a pair of additional euthytatic axes in each of the principal
planes of such a solid, are determined as in Sect. 23.
If there be a pair of such additional axes in each of the three principal
planes, they are normal to the faces of an irregular rhombic dodecahedron,
and to the edges of a rhombic octahedron.
If there be a pair of such additional axes in two of the three principal
planes, those axes are normal to the faces of an octahedron with a
rectangular or square base, and to the edges of an octahedron with a
rhombic or square base.
If there be a pair of such additional axes in one of the planes of
orthotatic symmetry only, those axes are normal to the lateral faces of a
right rhombic prism.
Section 26. — Of Cyboid Symmetry.
The case of Cyboid Symmetry is that in which the homotatic coefficients
are equal for three orthogonal axes, viz. —
(« 2 )  m = (y 2 ); (73 y) + 2 (X 2 ) = (y a) + 2 Ou 2 ) = (a 0) + 2 (t, 2 );
2 0uv)+(aX) = 2(vX) + (j3 j u) = 2(X i u) + ( 7 v) = O. (48.)
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 143
In this case, the principal metatatic axes coincide with the principal
euthytatic axes, which are normal to the faces of a cube; the diagonal
metatatic axes normal to the faces of a regular rhombic dodecahedron,
are euthytatic also; and there are, besides, four additional euthytatic
axes symmetrically situated between the first nine, and normal to the
faces of a regular octahedron, making in all thirteen euthytatic axes.
Section 27. — Of Monaxal Isotropy.
Monaxal Isotropy denotes the case in which the homotatic coefficients
are completely isotropic round one axis only. In this case, the principal
euthytatic axes are, the axis of isotropy, and every direction perpendi
cular to it ; and when there are additional axes, determined as in the
preceding sections, they are normal to the surface of a cone.
Section 28. — Of Complete Isotropy.
In the case of complete isotropy of the homotatic coefficients, every
direction is a euthytatic axis.
Section 29. — Probable Eelations between Euthytatic Axes and
Crystalline Forms.
In the preceding sections it has been shown what must be the nature
of the relations between the fifteen homotatic coefficients, for various
solids, having systems of euthytatic axes normal to the faces and edges
of the several Primitive Forms known in crystallography.
It is probable that the normals to Planes of Cleavage are euthytatic
axes of minimum elasticity.
It may also be considered probable, that in some cases, especially in
the tessular system, which corresponds to cyboi'd symmetry, and in the
case of the pyramidal summits of crystals of the rhombohedral system,
euthytatic axes correspond to symmetrical summits of crystalline forms.
In the icositetrahedral crystals of leucite and analcime, and the tetra
contaoctahedral crystals of diamond, there are twentysix symmetrical
summits, one pair corresponding to each of the thirteen axes of cybo'id
symmetry.
The following is a synoptical table of the various possible systems of
euthytatic axes, arranged according to their degrees and kinds of symmetry,
and of the crystalline forms to the faces and edges of which such systems
of axes are respectively normal.
144 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
Systems of Euthytatic Axes. Crystalline Forms.
Faces. Edges.
I. Asymmetry. Tetartoprismatic System.
1. Three unequal oblique axes, . Oblique hexahedron.
II. Symmetry about One Plane. Hemiprismatic System.
2. Two unequal oblique axes, and ) ,.,,.,,. „.,. . _..
\ , . \ Ri<ditrhornboidal prism, Oblique rhombic prism,
one rectangular axis, . ) a r ' *
wo equa < ( Oblique rhombic prism, Right rhomboidal prism,
oblique axis, . ) x *
III. Rhombic and Hexagonal Rhomboiiedral System.
Symmetry.
4. Three equioblique principal j
axes round one axis of > Rhombohedron, . . Rhombohedron.
symmetry, . . . . )
5. Three equioblique principal j
axes in one plane, normal > Hexagonal prism, . Hexagonal prism,
to axis of symmetry, . . )
6. Three pairs of secondary axes Pyramidal dodecahe Pyramidal dodecahe
in planes of symmetry, . dron, . . . dron.
IV. Orthoriiombic Symmetry. Prismatic and Pyramidal Systems.
7. Two equal oblique transverse \
axes normal to one longi > Right rhombic prism, . Rectangular prism,
tudinal axis, . . . )
8. Two pairs of secondary axes in Octahedron with rhom Octahedron with rectan
longitudinal planes, . . bic base, . . . gular base.
V. Orthogonal Symmetry.
9. Three orthogonal axes, not all Rectangular and square Right rhombic and
equal, prisms, . . . square prisms.
! Octahedron with rhom
bic base and rectan
gular prism.
,,_,.„ I Octahedron with square Octahedron with square
11. Two pairs of secondary axes, . j or rectangular base , . or rhombic base .
Same with 7. One pair of second ) . ' . .
arv axes \ ra S ut rhombic prism, . Rectangular prism.
VI. Cyboid Symmetry. Tessular System.
12. Three equal orthogonal axes, . Cube.
10 „. ,. , I Regular rhombic dode Cube and regular octahe
13. Six diagonal axes, . \ , j n
° ( cahedron, . . . dron.
14. Four symmetrical intermediate
axes,
Regular octahedron, . Rhombic dodecahedron.
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 145
VII. MONAXAL ISOTKOPY.
15. One axis of isotropy, . . Isotropic lamina?.
16. Innumerable transverse axes, . Istropic fibres.
17. Innumerable equioblique axes, Conical cleavage.
VIII. Complete Isotropy.
18. Innumerable axes of isotropy, . Amorphism.
Section 30. — Mutual Independence of the Euthytatic and
Heterotatic Axes, and of the Homotatic and Heterotatic
Coefficients.
The fifteen homotatic coefficients of the biquadratic surface, on which
the euthytatic axes depend, and the six heterotatic differences, coefficients
of the heterotatic ellipsoid, constitute twentyone independent quantities;
so that the euthytatic axes may possess any kind or degree of symmetry
or asymmetry, and the heterotatic axes any other kind or degree, in
the same solid.
Hence, if it be true that crystalline form depends on the arrangement
of euthytatic axes, it follows that two substances may be exactly alike
in crystalline form, and yet differ materially in the laws of their elasticity,
owing to differences in then respective heterotatic coefficients.
It may be observed, however, that this complete independence of those
two systems of axes and coefficients is mathematical only, and that their
physical dependence or independence is a question for experiment.
Section 31. — On Eeal and Alleged Differences between the Laws
of the Elasticity of Solids, and those of the Luminiferous
Force.
For every conceivable system of tasinomic coefficients in a solid, the
plane of polarisation of a wave of distortion is that which includes the
direction of the molecular vibration and the direction of its propagation,
being, in fact, the plane of distortion.
On the other hand, it appears to be impossible to avoid concluding,
from the laws of the diffraction of polarised light, as discovered by
Professor Stokes, and from those of the more minute phenomena of the
reflexion of light, as investigated theoretically by M. Cauchy and
experimentally by M. Jamin, that in planepolarised light the plane of
polarisation is perpendicular to the direction of vibration, or rather (to
avoid hypothetical language) to the direction of some physical phenomenon
K
146 AXES OF ELASTICITY AND CRYSTALLINE FORMS.
whose laws of communication are to a certain extent analogous to those
of a vibratory movement.
This constitutes an essential difference between the laws of the elastic
forces in a solid, and those of the luminiferous force.
In order to frame, in connection with the wave theory of light, a
mechanical hypothesis which should take that difference into account,
it has been proposed to consider the elasticity of the luminiferous medium
to be the same in all substances, and for all directions, or Pantatically
Isotropic ; and to ascribe the various retardations of light to variations in
the inertia of the mass moved in luminiferous waves, in different substances,
and for different directions of motion.*
Another essential difference between the laws of solid elasticity and
those of the luminiferous force is, that under no conceivable system of
tasinomic coefficients in a homogeneous solid, would the plane of distortion
in a wave be rotated continuously round the direction of propagation.
Much has been written, both recently and in former times, concerning
an alleged difficulty in the theories of waves, both of sound and of light,
arising from the physical impossibility of the actual divergence of waves
from, or their convergence to, a mathematical point. This impossibility
must be admitted; but the supposed difficulty to which it gives rise in
the theories of waves is completely overcome in Mr. Stokes' paper " On
the Dynamical Theory of Diffraction,"! in which that author proves, that
waves spreading from a focal space, or origin of disturbance, of finite
magnitude, and of any figure, sensibly agree in all respects with waves
spreading from an imaginary focal point, so soon as they have attained a
distance from the focal space which is large as compared with the
dimensions of that space ; so that the equations of the propagation of
waves spreading from imaginary focal points may be applied, without
sensible error, to all those cases of actual waves to which it is usual to
apply them.
The physical impossibility of focal points applies to light independently
of all hypotheses; for at such points the intensity would be infinite.
It appears to be worthy of consideration, whether this impossibility may
not be connected with the appearance of spurious disks of fixed stars
in the foci of telescopes.
Section 32. — On the Action of Crystals on Light.
If we set aside those actions on light to which there is nothing analogous
in the phenomena of the elasticity of homogeneous solids, the laws of
* Philosophical Magazine, June, 1851, December, 1853.
+ Cambridge Transactions, Vol. IX., Part 1.
AXES OF ELASTICITY AND CRYSTALLINE FORMS. 147
the refractive action of a crystal on light are in general of a more sym
metrical kind, or depend on fewer quantities than those of its elasticity.
Thus, the elasticity of a homogeneous solid depends on twentyone
quantities ; its crystalline form, on fifteen (the homotatic coefficients),
while its refractive action on homogeneous light in most cases is expres
sible by means of the magnitudes and directions of the orthogonal axes
of Fresnel's wavesurface, making in all six quantities. Crystals which
possess only rhombic or hexagonal symmetry in their euthytatic axes, are
usually monaxally isotropic in their action on light ; while crystals which
possess only cybo'id symmetry in their euthytatic axes, are completely
isotropic in their action on light.
From these remarks? however, there are exceptions, as in the case of
the extraordinary optical properties discovered by Sir David Brewster
in analcime, which, in its refraction as well as in its form, is cyboidally
symmetrical without being isotropic.
Note referred to at page 120, On Sylvestrian Umbrae.
Without attempting to enter into the abstract theory of the umbral
method, it may here be useful to explain the particular case of its appli
cation which is employed in this paper.
Let U be a quantity having an absolute value, constant or variable,
(such, for example, as any physical magnitude), and u, v, . . . &c.
a set of quantities, m in number, such that tJ is of them a homogeneous
rational function of the nth. degree. There are an indefinite number of
possible sets of m quantities satisfying this condition; and the quantities
of each set are related to those of each other set by m equations of the
first degree, called equations of linear transformation. Let
the
Up v v . . .
be two such sets.
^t (U . . .
development of
and let
M 2 > ^2>
denote the coefficient of u a v b .
(u + v+ . . .)»
U = 2{C a , 6 ,
• • • A l,a,6, • • • K V l ■
• ■}
= 2{C a . & ,
. . . A., , ... u, a v? .
2. a, b, • • • w 2 "2
• •}•
The two sets of coefficients A v A. z , are connected by linear equations
148
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
of transformation, the investigation of which is much facilitated by the
following process.
Let two sets, each of m symbols, a v [3 V &c. . . . a 2 , /3 2 , . . . &c,
be assumed such that
«i «i + & v x +
and that, consequently,
= a 2 u 2 + fi 2 v 2 +
(«1 M 1+/Vl+ •
■ .)*~2{€U •
. . <& & .
. ufvf .
• •}
= (a 2 u 2 + /3 2 v 2 + ■
• ) n =^K, b , ■
■ <A> & •
• «/^ 2 6 •
• •}•
Then, if the m equations of transformation between the two sets of
symbols a v & • • • and a 2 , (3 2 . . . be formed, and if from
them be deduced the equations between the two sets of products a x re fi^
, and a 2 a (3 2
, &c, and if in the latter system of
equations, there be substituted for each product a a /3 b . . . the
corresponding coefficient A a _ &> . . . , the result will be the system of
equations sought. Also, if any function of the products a°/S & . . .
be invariant (i e., a function whoso value, like that of the original function
U, is not altered by the transformation), the corresponding function of the
coefficients A will be invariant.
The symbols a, (3, &c, with reference to their relation to the coefficients
A, are called umbrce ; that is, factors of symbols, whose equations of transfor
mation are similar to those of the coefficients A. In the umbral notation,
umbrae are usually distinguished from symbols denoting actual quantities
by being enclosed in brackets thus :
(a), 08), &c. . . .
and each coefficient A is represented by enclosing in brackets that product
of umbrae with which it is covariant; thus :
a, I,
= (a a /3 6
•)•
The umbral notation is applied to abbreviate the expression of deter
minants in a manner of which the following are examples : —
a, (3, y, &c.
a, /3, y, &c.
denotes
(« 2 ) («/3)
(ay)
&c.
(«/3) (/3 2 )
(/3y)
&c.
(ay) (/3y)
(y 2 )
&c.
&c. &c.
&c.
&c.
AXES OF ELASTICITY AND CRYSTALLINE FORMS.
149
a, y, S, &c.
/5, y, S, &C.
denotes
(«/3)
(/3y)
CSS)
&c.
(«y)
(r)
(yS)
&c.
(« 8)
(yS)
(S 2 )
&c.
&c.
&c.
&c.
&c.
150 THE VIBRATIONS OF PLANEPOLARISED LIGHT.
VII— ON THE VIBRATIONS OF PLANEPOLARISED LIGHT.*
1. The important question, whether the direction of vibration in plane
polarised light is normal or parallel to the plane of polarisation, is
equivalent to this : — whether the velocity of propagation of a rectilinear
transverse vibratory movement in the medium which transmits light in
crystallised bodies is a function of the direction of vibration only, or of the
position of the plane, which includes the direction of vibration and the direction
of transmission.
The former of these views was adopted by Fresnel, as being necessary
to explain the phenomena of polarisation by reflexion; and in Mr. Green's
investigation of the laws of these phenomena, in which the conclusions
of Fresnel are shown to agree either exactly or approximately with the
consequences of strictly mechanical principles, the same supposition is
adopted.
But if we follow the generally received theory, that the different
velocities of differently polarised rays in crystalline bodies are due solely
to the different degrees of elasticity possessed by the vibrating medium
in different directions, Fresnel's supposition must be abandoned, and the
opposite one adopted. For if there is any proposition more certain than
others respecting the laws of elasticity, it is this : — that the transverse
elasticity of a medium, or the elasticity which resists distortion of the
particles, depends upon the position of the plane of distortion, being the
same for all directions of distortion in a given plane. This law is impli
citly involved in the researches of Poisson, of M. Cauchy, of Mr. Green,
and others on elasticity; and in a memoir read to the British Association
at Edinburgh, in 1850, and published in the Cambridge and Dublin
Mathematical Journal for February, 1851, I have shown that it is true
independently of all hypotheses respecting the constitution of matter,
being a necessary consequence of the conception of an elastic medium.
Now a wave of planepolarised light is a wave of distortion : the plane
of distortion is the plane which includes the direction of transmission and
the direction of vibration: the elasticity called into play depends on the
position of this plane; therefore, if the velocity of propagation depends
upon elasticity alone, the plane of distortion must be the plane of polaris
ation; and if a normal be drawn to that plane, the velocity of propagation
* Read before the Royal Society of Edinburgh, on December 2, 1850, and published
in the Philosophical Magazine, June, 1851.
THE VIBRATIONS OF PLANEPOLARISED LIGHT. 151
will be a function of the position of that normal, and not, as supposed
by Fresnel, of the direction of vibration itself.
2. Up to a very recent period, no experimental data existed adequate
to determine which of these suppositions is supported by facts ; for the
phenomena of double refraction are consistent with either ; and the theory
of polarisation by reflexion is not regarded as sufficiently certain to afford
the means of deciding this question. At length, however, the experimentum
cruris has been made by Professor Stokes, and the result is conclusive in
favour of the supposition of Fresnel.
In his paper on Diffraction (Cambridge Transactions, Vol. IX., Part 1),
Professor Stokes has shown, that on any conceivable theory of the
propagation of undulations of light, vibrations normal to the plane of
diffraction must be transmitted round the edge of an opaque body with less
diminution of intensity than vibrations in that plane. Therefore, Avhen
light, of which the vibrations are oblique to the plane of diffraction, is so
transmitted, the plane of vibration will be more nearly perpendicular to the
plane of diffraction in the diffracted ray than in the incident ray. He
has found by experiment, that when light of which the plane of polarisa
tion is oblique to the plane of diffraction is transmitted round the edge
of an opaque body, the plane of polarisation is more nearly parallel to the
plane of diffraction in the diffracted than in the incident ray. The
necessary conclusion is, that the direction of vibration in planepolarised
light is normal to the plane of polarisation; in other words, that the velocity
of light in crystallised media depends on the direction of vibration, as con
jectured by Fresnel.
This result of experiment is at variance with the necessary consequences
of the supposition, that the velocity of light depends on elasticity alone;
therefore, that supposition is inadequate to explain the phenomena of
polarised light.
3. Having considered what modifications must be introduced into our
hypothetical conceptions of the nature of the medium which transmits
light, to make them adequate to explain the facts which have thus been
established, I now offer the following suggestions.
In a paper read to the Royal Society of Edinburgh, and published in
their Transactions, Vol. XX., Part 1, I proposed, as a foundation for the
theory of heat, and of the elasticity of gases and vapours, an hypothesis
called that of molecular vortices ; and in a subsequent paper, already
referred to, I deduced from the same hypothesis some principles relative
to the elasticity of solids. I shall now show that Fresnel's conjecture
as to the direction of vibration in planepolarised light is a natural
consequence of that hypothesis.
The fundamental suppositions of the hypothesis of molecular vortices
are the following: —
152 THE VIBRATIONS OF PLANEPOLARISED LIGHT.
First. That each atom of matter consists of a nucleus or central physical
point enveloped by an elastic atmosphere, which is retained round it by
attraction; so that the elasticity of bodies is made up of two parts — one
arising from the diffused portion of the atmospheres, and resisting change
of volume only ; the other arising from the mutual actions of the nuclei,
and of the portions of atmosphere condensed round them, and resisting
not only change of volume, but also change of figure.
Secondly. That the changes of elasticity due to heat arise from the
centrifugal force of revolutions or oscillations among the particles of the
atomic atmospheres, diffusing them to a greater distance from their nuclei,
and thus increasing the elasticity which resists change of volume only, at
the expense of that which resists change of figure also.
Thirdly. That the medium which transmits light and radiant heat
consists of the nuclei of the atoms vibrating independently, or almost
independently, of their atmospheres; absorption being the transference of
motion from the nuclei to the atmospheres, and emission its transference
from the atmospheres to the nuclei.
This last supposition is peculiar to my own researches, the first two
having more or less resemblance to ideas previously entertained by others.
If an indefinitely extended vibrating medium, equally elastic in all
directions, consists of a system of atomic nuclei, tending to preserve a
certain configuration in consequence of their mutual attractions and
repulsions, it is well known that such a medium is capable of transmitting
two sorts of vibrations only, longitudinal and transverse, the latter alone
being supposed to be concerned in the phenomena of light. It is also
well known, that the square of the velocity of propagation of transverse
vibrations is directly proportional to a quantity called the transverse
elasticity of the medium, arising from the mutual actions of the nuclei,
and inversely proportional to its density — that is, to the sum of the masses
of all the nuclei contained in unity of volume. To account for the
immense velocity of light, the masses of the atomic nuclei must be sup
posed to be very small as compared with the mutual forces exerted by
them.
In stating the third supposition of the hypothesis, the nuclei are said
to vibrate independently, or almost independently, of their atmospheres;
for the absolute independence of their vibrations is probably an ideal case,
not realised in nature, though approached very nearly in the celestial
space, where the atomic atmospheres must be inconceivably rarefied.
As a pendulum is known to be accompanied in its oscillations by a
portion of the air in which it swings, so the nuclei probably in all cases
carry along with them in their vibrations a small portion of their atmos
pheres, which acts as a load, increasing the vibrating mass without increas
ing in the same proportion the elasticity, and consequently retarding
THE VIBRATIONS OF PLANEPOLARISED LIGHT. 153
the velocity of transmission. The amount of this load must depend on
the density of the atomic atmosphere; and, accordingly, we find that,
generally speaking, the most dense substances are those in which the
velocity of light is least.
Now, if we assume, what is extremely probable, that in crystallised
media the atomic atmospheres are not similarly diffused in all directions
round their nuclei, but are more dense in certain directions than in others,
we must at once conclude that in such media the velocity of propagation
of vibratory movement depends on the direction of vibration ; for upon that
direction depends the load of atmosphere which each nucleus carries along
with it.
4. Having thus shown that the conjecture of Fresnel, which has been
confirmed by the experiments of Professor Stokes, is a natural consequence
of the hypothesis of molecular vortices, I shall now prove that that
hypothesis leads to those mathematical laws of the transmission of light
in crystalline media which Fresnel discovered.
Considering it desirable in this paper to avoid lengthened algebraical
analysis, I shall with that view state, in the first place, certain known
geometrical properties of the ellipsoid, to which it will be necessary for
me to refer.
I. If a curved surface be described about a centre, such that the sum
of the reciprocals of the squares of any three orthogonal diameters is a
constant quantity, that surface, if no diameter is infinite, is an ellipsoid.
II. Every function of direction round a centre, whose variation from a
given amount varies as the reciprocal of the square of the diameter of
an ellipsoid described about that centre, is itself proportional to the
reciprocal of the square of the diameter of another ellipsoid described
about the same centre with the first, and having the directions of its axes
the same.
III. It follows from the last proposition, that if there be a function of
direction round a centre which is proportional to the reciprocal of the
square of the diameter of an ellipsoid of small excentricities, so that the
range of variation of the function is small as compared with its amount,
then any function of that function, whose range of variation is small also,
may be represented approximately by the reciprocal of the square of the
diameter of another ellipsoid, having its centre and the directions of its
axes the same with those of the first.
The square of the velocity of propagation of transverse vibrations is
proportional to the transverse elasticity of the medium divided by the
mean density; that is, by the sum of all the vibrating masses in unity
of volume. That sum is the sum of the masses of the nuclei, added to
the masses of atmosphere with which they are loaded. The atmospheric
load of each nucleus depends on, or is a function of, the density of the
154 THE VIBRATIONS OF PLANEPOLARISED LIGHT.
atmosphere adjoining the nucleus along the line in which the latter
vibrates. The mode of distribution of the atmospheres depends on the
attraction of the nuclei upon them, and therefore on the mode of arrange
ment of the nuclei. The mode of arrangement of the nuclei, when it
is symmetrical and uniform, may be expressed by means of their mean
intervals.
The mean interval of the nuclei is a function of direction, of such a
nature that its three values for any three orthogonal directions being
multiplied together give a constant result — viz., the space occupied (not
filled) by one nucleus, or the quotient of a given space by the number of
nuclei contained in it. Hence the sum of the values of the logarithm
of the mean interval for any three orthogonal directions is a constant
quantity; and that logarithm, therefore, is proportional to the reciprocal
of the square of the diameter of an ellipsoid, whose three axes may be
called the axes of atomic distribution. Therefore, the mean interval, the
atmospheric load of the nuclei, and the square of the velocity of propa
gation, for a given direction of transverse vibration, are all functions of
the reciprocal of the square of the diameter of an ellipsoid, and have
maxima and minima corresponding to its three axes, which are those of
atomic distribution.
Now, in all known crystalline media, the range of variation of these
quantities for different directions is very small compared with their
amount. Therefore, each of them may be approximately represented by
the reciprocal of the square of the diameter of an ellipsoid whose axes are
parallel to those of atomic distribution.
If, then, the directions of vibration in a given crystal which correspond
to the greatest and least velocities of transmission are known, let these
directions (which are at right angles to each other), and a third direction
at right angles to them both, be taken for the axes of an ellipsoid, the
lengths of those axes being inversely proportional to the corresponding
velocities of transmission. Then will the velocity of transmission of any
transverse vibratory movement be sensibly proportional to the reciprocal
of a diameter of that ellipsoid, drawn parallel to the direction of vibration.
And if a plane be drawn through the centre of the ellipsoid parallel to a
series of planewaves, the two axes of the elliptic section so made will
represent, in magnitude, the reciprocals of the greatest and least normal
velocities of transmission of waves parallel to that plane, and, in direction,
the corresponding directions of vibration.
This agrees exactly with the construction given by Fresnel, on which
his entire theory of double refraction is founded.
The degree of symmetry and uniformity of arrangement of the atoms
which is necessary in order that the mean interval may have a definite
value, and that three axes of distribution may exist, is the same which
THE VIBRATIONS OF PLANEPOLARISED LIGHT. 155
is necessary to the existence of rectangular axes of elasticity in a solid
body. It must extend round each point, throughout a space which is
large as compared with the sphere of appreciable molecular action.
The experiments of Sir David Brewster and of Fresnel on the action
of compressed glass on polarised light, show that rays polarised in a plane
normal to the direction of compression — that is to sa}^ vibrations parallel
to that direction — are accelerated. This indicates that the atmospheric
load on each vibrating nucleus in that direction is diminished, probably
by the displacement of a portion of the atmosphere out of that line.
5. Though I have assumed, in the course of this investigation, that the
luminiferous medium is equally elastic in all directions, I by no means
intend to assert that it is necessarily so in all substances ; but merely
that, in most known crystalline media, an atmospheric load on the vibrating
nuclei is the predominant cause of variation in the velocity of transmission
with the direction of transverse vibration.
It is remarkable that Fresnel, in his theory of the intensity of reflected
and refracted light, speaks of the particles of the luminiferous medium as
being more or less loaded in substances of greater or less refractive power.
He did not, however, apply this idea to double refraction, although he
adopted a theory, which, as we have seen, results from it.
The principles laid down in this paper are not compatible Avith the
prevalent idea of a luminiferous ether enveloping ponderable particles.
The fundamental idea from which they spring is the converse: that the
luminiferous medium is a system of atomic nuclei or centres of force, whose
office it is to give form to matter ; while the atmospheres by which they
are surrounded give, of themselves, merely extension.
156 AN OSCILLATORY THEORY OF LIGHT.
VIII— GENERAL VIEW OF AN OSCILLATORY THEORY OF
LIGHT.*
Section I. — Difficulties of the Present Hypothesis.
Notwithstanding the perfection to which the geometrical part of the
undulatory theory of light has been brought, it is admitted that great
difficulty exists in framing, to serve as a basis for the theory, a physical
hypothesis ■which shall at once be consistent with itself and with the
known properties of matter.
The present paper is a summary of the results of an attempt to diminish
that difficulty. All the conclusions stated have been deduced by means
of strict mathematical analysis ; and although it is impossible to read the
investigations before the British Association in detail, their results can
easily be verified by every mathematician who is familiar with the
undulatory theory in its present form.
It may be considered as established, that if we assume the supposition
that planepolarised light (out of the varieties of which all other light can
be compounded) consists in the wavelike transmission of a state of
motion, the nature and magnitude of which are functions of the direction
and length of a line transverse to the direction of propagation, we can
deduce from this supposition, with the aid of experimental data, and of
certain auxiliary hypotheses, the laws of the phenomena of the inter
ference of light, of its propagation in crystalline and uncrystalline
substances, of diffraction, of single and double refraction, of dispersion by
refraction, and of partial and total reflexion.
It has hitherto been always assumed, that the kind of motion which
constitutes light is a vibration from side to side, transmitted from particle
to particle of the luminiferous medium, by means of forces acting between
the particles. In order to account for the transmission of such transverse
vibrations, the luminiferous medium has been supposed to possess a kind
of elasticity which resists distortion of its parts, like that of an elastic
solid ; and in order to account for the nonappearance in ordinary cases of
effects which can be ascribed to longitudinal vibrations, it has been found
* Read before the British Association at Hull, September 10, 1^53, and published
in the PhilosojjJdcal Magazine, December, 1853.
AN OSCILLATORY THEORY OF LIGHT. 157
necessary to suppose further, that this medium resists compression with an
elasticity immensely greater than that with which it resists distortion;
the latter species of elasticity being, nevertheless, sufficiently great to
transmit one of the most powerful kinds of physical energy through
interstellar space, with a speed in comparison with which that of the
swiftest planets of our system in their orbits is appreciable, but no
"more.
It seems impossible to reconcile these suppositions with the fact, that
the luminiferous medium in interstellar space offers no sensible resistance
to the motions of the heavenly bodies.
A step towards the solution of this difficulty was made by Mr.
MacCullagh. The equations which he used to express the laws of the
propagation of light, when interpreted physically, denote the condition
of a medium whose molecules tend to range themselves in straight lines,
and when disturbed, to return to those lines with a force depending on the
curvature of the lines into which they have been moved. But even this
hypothesis requires the assumption that the elasticity of the luminiferous
medium to resist compression is immensely greater than the elasticity
which transmits transverse vibrations.
The difficulty just referred to arises from a comparison of the hypo
thesis of transverse vibrations with the observed phenomena of the world.
Another difficulty arises within the hypothesis itself. Fresnel originally
assumed, that in crystalline media, where the velocity of light varies with
the position of the plane of polarisation, the direction of vibration is
perpendicular to that plane. This is equivalent to the supposition, that
the velocity with which a state of rectilinear transverse vibration is
transmitted through such a medium, is a function simply of the direction
in which the particles vibrate. From this hypothesis he deduced the
form of that wavesurface . which expresses completely the law of the
propagation of planepolarised light through crystalline media, and he
obtained also a near approximation to the laws of the intensity of plane
polarised light reflected from singly refracting substances.
But it was afterwards demonstrated, that the elastic forces which
propagate a transverse movement in any medium must necessarily be
functions, not merely of the direction of the movement, but jointly of this
direction and the direction of propagation ; that is to say, of the position
of the plane containing these two directions. Consequently, if the various
velocities of variously polarised light in a doubly refracting medium, arise
from variations of elasticity in different directions, the direction of vibra
tion is in the plane of polarisation, contrary to the hypothesis of Fresnel.
Fresnel's wavesurface, and his approximate formulas for the intensity of
reflected light, are deducible from this supposition as readily as from his
original hypothesis; and Mr. MacCullagh obtained from it formula? for the
158 AN OSCILLATORY THEORY OF LIGHT.
intensity of light reflected from doubly refracting substances, agreeing
closely with the experiments of Sir David Brewster.
On the other hand, the formula? of M. Cauchy, and those of Mr. Green,
as modified by Mr. Haughton,* expressing the effects of reflexion on the
intensity and phase of polarised light, all of which are founded on the
supposition that the direction of vibration is perpendicular to the plane of
polarisation, have been shown to be capable, by the introduction of proper
constants, of giving results agreeing closely with those of the important
experiments of M. Jamin {Annales de Chimie et de Physique, 3rd Series,
Vol. XXIX., 1850); and it is difficult, if not impossible, to see how
such formulae could have been deduced from the opposite supposition.
But the true crucial experiment on this subject has been furnished by
the researches of Professor Stokes on the Diffraction of Polarised Light
(Camb. Trans., Vol. IX.). Whatsoever may be the nature of the motion
that constitutes light, if it can be expressed by a function of the direction
and length of a line perpendicular to the direction of propagation (which
may be called a transversal), it is certain that this motion will be more
abundantly communicated round the edge of an obstacle, when its trans
versal is parallel than when it is perpendicular to that edge ; so that the
effect of diffraction is, to bring every oblique transversal into a position
more nearly parallel to the diffracting edge. But it has been shown by
the experiments of Professor Stokes, that the effect of diffraction upon
every ray of light polarised in a plane oblique to the diffracting edge, is to
bring the plane of polarisation into a position more nearly perpendicular to
the diffracting edge. Therefore, the transversal of a ray of planepolarised
light (which, if light consists in linear vibratory movement, is the direction
of vibration) is perpendicular to the plane of polarisation.
Hence it follows, that, in a crystalline medium, the velocity of light
depends simply on the direction of the transversal characteristic of the
movement propagated, and not on the direction of propagation.
This conclusion is opposed to the laws of the propagation of transverse
vibrations through a crystalline elastic solid, or through any medium in
which the velocity of propagation depends on elasticity varying in different
directions. Therefore, the velocity of light depends on something not
analogous to the variations of elasticity in such a medium.
To solve this difficulty, the author of this paper some time since
suggested the hypothesis, that the luminiferous medium consists of particles
forming the nuclei of atmospheres of ordinary matter ; that it transmits
transverse vibrations by means of an elasticity which is the same in all
substances and in all directions ; and that the variations in the velocity of
the transmission of vibrations arise from variations in the atmospheric load,
which the luminiferous particles carry along with them in their vibrations,
* Philosophical Magazine, August, 1S53.
AN OSCILLATORY THEORY OF LIGHT. 159
and which is a function of the nature of the substance, and, in a crystalline
body, of the direction of vibration.
But although this hypothesis removes the inconsistency just pointed
out as existing within the theory itself, it leaves undiminished the
difficulty of conceiving a medium pervading all space, and possessed of an
elasticity of figure, at once so strong as to transmit the powerful energy
of light with its enormous velocity, and so feeble as to exercise no direct
appreciable effect on the motions of visible bodies.
Section II — Statement of the Proposed Hypothesis of
Oscillations.
The hypothesis now to be proposed as a groundwork for the undulatory
theory of light, consists mainly in conceiving that, the luminiferous
medium is constituted of detached atoms or nuclei distributed throughout
all space, and endowed with a peculiar species of polarity, in virtue of
which three orthogonal axes in each atom tend to place themselves
parallel respectively to the corresponding axes in every other atom ; and
that planepolarised light consists in a small oscillatory movement of each
atom round an axis transverse to the direction of propagation.
Such a movement would be transmitted through such a medium with
a velocity proportional, directly, to the . square root of the total rotative
force exercised by the luminiferous atoms in a given small space, upon
those in a given adjacent small space lying in the direction of propagation,
in consequence of a given amount of relative angular displacement round
the axis of oscillation ; and inversely, to the square root of the sum of
the moments of inertia round the axes of oscillation of the atoms contained
in a given space, loaded with such portions of molecular atmospheres
surrounding them as they may carry along with them in their oscilla
tions.
Then, denoting by
h, the velocity, in a given direction of planewaves, of oscillation round
transverse axes parallel to a given line;
C, a coefficient of polarity or rotative force for the given directions of
propagation and of axes ;
M, a coefficient of moment of inertia for the given direction of axes;
the above principle may be represented by this equation,
M
The coefficient of polarity in question is proper only to an axis of
100 AN OSCILLATORY THEORY OF LIGHT.
oscillation transverse to the direction of propagation. To account for the
stability of direction of the axes of the atoms, and also for the nonappear
ance, in ordinary cases, of phenomena capable of being ascribed to
oscillations round axes parallel to the direction of propagation, it is
necessary to suppose the corresponding coefficient for the latter species of
oscillations to be much greater than the coefficient for transverse axes of
oscillation.
It is evident, that how powerful soever the polarity may be which is
here ascribed to the atoms of the luminiferous medium, it is a kind of
force which must be absolutely destitute of direct influence on resistance
to change of volume or change of figure in the parts of that medium, or of
any body of which that medium may form part ; and that, consequently,
the difficulty, which in the hypothesis of vibrations arises from the
necessity of ascribing to the luminiferous medium properties like those of
an clastic solid, has no existence in the hypothesis of oscillations now
proposed. *
The luminiferous atoms may now be supposed to be diffused throughout
all space, and, as molecular nuclei, throughout all bodies ; the distribution
and motion of their centres being regulated by forces wholly independent
of that species of polarity which is the means of transmitting a state of
oscillation round those centres.
Section III. — Of the Diffraction of Planepolarised Light, and the
Relation of Axes of Oscillation to Planes of Polarisation.
In the diffraction of an oscillatory movement round transverse axes past
the edge of an obstacle, a law holds good exactly analogous to that
demonstrated by Professor Stokes for a transverse vibratory movement,
substituting only the axis of oscillation for the direction of vibration — that
is to say :
The direction of the axes of oscillation in the diffracted icave is the projection
of that of the axes of oscillation in the incident wave on a plane tangent to the
front of the diffracted wave.
Consequently, oscillations in the incident wave, round axes oblique to
the diffracting edge, give rise to oscillations in the diffracted wave round
axes more nearly parallel to the diffracting edge.
But the experiments of Professor Stokes .have proved, that light
polarised in a plane oblique to the diffracting edge, becomes, after
diffraction, polarised in a plane more nearly perpendicular to the diffracting
edge.
Therefore, the axes of oscillation in planepolarised light are perpendicular
to the plane of polarisation.
AN OSCILLATORY THEORY OF LIGHT. 161
Therefore, the velocity of transmission of oscillations round transverse
axes, through the luminiferous medium in a crystalline body, is a function
simply of the direction of the axes of oscillation.
Now, if the variations of the velocity of transmission arose from
variations of the coefficient of transverse polarity (denoted by C), they
would depend on the direction of propagation as well as upon that of the
axes of oscillation, so that the plane of polarisation would be that which
contains these two directions. Since the velocity of transmission depends
on the direction of the axes of oscillation only, it follows that its variations
in a given crystalline medium arise wholly from variations of the moment
of inertia of the luminiferous atoms, together with their loads of extraneous
matter.
Consequently, the coefficient of polarity, C, for transverse axes of oscilla
tion is the same for all directions in a given substance.
To account for the known laws of the intensity and phase of reflected
and refracted light consistently with the hypothesis of oscillations, it is
necessary to suppose, also, that this coefficient is the same for all substances ;
so that the variations of the velocities of light, and indices of refraction for
different media, depend solely on those of the moments of inertia of the
loaded luminiferous atoms.
There is reason to anticipate, that, upon further investigation, it will
appear that this condition is necessary to the stability of the luminiferous
atoms.
Section IV. — Of the Wavesurface in Crystalline Bodies.
Let the axes of coordinates be those of molecular symmetry in a
crystalline medium.
Let M v M 9 , M 3 , be coefficients proportional to the moments of inertia of
the luminiferous atoms with their loads of extraneous matter, round axes
parallel to x, y, z, respectively.
Let r be a radius vector of the diverging wavesurface in the direction
(«> A y)
Then the equation of that surface for polar coordinates is,
~   2 • ~ { (M 2 + M 3 ) cos 2 a + (M 3 + M x ) cos 2 /3 + (M x + M 2 ) cos 2 y }
+ ^ {M 2 M 3 cos 2 a + M 3 M x cos 2 /3 + M x M 2 cos 2 y } = ;
162 AN OSCILLATORY THEORY OF LIGHT,
and for rectangular coordinates,
^(x 2 + 2/ 2 + ^).(M 2 M 3 x 2 + M 3 M l2 / 2 + M 1 M 2 . 2 )
i{(M 2 +M3)x 2 +(M 3 +M 1 ) 2 / 2 +(M 1 +M 2 )x 2 }=:l.
The above equations are exactly those of Fresnel's wavesurface, with
the following semiaxes: —
Directions. Semiaxes.
x ' Vm 2 ' Vm 3 ;
y ' Vm 3 ' Vm^
the squares of the semiaxes of the wavesurface along each axis of
coordinates being inversely proportional to the moments of inertia of the
loaded luminiferous atoms in a given space round the other two axes of
coordinates.
The plane of polarisation at each point of the wavesurface is perpen
dicular to the direction of greatest declivity.
The equation of the indexsurface, whose radius in any direction is
inversely proportional to the normal velocity of the wave, is formed from
that of the wavesurface by substituting respectively,
r J ± !
' M x ' M 2 ' M 3 '
for
c , M 1? M 2 , M 3 ,
These equations are obtained on the supposition that the coefficient of
polarity for axes of oscillation parallel to the direction of propagation,
(which we may call A), is either very large, or very small, compared with
that for transverse axes. By treating the ratio of these quantities as
finite, there is obtained an equation of the sixth order, representing a
wavesurface of three sheets, differing somewhat from that of the propaga
tion of vibrations in an elastic crystalline solid ; inasmuch as the former
has always three circular sections, while the latter has none, unless it is
symmetrical all round one axis at least. By increasing the ratio ~
AN OSCILLATOEY THEOKY OF LIGHT. 163
without limit, this equation is made to approximate indefinitely to the
product of the equation of Fresnel's wavesurface by the following —
which represents a very large ellipsoidal wave of oscillations round axes
parallel to the direction of propagation.
Section V. — Of Reflexion and Effraction.
According to the proposed hypothesis of oscillations, the laws of the
phase and intensity of light reflected and refracted at the bounding
surface of two transparent substances, are to be determined by conditions
analogous to those employed in the hypothesis of vibrations by M. Cauchy
and Mr. Green. They are the consequences of the principle, that if we
have two sets of formula? expressing the nature and magnitude of the
oscillations in the two substances respectively, then either of those
formulae, being applied to a particle at the bounding surface, ought to give
the same results.
According to this principle, the following six quantities for a particle
at the bounding surface must be the same at every instant, when computed
by either of the two sets of formulae : —
The three angular displacements round the three axes of co
ordinates,
The three rotative forces round the same three axes.
There is, generally speaking, a change of phase when light undergoes
refraction or reflexion. It is known that we may express this change
of phase by subdividing each reflected or refracted disturbance into two,
of suitable intensities and signs; one synchronous in phase with the
corresponding incident disturbance, and the other retarded by a quarter
of an undulation. There are thus twelve quantities to be found — viz., the
amplitudes of the six components of the reflected disturbance, and those
of the six components of the refracted disturbance. To determine these
quantities there are twelve conditions — viz., the equality at every instant,
according to the formulas for either medium, of the total angular displace
ments, and of the total rotative forces, round each of the three axes of
coordinates, for the set of waves composed of the incident wave and those
synchronous with it, and for the set of waves retarded by one quarter of
an undulation.
164 AN OSCILLATORY THEORY OF LIGHT.
The results of these conditions have been investigated in detail for
singly refracting substances.
The indices of refraction of such substances are proportional to the
square roots of the moments of inertia of the loaded luminiferous atoms
in a given space. Thus, if the coefficients M', M" are proportional to
these moments in two given substances respectively, then the index of
refraction of the second substance relatively to the first is
M
/M"
In the case of light incident on a plane surface between two such
media, the axes of coordinates may be assumed respectively perpendicular
to the reflecting surface, perpendicular to the plane of reflexion, and along
the intersection of those two planes ; and oscillations round axes normal
and parallel to the plane of reflexion may be considered separately.
When the axes of oscillation are normal to the plane of reflexion — that
is to say, when the light is polarised in that plane — the formula} for the
intensities of the reflected and refracted light agree exactly with those of
Fresnel. When the reflexion takes place in the rarer medium, the
reflected light is retarded by half an undulation; when in the denser,
there is no change of phase, unless the reflexion is total, when there is a
certain acceleration of i hase depending on the angle of incidence. In the
last case, the disturbance in the second medium is an evanescent wave,
analogous to those introduced into the vibratory theory by M. Cauchy and
Mr. Green — that is to say, a wave in which the amplitude of oscilla
tion diminishes in proportion to an exponential function of the distance
from the bounding surface (called by M. Cauchy the modulus), and which
travels along that surface with a velocity less than the velocity of an
ordinary wave ; the square of the negative exponent of the modulus being
proportional to the difference of the squares of those velocities, divided by
the square of the velocity of an ordinary wave.
This is an evanescent wave of oscillation round transverse axes.
How large soever the coefficient of polarity for oscillations round
longitudinal axes may be, an evanescent wave of such oscillations may
travel along the bounding surface of a medium with any velocity, however
slow, provided the negative exponent of the modulus is made large enough.
Consequently, in framing the formulas to represent oscillations round axes
parallel to the plane of incidence, we must introduce in each medium two
such evanescent waves of suitable exponents and indeterminate amplitudes ;
one travelling along the surface with the incident wave, and the other a
quarter of an undulation behind it. The maximum amplitudes of oscilla
tion in these evanescent waves constitute four unknown quantities ; the
AN OSCILLATORY THEORY OF LIGHT. 1G5
amplitudes in the two ordinary reflected waves, and the two ordinary
refracted waves, differing by one quarter of an undulation, constitute four
more unknown quantities, making eight in all : four conditions having
been fulfilled by the waves polarised in the plane of incidence, there
remain to be fulfilled eight conditions — viz., the identity, as calculated
by the formulae for the first and second substance respectively, of the
following eight functions at the bounding surface ; the angular displace
ment, and the rotative forces, round each of the two axes in the plane
of incidence, for the incident wave, and the set of waves synchronous
with it, and for the set of waves retarded by one quarter of an undula
tion. These conditions are sufficient to determine the unknown quantities,
and to complete the solution of the problem.
The following is a general statement of the results of the solution when
the second medium is the denser. They agree with the results of the
experiments of M. Jamin, and are, in every respect, analogous to those
deduced from the hypothesis of vibrations by M. Cauchy, Mr. Green, and
Mr. Haughton.
Light polarised in a plane perpendicular to the plane of incidence, suffers
by reflexion at a perpendicular incidence no alteration of phase.
At a grazing incidence (or when the angle of incidence differs insensibly
from 90°), the phase, like that of light polarised in the plane of incidence,
is retarded by half an undulation.
The variation of phase with the angle of incidence is, in fact, con
tinuous; but it is, generally speaking, not appreciable by observation,
except in the immediate neighbourhood of an angle, called by M. Jamin
the principal incidence, where the retardation of phase is a quarter of an
undulation.
This angle differs by a very small amount, appreciable only in certain
substances, from the polarising angle, at which the intensity of light
polarised in a plane at right angles to the plane of incidence is a
minimum.
The " law of Brewster," that the tangent of the polarising angle is equal
to the index of refraction, is, theoretically, only approximately true; but
the error is quite inappreciable.
When the second medium is the less dense, the phase of the reflected
light is half an undulation in advance of its value when the second medium
is the denser.
In either case, light polarised in planes perpendicular to the plane of
incidence is less retarded — that is to say, is accelerated in phase — as
compared with light polarised in that plane, according to the following
table : —
166 AN OSCILLATORY THEORY OF LIGHT.
Angle of Relative
Incidence. Acceleration.
o
Perpendicular incidence, . . \ undulation.
Principal incidence, \ undulation.
Grazing incidence, . . .90
In the case of total reflexion, light polarised in planes perpendicular to
the plane of incidence, lias its phase more accelerated than light polarised
in that plane, by an amount to which the formulae of Fresnel give a close
approximation.
The proposed hypothesis has not yet been applied to reflexion from
doubly refracting crystals ; but there can be little doubt that it will be
found to represent the phenomena correctly.
Section VI. — Of Circular and Elliptic Polarisation.
Light polarised in a plane oblique to the angle of incidence is, generally
speaking, elliptically polarised after reflexion, the planepolarised com
ponents of the disturbance being in different phases.
According to the hypothesis of oscillations, circularly and elliptically
polarised light, being compounded of oscillations in different phases round
two transverse axes, consist in a sort of nutation of the longitudinal axis
of each luminiferous atom. The direction of this nutation, and the form
of the circle or ellipse described by the ends of the longitudinal axes, serve
to define the character of the light. The ellipse of nutation has its axes
in the same proportion with, but perpendicular in position to, those of the
elliptic orbit supposed to be described by each atom according to the
hypothesis of vibrations.
The molecular mechanism by which certain media transmit * right and
lefthanded circularly or elliptically polarised light with different velocities,
is still problematical, according to either hypothesis. The laws of the
phenomena, however, may be represented by means of the assumption^
that in the substances in question the extraneous load on the luminiferous
atoms is a function of the direction of nutation.
Section VII. — Of Dispersion.
If we assume the extent of sensible direct action of the polarity of the
luminiferous atoms to be appreciable, as compared with the length of a
wave, the velocity of propagation (precisely as with the vibratory
AN OSCILLATORY THEORY OF LIGHT. 167
hypothesis) is found to consist of a constant quantity, diminished by the
sum of a series in terms of the reciprocal of the square of the length of a
wave.
It may be doubted, however, whether this supposition is of itself
adequate to explain the phenomena of dispersion, and whether it may not
be necessary to assume, also, that the load upon the luminiferous atoms is
a function of the time of oscillation, as well as of the nature of the
substance and the position of the axes of oscillation.
In conclusion, it may be affirmed, that, as a mathematical system, the
proposed theory of oscillations round axes represents the laws of all the
phenomena which have hitherto been reduced to theoretical principles, as
well, at least, as the existing theory of vibrations ; while, as a physical
hypothesis, it is free from the principal objections to which the hypothesis
of vibrations is liable.
168 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
IX.— ON THE VELOCITY OF SOUND IN LIQUID AND SOLID
BODIES OF LIMITED DIMENSIONS, ESPECIALLY
ALONG PEISMATIC MASSES OF LIQUID.*
Intkoductory Kemarks.
1. The velocity of sound in elastic substances of different kinds, solid,
liquid, and gaseous, has been made the subject of numerous and careful
experiments, most of which are well known. The object of this investi
gation is to determine to what extent our present knowledge of the
condition and properties of elastic bodies, and of the laws of elasticity,
enables us to use those experiments as data for calculating the elasticity
of the materials ; and, also, to point out circumstances which, so far as I
am aware, have been insufficiently attended to, if not altogether overlooked,
in previous theoretical researches, and which must limit our power of
drawing definite conclusions from those experiments, until our knowledge
of molecular forces shall be in a more advanced state.
2. If it were possible for us to ascertain by experiment the velocities
of transmission of vibratory movements along the axes of elasticity of an
indefinitely extended mass of any substance, we could at once calculate
the coefficients of elasticity of that material ; for in such a mass we can
assign the direction of vibratory movement corresponding to each given
direction of transmission, and consequently the nature of the molecular
forces which are called into play, and whose intensity is indicated by the
velocity of transmission. In an uncrystallised medium, for instance, the
direction of vibration must either be exactly longitudinal or exactly
transverse with respect to the direction of transmission, so that we can
calculate from the velocity of transmission the longitudinal or the transverse
elasticity, as the case may be. In a crystalline medium having rectangular
axes of elasticity, the directions of vibration, though not always exactly
longitudinal or transverse, unless the direction of transmission coincides
with an axis, have still certain definite positions.
3. It is only in air and water, however, that such experiments are
possible. For other substances, the best experiments which it is prac
ticable for us to make, are those upon the transmission of nearly longi
tudinal vibrations along prismatic or cylindrical bodies. Were we able
* Eead before the British Association at Ipswich, July 3, 1851, and published in the
Cambridge and Dublin Mathematical Journal, Nov., 1851.
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 169
to ensure that the vibrations of those prisms and cylinders should be
exactly longitudinal, we might compute from their velocity of transmission,
as from that of such vibrations in an unlimited mass, the true longitudinal
elasticity. This we can do for gaseous substances, as M. Wertheim has
proved (Ann. de Chim. et de Phys., Se>. III., torn, xxiii.), by making the
organpipes in which they vibrate of proper construction.
In liquid and solid columns, on the other hand, it is impossible to
prevent a certain amount of lateral vibration of the particles, the effect
of which is to diminish the velocity of transmission in a ratio depending
on circumstances in the molecular condition of the superficial particles,
which are yet almost entirely unknown.
4. It has, indeed, been sometimes supposed, that the coefficient of
elasticity, as calculated from the vibrations of a solid rod, is that called
the weight of the modulus of elasticity — that is to say, the reciprocal of the
fraction by which the length of a rod is increased by a tension applied
to its ends of unity of weight upon unity of area; that coefficient being
less than the true coefficient of longitudinal elasticity, because the lateral
collapsing of the particles enables them to yield more in a longitudinal
direction to a given force, than if their displacements were wholly
longitudinal.
This conjecture, however, is inconsistent with the mechanics of vibratory
movement; and, accordingly, experiment has shown that the elasticity
corresponding to the velocity of sound in a rod agrees neither with the
modulus of elasticity, nor with the true longitudinal elasticity; although
it is, in some cases, nearly equal to the former of those quantities, and in
others to the latter.
5. In liquids, it has been shown by the experiments of M. "Wertheim
(Ann. de Chim. el de Phys., Se>. III., torn, xxiii.), that the velocity of sound
in a mass contained in a trough, and set in motion through an organpipe,
bears to that in an unlimited mass the ratio of ^2 to y/3. This has led
him to form the conjecture, that liquids possess a momentary rigidity
for very small molecular displacements as great in comparison with their
other elastic forces as that of solids. This conjecture, paradoxical as it
may seem, would indeed be necessary to account for the facts, if the
supposition I have already mentioned were true, that the velocity of
sound in a rod depends upon the modulus of elasticity. I shall show,
however, in the sequel, that if we suppose that at the free surface of
every mass of liquid, an atmosphere of its own vapour is retained by
molecular attraction under certain conditions of equilibrium, the ratio
mJ2 : *J3 between the velocities of sound in a prism and an unlimited
mass, is a consequence of the equations of motion in all cases in which
the liquid has any rigidity whatsoever, even although so small as to be
insensible by any means of observation; so that the supposition of a
170 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
rigidity for small displacements equal to that of solids becomes unneces
sary.
6. With respect to solids, all that theory is yet adequate to show us
is, that the velocity of sound along a rod must be less than in an unlimited
mass, a conclusion in accordance with experiment. The precise ratio
depends on properties of the superficial particles yet unknown.
General Equations of Vibratory Movement in Homogeneous
Bodies.
7. Having now stated generally the objects of this paper, I shall
proceed, in the first place, to the mathematical investigation of the integrals
of the general differential equations of vibratory movement in homo
geneous bodies; because, although those equations have already been
integrated by many mathematicians, it will be necessary in this paper to
introduce functions into the integrals which have hitherto been almost
totally neglected in such researches; having been applied only to the
theory of waves rolling by the influence of gravity, to that of total
reflexion, by Mr. Green (Camb. Trans., Vol. VI.), and by Professor Stokes
to represent the gradual extinction of sound by its conversion into
heat.
8. Let g represent the accelerating force of gravity :
D the weight of unity of volume of a homogeneous substance, having
orthogonal axes of elasticity whose directions are the same throughout
its extent ;
A p A 2 , A 3 , the coefficients of longitudinal elasticity for the axes of x,
y, z, respectively :
B 1? B 2 , B 3 , the coefficients of lateral elasticity ; and
C v C 2 , C 3 , those of rigidity for the planes of yz, zx, xy, respectively ;
£, 7), £, the displacements of a particle parallel to x, y, z, respectively.
Then, it is well known that the differential equations of small vibratory
movements are the following, when small quantities of the second order
are neglected :
. / D d? d? , n d? , n d?\ t
g dt 2 1 dz 2 3 dy
d' 2 n d 2 ?
+ (B 3 + C 3 ) £4 + (B 2 + C 2 ) f±
x •* 6I dz dy v l u dz dz
_/ D d? « <? d* LP i 2 \
~ Vlj'W + ^da* + 2 dy* + Ul rf7V
(1.)
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 171
\ g df "dx l l dy "dz/
of which the integrals are
I = ^{L^i^e.t + ax + (3y + yz+ k)}
n .= S {L 2 0(^ £ J + aa + j8y + y 2 + *)}
£ = 2{L 3 0(Ve.* + a^; + /3y + ys + *)}•
^ • (I)
Y ■ (2.)
The form of the function <£ being arbitrary, subject to a restriction
to be afterwards referred to, and 2 extending to any number of terms,
the coefficients of which fulfil the following conditions. Let
w 1 = A x a 2 + C 3 /3 2 + C 2 y 2
w , = C 3 a 2 + A. 2J 8 2 + C 1 y 2
u, 3 = C 2 a 2 + C 1 /3 2 + A 3 y
Pl = ^ 1 .+ C 1 )i8 7
Pi — ( B 2 + C 2) 7 a
p 3 = (B 3 + C 3 )«/3
9
(a.)
E
Then the following equations must be satisfied by the coefficients of
each set of terms in equation (2) :
= L x ( Wl  E) + L 2 p 3 + L 3 p 2 
= L lP3 + L 2 (a> 2 E) + L 3/3l [ . . (3.)
= L l Pi + L 2 P\ + L 3 ( W 3 — E ) „
By elimination we transform those equations as follows : let
G = w x + w 2 + (o 3 ,
H = (o. 2 w 3 + (o s w 1 \r w x Wo — p^ — p 2 — Pz> r* (b)
K =. w x Wo <*) z + 2 pj p 2 p 3 — w x p* — w 2 p 2 — h> z p 3 2 . J
172 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
Then for each set of values of a, /3, y, E has three values which are
the roots of the cubic equation,
= E 3 GE 2 + HEK; . . . (4.)
so that ^e has six values, three positive and three negative, of equal
arithmetical amount.
The absolute values of L v L 2 , L 3 , are arbitrary, but their mutual ratios
are fixed by the following equations :
L 1 {(w 1 E)^ 1 p : ,p. i }=L 2 {(w 2 E)p. 2 p3p 1 } "J
consequently, they have in general three sets of ratios for each set of
values of a, ft, y, corresponding to the three values of E.
9. The condition that the motions of the particles of the body must be
small oscillations restricts the variations of the displacements £, rj, £, within
certain limits. Now, as the time t increases ad infinitum, this can be
fulfilled only when each of those quantities is either a periodical circular
function of t, or a function developable into a sum or definite integral
of such functions. We may, therefore, make each of the functions <j> a
trigonometrical function of t. This being the case, those functions must
be either trigonometrical or exponential with respect to x, y, and z, or
compounded of both, being trigonometrical so far as a, (3, y, are real,
and exponential so far as they are imaginary.
If we suppose each of these coefficients to consist of a real and an
imaginary part, then each of their functions which enters into the equations
of condition, will also consist of a real and an imaginary part. Each of
the equations of condition thus becomes divided into two, which must be
separately satisfied.
Thus we arrive at the following results :
For the symbol <£ { }, put e 2 Wl { }; so as to make £, &c, trigono
metrical with respect to t. Let X be a line of such a length that
and let
a? + l 2 + c 2 = 1,
a = z(+a— a J — 1),
= £(T&&V1).
y = <( + c~ cV~ !)
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 173
also, let
Li = I + l's/ — 1> L 2 — m + m ' s/ — li L 3 = w + n'*/ — 1 ;
so that the displacements become
{a'x + b'y + c'zdz Vl(V e  iaxJycs)} 
rj = S {terms in m, m], £ = 2 {terms in w, w'}
> a)
J
Let the quantities in the equations of condition be thus represented
«i i\ ± iAV !> &c  ; pi = & ± 2W *> &c  ;
g = g ± aV i ; H = & ± &V 1 ; K = k ± kV 1
The equations of notation now become
Pl = A x (a 2  a' 2 ) + C 3 (6 2  6' 2 ) + C 2 (c 2  c' 2 )
^ 2 = C 3 (a 2  a' 2 ) + A 2 (6 2  b' 2 ) + C x (c 2  c' 2 )
j? 3 = C 2 (a 2  a' 2 ) + C^fi 2  6' 2 ) + A 3 (c 2  c' 2 )
< ?1 = (B 1 + C 1 )(&c6V)
#2 = (^2 + ^2) ( ca ~ c ' a ')
q 3 = (B 3 + C 3 )(aba'b')
Pi = 2 (A x a a' + C 3 6 6' + Co c c')
p 2 = 2 (C 3 a a' + A 2 6 6' + Cj c c')
j9 3 ' = 2 (C 2 a a' + C x b b' + A 3 c c')
?1 ' = (B 1 + C 1 )(6c' + 6 / c)
? 2 ' = (B 2 + C 2 )(ca' + c'a)
? 3 '=(B 3 + C 3 )(a&' + a'£)
— = Las before, or c = ^r
9 D
q =Pl+P 2 + p 3 , q' =pj + 2h + #$'
& = P 2 #5 +^1 + ^l2>2 ~ ^l 2 ~ ?2 2  ?3 2
P2P&P3P1P1P2 + 9i 2 + <li l + 9z 2
# =P 2 P3 +P2P3 +PzP\ + #$>i + Pi Pz+PlPi
2q 1 q 1 , 2q. 2 q 2 '2q 2 q 3 '
H =p l p. 2 p z +2q l q 2 q s p 1 q 1 2 p 2 q 2 2 p. 3 q^
M c )
He)
174 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
ftft'ft'  ft'ft ft' "ft'ft'ft
+Pi ft' 2 + ft ft' 2 + ft ft' 2 + 2 (ft.' ft ft' + ft'ft ft' + ft' ft ft')
K=pMp& + ^^'^IsPi^P^Pil^
 ft'ft ft  ftft'ft  Pi P2P3
 2 (fc'ft ft + ft ft' ft + ft ft ^s')
+ Kft 2 + iVft 2 +Kft 2 + 2 (/ ? lftft' + ^ftft' + ^3ftft')
Also, let
*! = (^  E) q x  p{q x '  ftft + ft'ft'
*2 = (ft  E ) ft " ft'ft'  ft ft + ft'ft'
*3 = (ft ~ E ) ft  ft'ft' ~ ft ft + ft'ft'
%l = (^  E) jfc' + ^'ft  q 2 2s  ft'ft
X 2 ' = (p 2  E) q 2 ' + ^ 2 '<7 2  ft ?/  ? 3 'ft
*s = (ft  E ) ft' + ft'ft  ft ft'  ft'ft J
Then the equations of condition relative to the coefficients become the
following : —
= E 8 flE2+ jEh,
= g'E 2  b'E + k', •
lt x + 1'X{= m% 2 + m'X 2 ' = nx z + w%' "
lx{ l'X x = mXo m'%2 =n% & '  n'x z . J
(8.)
(9.)
(10.)
The three original 'equations of condition are transformed into the
following six, to which (8), (9), (10) are equivalent : —
= I (p x — E) r I'pi + mq s + m'q 3 ' + nq 2 + n'q. 2
= l'(Pi — E) — lp x + m'q z — mq s ' + n'q 2 — nq 2
= I q z + l'q z ' + m (p 2 — E) + m'p 2 + nq x + n'q x
= l'q s  lq z ' + «i'(ft ~ E)  mp z ' + n'q x  nq x
= lq 2 + l'q 2 + mq x + m'ql + n{p 3  E) + rip z
= l'q 2  lq 2 + m'q x — mq x + n'(p s — E) — np z
y (10A.)
To give an intelligible result, the terms of the series in equation (7)
must be taken in pairs, with the imaginary exponents in each pair of
equal arithmetical value and opposite signs.
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
Hence equations (7) are equivalent to the following : —
— {a'x + b'y + cz) f 2 tt .
e x J I. cos— (^Z e . i — ax — by — cz)
2rr ") "1
+ T . sin — (^/e .t — ax — by — cz)r
r\ = S {terms in w, m'}, £= 2 {terms in w, ft'}
175
^ (H)
The above equations (11), together with the equations of condition (8),
(9), (10), or their equivalent (10A), and the equations of notation (c),
contain the complete representation of the laws of small molecular
oscillations in a homogeneous body of any dimensions and figure ; it being
understood that in the symbol of summation 2 are included as many
definite integrations as the problem may require with respect to inde
pendent variables of which the coefficients A, ^ z,a,b,c,a' ,b' ,c ,l,m,n,l' ,m ,n ,
are functions.
As there are fourteen coefficients, connected by seven equations — viz.,
a?\b 2 {c 2 =l, and the six equations of condition, the greatest number of
independent variables is limited to seven ; therefore, in the most general
case, the symbol 2 {...} in equations (11) may be replaced by
^ffflffl^v 0» %, e v dp e 6 , e 7 ) { . . } do v ae v ae 9 de„ ae„ ae & ae 7 , (i 2.)
V &c, being variables of which the coefficients are functions, and F an
arbitrary function.*
10. Let us consider the physical meaning of a single set of terms of
the sums in equations (11), containing but one set of values of the
coefficients. It represents a system of plane waves, the wave surfaces,
or planes of equal phase, in which are normal to the line whose direction
cosines are a, b, c. A is the length of a wave measured along that line.
2 7T I
7T (*Je .t — ax — by — cz)\ tan 1 y
~t (s/s .t — ax — by — cz) + tan 1 — >
A ' m
— (*/s .t — ax — by — cz) + tan 1 7
are the
phases of
vibration
for
%,
*To make the functions in equations (11) satisfy the conditions of equilibrium,
instead of those of oscillation, it is only necessary to make s = 0, and to substitute
h=0,k' = 0, for equations (8) and (9). Some additional functions, however, are necessary
in order to complete the values of £ , n, £.
176 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
^/e = J(—^ ) is the normal velocity of propagation, —j is the periodic
time of an oscillation of a particle
and the corresponding expressions in m and n are the semiamplitudes of
vibration parallel to x, y, z, respectively; a, b\ c, are proportional to the
directioncosines of a normal to a series of planes of equal amplitude of
vibration.
The trajectory of each particle affected by a single series of plane
waves is in general an ellipse, the position and magnitude of which are
2 7T
found as follows. Let (p denote the value of (y/e . t — ax — by — cz),
which makes the total displacement ^/{^ + tj 2 + £ 2 ) a maximum or
minimum. It is easily seen that
where
J2 + m 2 + n 2 _ p _ m >2 _ n t
1 1' + in m + /i n'
The values of £, tj, ^, calculated from <f> by equations (11), are the
coordinates of the extremities of the axes of the elliptic trajectory, referred
to the natural position of the particle as origin.
The processes of summation and definite integration denote the repre
sentation of an arbitrary manner of oscillation by the combination of a
definite or indefinite number of systems of plane waves.
Case of an Indefinitely Extended Medium.
11. Let the medium, in the first place, be supposed to be indefinitely
extended in all directions. This case having been thoroughly investigated
by MM. Poisson, Cauchy, Green, MacCullagh, Haughton, Stokes, and
others, I shall give merely an outline of the general results. The condition
that the motion shall consist of small oscillations, here makes it necessary
that the exponential factor in the displacements should in all cases be
equal to unity, and therefore that
a' = ; V = ; e = j
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
177
and, consequently", each of the accented symbols in equations (c) = 0.
Equation (9) vanishes, and the normal velocity of propagation for each
set of directioncosines a, b, c, has, generally speaking, three values, corre
sponding to the three values of E, roots of equation (8). Equations (10)
become
, ,, , , 1 1 1
I : m : n : : I : m : n : :  :  : — .
r, X., r.
• (14.)
consequently, the phases of £, rj, £, are simultaneous; so that >v /(^ 2 + Z /2 ),
x /(m 2 + m' 2 ), *J(n 2 + «' 2 ), are proportional to the directioncosines of a
rectilinear vibratory movement of the semiamplitude +J(l 2 \l' 2 \m 2 + m' 2
\n 2 \n' 2 ), which cosines have in general three sets of values corresponding
to the three values of E. It is easily shown that those three directions
are at right angles to each other. The number of coefficients being in
this case reduced to eleven, connected by six equations — viz., a 2 + 6 2 +c 2 = 1,
equation (8), and the proportional equation (14), which is equivalent to
four, the greatest number of definite integrations in the operation (12)
is restricted to five.
Thus it appears that the velocity of transmission of vibratory movement
through an indefinitely extended mass, has a set of definite values, not
exceeding three, for each position of plane waves. When the direction
of propagation coincides with an axis of elasticity, we find those values
to be :
For
vibrations
parallel
to
Velocity of propagation along
X .
X
m
%
m
V ■ ■
m
m
m
z . .
m
m
V(¥)
(15.)
When the substance is equally elastic in all directions, we have simply,
Velocities of propagation in any direction for longitudinal vibrations
V(t&
178 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
For transverse vibrations, in any direction perpendicular to that of
propagation,
m
Hence, experiments on the velocity of sound in an indefinitely extended
mass, or one so large as to be practically such, afford the means of directly
calculating the coefficients of elasticity.
General Case of a Body of Limited Dimensions.
12. It is not so, however, in a body of limited dimensions; for the
coefficients a, V , c, in the exponents of the exponential factors, are no
longer necessarily null, but have values which must depend on the mole
cular condition of the external surface of the body, and on the forces
applied to it. The velocity of propagation is no longer a function of the
directioncosines a, b, c, alone, but also of the coefficients a, b', c. It
has in general but one value, corresponding to the common root of the
equations (8) and (9). By substituting successively the two roots of
equation (9) — viz.,
6' + \(\r r
for E in equation (8), the latter is converted into two alternative equations
between the six quantities a, b, c, a', b', c\ showing the relations they must
have in order that equations (8) and (9) may have a common root.
In the only particular problems, however, of which I shall here give the
solutions, those relations are obvious without going through that process,
for they belong to a class of cases in which the three quantities g', ])', f{',
have a common factor; which being made = 0, the necessary conditions are
fulfilled.
It is obvious that in all cases the effect of the coefficients a', b', c, is
to diminish the velocity of propagation.
1 3. The following are the values of the three components of the velocity
of a particle :
2* 1
d£ JT2ir , y {a'z + b'y+e'z) C ^
^7= 2 I y V E  C { lsm—(^/£.taxbycz)
+ V cos ~ (J t . t ax  by  cz) } ] f ( 16 )
t.—^ {terms in m, m) ; y = 2 {terms in n, n).
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 179
The strains, or coefficients of relative molecular displacement, are as
follows :
Along
T2
tr —{a'x + b'y + c'z)
Longitudinal Strains.
■j {la —I' a) cos— (y/s .t — ax— by— cz)
\ (la + 1' a') sin —(^e.t — ax— by — cz) r
(1 11
p = 2 {terms in (in V — m b) } (m b + m' V) }
7 Y
5 = 2 {terms in (nc'—n'c), (nc\n'c)}
(17.)
In the
plane
yz
xy
z^dy Z L
Distortions.
2tt — (a'x + b'y + c'z)
dz di 1
C / 27T
1 (mc'—m'c \nV — n'b) cos — (^A . t— ax— by— cz)
+ (wc +m'c'f7i5 +?''^ / )sin — (^/s.t— ax— by— cz) r
dZ d£
j + 7 =2 {terms in (na' — na \lc—l'c),
(na {n'a \lc\l'c')}
jy 7
j +— = 2 {terms in (Ib' — l'b \ma —rn'a),
Lb if Cv JO
(lb \l'b'\ma +mV)}
J, (17 A.)
The pressures on the coordinate planes, arising from those strains
(using the notation of my paper on the Laws of Elasticity, Cambridge and
Dublin Mathematical Journal, February, 1851), are the following: —
On
yz
Normal.
n
*> — **f.*
—  — B„ 
dy 2 d
JZ
180 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
L2tt —(a'x + b'y + c'z) (/.,,, r ,
~e x  [A^la la)
\ 2 7T
+ B 3 (m V  m'6) + B 2 (« c'  n'en . cos zr^e.t — ax — byc .:)
+ (A x (/a + Va) + B 3 (m6 + m V) + B 2 (» c + »' c'))
sin   (Je .t — a x — ly — e .:
A
)}]
(18.)
The other normal pressures are found by substituting symbols accord
ing to the following table : —
Plane. Pressure. Coefficients.
yz . . . P x . . . A lt B 3 , B 2 ,
zx . . . P 2 . . . B 3 , A 2 , B v
■' y
B 2 , B p A 3 .
Tangential I'r
Plane of
Distortion Along On the plane
"
xy
fa
y*1
y*J
*=*®+fi
Case of an Uncrystallised Medium.
14. I shall now take the particular case of an uncrystallised medium,
in which the coefficients of elasticity are the same for all axes, and may
be represented thus : —
rigidity = C ; fluid elasticity = J ;
longitudinal elasticity A = 3 C + J,
lateral elasticity B = C + J = A— 2 C.
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 181
The position of the axes being in this case arbitrary, I shall take the
direction of propagation as the axis of x, so as to make
o=l, b = 0, c = 0.
To fulfil the condition that equations (8) and (9) shall have common
roots, we must make
a = 0,
being in this case a common factor of g', j', fe'.
The equations of notation (c) now become
i 7 2 = A6' 2 + C(lc' 2 )
^ 3 = Ac' 2 + C(l V 1 )
2l = (AC)6V; q 2 = 0; q, =
g = (A + 2C)(lZ/ 2 c' 2 )
ft = (2AC + C 2 )(1 b' 2 c' 2 ) 2
fe =AC 2 (1 6' 2 c' 2 ) 3
g' =0; jj'=0; fe'=0
'1 = (Pi — E )?i + fe'&'i r 2 = °; r s = °;
Xi=0; x:=(p 2  e)? 2 ' &'&; r s ,= (#$ E )&' &&'• ^
Hence it appears, that for an uncrystallised medium, equation (8) has
three roots, viz., —
r (d.)
one root . . E = A
two equal roots, each E = C
i(i  & ,a  n, \
) (1 _ tf* _ C '2). J
(19.)
So that the velocity of propagation is less than that in an unlimited mass,
in the ratio ^/(l — &' 2 — c' 2 ) : 1. Equation (9) disappears.
Equations (10) become
or
lx 1 = m'x% = w'r 3 '
— Vx 1 = ror 2 ' = nx 3 '
I : m : n' : : — I' : m : n
(20.)
i.i.I !
ti f a r' 3 J
(20A.)
182 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
Equations (10A) become
= I {p x — E) + m'jg + riq 2 '
= r(jpj  E)  m j g '  n j,'
= Z'g s ' + m (jp 2 — E) + n q x
= — lq^ + m {p, — E) + »'&
= l'q 2 ' + m q 1 + n (p 3  E)
=  lq 2 ' + m'q 1 + n'(jp 3  E) ^
15. It may be shown that the vibrations corresponding to the roots C
(1_Z/2_ C '2) canno t take place in a body of which the surface is free,
unless V = 0, c = 0, in which case they are reduced to ordinary transverse
vibrations. (See Appendix, No. II.)
Nearlylongitudinal Vibrations in an Uncrystallised Medium.
16. For the present, therefore, I shall confine the investigation to
the root
E = A (1  V  c' 2 ),
corresponding to the velocity of propagation
V—V{^< 1 '* *">}• • • (21 >
The vibrations to which this root is applicable may be called nearlylongi
tudinal; because in them the longitudinal component predominates, and
their velocity of transmission is a function of the longitudinal elasticity A.
This value being substituted for E in the expressions for X v &c, gives
' = — V I ; n = — c I }
. = V X ; ri = c X. J
. (22.)
Which values being substituted in equations (11), (16), (17), (18), give
the following results : —
For brevity's sake, let
also, let
$ = 2 { — e^ (Z' cos $  I sin 0) J .
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 183
Then the displacements are
£ = 2 {e* (Z cos (f> + V sin <p)} =
n = 2 {6' e^ (I' cos — Z sin <£)} =
£ = 2 {c' e^ (/' cos  J sin 0)} =
da
<Zs
(23.)
The velocities of the particles are :
^5 = 2  ^ ^ • «* (*' cos  Z sin 0) }
^=2J^ r /v /^ & '^G cos + rsin 0)} [" (24.)
dZ
=  ? \ ^ +/e . c e* (I cos $ + I' sin 0) J
The longitudinal strains :
dj
dx
=  2 j ^e^ (Z' cos  Z sin 0) J
^ = 2  ~6'2 e * (Z' cos  Z sin 0) j
^ = 2 j ^ c" 2 ^ (r cos  Z sin 0) }
a z (. A J
The total change of volume :
.xdvdz IX j
y (25.)
The distortions :
^ + ^ = 22 j ^h'c'e* (V cos rf>  Z sin 0) }
<Z,s tfy l A J
^ + ^ = 22J^c'^(Zcosri + Z'sin 9 f>)} V (25.)
dx dz I A J
^ + ^ = 22 ( ~ b'e+{l cos + r sin 0) }
dy dx IX J J
> (26.)
184 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
The pressures due to the displacements are as follows :
Normal Pressures.
Pj = 2r^ r ^{A(lPc /2 ) + 2C(&' 2 + c' 2 )}(r cos 0Z sin tf>)J
P 2 = 2 r^e^{A(l&' 2 r' 2 )2C(lc' 2 )}(rcos</>?sin0)]
P3 = 2r~ 7r e^{A(li /2 c' 2 )2C(l^ 2 )}(rcos«/,^sin^
The tangential pressures % Q 2 , Q 3 , are found by multiplying the
distortions by — C.
Let Rj, R 2 , R 3 , be the three components of the pressure exerted by
the particles of the body, in consequence of the molecular displacements,
at any part of its external surface, the normal to which makes with the
axes the angles a, /3, y Then
R x = P x cos a + Q 3 cos /3 + Q 2 cos 7
R 2 = Q 3 cos a + P 2 cos /3 + Q x cos y
R 3 = Q 2 cos a + Qi cos /3 + P 3 cos y ^
Should there be any surface along which the particles are constrained
to slide, it is obvious that at that surface the following condition must
be fulfilled :
= £ cos o + i? cos /3 + Z cos y ;
or if z x = / (x, y) be the equation of the surface,
f
(27/)
(28.)
J2. d z x
dx dy
Were we acquainted with the laws which determine the superficial
pressures in vibrating bodies, equations (27) would enable us to determine
the values which b' and c must have, in virtue of those laws, during the
transmission of sound in a limited mass of an uncrystallised material,
and thence the ratio ^/(l — Z>' 2 — c' 2 ) : 1, in which the velocity of sound in
such a body is less than in an unlimited mass of the same material.
Those laws, however, are as yet a matter of conjecture only.
Transmission of a Definite Musical Tone.
17. When the body transmits one or more definite musical tones (which
is the case in all experiments capable of yielding useful results), the
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
18/
50
velocity of propagation must be the same for all the elementary vibrations
into which the motion may be resolved: that is to say, 1 — b" 2 — c' 2 must
have the same value in all the terms of the sums 2. This affords the
means of simplifying the equations. Let
6'2 + c '2 = tf . i>  ] h cos q . c ' = hsin 6;
h being the same for all the terms in the sums 2. Then the velocity of
propagation is
' ^ = a/{tt (1 * 2) } • • • (29 °
and this factor may be removed outside the sign of summation.
2 7T
When but one musical tone is transmitted, the factor y also may be
removed outside that sign, and for 2 { } may be substituted a definite
integration,
2/F0 { . . . }d0,
F 6 being arbitrary.
We have also
r
e* = e x
h (y cos 6 + z sin 6)
(30.)
2tt
in which : ~ h, y, and z, are independent of 9, and may be treated as
A
constants in the definite integration.
Introducing these modifications into equations (23), &c, we find
$ = A (l' cos <j>  I sin <f) 2/«* FQdO
Displacements.
l = {l cos <p +1' sin <j>) 2/^ F OdO
r, = {V cos $  I sin tfh.'S/cos 6e*F 9d0
I = (l f cos <j>  l sin 0) h. 2/ sin 0e*F0d0
> (31.)
Velocities of the Particles.
~ = ^V £  {V cos  I sin 0)2/^ F0d0
ttf A
^ =  ^ ^e. (Z cos + r sin 0) A. 2/cos 0^ F 0c7
(it A
1SG VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
j^= ^Js.Qcos <p + r sin 0) ft. 2/ sin Oc^FOdO
dt
dx
Longitudinal Strain*.
27r (/' cos  I sin <j>) 2/e^ F6d0
p = ?£(T cos<j> l8m$)h 2 .'2fcoB 2 0e*F6d0
^? = ^ (7 cos  Z sin <f) h 2 . 2/ sin 2 Oe^FBdO.
clz A
CuJ/c Dilatation,
d% dri dZ
dx dy d :
*T ,v
{t cosfl sin <£) ( 1  /r) 2/e* F d 0.
Distortions.
7 7X A
," + ', = J (I' cos d>  I sin <h) h 2 /cos sin 0e*F0d0
dz dy A
^? + ^ = 1^(Z costf, + r sin 0)A2/ S in 0e*F0<Z0
J~ + P= y (' costf, + Z' sin 0)&2/cos de^FOdO.
Which, being multiplied by — C, give the tangential pressures
Qi» Q» Q 3 , on the coordinate planes.
Normal Pressures on the Coordinate Planes, due to the
Displacements.
P x = = (/' cos $  I sin <f) {A (1  /j 2 ) + 2 C/r} 2/ e ^ F d
p 2 = ^ (*' cos *  * sin 0) [{A (1  !r)  2 C} 2/e^ F 6 d
+ 2C/r2/sin 2 0^F0</0]
2tt
P 3 = — (T cos<j>l sin 0) [ {A (1  F)  2 C} 2/e^ F 0rf
+ 2C/?2/cos 2 0<^F0<2 0]. J
(31.)
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
187
Let R x , R 2 , R 3 , be the components of the pressure exerted by the body,
in consequence of the molecular displacements, at a point of its surface
normal to the direction (a, /3, y). Also, let
cos (3 — sin a cos X ,
cos y = sin a sin x ,
so as to make x the axis of polar coordinates, and xy the plane from which
longitudes X are measured. Then,
I> = ?£[cosa(rcos0Zsin0){A(l A 2 ) + 2CA 2 } Ife+VBdB
2 sin a (Z cos <f> + X sin <p) C /t 2/cos (0  X ) e^ F d 0]
2tt,
R 2 = — [  2 cos a (Z cos + Z' sin <p) C A 2/cos e^ F 0<Z
+ sina(rcos«/,/sin^){cos x (A(l/r)2C)v^F0 ( /0
+ 2 C /r 2/sin sin (0  x ) e^ F d 0}]
R = — [  2 cos a (/ cos + I' sin 0) C h 2/sin e^ F d
A
+ sin a (r cos tf>  / sin j,) {sin X (A (1 A 2 )  2 c) 2/«* F rf
 2 C /r 2/cos s i n (0 _ x ) c ^ F rf 0}] ,
[> (32.)
Let P' represent the normal pressure at the given point of the surface
due to molecular displacements : then
P' = R x cos a + sin a (II., cos X + R 3 sin X )
= P x cos 2 a + P 2 sin 2 a cos 2 X + P 3 sin 2 a sin 2 X
f 2 Q x sin 2 a cos X sin x + 2 Q 2 cos a sin a sin X
+ 2 Q 3 cos a sin a cos x
2tt /
= — [(? cosf—l sin 0) {(A (1 — lr) )(32A)
+ 2 C (/i 2 cos 2 a  sin 2 a) ) 2/V* F d
+ 2 C K sin 2 a 2/sin 2 (0  X ) c^ F 6d 0}
— 4(/cos0 + *' sin <p) C A cos a sin a 2/ C0S (0  x) e * FedB
188 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
Propagation of Sound by Nearlylongitudinal Vibrations along
a Horizontal Prism of Liquid contained in a Rectangular
Trough, investigated according to a Peculiar Hypothesis.
18. I shall now suppose the vibrating body to be a rectangular
horizontal prism of liquid contained in a trough of some substance so
dense, hard, and smooth, that the particles at the sides and bottom of the
trough are constrained to slide along those surfaces, and that the vertical
ends of the trough are capable of perfectly reflecting a wave of sound
travelling horizontally; so that the propagation of that wave may take
place as if in a trough of indefinite length ; and I shall investigate the
velocity of such a wave according to a peculiar hypothetical view of the
molecular condition of the upper surface of the liquid.
The axis of x being the horizontal axis of the trough, and parallel to
the direction of propagation, let that of y be transverse, and that of 2
vertical. Let the middle of the bottom of the trough be the origin of
coordinates, 2y 1 being its breadth, and z 1 the depth of liquid in it.
The conditions to be fulfilled at the bottom are, when
z = 0, a = ^ ir, and x = ~ i ~
Let
, h y cos 6 „ . , „
2/sin0^F0(Z0 = 2/sin0c A * F6dd =
at the sides, when
y = ± y v a = l ■ ir, and y_ = or 7r.
Let
2r
t „ — ^(±2/1 cos0 + 2Sin4)
2/cos0^F0 f Z0 = 2y"cos0e* F0<Z0 = O;
which conditions are fulfilled by making
cos = 0, sin = ± 1,
and putting for 2/F 6 d 6 a summation of two terms in which the signs
of the exponent are respectively positive and negative.
Thus we obtain
~^1,
£ = (I cos $ + V sin (p) \e A + e
7/ =
Z, = (I' cos — I sin (f)h\c x — e x / >
y (33.)
VELOCITY OF SOUND IX LIQUID AND SOLID BODIES. 189
The trajectory of each particle is an ellipse in a vertical longitudinal
plane ; the motion being direct in the upper part of the ellipse, because
the sien of — is the same with that of £. The axes are vertical and
° at
horizontal respectively, and have the following values : —
 V ,
hz  —hz
Horizontal axis = 2 J (I 2 + I" 2 ) . \e x
Vertical axis = 2 J (I 2 + l' 2 ) . h. \e * *  e * 7 ;
so that the motion is analogous to that of waves propagated by gravitation,
being entirely horizontal at the bottom of the trough, and elliptical else
where, the ellipse being larger and less eccentric as the height above the
bottom increases. The ratio of the axes, however, instead of approaching
equality as the depth of the trough increases (which is the case with waves
of gravitation), approaches 1 : h.
19. To determine this ratio, upon which the velocity of sound along
such a mass of liquid must depend, I shall assume the following hypo
thetical principles respecting the state of the particles at the upper surface :
First, That (as laid down in a previous paper, Cambridge and Dublin
Mathematical Journal, February, 1851) the elasticity of bodies is due
partly to the mutual actions of atomic centres producing elasticity both
of volume and figure, and partly to a mere fluid elasticity resisting change
of volume only, and exerted by atmospheres surrounding those centres ;
and that the effect of the mutual actions of the atomic centres in producing
pressure is very small in liquids, and absolutely inappreciable in gases
and vapours.
Secondly, That every liquid maintains at its surface, by molecular
attraction, an atmosphere of its own vapour, under these conditions —
that the total pressures of the liquid and vapour, and also their fluid
pressures, shall be equal at the bounding surface. (From this hypothesis
I have already deduced the form of an approximate equation between the
pressure and temperature of vapour at saturation.) The total pressure
of the vapour on the liquid is sensibly equal to its fluid pressure ; the
total pressure of the liquid on the vapour consists of its fluid pressure,
and a pressure due to atomic centres ; the latter quantity must, therefore,
be null.
Thirdly, That the pressure of the vapour follows that of the liquid
throughout its variations during the propagation of sound; so that the
portion of the pressure of the liquid on the vapour, due to atomic
centres, must continue null throughout these variations.
Let & be the mutual pressure of the liquid and its vapour in a state
190 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
of rest, then a» + P' is their momentary mutual pressure during the passage
of a wave of sound horizontally along the trough. The portion of P'
depending on the coefficient of rigidity C being made = 0, we shall
obtain an equation from which the value of h may be deduced.
Making the proper substitutions in equation (3 2 A), viz. —
cos a — 0, sin a = 1, cos ^ = 0, sin ^ = 1, \L = ± st h ..
cos 6 = 0, sin = ± 1, F 6 = 1, z = z v &c,
we find
/2r. _2» \
w + r'=w+~(rcos^/sin0){A(l/t 2 )2C}Vc x Zl +e x 7
A
The part of this depending on mere fluid elasticity, in which the liquid
is followed by the vapour, is
\dx dz
/ 2 t 2 *
2 7T ( h ~l TnZi
= u+ T (/' cos (p — I sin 0) . J (1 — Ir)\c x + e *
A
which, being subtracted, there remains for the part depending on atomic
centres,
/ 2* It \
A
Consequently,
= ^f (T cos  Z sin <j>) C (1  3 h°)\e * + e
1  3/i 2 = 0, or h = Jl . . . (33.)
is the equation of condition sought, arising from the state of the free
surface ; and this equation is independent of the amount of rigidity of
the liquid, requiring only that it shall be something, however small, while
that of the vapour is null.
It follows from this equation, that the velocity of propagation of sound
along a trough of liquid of the density D, and longitudinal elasticity A, is
+= J {&»}=&¥)• ■ <*>
or less than the velocity in an unlimited mass in the ratio of *J2 to y/3.
20. This is precisely the result arrived at by M. Wertheim from a
comparison of his numerous experiments on the propagation of sound in
water at various temperatures, from 15° to 60° Centigrade, in solutions of
various salts, in alcohol, turpentine, and ether (Ann. de Chim,, Ser. Ill,
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 191
torn, xxiii.), with those of M. Grassi on the compressibility of the
same substances (Comptes Rendus, XIX., p. 153), and with the experiments
of MM. Colladon and Sturm on the velocity of sound in an expanse of
water.
M. Wertheim having given this comparison in detail, I shall quote one
example only.
The velocity of sound in an unlimited mass of water, at the temperature
of 16° Centigrade, as ascertained by MM. Colladon and Sturm, was 1435
metres per second.
That of sound in water contained in a trough, the vibrations of which
were regulated by an organpipe, was found by M. Wertheim, at 15°
Centigrade, to be 11 734 metres per second.
The ratio of the squares of those quantities is 06G8G : 1, differing
from f by 00009 only.
REMARKS ON THE PROPAGATION OF SOUND ALONG SOLID RODS.
21. I refrain from giving, in the body of this paper, detailed investigations
of particular problems respecting the propagation of sound along a solid
prism or cylinder; for, in the present state of our knowledge of the
condition of the superficial particles of such bodies, the conclusions would
be almost entirely speculative and conjectural.
I may mention briefly, however, the following general results. If
we adopt for solids the same hypothesis as for liquids, then the ratio of
the velocity of sound in a rod of an uncrystallised material to that in an
unlimited mass has the following values :
For a rectangular prismatic rod, the lateral vibrations of the particles
of which are confined to planes parallel to one pair of faces of the prism,
but are perfectly free in other respects, the ratio is ^2:^/3, being the
same as for a liquid.
For a cylindrical rod, the surface being perfectly free, the ratio has
various values, from ^Jl : ^/f , approaching the less value as the diameter
of the rod diminishes, and the greater as it increases ad infinitum. (See
Appendix, No. I.)
22. These conditions, however, cannot be realised in practice; and
the lateral vibrations being more or less confined by the means used
in fixing the rods, we find that the ratio generally exceeds ,J'2 : ^3,
and sometimes approaches equality.
The following table illustrates this fact. I have selected, in the first
place, the experiments of M. Wertheim on tubes of crystal (Ann. de Chim.,
Ser. III., torn, xxiii.), because in them the coefficients of elasticity and
the velocities of sound were ascertained by experiments on the same
192
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
pieces of the material. To these I have added a calculation, founded
on a comparison of the experiments of M. Wertheim on the elasticity of
brass, with those of M. Savart on the velocity of sound in it, as being
the only other data of the kind now existing from which a satisfactory
conclusion can be drawn.
The coefficients of longitudinal elasticity, calculated by myself from
M. Wertheim's experiments, are extracted from my paper on elasticity
in the Cambridge and Dublin Mathematical Journal for February, 1851.
The quantities   for crystal are given as calculated by M. Wertheim.
For brass I have used the following data :
tj e — velocity of sound in brass rods ; mean of many experiments by
M. Savart =3560 metres per second.
D = density, in kilogrammes per cubic metre, 8395.
TABLE.
Crystal.
Longitudinal
Elasticity
A
Kilogrammes per
square millimetre.
jD
.'/
Kilogrammes
per square
millimetre.
llatio
1  /**
Tube No. L,
. 55142
53540
0970,
„ „ HI,
. 56110
54767
0976,
„ „ IV.,
. 61831
55973
0905,
v
. 66599
54898
0824,
Brass,
. 15625
10847
0694.
Concluding Remarks.
23. The chief positive results arrived at in this paper may be summed
up as follows: —
(i.) In liquid and solid bodies of limited dimensions, the freedom of
lateral motion possessed by the particles causes vibrations to be propagated
less rapidly than in an unlimited mass.
(u.) The symbolical expressions for vibrations in limited bodies are
distinguished by containing exponential functions of the coordinates as
factors; and the retardation referred to depends on the coefficients of
the coordinates in the exponents of those functions, which coefficients
depend on the molecular condition of the body's surface — a condition yet
imperfectly understood.
(in.) If we adopt the hypothetical principle, that at the free surface of a
vibrating mass of liquid the normal pressure depending on the actions of atomic
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 193
centres is always null, then we deduce from theory that the ratio of the
velocity of sound along a mass of liquid contained in a rectangular trough
to that in an unlimited mass is ^f'2 : ^3, that ratio being independent
of the specific rigidity of the liquid — a conclusion agreeing with our
present experimental knowledge.
24. I do not put forward the hypothetical part of these researches
as more than a probable conjecture; nor should I be justified in so doing
in the present state of our knowledge of molecular forces. I think,
however, that these investigations are sufficient to prove that we are
not warranted in concluding from M. Wertheim's experiments (as he is
disposed to do) that liquids possess a momentary rigidity as great as
that of solids, seeing that any amount of rigidity, howsoever small, will
account for the phenomena if we adopt certain suppositions as to mole
cular forces; and to show that our knowledge of those forces is not yet
sufficiently advanced to enable us to use experiments on sound as a means
of determining the coefficients of elasticity of solids.
Appendix. — No. I.
Propagation of Sound by NearlyLongitudinal Vibrations along
a Cylindrical Uncrystallised Eod.
Let the vibrating body be cylindrical round the axis of x, and let the
vibrations of all particles in a given circle round that axis be assumed
to be equal and simultaneous. Let r represent the distance of any particle
A
from the axis of x, and ^, the angle y r.
Then
To make the results of the definite integrations ^fFQdd independent
of the angle ^, we must have F $ = constant, and the limits of integra
tion and 2 tt.
The following are the definite integrals which enter into the solution of
this problem.
N
194 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES,
Let
2 7T
— — hr = k,
/■2t fja.fccos^ , p kcosS
e= <f«»iao = i\ = y * to
Jo Jo A
^L + i la*(r(»+i)) , JJ
9':
cosOe 7ccoa tdd
> l36.)
""■2 22n _!
pii
]
!».!>+ 1) i
O'
cos 2 Oe kcos <>dO
~ d& ~Jo l
r v j (2w+ i)* 2n ) "1
_ 2,r L* + 1 2 2n+1 i> + i)i> + 2) J J
the values of n comprehending all integers from 1 inclusive.
Those series have the following properties :
(i.) The term (n) of = term {n — 1) X —  , ; therefore, this series
always becomes convergent at the term for which n > \ k.
k 2
(n.) Term (n) of 0' = term (n — 1) X
series becomes convergent when n 2 — n > i W,
4 (n — l)w
; therefore, this
(in.) Term (w) of 0" = term (w — 1) X
(2n+ \)k 2
4(2» !).».(»+ 1)'
w
2 n  1 > 4"
therefore, it begins to converge when n 2 —
(iv.) Q'=k(eG").
(v.) Term (n) of 0"= term (n) of x —77 ; a ratio which is \ for
w ' 2 n + 2
the first term (w = 0), and approaches equality as n increases ; therefore,
0"
when ^ k is an inappreciably small fraction, —  = \ sensibl}'.
6'
And the larger k is, the more nearly is — =1.
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 195
The following table of a few numerical results illustrates this : —
¥
e
G"
e"
4
2<r
2*
e
10000
05000
05000
1
12661
07010
05537
i
3
13622
07741
05683
1
15661
09302
05490
1
22796
14843
06511
2
42523
30550
07160
3
• 71590
54238
07576
4
113019
93620
08284
The displacements in this case are as follows : —
£ = (I cos <p + X sin 0) G
rj = (I' cos (j) — I sin <f) h G' cos ^
£ = (7 cos — Z sin ^)/i6' sin x'>
(37.)
whence it appears that the two transverse displacements ij and £ compose
a radial displacement,
p = (I' cos <j> — I sin 0)A 6'.
. (37A.)
Therefore, the trajectory of each particle is an ellipse, in a plane passing
through the axis of the cylinder; and the axes of the ellipse are longi
tudinal and radial, and have the following values:
Longitudinal axis, = 2 A /(7 2 + P) . ")
L . (38.)
Eadial axis, . = 2 J{p + P) . h Q' j
If we now adopt the same hypotheses with respect to the outer surface
of the cylinder that have been used in the problem respecting liquids, we
shall have for the mutual pressure of the solid and its atmosphere of vapour
»+!?=» +^(l' cos <f>ltmfi{(A2V)(lh*)Q 1 2Ch*e i "}>
A
196 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
© 1? ©j" being the values of those integrals corresponding to the radius of
the cylinder.
The portion of this pressure depending on mere fluid elasticity is
w _j(^+^ + ^ =w + ^ _ Zs
\dx ay dz/ A *
which, being subtracted, leaves
= ~(r cos  J sin 0)C {©!  h 2 (S 1 + 2 ©/')} ;
therefore, according to the hypothesis adopted,
h* = 1 ttt,:. ■ • . (39.)
1 + 2 e x
and the velocity of sound along the cylinder is
Now, the limits of the ratios in the above formulae are the following :
• 1,
T . ., . 2 7r A r, ,
Lrmits of — — l = \ . . .
A
©/ J •
A V2 • • */i»
„ */(!]£) Jl . . Jl
That is to say, if the hypothesis already explained with reference to
liquids is applicable to a solid cylinder of an uncrystallised material,
the velocity of sound along such a cylinder, when its surface is perfectly
free, will be less than that in an unlimited mass in some ratio between
^ and J? 6 .
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 197
Appendix. — No. II.
General Equations of NearlyTransverse Vibrations.
The two equal roots of equation (8) in uncrystallised bodies, viz.
E = C (1  b' 2  c'%
correspond to what may be called nearlytransverse vibrations, propagated
with the velocity
x/ £ = V{^ (1 ~ & ' 2 " C ' 2) } ' * (41>)
Equations (20) in this case give no result; but equations (20A) are
reducible to the following two :
I = — m'b' — n' c' . . . . )
f • • (42);
I' — mb' + nc' )
the ratios m : n and m' : n' are arbitrary.
Equations (11), (16), (17), (18), become the following:
Displacements.
£ — 2 [e+ { (m! b' + ri c') cos + (m b' + n c) sin }]
t) = 2 {e^ (m cos + m sin <£) }
£ = 2 {e^ (n cos + n' sin cj>)}
Velocities of the Particles.
^ = 2^^Je.e* {(m'b' + n'c)sm<p + (mb' + nc')cos<p}J y ^
— = 2 j ^— ^/ g . e^ (— m sin + m' cos 0) >•
dt
dj
dx
= 2 j ^— a/ e . e^ ( — w sin + %' cos $>) r
Longitudinal Strains.
= _ 2 I^V {(»» &' + n cos + (m' b' + n' c') sin <p} \
198 VELOCITY OF SOUND IN LIQUID AND SOLID BODIES.
^ = 2 \~e* (mV cos <h + m'V sin d>)\
ay (A )
dz
2 \ t— e^ (w c' cos $ \ n' c sin </j) r
Cufo'c Dilatation.
dj, , <*JJ , dt __ Q
d re rf y rZ «
Distortions.
> (43.)
g+f!= 2 [x^ { ( n/ ( i+ ^ +m ' &v ) co ^
+ (n (1 + 0" + rub' A sin 0} 
+ (m (1 + Z>' 2 ) + nb' A sin 0} 
Which, being multiplied by — C, give the tangential pressures
%> ^2' **3'
Normal Pressures on the Coordinate Planes due to the
Displacements.
P x = 2 C . 2 1 ~ e* {(m V + n c) cos + {m V + w' c) sin </>} 
P2 — — 2 C . 2 1 j—e^ {m V cos <f> \ m b' sin 0} I
P3 — — 2 C . 2j ~ye^ {71 c' cos <j> \ n c sin 0}
P x + P 2 + P 3 = 0.
The normal pressure due to the displacements at any point of the
surface of a prism or cylinder described round x is
VELOCITY OF SOUND IN LIQUID AND SOLID BODIES. 199
P' = — 2 C . 2 I  e$ {(m b' cos'' ^ + n c sin 2 ^ + (m <:' + n b')
cos ^ sin ^J cos <p
+ ( wi' 6' cos" x + w' c' sin 2 y_ + (m' c' + n b') cos ^ sin y) sin 0} I (44.)
If this pressure is to be null at all points of the surface, we must have
b' = 0, c'=0, and, consequently, Z=0, l' = 0; and the motion is restricted
to common exactlytransverse vibrations, for which
E = Cand^a = >/ Cd)'
Nearlytransverse vibrations, therefore, cannot be transmitted along a
cylindrical or prismatic uncry stall ised body whose surface is absolutely
free.
PART II.
PAPERS RELATING TO ENERGY AND ITS TRANSFORMATIONS,
THERMODYNAMICS, MECHANICAL ACTION OF
HEAT IN THE STEAM ENGINE, &a
PART IT.
PAPERS RELATING TO ENERGY AND ITS TRANSFORMATIONS,
THERMODYNAMICS, MECHANICAL ACTION OF
HEAT IN THE STEAM ENGINE, &a
X.— ON THE ^CONCENTRATION OF THE MECHANICAL
ENERGY OF THE UNIVERSE.*
THE following remarks have been suggested by a paper by Professor
William Thomson f of Glasgow, on the tendency which exists in nature to
the dissipation or indefinite diffusion of mechanical energy originally
collected in stores of power.
The experimental evidence is every day accumulating, of a law which
has long been conjectured to exist, — that all the different kinds of physical
energy in the universe are mutually convertible; that the total amount
of physical energy, whether in the form of visible motion and mechanical
power, or of heat, light, magnetism, electricity, or chemical agency, or in
other forms not yet understood, is unchangeable ; the transformations of
its different portions from one of those forms of power into another, and
their transference from one portion of matter to another, constituting the
phenomena which are the objects of experimental physics.
Professor William Thomson has pointed out the fact, that there exists
(at least in the present state of the known world), a predominating
tendency to the conversion of all the other forms of physical energy into
heat, and to the uniform diffusion of all heat throughout all matter. The
form in which we generally find energy originally collected, is that of a
store of chemical power, consisting of uncombined elements. The com
bination of these elements produces energy in the form known by the
name of electric currents, part only of which can be employed in analysing
* Read before the British Association at Belfast, on September 2, 1S52, and published
in the Philosophical Magazine for November, 1852.
f ISIow Sir William Thomson.
MECHANICAL ENERGY OF THE UNIVERSE. 201
compounds, and thus reconverted into a store of chemical power; the
remainder is necessarily converted into heat : a part only of this heat can
be employed in analysing compounds, or in reproducing electric currents. If
the remainder of the heat be employed in expanding an elastic substance,
it may be entirely converted into visible motion, or into a store of visible
mechanical power (by raising weights, for example), provided the elastic
substance is enabled to expand until its temperature falls to the point
which corresponds to absolute privation of heat ; but unless this condition
be fulfilled, a certain proportion only of the heat, depending upon the
range of temperature through which the elastic body works, can be con
verted, the rest remaining in the state of heat. On the other hand, all
visible motion is of necessity ultimately converted, entirely into heat by
the agency of friction. There is thus, in the present state of the known
world, a tendency towards the conversion of all physical energy into the
sole form of heat.
Heat, moreover, tends to diffuse itself uniformly by conduction and
radiation, until all matter shall have acquired the same temperature.
There is, consequently, Professor Thomson concludes, so far as we
understand the present condition of the universe, a tendency towards a
state in which all physical energy will be in the state of heat, and that
heat so diffused that all matter will be at the same temperature ; so that
there will be an end of all physical phenomena. '
Vast as this speculation may seem, it appears to be soundly based on
experimental data, and to represent truly the present condition of the
universe, so far as we know it.
My object now is to point out how it is conceivable that, at some
indefinitely distant period, an opposite condition of the world may take
place, in which the energy which is now being diffused may be recon
centrated into foci, and stores of chemical power again produced from the
inert compounds which are now being continually formed.
There must exist between the atmospheres of the heavenly bodies a
material medium capable of transmitting light and heat ; and it may be
regarded as almost certain that this interstellar medium is perfectly
transparent and diathermanous ; that is to say, that it is incapable of
converting heat, or light (which is a species of heat), from the radiant into
the fixed or conductible form.
If this be the case, the interstellar medium must' be incapable of
acquiring any temperature whatsoever ; and all heat which arrives in the
conductible form at the limits of the atmosphere of a star or planet, will
there be totally converted, partly into ordinary motion, by the expansion
of the atmosphere, and partly into the radiant form. The ordinary
motion will again be converted into heat, so that radiant heat is the
ultimate form to which all physical energy tends ; and in this form it is,
202 MECHANICAL ENERGY OF THE UNIVERSE.
in the present condition of the world, diffusing itself from the heavenly
bodies through the interstellar medium.
Let it now be supposed, that, in all directions round the visible world,
the interstellar medium has bounds beyond which there is empty
space.
If this conjecture be true, then on reaching those bounds the radiant
heat of the world will be totally reflected, and will ultimately be recon
centrated into foci. At each of these foci the intensity of heat may be
expected to be such, that should a star (being at that period an extinct
mass of inert compounds) in the course of its motions arrive at that part
of space, it will be vaporised and resolved into its elements ; a store of
chemical power being thus reproduced at the expense of a corresponding
amount of radiant heat.
Thus it appears, that although, from what we can see of the known
world, its condition seems to tend continually towards the equable dif
fusion, in the form of radiant heat, of all physical energy, the extinction
of the stars, and the cessation of all phenomena ; yet the world, as now
created, may possibly be provided within itself with the means of recon
centrating its physical energies, and renewing its activity and life.
For aught we know, these opposite processes may go on together ; and
some of the luminous objects which we see in distant regions of space may
be, not stars, but foci in the interstellar ether.
GENERAL LAW OF THE TRANSFORMATION OF ENERGY. 203
XI.— ON THE GENERAL LAW OF THE TRANSFORMATION
OF ENERGY.*
Actual, or Sensible Energy, is a measurable, transmissible, and trans
formable condition, whose presence causes a substance to tend to change
its state in one or more respects. By the occurrence of such changes,
actual energy disappears, and is replaced by
Potential or Latent Energy; which is measured by the product of a
change of state into the resistance against which that change is made.
(The vis viva of matter in motion, thermometric heat, radiant heat, light,
chemical action, and electric currents, are forms of actual energy; amongst
those of potential energy are the mechanical powers of gravitation,
elasticity, chemical affinity, statical electricity, and magnetism).
The law of the Conservation of Energy is already known — viz., that the
sum of all the energies of the universe, actual and potential, is unchange
able.
The object of the present paper is to investigate the law according to
which all transformations of energy, between the actual and potential forms,
take place.
Let V be the magnitude of a measurable state of a substance;
U, the species of potential energy which is developed when the state V
increases;
P, the common magnitude of the tendency of the state V to increase,
and of the equal and opposite resistance against which it increases ; so
that—
dU = FdV; andP = ^. . . . (A.)
Let Q be the quantity which the substance possesses, of a species of
actual energy whose presence produces a tendency of the state V to
increase.
It is required to find how much energy is transformed from the actual
form Q to the potential form U, during the increment d V; that is to say,
the magnitude of the portion of d U, the potential energy developed, which
is due to the disappearance of an equivalent portion of actual energy of
the species Q.
The development of this portion of potential energy is the immediate
* Read before the Philosophical Society of Glasgow, on January 5, 1S53, and
published in the Proceedings of that Society, Vol. III., No. V.
204 GENERAL LAW OF THE TRANSFORMATION OF ENERGY.
effect of the presence in the substance of the total quantity Q of actual
energy.
Let this quantity be conceived to be divided into indefinitely small
equal parts d Q. As those parts are not only equal, but altogether alike
in nature and similarly circumstanced, their effects must be equal ; there
fore, the effect of the total energy Q must be equal simply to the effect of
Q
one of its small parts d Q, multiplied by the ratio yy
But the effect of the indefinitely small part d Q in causing development
of potential energy of the species U, during the increment of state d V, is
represented by —
whence it follows, that the effect of the presence of the total actual energy
Q, in causing transformation of energy from the actual form Q to the
potential form U, is expressed by the following formula : —
«^ v w
which is the solution required, and is the symbolical expression of the
General Law of the Transformation of Energy : —
The effect of the whole actual energy present in a substance, in causing
transformation of energy, is the sum of the effects of all its parts.
The difference between this quantity and the potential energy developed,
viz : —
( p «;nD« v <
dW
represents a portion of potential energy, clue to causes different from the
actual energy Q. This difference is null, when the resistance ( P = p\j)
against which the state V increases, is simply proportional to the total
actual energy Q.
It is next proposed to find the quantity of actual energy of the form Q,
which must be transmitted to the substance from without, in order that
its total actual energy may receive the increment d Q, and its state V at
the same time the increment d V.
This quantity is composed of three parts — viz., actual energy, which
preserves its form, dQ,; actual energy which transforms itself to some
unknown form, in consequence of the resistance which is offered to the
increase of the total actual energy, L d Q ; actual energy, already deter
GENERAL LAW OF THE TRANSFORMATION OF ENERGY. 205
mined, which transforms itself into potential energy of the form U,
p
Q . 7^ • d V ; the sum of these parts being —
d.Q = (l+L)dQ + Q.^.dV, . . (2.)
in which nothing remains to be determined except the function L.
If Ave subtract from the above formula the total potential energy
developed during the increment d Y, viz : —
■we obtain the algebraical sum of the energies, actual and potential, received
and developed by the substance during the changes d Q, d V; which is
thus expressed : —
d¥ = d.Qd.TJ = (l+L)dQ + (Q^l)p.dY. (B.)
This quantity must be the exact differential of a function of Q and V;
for otherwise it would be possible, by varying the order of the increments
d Q, d V, to change the sum of the energies of the universe.
It follows that —
*J±A(qA. \?q JLy
dVdQK^dq ) r  H 'dQ? r '
and, consequently, that
l=/(Q) + Q.^/.wv,
where /' (Q) is a function of Q and constants, the first derivative of/' (Q).
We find at length the following equation —
dV=d.Qd.\J = (l+f®) + Q.^fvdv)dQ+(Q±l)
= ^{q+/(Q) + (q^i)/p*v} • (3.)
which represents the algebraical sum of the energy, actual and potential,
received and developed by a substance, when the total actual energy of
the species Q, and the state V, receive respectively the increments d Q, d V.
It is to be observed, that in the last equation, the symbol J"P . d V
denotes a partial integral, taken in treating the particular value of Q, to
which it corresponds as a constant quantity ; while d . U represents the
real magnitude of the potential energy developed.
206 GENERAL LAW OF THE TRANSFORMATION OF ENERGY.
The application of the general law of the transformation of energy may
be extended to any number of kinds of energy, actual and potential, by
means of the following equation :
= 2 {(l +/(«) + Q.S^i/p<T>q{ + 2 {(SQ^ 1>*TJ
= rf jsQ + 2/(Q) + 2(2.Q^l)/p<Zv . (4.)
This equation is the complete expression of the general law of the
transformation of energy of all possible kinds, known and unknown. It
affords the means, so soon as the necessary experimental data have been
obtained, of analysing every development of potential energy, and referring
its several portions to the species of actual energy from which they have
been produced.
Amongst the consequences of this law, the author deduces that which
may be called the general principle of the maximum effect of engines.
An engine consists essentially in a substance whose changes of state,
and of actual energy, between given limits, are so regulated as to produce
a permanent transformation of energy.
Let Q x be the given superior limit of actual energy ; Q 2 , the inferior
limit.
To produce the maximum permanent transformation of energy from
the actual to the potential form, the substance must undergo a cycle of
four operations, viz : —
First Operation.
The substance, preserving the constant quantity Q l of actual energy,
passes from the state V A to the state V B , receiving from without the
following quantity of actual energy, which is converted into potential
energy: —
H «./o/v: p ' n 
Second Operation.
The substance passes from the superior limit of actual energy Q v to
the inferior limit Q 2 . Let V c be the value of the state V at the end of
this operation.
GENERAL LAW OF THE TRANSFORMATION OF ENERGY. 207
Third Operation.
The substance, preserving the constant quantity Q 2 of actual energy,,
passes from the state V c to the state V D , transmitting to external sub
stances the following quantity of actual energy, produced by the disap
pearance of potential energy: —
h *= q 4/v: p <* v 
Fourth Operation.
The substance is brought back to its original actual energy Q ls and
state V A , thus completing the cycle of operations.
In order that the second and fourth operations may be performed
without expenditure of energy, the following condition must be fulfilled : —
hi k VllY (for Q = Qa) = m /£ p d v (fOT Q = *«■
This being the case, the total expenditure of energy during a cycle of
operations will be H v being the quantity converted from the actual to
the potential form during the first operation; the energy lost will be H 2 ,
the quantity reconverted to the actual form, and transmitted to external
substances, during the third operation ; and the quantity of energy per
manently transformed from the actual to the potential form, that is to
say, the work done by the engine, will be —
H 1 H 2 =(Q 1 Q 2 )^^P^V(forQ = Q 1 ) . (6.)
Q
The ratio of this work to the total expenditure of energy is
H t — H 9 _ Q 2 — Q
(7.)
This principle is applicable to all possible engines, known and unknown.
In the sequel of the paper, the author gives some examples of the
application of the general principles of the transformation of energy to
the theory of heat, and to that of electromagnetism ; and deduces from
them, as particular cases, several laws already known through specific
researches.
The details of the application of these principles to the theory of
208 GENERAL LAW OF THE TRANSFORMATION OF ENERGY.
heat are contained in the sixth section of a memoir read before the Royal
Society of Edinburgh, " On the Mechanical Action of Heat."
The actual energy produced by an electric pile in unity of timo is
expressed by —
where M is the electromotive force, and u, the strength of the current.
The actual energy of an electric circuit is expressed by —
~Ru 2 ,
where R is the resistance of the circuit. This energy is immediately and
totally transformed into sensible heat.
The proportion of the actual energy produced in the pile, which is
transformed into mechanical work by an electrodynamic machine, is
represented by —
Q t Q 2 _ MRtt
Q 2 M '
The strength of the current is known to be found by means of the
equation —
MN
where N is the negative or inverse electromotive force of the apparatus
by means of which electricity is transformed into mechanical work.
Hence,
Qi  Q 2 _ n
Qi ~M"
The above particular forms of the general equation agree with formulae
already deduced from special researches by Mr. Joule and Professor
William Thomson.
OUTLINES OF THE SCIENCE OF ENERGETICS. 209
XII— OUTLINES OF THE SCIENCE OF ENEKGETICS.*
Section I. — What constitutes a Physical Theory.
An essential distinction exists between two stages in the process of
advancing our knowledge of the laws of physical phenomena; the first
stage consists in observing the relations of phenomena, whether of such
as occur in the ordinary course of nature, or of such as are artificially
produced in experimental investigations, and in expressing the relations
so observed by propositions called formal laws. The second stage consists
in reducing the formal laws of an entire class of phenomena to the
form of a science ; that is to say, in discovering the most simple system
of principles, from which all the formal laws of the class of phenomena
can be deduced as consequences.
Such a system of principles, with its consequences methodically
deduced, constitutes the physical theory of a class of phenomena.
A physical theory, like an abstract science, consists of definitions and
axioms as first principles, and of propositions, their consequences ; but
with these differences: — First, That in an abstract science, a definition
assigns a name to a class of notions derived originally from observation,
but not necessarily corresponding to any existing objects of real pheno
mena; and an axiom states a mutual relation amongst such notions, or
the names denoting them : while in a physical science, a definition states
properties common to a class of existing objects, or real phenomena ; and
a physical axiom states a general law as to the relations of phenomena.
And, secondly, That in an abstract science, the propositions first discov
ered are the most simple ; whilst in a physical theory, the propositions
first discovered are in general numerous and complex, being formal laws,
the immediate results of observation and experiment, from which the
definitions and axioms are subsequently arrived at by a process of reason
ing differing from that whereby one proposition is deduced from another
in an abstract science, partly in being more complex and difficult,
and partly in being, to a certain extent, tentative — that is to say,
involving the trial of conjectural principles, and their acceptance or
rejection, according as their consequences are found to agree or disagree
with the formal laws deduced immediately from observation and experi
ment.
* Read before the Philosophical Society of Glasgow on May 2, 1855, and published
in the Proceedings of that Society, Vol. III., No. VL
O
210 OUTLINES OF THE SCIENCE OF ENERGETICS.
Section II. — The Abstractive Method of forming a Physical
Theory distinguished from the Hypothetical Method.
Two methods of framing a physical theory may be distinguished,
characterised chiefly by the manner in which classes of phenomena are
defined. They may be termed, respectively, the abstractive and the
hypothetical methods.
According to the abstractive method, a class of objects or phenomena
is defined by describing, or otherwise making to be understood, and
assigning a name or symbol to, that assemblage of properties which is
common to all the objects or phenomena composing the class, as perceived
by the senses, without introducing anything hypothetical.
According to the hypothetical method, a class of objects or pheno
mena is defined, according to a conjectural conception of their nature, as
being constituted, in a manner not apparent to the senses, by a modifica
tion of some other class of objects or phenomena whose laws are already
known. Should the consequences of such a hypothetical definition be
found to be in accordance with the results of observation and experiment,
it serves as the means of deducing the laws of one class of objects or
phenomena from those of another.
The conjectural conceptions involved in the hypothetical method may
be distinguished into two classes, according as they are adopted as a pro
bable representation of a state of things which may really exist, though
imperceptible to the senses, or merely as a convenient means of expressing
the laws of phenomena ; two kinds of hypotheses, of which the former
may be called objective, and the latter subjective. As examples of objec
tive hypotheses may be taken, that of vibrations or oscillations in the
theory of light, and that of atoms in chemistry; as an example of a
subjective hypothesis, that of magnetic fluids.
Section III. — The Science of Mechanics considered as an
Illustration of the Abstractive Method.
The principles of the science of mechanics, the only example yet exist
ing of a complete physical theory, are altogether formed from the data of
experience by the abstractive method. The class of objects to which the
science of mechanics relates — viz., material bodies — are defined by
means of those sensible properties which they all possess — viz., the pro
perty of occupying space, and that of resisting change of motion. The
two classes of phenomena to which the science of mechanics relates are
distinguished by two words, motion and force — motion being a word
OUTLINES OF THE SCIENCE OF ENERGETICS. 211
denoting that which is common to the fall of heavy bodies, the flow of
streams, the tides, the winds, the vibrations of sonorous bodies, the
revolutions of the stars, and, generally, to all phenomena involving change
of the portions of space occupied by bodies ; and force, a word denoting
that which is common to the mutual attractions and repulsions of bodies,
distant or near, and of the parts of bodies, the mutual pressure or stress
of bodies in contact, and of the parts of bodies, the muscular exertions
of animals, and, generally, to all phenomena tending to produce or to
prevent motion.
The laws of the composition and resolution of motions, and of the
composition and resolution of forces, are expressed by propositions which
are the consequences of the definitions of motion and force respectively.
The laws of the relations between motion and force are the consequences
of certain axioms, being the most simple and general expressions for all
that has been ascertained by experience respecting those relations.
Section IV. — Mechanical Hypotheses in Various Branches
of Physics.
The fact that the theory of motions and motive forces is the only
complete physical theory, has naturally led to the adoption of mechanical
hypotheses in the theories of other branches of physics; that is to*say,
hypothetical definitions, in which classes of phenomena are defined con
jecturally as being constituted by some kind of motion or motive force
not obvious to the senses (called molecular motion or force), as when light
and radiant heat are defined as consisting in molecular vibrations, thermo
metric heat in molecular vortices, and the rigidity of solids in molecular
attractions and repulsions.
The hypothetical motions and forces are sometimes ascribed to hypo
thetical bodies, such as the luminiferous ether; sometimes to hypothetical
jparts, whereof tangible bodies are conjecturally defined to consist, such as
atoms, atomic nuclei with elastic atmospheres, and the like.
A mechanical hypothesis is held to have fulfilled its object, when, by
applying the known axioms of mechanics to the hypothetical motions and
forces, results are obtained agreeing with the observed laws of the classes
of phenomena under consideration; and when, by the aid of such a hypo
thesis, phenomena previously unobserved are predicted, and laws antici
pated, it attains a high degree of probability.
A mechanical hypothesis is the better the more extensive the range
of phenomena whose laws it serves to deduce from the axioms of
mechanics ; and the perfection of such a hypothesis would be, if it could,
212 OUTLINES OF THE SCIENCE OF ENERGETICS.
by means of one connected system of suppositions, be made to form a
basis for all branches of molecular physics.
Section V. — Advantages and Disadvantages of Hypothetical
Theories.
It is well known that certain hypothetical theories, such as the wave
theory of light, have proved extremely useful, by reducing the laws of a
various and complicated class of phenomena to a few simple principles,
and by anticipating laws afterwards verified by observation.
Such are the results to be expected from wellframed hypotheses in
every branch of physics, when used with judgment, and especially with
that caution which arises from the consideration, that even those hypo
theses whose consequences are most fully confirmed by experiment never
can, by any amount of evidence, attain that degree of certainty which
belongs to observed facts.
Of mechanical hypotheses in particular, it is to be observed, that their
tendency is to combine all branches of physics into one system, by making
the axioms of mechanics the first principles of the laws of all phenomena —
an object for the attainment of which an earnest wish was expressed by
Newton.*
In the mechanical theories of elasticity, light, heat, and electricity,
considerable progress has been made towards that end.
The neglect of the caution already referred to, however, has caused
some hypotheses to assume, in the minds of the public generally, as well
as in those of many scientific men, that authority which belongs to facts
alone ; and a tendency has, consequently, often evinced itself to explain
away, or set aside, facts inconsistent with these hypotheses, which facts,
rightly appreciated, would have formed the basis of true theories. Thus,
the fact of the production of heat by friction, the basis of the true theory
of heat, was long neglected, because inconsistent with the hypothesis of
caloric ; and the fact of the production of cold by electric currents, at
certain metallic junctions, the key (as Professor William Thomson recently
showed) to the true theory of the phenomena of thermoelectricity, was,
from inconsistency with prevalent assumptions respecting the socalled
" electric fluid," by some regarded as a thing to be explained away, and
by others as a delusion.
Such are the evils which arise from the misuse of hypotheses.
Utinam caetera naturae phenomena ex principiis mechanicis eodem argumentandi
genere derivare liceret.— {Phil. Nat. Prin. Math.; Protf.)
OUTLINES OF THE SCIENCE OF ENERGETICS. 213
Section VI. — Advantages of an Extension of the Abstractive
Method of framing Theories.
Besides the perfecting of mechanical hypotheses, another and an entirely
distinct method presents itself for combining the physical sciences into
one system; and that is, by an extension of the Abstractive Process
in framing theories.
The abstractive method has already been partially applied, and with
success, to special branches of molecular physics, such as heat, electricity,
and magnetism. We are now to consider in what manner it is to be
applied to physics generally, considered as one science.
Instead of supposing the various classes of physical phenomena to be
constituted, in an occult way, of modifications of motion and force, let
us distinguish the properties which those classes possess in common with
each other, and so define more extensive classes denoted by suitable
terms. For axioms, to express the laws of those more extensive classes
of phenomena, let us frame propositions comprehending as particular
cases the laws of the particular classes of phenomena comprehended
under the more extensive classes. So shall we arrive at a body of
principles, applicable to physical phenomena in general, and which, being
framed by induction from facts alone, will be free from the uncertainty
which must always attach, even to those mechanical hypotheses whose
consequences are most fully confirmed by experiment.
This extension of the abstractive process is not proposed in order to
supersede the hypothetical method of theorising; for in almost every
branch of molecular physics it may be held, that a hypothetical theory is
necessary, as a preliminary step, to reduce the expression of the phenomena
to simplicity and order, before it is possible to make any progress in
framing an abstractive theory.
Section VII. — Nature of the Science of Energetics.
«
Energy, or the capacity to effect changes, is the common characteristic
of the various states of matter to which the several branches of physics
relate; if, then, there be general laws respecting energy, such laws must
be applicable, mutatis mutandis, to every branch of physics, and must
express a body of principles as to physical phenomena in general.
In a paper read before the Philosophical Society of Glasgow, on the 5 th
of January, 1853 (seep. £08), a first attempt was made to investigate such
principles by defining actual energy and potential energy, and by demonstrat
ing a general law of the mutual transformations of those kinds of energy,
214 OUTLINES OF THE SCIENCE OF ENERGETICS.
of which one particular case is a previously known law of the mechanical
action of heat in elastic bodies, and another, a subsequently demonstrated
law which forms the basis of Professor William Thomson's theory of
thermoelectricity.
The object of the present ' paper is to present, in a more systematic
form, both these and some other principles, forming part of a science
whose subjects are, material bodies and physical phenomena in general,
and which it is proposed to call the Science of Energetics.
Section VIII. — Definitions of Certain Terms.
The peculiar terms which will be used in treating of the Science of
Energetics are purely abstract ; that is to say, they are not the names of
any particular object, nor of any particular phenomena, nor of any
particular notions of the mind, but are names of very comprehensive
classes of objects and phenomena. About such classes it is impossible to
think or to reason, except by the aid of examples or of symbols. General
terms are symbols employed for this purpose.
Substance.
The term "substance" will be applied to all bodies, parts of bodies,
and systems of bodies. The parts of a substance may be spoken of as
distinct substances, and a system of substances related to each other may
be spoken of as one complex substance. Strictly speaking, the term
should be "material substance;" but it is easily borne in mind, that in this
essay none but material substances are referred to.
Properly.
The term "property" will be restricted to invariable properties; whether
such as always belong to all material substances, or such as constitute
the invariable distinctions between one kind of substance and another.
Mass.
Mass means " quantity of substance." Masses of one kind of substance
may be compared together by ascertaining the numbers of equal parts
which they contain ; masses of substances of different kinds are compared
by means to be afterwards referred to.
Accident.
The term "accident" will be applied to every variable state of substances,
whether consisting in a condition of each part of a substance, how small
soever, (which may be called an absolute accident), or in a physical relation
OUTLINES OF THE SCIENCE OF ENERGETICS. 215
between parts of substances, (which may be called a relative accident).
Accidents to be the subject of scientific inquiry, must be capable of being
measured and expressed by means of quantities. The quantity, even of
an absolute accident, can only be expressed by means of a mentally
conceived relation.
The whole condition or state of a substance, so far as it is variable, is
a complex accident; the independent quantities which are at once necessary
and sufficient to express completely this complex accident, are independent
accidents. To express the same complex accident, different systems of
independent accidents may be employed; but the number of independent
accidents in each system will be the same.
Examples. — The variable thermic condition of an elastic fluid is a
complex accident, capable of being completely expressed by two independent
accidents, which may be any two out of these three quantities — the
temperature, the densihj, the pressure — or any two independent functions
of these quantities.
The condition of strain at a point in an elastic solid, is a complex acci
dent, capable of being completely expressed by six independent accidents,
which may be the three elongations of the dimensions and the three
distortions of the faces of a molecule originally cubical, or the lengths and
directions of the axes of the ellipsoidal figure assumed by a molecule
originally spherical ; or any six independent functions of either of those
systems of quantities.
The distinction of accidents into absolute and relative is, to a certain
extent, arbitrary; thus, the figure and dimensions of a molecule may be
regarded as absolute accidents when it is considered as a whole, or as
relative accidents when it is considered as made up of parts. Most kinds
of accidents are necessarily relative; but some kinds can only be considered
as relative accidents when some hypothesis is adopted as to the occult
condition of the substances which they affect, as when heat is ascribed
hypothetically to molecular motions; and such suppositions are excluded
from the present inquiry.
Accidents may be said to be homogeneous when the quantities expressing
them are capable of being put together, so that the result of the com
bination of the different accidents shall be expressed by one quantity.
The number of heterogeneous kinds of accidents is evidently indefinite.
Effort, or Active Accident.
The term "effort" will be applied to every cause which .varies, or tends
to vary, an accident. This term, therefore, comprehends not merely
forces or pressures, to which it is usually applied, but all causes of variation
in the condition of substances.
Efforts may be homogeneous or heterogeneous.
21 G OUTLINES OF THE SCIENCE OF ENERGETICS.
Homogeneous efforts are compared by balancing them against each other.
An effort being a condition of the parts of a substance, or a relation
between substances, is itself an accident, and may be distinguished as an
" active accident."
With reference to a given limited substance, internal efforts are those
which consist in actions amongst its parts; external efforts those which
consist in actions between the given substance and other substances.
Passive Accident.
The condition which an effort tends to vary may be called a "passive
accident" and when the word "accident" is not otherwise qualified,
''passive accident" may be understood.
Radical Accident.
If there be a quantity such that it expresses at once the magnitude of
the passive accident caused by a given effort, and the magnitude of the
active accident or effort itself, let the condition denoted by that quantity
be called a " radical accident."
[The velocity of a given mass is an example of a radical accident, for
it is itself a passive accident, and also the measure of the kind of effort
called accelerative force, which, acting for unity of time, is capable of
producing that passive accident.]
[The strength of an electric current is also a radical accident.]
Effort as a Measure of Mass.
Masses, whether homogeneous or heterogeneous, may be compared
by means of the efforts required to produce in them variations of some
particular accident. The accident conventionally employed for this pur
pose is velocity.
Work
u Work " is the variation of an accident by an effort, and is a term
comprehending all phenomena in which physical change takes place.
Quantity of work is measured by the product of the variation of the passive
accident by the magnitude of the effort, when this is constant; or by
the integral of the effort, with respect to the passive accident, when the
effort is variable.
Let x denote a passive accident ;
X an effort tending to vary it ;
W the work performed in increasing x from x to x x : then,
OUTLINES OF THE SCIENCE OF ENERGETICS. 217
W = f X Xdx, and
W = X (x x —£(,), if X is constant.
(10
Work is represented geometrically by the area of a curve, whereof the
abscissa represents the passive accident, and the ordinate, the effort.
Energy, Actual and Potential.
The term "energy" comprehends every state of a substance which
constitutes a capacity for performing work. Quantities of energy are
measured by the quantities of work which they constitute the means of
performing.
" Actual energy" comprehends those kinds of capacity for performing
work which consist in particular states of each part of a substance, how
small soever ; that is, in an absolute accident, such as heat, light, electric
current, vis viva. Actual energy is essentially positive.
"Potential energy" comprehends those kinds of capacity for performing
work which consist in relations between substances, or parts of substances;
that is, in relative accidents. To constitute potential energy there must be
a passive accident capable of variation, and an effort tending to produce
such variation; the integral of this effort, with respect to the possible
variation of the passive accident, is potential energy, which differs in work
from this — that in work the change has been effected, which, in potential
energy, is capable of being effected.
Let x denote an accident; x v its actual value; X, an effort tending
to vary it; x Q , the value to which the effort tends to bring the accident ;
then
X d x = U, denotes potential energy.
r
J x,
Examples of potential energy are, the chemical affinity of uncombined
elements ; the energy of gravitation, of magnetism, of electrical attraction
and repulsion, of electromotive force, of that part of elasticity which arises
from actions between the parts of a body, and, generally, of all mutual
actions of bodies, and parts of bodies.
Potential energy may be passive or negative, according as the effort in
question is of the same sign with the variation of the passive accident, or
of the opposite sign ; that is, according as X is of the same sign with dx,
or of the opposite sign.
218 OUTLINES OF THE SCIENCE OF ENERGETICS.
It is to be observed, that the states of substances comprehended under
the term actual energy, may possess the characteristics of potential energy
also ; that is to say, may be accompanied by a tendency or effort to vary
relative accidents ; as heat, in an elastic fluid, is accompanied by a ten
dency to expand; that is, an effort to increase the volume of the receptacle
containing the fluid.
The states to which the term potential energy is especially applied, aro
those which are solely due to mutual actions.
To put a substance into a state of energy, or to increase its energy, is
obviously a hind of work.
Section IX. — First Axiom.
All kinds of Work and Energy are Homogeneous.
This axiom means, that any kind of energy may be made the means of
performing any kind of work. It is a fact arrived at by induction from
experiment and observation, and its establishment is more especially due
to the experiments of Mr. Joule.
This axiom leads, in many respects, to the same consequences with the
hypothesis that all those kinds of energy which are not sensibly the results
of motion and motive force are the results of occult modifications of motion
and motive force.
But the axiom differs from the hypothesis in this, that the axiom is
simply the generalised allegation of the facts proved by experience, while
the hypothesis involves conjectures as to objects and phenomena which
never can be subjected to observation.
It is the truth of this axiom which renders a science of energetics
possible.
The efforts and passive accidents to which the branches of physics relate
are varied and heterogeneous ; but they are all connected with energy, a
uniform species of quantity which pervades every branch of physics.
This axiom is also equivalent to saying, that energy is transformable and
transferable (an allegation which, in the previous paper referred to, was
included in the definition of energy) ; for, to transform energy, means to
employ energy depending on accidents of one kind in putting a substance
into a state of energy depending on accidents of another kind ; and to
transfer energy, means to employ the energy of one substance in putting
another substance into a state of energy, both of which are kinds of work,
and may, according to the axiom, be performed by means of any kind of
energy.
OUTLINES OF THE SCIENCE OF ENERGETICS. 219
Section X. — Second Axiom.
TJie Total Energy of a Substance cannot he altered by the Mutual Actions of
its Parts.
Of the truth of this axiom there can be no doubt; but some difference
of opinion may exist as to the evidence on which it rests. There is ample
experimental evidence from which it might be proved; but independently
of such evidence, there is the argument, that the law expressed by this
axiom is essential to the stability of the universe, such as it exists.
The special application of this law to mechanics is expressed in two
ways, which are virtually equivalent to each other, the principle of vis viva,
and that of the equality of action and reaction. The latter principle is
demonstrated by Newton, from considerations connected with the stability
of the universe (Principia, Scholium to the Laws of Motion) ; for he shows,
that but for the equality of action and reaction, the earth, with a continually
accelerated velocity, would fly away through infinite space.
It follows, from the second axiom, that all work consists in the transfer
and transformation of energy alone; . for otherwise the total amount of
energy would be altered. Also, that the energy of a substance can be
varied by external efforts alone.
Section XL — External Potential Equilibrium.
The entire condition of a substance, so far as it is variable, as explained
in Sect. VIII., under the head of accident, is a complex accident, which
may be expressed in various ways by means of different systems of
quantities denoting independent accidents ; but the number of independent
accidents in each system must be the same.
The quantity of work required to produce any change in the condition
of the substance, that is to say, the potential energy received by it from
without during that change, may in like manner be expressed in different
ways by the sums of different systems of integrals of external efforts, each
integrated with respect to the independent accident which it tends to
augment ; but the number of integrals in each system, and the number of
efforts, like the number of independent accidents, must be the same ; and
so also must the suras of the integrals, each sum representing the same
quantity of work in a different way.
The different systems of efforts which correspond to different systems of
independent accidents, each expressing the same complex accident, may
220 OUTLINES OF THE SCIENCE OF ENELIGETICS.
be called equivalent systems of efforts; and the finding of a system of efforts
equivalent to another may be called conversion of efforts. *
When the law of variation of potential energy, by a change of condition
of a substance, is known, the system of external efforts corresponding to
any system of independent accidents is found by means of this principle :
Each effort is equal to the rate of variation of the potential energy with respect
to the independent accident which that effort tends to vary; or, symbolically,
X£ .... (,)
External Potential Equilibrium of a substance takes place, when the
external effort to vary each of the independent accidents is null ; that is to say,
when the rate of variation of the potential energy of the substance with the
variation of each independent a null.
For a given substance there are as many conditions of equilibrium, of
the form
 = 0, .... (3.)
as there are independent accidents in the expression of its condition.
The special application of this law to motion and motive force consti
tutes the principle of virtual velocities, from which the whole science of
statics is deducible.
Section XII. — Internal Potential Equilibrium.
The internal potential equilibrium of a substance consists in the equili
brium of each of its parts, considered separately ; that is to say, in the
nullity of the rate of variation of the potential energy of each part with
respect to each of the independent accidents on which the condition of
such part depends.
Examples of particular cases of this principle are, the laws of the
equilibrium of elastic solids, and of the distribution of statical electricity.
Section XIII. — Third Axiom.
The Effort to Perform Work of a Given Kind, caused by a Given Quantity
of Actual Energy, is the Sum of the Efforts caused by the Parts, of that
Quantity.
A law equivalent to this axiom, under the name of the "General
* The conversion of efforts in physics is connected with the theory of lineal trans
formations in aliiehra.
OUTLINES OF THE SCIENCE OF ENERGETICS. 221
Law of the Transformation of Energy," formed the principal subject
of the previous paper already referred to. (See p. £03.)
This axiom appears to be a consequence of the definition of actual
energy, as a capacity for performing woi'k possessed by each part of a
substance independently of its relations to other parts, rather than an
independent proposition.
Its applicability to natural phenomena arises from the fact, that there
are states of substances corresponding to the definition of actual energy.
The mode of applying this third axiom is as follows : —
Let a homogeneous substance possess a quantity Q, of a particular kind
of actual energy, uniformly distributed, and let it be required to determine
the amount of the effort arising from the actual energy, which tends to
perform a particular kind of work "W, by the variation of a particular
passive accident x.
The total effort to perform this kind of work is represented by the rate
of its increase relatively to the passive accident, viz., —
d X '
Divide the quantity of actual energy Q into an indefinite number of
indefinitely small parts SQ; the portion of the effort X due to each of
those parts will be
and adding these partial efforts together, the effort caused by the whole
quantity of actual energy will be
„dX _ d*W , x
If this be equal to the effective effort X, then that effort is simply
proportional to, and wholly caused by, the actual energy Q. This is the
case of the pressure of a perfect gas. and the centrifugal force of a moving
body.
If the effort caused by the actual energy differs from the effective effort,
their difference represents, when the former is the less, an additional
effort,
and when the former is the greater, a counter effort }■ (5.)
due to some other cause or causes.
222 OUTLINES OF THE SCIENCE OF ENERGETICS.
Section XIV. — Rate of Transformation; Metamorphic Function.
The effort to augment a given accident x, caused by actual energy of
a given kind Q, may also be called the "rate of transformation" of tho
given kind of actual energy, with increase of the given accident ; for the
limit of the amount of actual energy which disappears in performing work
by an indefinitely small augmentation dx, of the accident, is
dII = Q d ~dx .... (6.)
a v^j
n <?w , aw
— QjTTj  dx = Qd
dQ,dx aQ
The last form of the above expression is obviously applicable when the
work W is the result of the variation of any number of independent
accidents, each by the corresponding effort. For example, let x, y, z, &c,
be any number of independent accidents, and X, Y, Z, &c, the efforts to
augment them ; so that
dV? = Xdx + Ydy + Zdz + &c.
Then,
7TT ._ f d X 7 dY , dZ , . ) /n .
'**il$*' + i$'t+i$" + * J ' (7 >
= Q d 777 , as before.
d*4
The function of actual energy, efforts, and passive accidents, denoted by
Si?'. • • ■«
whose variation, multiplied by the actual energy, gives the amount of
actual energy transformed in performing the work d W, may be called the
" Metamorphic Function " of the kind of actual energy Q, relatively to
the kind of work W.
When this metamorphic function is known for a given homogeneous
substance, the quantity H of actual energy of the kind Q transformed to
the kind of work W, during a given operation, is found by taking the
integral
H
JQdF (9.)
The transformation of actual energy into work by the variation of
passive accidents is a reversible operation; that is to say, if the passive
OUTLINES OF THE SCIENCE OF ENEEGETICS. 223
accidents be made to vary to an equal extent in an opposite direction,
potential energy will be exerted upon the substance, and transformed
into actual energy: a case represented by the expression (9) becoming
negative.
The metamorphic function of heat relatively to expansive power, was
first employed in a paper on the Economy of Heat in Expansive Machines,
read before the Royal Society of Edinburgh in April, 1851. (Trans. Boy.
Soc. Edin., Vol. XXI.)
The metamorphic function of heat relatively to electricity was employed
by Professor William Thomson, in a paper on ThermoElectricity, read before
the Eoyal Society of Edinburgh in May, 1854 (Trans. Boy. Soc. Edin.,
Vol. XXL), and was the means of anticipating some most remarkable laws,
afterwards confirmed by experiment.
Section XV. — Equilibrium of Actual Energy ; Metabatic
Function.
It is known by experiment, that a state of actual energy is directly
transferable ; that is to say, the actual energy of a particular kind (such
as heat), in one substance, may be diminished, the sole work performed
being an equal augmentation of the same kind of actual energy in another
substance.
Equilibrium of actual energy of a particular kind Q between substances
A and B, takes place when the tendency of B to transfer this kind of
energy to B is equal to the tendency of B to transfer the same kind of
energy to A.
Laws respecting the equilibrium of particular kinds of actual energy
have been ascertained by experiment, and in some cases anticipated by
means of mechanical hypotheses, according to which all actual energy con
sists in the vis viva of motion.
The following law will now be proved, respecting the equilibrium of
actual energy of all possible kinds : —
Theorem. — If equilibrium of actual energy of a given kind take
PLACE BETWEEN A GIVEN PAIR OF SUBSTANCES, POSSESSING RESPECTIVELY
QUANTITIES OF ACTUAL ENERGY OF THAT KIND IN A GIVEN RATIO, THEN
THAT EQUILIBRIUM WILL SUBSIST FOR EVERY PAIR OF QUANTITIES OF
ACTUAL ENERGY BEARING TO EACH OTHER THE SAME RATIO.
Demonstration. — The tendency of one substance to transfer actual energy
of the kind Q to another, must depend on some sort of effort, whose
nature and laws may be known or unknown. Let Y A be this effort for
224 OUTLINES OF THE SCIENCE OF ENERGETICS.
the substance A, Y B the corresponding effort for the substance B. Then
a condition of equilibrium of actual energy is
Y A = Y B (10.)
The effort Y may or may not be proportionate to the actual energy Q
multiplied by a quantity independent of Q,
Case first. — If it is so proportional, let
K being independent of Q ; then the condition of equilibrium becomes
or
Qb _ Kj,
a ratio independent of the absolute amounts of actual energy.
Case second. — If the effort Y is not simply proportional to the actual
energy Q, the portion of it caused by that actual energy, according to
the principle of Sect, XIII., deduced from the third axiom, is, for each
substance,
o' /v
and a second condition of equilibrium of actual energy is furnished by the
equation
*& = *& ' • ' (11 ' )
In order that this condition may be fulfilled simultaneously with the con
dition (10), it is necessary that
<ZQ A _ dQ B
that is to say, that the ratio of the quantities of actual energy in the two
substances should be independent of those quantities themselves ; a con
dition expressed, as before, by
?i = ?? (11.)
Q.E.D.
This ratio is a quantity to be ascertained by experiment, and may be
OUTLINES OF THE SCIENCE OF ENERGETICS. 225
called the ratio of the specific actual energies of the substances A and
B, for the kind of energy under consideration.
The function
K  K e, .... (i2 v
whose identity for the two substances expresses the condition of equili
brium of the actual energy Q between them, may be called the " meta
batic FUNCTION " for that kind of energy.
In the science of thermodynamics, the metabatic function is absolute
temperature; and the factor K is real specific heat. The theorem stated
above, when applied to heat, amounts to this : that the real specific heat of a
substance is independent of its temperature.
Section XVI. — Use of the Metabatic Function; Transformation
of Energy in an aggregate.
From the mutual proportionality of the actual energy Q, and the meta
batic function 9, it follows that the operations
are equivalent ; and that the latter may be substituted for the former in
all the equations expressing the laws of the transformation of energy.
"We have therefore
dX_ dX_ d?W ,
q dq e Te deTx ' ■ (W
for the effort to transform actual energy of the kind Q into work of the
land W, when expressed in terms of the metabatic function ; and
dn = 9d~, .... (H.)
for the limit of the indefinitely small transformation produced bj» an
indefinitely small variation of the accidents on which the kind of work
W depends.
There is also a form of metamorphic function,
♦=£=/" = "• ■ : w
22G OUTLINES OF THE SCIENCE OF ENERGETICS.
suited for employment along with the metabatic function, in order to find,
by the integration
B. = fed<t>, .... (1G.)
the quantity of actual energy of a given kind Q transformed to the kind
of work W during any finite variation of accidents.
The advantage of the above expressions is, that they are applicable not
merely to a homogeneous substance, but to any heterogeneous substance or
aggregate, which is internally in a state of equilibrium of actual and potential
energy; for throughout all the parts of an aggregate in that condition, the
metabatic function 6 is the same, and each of the efforts X, &c, is the
same, and consequently the metamorphic function <p is the same.
" Carnot's function " in thermodynamics is proportional to the reciprocal
of the metabatic function of heat.
Section XVII. — Efficiency of Engines.
An engine is a contrivance for transforming energy, by means of the
periodical repetition of a cycle of variations of the accidents of a sub
stance.
The efficiency of an engine is the proportion which the energy perma
nently transformed to a useful form by it, bears to the whole energy com
municated to the working substance.
In a perfect engine the cycle of variations is thus : —
I. The metabatic function is increased, say from 6 to 9 V
II. The metamorphic function is increased by the amount A <f>.
III. The metabatic function is diminished from Q x back to 6 .
IV. The metamorphic function is diminished by the amount A (p.
During the second operation, the energy received by the working sub
stance, and transformed from the actual to the potential form is B x A 0.
During the fourth operation energy is transformed back, to the amount
O A 0. So that the energy permanently transformed during each cycle
a a
is (9 1 — ) A <f> ; and the efficiency of the engine — — °.
"i
Section XVIII. — Diffusion of Actual Energy; Irreversible or
Frictional Operations.
There is a tendency in every substance, or system of substances, to the
equable diffusion of actual energy; that is to say, to its transfer between the
OUTLINES OF THE SCIENCE OF ENERGETICS. 227
parts of the substance or system, until the value of the metabatic function
becomes uniform.
This process is not directly reversible; that is to say, there is no such
operation as a direct concentration of actual energy through a tendency of
the metabatic function to become unequal in different parts of a substance
or system.
Hence arises the impossibility of using the energy reconverted to the
actual form at the lower limit" of the metabatic function in an engine.
There is an analogy in respect of this property of irreversibility, between
the diffusion of one kind of actual energy and certain irreversible trans
formations of one kind of actual energy to another, called by Professor
William Thomson, " Frictional Phenomena " — viz., the production of heat
by rubbing, and agitation, and by electric currents in a homogeneous sub
stance at a uniform temperature.
In fact, a conjecture may be hazarded, that immediate diffusion of the
actual energy produced in frictional phenomena, is the circumstance which
renders them irreversible ; for, suppose a small part of a substance to have
its actual energy increased by the exertion of some kind of work upon it,
then, if the increase of actual energy so produced be immediately diffused
amongst other parts, so as to restore the uniformity of the metabatic
function, the whole process will be irreversible. This speculation, how
ever, is, for the present, partly hypothetical ; and, therefore, does not,
strictly speaking, form part of the science of energetics.
Section XIX. — Measurement of Time.
The general relations between energy and time must form an important
branch of the science of energetics; but for the present, all that I am
prepared to state on this subject is the following DEFINITION OF EQUAL
times : —
Equal times are the times in which equal quantities of the same kind of
work are performed by equal and similar substances, under loholly similar
circumstances.
Section XX. — Concluding Eemarks.
It is to be observed, that the preceding articles are not the results of a
new and hitherto untried speculation, but are the generalised expression
of a method of reasoning which has already been applied with success to
special branches of physics.
£28 OUTLINES OF THE SCIENCE OF ENERGETICS.
In this brief essay, it has not been attempted to do more than to give
an outline of some of the more obvious principles of the science of ener
getics, or the abstract theory of physical phenomena in general ; a science
to which the maxim, true of all science, is specially applicable — that its
subjects are boundless, and that they never can, by human labours, be
exhausted, nor the science brought to perfection.
THE PHRASE "POTENTIAL ENERGY." 229
XIII.— ON THE PHEASE "POTENTIAL ENERGY," AND ON
THE DEFINITIONS OF PHYSICAL QUANTITIES*
1. In the course of an essay by Sir John Herschel "On the Origin of
Force," which appeared some time ago in the Fortnightly Review, and has
lately been republished in a volume, entitled Familiar Lectures on Scientific
Subjects, the opinion is expressed that the phrase " Potential Energy " is
" unfortunate, inasmuch as it goes to substitute a truism for the announce
ment of a great dynamical fact" (Familiar Lectures, page 469).
2. There is here no question as to the reality of the class of relations
amongst bodies to which that phrase is applied, nor as to any matter of
fact concerning those relations, but as to the convenient and appropriate
use of language. This is a sort of question in the discussion of which
authority has much weight ; and when an objection to the appropriateness
of a term is made by an author who is not less eminent as a philosopher
than as a man of science, and whose skill in the art of expressing scientific
truth in clear language is almost unparalleled, it becomes the duty of those
who use that term to examine carefully their grounds for doing so.
3. As the phrase "Potential Energy," now so generally used by writers on
physical subjects, was first proposed by myself in a paper " On the General
Law of the Transformation of Energy," f read before the Philosophical
Society of Glasgow, on the 5th of January, 1853 (seep. 203), I feel that
the remark of Sir John Herschel makes it incumbent upon me to explain
the reasons which led me, after much consideration, to adopt that phrase
for the purpose of denoting all those relations amongst bodies, or the parts
of bodies, which consist in a power of doing work dependent on mutual
configurations.
4. The kind of quantity now in question forms part of the subject of
the thirty ninth proposition of Newton's Principia; but it is there repre
sented by the area of a figure, or by symbols only, and not designated
by a name ; and such is also the case in many subsequent mathematical
writings.
5. The application of the word "force" to that kind of quantity is open
* Pvead before the Philosophical Society of Glasgow on Jan. 23, 1867, and published
in the Proceedings of that Society, Vol. VI., No. III.
t Viz.,— that the effect of the presence of a quantity of actual energy, in causing
transformation of energy between the actual and the potential forms, is the sum of the
effects of all the parts of that quantity.
230 THE PHRASE "POTENTIAL ENERGY."
to the objection, that when "force" is taken in the sense in which Newton
defines "vis motrix," the power of performing work is not simply force,
but force multiplied by space. To make such an application of the word
"force," therefore, would have been to designate a product by the name
properly belonging to one of its factors, and would have added to
the confusion which has already arisen from the ambiguous employ
ment of that word.
6. The word " power," though at first sight it might seem very appro
priate, was already used in mechanics in at least three different senses : —
viz., first, the power of an engine, meaning the rate at which it performs
work, and being the product of force and space divided by time; secondly,
the power, in the sense of effort or pressure, which drives a machine ; and
thirdly, "mechanical powers," meaning certain elementary machines. Thus
"'power" was open to the same sort of objection with "force."
7. About the beginning of the present century, the word "energy" had
been substituted by Dr. Thomas Young for "vis viva," to denote the
capacity for performing work due to velocity ; and the application of the
same word had at a more recent time been extended by Sir William
Thomson to capacity of any sort for performing work. There can be no
doubt that the word "energy" is specially suited for that purpose; for not
only does the meaning to be expressed harmonise perfectly with the
etymology of avipyua, but the word "energy" has never been used in
precise scientific writings in a different sense; and thus the risk of
ambiguity is avoided.
8. It appeared to me, therefore, that what remained to be done, was to
qualify the noun "energy" by appropriate adjectives, so as to distinguish
between energy of activity and energy of configuration. The wellknown
pair of antithetical adjectives, " actual " and " potential," seemed exactly
suited for that purpose ; and I accordingly proposed the phrases " actual
energy " and " potential energy," in the paper to which I have referred.
9. I was encouraged to persevere in the use of those phrases, by the
fact of their being immediately approved of and adopted by Sir William
Thomson ; a fact to which I am disposed to ascribe, in a great measure,
the rapid extension of their use in the course of a period so short in
the history of science as fourteen years.* I had also the satisfaction
of receiving a very strong expression of approval from the late Professor
Baden Powell.
10. Until some years afterwards I was not aware of the fact, that the
idea of a phrase equivalent to " potential energy," in its purely mechanical
sense, had been anticipated by Carnot, who, in an essay on machines in
general, employed the term "force vive virtuelle," of which "potential
* Sir William Thomson and Professor Tait have lately substituted the word "kinetic "
for "actual."
THE PHRASE "POTENTIAL ENERGY." 231
energy" might be supposed to be almost a literal translation. That coin
cidence shows how naturally the phrase " potential energy," or something
equivalent, occurs to one in search of words appropriate to denote that
power of performing work which is due to configuration, and not to activity.
11. Having explained the reasons which led me to propose the use of
the phrase " potential energy," I have next to make some observations on
the objection made by Sir John Herschel to that phrase, that " it goes to
substitute a truism for a great dynamical fact."
12. It must be admitted that the use of the term "potential energy"
tends to make the statement of the law of the conservation of energy wear,
to a certain extent, the appearance of a truism. It seems to me, however,
that such must always be the effect of denoting physical relations by words
that are specially adapted to express the properties of those relations ; or,
what amounts virtually to the same thing, of drawing up precise and com
plete definitions of physical terms. Let A and B denote certain conceivable
relations, and let them be precisely and completely defined ; then, from the
definitions follows the proposition, that A and B are related to each other
in a certain way; and that proposition wears the appearance of a truism,
and is virtually comprehended in the definitions. But it is not a bare
truism ; for when with the definitions are conjoined the two facts, ascer
tained by experiment and observation, that there are relations amongst
real bodies corresponding to the definition of A, and that there are also
relations amongst real bodies corresponding to the definition of B, the pro
position as to relation between A and B becomes not a bare truism, but a
physical fact. In the present case, for example, "actual energy" and
"potential energy" are defined in such a way as to make the proposition :
That what a body or a system of bodies gains in one form of energy through
mutual actions, it loses in the other form — in other words, that the sum of
actual and potential energies is "conserved" — follow from the definitions,
so as to sound like a truism ; but when it is proved by experiment and
observation that there are relations amongst real bodies agreeing with the
definitions of " actual energy" and " potential energy," that which otherwise
would be a truism becomes a fact.
1 3. A definition cannot be true or false ; for it makes no assertion, but
says, " let such a word or phrase be used in such a sense;" but it may
be real or fantastic, according as the description contained in it corresponds,
or not, to real objects and phenomena; and when, by the aid of experi
ment and observation, a set of definitions have been framed which possess
reality, precision, and completeness, the investing of a physical fact with
the appearance of a truism is often an unavoidable consequence of the
use of the term so defined.
14. In the case of physical quantities in particular, the definition involves
a rule for measuring the quantity ; and the proof of the reality of the
232 THE PHRASE "POTENTIAL ENERGY."
definition is the fact, that the application of the rule to the same quantity
under different circumstances gives consistent results, which it would not
do if the definition were fantastic ; and hence the definitions of a set of
physical quantities necessarily involve mathematical relations amongst
those quantities, which, when expressed as propositions and compared
with the definitions, wear the appearance of truisms, and are at the same
time statements of fact.
15. In illustration of the foregoing principles, it may be pointed out
that there is a certain set of definitions of the measurement of time, force,
and mass, which reduce the laws of motion to the form of truisms, thus —
I. Let " equal times " mean the times in which a moving body, under
the influence of no force, describes equal spaces. This definition is
proved to be real by the fact, that times which are equal when compared
by means of the free motion of one body, are equal when compared by
means of the free motion of any other body. If the definition were
fantastic, times might be equal as measured by the free motion of one
body, and unequal as measured by that of another.
II. Let "fwce" mean a relation between a pair of bodies such that
their relative velocity changes, or tends to change, in magnitude or
direction, or both ; and let " equal forces " mean those which act when
equal changes of the relative velocity of a given pair of bodies occur in
equal times. This definition is proved to be real by the fact, that the
comparative measurements of forces made in different intervals of time
are consistent with each other, which would not be the case if the
definition were fantastic.
III. Let the " mass " of a body mean a quantity inversely proportional
to the change of velocity impressed on that body in a given time by a
given force. This definition is proved to be real by the fact, that the
ratio of the masses of two given bodies is found experimentally to be
always the same, when those masses are compared by means of the
velocities impressed on them by different forces, and in different times ;
and is also the same, whether each of the masses is measured as a whole
or as the sum of a set of parts.
Assuming those definitions as merely verbal, without reference to
their reality, the laws of motion take the form of verbal truisms; but
when experiment and observation inform us that permanent relations
exist amongst real bodies and real events corresponding to the definitions,
those apparent truisms become statements of fact.
16. One of the chief objects of mathematical physics is to ascertain,
by the help of experiment and observation, what physical quantities or
functions are "conserved." Such quantities or functions are, for example —
I. The mass of every particle of matter, conserved at all times and
under all circumstances.
THE PHRASE " POTENTIAL ENERGY." 233
II. The resultant momentum of a body, or system of bodies, conserved
so long as internal forces act alone.
III. The resultant angular momentum of a body or system of bodies,
conserved so long as internal forces act alone.
IV. The total energy of a body, or system of bodies, conserved so long
as internal forces act alone.
V. The tliermodynamic function, conserved in a body while it neither
receives nor gives out heat.
In defining such physical quantities as those, it is almost, if not quite,
impossible to avoid making the definition imply the property of con
servation ; so that when the fact of conservation is stated, it has the form
of a truism.
1 7. In conclusion, it appears to me that the making of a physical law
wear the appearance of a truism, so far from being a ground of objection
to the definition of a physical term, is rather a proof that such definition
has been framed in strict accordance with reality.
234 THE MECHANICAL ACTION OF HEAT.
XIV.— ON THE MECHANICAL ACTION OF HEAT, ESPECIALLY
IN GASES AND VAPOURS. *
Introduction — Summary of the Principles of the Hypothesis
of Molecular Vortices, and its Application to the Theory
of Temperature, Elasticity, and Real Specific Heat.
The ensuing paper forms part of a series of researches respecting the
consequences of an hypothesis called that of Molecular Vortices, the
object of which is, to deduce the laws of elasticity, and of heat as connected
with elasticity, by means of the principles of mechanics, from a physical
supposition consistent and connected with the theory which deduces the
laws of radiant light and heat from the hypothesis of undulations. Those
researches were commenced in 1842, and after having been laid aside
for nearly seven years, from the want of experimental data, were resumed
in consequence of the appearance of the experiments of M. Regnault on
gases and vapours.
The investigation which I have now to describe, relates to the mutual
conversion of heat and mechanical power by means of the expansion and
contraction of gases and vapours.
In the introduction, which I here prefix to it, I purpose to give such a
summary of the principles of the hypothesis as is necessary to render the
subsequent investigation intelligible.
The fundamental suppositions are the following : —
First, That each atom of matter consists of a nucleus, or central physical
point, enveloped by an elastic atmosphere, which is retained in its position by
forces attractive towards the nucleus or centre.
Suppositions similar to this have been brought forward by Franklin,
iEpinus, Mossotti, and others. They have in general, however, conceived
the atmosphere of each nucleus to be of variable mass. I have treated
it, on the contrary, as an essential part of the atom. I have left the
question indeterminate, whether the nucleus is a small body of a character
distinct from that of the atmosphere, or merely a portion of the atmosphere
in a highly condensed state, owing to the mutual attraction of its parts.
According to this first supposition, the boundary between two con
* Read before the Royal Society of Edinburgh on Feb. 4, 1850, and published in
the Transactions of that Society, Vol. XX., Part I. (See also p. 16.)
THE MECHANICAL ACTION OF HEAT. 235
tiguous atoms of a body is an imaginary surface at which the attractions
of all the atomic centres of the body balance each other; and the elasticity
of the body is made up of two parts : First, the elasticity of the atomic
atmospheres at the imaginary boundaries of the atoms, which I shall call
the superficialatomic elasticity; and, secondly, the force resulting from
the mutual actions of distinct atoms. If the atmospheres are so much con
densed round their nuclei or centres, that the superficialatomic elasticity
is insensible, and that the resultants of the mutual actions of all parts of
the distinct atoms are forces acting along the lines joining the nuclei or
centres, then the body is a perfect solid, having a tendency to preserve
not only a certain bulk, but a certain figure ; and the elasticity of figure,
or rigidity, bears certain definite relations to the elasticity of volume.
If the atmospheres are less condensed about their centres, so that the
mutual actions of distinct atoms are not reducible to a system of forces
acting along the lines joining the atomic centres, but produce merely a
cohesive force sufficient to balance the superficialatomic elasticity, then
the condition is that of a perfect liquid ; and the intermediate conditions
between this and perfect solidity constitute the gelatinous, plastic, and
viscous states.
When the mutual actions of distinct atoms are very small as compared
with the superficialatomic elasticity, the condition is that of gets or vapour;
and when the substance is so far rarefied that the influence of the atomic
nuclei or centres in modifying the superficial elasticity of their atmos
pheres is insensible, it is then in the state of perfect gas.
So far as our experimental knowledge goes, the elasticity of a perfect
gas, at a given temperature, varies simply in proportion to its density.
I have therefore assumed this to be the law of the elasticity of the
atomic atmospheres, ascribing a specific coefficient of elasticity to each
substance.
The second supposition, being that from which the hypothesis of mole
cular vortices derives its name, is the following : — That the elasticity due
to heat arises from the centrifugal force of revolutions or oscillations among the
particles of the atomic atmospheres ; so that quantity of heat is the vis viva of
those revolutions or oscillations.
This supposition appears to have been first definitely stated by Sir
Humphry Davy. It has since been supported by Mr. Joule, whose
valuable experiments to establish the convertibility of heat and mechanical
power are well known. So far as I am aware, however, its consequences
have not hitherto been mathematically developed.
To connect this hypothesis with the undulatory theory of radiation, I
have introduced a third supposition : That the medium which transmits light
and radiant heat consists of the nuclei of the atoms, vibrating independently,
or almost independently, of their atmospheres; so that the absorption of
236 THE MECHANICAL ACTION OF HEAT.
light and of radiant heat, is the transference of motion from the nuclei to
their atmospheres; and the emission of light and of radiant heat, the
transference of motion from the atmospheres to their nuclei.
Although in all undulations of sensible length and amplitude, such as
those of sound, the nuclei must carry their atmospheres along with them,
and vibrating thus loaded, produce a comparatively slow velocity of
propagation; yet, in all probability, the minute vibrations of light and
radiant heat may be performed by the atomic nuclei in transparent and
diathermanous bodies, without moving the atmospheres more than by
that amount which constitutes absorption; and those vibrations will
therefore be transmitted according to the laws of the elasticity of perfect
solids, and with a rapidity corresponding to the extreme smallness of the
masses set in motion, as compared with the mutual forces exerted by them.
This supposition is peculiar to my own view of the hypothesis, and is,
in fact, the converse of the idea hitherto adopted, of an ether surrounding
ponderable particles.
The second and third suppositions involve the assumption, that motion
can be communicated between the nuclei and their atmospheres, and
between the different parts of the atmospheres; so that there is a tendency
to produce some permanent condition of motion, which constitutes equili
brium of heat. It is now to be considered what kind of motion is capable
of producing increase of elasticity, and what are the conditions of perma
nency of that motion.
It is obvious, that the parts of the atomic atmospheres may have
motions of alternate expansion and contraction, or of rectilinear oscillation
about a position of equilibrium, without affecting the superficial atomic
elasticity, except by small periodical changes. Should they have motions,
however, of revolution about centres, so as to form a group of vortices, the
centrifugal force will have the effect of increasing the density of the
atmosphere at what I have called the bounding surfaces of the atoms, and
thus of augmenting the elasticity of the body.
In this summary, I shall not enter into the details of mathematical
analysis, but shall state results only. The following, then, are the con
ditions which must be fulfilled, in order that a group of vortices, of small
size as compared with the bulk of an atom, and of various diameters, may
permanently coexist, whether side by side, or end to end, in the atomic
atmospheres of one substance, or of various substances mixed.
First, The mean elasticity must vary continuously, which involves the
condition, that at the surface of contact of two vortices of different
substances, side by side, or end to end, the respective densities at each
point of contact must be inversely proportional to the coefficients of
elasticity. Hence, the specific gravities of the atmospheric parts of all
substances, under precisely similar circumstances as to heat and molecular forces
THE MECHANICAL ACTION OF HEAT. 237
(a condition realised in perfect gases at the same pressure and temperature),
are inversely proportional to the coefficients of atmospheric elasticity. Therefore,
let juL represent the mass of the atmosphere of one atom of any substance,
b its coefficient of elasticity, and n the number of atoms which, in the
state of perfect gas, occupy unity of volume under unity of pressure at
the temperature of melting ice; — then
n fib (I.)
is a constant quantity for all substances.
Secondly, The superficial elasticity of a vortex must not be a function
of its diameter : to fulfil which condition, the linear velocity of revolution
must be equal throughout all parts of each individual vortex.
Thirdly, In all contiguous vortices of the same substance, the velocities
of revolution must be equal; and in contiguous vortices of different
substances, the squares of the velocities must be proportional to the
coefficients of elasticity of the molecular atmospheres.
The second and third conditions are those of equilibrium of heat, and
are equivalent to this law : —
Temperature is a function of the square of the velocity of revolution in
the molecular vortices, divided by the coefficient of elasticity of the atomic atmos
pheres; or
Temperature = ( —J, . . . (II.)
where w represents that velocity.
The mean elasticity which a vortex exerts endways is not affected by
its motion, being equal to
b P , (HI.)
where p is its mean density. The superficial elasticity at its lateral
surfaces, however, is expressed by
% + *p (iv.)
The additional elasticity ■—*, being that which is due to the motion, is
independent of the diameter. The divisor #,(the force of gravity) is
introduced, on the supposition of the density p being measured by
weight.
Supposing the atmosphere of an atom to be divided into concentric
spherical layers, it may be shown that the effect of the coexistence of a
great number of small vortices in one of those layers whose radius is
r, and mean density p, is to give it a centrifugal force, expressed by
^ (V.)
gr v
238 THE MECHANICAL ACTION OF HEAT.
which tends to increase the density and elasticity of the atmosphere at
the surface, which I have called the boundary of the atom. The layer
is also acted upon by the difference between the mean elasticities at its
two surfaces, and by the attraction towards the atomic centre; and these
three forces must balance each other.
I have integrated the differential equation which results from this
condition, for substances in the gaseous state, in which the forces that
interfere with the centrifugal force and atmospheric elasticity are com
paratively small ; and the result is
p = J s D (5 +1 )( 1  F ) + /( D ) • < VL >
P is the entire elasticity of the gas, and D its mean density. M repre
sents the total mass of an atom, measured by weight, and /.i that of its
atmospheric part; so that D is the mean density of the atomic
atmospheres.
/ (D) denotes the effect of the mutual actions of separate atoms.
The first term represents the superficialatomic elasticity. F denotes
the effect of the attraction of the nucleus in modifying that elasticity,
and can be represented approximately by a converging series, in terms of
ID 2
the negative powers of „ r + 1, commencing with the inverse square,
the coefficients being functions of the density D.
By using the first term of such a series, and determining its coefficient
and the quantity /(D) empirically, I have obtained formula? agreeing
closely with the results of M. Eegnault's experiments on the expansion
of atmospheric air, carbonic acid, and hydrogen.
In a perfect gas, the above expression is reduced to
p=i H D W> + 1 > < VIL >
Let n, as before, denote the number of atoms of a substance which, in
the state of perfect gas, occupy unity of volume under unity of pressure,
at the temperature of melting ice, so that wM is its specific gravity in
that state : then,
p = ^ J (sT* + 1 ) • ( VIIL >
The factor by which ^= is here multiplied, fulfils the condition of
THE MECHANICAL ACTION OF HEAT. 239
being a function of  , and of constants which are the same for all
substances, and is, therefore, fitted for a measure of temperature. It
obviously varies proportionally to the pressure of a perfect gas of a given
density, or its volume under a given pressure.
Let r, therefore, denote temperature, as measured from an imaginary zero,
C degrees of the scale adopted, below the temperature of melting ice,
at which
3gb
Then, for all substances
and in perfect gases • O^*)
»lff
r may be termed absolute temperature, and the point from which it is
measured, the absolute zero of temperature. This, as I have observed, is an
imaginary point, being lower than the absolute zero of heat by the
quantity Ciifib, which is the same for all substances.
The value of C, or the absolute temperature of melting ice, as determined
from M. Eegnault's experiments, is
274°6 Centigrade,
being the reciprocal of
000364166 per Centigrade degree,
the value to which the coefficients of dilatation of gases at the temperature
of melting ice approximate as they are rarefied.
For Fahrenheit's scale C = 494°28.
In the sequel I shall represent temperatures measured from that of
melting ice by
T = r  C.
We have now to consider the absolute quantity of heat, or of mole
cular vis viva which corresponds to a given temperature in a given substance.
It is obvious that
/AW 2
represents, in terms of gravity, the portion of vis viva, in one atom, due
240
THE MECHANICAL ACTION OF HEAT.
to the molecular vortices ; but besides the vortical motion, there may be
oscillations of expansion and contraction, or of rectilinear vibration about a
position of equilibrium. The velocity with which these additional motions
are performed will be in a permanent condition, when the mean value
of its square, independent of small periodic changes, is equal throughout
the atomic atmosphere. We may therefore represent by
fX V L _ fX w*
27 "27'
(X.)
the total vis viva of the atomic atmosphere. To this we have to add that
of the nucleus, raising the quantity of heat in one atom to
Mr 2
2<7
= <Z>
while the quantity of heat in unity of weight is y .
(XI.)
2? H
The coefficient k (which enters into the value of specific heat) being
the ratio of the vis viva of the entire motion impressed on the atomic
atmospheres by the action of their nuclei, to the vis viva of a peculiar kind of
motion, may be conjectured to have a specific value for each substance
depending, in a manner as yet unknown, on some circumstance in the
constitution of its atoms. Although it varies in some cases for the same
substance in the solid, liquid, and gaseous states, there is no experimental
evidence that it varies for the same substance in the same condition. In
the investigation which follows, therefore, I have treated it as sensibly
constant.
The following, then, are the expressions for quantity of heat in terms
of temperature : in one atom, —
v 1 __ 3fcM , n ..
In unity of weight,
3k
Q = 2^ = 2^ r  Cw ^
1
Y (xii.)
J
Real specific heat is, consequently, expressed by the following equa
tions : —
THE MECHANICAL ACTION OF HEAT.
241
For one atom, —
dq
dr~
3kM
2Crc//
For unity of weight, —
dQ
'dr ~
3k
2Cwu
y (xiii.)
For so much of a perfect gas as occupies unity of volume
under unity of pressure at the temperature of melting
ice, —
dq 3kM
d<
2C
fi
The laws established experimentally by Dulong, that the specific heats
of simple atoms, and of certain groups of compound atoms, bear to each
other simple ratios, generally that of equality, and that the specific
heats of equal volumes of all simple gases are equal, show that the specific
factor depends on the chemical constitution of the atom, and thus
M
confirm the conjecture I have stated respecting the coefficient k.
As I shall have occasion, in the investigation which follows, to refer
to and to use the equation for the elasticity of vapours in contact with
their liquids, which I published in the Edinburgh New Philosophical
Journal for July, 1849, I shall here state generally the nature of the
reasoning from which it was deduced.
The equilibrium of a vapour in contact with its liquid depends on three
conditions :
First, The total elasticity of the substance in the two states must be
the same.
Secondly, The superficial atomic elasticity must vary continuously;
so that if at the surface which reflects light there is an abrupt change
of density (which seems almost certain), there must there be two densities
corresponding to the same superficialatomic elasticity.
Thirdly, The two forces which act on each stratum of vapour parallel
to the surface of the liquid — namely, the preponderance of molecular
attraction towards the liquid, and the difference of the superficialatomic
elasticities at the two sides of the stratum — must be in equilibrio.
Close to the surface of the liquid, therefore, the vapour is highly
condensed. The density diminishes rapidly as the distance from the
liquid increases, and at all appreciable distances has a sensibly uniform
value, which is a function of the temperature and of certain unknown
molecular forces.
Q
242
THE MECHANICAL ACTION OF HEAT.
The integration of a differential equation representing the third condition
of equilibrium, indicates the form of the approximate equation,
P v
Log P = a  ^ 
T T"
(XIV.)
the coefficients of which have been determined empirically by three
experimental data for each fluid. For proofs of the extreme closeness
with which the formula? thus obtained agree with experiment, I refer to
the Journal in which they first appeared.
I annex a table of the coefficients for water, alcohol, ether, turpentine,
petroleum, and mercury, in the direct equation, and also in the inverse
formula,
J
• A 1
y 4 r
ay
(XV.)
by which the temperature of vapour at saturation may be calculated from
the pressure. In the ninth and tenth columns are stated the limits
between which the formulae have been compared with experiment.
For turpentine, petroleum, and mercury, the formula consists of two
terms only.
LokP
/3
a — — .
(XVI.)
the small range of the experiments rendering the determination of y
impossible.
The following are some additional values of the constant a for steam,
corresponding to various units of pressure used in practice.
Units of Pressure.
Atmospheres of 7 GO millimetres of mercury,
= 29*922 inches of mercury,
= 14" 7 lbs. on the square inch,
= 1 , 0333 kilogrammes on the square centimetre,
Atmospheres of 30 inches of mercury,
= 761  99 millimetres,
= 14  74 lbs. on the square inch,
= T036 kilogrammes on the sqv
Kilogrammes on the square centimetre,
Kilogrammes on the circular centimetre,
Pounds avoirdupois on the square inch,
Pounds avoirdupois on the circular inch,
Pounds avoirdupois on the square foot,
Values of a.
4950433
are centimetre, 4949300
4964658
4859748
6117662
6012752
8276025
All the numerical values of the constants are for common logarithms.
THE MECHANICAL ACTION OF HEAT.
243
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244 THE MECHANICAL ACTION OF HEAT.
Section I. — Of the Mutual Conversion of Heat and Expansive
Power.
1. The quantity of heat in a given mass of matter, according to the
hypothesis of molecular vortices, as well as every other hypothesis which
ascribes the phenomena of heat to motion, is measured by the mechanical
power to which that motion is equivalent, that being a quantity the
total amount of which, in a given system of bodies, cannot be altered by
their mutual actions, although its distribution and form may be altered.
This is expressed in equation XII. of the introduction, where the quantity
v 2
of heat in unity of weight, Q, is represented by the height  , from which
a body must fall in order to acquire the velocity of the molecular oscilla
tions. This height, being multiplied by the weight of a body, gives the
mechanical power to which the oscillations constituting its heat are
equivalent. The real specific heat of unity of weight, as given in equa
tion (XIII.) of the introduction,
d Q 3 h
dr 2Cn
/•
represents the dijdli of fall, which is equivalent to one degree of rise of
temperature in any given weight of the substance under consideration.
We know, to a greater or less degree of precision, the ratios of the
specific heats of many substances to each other, and they are commonly
expressed by taking that of water at the temperature of melting ice as
unity ; but their actual mechanical values have as yet been very imper
fectly ascertained, and, in fact, the data necessary for their determination
are incomplete.
2. Mr. Joule, indeed, has made several very interesting series of
experiments, in order to ascertain the quantity of heat developed in
various substances by mechanical power employed in different ways — viz.,
by electric currents excited by the rotation of a magnet, by the forcing
of water through narrow tubes, by the agitation of water and oil with a
paddle, by the compression of air, and by the friction of air rushing
through a narrow orifice. The value of the depth of fall equivalent
to a rise of one degree of Fahrenheit's scale in the temperature of a mass
of water, as determined by that gentleman, varies, in the different series
of experiments, between the limits of 760 feet and 890 feet, the value
in which Mr. Joule appears to place the greatest confidence being about
780 feet.
Although the smallness of the differences of temperature measured in
those experiments renders the numerical results somewhat uncertain,
THE MECHANICAL ACTION OF HEAT. 245
it appears to me that, as evidence of the convertibility of heat and
mechanical power, they are unexceptionable. Nevertheless, there is
reason to believe that the true mechanical equivalent of heat is consider
ably less than any of the values deduced from Mr. Joule's experiments;
for in all of them there are causes of loss of power the effect of which
it is impossible to calculate. In all machinery, a portion of the power
which disappears is carried off by waves of condensation and expansion,
along the supports of the machine, and through the surrounding air :
this portion cannot be estimated, and is, of course, not operative in
producing heat within the machine. It is also impossible to calculate,
where friction is employed to produce heat, what amount of it has been
lost in the production of electricity, a power which is, no doubt, conver
tible into heat, but which, in such experiments, probably escapes without
undergoing that conversion. To make the determination of the mechanical
equivalent of heat by electromagnetic experiments correct, it is necessary
that the whole of the mechanical power should be converted into magnetic
power, the whole of the magnetic power into what are called electric
currents, and the whole of the power of the electric currents into heat,
not one of Avhich conditions is likely to be exactly fulfilled. Even in
producing heat by the compression of air, it must not be assumed that
the whole of the mechanical power is expended in raising the tempera
ture.
3. The best means of determining the mechanical equivalent of
heat are furnished by those experiments in which no machinery is
employed. Of this kind are experiments on the velocity of sound in air
and other gases, which, according to the received and wellknown theory
of Laplace, is accelerated by the heat developed by the compression of
the medium.
The accuracy of this theory has lately been called in question. There
can be no doubt that it deviates from absolute exactness, in so far that
the magnitude of the displacements of the particles of air is neglected
in comparison with the length of a wave. It appears to me, however,
that the AstronomerEoyal, in his remarks on the subject in the London
and Edinburgh Philosophical Magazine for July, 1849, has shown, in a
satisfactory manner, that although the effect of the appreciable magnitude
of those displacements, as compared with the length of a wave of sound,
is to alter slowly the form of the function representing the wave, still
that effect is not sufficiently great to make Laplace's theory practically
erroneous. I have, therefore, in the sequel, adhered to the experiments
of Dulong, and to those emoted by Poisson, on the velocity of sound, as
the best data for determining the mechanical equivalent of heat.
4. The expression already given for the real specific heat of unity of
weight of a given substance may be resolved into two factors, thus : —
246 THE MECHANICAL ACTION OF HEAT.
dQ, 1_ 3&M (l)
dr C«M 2/i ' ' k ''
The first factor, ~ — r>, may be considered in general as a known quantity;
CwM
for C represents, as already stated, 274"6 Centigrade degrees, the absolute
temperature of melting ice, and n M the theoretical weight, in the per
fectly gaseous state, of unity of volume of the substance, under unity of
pressure, at that temperature; or what is the same thing, — ^ is the
height of an imaginary column of the substance, of uniform density, and
at the temperature of melting ice, whose pressure by weight upon a given
area of base is equal to its pressure by elasticity, supposing it to be
. 3&M .
perfectly gaseous. The determination of the ratio — — is necessary to
complete the solution of the problem.
5. The relation now to be investigated between heat and mechanical
power, is that which exists between the power expended in compressing
a body into a smaller volume, and the increase of heat in consequence of
such a compression ; and, conversely, between the heat which disappears,
or, as it is said, becomes latent during the expansion of a body to a greater
volume, and the mechanical power gained or developed by that expansion.
Those phenomena, according to the hypothesis now under consideration,
as well as every hypothesis which ascribes heat to motion, are simply the
transformation of mechanical power from one shape into another.
It is obvious, in the first place, without the aid of algebraical symbols,
that the general effect of the compression of an oscillating atomic atmos
phere, or molecular vortex, must be to accelerate its motion, and of its
dilatation, to retard its motion; for every portion of such an atmosphere
is urged toward the nucleus or atomic centre by a centripetal force equal
to the centrifugal force arising from the oscillation; so that when, by
compression, each portion of the atmosphere is made to approach the centre
by a given distance, the vis viva of its motion will be increased by the
amount corresponding to the centripetal force acting through that dis
tance ; and, conversely, when by expansion each portion of the atmosphere
is made to retreat from the centre, the vis viva of its motion will be
diminished by a similar amount.
It is not, however, to be taken for granted, that all the power expended
in compressing a body appears in the form of heat. More or less power
may be consumed or developed by changes of molecular arrangement,
or of the internal distribution of the density of the atomic atmospheres;
and changes of molecular arrangement or distribution may develop or
consume heat, independently of changes of volume.
6. We shall now investigate, according to the hypothesis of mole
THE MECHANICAL ACTION OF HEAT. 247
cular vortices, the amount of heat produced by an indefinitely small
compression of one atom of a body in that state of perfect fluidity which
admits of the bounding surface of the atom being treated as if it were
spherical : its radius being denoted by R, and the radius of any internal
spherical layer of the atmosphere by multiplying R by a fraction u.
I shall denote by the ordinary symbol of differentiation d, such changes
as depend on the various positions of portions of the atomic atmosphere
relatively to each other, when changes of volume and temperature are not
taken into consideration; while by the symbol 8 of the calculus of
variations, I shall represent such changes as arise from the variations of
volume and temperature.
Let us consider the case of an indefinitely thin spherical layer of the
atomic atmosphere, whose distance from the nucleus is Rw, its thickness
R d u, its area 4 tt R 2 u 2 , and its density ^ : D \p (u, D, t).
The weight, then, of this layer is
4 ttR 3 ^ Dm 2 ?/,(«, D, T )du.
Its velocity of oscillation is v, and having, in virtue of that velocity, a
mean centrifugal force, as explained in the introduction (Equation V.),
equal to
/ t' 2 2Q \
its weight X ( ? p = y~— )
° \ g k R u kliu/
it is kept in equilibrio by an equal and opposite centripetal force, arising
from attraction and elastic pressure, which is consequently represented by
4ttR 2 £^Dmi/,(w, D > T)du
M gk
= 8 7t R 2 r^ Q D u $ («, D, r) d u.
Let the mean density of the atom now be increased by the indefinitely
small quantity § D. Then the layer will approach the nucleus through
the distance — S(Rw)=— wSR — RSm, and being acted upon through
that distance by the centripetal force already stated, the vis viva of
oscillation will be increased by a quantity corresponding to the mechanical
power (that is to say, the heat), represented by the product of that
distance by that force, or by
8ttR 2 t^QDm^ («, J),T)duxd(Ru)
248 THE MECHANICAL ACTION OF HEAT.
= 8»^QD^D,r)^ + ^.
which, because
SR 1 SD , 47rE 3 D __ .
— = —  . jp and = M, is equal to
+ QM.^frD,r)tf(^8^)*iL
"We must suppose that the velocity of oscillation is equalised throughout
the atomic atmosphere, by a propagation of motion so rapid as to be
practically instantaneous.
Then, if the above expression be integrated with respect to du, from
u = to u=l, the result will give the whole increase of heat in the
atom arising from the condensation SD; and dividing that integral by
the atomic weight M, we shall obtain the corresponding development of
heat in unity of weight. This is expressed by the following equation : —
8Q' = *<*&{irfo du • ** ( * D ' r)
3 ( l du.vBu\l,(u t 'D, r) . (2.)
The letter Q' is here introduced to denote, when negative, that heat
which is consumed in producing changes of volume and of molecular
arrangement ; and when positive, as in the above equation, the heat which
is produced by such changes.
The following substitutions have to be made in equation (1) of this
Section :
For Q is to be substituted its value, according to equation XII. of the
introduction; or abbreviating Cn/nb into k, —
«=rab<'— > • • • < 3)
The value of the first integral in equation (2) of this Section is
/•i 1
I du . u 2 \p (u, D, t) = x.
JO "J
The value of the second integral,
— 31 du . uBuip (u, D, t)
J o
remains to be investigated. The first step in this inquiry is given by
THE MECHANICAL ACTION OF HEAT.
219
the condition, that whatsoever changes of magnitude a given spherical
layer undegoes, the portion of atmosphere between it and the nucleus is
invariable. This condition is expressed by the equation
„ A + 8 r 4 + § D ^)fd u . M 2 + (n, D, r), (4.)
du ' dr
from which it follows that
d~D,
Su = 
1 T — (8 r f + S D A ) f\* » . ^ ( w , D, r),
u, D, t) \ dr a D/J o
u 2 \p (m, D, r)
and, consequently, that
— 3 I du . u$uip (u, D, r) =
•Jo
Hence, making
9 I 1 — I" du. u 2 xL (u, D, r) = U. . . (5.)
J U J
The second integral in equation (2) is transformed into
By means of those substitutions we obtain for the mechanical value of
the heat developed in unity of weight of a fluid by any indefinitely small
change of volume or of molecular distribution —
SQ' =
r — k
CnM
(SD(1 + ^WC^
dW
or taking V = ==r to denote the volume of unity of weight
of the substance,
C»M\ \V d V/ d t
(6.)
Of this expression, the portion
k SD
T.C SV
CnM ' Y~ repre "
CnM D
sents the variation of heat arising from mere change of volume.
tk gyfg = JLZJLgr)45, denotes the variation of heat pro
CnM dY CnM dD
duced by change of molecular distribution dependent on change of volume.
250 THE MECHANICAL ACTION OF HEAT.
— — —  8 t —, — expresses the variation of heat due to change of mole
(jnM dr
cular distribution dependent on change of temperature.
7. The function U is one depending on molecular forces, the nature
of which is as yet unknown. The only case in which it can be calculated
directly is that of a perfect gas. Without giving the details of the integra
tion, it may be sufficient to state, that in this case
1
y . . . (7.)
T
and, therefore, that
dU _ K clU _
dr ~ t 25 dV~ J
In all other cases, however, the value of this function can be determined
indirectly, by introducing into the investigation the principle of the
conservation of vis viva.
Suppose a portion of any substance, of the weight unity, to pass through
a variety of changes of temperature and volume, and at length to be
brought back to its primitive volume and temperature. Then the absolute
quantity of heat in the substance, and the molecular arrangement and
distribution, being the same as at first, the effect of their changes is
eliminated ; and the algebraical sum of the vis viva expanded and produced,
whether in the shape of expansion and compression, or in that of heat, must be
equal to zero: that is to say, if, on the whole, any mechanical power has
appeared, and been given out from the body, in the form of expansion,
an equal amount must have been communicated to the body, and must
have disappeared in the form of heat ; and if any mechanical power
has appeared and been given out from the body in the form of heat, an
equal amount must have been communicated to the body, and must
have disappeared in the form of compression. This principle expressed
symbolically is
A n + A Q' = 0, . . . . (8.)
where n, when positive, represents expansive power given out, when
negative, compressive power absorbed; and Q' represents, when positive,
heat given out, when negative, heat absorbed.
To take the simplest case possible, let the changes of temperature and
of volume be supposed to be indefinitely small, and to occur during
distinct intervals of time, so that t and V are independent variables;
let the initial absolute temperature be t, the initial volume V, and the
initial total elasticity P ; and let the substance go through the following
four changes :
THE MECHANICAL ACTION OF HEAT. 251
First, Let its temperature be raised from r to r + 8 r, the volume
remaining unchanged. Then the quantity of heat absorbed is
 ( d ® T ~ K dJJ
dT \dr CmM dr
and there is no expansion nor compression.
Secondly, Let the body expand, without change of temperature, from
the volume V to the volume V + S V. Then the quantity of heat
absorbed is
+ Sric/l d nT , d\J
while the power given out by expansion is
SY(P + ^Sr).
Thirdly, Let the temperature fall from r + 8 r, to its original value r,
the volume V + S V continuing unchanged ; then the heat given out is
+ «'(£wh £<" + £">>
and there is no expansion nor compression.
Fourthly, Let the body be compressed, without change of temperature, to
its original volume V ; then the heat given out is
+ 6V C7iMVV dYJ'
while the power absorbed in compression is — 8 V . P. The body being
now restored in all respects to its primitive state, the sum of the two
portions of power connected with change of volume, must, in virtue of the
principle of vis viva, be equal to the sum of the four quantities of heat
with their signs reversed. Those additions being made, and the sums
divided by the common factor S V 8 t, the following equation is obtained :
dV _ 1 (1 dU\
dr C»M\V dVJ' ' ' ' K ' J
The integral of this partial differential equation is
U = 0.r + fdV^CnM^). . . (10.)
252
THE MECHANICAL ACTION OF HEAT.
Now <f).r being the same for all densities, is the value of U for the
perfectly gaseous state, or ; for in that state, the integral = 0. The
values of the partial differential coefficients are accordingly
dV
dY ~
1
V
—
c
dr
dJJ
dr ~

K
—
CnM fdY
d 2 ~P
dr*
i
J
and they can, therefore, be determined in all cases in which the quantity
ic = Cii{xb, and the law of variation of the total elasticity with the volume
and temperature are known, so as to complete the data required in order
to apply equation 6 of this section to the calculation of the mechanical
value of the variations of heat, due to changes of volume and molecular
arrangement.
The total elasticity of an imperfect gas, according to equations VI. and
XII. of the introduction, being
p uiavO F M)+'<w
its first and second partial differential coefficients with respect to the
temperature arc
dV
dr
d?V
dr 2
i
C n M V
1
0
1+T
a t
d t d r
C n M V
Consequently, for the imperfectly gaseous state,
u
du
dY
'M^l
)j<ivX T) >0
l
A'
nr>fc
y (I 2 )
— = i
a r r"
*£+'£)'
*)/<
rt
8. It is to be observed that the process followed in ascertaining the
nature of the function U is analogous to that employed by M. Carnot
in his theory of the motive power of heat, although founded on contrary
THE MECHANICAL ACTION OF HEAT. 253
principles, and leading to different results. Carnot, in fact, considers
heat to be something of a peculiar kind, whether a condition or a substance,
the total amount of which in nature is incapable of increase or of
diminution. It is not, therefore, according to his theory, convertible into
mechanical power ; but is capable, by its transmission through substances
under parti cidar circumstances, of causing mechanical power to be developed.
He supposes a body to go through certain changes of temperature
and volume, and to return at last to its primitive volume and temperature,
and conceives, in accordance with his view of the nature of heat, that it
must have given out exactly the same quantity of heat that it has absorbed.
The transmission of this heat he regards as the cause of the production
of an amount of mechanical power depending on the quantity of heat
transmitted, and on the temperature at which the transmission has taken
place. According to these principles, a body, having received a certain
quantity of heat, is capable of giving out, not only all the heat it has
received, but also a quantity of mechanical power which did not before exist.
According to the theory of this essay, on the contrary, and to every
conceivable theory which regards heat as a modification of motion, no
mechanical power can be given out in the shape of expansion, unless the
quantity of heat emitted by the body in returning to its primitive tem
perature and volume is less than the quantity of heat originally received ;
the excess of the latter quantity above the former disappearing as heat,
to appear as expansive power, so that the sum of the vis viva in these two
forms continues unchanged.
Section II. — Of Eeal and Apparent Specific Heat, especially in
the State of Perfect Gas.
9. The apparent specific heat of a given substance is found by adding
to the real specific heat (or the heat which retains its form in producing
an elevation of one degree of temperature in unity of weight) that
additional heat which disappears in producing changes of volume and of
molecular arrangement, and which is determined by reversing the sign
of Q 1 in equation 6 of Sect. I (so as to transform it from heat evolved
to heat absorbed), and taking its total differential coefficient with respect
to the temperature. Hence, denoting total apparent specific heat by K,
K = dQ_ d. Q 1 _dQ dQ 1 dQ 1 dV
dr dr dr dr dV ' d
r
1 (31M, , ,/tiV/l dV\ dV\) ,„„.
251 THE MECHANICAL ACTION OF HEAT.
Another mode of expressing this coefficient is the following :
Denote the ratio . M by N, and the real specific heat by
ft = ClM' .... (H.)
then
K i {1 +N(r  k) ( 7z7 ( y  <nr )  7/ J }. (15.)
The value of ■=— is to be determined from the conditions of each par
ticular case, so that each substance may have a variety of apparent specific
heats, according to the manner in which the volume varies with the
temperature.
dV
If the volume is not permitted to vary, so that —  = 0, there is
a t
obtained the following result, being the apparent specific heat at constant
volume : —
*'o^(ifr'»£)*( l *<'")i7> ( 10 »
10. Then the substance under consideration is a perfect gas, it has
already been stated, equation (7), that
d]J k dV _
dr == r 2 ' d V ~ '
and because the volume of unity of weight is directly as the absolute
temperature and inversely as the pressure.
1 cTV _ 1 1 dP_
V dr ~ r P dr ' " " ( '^
Hence, the following are the values of the apparent specific heats of
unity of weight of a theoretically perfect gas under different circum
stances : —
General value of the total apparent specific heat :
THE MECHANICAL ACTION OF HEAT.
255
K M1 +
c»m In t
**&+4h)}
= chi { s + (r _ k) ( ? + r " ^7) 1
Apparent specific heat at constant volume : y (18.)
*jot{s+!S»0+»(:s))
Apparent specific heat under constant pressure :
^ = o^m(s + 1 ?) = s { 1 + n ( 1 ?)} ,
The ratio of the apparent specific heat under constant pressure to the
apparent specific heat at constant volume, is the following : —
1 +N
(ia
1 
l+N 
= 1 +N
1 +N
;5)
(19.)
The value of k is unknown, and, as yet, no experimental data exist
from which it can be determined. I have found, however, that practically,
results of sufficient accuracy are obtained by regarding k as so small in
comparison with
that , and ic fortiori —,, may be neglected in
T T~
calcvdation.
Thus are obtained the following approximate results for perfect gases,
and gases which may without material error be treated as perfect.
General value of the total apparent specific heat : —
_ _L M r dY
CnM\W + Y dr
= fe+P
dY
1
1 _ r dV
N + P dr
CwM
Apparent specific heat at constant volume
1
K, =
%
y (20.)
CnMN
being equal to the real specific heat.
Apparent specific heat under constant pressure :
K„ =
cin(i + i)=ft(i+N).
256 THE MECHANICAL ACTION OF HEAT.
Eatio of those two specific heats : —
 P =1 + N (21.)
This ratio is the quantity called by Poisson y, in his researches on the
propagation of sound.
11. It is unnecessary to do more than to refer to the researches
of Poisson, and to those of Laplace, for the proof that the effect of the
production of heat by the compression of air is the same as if the elasticity
varied in proportion to that power of the density whose index is the
ratio of the two specific heats ; so that the actual' velocity of sound is
greater than that which it would have if there were no such development
of heat, in the proportion of the square root of that ratio.
The following is the value of the velocity of sound in a gas, as given
by Poisson in the second volume of his TraiU de M6caniqu& : —
B =^.y.(l+ET)^, . . . (22.)
where a denotes the velocity of sound, g the velocity generated by gravity
in unity of time, E the coefficient of increase of elasticity with temperature,
at the freezing point of water, T the temperature measured from that
point, m the specific gravity of mercury, A that of the gas at the tem
perature of melting ice, and pressure corresponding to a column of
mercury of the height h. It follows that the ratio y is given by the
formula
Y = l + Nneari y = gm/(( ^ ET) . . . (23.)
Calculations have been made to determine the ratio y from the velocity
of sound ; but as many of them involve erroneous values of the coefficient
of elasticity E, the experiments have to be reduced anew.
The following calculation is founded on an experiment quoted by
Poisson on the velocity of sound in atmospheric air, the values of E, w,
and A being taken from the experiments of M. Eegnault.
a = 34089 metres per second.
g = 9 m S0896. h = m 76. T = 15 0< 9 Centigrade.
in.
E = 0003665; ~ = 10513.
A
Consequently, for atmospheric air,
y = 1401.
THE MECHANICAL ACTION OF HEAT.
257
The results of a reduction, according to correct data, of the experiments
of Dulong upon the velocity of sound in atmospheric air, oxygen, and
hydrogen, are as follows : —
Atmospheric air,
Oxygen, .
Hydrogen,
7
1410
1426
1426
Thus it appears, that for the simple substances, oxygen and hydrogen,
the ratio N is the same, while for atmospheric air it is somewhat smaller*
12. The ordinary mode of expressing the specific heats of gases is
to state their ratios to that of an equal volume of atmospheric air at the
same pressure and temperature.
When  is a very small fraction, specific heats of unity of volume of a
perfect gas are given by the equations —
M K n =
n M K t
1
CN
1 /l
SGr +
(24.)
That is to say, the specific heat of unity of volume at constant volume
is inversely proportional to the fraction by which the ratio of the two
specific heats exceeds unity; a conclusion already deduced from experiment
by Dulong.
The following is a comparison of the ratios of the apparent specific
heats under constant pressure, of unity of volume of oxygen and hydrogen
* The following are some additional determinations of the value of y for atmospheric
air, founded upon experiments on the velocity of sound : —
Observers.
Bravals and Martins : mean of several experi
ments at temperatures varying from 5° to
11° Centigrade, reduced to 0° (Comptes
JRendus, xix.) ......
Moll and Van Beeh : reduced to
Stampfer and Myrbach: reduced to 0° (not
corrected for moisture) ....
Academic des Sciences, 173S : (not corrected )
for moisture) . . . . . . S
A variation of one metre per second in the velocity of sound at 0° corresponds to a
variation of '0085 in the value of y.
K
T
Centigrade.
a
Metres per second.
y
0°
33237
140955
0°
33225
140853
1 0'
33296
141456
G°l
33710
141S
258 THE MECHANICAL ACTION OF HEAT.
respectively, to that of atmospheric air, as deduced from equation (24),
with those determined experimentally by De la Roche and Berard: —
_ A . wMK p (Gas)
Ratio ■
n M K r (Atmos. air)
Gas. By Theory. By Experiment.
Oxygen, . . . 0973 0*9705
Hydrogen, . . . 0973 09033
This comparison exhibits a much more close agreement between theory
and experiment than has been hitherto supposed to exist, the errors in
the constants employed having had the effect of making the ratio 1+N
seem greater for atmospheric air than for oxygen and hydrogen, while in
fact it is smaller.
To treat the other substances on which both M. Dulong and MM. De
la Roche and Berard made experiments as perfect gases, would lead to
sensible errors. I have, therefore, confined my calculations for the
present to oxygen, hydrogen, and atmospheric air.
13. The heat produced by compressing so much of a perfect gas as
would occupy unity of volume under the pressure unity, at the temperature
0° Centigrade, from its actual volume nMY 1 = pp> into a volume which
is less in a given ratio ^ (when k is neglected as compared with r), is
expressed by the following motion : —
sV s
«MQ' = i( 1 dY.^ = nM\ 1 t Vds, . (25.)
v 1
being, in fact, equal to the mechanical power used in the compression.
When the temperature is maintained constant, this becomes
»MQ' (T) = £log e .i . . (26.)
which is obviously independent of the nature of the gas.
Hence, equal volumes of all substances in the state of perfect gas, at the same
pressure, and at equal and constant temperatures, being compressed by the same
amount, disengage equal quantities of heat; a law already deduced from
experiment by Dulong.
14. The determination of the fraction N affords the means of
calculating the mechanical or absolute value of specific heat, as defined by
equation (1), section first. The data for atmospheric air being taken as
follows : —
N =: 04, C = 274°6 Centigrade,
THE MECHANICAL ACTION OF HEAT. 259
—== := height of an imaginary column of air of uniform density, at the
n M
temperature 0° Cent., whose pressure by weight on a given base is equal to
its pressure by elasticity,
= 7990 metres, =26214 feet :—
the real specific heat of atmospheric air, or the depth of fall equivalent
to 1 Centigrade degree of temperature in that gas, is found to be
C»H
= 7274 metres = 238*60 feet. . (27.)
The apparent specific heat of atmospheric air, under constant pressure,
according to De la Eoche and Berard, is equal to that of liquid water at
0° Centigrade x 02669. The ratio of its real specific heat to the apparent
specific heat of water at 0° Centigrade is, therefore,
•2669 x ~= 1906.
1"4
And, consequently, the mechanical value of the apparent specific heat of
liquid water, at the temperature of melting ice, is
; n '7 = 38T64 metres = 1252 feet per Centigrade degree, ( . x
•1906 > [to.)
or 695*6 feet per degree of Fahrenheit's scale. )
This quantity we shall denote by K w . It is the mechanical equivalent
of the ordinary thermal unit
I have already pointed out (in article 2 of the first section) the causes
which tend to make the apparent value of the mechanical equivalent of
heat, in Mr. Joule's experiments, greater than the true value. The
differences between the result I have just stated, and those at which he
has arrived, do not seem greater than those causes are capable of producing
when combined with the uncertainty of experiments, like those of Mr.
Joule, on extremely small variations of temperature.
15. Besides the conditions of constant volume and constant pressure,
there is a third condition in which it is of importance to know the
apparent specific heat of an elastic fluid — namely, the condition of vapour
at saturation, or in contact with its liquid.
The apparent specific heat of a vapour at saturation is the quantity of
heat which unity of weight of that vapour receives or gives out, while
its temperature is increased by one degree, its volume being at the same
time compressed so as to bring it to the maximum pressure corresponding
to the increased temperature.
2G0 THE MECHANICAL ACTION OF HEAT.
It has been usually taken for granted, that this quantity is the same,
with the variation for one degree of temperature, of what is called the
total heat of evaporation. Such is, indeed, the case according to the theory
of Carnot; but I shall show that, according to the mechanical theory of
heat, these two quantities are not only distinct, but in general of con
trary signs.
I shall, for the present, consider such vapours only as may be treated
in practice as perfect gases, so as to make the first of the equations (20)
applicable.
It has been shown that the logarithm of the maximum elasticity of a
vapour in contact with its liquid may be represented by the expression
Log. P = a  £  <.
T T
The coefficients a, /3, y, being those adapted for calculating the common
logarithm of the pressure, I shall use the accented letters a, /3', y, to
denote those suited to calculate the hyperbolic logarithm, being equal
respectively to the former coefficients X 2 , 3025851.
Then for vapour at saturation,
11. _ £ + Ix (29 ,
Prfr _ r + J*' ■ ■ ■ (>)
Making this substitution in the general equation (21,) wo find the
following value for the apparent specific heat of perfectly gaseous vapour
at saturation :
K s ^h + P (  = h(l+N. ? )
. (30.)
>{i+*(if£)}
C n M \N ^ T r 2 /
16. For the vapours of which the properties are known, the negative
terms t>f this expression exceed the positive at all ordinary temperatures,
so that the kind of apparent specific heat now under consideration is a
negative quantity : — that is to say, that if a given weight of vapour at
saturation is increased in temperature, and at the same time maintained
by compression at the maximum elasticity, the heat generated by the
compression is greater than that which is required to produce the elevation
of temperature, and a surplus of heat is given out; and on the other
hand, if vapour at saturation is allowed to expand, and at the same time
THE MECHANICAL ACTION OF HEAT. 261
maintained at the temperature of saturation, the heat which disappears
in producing the expansion is greater than that set free by the fall of
temperature ; and the deficiency of heat must be supplied from without,
otherwise a portion of the vapour will be liquefied in order to supply the heat
necessary for the expansion of the rest.
This circumstance is obviously of great importance in meteorology, and
in the theory of the steamengine. There is as yet no experimental
proof of it. It is true that, in the working of noncondensing engines,
it has been found that the steam which escapes is always at the tem
perature of saturation corresponding to its pressure, and carries along with
it a portion of water in the liquid state ; but it is impossible to distinguish
1 ictween the water which has been liquefied by the expansion of the steam,
and that which has been carried over mechanically from the boiler.
The calculation of the proportion of vapour liquefied by a given
expansion, requires the knowledge of the latent heat of evaporation, which
forms the subject of the next section.
Section III— Of the Latent and Total Heat of Evaporation,
ESPECIALLY FOR WATER.
1 7. The latent heat of evaporation of a given substance at a given
temperature, is the amount of heat which disappears in transforming
unity of weight of the substance from the liquid state, to that of vapour
of the maximum density for the given temperature, being consumed in
producing an increase of volume, and an unknown change of molecular
arrangement.
It is obvious, that if the vapour thus produced is reconverted into the
liquid state at the same temperature, the heat given out during the lique
faction must be equal to that consumed during the evaporation; for as
the sum of the expansive and compressive powers, and of those dependent
on molecular arrangement during the whole process, is equal to zero,
so must the sum of the quantities of heat absorbed and evolved.
The heat of liquefaction, at a given temperature, is therefore equal
to that of evaporation, with the sign reversed.
18. If to the latent heat of evaporation at a given temperature, is
added the quantity of heat necessary to raise unity of weight of the
liquid from a certain fixed temperature, (usually that of melting ice), to
the temperature at which the evaporation takes place, the result is
called the total heat of evaporation from the fixed temperature chosen.
According to the theory of Carnot, this quantity is called the constituent
heat of vapour ; and it is conceived, that if liquid at* the temperature oi
melting ice be raised to any temperature and evaporated, and finally
262 THE MECHANICAL ACTION OF HEAT.
brought in the state of vapour to a certain given temperature, the whole
heat expended will be equal to the constituent heat corresponding to that
given temperature, and will be the same, whatsoever may have been the
intermediate changes of volume, or the temperature of actual evaporation.
According to the mechanical theory of heat, on the other hand, the
quantity of heat expended must vary with the intermediate circumstances;
for otherwise no power could be gained by the alternate evaporation and
liquefaction of a fluid at different temperatures.
19. The law of the latent and total heat of evaporation is immediately
dcducible from the principle of the constancy of the total vis viva in the
two forms of heat and expansive power, when the body has returned to
its primitive density and temperature, as already laid down in article 7.
That principle, when applied to evaporation and liquefaction, may be
stated as follows : —
Let a portion of fluid in the liquid state be raised from a certain
temperature to a higher temperature : let it be evaporated at the higher
temperature : let the vapour then be allowed to expand, being maintained
always at the temperature of saturation for its density, until it is restored
to the original temperature, at which temperature let it be liquefied : —
then, the excess of the heat absorbed by the fluid above the heat given out, will
he equal to the expansive powt v generated.
To represent those operations algebraically, — let the lower absolute
temperature be t ; the volume of unity of weight of liquid at that
temperature r , and that of vapour at saturation V : let the pressure of
that vapour be P ; the latent heat of evaporation of unity of weight L ;
and let the corresponding quantities for the higher absolute temperature
t v be v v V v P x , L r Let K L represent the mean apparent specific heat
of the substance in the liquid form between the temperatures r and r r
Then,—
First, Unity of weight of liquid being raised from the temperature r
to the temperature r v absorbs the heat,
K l( t i  r o)>
and produces the expansive power,
J dv.V.
Secondly, It is evaporated at the temperature r v absorbing the heat
and producing the expansive power,
THE MECHANICAL ACTION OF HEAT. 263
P x (V x  vj.
Thirdly, The vapour expands, at saturation, until it is restored to the
original temperature r. In this process it absorbs the heat,
r
dr . K 3 ,
and produces the expansive power
I dV.F.
Fourthly, It is liquefied at the original temperature, giving out the heat
\y
and consuming the compressive power,
Po(V *'o)
The equation between the heat which has disappeared, and the expansive
power which has been produced, is as follows : —
L x  L + K t (r x r )  dr . K s
= P x (V x  v x )  P (Y  v ) + /\ * . P +/ ' ° <Z V . P.
y (3i.)
j
■V
If the vapour be such that it can be regarded as a perfect gas without
^forK s ,andof^
d V r
sensible error, the substitution of ft + P r for K s , and of n M = HNr
for P V, transforms the above to
L x  L + {K L  k (1 + N)} (r,  r ) ^j
fix / P o \ (32.)
= P 1 t; 1 + P » +J dv.? = j dP.v I
In almost all cases which occur in practice, v is so small as compared
with V, that — [ d P . v may be considered as sensibly = ; and, there
fore, (sensibly)
L 1 + K t (r 1 r ) = L to + fc(l+N)(r 1 r ). . (33.)
2G4 THE MECHANICAL ACTION OF HEAT.
Now this quantity, which I shall denote by H, is the total heat required
to raise unity of weight of liquid from r to rj of absolute temperature,
and to evaporate it at the latter temperature. Therefore, the total heat of
evaporation, where the vapour may be treated as a perfect gas, increases sensibly
at an unifwm rate icith the temper attire of evaporation; and the coefficient of
Us increase with temperature is equal to the apparent specific heat of the vapour
at constant pressure, lt(l + X).
20. There have never been any experiments from which the apparent
specific heat of steam under constant pressure can be deduced in the
manner in which that of permanent gases has been ascertained.
The experiments of M. Eegnault, however, prove that the total heat
of evaporation of water increases uniformly with the temperature from
0° to 200° Centigrade, and thus far fully confirm the results of this theory.
The coefficient of increase is equal to
K w x 0305.
Its mechanical value is, consequently,
1 16*4 metres = 382 feet per Centigrade degree, or
212 feet per degree of Fahrenheit.
(34.)
Although the principle of the conservation of vis viva has thus enabled
us to ascertain the law <>f increase of the total heat of evaporation, it does
not enable us to calculate a priori the constant L of the formula, being
the latent heat of evaporation at the fixed temperature from which
the total heat is measured ; for the changes of molecular arrangement
which constitute evaporation are unknown.
"When the fixed temperature is that of melting ice, M. Eegnault's
experiments give 606 5 Centigrade degrees, applied to liquid water as the
value of this constant : so that
H = K w (606°5 + 30:)T°),
for the Centigrade scale,
H = K w (l091°7 + 305 (T°  32°)\
for Fahrenheit's scale,
(35.)
is the complete expression for the heat required to raise unity of weight
of water from the temperature of melting ice to T° above the ordinary
zero, and to evaporate it at the latter temperature. This formula has
been given by M. Eegnault as merely empirical ; but we have seen that
it closely represents the physical law when quantities depending on the
expansion of water are neglected.
THE MECHANICAL ACTION OF HEAT. 2G5
It must be remarked, that the unit of heat in M. Regnault's tables is
not precisely the specific heat of water at 0° Centigrade, but its mean
specific heat between the initial and final temperatures of the water in the
calorimeter. The utmost error, however, which can arise from this
circumstance, is less than y^ of the total heat of evaporation, so that it
may safely be neglected.
The coefficient 305 K w = 382 feet per Centigrade degree, is the
apparent specific heat of steam at constant pressure ; that is to say, for
steam, —
f{ + — — m = 382 feet per Centigrade degree, but ~ — ^ = 153 ft.
1
Therefore, the real specific heat of steam is K = 7. — ^r^p =229
1 C 11 JV1 N
feet per Centigrade degree = 127*4 feet per deg. of Fahrenheit
153 _ 2
229 ~ 3'
> (36.)
153 °
= K W X 183,andX = — =
The quantity — I d P . v has been neglected, as already explained, in
these calculations, on account of its smallness. When r = C, or the
fixed point is 0° Centigrade, this integral is nearly ecpial to
which for steam, is equal to
— T\ x 122 V  t
v 1
For a pressure of eight atmospheres, ~= = —7 nearly, r a = 445 0, 5
(T = 170°9 Cent.); consequently,  v P 2 =  K w X 0°22 Cent., a
quantity much less than the limit of errors of observation in experiments
on latent heat. This shows that in practice we are justified in overlook
ing the influence of the volume of the liquid water on the heat of
evaporation.
Section IV. — Or the Mechanical Action of Steam treated as a
Perfect Gas, and the Power of the Steam Engine.
21. In the present limited state of our experimental knowledge of the
density of steam at pressures differing much from that of the atmosphere.
200 THE MECHANICAL ACTION OF HEAT.
it is desirable to ascertain whether any material error is likely to arise
from treating it as a perfect gas. For this purpose the ratio of the
volume of steam at 100° Centigrade under the pressure of one atmosphere,
to that of the water which produces it at 4 0, 1 Centigrade, as calculated
theoretically on the supposition of steam being a perfect gas, is to be
compared with the actual ratio.
The weight of one volume of water at 4 0 l Centigrade being taken
as unity, that of half a volume of oxygen at 0° Centigrade, under
the pressure of one atmosphere, according to the experiments of M.
Eegnault, is 0*000714900
That of one volume of hydrogen, .... 0000089578
The sum being, 0000804478
The reciprocal of this sum being multiplied by »,„ = 1'3 64 16 6, the
ratio of dilatation of a perfect gas from 0° to 100° Centigrade, the result
gives for the volume of steam of saturation at 100° Centigrade, as com
pared with that of water,
At 4°l 109572
And for its density, .... 000058972
The agreement of those results with the known volume and density of
steam is sufficiently close to show, that at pressures less than one atmos
phere, it may be regarded as a gas sensibly perfect ; from which it may be
concluded, that, in the absence of more precise data, the errors arising
from treating it as a perfect gas at such higher pressures as occur in
practice, will not be of much importance.
Representing, then, by v the volume of unity of weight of water at
4 "1 Centigrade, that of unity of weight of steam at any pressure and
temperature will be given by the formula
_ 1696e<o r
\  — ^y . p . • • • (38.)
(o representing the number of units of weight per unit of area in the
pressure of one atmosphere, and (r) the absolute temperature at which the
pressure of saturation is one atmosphere ; being for the Centigrade scale
374°6, and for Fahrenheit's scale 674 0> 28.
The mechanical action of unity of weight of steam at the temperature r
and pressure P, during its entrance into a cylinder, before it is permitted
to expand, is represented by the product of its pressure and volume,
or by
THE MECHANICAL ACTION OF HEAT. 207
PV, 16 ^., . . . (39.)
The coefficient — yr — represents a certain depth of fall per degree of
absolute temperature, and is the same with the coefficient ~ — ^., already
referred to.
By taking the following values of the factors: — v = 0*016 cubic foot
per pound avoirdupois, w = 2117 pounds avoirdupois per square foot, we
find this coefficient to be
153*35 feet = 46*74 metres per Centigrade degree,
85*19 feet per degree of Fahrenheit.
(40.)
This determination may be considered correct to about 12 1 0o part.
When French measures are used in the calculation, the following is the
result : —
v = 1 cubic centimetre per gramme,
to = 1033*3 grammes per square centimetre.
1
,, = 46*78 metres per Centigrade degree,
C»M
153*48 feet, or 85*27 feet per degree of Fahrenheit.
(41.)
The difference, which is of no practical importance in calculating the
power of the steamengine, arises in the estimation of the density of
liquid water.
22. Unit of weight of steam at saturation, of the elasticity P x and
volume V 19 corresponding to the absolute temperature r v being cut off
from external sources of heat, it is now to be investigated what amount
of power it will produce in expanding to a lower pressure P 2 and
temperature r 2 .
It has already been shown, at the end of the second section, that if
vapour at saturation is allowed to expand, it requires a supply of heat
from without to maintain it at the temperature of saturation, otherwise a
portion of it must be liquefied to supply the heat required to expand the
rest. Hence, when unity of weight of steam at saturation, at the pressure
Pj and volume V v expands to a lower pressure P, being cut off from
external sources of heat, it will not occupy the entire volume V corre
sponding to that pressure, according to equation (38), but a less volume,
S = mV, where m represents the weight of water remaining in the
gaseous state, the portion 1 — m having been liquefied during the expan
2GS
THE MECHANICAL ACTION OF HEAT.
sion of the remainder. The expansive action of the steam will, therefore,
be represented by
S
%ZS.P (42.)
/:
The law of variation of the fraction m flows from the following con
siderations : — Let d m represent the indefinitely small variation of m
corresponding to the indefinitely small change of temperature S r ; L,
the latent heat of evaporation of unity of weight ; K s , as in equation (30),
the specific heat of vapour at saturation, which is a negative coefficient
varying with the temperature ; then we must have
— L o m = m EL o r, or — = — T  S t,
in L
in order that the heat produced by the liquefaction of $ m may be equal
to the heat required to expand m. Hence making, according to
equation (30) —
K s Sr = 1i(Sr + N^Sv),
and Sr = y8V^
1
+
2 V
we obtain
m " IA ft'
27
 + I  1
) v'
and denoting the coefficient of
(43.)
SV
by  v,
d log. m
d log.
V
and because
d log.
V
d log.
P
d log.
m
d log.
P
d log.
S
V
cl log. S
V ' d log. V
P...I 2 V
T + T 2
+ V 1 
1
/3'
+
2y
d log. P
= 0v) 1
1
^4
T T~
2y
^ (44.)
THE MECHANICAL ACTION OF HEAT. 269
As the mean temperature of the liquid thus produced, more or less
exceeds that of the remaining vapour, a small fraction of it will be
reconverted into vapour, if the expansion is carried on slowly enough ;
but its amount is so small, that to take it into account would needlessly
complicate the calculation, without making it to any material extent more
accurate.
23. The extreme complexity of the exponent a, considered as a function
of the pressure P, would render a general formula for the expansive action
/PfZS very cumbrous in its application. For practical purposes, it is
sufficient to consider the exponent o as constant during the expansion
which takes place in any given engine, assigning it an average value
suitable to the part of the scale of pressures in which the expansion takes
p>lace. For engines in which the steam is introduced at pressures not
exceeding four atmospheres, I conceive that it will be sufficiently accurate
to make tr = —  } while for engines in which the initial pressure lies
5
between four and eight atmospheres, the suitable value is a = .
The utmost error which can arise from using these exponents is
about yijj of the whole power of the engine, and that only in extreme
cases. Making, therefore,
* = *>$
we obtain for the value of the expansive action of unity of weight of
steam,
V,
1
P,V
y (45.)
^iU l ' l ' s )
s
s being used to denote ==^, or the ratio of the volumes occupied by steam
at the end and at the beginning of the expansion respectively.
A table to facilitate the computation is given in the Appendix.
The gross mechanical action of unity of weight of steam on one side of
the piston is found by adding to the above quantity the action of the
steam before it begins to expand, or P x V v and is therefore,
(46.)
270 THE MECHANICAL ACTION OF HEAT.
The values of the coefficients and exponent being
1
a
!_1
1  a
1  <7
x —  .
a
For initial pressures between
1 and 4 atmospheres,
. 7
6
1
6'
4 and 8 atmospheres,
. 6
5
1
5'
24. The following deductions have to be made from the gross action,
in order to obtain the action effective in overcoming resistance.
First, For loss of power owing to a portion of the steam being employed
in filling steampassages, and the space called the clearance of the cylinder
at one end. Let the bulk of steam so employed be the fraction c S 2 of
the space filled by steam at the end of the expansion ; then the loss of
power from this cause is
PjCSo = csP 1 V 1 .
Secondly, For the pressure on the opposite side of the piston, of the steam
which escapes into the condenser, or into the atmosphere, as the case may
be. Let P 3 be the pressure of this steam ; the deduction to be made for
its action is
P 3 S.,(1 c)= P 8 V t (l  c)s.
These deductions having been made, there is obtained for the effect of
unity of weight of water evaporated,
VijP, (y^  ^J ~ » cs)  P,(l  c)s) (47.)
25. The effect of the engine in unity of time is found by multiplying
the above quantity by the number of units of weight of water evaporated
in unity of time.
If this number be denoted by W,
"W S 2 (1  c) = W V 1 (1  c) s = Au, . . (48.)
will represent the cubical space traversed by the piston in unity of time,
A denoting the area of the piston, and u its mean velocity.
Now, let the whole resistance to be overcome by the engine be reduced,
by the principles of statics, to a certain equivalent pressure per unit of area
of piston, and let this pressure be denoted by P. Then,
THE MECHANICAL ACTION OF HEAT. 271
E A u = E W Vj (1  c) 5, . . . (49.)
expresses the effect of the engine in terms of the gross resistance.
We have now the means of calculating the circumstances attending the
"working of a steamengine, according to the principle of the conservation
of vis viva, or, in other words, of the equality of power and effect, which
regulates the action of all machines that move with an uniform or
periodical velocity.
This principle was first applied to the steamengine by the Count de
Pambour ; and, accordingly, the formulae which I am about to give only
differ from those of his work in the expressions for the maximum pressure
at a given temperature, and for the expansive action of the steam, which
are results peculiar to the theory of this essay.
In the first place, the effect as expressed in terms of the pressure, is to be
equated to the effect as expressed in terms of the resistance, as follows: —
ea^kwv^i c)s = wVijp^j^ yzt^ s a ~ cs )
P 3 (lc)s} . . . (50.)
This is the fundamental equation of the action of the steamengine, and
corresponds with equation A. of M. de Pambour's theory.
26. Dividing both sides of equation (50) by the space traversed by
the piston in unity of time, "W V x (1 — c) s, and transferring the pressure
of the waste steam, P 3 , to the first side, we obtain this equation : —
iL
cs
R+p 3= p 1 1 ' ll'). ■ ■ («•)
which gives the means of determining the pressure P x at which the steam
must enter the cylinder, in order to overcome a given resistance and
counterpressure with a given expansion ; or, supposing the expansion s to
be variable at pleasure, and the initial pressure P x fixed, the equation
gives the means of finding, by approximation, the expansion best adapted
to overcome a given resistance and counterpressure.
The next step is to determine, from equations (XV.) of the introduction
and (38) of this section, the volume V 1 of unity of weight of steam
corresponding to the maximum pressure P r Then equation (48) gives
the space traversed by the piston in unity of time, which, being multiplied
by the resistance E per unit of area of piston, gives the gross effect of
the engine.
272 THE MECHANICAL ACTION OF HEAT.
27. If, on the other hand, the space traversed by the piston in unity
of time is fixed, equation (48) gives the means of determining, from the
evaporating power of the boiler W, either the volume Y 1 of unity of
weight of steam required to work the engine at a given velocity with
a given expansion, or the expansion s proper to enable steam of a given
initial density to work the engine at the given velocity. The initial
pressure P x being then determined from the volume Y v the resistance
which the engine is capable of overcoming with the given velocity is to be
calculated by means of equation (51).
28. This calculation involves the determination of the pressure P 2
from the volume V 1 of unity of weight of steam at saturation, which can
only be done by approximation. The following formula will be found
useful for this purpose : —
12
Fi^Hv/ 1 • • • • ( 52 )
where to represents the pressure of one atmosphere, V the volume of
steam of saturation at that pressure (being 1G96 times the volume of
water at 4°'l Cent., or 27*130 cubic feet per pound avoirdupois), and V,
the volume of steam of saturation at the pressure P r This formula is
only applicable between the pressures of one and eight atmospheres : that
is to say, when the volume of steam is not greater than 27 cubic feet per
pound, nor less than 4, and the temperature not lower than 100° Centigrade,
nor higher than 171° Centigrade (which correspond to 212° and 340°
Fahrenheit).
The greatest error in computing the pressure by means of this formula
is about Jg of an atmosphere, and occurs at the pressure of four atmos
pheres, so that it is pj of the whole pressure. This is sufficiently
accurate for practice, in calculating the power of steamengines ; but
should a more accurate result be required, the approximate value of the
pressure may be used to calculate the temperature by means of equation
(XV.) ; and the temperature thus determined, (which will be correct to ~ of
a Centigrade degree), may then be used in conjunction with the volume to
compute a corrected value of the pressure, according to equation (38).
The pressure, as thus ascertained, will be correct to ^wo of its amount,
which may be considered the greatest degree of accuracy attainable.
The most convenient and expeditious mode, however, of computing the
pressure from the volume, or vice versd, is by interpolation from the table
given in the Appendix to this paper.
29. The resistance denoted by R may be divided into two parts ; that
which arises from the useful work performed, and that which is independent
of it, being, in fact, the resistance of the engine when unloaded. Now it
is evident, that the maximum usrful effect of the steam has been attained,
THE MECHANICAL ACTION OF HEAT. 273
as soon as it has expanded to a pressure which is in equilibrio with the
pressure of the waste steam added to the resistance of the engine when
unloaded ; for any further expansion, though increasing the total effect,
diminishes the useful effect. Therefore, if we make
R = K'+/,
R' being the resistance arising from the useful work, and / the resistance
of the engine when unloaded, both expressed in the form of pressure
on the piston, the expansion corresponding to the maximum of useful
effect will take place when
P 2 = P 3 +/>
the corresponding ratio of expansion being ! ,_ .
\p.+//
The maximum useful effect with a given pressure on the safetyvalve
has been so fully discussed by M. de Pambour, that it is unnecessary to
do more than to state that it takes place when the initial pressure
in the cylinder is equal to that at the safetyvalve : that is to say, when
it and the useful resistance are the greatest that the safetyvalve will
permit.
30. Annexed is a table of the values of some of the quantities which
enter into the preceding equations in the notation of the Count de
Pambour's works : —
Expression in the Notation Equivalent Expression in
of this Paper. M. tie Pambour's Notation.
R = R'+/ (1 + 8)r+/
Aw av
W . . . S x weight of one cubic
foot of water.
P 3 . . p
I + c
V + c
I' + c
31. As an illustration, I shall calculate the maximum useful effect of
one pound, and of one cubic foot of water, in a Cornish doubleacting
S
27i THE MECHANICAL ACTION OF HEAT.
engine, in the circumstances taken by M. de Pambour as an example for
that kind of engine, that is to say, —
Clearance onetwentieth of the stroke, or c = —
Eesistance not depending on the useful load, / = 72 lbs. per sq. ft.
Pressure of condensation, . . . . P 3 = 576 lbs. „ „
Consequently, to give the maximum useful effect,
P 2 = P S +/ = 048 lbs. „ „
Total pressure of the steam when first admitted, V 1 — 7200 lbs. „ „
Volume of 1 lb. of steam V x = 8*7825 cubic feet.
Therefore P x Vj = G3234 lbs. raised one foot.
P 7200
— 1 = — — ; and, consequently,
P 2 04o
Expansion to produce the maximum useful effect s = (rr ) ' — 7'877.
Space traversed by the piston during the action of one pound of steam,
= V x (1  c) s = G58SG cubic feet.
Gross effect of one pound of steam, in pounds raised one foot high,
1
= PjV, (7 6s' 6  £)  P 3 V 1 (1  c)s = 112001
Deduct for resistance of engine when unloaded f\ 1 (1 — c)s = 4744
Effect of one pound of steam in overcoming resistance depend"! io7°60
ing on useful load, . . . . . . J
This being multiplied by G2£, gives for the effect of one cubic
foot of water evaporated, in pounds raised one foot, . G,703,750
It is here necessary to observe, that M. de Pambour distinguishes the
useful resistance into two parts, the resistance of the useful load indepen
dently of the engine, and the increase in the resistance of the engine
arising from the former resistance, and found by multiplying it by a
constant fraction, which he calls $. In calculating the net useful effect, he
takes into account the former portion of the resistance only ; consequently :
Net useful effect as defined by M. de Pambour
THE MECHANICAL ACTION OF HEAT. 275
Gross effect — f Y, (1 — c) s /t , . N
= i + g • • <"•>
The value of S, for doubleacting steamengines generally, is considered
by M. de Pambour to be y ; consequently, to reduce the effect of one
cubic foot of water, as calculated above, to that which corresponds with his
definition, we must deduct £, which leaves,
5,865,781 lbs. raised one foot.
M. de Pambour's own calculation gives,
6,277,560,
being too large by about onefifteenth.
32. In order to show the limit of the effect which may be expected
from the expenditure of a given quantity of heat in evaporating water,
and also to verify the approximate method employed in calculating the
expansive action of the steam, I shall now investigate the maximum gross
cjfed, including resistance of all kinds, producible by evaporating unity of
weight of Avater at a higher temperature and liquefying it at a lower, and
compare, in two examples, the power produced with the heat which
disappears during the action of the steam, as calculated directly.
To obtain the maximum gross effect, the steam must continue to act
expansively until it reaches the pressure of condensation, so that P 2 = P 3 .
The clearance must also be null, or c = 0. Making those substitutions in
the formula (47), we find, for the maximum gross effect of unity of weight
of water, evaporated under the pressure P x , and liquefied under the
pressure P OJ
, ( M ^ST"
p^rV 1  8 ''  P ' Y ' i  . (55)
In order to calculate directly the heat which is converted into power in
this operation, let r v r 2 , respectively represent the absolute temperatures
of evaporation and liquefaction, and L 2 the latent heat of evaporation at
the lower temperature r 2 ; then the total heat of evaporation at t v starting
from t as the fixed point, by equation (33), is
H 2 , , = L 2 + 305 K w (r,  r 2 ).
This is the heat communicated to the water in raising it from r 2 , to t x and
evaporating it. Now a weight 1 — m of the steam is liquefied during the
expansion at temperatures varying from r a to r 2 , so that it may be looked
upon as forming a mass of liquid water approximately at the mean
276 THE MECHANICAL ACTION OF HEAT.
temperature ^— — , and from which a quantity of heat, approximately
a
represented by
K w (im)^p,
must be abstracted, to reduce it to the primitive temperature r 2 .
Finally, the weight of steam remaining, m, has to be liquefied at the
temperature r 2 , by the abstraction of the heat
mL 2 .
The difference between the heat given to the water, and the heat
abstracted from it, or
Hjs,,  K w (l  m) Tl T *  mL 2
= (1  m)U + K w (305  1  J =^)(r 1  r 2 )
> ( 56
is the heat which has disappeared, and ought to agree with the expression
(55) for the power produced, if the calculation has been conducted
correctly.
As a first example, I shall suppose unity of weight of water to be
evaporated under the pressure of four atmospheres, and liquefied under
that of half an atmosphere ; so that the proper values of the coefficients
and exponent are
1 7 1 l
= 7, 1 — a =
1 a ' T
The data, in this case, for calculating the power are,
P x = 84G8 lbs. per square foot.
Vj = 7  58 4 cubic feet for 1 lb. of steam.
P x V x = 64221 lbs. raised one foot.
P 1 "
— = , whence s = 8 7 = 5 '9 44.
Ji b
Maximum possible effect of one pound of water,
= P 1 V 1 x7H Qm = 115600 lbs. raised one foot.
Being the mechanical equivalent of 9 2° '3 Centigrade applied to one
pound of liquid water at 0° C; or,
THE MECHANICAL ACTION OF HEAT. 277
92°3 K w .
Maximum possible effect of one cubic foot of water, 7,225,000 lbs. raised
one foot.
In order to calculate directly the heat converted into power, we have,
Tj = C + 144°1 Cent. r 2 = C + 81°'7.
L 2 = 549°7 K w .
H 2n = 5 6 8 0 7 K w = heat expended in the boiler.
1 — m = '14 nearly = proportion of steam liquefied during the expansion.
The heat converted into mechanical power, as calculated from these
data, is found to be,
01°'6K W ,
differing by only o, 7 from the amount as calculated from the power
produced.
The direct method, however, is much less precise than the other, and is
to be regarded as only a verification of the general principle of calculation.
92'3
The heat rendered effective, in the above example, is , or less than
onesixth of that expended in the boiler.
As a second example, I shall suppose the steam to be produced at a
pressure of eight atmospheres, and to expand to that of one atmosphere.
In this case,
Pj = 16936 lbs. per square foot.
Vj = 4'03 cubic feet per lb. of steam.
V l Y 1 = 68252 lbs. raised one foot.
PI 6
£?= * .s = 5657 = 8*.
Maximum possible effect of one pound of water,
= P 1 V 1 xen (q)0 = 119 ' 942 lbs  raised one foot '
Being the equivalent of 95 0 8 K w (Centigrade).
Maximum possible effect of one cubic foot of water = 7,496,375 lbs.
raised one foot.
The data for calculating directly the heat rendered effective are,
278 THE MECHANICAL ACTION OF HEAT.
Tl = C + 170 o, 9 Cent, r, = C + 100°.
L 2 = 537° K w .
Hojj = 558 0, 6 K w = heat expended in the boiler.
1 — m = '148 nearly = steam liquefied during the expansion.
Whence, the heat converted in power, as calculated directly, is
95°S K w ,
agreeing with the calculation from the power produced.
95"8
In this example, the heat rendered effective is  , or somewhat more
than onesixth of that expended in the boiler.
33. The results of the calculations of maximum possible effect, of
which examples have just been given, are limits which may be approached
in practice by Cornish and similar engines, but which cannot be fully
realised ; and yet it has been shown, that in those theoretical cases only
about onesixth of the heat expended in the boiler is rendered effective.
In practice, of course, the proportion of heat rendered effective must be
still smaller; and, in fact, in some unexpansivc engines, it amounts to
only onct ve a f {/fourth, or even less.
Dr. Lyon Playfair, in a memoir on the Evaporating Power of Fuel, has
taken notice of the great disproportion between the heat expended in the
steamengine and the work performed. It has now been shown that this
waste of heat is, to a great extent, a necessary consequence of the nature
of the machine. It can only be reduced by increasing the initial pressure
of the steam, and the extent of the expansive action; and to both of those
resources there are practical limits, which have already, in some instances,
been nearly attained.
APPENDIX TO THE FOURTH SECTION.
Containing Tables to be used in Calculating the Pressure,
Volume, and Mechanical Action of Steam,
Treated as a Perfect Gas.
The object of the First of the annexed Tables is to facilitate the calcula
tion of the volume of steam of saturation at a given pressure, of the
pressure of steam of saturation at a given volume, and of its mechanical
action at full pressure.
THE MECHANICAL ACTION OF HEAT. 279
The pressures are expressed in pounds avoirdupois per square foot, and
the volumes by the number of cubic feet occupied by one pound avoirdu
pois of steam, when considered as a perfect gas ; those denominations
being the most convenient for mechanical calculations in this country.
The columns to be used in determining the pressure from the volume,
and vice versd, are the third, fourth, sixth, and seventh.
The third column contains the common logarithms of the pressures of
steam of saturation for every fifth degree of the Centigrade thermometer,
from — 30° to + 260°: that is to say, for every ninth degree of
Fahrenheit's thermometer, from — 22° to + 500°.
The fourth column gives the differences of the successive terms of the
third column.
The sixth column contains the common logarithms of the volume of
one pound of steam of saturation corresponding to the same temperatures.
The seventh column contains the differences of the successive terms of
the sixth column, which are negative ; for the volumes diminish as the
pressures increase.
By the ordinary method of taking proportional parts of the differences,
the logarithms of the volumes corresponding to intermediate pressures,
or the logarithms of the pressures corresponding to intermediate volumes,
can be calculated with great precision. Thus, let X + h be the logarithm
of a pressure not found in the table, X being the next less logarithm
which is found in the table ; let Y be the logarithm of the volume cor
responding to X, and Y — h the logarithm of the volume corresponding to
X + h; let H be the difference between X and the next greater logarithm
in the table, as given in the fourth column, and K the corresponding
difference in the seventh column ; then by the proportion
H : K : : h : i
either Y — Jc may be found from X f h, or X + h from Y  k
In the fifth and eighth columns respectively, are given the actual
pressures and volumes corresponding to the logarithms in the third and
sixth columns, to five places of figures.
In the ninth column are given the values of the quantity denoted by
P Vj in the formulae, which represents the mechanical action of unity of
weight of steam at full pressure, or before it has begun to expand, in
raising an equal weight. Those values are expressed in feet, being the
products of the pressures in the fifth column by the volumes in the
eighth, and have been found by multiplying the absolute temperature in
Centigrade degrees by 153*48 feet. Intermediate terms in this column,
for a given pressure or a given volume, may be approximated to by the
method of differences, the constant difference for 5° Centigrade being 767'4
280 THE MECHANICAL ACTION OF HEAT.
feet ; but it is more accurate to calculate them by taking the product of
the pressure and volume.
When the pressure is given in other denominations, the following
logarithms are to be added to its logarithm, in order to reduce it to
pounds avoirdupois per square foot : —
For Millimetres of mercury, . . . 0*44477
„ Inches of mercury, .... 1 '84960
„ Atmospheres of 7 GO millimetres, . . 3*32559
„ Atmospheres of 30 inches, . . . 33 2 G 72
. Kilogrammes on the square centimetre, . 331136
„ Kilogrammes on the circular centimetre, 3*41 G 27
„ Kilogrammes on the square metre, . 1*31136
„ Pounds avoirdupois on the square inch, . 2*15836
„ Pounds avoirdupois on the circular inch, 2*26327
To reduce the logarithm of the number of cubic metres occupied by one
kilogramme to that of the number of cubic feet occupied by one pound
avoirdupois, add 1*20463.
The logarithms are given to five places of decimals only, as a greater
degree of precision is not attainable in calculations of this kind.
The Second Table is for the purpose of calculating the mechanical action
of steam in expansive engines.
The first column contains values of the fraction of the entire capacity
of the cylinder which is filled with steam before the expansion commences
(being the quantity  of the formulas), for every hundredth part, from
1*00, or the whole cylinder, down to 0*10, or onetenth.
If I be the entire length of stroke, V the portion performed at full
pressure, and c the fraction of the entire capacity of the cylinder allowed
for clearance, then
1 _
I' s C , 1 „ , V
i=Tzre and=(i ,) + ,.
The entire capacity of the cylinder is to be understood to include clearance
at one end only.
The second column gives the reciprocals of the quantities in the first,
or the values of the ratio of expansion s.
THE MECHANICAL ACTION OF HEAT. 281
The third and fourth columns, headed Z, give the values of the
quantity
1 C7 1 ~i
1  a 1  a
of article 23, which represents the ratio of the entire gross action of the
steam to its action at full pressure, without allowing for clearance. The
third column is to be used for initial pressures of from one to four
atmospheres ; and the fourth for initial pressures of from four to eight
atmospheres.
The deduction to be made from the quantity Z for clearance is c s, or
the product of the fraction of the cylinder allowed for clearance by the
ratio of expansion. Hence, to calculate from the tables the net mechanical
action of unity of weight of steam, allowing for the counterpressure of the
waste steam P 3 , as well as for clearance, we have the formula
p 1 y 1 (z cs )p 3 v 1 (i c ),,
being equivalent to the formula (47) of this paper.
282
THE MECHANICAL ACTION OF HEAT.
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THE MECHANICAL ACTION OF HEAT. 283
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284
THE MECHANICAL ACTION OF HEAT.
TABLE II. — Expansive Action of Steam.
(1.)
(2.)
(3.)
(4.)
(1.)
(2.)
(3.)
(4.)
Fraction of
Coefficient of Gross
Fraction of
Coefficient of Cros3
Cylinder
Action = Z.
i rider
Action = Z.
filled with
Steam at full
Pressuro
Patio of
Expansion
Cnitj il Pres
sure One
Initial Pres
sure Four
filled with
Steam at full
Pressure
Ratio of
Expansion
Initial Pres
sure One
Initial Pros
sure Four
1
'
to Four
to Eight
1
= •'•
to Four
to Eight
Atmos
Atmos
Atmos
— s'
pheres.
ph( l
s
pheres.
pheros.
100
1000
1000
1000
•54
1852
1586
1 5S0
■99
1010
1010
1010
•53
1887
1G02
1596
•98
1020
1020
1 020
•52
1 923
1 620
1613
•97
1031
1030
1 030
•51
1961
1637
1G30
•9G
1 042
1041
1041
•50
2 000
1 655
1 647
•95
1 053
1051
1051
•49
2 041
1G73
L665
•94
1064
10G2
10G2
•48
2083
1691
1683
•93
1075
1072
L072
•47
212S
1709
1701
•92
1087
1083
10S3
•46
2174
1728
1719
•91
1 099
1094
1 093
•45
2*222
1748
1738
•90
1111
1104
1104
•44
, 2273
1767
1757
•89
1124
1115
1115
•43
2326
1787
1777
•88
1136
1126
1126
•42
2 381
1803
1 79G
•87
1149
1138
1137
•11
2439
1S29
1817
•86
11G3
1149
1149
•40
2500
1850
1837
•85
1176
11G0
11G0
•39
2564
1871
1858
•84
1190
1172
1171
•38
2632
1894
1880
•83
1205
1183
11S3
•37
2703
1916
1902
•82
1 220
1195
1195
•36
2778
1939
1924
•81
1 235
1207
1 206
•35
2857
1963
1947
•80
1250
1219
1218
•34
2941
1987
1970
•79
1266
1231
1230
•33
3 030
2012
1994
•78
1282
1243
1242
•32
3125
2038
2 019
•77
1 209
1 256
1 255
•31
3225
2 064
2 044
•76
1316
1268
12G7
•30
3333
2091
2 070
•75
1 333
1281
1280
•29
344S
2119
2 097
•74
1351
1294
1 292
•28
3 571
2147
2124
•73
1370
1 307
1305
•27
3704
2176
2152
•72
1389
1320
1318
•26
3S46
2207
2181
•71
1408
1333
1331
•25
4000
2238
2211
•70
1429
1346
1344
•24
4167
2270
2242
•69
1449
13G0
1358
•23
434S
2 304
2273
•6S
1471
1 374
1371
•22
4545
2338
2306
•67
1493
1387
1385
•21
4762
2374
2341
•66
1515
1401
1399
•20
5 000
2412
2376
•65
1538
1416
1413
•19
5203
2451
2413
•64
1563
1430
1427
•18
5556
2492
2452
•63
1587
1445
1441
•17
58S2
2534
2492
•62
1613
1459
1 456
•16
6250
2579
2434
•61
1640
1474
1471
•15
6667
2626
2579
•60
1667
1490
1486
•14
7143
2676
2626
•59
1695
1505
1501
•13
7692
2730
2675
•58
1724
1521
1516
•12
8333
2786
2728
•57
1754
1537
1532
•11
9 091
2847
2784
•56
1786
1553
1547
•10
10 000
2912
2845
•55
1818
1569
1563
THE MECHANICAL ACTION OF HEAT. 285
XV.— NOTE AS TO THE DYNAMICAL EQUIVALENT OF
TEMPERATURE IN LIQUID WATER, AND THE SPECIFIC
HEAT OF ATMOSPHERIC AIR AND STEAM:
Being a Supplement to a Paper On the Mechanical Action
of Heat.*
33. In my paper on the Mechanical Action of Heat (see p. *23Jt), published
in the first part of the twentieth volume of the Transactions of Hie Royal
Society of Edinburgh, some of the numerical results depend upon the
dynamical equivalent of a degree of temperature in liquid water. The
value of that quantity which I then used, was calculated from the experi
ments of De la Roche and Be>ard on the apparent specific heat of
atmospheric air under constant pressure, as compared with liquid water.
The experiments of Mr. Joule on the production of heat by friction
give, for the specific heat of liquid water, an equivalent about oneninth
part greater than that which is determined from those of De la Roche
and B6rard. I was formerly disposed to ascribe this discrepancy, in a
great measure, to the smallness of the differences of temperature measured
by Mr. Joule, and to unknown causes of loss of power in his apparatus,
such as the production of sound and of electricity; but, subsequently
to the publication of my paper, I have seen the detailed account of
Mr. Joule's last experiments in the Philosophical Transactions for 1850,
which has convinced me, that the uncertainty arising from the smallness
of the elevations of temperature, is removed by the multitude of experi
ments (being forty on water, fifty on mercury, and twenty on cast iron) ;
that the agreement amongst the results from substances so different,
shows that the error by unknown losses of power is insensible, or nearly
so; and that the necessary conclusion is, that the dynamical value
assigned by Mr. Joule to the specific heat of liquid water — viz., 772 feet
per degree of Fahrenheit — does not err by more than two, or, at the
utmost, three feet; and therefore, that the discrepancy originates chiefly
in the experiments of De la Roche and Berard.
I therefore take the earliest opportunity of correcting such of my
calculations as require it, so as to correspond with Mr. Joule's equivalent.
* Eead before the Eoyal Society of Edinburgh on December 2, 1850, and published
in the Transactions of that Society, Vol. XX., Part II.
286 THE MECHANICAL ACTION OF HEAT.
They relate to the specific heat of atmospheric air as compared with
liquid water, and to that of steam, and are contained in the second and
third sections of my paper, Articles 14 and 20 ; equations 28, 34, and 36.
Specific Heat of Atmospheric Air as Compared with liquid
Water. — (Section II., Article 14.)
The dynamical values of the specific heat of atmospheric air are
calculated independently from the velocity of sound, without reference
to the specific heat of liquid water; and from the closeness of the agree
ment of the experiments of MM. Bravais and Martins, Moll and Van
Beek, Stampfcr and Myrbach, Wertheim and others, it is clear that
the limits of error are about ^ for the velocity of sound, j^ for the
ratio, and from  4 T  to  5 V for the dynamical values of the specific heat of air,
at constant volume and constant pressure. Those values, as given by
equation (27), are —
Real specific heat, —
ft = 23S'GG feet = 7274 metres per Centigrade degree.
= 132G feet per degree of Fahrenheit.
Apparent specific heat under constant pressure, —
K p = 334 fcet= 1018 metres per Centigrade degree,
= 185*6 feet per degree of Fahrenheit.
The ratio of these two quantities being taken as
^ p =l +N= 14.
R
The dynamical equivalent of the specific heat of liquid water, as
determined by Mr. Joule, is
K w = 13896 feet = 42354 metres per Centigrade degree,
= 772 feet per degree of Fahrenheit.
The specific heat of air, that of liquid water being taken as unity, has
therefore the following values : —
Real specific heat, —
It 1326
IF— rnn = 1< 1 I.
K w 772
THE MECHANICAL ACTION OF HEAT. 287
Apparent specific heat under constant pressure, —
K P . 1856 nnlnt
—  = = ,o 404
K w 772 V ~ Wi 
This last quantity, according to De la Eoche and Berard, is
02669
The discrepancy being . . . . 0'0265
or oneninth of the value, according to Mr. Joule's equivalent.
Specific Heat of Steam. — (Section III, Art. 20.)
The apparent specific heat of steam (equations 34 and 36) as a gas
under constant pressure, is equal to that of liquid water x0'305. Its
dynamical value is, therefore,
K P = fc + ?1 Vr = 13896 x 030.3
C n M
= 42283 feet = 12918 metres per Cent, degree.
But
— — — r = 153*48 feet = 46  78 metres per Cent, decree.
Therefore, the real specific heat is
ft = 269*35 feet = 82  40 metres per Cent, degree.
Or, that of liquid water being taken as unity,
A = 2^ = 0194
K w 13896 U1J *
The ratio of these two values of the specific heat of steam is
l+N=l57.
Their dynamical equivalents for Fahrenheit's scale are,
ft = 14964 feet, . . . K P = 235'46 feet.
Neither the formula? in the fourth Section, respecting the working of the
steamengine, nor the tables at the end of the paper, require any alteration ;
for the action of steam at full pressure being calculated from data
independent of its specific heat, is not at all affected by the discrepancy I
have mentioned ; and the expansive action is not affected to an extent
appreciable in practice.
2SS THE MECHANICAL ACTION OF HEAT.
XVI.— ON THE POWER AND ECONOMY OF SINGLEACTING
EXPANSIVE STEAMENGINES :
Being a Supplement to the Fourth Section of a Paper On the
Mechanical Action of Heat.*
34. The objects of this" paper are twofold : First, To compare the
results of the formula? and tables relative to the power of the steam
engine, which have been deduced from the dynamical theory of heat,
with those of experiments on the actual duty of a large Cornish engine
at various rates of expansion ; and, Second!;/, To investigate and explain
the method of determining the rate of expansion, and, consequently,
the dimensions and proportions of a Cornish engine, which, with a given
maximum pressure of steam in the cylinder, at a given velocity, shall
perform a given amount of work at the least possible pecuniary cost,
taking into account the expense of fuel, and the interest of the capital
required for the construction of the engine.
This problem is solved with the aid of the tables already printed, by
drawing two straight lines on a diagram annexed to this paper.
The merit of first proposing the question of the economy of expansive
engines in this definite shape belongs, I believe, to the Artizan Club,
who have offered premiums for its solution ; having done so (to use their
own words) ''with a view to enable those who, from their position,
cannot take part in the discussions of the various scientific societies, to
give the profession the benefit of their studies and experience." The 5th
of April is the latest day fixed by them for receiving papers; and as
this communication cannot possibly be read to a meeting before the 7th
April, nor published until some months afterwards, I trust I may feel
confident that it will not be considered as interfering with their design.
Formula Applicable to the Cornish Engine.
35. The equations of motion of the steamengine, in this and the
original paper, are the same in their general form with those of M. de
* Eead before the Royal Society of Edinburgh on April 21, 1851, and published in
the Transactions of that Society, Vol. XX., Part II.
SINGLEACTING EXPANSIVE STEAMENGINES. 289
Pambour. The differences consist in the expressions for the pressure
.and volume of steam, and for the mechanical effect of its expansion ;
the former of which were deduced from a formula suggested by peculiar
hypothetical views, and the latter from the dynamical theory of heat.
Those equations are Nos. (50) and (51) of the original paper. (Seep.
971.) I shall now express them in a form more convenient for practical
use, the notation being as follows : —
Let A be the area of the piston ;
/, the length of stroke ;
v, the number of double strokes in unity of time ;
c, the fraction of the total bulk of steam above the piston when down,
allowed for clearance, and for filling steampassages; so that the total
bulk of steam at the end of the effective stroke is
... . . . (a.)
1 — c
I', the length of the portion of the stroke performed when the steam is
cut off.
s, the ratio of expansion of the steam, so that
(b.)
Let W be the weight of steam expended in unity of time.
Pj, the pressure at which it enters the cylinder.
A" , the volume of unity of weight of steam at saturation at the pressure
P x j which may be found from Table I. of the Appendix to the original
paper. (See p. 282)
F, the sum of all the resistances not depending on the useful load,
reduced to a pressure per unit of area of piston; whether arising from
imperfect vacuum in the condenser, resistance of the airpump, feedpump,
and coldwater pump, friction, or any other cause.
E, the resistance arising from the useful load, reduced to a pressure per
unit of area of piston.
Z, the ratio of the total action of steam Avorking at the expansion s,
to its action without expansion. Values of this ratio are given in the
second table of the Appendix to the original paper.
Then the following are the two fundamental equations of the motion of
the steamengine as comprehended in equation (50) of the original paper. .
First, Equality of power and effect, —
T
1
J'
\
s
= (!■
;h
+
;
1
 c
f
V
s
I
= T^
 c
J
290 SINGLEACTING EXPANSIVE STEAMENGINES.
R A I a = W V, {P 2 (Z  c s)  F (1  c) *}. . (c.)
Secondly, Equality of two expressions for the weight of steam expended
in unity of time, —
w = v ,f* n . (d.)
\j(l  c)s
From these two equations is deduced the following, expressing the
ratio of the mean load on the piston to the initial pressure of the steam : —
li + F _ Z — cs . .
being equivalent to equation (51).
In computing the effect of Cornish engines these formulae require to be
modified, owing to the following circumstances : —
The terms depending on the clearance c have been introduced into
equations (c), (//), on the supposition that the steam employed in filling
the space above the piston at the top of its stroke is lost, being allowed
to escape into the condenser, without having effected any work; so
that a weight of steam Wcs is wasted, and an amount of power
WVj (Pj — F)ox lost, in unity of time. But in Cornish engines this is
not the case; for by closing the equilibriumvalve at the proper point of
the up or outdoor stroke, nearly the whole quantity of steam necessary
to fill the clearance and valveboxes may be kept imprisoned above the
piston, so as to make the loss of power depending on it insensible in
practice. This portion of steam is called a cushion, from its preventing
a shock at the end of the upstroke; and, as Mr. Pole in his valuable
work on the Cornish engine has observed, its alternate compression and
expansion compensate each other, and have no effect on the duty of the
engine. The proper moment of closing the equilibriumvalve is fixed by
trial, which is, perhaps, the best way; but if it is to be fixed by theory,
the following is the proper formula : let I" be the length of the portion
of the upstroke remaining to be performed after the equilibriumvalve
has been closed : then —
1" _ C(81) m
I ' I  c ' ' ' * u ' ;
A slight deviation from this adjustment will produce little effect in
practice, if the fraction c is small.
In forming the equations of motion, therefore, of the Cornish engine,
we may, without material error, in practice omit the terms denoting a
waste of steam and loss of power due to clearance and filling of steam
passages ; and the results are the following : —
Equation of effect and power in unity of time : —
SINGLEACTING EXPANSIVE STEAMENGINES. 291
Useful effect E = E AZ» = W V, {P x Z  F}. . (57.)
"Weight of steam expended in unity of time : —
w = 4^ ( 58 o
From those two fundamental equations the following are deduced: —
Ratio of mean load on piston to maximum pressure, —
R + F _ Z
Pi = »
(59.)
Duty of unity of weight of steam, —
WV^ZF), • • (6Q.)
which, being multiplied by the number of units of weight of steam
produced by a given weight of fuel, gives the duty of that weight of fuel.
Weight of steam expended per stroke, —
^ = ^ (61.)
In fact, it is clear that if any five quantities out of the following seven
be given, the other two may be determined by means of the equations :
R + F, the mean load on unit of area of piston.
P p the maximum pressure of steam in the cylinder.
s, the ratio of expansion.
W, the weight of steam produced in unity of time.
A, the area of the piston.
/, the length of stroke.
n, the number of strokes in unity of time.
The other quantities, E, Y v Z, are functions of those seven.
Comparison of the Theory with Mr. Wicksteed's Experiments.
36. In order to test the practical value of this theory, I shall compare
its results with those of the experiments which were made by Mr.
Wicksteed on the large Cornish pumping engine, built under the direction
of that eminent engineer, by Messrs. Harvey and West, for the East London
292 SINGLEACTING EXPANSIVE STEAMENGINES.
WaterWorks at Old Ford, and which were published in 1841. The
dimensions and structure of the engine, and the details of the experiments,
are stated with such minuteness and precision, that there is none of that
uncertainty respecting the circumstances of particular cases, which is the
most frecpient cause of failure in the attempt to apply theoretical principles
to practice.
The enoine was worked under a uniform load at five different rates of
expansion successively. The number of strokes, and the consumption
of steam during each trial, having been accurately registered, Mr. ^Wieksteed
gives a tabic showing the weight of steam consumed per stroke for each
of the five rates of expansion. I shall now compute the weight of steam
per stroke theoretically, and compare the results.
Throughout these calculations I shall uniformly use the foot as the
unit of length, the avoirdupois pound as that of weight, and the hour as
that of time. Pressures are consequently expressed in pounds per square
foot for the purpose of calculation; although in the table of experimeuts
I have reduced them to pounds per square inch, as being the more
familiar denomination.
The data respecting the dimensions and load of the engine, which are
constant throughout the experiments, are the following : —
Area of piston, . .... A =: 34'854 square feet.
Stroke, . ... 7 = 10 feet.
Cubic space traversed by piston during one
down stroke, . . . . = A I — 3 48 '5 4 cubic feet
Clearance and valveboxes, .... 18  00 „
Sum, . 30054
Therefore, c = 0*05
E = useful load of piston, . . • = 15970 lbs. per sq.ft.
F = additional resistance, . . • =200*0 „
R + F = total mean pressure on piston, . . = 18030 „
The mode of calculation is the following : —
Mr. Wicksteed states the fraction  of the stroke performed at full
pressure in each experiment. From this the ratio of expansion s is
computed by equation (b), giving in this case
 = 095  7 + 005.
s I
SINGLEACTING EXPANSIVE STEAMENGINES. 293
The value of Z corresponding to s is then found by means of the third
column of table second; that column being selected because the initial
pressures were all below four atmospheres. This affords the means of
determining the initial pressure of the steam by equation (59), viz. —
P x = (E + F) = 18636 . ,
By using table first according to the directions prefixed to it, the
volume of one pound of steam at the pressure P p in cubic feet, is
calculated, and thence, by equation (60), the weight of steam per stroke,
according to theory, which is compared with the weight as ascertained
by experiment.
Further, to illustrate the subject, the useful effect, or duty of a pound
of steam, is computed according to the theory and the experiments
respectively, and the results compared.
The following table (See j>. ,!UJ t ) exhibits the results.
This comparison sufficiently proves that the results of the theory are
practically correct.
It is remarkable, that in every instance except one (experiment E), the
experimental results show a somewhat less expenditure of steam per
stroke, and a greater duty per pound of steam, than theory indicates.
This is to be ascribed to the fact, that although the action of the steam
is computed theoretically, on the assumption that during the expansion
it is cut off from external sources of heat, yet it is not exactly so in
practice; for the cylinder is surrounded with a jacket or casing communi
cating with the boiler, in which the temperature is much higher than
the highest temperature in the cylinder, the pressure in the boiler being
more than double the maximum pressure of the steam when working,
as columns (2) and (5) show. There is, therefore, a portion of steam, of
whose amount no computation can be made, which circulates between the
boiler and the jacket, serving to convey heat to the cylinder, and thus
augment by a small quantity the action of the steam expended ; and
hence the formulae almost always err on the safe side.
Supposing one pound of the best Welsh coals to be capable (as found
by Mr. Wicksteed) of evaporating 9'493 lbs. of water at the pressure
in the boiler during the experiment F, then the duty of a Cornish bushel,
or 94 lbs. of such coals, in the circumstances of that experiment would be —
By theory, 88,288,000 ft. lbs.
By experiment, 90,801,000 „
Difference, . . + 2,513,000 „
294
SINGLEACTING EXPANSIVE STEAMENGINES.
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SINGLEACTING EXPANSIVE STEAMENGINES. 295
Economy of SingleActing Expansive Engines.
37. By increasing the ratio of expansion in a Cornish engine, the
quantity of steam required to perform a given duty is diminished ; and
the cost of fuel, and of the boilers, is lowered. But at the same time,
as the cylinders and every part of the engine must be made larger, to
admit of a greater expansion, the cost of the engine is increased. It thus
becomes a problem of maxima and minima to determine what ratio of
expansion ought to be adopted under given circumstances, in order that
the sum of the annual cost of fuel, and the interest of the capital
employed in construction, may be the least possible, as compared with the
work done.
That this problem may admit of a definite solution, the following five
quantities must be given : —
Pj, the initial pressure in the cylinder.
F, the resistance not depending on the useful load.
I n, the amount of the length of the effective strokes made in unity of
time.
k, the annual cost of producing unity of weight of steam in unity of
time, which consists of two parts — the price of fuel, and the interest of
the cost of the boilers.
k, the interest of the cost of the engine, per unit of area of piston.
Hence the annual expenditure to be taken into consideration, reduced
to unity of weight of steam, is ,
And the useful effect of unity of weight of steam being
V^ZFs),
the problem is to determine the ratio of expansion s, so that
V^ZFs)
V s
In
shall be a maximum.
Dividing the numerator of this fraction by V t P 1? and the denominator
by r 1 , both of which are constants in this problem, we find that it will
296 SINGLEACTING EXPANSIVE STEAMENGINES.
be solved by making the ratio
7 F
Z — =rS
Py .... (G2.)
a maximum.
The algebraical solution would be extremely complicated and tedious.
The graphic solution, on the other hand, is very simple and rapid, and
sufficiently accurate for all practical purposes ; and 1 have therefore
adopted it.
In the diagram (See Piatt II., Fig. I), the axis of abscissa; — XO
+ X, is graduated from towards + X into divisions representing
ratios of expansion, or values of & The divisions of the axis of ordinates
OY represent values of Z. The curve marked "locus of Z," is laid
down from the third column of Table II. of the Appendix to the original
paper, being applicable to initial pressures not exceeding four atmospheres.
Through the origin draw a straight line BOA, at such an inclination
F
to — X + X that its ordinates are represented by s. Then the
*■ i
ordinates measured from this inclined line to the locus of Z represent the
F
value of the numerator Z — s, of the ratio (62), corresponding to the
* i
various values of .*.
Take a point at C on the line B A, whose abscissa, measured along
— X, represents — 7 . Then the ordinates, measured from BOA,
k V j
of any straight line drawn through C, vary proportionally to the denomi
nator — — + s of the ratio (62).
/,' V ,
Through the point C, therefore, draw a straight line C T, touching the
locus of Z : then the ratio (62) is a maximum at the point of contact T,
and the abscissa at that point represents the ratio of expansion required.
Example.
i
38. To exemplify this method, let us take the following data: —
Greatest pressure in the cylinder P x = 20 lbs. per square inch, = 2880
lbs. per square foot.
The corresponding value of V x is 20  248 cubic feet per pound of steam.
To obtain this initial pressure in the cylinder, it will be necessary to
have a pressure of about 50 lbs. per square inch in the boiler.
F, resistance not depending on the useful load = 2 lbs. per square inch,
= 288 lbs. per square foot, = T V P r
SINGLEACTING EXPANSIVE STEAMENGINES. 21)7
In, amount of clown strokes, = 4800 feet per hour; being the average
speed found to answer best in practice.
To estimate h, the annual cost of producing one pound of steam per
hour, I shall suppose that the engine works 6000 hours per annum; that
the cost of fuel is one penny per 100 lbs. of steam;'" that the cost of
boiler for each pound of steam per hour is 0*016 ton, at £27, = £0*432;
and that the interest of capital is five per cent, per annum. Hence h is
thus made up —
Fuel for 6000 lbs. of steam at 0*01d., . . . £0*2500
Interest on £0*432, at 5 per cent,, . . . 0*0216
h = £0*2716
Estimating the cost of the engine at £250 per square foot of piston,
we find /; = 5 per cent, per annum on £250 = £12*5.
, h „ , ^. ~ h 1 n _ , . .
and =0*0217; ==■ = 5*144.
I; k V j
The line E O A, then, is to be drawn so that its ordinates are
F 1
p; 5 = To 5 
The point C is taken on this line, at £= = 5*144 divisions of the
axis of abscissa? to the left of O Y.
The tangent C T being drawn, is found to touch the locus of Z at 2*800
divisions to the right of O Y.
Then s = 2*800 is the ratio of expansion sought, corresponding to the
greatest economy.
If we make c=r0'05, as in Mr. Wicksteed's engine, then the fraction of
the stroke to be performed at full pressure is
 = 0*323,
I
being nearly the same as in experiment F.
The mean resistance of the useful load per square foot of piston is
E = — R  F =1713*6 lbs.
s 1
* This estimate is maile on the supposition that coals capable of producing nine times
their weight of steam are worth about 16s. 9d. per ton.
298 SINGLEACTING EXPANSIVE STEAMENGINES.
The duty of one square foot of piston per liour —
B,ln, = 8,225,300 footlbs.
And one horsepower being 1,980,000 footlbs. per hour, the real horse
power of the engine is
4  154 per square foot of piston.
The duty of one pound of steam is
RV 1 s = 97,154 footlbs.
To give an example of a special case, let the duty to be performed be
198,000,000 footpounds per hour, being equal to 100 real horsepower,
for G,000 hours per annum. This being called E, we find from the above
data that the area of piston required is
i;
A = _ . = 21072 square feet,
li / n
The consumption of steam per hour is
W =,,!': = 2038 lbs.,
Jtt V x .S'
which requires 2038 x 001 G = 32608 tons of boilers.
The expenditure of steam per annum is
2038 x G000 = 12,228,000 lbs.
Hence we have the following estimate : —
Cost of engine, 24*072 square feet of piston at
£250, £6018000
Cost of boilers, 32G08 tons at £27, . . . 880*416
Total capital expended, . . . £6898416
Interest at five per cent, per annum, . . . 344921
Cost of fuel per annum, 12,228,000 lbs. of steam
at 001d., 509500
Annual cost for interest and fuel, . . £854*421
I wish it to be understood that the rates I have adopted in the fore
SINGLEACTING EXPANSIVE STEAMENGINES. 299
going calculations, for interest, cost of fuel, and cost of construction, are
not intended as estimates of their average amount, nor of their amount in
any particular case ; but are merely assumed in order to illustrate, by a
numerical example, the rules laid down in the preceding article. It is,
of course, the business of the engineer to ascertain those data with
reference to the special situation and circumstances of the proposed work;
and having clone so, the method explained in this paper will enable him
to determine the dimensions and ratio of expansion which ought to be
adopted for the engine, in order that it may effect its duty with the
greatest possible economy.
300 ECONOMY OF HEAT IN EXPANSIVE MACHINES.
XVII.— ON THE ECONOMY OF HEAT IN EXPANSIVE
MACHINES :*
Forming the Fifth Section of a Paper On the
Mechanical Action of Heat.
39. A MACHINE working by expansive power consists essentially of a
portion of some substance to which heat is communicated, so as to expand
it, at a higher temperature, being abstracted from it, so as to condense it
to its original volume, at a lower temperature. The quantity of heat
given out by the substance is less than the quantity received; the
difference disappearing as heat, to appear in the form of expansive power.
The heat originally received by the working body may act in two ways:
to raise its temperature, and t<< expand it. The heal given out may also
act in two ways: to lower the temperature, and to contract the body.
Now, as the conversion of heat into expansive power arises from changes
of volume only, and not from changes of temperature, it is obvious, that
the proportion of the heat received which is converted into expansive
power will lie the greatest possible, when the reception of heat, and its
emission, each take place at a constant temperature.
40. Carnot was the first to assert the law, that the ratio of the maximum
mechanical effect to the vial heat expended in an expansive machine, is a
function solely of tin fir,, temperatures at which the loot is respectively received
ami emitted, and is independent of (he nature of the working substance, But
his investigations not being based on the principle of the dynamical
convertibility of heat, involve the fallacy that power can be produced out
of nothing.
•41. The merit of combining Carndt's Law, as it is termed, with that
of the convertibility of heat and power, belongs to Mr. Clausius and
Professor William Thomson; and in the shape into which they have
brought it, it may be stated thus : —
The maximum proportion of heat converted in la expansive power by any
machine, is a function solely of the temperatures at which heat is received and
emitted by the working substance; which function for each pair of temperatures
is the same for all substances in nature.
* Bead before the Eoyal Society of Edinburgh on April 21, 1851, and published in
the Transactions of that Society, Vol. XX., Part II.
in which the negative sign denotes aDsorpuou, ana cue jjustvttg emission.
Bn.l.
ECONOMY OF HEAT IN EXPANSIVE MACHINES. 301
This law is laid down by Mr. Clausius. as it originally had been by
Carnot, as an independent axiom; and I had at first doubts as to the
soundness of the reasoning by which he maintained it. Having stated
those doubts to Professor Thomson, I am indebted to him for having
induced me to investigate the subject thoroughly; for although I have
not yet seen his paper, nor become acquainted with the method by which
he proves Carnot's law, I have received from him a statement of some
of his more important results.
42. I have now come to the conclusions, — First, That Carndt's Law
is not an independent principle in the theory of heat; hut is deducible, as a
consequence, from the equations of the mutual conversion of heat and expansive
power, as given in the first section of this paper.
Secondly, That the function of the temperatures of reception and emission,
which expresses the maximum ratio of the heat converted into power to the total
heat received by the working body, is the ratio of the difference of those temper
atures to the absolute temperature of reception diminished by the constant, which
I have called tc = Cn/j.b ; and which must, as I have shown in the intro
duction, be the same for all substances, in order that molecular equilibrium
may be possible.
43. Let abscissa^ parallel to X in the diagram, Plate II, Fig. 2,
denote the volumes successively assumed by the working body, and
ordinates, parallel to Y, the corresponding pressures. Let r x be the
constant absolute temperature at which the reception of heat by the body
takes place : t , the constant absolute temperature at which the emission
of heat takes place. Let A B be a curve such that its ordinates denote
the pressures, at the temperature of reception t v corresponding to the
volumes denoted by abscissa\ Let D C be a similar curve for the temper
ature of emission r . Let A D and B C be two curves, expressing by
their coordinates how the pressure and volume must vary, in order
that the body may change its temperature, without receiving or emitting
heat ; the former corresponding to the most condensed and the latter to
the most expanded state of the body, during the working of the machine.
The quantity of heat received or emitted during an operation on the
body involving indefinitely small variations of volume and temperature,
is exj>ressed by adding to equation (G) of section fourth the heat due to
change of temperature only, in virtue of the real specific heat. "We thus
obtain the differential equation
— fe £ r,
in which the negative sign denotes absorption, and the positive emission.
302 ECONOMY OF HEAT IN EXPANSIVE MACHINES.
If wo now put for } v , , , their values according to equation (11),
d V (It
we find
8Q'_SQ= _( T _«)^?.gy
_ jfc + 1 (« _ «*) + (T _ .) '' j™ ,, V \» T . (63.)
(. C«M\t rv d tJ (It )
The first term represents the variation of heat due to variation of
volume only; the second, that due to variation of temperature. Let
us now apply this equation to the cycle of operations undergone by the
working body in an expansive machine, as denoted by the diagram.
First operation. — The body, being at first at the volume V A and pressure
P A , is made to expand, by the communication of heat at the constant
temperature t v until it reaches the volume V B and pressure P B , A B being
the locus of the pressures.
Here S r = ; therefore, the total heat received is
H 1 = Q' 1 = (r 1  K )J v
v
1 dV
d
r
= 0"! *){<P(V*>tJ *(V*r,)}.J
(a.)
Set: lion. — The body, being prevented from receiving or emitting
heat, exj lands until it falls to the temperature r , the locus of the pressures
being the curve B C. During this operation the following condition
must be fulfilled, —
= 8 q  8 < >.
which, attending to the fact that V is now a function of r, and trans
forming the integrals as before, gives the equation
= ,! + cTm \r ~ ?) + < r  K) KTr + ,77 • 2 v) * <*■ T >
This equation shows that
(V. r x )  96 (V c , r ) = ^ (r r r ). . . (b.)
Third operation. — The body, by the abstraction of heat, is made to
contract, at the constant temperature r , to the volume V D and pressure P Dr
which are such as to satisfy conditions depending on the fourth operation.
C D is the locus of the pressures. The heat emitted is evidently
H = Q' = (r  k) {cp (V c , r )  d> (V w r )}. . (&)
ECONOMY OF HEAT IN EXPANSIVE MACHINES. 303
Fourth operation. — The body, being prevented from receiving or emitting
heat, is compressed until it recovers its original temperature t v volume V A , ,
and pressure P A ; the locus of the pressures being DA. During this
operation, the same conditions must be fulfilled as in the second operation ;
therefore,
<P ( Y a, O  </> (V D , r ) = xP (t v r ). . . (d.)
ib being the same function as in equation (b).
By comparing equations (b) and (d), we obtain the relation which must
subsist between the four volumes to which the body is successively brought,
in order that the maximum effect may be obtained from the heat. It is
expressed by the equation
tiVr,^)  c/>(Y x ,T 1 ) = ci>(Y c ,T ) <P(\\,T ). • (64.)
From this and equations (a) and (c), it appears that
Ho _ To — K
H l T l  *
(65.)
That is to say : ivhen no heat is employed in producing variations of tempera
ture, the ratio of the heat received to the heat emitted by the working body of an
ivi machine, is equal to that of the absolute temperatures of reception and
emission, each diminished by the constant k, which is the same for all substances.
Hence, let
n =  q\  Q' = h x  h
denote the maximum amount of power which can be obtained out of
the total heat H^ in an expansive machine working between the tempera
tures t x and r . Then
£ = ^=A .... (66.)
H i T i ~ K
being the law which has been enunciated in Article 42, and which is
deduced entirely from the principles already laid down in the introduction
and first section of this paper.
The value of the constant K is unknown; and the nearest approximation
to accuracy which we can at present make, is to neglect it in calculation,
as being very small as compared with r.
44. This approximation having been adopted, I believe it will be
found that the formula (66), although very different in appearance from
that arrived at by Professor Thomson, gives nearly the same numerical
results. For example : let the machine work between the temperatures
140° and 30° Centigrade : then
304 ECONOMY OF HEAT IX EXPANSIVE MACHINES.
r, = 414°G, r = 304°'6,
and
" = 02653.
Trofessor Thomson has informed me, that for the same temperatures
he finds this ratio to be 0*271 3.*
45. To make a steamengine work according to the conditions of
maximum effect here laid down, the steam must enter the cylinder frorn^
the boiler without diminishing in pressure, and must be worked expan
sively down to the pressure and temperature of condensation. It must
then be so far liquefied by conduction alone, that on the liquefaction
being completed by compression, it may be restored to the temperature
of the boiler by means of that compression alone. These conditions
are unattainable in steamengines as at present constructed, and different
from those which form the basis of the formulas and tables in the
fourth section of this paper; hence it is found, both by experiment
and by calculation from those formulas, that the proportion of the total
heat converted into power in any possible steamengine is less than that
indicated by equation (GG).
The annexed table illustrates this.
The heat transformed into power, as given in the fifth column, has
been reduced to Centigrade degrees in liquid water, by dividing the duty
of a pound of steam by Mr. Joule's equivalent, 1389G feet per Centigrade
degree. Hence, the first two numbers in that column are less than those
given in Art. 32, which were computed from too small an equivalent.
The first two cases fulfil the conditions required by Canmt's law in
every respect except one : — viz., that the steam remaining at the end
of the stroke, instead of being partially liquefied by refrigeration, and
then reduced to water at the temperature of the boiler by compression,
is supposed to be entirely liquefied by refrigeration. This occasions the
loss of the heat necessary to raise the water from the temperature of
the condenser to that of the boiler; but at the same time, there is a
gain of the power which would be required to liquefy part of the
steam by compression, and those two quantities partially compensate
for each other's effects on the ratio of the power to the heat expended,
so that although it is below the maximum, the difference is small.
* From information which I have received from Professor Thomson subsequently to
the completion of this paper, it appears that his formula becomes identical with the
approximate formula here proposed, on making the function called bj' him fc = — , J
being Joule's equivalent.
Mr. Joule also, some time since, arrived at this approximate formula in the
particular case of a perfect gas.
ECONOMY OF HEAT IN EXPANSIVE MACHINES.
305
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306 ECONOMY OF HEAT IN EXPANSIVE MACHINES.
In the third and fourth examples, founded on the calculated and
observed duty of Mr. Wicksteed's engine during experiment F, the actual
ratio is less than half the maximum. This waste of heat is to he
ascribed to the following causes : —
First, The mode of liquefaction, which has already been referred to.
Secondly, The initial pressure in the cylinder is but 18 "9 3 lbs. on the
square inch, while that in the boiler is 457; so that although the steam
is produced at 135°2 Centigrade, it only begins to work at 107°'2G.
This great fall of pressure is accounted for by the fact, that the steam
for each stroke, which is produced in the boiler in about seven or eight
seconds, escapes suddenly into the cylinder in a fraction of a second.
Thirdly, The expansive working of the steam, instead of being continued
down to 30° Centigrade, the temperature of the condenser, stops at a
much higher temperature, 74°GG. This is the most important cause
of loss of power.
If we now take for r x and r the absolute temperatures at the beginning
and end of the expansive working, and calculate the maximum duty of
one pound of steam by Carn&t's Law between those temperatures, we find, —
t x = 107°2G + 274°G = 381°8G
Tq = 74°GG + 274°G = 349 '26
~ = 008f) ! 2
K 1 =5U '5 i .:Il= 48°'22
To this has to be added the duty, at full pressure,
of steam at r , diminished by onethird for back
pressure and friction, and by OHefifteenth for
liquefaction in the cylinder, = . . . . 23 0, 14
The whole amounting to . . . 71° # 3G
Which agrees very nearly with 73°'23, the observed duty, and almost
exactly with 71°*2, the duty as calculated by the formulae and tables of
section fourth.
These examples show clearly the nature and causes of the waste of heat
in the steamengine.
ABSOLUTE ZERO OF THE PERFECT GAS THERMOMETER. 307
XVIIL— ON THE ABSOLUTE ZERO OF THE PERFECT GAS
THERMOMETER : *
Being a Note to a Paper On the Mechanical Action of Heat.
{Seep. 23 If.)
Temperature being measured by the pressure of a perfect gas at constant
density, the absolute zero of temperature is that point on the thermometric
scale at which, if it were possible to maintain a perfect gas at so low a
temperature, the pressure would be null.
The position of this point is of great importance, both theoretically and
practically; for by reckoning temperatures from it, the laws of phenomena
depending on heat are reduced to a more simple form than they are when
any other zero is adopted.
As we cannot obtain any substance in the perfectly gaseous condition
(that is to say, entirely devoid of cohesion), Ave cannot determine the
position of the absolute thermometric zero by direct experiment, which
furnishes us with approximate positions only. Those approximate posi
tions are always too high ; because the effect of cohesion is to make the
pressure of a gas diminish more rapidly with a diminution of temperature,
than if it were devoid of cohesion.
As a gas is rarefied, the cohesion of its particles diminishes, not only in
absolute amount, but also in the proportion which it bears to the pressure
due to heat. The gas, therefore, approaches more and more nearly to the
state of a perfect gas as its density diminishes ; and from a series of
experiments on the rate of increase of its elasticity with temperature, at
progressively diminishing densities, may be calculated the positions of a
series of points on the thermometric scale, approaching more and more
nearly to the true absolute zero.
By observing the law which those successive approximations follow, the
true position of the absolute zero can be determined.
Having performed this operation by means of a graphic process, soon
after the publication of the experiments of M. Regnault on the elasticity
and expansion of gases, I stated the result in a paper on the Elasticity of
Vapours (Edinburgh New Philosophical Journal, July, 1849), {See p. 1). and
* Read before the Royal Society of Edinburgh on January 4, 1333, and pnblis! ed
in the Transactions of that Society, Vol. XX. Part IV.
80S ABSOLUTE ZERO OF THE TERFECT GAS THERMOMETER.
also in a paper on the Mechanical Action of Heat (Trans. Royal Sue. Edin.,
Vol. XX., Part I.), (See p. ..'■'.'/' — viz., that the absolute zero is —
274°G Centigrade, .) below ^ tc rature of me lting ice ;
or 494°28 Fahrenheit, J
or 4G2 0 28 below the ordinary zero of Fahrenheit's scale.
To enable others to judge of the accuracy of this result, 1 shall now
explain the method by which it was obtained.
Let E denote the mean rate of increase, per degree, between the
freezing and boiling points, of the pressure of a gas whose volume is
maintained constant. Then the reciprocal of this coefficient, , is an
approximation to the number of degrees below the freezing point, at
which the absolute zero is situated.
The experimental data in the following table were copied from the
memoirs of M. Regnault on the Expansion of Gases. The numbers in
the first column designate the series of experiments. The second column
contains the pressures of the gases at the freezing point. The third
column contains the mean coefficients of increase of pressure per Centi
grade degree, between 0° sod 100° Centigrade. The fourth column
contains the reciprocals of those coefficients, with the negative Bign, being
approximate positions of the absolute zero, in Centigrade degrees, below
the temperature of melting ice. The gases employed were atmospheric air
and carbonic acid.
The approximate positions of the absolute zero contained in this table
wen* laid down on ;i diagram, in which they were marked by crosses.
The longitudinal divisions representing Centigrade degrees divided into
tenths; the transverse divisions, atmospheres of pressure a1 (l Centigrade,
also divided into tenths. The positions of the crosses indicating at
once the pressures in the second column of the table, and the approximate
zeros in the fourth ; and the numbers affixed to them corresponding with
those in the first column.
As the effect of cohesion is greater and more easily eliminated in car
bonic acid gas than in atmospheric air, the determination of the true
absolute zero was made from the experiments on the former gas. The
approximate positions of the absolute zero for carbonic acid lie nearly in a
straight line. A straight line having been drawn so that it should, as
nearly as possible, traverse them, was found to intersect the line corre
sponding to the zero of pressure, that is, to the state of perfect gas, at a
point on the scale of temperatures 274  6 Centigrade degrees below the
temperature of melting ice, which point was accordingly taken as the true
absolute zero of the perfect gas thermometer.
ABSOLUTE ZERO OF THE PERFECT GAS THERMOMETER.
309
No.
Pressure at 0° Centigrade
in Atmospheres.
Coefficient of Increase of
Elasticity with Temperature
= E.
Approximate Positions
of the Absolute Zero
in Centigrade Degrees
1
~ E"
CARBONIC ACID.
1.
099S0
00036S5G
27133
o
11857
0036943
27063
3.
22931
00037523
26650
4,
47225
0003S59S
259 OS •
ATMOSPHERIC AIR.
1.
01444
00036482
27411
o
02294
0003G513
27388
3.
03501
00036542
27366
4.
04930
00365S7
27332
5.
04937
0036572
27343
6.
10000
0003G650
27285
7.
22084
0003G700
27203
8.
22270
00036800
27174
9.
28213
0036S94
27105
10.
4S100
0037091
269 61
So far as their irregularity permits, the experiments on atmospheric air
confirm this result, for the approximate positions of the absolute zero
deduced from them evidently tend towards the very same point on the
diagram with those deduced from the experiments on carbonic acid.
The values of the coefficient of dilatation and of increase of pressure of
a perfect gas, per degree, in fractions of its volume and pressure, at the
temperature of melting ice, are, accordingly,—
For the Centigrade Scale 7 —— = 0003641 CO
° 2740
For Fahrenheit's Scale
1
494L'S
000202314.
810 THE MECHANICAL ACTION OF HEAT.
XIX.— OX THE MECHANICAL ACTION OF HEAT.*
Section VI. — A Review of the Fundamental Principles of the
Mechanical Theory of Heat; with Remarks on the
Thermic Phenomena of Currents of Elastic
Fluids, as illustrating those
Principles.
4G. I have been induced to write this section in continuation of a
paper on the Mechanical Action of Heat, by the publication (in the
Philosophical Magazine for December, 1852, Supplementary Number) of a
series of experiments by Mr. Joule and Professor "William Thomson, On
the Thermal Effects experienced by Air in rushing through Small Apertures.
Although those authors express an intention to continue the experiments
on a large scale, so as to obtain more precise results; yet the results
already obtained are sufficient to constitute the first step towards the
experimental determination of that most important function in the theory
of the mechanical action of heat, which has received the name of Carnot's
function.
By the theoretical investigations of Messrs. Clausius and Thomson, —
which are based simply on the fact of the convertibility of heat and
mechanical power, the determination of their relative value by Mr. Joule,
and the properties of the function called temperature, without any definite
supposition as to the nature of heat, — Carnot's function is left wholly
indeterminate.
By the investigations contained in the previous sections of this paper,
and in a paper on the Centrifugal Theory of Elasticity (See p. ^9), — in
which the supposition is made, that heat consists in the revolutions of
what are called molecular vortices, so that the elasticity arising from heat
is in fact centrifugal force, — a form is assigned to Carnot's function; but
its numerical values are left to be ascertained by experiment.
The recent experiments of Messrs. Joule and Thomson serve (so far
as the degree of precision of their results permits) at once to determine
numerical values of Carnot's function for use in practice, and to test the
* Read before the Eoyal Society of Edinburgh on January 17, 1853, and pub
lished in the Transactions of that Society, Vol. XX., Part IV.
THE MECHANICAL ACTION OF HEAT. oil
accuracy with which the phenomena of heat are represented by the
consequences of the hypothesis of molecular vortices, from which the
investigation in this paper sets out.
SubSection 1. — Properties of Expansive Heat.
47. To show more clearly the nature of the questions, towards the
decision of which these experiments are a step, I shall now briefly review
the fundamental principles of the theory of heat, and the reasoning on
which they are based; and the object of this being illustration rather
than research, I shall use algebraical symbols no further than is absolutely
necessary to brevity and clearness, and shall follow an order of investigation
which, though the same in its results with that pursued in the previous
sections of this paper, is different in arrangement.
By a mind which admits as an axiom that, in the present order of
things, physical power cannot be annihilated, nor produced out of nothing,
the law of the mutual convertibility of heat and motive power must be
viewed as a necessary corollary from this axiom, and Mr. Joule's experi
ments, as the means of determining the relative numerical value of those
two forms of power. By a mind which does not admit the necessity of
the axiom, these experiments must be viewed also as the proof of the law.
This law was virtually, though not expressly, admitted by those who
introduced the term latent heat into scientific language; for Avhen divested
of ideas connected with the hypothesis of a subtle fluid of caloric, and
regarded simply as the expression of a fact, this term denotes heat which
has disappeared during the appearance of expansive power in a mass of
matter, and which may be made to reappear by the expenditure of an
equal amount of compressive power.
48. Without for the present framing any mechanical hypothesis as to
the nature of heat, let us conceive that unity of weight of any substance
occupying the bulk V under the pressure P, and possessing the absolute
quantity of thermometric heat whose mechanical equivalent is Q, undergoes
the indefinitely small increase of volume d V; and let us investigate how
much heat becomes latent, or is converted into expansive power, during
this process; the thermometric heat being maintained constant, so that
the heat which disappears must be supplied from some external source.
During the expansion d V, the body, by its elastic pressure P, exerts
the mechanical power P d V. Part of this power is produced by mole
cular attractions and repulsions; and although this part may be modified
by the influence of heat upon the distribution of the particles of the body,
it is not the direct effect of heat. The remainder must be considered as
directly caused by the heat possessed by the body, of which the pressure
312 THE MECHANICAL ACTION OF HEAT.
P is a function; and to this portion of the power developed, the heat
which disappears during the expansion must be equivalent.
To determine the portion of the mechanical power P<ZV which is
the effect of heat, let the total heat of the body, Q, be now supposed to
vary by an indefinitely small quantity d Q. Then the mechanical power
of expansion VdY will vary by the indefinitely small quantity
This is the development of power for the expansion dV, caused by each
indefinitely small portion rf'Q of the total heat possessed by the body;
and, consequently, the whole mechanical power for the expansion dY due
to the whole heat possessed by the body Q, is expressed as follows: —
«ii" (67 »
and this is the equivalent of the heat transformed into mechanical power,
or the latent heat of expansion of unity of weight, fur the small increment
of volume d V, at the volume V and total heat Q.
Now, a part only of this power, viz. —
P d Y,
is visible mechanical energy, expended in producing velocity in the
expanding body itself, or in overcoming the resistance of the bodies which
inclose it. The remainder
(„;:;; .•).. v. . w
r/O
is, therefore, expended in overcoming molecular attraction.
Molecular attraction depends on the density and distribution of the
particles of the body; and is, consequently, a function of the volume and
total heat of unity of weight. It is, therefore, possible to find a potential
S, being a function of V and Q, of such a nature, that the difference
between its two values
S 2 — 9j,
corresponding respectively to two sets of values of the volume and total
heat (V 15 Q, and V„ Q 2 ), shall represent the power which is the equivalent
of the heat consumed in overcoming molecular attraction, during the
passage of the body from the volume Y 1 and heat Q x to the volume V 2
THE MECHANICAL ACTION OF HEAT. 313
and heat Q 2 . The form of the expression (68) shows that this potential
has the following property : —
The integration of which partial differential equation gives the following
value for the potential of molecular action: — ■
S = /(Q^!P)eZV + tf>(Q), • • (70.)
<j) (Q) being some unknoAvn function of the heat only, and the integral
being taken as if the heat Q were constant.
The heat which disappears in overcoming molecular action, during a
small increase of total heat d Q, while the volume remains constant, is
expressed as follows: —
the heat Q being treated as a constant in the integration.
If we now investigate the entire quantity of heat, both sensible and
latent, which is consumed by a body during a simultaneous small change
of total heat d Q and volume d V, we find the following results : —
Sensible heat (which retains its condition), . = d Q
Latent heat, or heat which disappears in
overcoming molecular action, . . yf\ ^ ^ • Tv "
Latent heat equivalent to the visible me
chanical effect, P d V
The amount being
^ + ^S + P.V=(l + ^)iQ+(^ + p).V =
(72.)
This formula expresses completely the relations between heat, mole
cular action, and expansion, in all those cases in which the expansive
power developed, P d V, is entirely communicated to the bodies inclosing
the substance which expands.
49. The following coefficients are contained in, or deducible from it,
314
THE MECHANICAL ACTION OF HEAT.
The ratio of the specific heat at constant volume to the real specific
heat : —
T = 1+ s = 1 + Q /^ dV + ^ (Q) ' • (73)
The coefficient of latent heat of expansion at constant heat :—
dS
d\
+ P = Q
dV
(74.)
The ratio of the specific heat at constant pressure to the real specific
heat is found as follows. To have the pressure constant we must have
dV lrt dF ITT , dV
a (^ d\ d Q
dV
dQ
dl y
dX
consequently, the ratio in question is,
dV
dX
dVY
\d ( )
^ (75.)
50. In order to investigate the laws according to which heat is con
verted into mechanical power, in a machine working by the expansion of
an elastic body, it will be convenient to use a function,
F= j^dY(Q = const.),
of such a nature that the difference between two of its values, correspond
ing to different volumes of the body at the same total heat, represents the
ratio of the heat converted into power by expansion between those volumes,
to the given constant total heat. I shall call this function a heatpotential.
Introducing this function into equation (72), we find for the total heat
consumed by a body during the increase of total heat d Q, and the expan
sion d V,
dQ, + d . S + P<7V = (l + f . (Q) W + Qd . F (7G.)
THE MECHANICAL ACTION OF HEAT. 315
(
d F dF
observing that d . F = T — d Q + T, r d V
« y a V
"OS" )'« + £«•>
Let us now suppose that the body changes its volume without either
losing or gaining heat by conduction. This condition is expressed by the
equation
= (1 +$' .Q)dQ + Qd. F,
from which we deduce the following,
i.rl+^W.iq. . . (77.)
which expresses the following theorem : —
When the quantity of heat in a body is varied by variation of volume only, the
variation of the heatpotential depends on the heat only, and is independent of the
volume.
In order that a machine working by the expansive power of heat may
produce its greatest effect, all the heat communicated from external bodies
should be employed in producing expansive power, and none in producing
variations of the quantity of heat in the body; for heat employed for the
latter purpose would be wasted, so far as the production of visible motion
is concerned. To effect this the body must receive heat by conduction,
and convert it into expansive power, while containing a certain constant
quantity of heat Q : ; give out by conduction heat produced by compression,
while containing a smaller constant quantity of heat Q 9 ; and change
between those two quantities of thermometric heat by means of changes
of volume only, without conduction. For this purpose a cycle of opera
tions must be performed similar to that described by Carnot, as
follows : —
(I.) Let F A be the initial value of the heatpotential ; let the body
expand at the constant heat Q l till the heatpotential becomes F B . Then
the heat received and converted into expansive power is
H a = Q, (F B  F A ).
(II.) Let the body further expand without receiving or emitting heat
till the quantity of heat in it falls to Q 2 ; the heatpotential varying
according to equation (77), and becoming at length F c . The heat con
verted into expansive power in this operation is
Q,  Q,
31G THE MECHANICAL ACTION OF HEAT.
(III.) Let the body be compressed, at the constant heat Q„ till the
heatpotential becomes F D ; a quantity differing from the initial heat
potential F v by as much as F c differs from F B . In this operation the
following amount <>;' power is reconverted into heat, and given out by
conduction : —
IL = Q, (F c  F„).
(IV.) Let the body be further compressed, till the heatpotential returns
to F A , its original value. Then, by the power expended in this com
pression alone, without the aid of conduction, the total heat of the body
will be restored to its original amount, exactly reversing the operation II.
At the end of this cycle of operations, the following quantity of heat
will have been converted into mechanical power: —
H x  H 2 = Q, (F B  F A )  Q 2 (F c  F D ),
but it is obvious that the difference between the heatpotentials is the
same in the first and third operations ; therefore, the useful effect is
simply
11,11,, = ^, t>,KF u F A ), 1
while the whole heat expended is, ' . . (78.)
H^Q^F, V J
Hence, the ratio of the heat convert* J into mechanical effect, in cm expansive
machine, working to the greatest advantage, to the whole heat expended, is the
same with that which the difference between the quantities of heat possessed by the
expansive body during the i
to the quantity of heat possessed by it during the operation of receiving .
and is independent of the nature and condition of the body.
This theorem is thus expressed symbolically, —
H,  H., Effect _ Q,  Q.,
\l i i teat expended Q x
(79.)
51. When a body expands without meeting with resistance, so that all
its expansive power is expended in giving velocity to its own particles,
and when that velocity is ultimately extinguished by friction, then a
quantity of heat equivalent to the expansive power is reproduced.
The heat consumed is expressed by taking away the term representing
the expansive power, ~P dY, from the expression (72), the remainder of
which consists merely of the variation of actual heat, and the heat expended
in overcoming molecular attraction, viz. : —
/ d S\ , „ d S , ,,
THE MECHANICAL ACTION OF HEAT. 317
This expression is a complete differential, and may be written thus : —
d (Q + S) = d { Q + <p (Q) + (Q ~  l) j P rf v (80.)
(Q being treated as a constant in performing the integration I P d X ).
Its integral, Q + 8, the sum of the heat of the body, and of the potential
of its molecular actions, is the same quantity which I have denoted by the
symbol ^F in the tenth article of a paper on the Centrifugal Theory of
Elasticity (See p. G2), and whose differences are there stated to repre
sent the total amount of power which must be exercised on a body,
whether in the form of expansive or compressive power, or in that of heat,
to make it pass from one volume and temperature to another. This
integral corresponds also to the function treated of by Professor William
Thomson in the fifth part of his paper on the Dynamical Theory of Heat,
under the name of " Total Mechanical Energy."
52. We have now obtained a system of formula 3 , expressing all the
relations between heat and expansive power, analogous to those deduced
from a consideration of the properties of temperature, by Messrs. Clausius
and Thomson, and from the hypothesis of molecular vortices in the
previous sections of this paper; but, in the present section, both the
theorems and the investigations are distinguished from former researches
by this circumstance — that they are independent, not only of any hypo
thesis respecting the constitution of matter, but of the properties, and even
of the existence, of such a function as temperature; being, in fact, simply
the necessary consequences of the following
" Definition of Expansive Heat.
Let the term Expansive Heat he used to denote a hind of physical energy
•convertible with, and measurable by, equivalent quantities of mechanical power,
and augmenting the expansive elasticity of matter in which it is present.
52a. It is further to be remarked, that the theorems and formulae in
the preceding articles of this section are applicable, not only to heat and
expansive power, but to any two directly convertible forms of physical
energy, one of which is actual, and the other potential. They are, in fact,
the principles of the conversion of energy in the abstract, when interpreted
according to the following definitions of the symbols : —
318 THE MECHANICAL ACTION OF HEAT.
Let Q denote the quantity of a form of actual physical energy present
in a given body ;
V, a measurable state, condition, or mode of existence of the body,
whose tendency to increase is represented by
P, a force, depending on the condition V, the energy Q, and permanent
properties of the body, so that
P d V is the increment of a form of potential energy, corresponding to
a small increment d V of the condition V.
Let d& be the quantity Avhcrcby the increment of potential energy
P(7V falls short of the quantity of actual energy of the form Q, which is
converted into the potential form by the change of condition d V.
Then, as in equation (G9),
dX ~ H dQ '
an equation from which all those in the previous articles are deducible,
and which comprehends the whole theory of the mutual conversion of the
actual form of energy Q, and the potential form I "PdV, whatsoever those
forms may be, when no other form of energy interferes. The application
of these principles to any form, or any number of forms, of actual and
potential energy, is the subject of a paper read before the Philosophical
►Society of Glasgow, on the 5th January, 18.33, and published in the
Philosophical Magazine for February, 1853. (See p. 203.)
Si BSECTION 2. — PROPERTIES OF TEMPERATURE.
53. Still abstaining from the assumption of any mechanical hypothesis,
let us proceed a step beyond the investigation of the foregoing articles,
and introduce the consideration of temperature — that is to say, of an
arbitrary function increasing with heat, and having the following pro
perties : —
Definition of Equ.d Temperatures.
Two portion; of matter are said to have equal temperatures ivhen neither tends
to communicate heat to the other.
Corollary.
All bodies absolutely destitute of heat have equal temperatures.
The ratio of the real specific heats of two substances is that of the
THE MECHANICAL ACTION OF HEAT. 319
quantities of heat which equal weights of them possess at the same
temperature.
Theorem.
The ratio of the real specific heats of any pair of substances is the same at all
temperatures.
For, suppose equal weights of a pair of homogeneous substances to be
in contact, containing heat in such proportions as to be in equilibrio.
Then, let additional portions of each substance, of equal weight, and
destitute of heat, be added to the original masses ; so that the quantities
of heat in unity of weight may be diminished in each substance, but may
continue to be in the same ratio. Then, if the equality of temperature do
not continue, portions of heat which were in equilibrio must have lost that
equilibrium, merely by being transferred to other particles of a pair of
homogeneous substances, which is absurd. Therefore, the temperatures
continue equal.
It follows, that the quantity of heat in unity of weight of a substance
at a given temperature, may be expressed by the product of a quantity
depending on the nature of the substance, and independent of the
temperature, multiplied by a function of the temperature, which is the
same for all substances.
Let r denote the temperature of a body according to the scale adopted ;
k, the position, on the same scale, of the temperature corresponding to
absolute privation of heat ; ft, a quantity depending on the nature of the
substance, and independent of temperature. Then the quantity of heat
in unity of weight may be expressed as follows : —
Q = feO/,.r^. K ). . . . (81.)
54. If we introduce this notation into the formula (79), which expresses
the proportion of the total heat expended which is converted into useful
power by an expansive machine working to the best advantage, the
quantity ft, peculiar to the substance employed, disappears, and we obtain
Carnot's theorem, as modified by Messrs. Clausius and Thomson — viz.,
that this ratio is a function solely of the temperatures at which heat is
received and emitted respectively, and is independent of the nature of the
substance ; or symbolically,
Effect _ Qi^Q. 2 _ \p . Tj  }p . r, , g2 .
Heat expended Q x \f; . r x — \p . k
55. Let us now apply the same notation to the formula (G7) for the
latent heat of a small expansion, d V, at constant heat, viz : —
320
THE MECHANICAL ACTION OF HEAT.
we have evident lv
d? 1 dV
1 dv
dQ ~ dQ dr ~
\l \jj . T ll 7
dr
and. consequently, the heat which disappears by the expansion dX is
dV
dr
T
d V,
dQ \f/
from which formula the specific quantity ft has disappeared.
Now, in the notation of Professor Thomson we have
\p . T — Tp . K _ «J
where J is Joule's equivalent, and ju. a function of the temperature, the
same for all substances, to be determined empirically ; and, consequently,
hyp. log. (i// . t — i// . k) = 1 J ' fx d .
if
/J
ip . t — ifj . k = e
and
(84.)
sr
^
j
Q = fe (i/, . t  i£ . k) = ft
These expressions will be re by those who have studied Professor
Thomson's papers on the dynamical theory of heat. By introducing the
value given above of the quantity of heat in unity of weight, into the
formula' of the preceding articles of this section, they are at once trans
formed to those of Professor Thomson, and in particular, the formulae (79)
and (82) become the following: —
1 "I 7 1 ~,
j) " dr J,
* Effect of Machine
Heat expended
M
= 1  £
J
k
d
,"
(85.)
* It is to be observed, that in Professor Thomson's notation, heat is supposed to be
measured by an arbitrary unit, whose ratio to a unit of mechanical power is denoted by
J ; while in this paper, the same unit is employed in expressing quantities of heat and
of mechanical power.
THE MECHANICAL ACTION OF HEAT. 321
SubSection 3. — On the Hypothesis of Molecular Vortices.
56. The use of a mechanical hypothesis in the theory of heat, as in
other branches of physics, is to render it a branch of mechanics, the only
"complete physical science ; and to deduce its principles from the laws of
force and motion, Avhich are better understood than those of any other
phenomena.
The results of the investigations in the £>receding part of this section
are consistent alike with all conceivable hypotheses which ascribe the
phenomena of heat to invisible motions amongst the particles of bodies.
Those investigations, however, leave undetermined the relation between
temperature and quantity of heat, except in so far as they show that it
must follow the same law of variation in all substances.
By adopting a definite hypothesis, we are conducted to a definite
relation between temperature and quantity of heat; which, being intro
duced into the formulae, leads to specific results respecting the phenomena
of the mutual transformation of heat and visible mechanical power ; and
those results, being compared with experiment, furnish a test of the
soundness of the hypothesis.
Thus, the hypothesis of molecular vortices, which forms the basis of the
investigations in the first five sections of this paper, and in a paper on the
centrifugal theory of elasticity, leads to the conclusion, that, if tempera
ture be measured by the expansion of a perfect gas, the total quantity of
heat in a body is simply proportional to the elevation of its temperature
above the temperature of absolute privation of heat ; or, in the notation
of the preceding article,
xp . r = r, \f/ . r = 1,
and
Q = fc (r  k), . . . . (86.)
ft being the real specific heat of the body.
If this value be substituted for the quantity of heat Q, in all the
formulae, from (67) to (80) inclusive, which are founded simply on the
definition of expansive heat, it reproduces all the formula? which, in this
and the other paper referred to, have been deduced directly from the
hypothesis. In the sequel, I shall apply one of these formulae to the
calculation, from the experiments of Professor Thomson and Mr. Joule on
the heating of currents of air by friction, of approximate values of the
absolute temperature corresponding to total privation of heat, that the
mutual consistency of those values may serve as a test of the soundness of
the hypothesis, and the accuracy of the formulae deduced from it.
57. Before proceeding further, it may be desirable to point out how far
x
322 THE MECHANICAL ACTION OF HEAT.
this hypothesis agrees with, and how far it differs from, that proposed by
Mr. Herapath and Mr. Waterstom, which supposes bodies to consist of
extremely small and perfectly clastic particles, which fly about in all
directions with a velocity whose halfsquare is the mechanical equivalent
of the heat possessed by unity of weight, and are prevented from dispersing
by their collisions with each other and with the particles of surrounding
bodies. Let v be the velocity of motion, then
9
represents the heat possessed by unity of weight, expressed in terms of
the force of gravity.
The expansive pressure due to such motions is found by conceiving a
hard, perfectly elastic plane of the area unity to be opposed to the collision
of the particles, and calculating the pressure which would be required to
maintain its position against them. If all the particles were to strike and
rebound from such a plane at right angles, the pressure would bi
represented thus:
1_
9 ' T
»
where V is the volume which contains so many particles as amount to
unity of weight. But the particles are supposed to tly in equal numbers
in all directions. Then, if 9 denote the angle of incidence on the plane
/
sin <1 9 . „ , »
= sin 9 I' 9
—
sin 6 d
represents the proportion of the whole particles which fly in those direc
tions which make the angle with the normal to the plane. Of this
proportion, again, the fraction cos 9 only strikes the plane; while the
force of the blow also is less than that of a normal blow in the ratio
cos 0:1. Hence, the mean force of collision is
/
.7T
2 1
cos 2 9 sin 9 (I 9 = ^
6
of the force of a perpendicular collision ; so that the expansive pressure i&
represented by
1 *2 ^_2 Q
3 ' cj ' V ~ 3 ' V
THE MECHANICAL ACTION OF HEAT. 323
Hence, according to this hypothesis, we should have for a perfect gas
P V =  Q,
o
or the product of the pressure and volume of a mass of a perfect gas
equal to twothirds of the mechanical equivalent of its total heat.
It is known, however, that the product of the pressure and volume of a
mass of sensibly perfect gas is only about fourtenths of the equivalent of
its total heat. The hypothesis, therefore, requires modification.
By supposing the particles to attract each other, or to be of appreciable
bulk compared with the distances between them, the ratio in question is
diminished; but either of these suppositions is inconsistent with the
perfectly gaseous condition. *
It appears to me, that, besides this difficulty connected with the gaseous
condition, there exists also great difficulty in conceiving how the hypothesis
can be applied to the solid condition, in which the particles preserve
definite arrangements. The limited amount of time and attention,
however, which I have hitherto bestowed on this hypothesis, is not
sufficient to entitle me to pronounce whether these difficulties admit of a
solution.
58. The idea of ascribing expansive elasticity to the centrifugal force of
vortices or eddies in elastic atmospheres surrounding nuclei of atoms,
originated with Sir Humphry Davy. The peculiarity of the view of the
hypothesis taken in this paper consists in the function ascribed to the
nuclei or central physical points of the atoms, which, besides retaining the
atmospheres round them by their attraction, are supposed, by their actions
on each other, to constitute the medium which transmits radiant heat and
light ; so that heat is radiant or thermometric, according as it affects the
nuclei or their atmospheres.
In this form the hypothesis of molecular vortices is not a mere special
supposition to elucidate the theory of expansive heat, but becomes
connected with the theory of the elasticity of matter in all conditions,
from solid to gaseous, and with that of the transmission of radiations.
I have already investigated mathematically the consequences of this
hypothesis by two different processes, which are necessarily somewhat
complicated.
When the question, however, is confined to the relations between tem
peratures and quantities of heat, a more simple process may be followed,
analogous to that which has been applied in the preceding article to the
hypothesis of molecular collisions.
If a mass of elastic fluid, so much rarefied that the effect of molecular
attraction is insensible, be entirely filled with vortices, eddies, or circu
lating currents of any size and figure, so that every particle moves with
324 THE MECHANICAL ACTION OF HEAT.
the common velocity iv, then, if the planes of revolution of these eddies
be uniformly distributed in all possible positions, it follows, from reasoning
precisely similar to that employed in the preceding article, that the
pressure exerted by the fluid against a plane, in consequence of the cen
trifugal force of the eddies, has the following value in terms of gravity : —
\'?l (870
3 g V
or twothirds of the hydrostatic pressure due to the velocity of the eddies
w j V being, as before, the volume occupied by unity of weight.
It is, however, reasonable to suppose, that the motion of the particles of
atomic atmospheres does not consist merely in circulating currents ; but
that those currents afe accompanied with a certain proportionate amount
of vibration, — a kind of motion which docs not produce centrifugal force.
To these Ave have to add the oscillations of the atomic nuclei, in order to
obtain the mechanical equivalent of the whole molecular motions ; which
is thus found to be expressed for unity of weight by
I f • = Q, • • ■ • (88.)
2g
2 k
h being a specific coefficient. Hence it follows (denoting — by N), that
the expansive pressure due to molecular motions in a perfect gas is equal
to the mechanical equivalent of those motions in unity of volume multi
plied by a specific constant
N •  (89.)
The coefficient N has to be determined by experiment ; its value for
atmospheric air is known to be between 0'4 and 0*41.
In order to account for the transmission of pressure throughout the
molecular atmospheres, it is necessary to suppose them possessed of a
certain amount of inherent elasticity, however small, varying proportionally
to density, and independent of heat. Let this be represented by
h
V'
then
P=(NQ + A)i . • (90.)
is the total pressure of a perfect gas.
Equilibrium of heat and pressure between portions of two different
perfect gases in contact requires that the pressures independent of heat
THE MECHANICAL ACTION OF HEAT. 325
and the pressures caused by heat, shall separately be in equilibria Let
the suffixes a and b be used to distinguish quantities relative to two
different substances in the perfectly gaseous condition. Then the first
condition of equilibrium is expressed as follows : —
h\ , N (h
(4) w =(£)<»» • (9L)
that is to say, the densities of two perfect gases in equilibrio are inversely
proportional to the coefficients of elasticity of their atomic atmospheres.
The second condition is expressed as follows : —
("*)« = &*)»
which, being taken in connection with the first condition, gives
(!«)«=(!«)» • < 93 >
Now, by equation (90), we have
Hence the condition of equilibrium of heat between two perfect gases is
(^)(°> = (i>>> • • • o*>
consequently, temperature may be measured by the product of the pressure and
volume of a perfect gas, divided by a coefficient, which is proportioned to the
volume of the gas at a standard pressure and temperature.
Temperatures thus measured are reckoned from the point known as the
zero of gaseous tension, or absolute zero of a perfect gas thermometer, 274°*6
Centigrade below the temperature of melting ice.
Let V denote the volume of unity of weight of a perfect gas, at a
standard pressure P , and absolute temperature t ; then any other absolute
temperature has the following value : —
PV
t — r,
^(NQ + A), • • (94.)
op v ~p v
x v x v
while the absolute temperature of total privation of heat is
K — T o p v "
r v
(94a.)
320 THE MECHANICAL ACTION OF HEAT.
Hence it appears that quantity of heat in unity of weight bears the
following relation to temperature, —
Q = i(PT*) = ^.(r.4 • • (95.)
in which, if we substitute the symbol of real specific heat,
» p V
ft =iv • • • • ^
we obtain the formula already given (8*6) for the relation between heat
and temperature.*
59. The introduction of the value given above of the quantity of heat
in terms of temperature, into the formula (67), gives for the latent heat
of a .small expansion d V at constant temperature
(r K )7 > <>V. . . . (97 .)
The formula' (79) and (82), for the proportion of heart rendered available
by an expansive engine working to the greatest advantage, becomes
rj  r 2
(98.)
or the ratio of the difference between the temperatures of receiving and
emitting heat, to the elevation of the former temperature above that of
total privation of heat. This is the law already arrived at by a different
process in Section V of this paper.
When the same substitution is made in equation (80), which represents
the total energy, whether as heat or as compressive power, which must be
applied to unity of weight of a substance to produce given changes of
heat and volume, the following result is obtained : —
d . ¥ = dQ + d . S = I ft +/'(t) + (r  k)J~ d V I dr
= d . { ft r +/(r) + ((t  k) f~ T  l) / P d V } . (99.)
As it cannot be simplified, it is unnecessary here to recapitulate the
investigation, which leads to the conclusion that the functions / (r) and
/ (t) have the following values : —
* See Appendix, Note A, p. 33G.
THE MECHANICAL ACTION OF HEAT. 327
f{r) = * N (k hyp. log. r + £) ; /' (r) = ft N (^  J). (99 A.)
We have thus reproduced equation (26) of the paper formerly referred to,
on the Centrifugal Theory of Elasticity. (See p. 49.)
The coefficient of the variation of temperature in the first form of
equation (99) is the specific heat of the suhstance at constant volume.
Denoting this by K v , the formula becomes
d . * = K v . d t +  (r  k) ^f  P  d V. . (100.)
SubSection 4. — Thermic Phenomena of Currents of
Elastic Fluids.
60. When a gas previously compressed is allowed to escape through
small apertures, as in the experiments of Mr. Joule and Professor Thomson,
and has its velocity destroyed entirely by the mutual friction of its particles,
without impediment from any other substance, and without conduction of
heat to or from any other substance ; then its condition is expressed by
making
d ¥ = 0,
that is to say,
1 f (d? P\ r/P)
If we assume (as is really the case in the experiments) that the specific
heat of the gas at constant volume does not sensibly vary within the
limits of the experiments as to temperature and volume, so that K T is
sensibly constant, and also that the variation of temperature is very
small as compared with the absolute temperatures, then we have the
following approximate integral :
i i
which represents the cooling effect of an expansion from the volume V t to
the volume V 2 .
If it were possible to obtain any substance in the state of perfect gas to
be used in experiments of this kind, the first integral in the above expres
sion would disappear, because for a perfect gas
dr t
328 THE MECHANICAL ACTION OF HEAT.
and as the other terra is negative, the result would be a slight heating
dP , , P ,
effect. As no gas, however, is perfect, and as — always exceeds — , the
(I T T
mode of reducing the experimental data is to calculate the value of the
first term, which represents the effect of cohesion, from the known pro
perties of the gas, to subtract from it the actual cooling, and from the
remainder to compute values of k, the temperature of absolute privation of
heat, according to the following formula : —
v
2 riP
(103.)
r i T dY
K T J v dr
When the gas is nearly perfect, as in the case of atmospheric air, it is
unnecessary to take into consideration its deviation from the perfect
condition in computing the integral in the denominator, whose approxi
mate value is found to be
P V V P
 ° —  . hyp. log. ~ = N . hyp. log.  nearly (t being nearly constant),
K v r Vj 1 2
and K v nearly = ft.
The value of the integral in the numerator is found as follows : —
The centrifugal theory of elasticity indicates that the pressure of an
imperfect gas may be represented by the following formula : —
P = P »v{r + A » A ;r' &C }' ' (1M0
where V is the volume in the perfectly gaseous state, at a standard
pressure P , and absolute temperature r , and A , A 1} &c, are a series of
functions of the density, to be determined empirically. From this formula
it is easily seen that
dP P_ p V f A 2A )
so that the first term in the numerator of the expression (103) has the
following value : —
i ii
p v
in which — ^— ° = N r nearly.
THE MECHANICAL ACTION OF HEAT. 329
In order to represent correctly the result of M. Regnault's experiments
on the elasticity and expansion of gases, it was found sufficient to use, in
the formula for the pressure (104), the first three terms; and the functions
of the density which occur in these terms, as determined empirically from
the experiments, were found to have the following values, in which the
unit of volume is the theoretical volume of unity of weight of air under
the pressure of one atmosphere, at the temperature of melting ice, * and
the values of the constants are given for the Centigrade scale.
v =t (v) ? ; v 1 = "G) f • • < 107 >
Com. log. b — 38181545 ; Com. log. a = 0317616S.
Hence it appears that the integrals in the formula (106) have the
following values : —
/>y=«.A.(^ ! !/>T=¥.t. ? .A.(^)*a M i.)
i i
in which the common logarithms of the constants are
Com. log. 2 b = 21101845 ; log. — .  = 24017950 ;
3 r
and these values suit any scale of temperatures.
In calculating, for use in these formulae, the densities — from the observed
pressures, it is sufficiently near the truth, in the case of air, to use the
approximate equation
1 T
— ° . P (in atmospheres).
V T
The common logarithm of r , the absolute temperature of melting ice,
for the Centigrade scale, is 2  4387005.
The constant N for atmospheric air is 0"4 nearly ; therefore
Com. log. (N X hyp. log. 10) = 19642757.
The following, therefore, is the approximate value of the formula (103)
to be used (with the numerical constants already given) in reducing the
experiments of Mr. Joule and Professor Thomson on atmospheric air, so
as to obtain approximate values of the absolute temperature of total
privation of heat : —
* This unit of volume is greater than the actual volume of air, under the circum
stances described, in the ratio of 1 00085 to 1.
330 THE MECHANICAL ACTION OF HEAT.
={^(jg.0)»A.lP*)«^A.CP»))XA^
4 N hyp. log. 10 x A . com. log. ,,. . (106.)
In using this formula, the mean absolute temperature should be taken
as the value of r.
The following table shows the values of the quantity k, computed from
ten mean experimental data, taken respectively from the first ten series of
experiments described in the recent paper of Messrs. Joule and Thomson,
in the supplementary number of the Philosophical Magazine for December,
L852. The temperatures in the table, for the sake of convenience, are
reduced to the Centigrade scale, because that scale has been used through
out the previous sections of this paper.
The final pressure in each case was thai of the atmosphere.
Professor Thomson and Mr. Joule have expressed the opinion, which is
undoubtedly correct, that those experiments in which the largest quantities
of air Avere used were the least liable to error from disturbing causes, such
as the conduction of heat.
Now, it may 1 bserved in the table, that the calculated values
of k are generally greatest, and the discrepancies amongst them least,
for the experiments in which most air was used. To illustrate this, the
results of the last eight series are arranged below in the order of the
quantities of air i mployed.
Cubic inches 1 1>4 ^ ^ g . fi &l ^ n . 2 {] ..,
per second, )
Values of ic, 1683 1'762 2'09 2228 151 2087 2345 2'14
It is further to be remarked, that the discrepancy between the highest
and the lowest of the values of k is
2°345  l°08 = r265 Centigrade:
a quantity which corresponds to a difference of less than one threehundredth
part in computing the proportion of heat converted into mechanical power
by any ordinary expansive engine, according to the formula (98), which
has been deduced from the hypothesis of molecular vortices.
The experiments, therefore, may 1 »e considered as tending to prove, that
the formulas deduced from this hypothesis are sufficiently correct for
practical purposes; and also as affording a strong probability that the
principles to which it leads are theoretically exact, and that the tempera
ture of absolute privation of heat is a real fixed point on the scale,
THE MECHANICAL ACTION OF HEAT.
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332 THE MECHANICAL ACTION OF HEAT.
somewhat more than two Centigrade degrees above the absolute zero of a
perfect gasthermometer (which is, of course, an imaginary point) ; that is
to say, about 272h Centigrade degrees, or 490 degrees of Fahrenheit,
below the temperature of melting ice.
If these conclusions be correct, it follows, that when the temperatures
T t and T.„ between which an expansive engine works, are measured from
the ordinary zero points of the Centigrade and of Fahrenheit's scales
respectively, the following are the utmost proportions of the total heat
expended which it can be made to convert into mechanical power : —
For the Centigrade scale,
For Fahrenheit's scale,
T,  T 2
Tj + 2721
T x + 458£ J
(109.)
In the fifth section of this paper, where a comparison is made between
the actual duty of the Cornish engine at Old Ford, as determined by Mr.
Wicksteed, and the greatest possible duty which could be obtained from a
given quantity of heat by a theoretically perfect engine working between
the same temperatures, the constant k is treated as being so small that it
may be neglected in practice. If the value of k is really 2 0, 1 Centigrade,
as computed above, the calculated maximum theoretical duty in Section \
is too small by about one onehundredandninetieth part of its amount,, —
a quantity of no practical importance in such calculations.
61. It may be anticipated, that when Mr. Joule and Professor Thom
son shall have performed experiments on the thermic phenomena exhibited
by air in more copious currents, and by gases of more definite composition,
and more simple laws of elasticity, much more precise results will be
obtained.
When a gas deviating considerably from the perfectly gaseous condition,
or a vapour near the point of saturation, is employed, it will no longer be
sufficiently accurate to treat the specific heat at constant volume as a con
stant quantity, nor the cooling effect as very small. It will, therefore, be
necessary to employ, for the reduction of the experiments, the integral
form of equation (99) — that is to say,
= A ^ = A  h t + fe X k (hyp. log. r + J
+ (('^/ r 0f p '' v }
THE MECHANICAL ACTION OF HEAT. 335
 k { A (^ rf V  k N (A . 5 + A hj T . log. t) } . (110.)
62. Preliminary to the application of this equation, it is necessary to
determine the mechanical value of the real specific heat fe. Supposing the
law which connects the pressure, density, and temperature of the gas to
be known, it is sufficient for this purpose to have an accurate experimental
determination, either of the apparent specific heat at constant pressure for
a given temperature, or the velocity of sound in the gas under given
circumstances.
First, let us suppose that the apparent specific heat at constant pressure
is known.
The value of this coefficient (Centrifugal Theory of Elasticity, Art. 1 2) is
( dV
\ dr
v dv ;
In order that the lower limit of the integral may correspond with the
condition of perfect gas, it is convenient to transform it into one in terms
of the density. Let D be the weight of unity of volume, then
/£"#•£<*■ < 1I1A >
If, then, we have the pressure of the gas undeu consideration expressed
by the following approximate formula : —
B ^U+v*}
The following will be the values of the functions of the pressure which
enter into the above equation : —
:p P o V f 1 A t 1 ^P P V A, _ , pv A 1 ]
334
THE MECHANICAL ACTION OF HEAT.
) dr> ~ i D 2 " dr** + " r i D
( dF . S
\dr,
— = p v
d p ° °
,/ v
1 + AA»
rf. A D 1 < Z . A 1 D
To illustrate the application of these formula?, let us calculate the
difference between the real specific heat and the apparent specific heat, at
constant pressure, of carbonic acid gas. at the temperature of melting ice,
and at the density which, it' the gas were perfect, would correspond to a
pressure of one atmosphere al the temperature of melting ice. Let this
density be denoted by D , and its reciprocal by V . As the constants have
been deduced from M. Regnault's experiments, the calculations will be
made in French measures and for the latitude of Paris
The actual density of carbonic acid at 0° Centigrade, and under one
atmosphere of pressure, exceeds the theoretical density, in the perfectly
gaseous state, in the ratio of 1'0065 to 1 nearly. Hence, the height
of a homogeneous atmosphere of actual carbonic acid at 0° Centigrade
being ......... 5225*5 metres,
the corresponding height in the state of perfect gas is P V =5259  5
P V
and "  = 19*53 metres per Centigrade i  02*84 !■
r
The functions which express the influence of density on thejdeviation
of carbonic acid gas from the perfectly gaseous state, have the following
values : —
b
D
A T)
A 1 = «.
when
>
Com. log. h = 3*1083932; Com. log. a = 03344538
6 = 000128349 a =2*16:
Mine.)
B n d.D
D d
J n dDJ A 1]D . ^ « DflJ rfD'^o^
= 2b
D d
D ' rfD'
A,D
i>
For the purposes of a first approximation, we may assume that the
THE MECHANICAL ACTION OF HEAT. 335
value of k already found is sufficiently near the truth — viz., 2 0, 1 Centi
grade, so that, in the present instance, t — k = 2 7 2° 5 Centigrade.
Then we find the following results when r = r , and D=D :
Metres. Feet.
( T  K ) P °  V ° .  7 = per Centigrade degree, . . . 0*145 048
/' <T P
( T — K \\ —  d V =■ per Centigrade degree, . . . 0*150  49
J tl t
Sum = K v — ll = excess of apparent specific heat at con
stant volume above real specific
heat 0295 097
,/r
r
(j _ K ) T/ = difference between apparent specific
heats at constant volume and at
constant pressure, . . . 19'565 64  19
K r — ft = excess of apparent specific heat at con
stant pressure above real specific
heat 198G0 6516
4 of the above quantities are of course the corresponding quantities for
Fahrenheit's scale.
Secondly, If the velocity of sound in the gas is given, let this = u. Then
Ave know that
v? = q  P K '' • • • (112.)
9 dD K v V ;
in which
d?
d
P p y f r , d.A D ld^AjD) . A)
So that from the velocity of sound we can calculate the ratio of the specific
heats at constant pressure and at constant volume. Let this ratio be
denoted by y, and let
K v = fc+ c; K p = ft + c';
then
fc + c
; ana k = 
7
= l±^; andft = ^ C . . (H2B.)
B+C y — 1
i.3G
THE MECHANICAL ACTION OF HEAT.
in which c and c are to be calculated as above.*
G3. In using the formula (110) for a gas whose pressure is represented
by the formula
the integrals may be transformed so as to be taken, with respect to the
density, as in the preceding article. Thus we obtain
*J(^*)"4*vS*)»=
 fl.^Dn: PoV A(^hy P .log.I)+^/; V ;^D) (113.)
For carbonic acid, the first of these formula; becomes simply
+
^gQ^KOw.
J
and the second, )■ (113 A.)
n „ fl, . D, a /D t DA )
+ P Y u . ) hvp.log. I) ' Do (^^)^
APPENDIX.
Note A. (to Article 58). — Since this section was read, the theoretical
views relative to the relation between heat and temperature contained in
it and the previous sections of this paper, have received a strong confir
mation by the publication by M. Eegnault of the fact, that he has found
the specific heat of air to be sensibly constant at all temperatures
from — 30° Centigrade to + 225°, and at all pressures from one to ten
atmospheres (Comptes Rcnclus, April 18, 1853); so that equal lengths on
the scale of the air thermometer represent equal quantities of heat.
Note B. (to Article 62). — Until very recently, there existed no exact
* See Appendix, Note B.
THE MECHANICAL ACTION OF HEAT.
337
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338 THE MECHANICAL ACTION OF HEAT.
experimental determination of the specific heat of any gas. The specific
heat of air at constant pressure, as compared with that of water, was
calculated theoretically in the previous part of this paper, from Joule's
equivalent and the velocity of sound, and found to be 0'24. This value
has since been confirmed very closely by Mr. Joule's experiments, whose
mean result was 0'23, and still more exactly by M. Kegnault's experiments,
already referred to, which give the value 02379. The table {See p. 337),
shows the results of the application of the formula? of this paper to the
specific heats of five different gases at constant pressure, selected from M.
Kegnault's table (Comptes Rendus, April 1 8), as being those in which the
velocity of sound can be computed, and has been determined experi
mentally. The table shows also a comparison of the calculated and
observed velocities of sound. This table appeared originally, in French
measures, in the Philosophical Magazine for June, 1853: the metres are
here reduced to feet. K p , K v , and K w , are expressed in feet of fall per
Centigrade degree. K w (Joule's equivalent) = 1389'G
The real specific heat of carbonic acid gas is 235*5 feet of fall per
Centigrade degree. That of the other gases does not differ from the
apparent specific boat at constant volume by an amount appreciable in
practice.
ON THERMODYNAMICS.
339
XX.— ON THE GEOMETRICAL REPRESENTATION OF
THE EXPANSIVE ACTION OF HEAT, AND THE
THEORY OF THERMODYNAMIC ENGINES *
Section I. — Introduction and General Theorems.
1. The first application of a geometrical diagram to represent the expan
sive action of heat was made by James Watt, when he contrived the
wellknown steamengine indicator, subsequently altered and improved
by others in various ways. As the diagram described by Watt's Indicator
is the type of all diagrams representing the expansive action of heat, its
general nature is exhibited in Fig. 1.
Let abscissas, measured along, or par
allel to, the axis X represent the vol
umes successively assumed by a given
mass of an elastic substance, by whose
alternate expansion and contraction heat
is made to produce motive power ; V A
and V B being the least and greatest
volumes which the substance is made to
assume, and OV any intermediate vol
ume. For brevity's sake, these cmantities
will be denoted by V A , Y B , and V, respec
tively. Then V B — V A may represent the
space traversed by the piston of an engine
during a single stroke.
Let ordinates, measured parallel to the axis Y, and at right angles to
X, denote the expansive pressures successively exerted by the substance
at the volumes denoted by the abscissae. During the increase of volume
from V A to V B , the pressure, in order that motive power may be produced,
must be, on the whole, greater than during the diminution of volume from
V B to V A ; so that, for instance, the ordinates V P x and V P 2 , or the symbols
* Read before the Royal Society of London on January 19, 1834, and published in
the Philosophical Transactions for 1854.
340 ON THERMODYNAMICS.
P, and P , may represent the pressures corresponding to a given volume
V during the expansion and contraction of the substance respectively.
Then the area of the curvilinear figure, or Indicatordiagram, A P x B P 2 A,
will represent the motive power, or " potential energy," developed or given
out during a complete stroke, or cycle of changes of volume of the elastic
substance. The algebraical expression for this area is
P,) i V (l.)
The practical use of such diagrams, in ascertaining the power and the
mode of action of the steam in steamengines, where the curve A P t B P 2 A
is described by a pencil attached to a pressuregauge on a curd whose
motion corresponds with that of the piston, is sufficiently well known.
2. It appears that the earliest application of diagrams of energy (as they
may be called) to prove and illustrate the theoretical principles of the
mechanical action of heat, was made either by Carnot, or by M. Clapeyron
in his account of Carnot's theory ; but the conclusions of those authors
were in a great measure vitiated by the assumption of the substantiality
of heat.
In the fifth section of a paper on the Mechanical Action of Heat,
published in the Transactions of the Royal Society of Edinburgh (Seep.
300), a diagram of energy is employed to demonstrate the general law of
the economy of heat in thermodynamic engines according to the correct
principle of the action of such machines — viz., that the area of the diagram
represents at once the potential energy or motive power which is de
veloped at each stroke and the mechanical equivalent of the actual energy,
or heat, Avhich permanently disappears.
As the principles of the expansive action of heat are capable of being
presented to the mind more clearly by the aid of diagrams of energy than
by means of words and algebraical symbols alone, I purpose, in the present
paper, to apply those diagrams partly to the illustration and demonstration
of propositions already proved by other means, but chiefly to the solution
of new questions, especially those relating to the action of heat in all
classes of engines, whether worked by air, or by steam, or by any other
material ; so as to present, in a systematic form, those theoretical principles
which are applicable to all methods of transforming heat to motive power
by means of the changes of volume of an elastic substance.
Throughout the whole of this investigation, quantities of heat, and
coefficients of specific heat, are expressed, not by units of temperature in
a unit of weight of water, but by equivalent quantities of mechanical
power, stated in footpounds, according to the ratio established by Mr.
Joule's experiments on friction (Phil. Trans., 1850); that is to say,
ON THERMODYNAMICS.
341
772 footpounds per degree of Fahr., or
1389  6 footpounds per Centigrade degree,
applied to one pound of liquid water at atmospheric temperatures.
3. Of Isothermal Curves, and Curves of No Transmission of Heat.
A curve described on a diagram of energy, such that its ordinates
represent the pressures of a homogeneous substance corresponding to
various volumes, while the total sensible or actual heat present in the body
is maintained at a constant value, denoted, for example, by Q, may be
called the isothermal curve of Q for the given substance (See Fig. 2).
Suppose, for instance, that the coordinates of the point A, V A and P A ,
represent respectively a volume and a pressure of a given substance, at
which the actual heat is Q ; and the coordinates of the point B — viz., V B
and P B , another volume and pressure ' at which the actual heat is the
same ; then are the points A and B situated on the same isothermal
curve Q Q.
On the other hand, let the substance be allowed to expand from the
volume and pressure V A , P A ,
without receiving or emitting
heat; and when it reaches a
certain volume, V c , let the
pressure be represented by
P c , which is less than the
pressure would have been
had the actual heat been
maintained constant, because
by expansion heat is made
to disappear. Then C will
be a point on a certain curve
N N passing through A,
which may be called a Curve
of No Transmission.
It is to be understood, that "during the process last described, the
potential energy developed during the expansion, and which is represented
by the area A C V c V A , is entirely communicated to external substances ;
for if any part of it were expended in agitating the particles of the
expanding substance, a portion of heat would be reproduced by friction.
If o o o be a curve whose ordinates represent the pressures corresponding
to various volumes when the substance is absolutely destitute of heat, then
this curve, which may be called the Curve of Absolute Cold, is at once an
isothermal curve and a curve of no transmission.
So far as we yet know, the curve of absolute cold is, for all substances,
an asymptote to all the other isothermal curves and curves of no trans
Fig. 2.
342 ON THERMODYNAMICS.
mission, which approach it and each other indefinitely as the volume of
the substance increases without limit.
Note. — The following remarks are intended to render more clear the
precise meaning of the term Total Actual Heat.
The Total Actual Heat of a given mass of a given substance at a given
temperature, is the cpiantity of physical energy present in the mass in the
form of heat under the given circumstances.
If, for the purpose of illustrating this definition, we assume the hypo
thesis that heat consists in molecular revolutions of a particular kind,
then the Total Actual Heat of a mass is measured by the mechanical
power corresponding to the vis viva of those revolutions, and is repre
sented by
 2 . m v 2 ,
m being the mass of any circulating molecule, and i> 2 the mean square of
its velocity.
But the meaning of the term Total Actual Heat may also be illustrated
without the aid of any hypothesis.
For this purpose, let us take the ascertained fact of the production of
heat by the expenditure of mechanical power in friction, according to the
numerical proportion determined by Mr. Joule; and let E denote the
quantity of mechanical power which must be expended in friction, in
order to raise the temperature of unity of weight of a given substance
from that of absolute privation of heat to a given temperature r.
During this operation, let the several elements of the external surface
of the mass undergo changes of relative position expressed by the
variations of quantities denoted generally by^, and let the increase of each
such quantity as p be resisted by an externallyapplied force such as P.
Then, during the elevation of temperature from absolute cold to r, the
energy converted to the potential form in overcoming the external pressures
P will be
fvdp.
Also, let the internal particles of the mass undergo changes of relative
position, expressed by the variations of quantities denoted generally by r,
and let the increase of each such quantity as r be resisted by an internal
molecular force such as R :
Then the energy converted to the potential form in overcoming internal
molecular forces will be
2 . I E d r.
ON THERMODYNAMICS. 343
Subtracting these quantities of energy converted to the potential form
by means of external pressures and internal forces, from the whole power
converted into heat by friction in order to raise the temperature of the
mass from that of absolute privation of heat to the given temperature
t, we find the following result : —
Q = E2.[p<Zp2.(
Hdr
and this remainder is the quantity of energy which retains the form of heat
in unity of weight of the given substance at the given temperature ; that
is to say, the Total Actual Heat.
It is obvious that Total Actual Heat cannot be ascertained directly ;
first, because the temperature of total privation of heat is unattainable ;
and, secondly, because the molecular forces R are unknown.
It can, however, be determined indirectly from the latent heat of
expansion of the substance. For the heat which disappears during the
expansion of unity of weight of an elastic substance at constant actual
heat from the volume V A to the volume V B , under the constant or variable
pressure P, is expressed (as will be shown in the sequel) by
q /q/,> V;
so that from a sufficient number of experiments on the amount of heat
transformed to potential energy by the expansion of a given substance,
the relations, for that substance, between pressure, volume, and total
actual heat, may be determined.
4. Proposition I. — Theorem. The mechanical equivalent of the heat
absorbed or given out by a substance in passing from one given state as to
pressure and volume to another given state, through a series of states represented
by the coordinates of a given curve on a diagram of energy, is represented by
the area included between the given curve and two curves of no transmission of
heat drawn from its extremities, and indefinitely prolonged in the direction
representing increase of volume.
(Demonstration : see Fig. 3.) Let the coordinates of any two points,
A and B, represent respectively the volumes and pressures of the substance
in any two conditions ; and let a curve of any figure, A C B, represent,
by the coordinates of its points, an arbitrary succession of volumes and
pressures through which the substance is made to pass, in changing from
the condition A to the condition B. From the points A and B respectively
let two curves of no transmission AM, BN, extend indefinitely towards X;
then the area referred to in the enunciation is that contained between
344
ON THERMODYNAMICS.
the given arbitrary curve A C B and the two indefinitely prolonged curves
of no transmission; areas above the curve AM bciii^ considered as
representing heat absorbed by the substance, and those below heat
given out.
To fix the ideas, let us, in the first place, suppose the area MACBN
Re. 3.
to be situated above A M. After the substance has reached the state B,
let it be expanded according to the curve of no transmission BN, until
its volume and pressure are represented by the coordinates of the point D'.
.Next, let the volume V„ be maintained constant, while heat is abstracted,
until the pressure falls so as to be represented by the ordinate of the
point D, situated on the curve of no transmission A M. Finally, let the
substance be compressed, according to this curve of no transmission, until
it recovers its primitive condition A. Then the area AcBD'DA, which
represents the whole potential energy developed by the substance during
one cycle of operations, represents also the heat which disappears, that is.
the difference between the heat absorbed by the substance during the
change from A to 1!. and emitted during the change from D' to D; for
if this were not so, the cycle of operations would alter the amount of
energy in the universe, which is impossible.
The farther the ordinate V r ,PD' is removed in the direction of X. the
smaller does the heat emitted during the change from D' to D become;
and, consequently, the more nearly does the area ACBD'DA approximate
to the equivalent of the heat absorbed during the change from A to B;
to which, therefore, the area of the indefinitely prolonged diagram
M A C B N is exactly equal. Q. E. D.
It is easy to see how a similar demonstration could have been applied,
■mutatis mutandis, had the area lain below the curve AM. It is evident
also, that when this area lies, part above and part below the line A M.
the difference between these two parts represents the difference between
ON THERMODYNAMICS. 345
the heat absorbed and the heat emitted during different parts of the
operation.
5. First Corollary. — Theorem. The difference between the whole, heat
absorbed, and the whole expansive power developed, during the operation repre
sented by any curve, such as A C B, on a diagram of energy, depends on the
initial and final conditions of the substance alone, and not on the intermediate
process.
(Demonstration.) In Fig. 3, draw the ordinates AV A , BV B parallel to
Y. Then the area V A A C B V„ represents the expansive power
developed during the operation ACB; and it is evident that the difference
between this area and the indefinitelyprolonged area MAC B N, which
represents the heat received by the substance, depends simply on the
positions of the points A and B, which denote the initial and final
conditions of the substance as to volume and pressure, and not on the
form of the curve ACB, which represents the intermediate process. Q.E.D.
To express this result symbolically, it is to be considered, that the
excess of the heat or actual energy received by the .substance above the
expansive power or potential energy given out and exerted on external
bodies, in passing from the condition A to the condition B, is equal to
the whole energy stored "jp in the substance during this operation, which
consists of two parts, viz. —
Actual energy; being the increase of the actual or sensible heat of the
substance in passing from the condition A to the condition B, which is to
be represented by this expression,
A.Q = Q B Q A ;
Potential energy; 1 icing the power which is stored up in producing
changes of molecular arrangement during this process; and which it appears
from the theorem just proved, must be represented, like the actual energy,
by the difference between a function of the volume and pressure corre
sponding to A, and the analogous function of the volume and pressure
corresponding to B ; that is to say, by an expression of the form,
Let
AS = S B S A .
H A>B = areaMACBX
represent the heat received by the substance during the operation
A C B, and
P d V = area V A A C B V B
the power or potential energy given out.
346
ON THERMODYNAMICS.
Then, the theorem of this article is expressed as follows : —
,V„
H AlB  I
PrfV = Q B Q A + S B S A = AQ + A.S, (2.)
being a form of the general equation of the expansive action of heat, in
which the potential of molecular action, S, remains to be determined.
6. Second Corollary (see Fig. 4). — The Latent Heat of Expansion of
a substance from one given volume V A to another V B , for a given amount
of actual heat Q;
Fig. 4.
that is to say, the heat which must be absorbed by the substance in expand
ing from the volume V A to the volume V B , in order that the actual heat Q
may be maintained constant, is represented geometrically as follows:
Let Q Q be the isothermal curve of the given actual heat Q on the
diagram of energy; A, B two points on this curve, whose coordinates
represent the two given volumes and the corresponding pressures. Through
A and B draw the two curves of no transmission A M, B N, produced
indefinitely in the direction of X. Then the area contained between the
portion of isothermal curve AB, and the indefinitelyproduced curves AM,
B N, represents the mechanical equivalent of the latent heat sought, Avhose
symbolical expression is formed from equation (2) by making Q,. — Q A = 0,
and is as follows: —
H A , B (for Q = const.) = J ^ P tf V + S B  S A . . (3.)
Section II. — Propositions Belative to Homogeneous Substances.
7. Proposition II.— Theorem. In Fig. 5, let A x A 2 M, B x B 2 N he
any two curves of no transmission, indefinitely extended in the direction of X,
ON THERMODYNAMICS.
347
intersected in the points A v B v A 2 , B 2 , by two isothermal curves Q x A x B x Q x ,
Q A B Q , which are indefinitely near to each other; that is to say, which
correspond to two quantities of actual heat, Q x and Q 2 , differing by an indefinitely
small quantity Q x — Q 2 = SQ.
Then the elementary quadrilateral area, A 1 B x B 2 A 2 , bears to the whole
indefinitelyprolonged area MA 1 B 1 N, the same proportion which the indefi
nitely small difference of actual heat SQ bears to the ivhole actual heat Q : ; or
area A x B a B 2 A 2 S Q
area M A x B 1 N ~~ Q x '
(Demonstration.) Draw the ordinates A 1 V Al , A 2 V A2 , B^^, B 2 V B2 .
Suppose, in the first place, that 8 Q is an aliquot part of Q 1? obtained by
dividing the latter quantity by a very large integer n, which we are at
liberty to increase without limit.
The entire indefinitelyprolonged area M A 1 B 1 N represents a quantity
of heat which is converted into potential energy during the expansion of the
substance from V Al to V Bl , in consequence of the continued presence of the
total actual heat Q x ; for if no heat were present no such conversion would
take place. Mutatis mutandis, a similar statement may be made respecting
the area MA.,B,K By increasing without limit the number n and
diminishing 8 Q, we may make the expansion from V A2 to V B2 as nearly
as we please an identical phenomenon with the expansion from V Al to V Bl .
The quadrilateral A x B l B 2 A 2 represents the diminution of conversion of
heat to potential energy, which results from the abstraction of any one
whatsoever of the n small equal parts § Q into which the actual heat Q :
is supposed to be divided, and it therefore represents the effect, in
conversion of heat to potential energy, of the presence of any one
of those small portions of actual heat. And as all those portions
8 Q, are similar and similarly circumstanced, the effect of the presence
of the whole actual heat Q x in causing conversion of heat to potential
348 ON THERMODYNAMICS.
energy, will be simply the sum of the effects of all its small portions,
and will bear the same ratio to the effect of one of those small portions
which the whole actual heat bears to the small portion. Thus, by
virtue of the general law enunciated below and assumed as an axiom,
the theorem is proved when SQ is an aliquot part of Q l ; but SQ is
either an aliquot part, or a sum of aliquot parts, or may be indefinitely
approximated to by a series of aliquot parts; so that the theorem is
universally true. Q. E. D.
The symbolical expression of this theorem is as follows : when the
actual heat Q l5 at any given volume, is varied by the indefinitely small
quantity 8 Q, let the pressure vary by the indefinitely small quantity
(IP
yr B Q ; then the area of the quadrilateral A x ~B 1 B 2 A 2 will be rcpre
(l v^
sented by
v
V A, 1
and, consequently, that of the whole figure MAjBjN", or the latent heat
of expansion from V Al to V Bl , at Q u by
V
' "' clV
■V; . . . (4.)
^ A 1
a result identical with that expressed in the sixth section of a paper
published in the Transactions of th Royal Society of Edinburgh. (Seep. 310)
The demonstration of this theorem is an example of a special application
of the following
General Law of the Transformation of Energy.
The effect of the presence, in a substance, of a quantify of actual energy,
in causing transformation of energy, is the sum of the effects of all its parts: —
a law first enunciated in a paper read by me to the Philosophical Society
of Glasgow on the 5th of January, 1853. (See p. 203.)
8. General Equation of the Expansive Action of Heat.
The two expressions for the latent heat of expansion at constant
actual heat, given in equations (3) and (4) respectively, being equated,
furnish the means of determining the potential energy of molecular action
S, so far as it depends on volume, and thus of giving a definite form to
the general equation (2).
The two expressions referred to may be thus stated in words: —
ON THERMODYNAMICS. 349
I. The heat which disappears in producing a given expansion, while
the actual heat present in the substance is maintained constant, is equiva
lent to the sum of the potential energy given out in the form of expansive
power, and the potential energy stored up by means of molecular attractions.
II. It is also equivalent to the potential energy due to the .action during
d P
the expansion of a pressure Qttt, at each instant equal to what the
pressure would be, if its actual rate of variation with heat at the instant
in question were a constant coefficient, expressing the ratio of the whole
pressure to the whole actual heat present.
The combination of these principles, expressed symbolically, gives the
following result :
V V
H A , B (for Q = const.) = Q f ^d\ = f ' P dV + S B  8 A ;
* A ' A
whence Ave deduce the following general value for the potential of mole
cular action : —
S = /( Q ff P )<* V + * Q ' * • < 5 >
in which <£.Q denotes some function of the total actual heat not depending
on the density of the substance. This value being introduced into
equation (2), produces the following : —
H A ,J PiV = Q.Q 4 + S,S A
= Qb  Qa + <j> . Q B  <j> . Q A + / "(q ~ p)tf v = ¥ B  * A . (6.)
' A
The symbol ¥" = Q + S is used to denote the sum of the actual energy
of heat, and the potential energy of molecular action, present in the
substance in any given condition.
The above is the General Equation of the Expansive Action of
Heat in a Homogeneous Substance, and is the symbolical expression
of the Geometrical Theorems I. and II. combined.
When the variations of actual heat and of volume become indefinitely
small, this equation takes the following differential form : —
d.¥ = d.KFdV = dQ + d.S
= (l + '.Q + Q^/ P(J v)iQ + (QfP>T )
otherwise
y (<)
*.H£.*Q + Qg.iV.
350
ON THERMODYNAMICS.
The coefficient of d Q in the above expressions, viz. —
(8.)
is the ratio of the apparent specific heat of the substance at constant
volume to its real specific heat ; that is, the ratio of the whole heat
consumed in producing an indefinitely small increase of actual heat, to
the increase of actual heat produced.
These general equations are here deduced independently of any special
molecular hypothesis, as they also have been, by a method somewhat
different, in the sixth section of a paper previously referred to. Equations
equivalent to the above have also been deduced from the hypothesis
of molecular vortices, in the paper already mentioned, and in a paper on
the Centrifugal Theory of Elasticity. (See p. 4$.)
9. First Corollary from Proposition II. — Theorem. If a succession of
isothermal curves corresponding to quantities of heal diminishing by equal small
differences $ Q, be drawn across any pair of curves of no transmission, they ivill
cut off a series of equal small quadrilaterals.
Second Corollary. — Theorem. In Fig. G, let ADM,BCN be any two
curves of no transmission, indefinitely prolonged in the direction of X, and let
Fig. 6.
any two isothermal curves Q t Q 15 Q 2 Q 2 , corresponding respectively to any two
quantities of actual heat Q x , Q 2 , be drawn across them. Then will the
indefinitelyprolonged areas MABN, M DCN, bear to each other the simple
ratio of the quantities of actual heat, Q v Q 2 .
Or, denoting those areas respectively by ~H_ V H 2 —
H
Hi ~ Q/
 .... (9 ,
This corollary is the geometrical expression of the law of the maximum
ON THERMODYNAMICS. 351
efficiency of a perfect thermodynamic engine, already investigated by
other methods. In fact, the area MABN represents the whole heat
expended, or the latent heat of expansion, the actual heat at which heat
is received being Q t ; M D C N, the heat lost, or the latent heat of com
pression, which is carried off by conduction at the actual heat Q 2 ; and
ABCD (being the indicatordiagram of such an engine), the motive
power produced by the permanent disappearance of an equivalent quantity
of heat ; and the efficiency of the engine is expressed by the ratio of the
heat converted into motive power to the whole heat expended, viz. : —
ABCD HtHg, QtQg (w)
MABN H x Q x " ' K ''
1 0. Third Corollary (of Thermodynamic Functions).
If the two curves of no transmission in Fig. 6, ADM, BCN, be
indefinitely close together, the ratio of the heat consumed in passing from
one of those curves to the other to the actual heat present, will be the
same, whatever may be the form and position of the curve indicating the
mode of variation of pressure and volume, provided it intersects the two
curves of no transmission at a finite angle; because the area contained
between this connecting curve and the two indefinitelyprolonged curves
of no transmission will differ from an area whose upper boundary is an
isothermal curve, by an indefinitely small area of the second order.
To express this symbolically, let
f = 8F,
be the ratio in question, for a given indefinitelyclose pair of curves of no
transmission. Let the change from one of these curves to the other be
made by means of any indefinitelysmall changes of actual heat and of
volume, SQ, SV. Then by the general equation (7), the following
quantity —
+ m SY = m^ + '^ v ' ' ' (11)
is constant for a given pair of indefinitelyclose curves of no transmission,
and is, therefore, the complete variation of a function, having a peculiar
constant value for each curve of no transmission, represented by the
following equation : —
352
ON THERMODYNAMICS.
This function, which I shall call a thermodynamic function, has the
following properties : —
H=(QdF, .... (13.)
is equivalent to the general equation (6) ;
<1F = 0, . . . . (14.)
is the equation common to all curves of no transmission; and
F = a given constant, . . . (14A.)
is the equation of a particular curve of no transmission.
11. Proposition 111. — Problem. Let it be supposed find for a given
substance, the forms of all pc \ermal cm . hut of only one
curve of no tra ; it is required /■ determination of
points, another curve of no transmi sing through a given point, situated
anywhere out of the known curve.
(Solution : see Fig. 7.) Let L M be the known curve of no trans
mission ; B the given point. Through B draw an isothermal curve
Q 1 ABQ 1 , cutting LM in A. o, being the quantity of heat to which this
curve corresponds, draw, indefinitely near to it, the isothermal curve q l q v
corresponding to the quantity of heat Q, — 2 Q, where c Q is an indefinitely
small quantity. Draw any other pair of indefinitely close isothermal
Kg. 7.
curves Q„ Q 2 , q 2 q 2 , corresponding to the quantities of heat Q 2 Q 2 — $ Q; 2 Q
being the same as before. Let D be the point where the isothermal curve
Q 2 Q 2 cuts the known curve of no transmission. Draw the ordinates
A V A , B V B parallel to Y, enclosing, with the isothermal curves of Q x
and Qj — S Q, the small quadrilateral A B b a. Draw the ordinate D V D
ON THERMODYNAMICS. 353
parallel to Y, intersecting the isothermal curve of Q 2 — § Q in d.
Lastly, draw the ordinate C V c in such a position as to cut off from the
space between the isothermal curves of Q 2 and Q 2 — 8 Q a quadrilateral
D C c d, of area equal to the quadrilateral ABba.
Then will C, where the last ordinate intersects the isothermal curve of
Q 2 , approximate indefinitely to the position of a point in the curve of no
transmission passing through the given point B, when the variation of
actual heat S Q is diminished without limit. And thus may be determined
to as close an approximation as we please, any number of points in the
curve of no transmission NBE which passes through any given point B,
when any one curve of no transmission L M is known.
(Demonstration.) For when the variation § Q diminishes indefinitely,
the curves q x q v q. 2 q 2 , approach indefinitely towards the curves Q x Q 15 Q 2 Q 2
respectively; and the small quadrilaterals bounded endways by the
ordinates approximate indefinitely to the small quadrilaterals bounded
endways by the curves of no transmission ; which latter pair of quadri
laterals are equal, by the first corollary of Proposition II.
The symbolical expression of this proposition is as follows : —
Let V A , V B , V c , V D , be the volumes corresponding to the four points of
intersection of a pair of isothermal curves with a pair of curves of no
transmission; A and B being on the isothermal curve of Q p C and D on
that of Q 2 , A and D on one of the curves of no transmission, B and C on
the other ; then
C § d v (for Q = Ql) = C* 8 '' v (for Q = Qi> ' 1
' a v i> y (is.)
or
F — F = F — F
J B L A 1 C I D
The second form of this equation is in the present case identical, because
F = F • F = F
12. Proposition IV. — Problem (see Fig. 8). The forms of all
isothermal curves for a given substance being given, let E F be a curve of any
form, representing an arbitrarily assumed succession of pressures and volumes.
It is required to find, by the determination of points, a corresponding curve
passing through a given point B, such that the quantity of heat absorbed or
emitted by the substance, in passing from any given isothermal curve to any other,
shall be the same, ichether the pressures and volumes be regulated according to
the original curve E F, or according to the curve passing through the point B.
(Solution.) The process by which the latter curve is to be deduced
z
354 ON THERMODYNAMICS.
from the former is precisely the same with that by Avhich one curve of no
transmission is deduced from another in the last problem.
«*\
<Di^
\"~~"~~~><1L
S ^~ H a
p
— ^£__ ^^\
^c
0.
H ^^^
**" ~^5jm
^Kl
L
Fig. 8.
(Demonstration.) Let GBH be the required curve. This curve, and
the curve EF, in their relation to each other, may be called Curves of
Equal Transmission. Through B draw the isothermal curve Q x Q 19 inter
secting the curve E F in A. Draw also any other isothermal curve Q 2 Q 2 ,
intersecting E F in D and G H in C. Through A, B, C, D, respectively,
draw the four indefinitelyprolonged curves of no transmission, AK,
intersecting Q 2 Q 2 in d, B L, intersecting Q 2 Q 2 in c, C M, and D N.
Conceive the whole space between the isothermal curves Q, Q t , Q 2 Q 2 , to
be divided by other isothermal curves, into a series of indefinitelynarrow
stripes, corresponding to equal indefinitelysmall variations of actual heat.
Then, by the construction of the solution, the quadrilaterals cut from those
stripes by the pair of curves E F, G H are all equal ; and so also are the
quadrilaterals cut from the stripes by the pair of curves of no transmission,
A K, B L. Therefore the area A B C D is equal to the area A B c d. The
indefinitelyprolonged areas, M C D N, L c d K, are evidently equal ;
therefore, adding this pair of equal areas to the preceding, the pair of
indefinitelyprolonged areas LEAK. MCBADN are equal. Subtracting
from each of these areas the part common to both, ABR, and adding to
each the indefinitely prolonged area K K C M, we find, finally, that the
indefinitelyprolonged areas KADN, LBCM are equal.
But the former of those areas (by Prop. I.) represents the mechanical
equivalent of the heat absorbed by the substance in passing from the
actual heat Q 2 to the actual heat Q x through a series of pressures and
volumes represented by the coordinates of the curve E F ; and the latter,
the corresponding quantity for the curve G H ; therefore those curves
are, with respect to each other, curves of equal transmission, which was
required.
The algebraical expression of this result is that the equation (15) holds
ON THERMODYNAMICS.
>oa
for any pair of curves of equal transmission, as well as for a pair of curves
of no transmission ; or, in other terms, let F A , F B , F c , F D be the thermo
dynamic functions for the curves of no transmission passing through the
four points where a pair of isothermal curves cut a pair of curves of equal
transmission : A, B being on the upper isothermal curve ; C, D on the
lower; A, D on one curve of equal transmission, B, C on the other: then,
F. = K  F r
(16.)
13. Proposition V. — Theorem. The difference between the quantities
of heat absorbed by a substance in passing from one given amount of actual heat
to another, at two different constant volumes, is equal to the difference between
the two latent heats of expansion in passing from one of those volumes to the
other at the two different amounts of actual heat respectively, diminished by the
corresponding difference between the quantities of expansive power given out.
(Demonstration: see Fig. 9.) Let Q t Q a be the isothermal curve of the
higher amount of actual heat; Q 2 Q 2 that of the lower. Let V A , V B be
Fig. 9.
the two given volumes. Draw the two ordinates V A a A, V B b B, and the
four indefinitelyprolonged curves of no transmission A M, a m, B 1ST, b n.
the quantities of heat absorbed, in passing from the actual heat Q 2 to the
actual heat Q v at the volumes V A and V B , are represented respectively
by the indefinitelyprolonged areas MAam, NBbn. Then adding to
each of those areas the indefinitelyprolonged area n iBAM (observing
that the space below the intersection R is to be treated as negative), we
find for their difference
NBJn MAam = NBAM ubBAam =(NBAM nbam)
(V B BAV A V B 5«V A );
but NBAM and n b a m represent the latent heats of expansion from
V A to V B , at the actual heats Q x and Q 2 respectively ; and V B B A V A and
856
ON THERMODYNAMICS.
V B b a V A represent the power given out by expansion from V A to V,, at
the actual heats Q x and Q 2 respectively : therefore, the proposition is
proved. Q. E. D.
This proposition, expressed symbolically, is as follows : A Q being the
difference of actual heat, Q x — Q 2 , let A (Q + S A ) be the heat absorbed
in passing from Q 2 to Q t at the volume V A , and A (Q + S„) the corre
sponding quantity ;tt the volume V B ; A S A and A S B representing quantities
of potential energy stored up in altering molecular arrangement. Then
»H«i(^l)/'MT. . (17.)
A
14. Of Curves of Free Expansion.
In all the preceding propositions, the whole motive power developed
by an elastic substance in expanding is supposed to be communicated
to external bodies; to a piston, for example, which the substance causes
to move and to overcome the resistance of a machine.
Let us now suppose that as much as possible of the motive power
developed by the expansion is expended in agitating the particles of the
expanding substance itself, by whose mutual friction it is finally recon
verted into heat (as when compressed air escapes freely from a small
orifice); and let us examine the properties of the curves which, on a
diagram of energy, represent the law of expansion of the substance under
these circumstances, and which may be called Curves of free Expansion.
15. Proposition VI. — Theorem. If from two points on a curve of free
expansion there be drawn tiro straight lines perpendicular to and terminating
at the axis of ordinates, and also tv:o curves of no transmission, indefinitely
prolonged away from the origin of coordinates ; then the area contained between
the curve of free expansion, the two straight
lines, and the axis of ordinates, will be
erpial to the area contained between the
curve of free expansion and the two
indefinitelyprolonged curves of no trans
mission.
(Demonstration.) Let FF (Fig. 10)
be a curve of free expansion ; G H
any two points in it ; G V G , H V H
ordinates ; G P G , H P n lines per
pendicular to O Y ; G M, HN
■* curves of no transmission, indefinitely
Fig. 10. prolonged in the direction of X.
Then the indefinitelyprolonged area MGHN represents the heat which
would have to be communicated to the substance, if the motive power
ON THERMODYNAMICS.
357
developed were entirely transferred to external bodies, while the area
Y,. G H V H represents that motive power. The excess of the rectangular
area P H H V H above the area P G G V G 0, is the power necessarily given
out by the elastic fluid in passing from a vessel in which the pressure is
P G and volume V G , to a vessel in which the pressure is P H and volume V H .
The remainder of the expansive power, represented by the area P G H P H ,
by the mutual friction of the particles of the expanding substance, is
entirely reconverted into heat, and is exactly sufficient (by the definition
of the curve of free expansion) to render the communication of heat to
the substance unnecessary; from which it follows, that this area is equal
totheareaMGHX. Q. E. D.
The equation of a curve of free expansion is
d (¥ + P V) = 0.
(17 a.)
16. Corollary. — In Fig. 11, the same letters being retained as in the
last figure, through G draw an isothermal curve Q L Q x , which the line P H H
produced cuts in h;
and from h draw the indefinitelyprolonged curve of no transmission, hn.
Then because, by the proposition just proved, the areas P G G H P H and
M G H N are equal, it follows that the indefinitelyprolonged area, MGhn,
which represents the latent heat of expansion at the constant actual
heat Q 1? from the volume V G to the volume Y h , exceeds P G G h P H , by the
indefinitelyprolonged area NH/j n, which represents the heat which the
substance would give out, in falling, at the pressure P H , from the actual
heat Q t to the actual heat corresponding to the point H on the curve of
free expansion passing through G. Subtracting from this area the excess
of the rectangle P H Y h above the rectangle P G V G , we obtain the excess of
the area M G h n above the area V G G h Y h .
This conclusion may be thus expressed : — Let Q 2 be the actual heat for
358
ON THERMODYNAMICS.
K,
the point H ; ~ the ratio of specific heat at the constant pressure P H to
real specific heat ; then
/
Qa
■rfQP H V A + PaV fl
(^ 1 )L ApdV ( f0rQ = Ql); r • (18.)
otherwise : —
Qi r P o
Equation (18) may he used, either to find points in the curve of free
expansion which passes through G, when the isothermal curves and the
curves of no transmission are known; or to deduce theoretical results
from experiments on the form of curves of free expansion, such as those
which have been for some time carried on by Mr. Joule and Professor
"William Thomson.
Considered geometrically, these experiments give values of the area
XH/ni, The area
,P <;
P fl G/»P H = VdT
' I\,
is known in each case from previous experiments on the properties of
the gas employed ; and this area, by Proposition VI., is equal to the area
MG/iHN; to which, adding the area N H h n, ascertained by experiment,
we obtain the area MG/j n, that is, the latent heat of expansion from the
volume V to the volume Y h , at the constant actual heat Q 1} denoted
symbolically by
* G
Now the problem to be solved is of this kind. "We know the differences
of actual heat corresponding to a certain series of isothermal curves for
the substance employed ; and we have to ascertain the absolute quantities
of actual heat corresponding to those curves. Of the above expression
for the area M G h n, therefore, the factor Q t is to be determined, while
the other factor, being the difference between two thermodynamic
ON THERMODYNAMICS.
359
functions, is known ; and the experiments of Messrs. Thomson and Joule,
by giving the value of the product, enable us to calculate that of the
unknown factor, and thence to determine the point on the thermometric
scale corresponding to absolute privation of heat.
1 7. Proposition VII. — Problem. To determine the ratio of the apparent
specific heats of a substance at constant volume and at constant pressure, for a
given pressure and volume; the isothermal curves and the curves of no
transmission being known.
(Solution.) In Fig. 12, let A be the point whose coordinates represent
the given volume V A and pressure P A ; Q A Q the isothermal curve passing
Fis. 12.
through A; qq another isothermal curve, very near to Q Q. Through A
draw the ordinate V A A a parallel to Y, cutting qq in a; draw also A B
parallel to X, cutting q q in B. From A, a, B, draw the three indefi
nitelyprolonged curves of no transmission A M, a in, B N.
Then the heat absorbed in passing from the actual heat Q to the actual
heat q, at the constant volume V A , is represented by the indefinitely
prolonged area MAaro, while at the constant pressure P A it is represented
by the area MABN. Let the curve qq be supposed to approximate
indefinitely to QQ. Then will the threesided area A«B diminish
indefinitely as compared with the areas between the curves of no trans
mission A M, a m, B N ; and, consequently, the area MABN will approxi
mate indefinitely to the sum of the areas MA am and m«BN; the
ultimate ratio of which sum to the area M A a m is therefore the required
ratio of the specific heats. Now in a B N, as qq approaches Q Q, approxi
mates indefinitely to the latent heat of the small expansion V B — V A at
the actual heat Q, and this small expansion bears ultimately to the
increment of pressure P a — P A , the ratio of the subtangent of the isothermal
curve Q Q to its ordinate at the point A.
The symbolical expression of this proposition is as follows : — Let o Q
denote the indefinitelysmall difference of actual heat between the
360
ON THERMODYNAMICS.
isothermal curves Q Q, qq; B V the indefinitelysmall variation of volume
V B — V A ; B P the indefinitely small variation of pressure P a — P A ;
the quantities of heat required to produce the variation BQ, at the
constant volume V A , and at the constant pressure P A respectively.
Then
d?
BY =
SP
d?
dV
dQ
d P
" dY
■ BQ;
and
^.8Q=^.8Q + Qi£.SV
consequently.
dQ
,/p
r~ (19.)
_ ' ; ?
J
equations agreeing with equation (31) of a paper on the Centrifugal Theory
of Elasticity before referred to.
18. First Corollary. — As the curves AM, am, BN approximate indefi
nitely towards parallelism, and the point a towards C, where am intersects
A B, the ratio of the areas MABN: MAaih, approximates indefinitely
to that of the lines AB:AC, which are ultimately proportional, respectively,
to the subtangents of the isothermal curve and the curve of no transmission
passing through A. Therefore,
Kv
Subtangent of Isothermal Curve
Subtangent of Curve of No Transmission"
(20.)
19. Second Corollary.— Velocity of sound. The subtangents of different
curves at a given point on a diagram of energy being inversely proportional
to the increase of pressure produced by a given diminution of volume
according to the respective curves, are inversely proportional to the squares
of the respective velocities with which waves of condensation and rare
faction will travel when the relations of pressure to volume are expressed
by the different curves. Therefore, if there be no sensible transmission
of heat between the particles of a fluid during the passage of sound, the
ON THERMODYNAMICS.
3G1
square of the velocity of sound must be greater than it would have been
had the transmission of heat been instantaneous in the ratio of the
subtangent of an isothermal curve to that of a curve of no transmission
at the same point, or of the specific heat at constant pressure to the
specific heat at constant volume.
This is a geometrical proof of Laplace's law for all possible fluids. The
same law is deduced from the hypothesis of molecular vortices in the
paper before referred to on the Centrifugal Theory of Elasticity.
20. Proposition VIII. — Problem. The isothermal curves for a given
substance being knoum, and tin quantities of heat required to produce all variations
of actual heat at a given constant volume; it is required to find any number of
points in a curve of no transmission passing through a given point in the ordinate
corresponding to that volume.
(Solution.) In Fig. 13, let V A A X be the given ordinate; Q 1 Q 1 , A 2 Q,
isothermal curves meeting it in A lf A 2 , respectively; and let it be required.
Fig. 13.
for example, to find the point where the curve of no transmission passing
through A 1 intersects the isothermal curve A 2 Q 2 . On the line V A A 2 A 1?
as an axis of abscissa?, describe a curve C C, whose ordinates (such as
A 2 C 2 , a A c A , &c.) are proportional to the specific heat of the substance at
the constant volume V A , and at the degrees of actual heat corresponding
to the points where they are erected, divided by the corresponding rate
of increase of pressure with actual heat ; so that the area of this curve
between any two ordinates (e.g., the area a 4 c 4 c 3 a 3 ) may represent the
mechanical equivalent of the heat absorbed in augmenting the actual
362 ON THERMODYNAMICS.
heat from the amount corresponding to the lower ordinate to that corre
sponding to the higher (e.g., from the amount corresponding to « 4 to that
corresponding to a' 3 ).
Very near to the isothermal curve A 2 Q 2 , draw another isothermal curve
a 2 #2, and let the difference of actual heat corresponding to the interval
between these curves be 3 Q. Draw a curve D D, such that the part cut
off by it from each ordinate of the curve C C shall bear the same proportion
to the whole ordinate which the difference 8 Q bears to the whole actual
heat corresponding to the ordinate ; for example, let
A^q : A7D x : : Qi = 8 Q
ATO; : A^TI, : : Q 2 : 8 Q, &C.
Then draw an ordinate V B B b, parallel to Y, cutting off from the
space between the isothermal curves A, Q 2 , a 2 q 2 , a quadrilateral area
A 2 B b a 2 equal to A { D 4 D 2 A 2 , the area of the curve D D between the
ordinates at A, and A..
Then, if the difference 8 Q be indefinitely diminished, the point B will
approximate indefinitely to the intersection required of the isothermal
curve A 2 Q 2 with the curve of no transmission passing through A 4 ; and
thus may any number of points in this curve of no transmission be found.
(Demonstration.) Let A! M] be the curve of no transmission required.
Let « 3 c 3 , « 4 c 4 be any two indefinitelyclose ordinates of the curve C C,
corresponding to the mean actual heat Q 3 4 . Let a 3 m 3 , a 4 w? 4 be curves of
no transmission, cutting the curves a., q 2 , A, Q 2 , so as to enclose a small
quadrilateral area e. Then, by the construction, and Proposition I.,
The area a 3 c 3 c 4 « 4 = the indefinitelyprolonged area m 3 a 3 « 4 m A ;
and by the first corollary of the second proposition and the construction,
the area e 8 Q area a 3 d 3 d 4 a 4
m 3 a 3 a 4 m i Q 3 4 area a 3 c s c 4 a^
Therefore, the area e = the area a 3 d 3 </ 4 a 4 ; but the area A x D x D 2 A 2 is
entirely made up of such areas as a 3 d 3 d i a 4) to each of which there corre
sponds an equal area such as e; and when the difference 3 Q is indefinitely
diminished, the area A 2 B b a 2 approximates indefinitely to the sum of all
the areas such as e, that is, to equality with the area AjDjD^o. Q.E.D.
The symbolical expression for this proposition is found as follows : —
rQi K
The area A x D x D 2 A<, ultimately = 8 Q . / ^ . d Q (for V = Y A ) ;
J q KQ,
ON THERMODYNAMICS. 363
V
The area A 2 B b a. 2 ultimately = % Q . I TfidV (f ov Q = Q2) ;
divide both sums by S Q and equate the results ; then,
r*B f ;p A? 1
I" % " v (for Q  Qi> " L h " Q (for v  v  ) ' (21)
which denotes the equality of two expressions for the difference, F x — F a
between the thermodynamic functions for the curve of no transmission
A x M, and for that passing through the point A 2 .
When the relations between pressure, volume, and heat, for a given
substance, are known, the equation (21) may be transformed into one
giving the volume V B corresponding to the point at which the required
curve of no transmission cuts the isothermal curve of Q..
Suppose, for instance, that for a perfect gas
P V = N Q sensibly ; and \ Y = 1 sensibly; . (22.)
N being a constant (whose value for simple gases and for atmospheric air
and carbonic oxide is about  41); then the thermodynamic function for
a perfect gas is sensibly
F = hyp. log. Q + N hyp. log. V; . . (22a.)
and equation (21) gives, for the equation of a curve of no transmission,
X, = a>". . . . (23.)
whence
P„
P
! = (r)"" 1_Sk  • • (24)
Equations (23) and (24) are forms of the equation of a curve of no
transmission for a perfect gas, according to the supposition of Mayer ;
and are approximately true for a perfect or nearly perfect gas on any
supposition.
According to the hypothesis of molecular vortices, the relations between
pressure, volume, and actual heat, for a perfect gas, are expressed by these
equations : —
PV = NQ + / i; I =l + ^^ ; . (25.)
T)G4 ON THERMODYNAMICS.
where h is a very small constant, which is inversely proportional to the
specific gravity of the gas, and whose value, in the notation of papers on
the hypothesis in question, is
h = Nlr, . . . . (25A.)
k being the height, on the scale of a perfect gasthermometer, of the point
of absolute cold above the absolute zero of gaseous tension. Hence we
find, for the thermodynamic function of a perfect gas,
F = hyp. log. Q  y^r h + N hyp. log. V, . (26.)
and for the equation of a curve of no transmission,
1 \ h h )
 B = (9AN . e t » Q* .+ h N Q x + A I _ /07)
V . \\j.tJ
For all practical purposes yet known, these equations may be treated as
sensibly agreeing with equation (23), owing to the smallncss of h as
compared with N Q,
Section III. — Of the Efficiency of Thermodynamic Engines,
Worked by the Expansion and Condensation of
Permanent Gases.
21. The efficiency of a thermodynamic engine is the proportion of
the whole heat expended which is converted into motive power ; that is to
say, the ratio of the motive power developed to the mechanical equivalent
of the whole heat consumed.
To determine geometrically the efficiency of a thermodynamic engine,
it is necessary to know its true indicatordiagram ; that is to say, the
curve whose coordinates represent the successive volumes and pressures
which the elastic substance working the engine assumes during a complete
revolution. This true indicatordiagram is not necessarily identical in
figure with the diagram described by the engine on the indicatorcard ;
for the abscissae representing volumes in the latter diagram include not
only the volumes assumed by that portion of the elastic substance which
really performs the work by alternately receiving heat while expanding,
and emitting heat while contracting, in such a manner as permanently to
transform heat into motive power, but also the volumes assumed by that
portion of the elastic substance, if any, which acts merely as a cushion for
transmitting pressure to the piston, undergoing, during each revolution, a
ON THERMODYNAMICS.
3G5
scries of changes of pressure and volume, and then the same series in an
order exactly the reverse of the former order, so as to transform no heat
permanently to power.
The thermodynamic engines to be considered in the present section,
are those in which the elastic substance undergoes no change of condition.
We shall, in the first place, investigate the efficiency of those which work
without the aid of the contrivance called an ; ' economiser " or " regenerator,"
and afterwards, those which work with the aid of that piece of apparatus.
22. Lemma. — Problem. To determine the true from the apparent
indicatordiagram of a thermodynamic engine ; the portion of the elastic
substance which acts as a cushion being known, and the law of its changes of
pressure and volume.
X
Fig. 14
(Solution.) In Fig. 14, let abed be the apparent indicatordiagram.
Parallel to X draw H a and L c, touching this diagram in a and c
respectively ; then those lines will be the lines of maximum and minimum
pressure. Let H E and L G be the volumes occupied by the cushion at the
maximum and minimum pressures respectively : draw the curve E G,
such that its coordinates shall represent the changes of volume and
pressure undergone by the cushion during a revolution of the engine. Let
K F d b be any line of equal pressure, intersecting this curve and the
apparent indicatordiagram ; so that K b, K d shall represent the two
volumes assumed by the whole elastic body at the pressure K, and K F
the volume of the cushion at the same pressure. On this line take
JB = Iff) = ~KF;
then it is evident that B and D will be two points in the true indicator
diagram ; and in the same manner may any number of points be found.
The area of the true diagram ABCD is obviously equal to that of the
apparent diagram abed.
3G6 ON THERMODYNAMICS.
23. Proposition IX. — Problem. The true Indicatordiagram of a
thermodynamic engine worked by the expansion and contraction of a substance
which does not change its condition, and without a regenerator, being given, it is
required to determine the efficiency of the engine.
(Solution.) In Fig. 15, let A a a Bb' b A be the given true indicator
diagram. Draw two curves of no transmission, A M, B N, touching this
figure at A and B respec
tively, and indefinitely pro
duced towards X. Then
during the process denoted
by the portion Aaa'B of
the diagram the elastic sub
stance is receiving heat, and
the mechanical equivalent of
the total quantity received
r is represented by the in
Firr. 15. definitely  prolonged area
MAaa'BN; during the
process denoted by the portion Bb'bA of the diagram, the substance is
giving out heat, and the mechanical equivalent of the total heat given out
is represented by the indefinitelyprolonged area M A b b' B N ; while the
difference between those areas, that is, the area of the indicatordiagram
itself, represents at once the heat which permanently disappears and the
motive power given out. The EFFICIENCY of the engine is the ratio of
this last quantity to the total heat received by the elastic substance during
a revolution ; that is to say, it is denoted by the fraction,
area A a a' B b' b A
area M A a «' B N'
To express this result symbolically, find the limiting points A and B by
combining the equation of the indicatordiagram Avith the general equation
of curves of no transmission, viz. : —
d F = 0.
Then draw two indefinitelyclose and indefinitelyprolonged curves of no
transmission, a b m, a V m, through any part of the diagram, cutting out of
it a quadrilateral stripe, a b V a. Let Q x be the mean actual heat corre
sponding to the upper end ad of this quadrilateral stripe; Q 2 , that
corresponding to the lower end, b V.
The area of this indefinitelynarrow stripe, representing a portion of
the heat converted into motive power, is found, according to the principles
and notation of the third corollary to Proposition II. and of Proposition
ON THERMODYNAMICS.
3G7
III., by multiplying the difference between the actual heats by the
difference between the thermodynamic functions for the curves of no
transmission that bound the stripe, thus : —
SE = (Q 1 Q 2 )SF:
while the area of the indefinitelyprolonged stripe, mactfm', representing
part of the total heat expended, is, according to the same principles,
and that of the indefinitelyprolonged stripe m b V m', representing part
of the heat given out, is
o H 2 = Q 2 o F.
Integrating these expressions we find the following results : —
Whole heat expended,
F B
H 1 = f B Q^F;
Heat given out.
Motive power given out,
F,
H„ = ( B Q,dF;
F
E = H 1 H 2 = f "(QiQ^ZF;
(28.)
Efficiency,
J V
QtdF
J
formulae agreeing with equation (28) of a paper on the Centrifugal Theory
of Elasticity (Seep. 63); it being observed that the symbol F in the last
mentioned paper denotes, not precisely the same quantity which is denoted
by it in this paper, and called a thermodynamic function, but the pro
duct of the part of that function which depends on the volume by the
real specific heat of the substance.
24. First Corollary. Maximum efficiency between given limits of actual heat.
When the lushest and lowest limits of actual heat at which the engine
3GS
ON THERMODYNAMICS.
can work are fixed, it is evident that the greatest possible efficiency of an
engine without a regenerator will be attained when the whole reception of
heat takes place at the highest limit, and the whole emission at the lowest;
so that the true indicatordiagram is such a quadrilateral as is shown in
Fig. 6, and referred to in the second corollary of Proposition II.; bounded
above and below by the isothermal curves denoting the limits of actual
heat, and. laterally, by any pair of curves of no transmission. The
•efficiency in this case, as has been already proved in various ways, is
represented by
E
(29.)
being the maximum efficiency possible between the limits of actual heat,
Q x and Q,.
25. Second Corollary. — Problem. To drew the diagram of greatest
efficiency of a thermodynamic engine without a regenerator, when the extent of
variation of volume is limited, as well as that of the ra notion of actual heat.
(Solution.) In Fig. 16, let
Qi ( i i • ( I ■• Q2 ^ e the isothermal
curves denoting the limits of
actual heat; Y A , Y„ the limits
of volume. Draw the ordinates
\\ I > A. V B C B, intersecting the
isothermal curves in the points
A.IU',D. Through A and C
respectively draw the curves of
no transmission, A M cutting
Q2Q2 in d, and ( ' \ cutting C^Q,
in b. Then will AbCd be the
diagram required An anal
ogous construction would give the diagram of greatest efficiency when the
variations of pressure and of actual heat are limited ; as in the airengine
proposed by Mr. Joule.
26. Of the use of the Economizer or Regenerator in Thermodynamic Engines.
As the actual heat of the elastic substance which works a thermo
dynamic engine requires to be alternately raised and lowered, it is obvious
that unless these operations are performed entirely by compression and
expansion, without reception or emission of heat (as in the case of
maximum efficiency described in the first corollary of Proposition IX.),
part, at least, of the heat emitted during the lowering of the actual heat
may be stored up, by being communicated to some solid conducting
substance, and used again by being communicated back to the elastic
substance, when its actual heat is being raised. The apparatus used for
ON THERMODYNAMICS. 3G9
this purpose is called an economiser or regenerator, and was first invented
about 181 G, by the Rev. Robert Stirling. In the airengine proposed
by him, it consisted of a sheetmetal plunger surrounded by a wire
grating or network; in that of Mr. James Stirling, it is composed of thin
parallel plates of metal or glass through which the air passes longitudinally,
and in the engine of Captain Ericsson of several sheets of wire gauze.
A regenerator may be regarded as consisting of an indefinite number
of strata with which the elastic substance is successively brought into
contact; each stratum serving to store up and give out the heat required
to produce one particular indefinitelysmall variation of the actual heat
of the working substance.
A perfect regenerator is an ideal apparatus of this kind, in which the
mass of material is so large, the surface exposed so extensive, and the
conducting powers so great as to enable it to receive and emit heat
instantaneously without there being any sensible difference of temperature
between any part of the regenerator and the contiguous portion of the
working substance; and from which no appreciable amount of heat is lost
by conduction or radiation. In theoretical investigations it is convenient,
in the first place, to determine the saving of heat effected by a perfect
regenerator, and afterwards to make allowance for the losses arising from
the nonfulfiment of the conditions of ideally perfect action; losses which,
in the present imperfect state of our knowledge of the laws of the
conduction of heat, can be ascertained by direct experiment only.*
27. Proposition X. — Problem. The true indicatordiagram of any
thermodynamic engine being given, to determine the amount of heat saved by a
perfect regenerator.
(Solution.) Let ABCD (in Fig. 1 7) be the given indicatordiagram.
Across it draw any two indefinitelyclose isothermal curves, q 1 q x inter
secting it in a, b, and q 2 q 2 intersecting it in d, c To the stripe between
those two curves, speaking generally, a certain layer or stratum of the
regenerator corresponds, which receives heat from the working substance
during the change from b to c, and restores the same amount of heat
during the change from d to a. The amount of heat economised by the
layer in question is thus found. Through the four points a, b, c, d, draw
the indefinitelyprolonged curves of no transmission, ah, bl, cm, dn; then
the smaller of the two indefinitelyprolonged areas, lb cm, Icadn, represents
the heat saved by the layer of the regenerator corresponding to the
indefinitelynarrow stripe between the isothermal curves q x q 1 and q 2 q 2 .
* It is true that the problem of the waste of heat in the action of the regenerator
is capable of a hypothetical solution by the methods of Fourier and Poisson; and I
have by these methods obtained formulae which are curious in a mathematical point of
view; but owing to our ignorance of the absolute values and laws of variation of the
coefficients of conductivity contained in these formulae, they are incapable of being
usefully applied ; and I therefore for the present refrain from stating them.
2 A
370
ON THERMODYNAMICS.
Draw two curves of no transmission, BL, DN, touching the diagram;
and through the points of contact, B and I), draw the isothermal curves,
X
Fig. 17.
Qj Q, cutting the diagram in A and B, and Q 2 Q 2 cutting it in C and D.
Then because, during the whole of the change from D through A to B,
the Avorking substance is receiving heat, and during the whole of the
change from B through C to D, emitting heat, the regenerator can have
no action above the isothermal curve QjQp nor below the isothermal
curve Q 2 Q .
The whole of the diagram between these curves is to be divided by
indefinitelyclose isothermal curves into stripes like abed; and the saving
of heat effected by the layer of the regenerator corresponding to each
stripe ascertained in the manner described, when the whole saving may
be found by summation or integration.
The symbolical expression of this result is as follows : Let the points
of contact, B, D, which limit the action of the regenerator, and the
corresponding quantities of actual heat, Q 15 Q.„ be found, as in Proposition
IX., by means of the equation dF = 0.
Then, the saving of heat
■Qi
Q,
= L q w: dq = L It + %iqJ^ (3(X)
Qs
Q 2
.7 T?
care being taken, when 'y nas different values for the same value of Q,
ON THERMODYNAMICS.
371
corresponding respectively to the two sides of the diagram, to choose the
smaller in performing the integration.
28. Corollary. — It is evident that the regenerator acts most effectually
when the outlines of the indicatordiagram from A to D, and from B to C,
are portions of a pair of curves of equal transmission (determined as in
Proposition IV.) ; for then, if the operation of the regenerator is perfect,
the changes from B to C and from D to A will be effected without
expenditure of heat; the. heat transmitted from the working substance
to a given stratum of the regenerator, during any part, such as be, of the
operation B C, being exactly sufficient for the corresponding part, da, of
d . F
the operation D A. In this case for each value of Q between Q, and
d^
Q 2 , has the same value at either side of the diagram.
In fact, the effect of a perfect regenerator is, to confer upon any pair of
curves of equal transmission the properties of a pair of curves of no
transmission.
29. Proposition XL — Theorem. The greatest efficiency of a thermo
dynamic engine, working between given limits of actual heat, with a perfect
regenerator; is equal to the greatest efficiency of a thermodynamic engine, working
between the same limits of actual heat without a regenerator.
(Demonstration.) In Fig. 1 8, let Q x Q v Q 2 Q. 2 be the isothermal curves
denoting the given limits of actual heat. Let AD, B C be a pair of
curves of equal transmission of any form. Then by the aid of a perfect
regenerator, the whole of the heat given out by the elastic substance during
the operation B C may be stored up, and given out again to that substance
in such a manner as to be exactly sufficient for the operation DA; so
that the whole consumption of heat in one revolution by an engine whose
Fig. 18.
indicatordiagram is A B C D, may be reduced simply to the latent heat
of expansion during the operation AB, which is represented by the
372 ON THERMODYNAMICS.
indefinitelyprolonged area M A B N, A d M and B c N being curves of no
transmission. The efficiency of such an engine is represented by
the area A B C D
the area MABY
Now the maximum efficiency of an engine without a regenerator,
working between the same limits of actual heat, is represented by
the area AB cd Q, — Q 2
the area MABN Q,
and from the mode of construction of curves of equal transmission,
described in Proposition IV., it is evident that
the area A B C D = the area ABn/;
hence the maximum efficiencies, working between the given limits of actual
heat, Qj and {}.» are equal, with or without a perfect regenerator. Q.E.D.
30. Advantage of a Regenerator.
It appears from this theorem that the advantage of a regenerator is, not
to increase the maximum efficiency of a thermodynamic engine between
given limits of actual heat, but to enable that amount of efficiency to be
attained with a less amount of expansion, and, consequently, with a
smaller engine.
Suppose, for instance, that to represent the isothermal curves, and the
curves of no transmission, for a gaseous substance, Ave adopt the approxi
mate equations already given in Article 20, viz. : —
For the isothermal curve of Q, P V = N Q ;
]  1 I (31.)
V /n \  . /P V f I
For a curve of no transmission ~ = (   J ^ ' = ( ~ ) 1 + * ' ; j
and let us compare the forms of the indicatordiagrams without and with
a regenerator, for a perfect airengine, working between given limits as to
actual heat, defined by the isothermal curves Q 2 Q 1} ()., Q., in Fig. 19.
The amount of expansion at the higher limit of heat being arbitrary,
let us suppose it to be from the volume V A to the volume V B , corre
sponding respectively to the points A and B, and to be the same in all
cases, whether with or without a regenerator.
The engine being without a regenerator, the diagram corresponding to
the maximum efficiency has but one form, viz., A B c d, where Be, A d are
curves of no transmission. Hence, in this case, there must be an additional
expansion, from the volume V B to the volume
ON THERMODYNAMICS.
373
V. = V,
Qi\n
Q 2 / '
(32.)
for the purpose merely of lowering the actual heat of the air without loss
of heat ; and the engine must be made large enough to admit of this
expansion, otherwise heat will lie wasted.
Fig. 19.
On the other hand, if the engine be provided with a perfect regenerator,
any pair of curves of equal transmission passing through A and B will
complete a diagram of maximum efficiency. The property of a pair of
these curves being, as shown in Proposition IV., that the difference of their
thermodynamic functions,
AF
( = I r= d V, when Q is constant j,
is the same for every value of Q, it follows, that for a gas, according to
the approximate equation (23), the property of a pair of curves of equal
transmission is, that the volumes corresponding to the intersections of the
two curves by the same isothermal curve, are in a ratio which is the same
for every isothermal curve. Thus, let V a , Y b be such a pair of volumes,
then this equation
v.
V.'
(33.)
defines a pair of curves of equal transmission. From this and from
equation (31) it follows, that for such a pair of curves
P P
(34.)
374 OX THERMODYNAMICS.
If one of the curves, or lines, of equal transmission is a straight line of
equal volumes, that is, an ordinate A D parallel to Y, then the other is
an ordinate B C, parallel to Y also. Then ABCI) is the diagram of
maximum efficiency for an airengine with a perfect regenerator, when the
air traverses the regenerator without alteration of volume; and by adopting
this diagram, the additional expansion from V B to V c is dispensed with.
If one of the curves, or lines, of equal transmission is a straight line of
equal pressures A D' parallel to X, then the other also is a straight line
of equal pressures BC. The diagram thus formed, A BCD', is suitable,
when the air, as in Ericsson's engine, has to traverse the regenerator
without change of pressure.
It must be observed, that no finite mass, or extent of conducting
surface, will enable a regenerator to act with the ideal perfection assumed
in Propositions X. and XL, and their corollaries.
Owing to the want of a general investigation of the theory of the action
of the regenerator based on true principles, those who have hitherto
written respecting it have either exaggerated its advantages or unduly
depreciated them. From this remark, however, must be excepted a
calculation of the expenditure of heat in Captain Ericsson's engine, by
Professor Barnard of the University of Alabama.*
31. General Hi urn 1 1,< on the preceding Propositions.
The eleven preceding propositions, with their corollaries, are the
geometrical representation of the theory of the mutual transformation of
heat and motive power, by means of the changes of volume of a homo
geneous elastic substance which does not change its condition. All these
propositions are virtually comprehended in the first two, of which, perhaps,
the most simple enunciations are the following : —
I. The mechanical equivalent of the heat absorbed or given out by a
substance in passing from one given state as to pressure and volume to
another given state, through a series of states represented by the co
ordinates of a given curve on a diagram of energy, is represented by the
area included between the given curve and two curves of no transmission
of heat drawn from its extremities, and indefinitely prolonged in the
direction representing increase of volume.
II. If across any pair of curves of no transmission on a diagram of
energy there be drawn any series of isothermal curves at intervals corre
sponding to equal differences of actual heat, the series of quadrilateral
areas thus cut off from the space between the curves of no transmission
will be all equal to each other.
These two propositions are the necessary consequences of the definitions
of isothermal curves and curves of no transmission on a diagram of energy,
and are the geometrical representation of the application to the particular
* SUliman's Journal, September, 1853.
ON THERMODYNAMICS. 375
case of heat and expansive power, of two axioms respecting Energy in the
abstract, viz.: —
I. The sum of Energy in the Universe is unalterable.
II. The effect, in causing Transformation of Energy, of the whole
quantity of Actual Energy present in a substance, is the sum of the effects
of all its parts.
The application of these axioms to Heat and Expansive Power virtually
involves the following definition of expansive heat : —
Expansive Heat is a species of Actual Energy, the presence of iddch in a
substance affects, and in general increases, its tendency to expand.
And this definition, arrived at by induction from experiment and
observation, is the foundation of the theory of the expansive action of
heat.
Section IV. — Of Temperature, the Mechanical Hypothesis of
Molecular Vortices, and the Numerical Computation
of the Efficiency of AirEngines.
32. In order to apply the propositions of the preceding articles to
existing substances, besides experimental data sufficient for the determina
tion, direct or indirect, of the isothermal curves and curves of no
transmission, it is necessary also to know the relation, for the substance in
question, between the quantity of heat actually present in it under any
circumstances, and its temperature ; a quantity measured by the product
of the pressure, volume, and specific gravity of a mass of perfect gas, when
in such a condition that it has no tendency to communicate heat to, or to
abstract heat from, the substance whose temperature is ascertained.
The nature of the relation between heat and temperature has been
discussed in investigations already published, as a consequence deducible
from a hypothesis respecting the molecular constitution of matter, with
the aid of data supplied by the experiments of Messrs. Thomson and
Joule and of M. Eegnault. Nevertheless, it seems to me desirable to
add here a few words respecting the grounds, independent of direct
experiment, for adopting the hypothesis of molecular vortices as a probable
conjecture, the extent to which, by the aid of this hypothesis, the residts
of experiment were anticipated, and its use, in conjunction with the results
of experiment, as a means of arriving at a knowledge of the true law of
the relation between temperatures and total quantities of heat.
To introduce a hypothesis into the theory of a class of phenomena, is to
suppose that class of phenomena to be, in some way not obvious to the
senses, constituted of some other class of phenomena with whose laws we
are more familiar. In thus framing a hypothesis, we are guided by some
376 ON THERMODYNAMICS.
analogy between the laws of the two classes of phenomena : we conclude,
from this analogy of laws, that the phenomena themselves are probably
alike. This act of the mind is the converse of the process of ordinary
physical reasoning; in which, perceiving that phenomena are alike, we
conclude that their laws are analogous. The results, however, of the
latter process of reasoning may be certainly true, while those of the former
can never be more than probable ; for how complete soever the analogy
between the laws of two classes of phenomena may be, there will always
remain a possibility of the phenomena themselves being unlike. A
hypothesis, therefore, is incapable of absolute proof; but the agreement
of its results with those of experiment may give it a high degree of
probability.
The laws of the transmission of radiant heat are analogous to those of
the propagation of a transverse oscillatory movement. The laws of
thermometric heat are analogous to those of motion, inasmuch as both
are convertible into mechanical effect ; and motion, especially that of
eddies in liquids and gases, is directly convertible into heat by friction.
If, guided by these analogies, we assume as a probable hypothesis that
heat consists in some kind of molecular motion, we must suppose that
thermometric heat is such a molecular motion as will cause bodies to tend
to expand; that is to say, a motion productive of centrifugal force. Thus
we are led to the hypothesis of Molecular Vortices.
This hyp i :hesis, besides the principles already enunciated, of the
mutual tram formation of heat and motive power in homogeneous sub
stances, leads to the following special conclusion respecting the
Eelation between Temperature and Actual Heat: —
When the temperature of a substance, as measured by a perfect gas ther
mometer, rises by equal increments, the actual heat present in the substance rises
also by equal increments — a principle expressed symbolically by the equation
Q = h (r  k), . . . . (35.)
where Q is the actual heat in unity of weight of a substance, r its
temperature, measured from the absolute zero of gaseous tension, k the
temperature of absolute cold, measured from the same point, and f> the
real specific heat of the substance, expressed in terms of motive power.*
The enunciation of this law was originally an anticipation of the results
of experiment; for when it appeared no experimental data existed by
which its soundness could be tested.
Since then, however, one confirmation of this law has been afforded
* The hypothesis of Mayer amounts to supposing that » = 0, or that the zero of
gaseous tension coincides with the point of absolute cold.
ON THERMODYNAMICS. o77
by the experiments of M. Regnault, showing that the specific heat of
atmospheric air is sensibly constant at all temperatures and at all densities
throughout a very great range; and another, by the experiments of Messrs.
Joule and Thomson, referred to in Proposition VI., on the thermic
phenomena of gases rushing through small apertures, which not only
verify the theoretical principle, but afford the means of computing approxi
mately the position k of the point of absolute cold on the thermometric
scale.
According to this relation between temperature and heat, every
isothermal curve on a diagram of energy is also a curve of equal tempera
ture. The isothermal curve, for example, corresponding to a constant
quantity of actual heat, Q, corresponds also to a constant absolute
temperature,
r = fH K (36.)
The curve of absolute cold is that of the absolute temperature k.
Any series of isothermal curves at intervals corresponding to equal
differences of heat, correspond to a series of equidistant temperatures.
Hence we deduce
Proposition XII. — Theorem. Everything that has been predicated, in
the propositions of the preceding articles, of the mutual proportions of quantities
of actual heat and their differences, may be predicated also of the mutual
proportions of temperatures as measured from the point of absolute cold, and
their differences.
The symbolical expression of this theorem is, that in all the equations
of the preceding sections, we may make the following substitutions: —
Qa _ r 2  k _ (A, g, or d) Q _ (A, g, or d) r ^
Ql T X — K ' Q r — K
This theorem is not, like those which have preceded it, the consequence
of a set of definitions. It is a law known by induction from experiment,
aided by a hypothesis or conjecture, with the results of which those of
experiment have been found to agree.
It is true that the theorem itself might have been stated in the form of
a definition of degrees of temperature; but then induction from experiment
would still have been required, to prove that temperature, as measured in
the usual way, agrees with the definition.
By substituting symbols according to the above theorem, and making
378
ON THERMODYNAMICS.
the general equation of the expansive action of heat is made to take the
following form: —
A . ¥ = A . H  IPdV = AQ+A.S = fc.Ar«f A/ . 
which agrees with the equation deduced directly from the hypothesis of
molecular vortices, if we admit that
and, consequently,
/.<r=ftNie (hyp. log. r +
/\r = *N
(37 a.)
J
The differential form of equation (37) is
d .* = d .H  VdX = dQ + d .* = K v o? r
(38.)
where
K v = k+/'.r + (rK) //r ,
rfV.
The expression for the thermodynamic function denoted by F takes
the form
r l +£./'. r
(39.)
but a more convenient thermodynamic function, bearing the same relation
to temperature as reckoned from the point of absolute cold, which the
function F does to actual heat, is formed by multiplying the latter by the
real specific heat fe, thus: —
$ = kF= fii+/^, r+ (''l\,Y, . (40.)
J r — K Jut
which, being introduced into the general equation, transforms it to
. (10 A.)
A.¥= f(rK)i$ fvdY
ON THERMODYNAMICS. 370
33. Of the Numerical Computation of the Efficiency of AirEngines, with or
without a perfect Regenerator.
The relation between temperature and heat being known, the preceding
propositions can be applied to determine the efficiency, and other circum
stances relative to the working of thermodynamic engines. To exemplify
this application of the theory, let the substance working the engine be
atmospheric air, and let the real indicatordiagram be such as to develop
the maximum efficiency between two given absolute temperatures r x and r 2 ,
being a quadrilateral, as in Fig. 1 9, of which two sides are portions of the
isothermal curves of those temperatures, and the other two portions of a
pair of curves of equal transmission, of such a form as may be best suited
to the easy working of the engine. Should these curves be curves of no
transmission, a regenerator may be dispensed with. In every other case a
regenerator is necessary, to prevent waste of heat; and for the present,
its action will be assumed to be perfect, as the loss which occurs from its
imperfect action cannot be ascertained except by direct experiment.
In this investigation it is unnecessary to use formula? of minute
accuracy ; and for practical purposes those will be found sufficient which
treat air as a perfect gas, whose thermometric zero of pressure coincides
with the point of absolute cold, viz. —
272 J° Centigrade, or.
! below melting; ice ; *
490i° Fahrenheit, 3
whose real specific heat is equal to its specific heat at constant volume, being
, 2'M'G feet of fall per Centigrade degree, Gi
ft = K v =
I 130  3 feet of fall per degree of Fahrenheit;
whose specific heat at constant pressure (as determined by M. Regnault)
is  23S x the specific heat of liquid water; or
c 3 30  8 feet of fall per Centigrade degree, or
K P = \
i. 1838 feet of fall per degree of Fahrenheit;
the ratio of these two quantities being
?= 1+X=141,
as calculated from the velocity of sound.
* This estimate of the position of the point of absolute cold is to be considered as
merely approximate, recent experiments and calculations having shown that it may
possibly be too high by about 1^° Centigrade. It is, however, sufficiently correct for
all practical purposes.
380
ON THERMODYNAMICS.
The volume occupied by an avoirdupois pound of air, at the temperature
of melting ice, under the pressure of one pound on the square foot, as
calculated from the experiments of M. Regnault, is
P V = 2G2144 cubic feet.
This represents also the length in feet of a column of air of uniform
density and sectional area, whose weight is equal to its elastic pressure on
the area of its section at the temperature of melting ice.
Tt will be found convenient, in expressing the temperature, as measured
from the point of absolute cold, to make the following substitution : —
k = T +T fl
(41.)
where T represents the temperature as measured on the ordinary scale
from the temperature of melting ice, and T the height of the temperature
of melting ice above the point of absolute cold, as already stated.
Then we have
P V
IN B  T •
(41 A.)
According to these data, the equation of the isothermal curve of air for
any temperature T is
PV = P V .^l° = Nfe(T + T ). . (42.)
The thermodynamic functions are —
For quantities of actual heat,
F = hyp. log. Q + N hyp. log. V;
For temperatures,
$ = hF + constant = K v {hyp. log. (T + T ) + N hyp. log. V]
= K v hyp. log. (T + T^ + P „0 ° • hyp. log. V;
1 o
consequently, the equation of any curve of no transmission is
$ = constant ;
otherwise
N 1 + N
(T + T ) . V = const.; orP . V = const.; K (43.)
y (42a.)
or
(T + T ) . PI + N  constant ;
ON THERMODYNAMICS.
381
111 W
liicli
N = 041, 1 + N = 141,
N
1 + N
02908.
The maximum possible efficiency between any two temperatures T x and
T., is given by the universal formula,
H t  H, T,  T,
H x
T x + T
(44.)
The latent heat of expansion of unity of weight of air at a given
constant temperature T v from the volume V A to the volume V B , is sensibly
equivalent simply to the expansive power developed, being given by the
following formula : —
H x = (T, + T )
(*,*a) = P V„
,Y
V
Tx + T
hyp. log.
c
Pd v.
(45.)
Let Y a and V & be the volumes corresponding to the points at which any
isothermal curve intersects a given pair of curves of no transmission, or of
equal transmission ; then the ratio of these volumes,
V'
(46.)
is constant for every such pair of points on the given pair of curves ;
because the difference of the thermodynamic functions, which is pro
portional to the logarithm of this ratio, is constant.
Hence, if in Fig. 19 a, two isothermal curves, T^, T 2 T 2 , be the upper
and lower boundaries of an y
indicatordiagram of maximum
energy for an airengine, AaD
an arbitrary curve bounding
the diagram at one side, and B
the other limit of the expan
sion at the higher temperature ;
the fourth boundary, being a
curve of equal transmission to
AftD, may be described by
this construction; draw any
isothermal curve 1 1 cutting A a D in a, and make
V A :V B ::V ■ V,. . (47.)
Fis 19 A.
382 ON THERMODYNAMICS.
then will b be a point in the curve sought, B b C.
Suppose, for example, that the form assumed for AaD is a hyperbola,
concave towards Y, and haviug the following equation : —
. . . (47 A.)
a /s  v;
v
in which a and (3 are two arbitrary constants; and let the ratio v " = r.
Then must the curve B b C be another hyperbola concave towards Y,
having for its equation
p. = r ^. ■ • • («»)
The total expenditure of heat, per pound of air per stroke, in a perfect
airengine, is the latent heat of expansion from V A to V B , given by
equation (45).
The heat to be abstracted by refrigeration is the latent heat of com
pression from V c to V D j and is found by substituting in the same
equation the lower temperature T 2 for the higher temperature T r
The indicated work per pound of air per stroke, being the difference
between those two quantities, is found by multiplying the range of
temperature by the difference of the thermodynamic functions l l> for the
curves AD, BC, or by multiplying the latent heat of expansion by the
efficiency, and has the following value : —
E^^H^^T,).^^)
Po^o.^V^^Plog.^. • • (48.)
o ' o • nn • 'vr —&■ y
The heat alternately stored up and given out by the regenerator
(supposing it to work perfectly), is to be computed as follows : — Let the
arbitrary manner in which volume is made to vary with temperature, on
either of the curves D a A, C b B, be expressed by an equation
then the thermodynamic function $ takes the form
$ = K v hyp. log. (T + T ) + P ^° hyp. log. V . T :
and the total heat stored up and given out per pound of air per stroke, is
ON THERMODYNAMICS. 383
T
/ 1 (T + T )^^T = K V (T 1 T 2 )
T
f
 ' rp
For example, if, as before,
+ gAJ H? + t )V .? dT (49)
ft  V a
be the equation of the curve D A, then
P (T + T )
V„ =
T + T o(l+pV)'
x x v
and the heat stored up per pound of air per stroke, is
K v (T,  T 2 ) + « . hyp. log. <{ ) ^ V (49 a.)
L T 3 + T (l + ^)j
33A. Numerical Examples?.
To illustrate the use of these formula*, let us take the following;
example : —
Temperature of receiving heat,
T x = 343°3 Centigrade.
T l + T = 615°8 Centigrade.
Temperature of emitting heat,
T 2 = 35°4 Centigrade.
T 2 + T = 307°9 Centigrade.
Ratio of Effective Expansion,
Y. _ lo _ Pa = ?d = 3
V A V D P B P c 2'
From these data are computed the following results : —
3S4 OX THERMODYNAMICS.
Maximum Efficiency. —
307°'9 1
G15°8 2
Heat expended, or latent heat of expansion, —
G15S 3
H, = P \ x ^^ x hyp. log. 
= 24020 footpounds per pound of working air per stroke.
Heat abstracted by refrigeration, —
H 2 = P V X y^ X hyp. log. 
= 12010 footpounds per pound of working air per stroke.
Work performed, —
H 1 H, = P V o x ^x h yPlog
= 12010 footpounds per pound of working air per stroke.
To exemplify the computation of the heat stored by a perfect regenerator,
let it be supposed, in the first place, that the indicatordiagram resembles
ABC'D' in Fig. 1 9, where the curves of equal transmission are represented
by a pair of lines of constant pressure. Then the heat to be stored is
K p (T, — l\)) = 101,800 footpounds per pound of working air per stroke.
(Secondly, let the diagram resemble ABCD in Fig. 19, where the
curves of equal transmission are represented by a pair of lines of constant
volume. Then the heat to be stored is
K V (T 2 — T 2 ) = 72,233 footpounds per pound of working air per stroke.
Thirdly, let the curves of equal transmission, as in a recent example, be
hyperbolas, concave towards O Y, and let the arbitrary constant a have
the following value, —
a — . P V = 26214*4 footpounds;
then the heat to be stored, according to equation (49 a), is
72,233 + 262144 x hyp. log.^ = 72,233 +11,157
= 83,390 footpounds per pound of working air per stroke.
ON THERMODYNAMICS. 385
The large proportions borne by these quantities to the whole heat
expended, show the importance of efficient action in the regenerator to
economy of fuel. The quantity of heat to be stored, however, becomes
smaller, as the curves of equal transmission approach those of no trans
mission, for which it is null. The additional expansion requisite in this
last case is found by the following computation : —
/% + T \s
" VT 2 + Tj
l
2 041
= 5423,
the result of which shows the great additional bulk of engine required, in
order to obtain the maximum efficiency without a regenerator.
Supposing one pound of coal, by its combustion, to be capable of com
municating heat to the air working in an engine corresponding with the
above example, to an amount equivalent to
6,000,000 footpounds
(an amount which would evaporate about 7 lbs. of water), the maximum
theoretical duty of one pound of such coal in such an engine, without
waste of heat or power, would be
3,000,000 footpounds,
corresponding to
3,000,000
= 249 strokes of a pound of working air, with the effective
12,010
. 3
expansion .
u
The deductions to be made from this result in practice must, of course,
be determined by experience.
Section V. — Propositions Relative to a Heterogeneous Mass, or
Aggregate, especially in VapourEngines.
34. The heterogeneous mass to which the present investigation refers,
is to be understood to mean an aggregate of portions of different in
gredients, in which each ingredient occupies a space, or a number of
spaces, of sensible magnitude.
The results arrived at are not applicable to mixtures in which there is a
complete mutual diffusion of the molecules of the ingredients, so that every
space of appreciable magnitude contains every ingredient in a fixed pro
portion. A mixture of this kind, when the relations between its pressure,
2b
38G ON THERMODYNAMICS.
volume, heat, and temperature are known, may be treated, so far as regards
the expansive action of heat, as a homogeneous substance.
The ingredients of an aggregate are heterogeneous with respect to the
expansive action of heat, when either their specific heats, or their volumes
for unity of weight at a given pressure and temperature, or both these
classes of quantities, arc different.
Hence a portion of a liquid, and a portion of its vapour, enclosed in
the same vessel, though chemically identical and mutually transformable,
are heterogeneous, and are to be treated as an aggregate, with respect to
the expansive action of heat.
M. Clausius and Professor William Thomson have applied their formulae
to the aggregate composed of a liquid and its vapour, and have pointed out
certain relations which must exist between the pressure and density of a
liquid and its vapour, and the latent heat of evaporation.
I shall now apply the geometrical method of this paper to the theory
of the expansive action of heat in an aggregate, especially that consisting
of a liquid and its vapour. The total volumes are, for the present,
supposed not to be large enough to exhibit any appreciable differences
of pressure due to gravitation.
35. Proposition XIII. — Theorem. In an aggregate in equilibria, the
pressure of each ingredient must be the same; and the quantity of heat in unity
of weight of each ingredient must fa inversely proportional to its real specific
heat; that is to say, the temperature must be equal.
The following is the symbolical expression of this theorem, with certain
conclusions to which it leads : —
Let t — k be the common temperature of the ingredients, as measured
from the point of absolute cold ;
P, their common pressure ;
n v w 2 , ?? 3 , &c.j their proportions by weight, in unity of weight of tin
aggregate ;
v v v 2 , v z , &c, the respective volumes of unity of weight of the several
ingredients.
V, the volume of unity of weight of the aggregate ;
q v q 2 , q 3 , Sec, the respective quantities of actual heat in unity of weight
of the several ingredients;
fci> fc2> ^3> c ^' c ' their respective real specific heats ;
0, the quantity of heat in unity of weight of the aggregate ;
«1>, a thermodynamic function for the aggregate.
ON THERMODYNAMICS. 387
Then these quantities are connected by the following equations: —
2 . n = 1 (50.)
V=.2..»«. (51.)
T  I.   —  cVC. . . . {0^.)
**1 "2 ^3
Q = S . n J = (r  k) . 2 . n Is. . . . (53.)
t. p''» +/, ' ,) .i,'+fg.<T. . (54.) '
It is evident that all these equations hold, whether the proportions of
the ingredients n v &c, are constant, as in an aggregate of chemically
distinct substances, or variable, as in the aggregate of a liquid and its
vapour.
Let 3H be the heat which disappears in consequence of a small
expansion of aggregate at constant temperature, represented by
SV = 2.S«, .... (55.)
$ u representing any one of the parts arising from the changes undergone
by the different ingredients, of which the whole expansion of the aggregate
S V is made up.
Then
gH = 2[(rK)^.S«}; . • (56.)
the same for every ingredient, as well as the tei
perature ; therefore, the factor (r  k) j is the same for every ingredient.
but the pressure P is the same for every ingredient, as well as the tem
(V?
(It
and, consequently, for the whole aggregate; that is to say,
SH = (r K )^.SV=(r K )S$. . (57.)
(It
This equation shows that the relation of temperature to the mutual
transformation of heat and expansive power is the same in an aggregate
as in a homogeneous substance.
Consequently, if we define isothermal curves for an aggregate to be cm
of constant temperature, we arrive at the following conclusion: —
Proposition XIV. — Theorem. Isothermal curves on the diagram of
energy of an aggregate have the same properties, with reference to the mutual
transformation of heat and expansive power, with those on the diagram of energy
of a homogeneous substance.
It is unnecessary to enunciate separately a similar proposition for
388 ON THERMODYNAMICS.
curves of no transmission; for the demonstration of Proposition I., on
which all their properties depend, is evidently appli cable to an aggregate,
constituted in any manner.
Hence it appears, that if the isothermal curves for an aggregate be drawn
according to the above definition, all the propositions proved in this
paper respecting homogeneous substances become true of the aggregate.
36. Proposition XV. — Theorem. Every isothermal line for an aggre
gate of a liquid and its vapour, is a straight line of equal pressure, from the volume
corresponding to complete liquefaction to the volume corresponding to complete
evaporation.
This is a fact known by experiment. The theorem is equivalent to
a statement, that the pressure of a liquid and its vapour in contact with
each other is a function of the temperature only.
Corollary. — Theorem. At any given temperature, the volume of an aggre
gate of liquid and vapour is arbitrary between and up to the limits of total
liquefaction and total evaporation.
To express this symbolically, let P be the pressure of an aggregate of
liquid and vapour corresponding to the absolute temperature r ; and
unity of weight being the quantity of the aggregate under consideration,
let v be the volume corresponding to complete liquefaction, v that
corresponding to complete evaporation, and V the actual volume at any
time; let n be the proportion of liquid, and 1 — n that of vapour, corre
sponding to the aggregate volume V ; then
V — nv + (1  n)v', . . . (58.)
and V may have any value not less than v nor greater than v, while P
and r remain constant; the proportion of liquid, n, being regulated
according to the foregoing equation.
37. Proposition XVI. — Problem. The density of a liquid and of Us
vapour, when in contact at a given temperature, being given, and the isothermal
lines of the aggregate, it is required to determine the latent heat of evaporation
of unity of weight of the fluid.
(Solution.) The densities of the liquid and of its vapour are respectively
the reciprocals of the volumes of total liquefaction and total evaporation
of unity of weight, abovementioned. In Fig. 20, let the abscissae Ov, Ov
represent these volumes, and the equal ordinates, vA, v'B, the pressure
corresponding to the given temperature; so that AB, parallel to OX, is
the isothermal line of the aggregate for that temperature. Suppose two
curves of no transmission AM, BN, to be drawn from A and B respectively,
and indefinitely prolonged towards X; then the indefinitelyprolonged
area MABN represents the mechanical equivalent of the latent heat
sought, and this area is to be computed in the following manner : — Draw
a second isothermal line, a b, indefinitely near to A B, at an interval
ON THERMODYNAMICS.
389
A 1
3
N
M
X
Fv'. 20.
corresponding to the indefinitelysmall difference of temperature dr; then,
ultimately,
d t : t — k : : area A B b a : area MABN;
or, symbolically,
L
d~P
latent heat of evaporation = (r — k) = (v — v). (59.)
This is simply the application of Propositions I. and II. to the aggregate
of a liquid and its vapour, mutatis mutandis.
(Remarks?) — The existence of a necessary relation between the density,
pressure, and temperature of a vapour and its liquid in contact, and the
latent heat of evaporation, was first shown by Carnot. If for r — k in
the preceding equation be substituted, according to Professor Thomson's
notation, , J being "Joule's equivalent" and /x "Carnot's function," the
,u
equation is transformed into that deduced by Messrs. Clausius and
Thomson from the combination of Carnot's theory with the law of the
mechanical convertibility of heat.
38. Corollary. — The volume occupied by unity of weight of vapour at
saturation may be computed from its latent heat of evaporation by means
of the inverse formula, —
L
(r  k)
dj>'
It
(GO.)
the latent heat, L, being, of course, always stated in units of motive power.
The want of satisfactory experiments on the density of vapours of any
kind, has hitherto prevented the use of the direct formula (59).
390 ON THERMODYNAMICS.
It is otherwise, however, with the inverse formula (60), at all events
in the case of steam; for, so far as we are yet able to judge, the experi
ments of M. Kegnault have determined the latent heat of evaporation of
water with accuracy throughout a long range of temperature.
M. Clausius, applying to those experimental data a formula founded on
the supposition of Mayer (that is to say, similar to the above, with the
exception that k is supposed = 0), has calculated the densities of steam
at certain temperatures, so as to show how much they exceed the densities
calculated from the pressures and temperatures on the supposition that
steam is a perfect gas. From these calculations he concludes, that
either the supposition of Mayer is erroneous, or steam deviates very much
at high densities from the condition of a perfect gas.
In the following table, the value of k is supposed to be 2 0, 1 Centigrade,
and use has been made of the formula for calculating the pressure of steam
and other vapours at saturation, first published in the Edinburgh Neiv
Philosophical Journal for July, 1849 (Sec p. 1), viz. —
log.P = a£^. • • • (61.)
This table exhibits, side by side, the volume in cubic feet occupied by
one pound avoirdupois of steam at every twentieth Centigrade degree,
from  20° to + 200° (that is, from 4° to + 500' Fahrenheit)— first, as
extracted from a table for computing the power of steamengines, in the
Transactions of the Royal Society of Edinburgh, Vol XX. (See p. 2S2), which
was calculated on the supposition that steam is a perfect gas; and, secondly,
as computed by equation (GO) from the latent heat of steam as determined
by M. Regnault The excess of the former quantity above the latter is
also given in each case, with its ratio to the second value of the volume.
For convenience sake, a column is added containing the pressures of
steam corresponding to the temperatures in the table in pounds per
scpiare foot.
The fourth column of this table could easily be extended and filled up,
so as to replace the column of volumes of steam for every fifth Centigrade
degree in the table previously published; but it would be unadvisable to
do so at present, for the following reasons : —
First, the value of the constant k is still uncertain.""
Secondly, the results of M. Regnault's direct experiments, on the density
of steam and other vapours, may soon be expected to appear.
* It is probable that x. may be found to be inappreciably small ; in which case the
numbers in column (4) will have to be diminished to an extent varying from T l,,
to T J „ of their amount.
ON THERMODYNAMICS. 391
Table of Computed Volumes of 1 lb. Avoirdupois of Steam.
Temperature.
Volume sup
posed a Per
fect Gas.
Volume com
puted from
Latent Heat.
Difference.
Ratio of
Difference to
lesser Value
of Volume.
Pressure.
Fahrenheit.
Centigrade.
Deg.
Deg.
Cubic feet.
Cubic feet.
Cubic feet.
lb. per square
foot.
 4
20
15757
15718
39
00025
24799
+ 32
3390 4
33772
132
00039
12431
68
+ 20
93681
93450
231
0025
48265
104
40
31488
313 56
132
0042
15334
140
60
123 65
12263
102
00083
41533
176
80
55 05
5419
86
00158
98867
212
100
27166
2647S
068S
00260
21164
248
120
14596
14 076
0520
00369
41493
284
140
8420
8004
0416
00502
75570
320
160
515S
4838
0320
0661
12931
356
180
3326
3071
0255
0830
20979
392
200
2241
2 033
020S
1023
32512
428
220
1568
1396
0172
0T232
48425
464
240
1134
0990
0144
01455
69680
500
260
0843
722
121
01676
97275
Col. (1.)
(2.)
(3.)
(4.)
(5.)
(6.)
(7.)
Thirdly, it is possible that the values of the latent heat of evaporation
of water, as deduced from M. Eegnault's experiments, may still have to
undergo some correction ; because, according to the theoretical definition
of the latent heat of evaporation, the liquid is supposed to be under the
pressure of an atmosphere of its own vapour, which atmosphere, as it
increases in bulk, performs work of some kind, such as lifting a piston;
whereas, in M. Regnault's experiments, the water is pressed by an atmo
sphere of mingled steam and air, whose united pressure is that corre
sponding to the tenrperature of internal ebullition of the water; so that the
pressure of the steam alone on the surface of the water, which regulates
the superficial evaporation, may be less than the maximum pressure
corresponding to the temperature of ebullition; and this steam, moreover,
has no mechanical work to perform, except to propel itself along the
passage leading to the calorimeter, and to agitate the water in the latter
vessel. Under these circumstances, it is possible, though by no means
certain, that the latent heat of evaporation of water, as deduced from M.
Eegnault's experiments, may be somewhat smaller than that which corre
sponds to the theoretical definition, especially at high pressures; and a
doubt arises as to the precise applicability of the formulas (59) and (60)
to those experimental results, which cannot be solved except by direct
experiments on the density of steam.
Notwithstanding this doubt, however, the preceding table must be
regarded as adding a reason to those already known, for believing that
392
ON THERMODYNAMICS.
saturated steam of high density deviates considerahly from the laws of
the perfectly gaseous condition.*
39. Proposition XVII. — Problem. The isothermal lines for a liquid and
Us vapour, and the apparent specific heat of the liquid at all temperatures hcing
given, and the expansion of the liquid by heat being treated as inappreciably
small: to determine a curve of no transmission for the aggregate, passing
through a given point on the ordinate ichose distance from the origin approxi
mately represents the volume of the liquid,
(Solution.) In Fig. 21, let Ov represent the volume of the liquid
assumed to be approximately constant for all temperatures under con
sideration; let jjA be an ordinate parallel to Y, and let the heat
Fig. 21.
consumed by the liquid in passing from the temperature corresponding to
any point on this ordinate to that corresponding to any other point, be
known; let the isothermal lines for the aggregate of liquid and vapour,
all of which are straight lines of equal pressure parallel to X, such as
AT 1? «BT 2 , be known. Then to draw a curve of no transmission through
any point A on the ordinate vA, the same process must be followed as in
Proposition VIII.
To apply to this case the symbolical representation of Proposition VIIL,
viz., equation (21), let r x be the absolute temperature corresponding to
the point A (that is, to the isothermal line A T x ) ; r 2 that corresponding
to any lower isothermal line aBT 9 ; V B the volume of the aggregate of
liquid and vapour corresponding to the point B, where the curve sought,
AM, intersects the latter isothermal line; K L the apparent specific heat
* Evidence in favour of this opinion is afforded by the experiments recorded by
Mr. C. W. Siemens (Civil Engineer and Architect's Journal). A remarkable cause,
however, of uncertainty in all such experiments has lately been investigated by
Professor Magnus (Poggendorff's Annalen, 1853, No. 8), viz., a power which solid
bodies have of condensing, by attraction on" their surfaces, appreciable quantities
of gases.
ON THERMODYNAMICS. 393
of the liquid; then, making the proper substitutions of the symbols of
temperature for those of heat, and observing that the operation
/ dV
'v A
is in this case equivalent to multiplication by V B — v, we have
l^ = f T (Y B v)(iovr = r. 1 )=f 1 ^ K dr, . (62.)
being an equation between two expressions for the difference between the
thermodynamic functions $ for the curve A B, and for that which passes
through a.
If the specific heat of the liquid is approximately constant, this
equation becomes
A * = ~ (V B  v) (for t = t.J = K t hyp. log. *LZ*> (63.)
»T T 2 — K
40. Corollary. — Problem. The same data being given as in the preceding
problem, and the expansion of the liquid by heat neglected, a mass of liquid,
having been raised from the absolute temperature t 2 to the absolute temperature r l7
is supposed to be allowed to evaporate partially, under pressure, without receiving
or emitting heat, until its temperature falls again to t 2 , at which temperature it
is liquefied under constant pressure by refrigeration : it is required to find the
power developed.
(Solution.) The power developed is represented by the area of the
threesided diagram of energy in Fig. 21, A B a ; that is to say, by
T 2 T 2
which, if K L is nearly constant, becomes
K L j * hyp. log. ^^ .dr = K L { (r,  k)
r 9
 (r 2 k) . (l + hyp. log. J^) }• • . (65.)
41. Numerical Example.
Let one pound avoirdupois of water be raised, in the liquid state, from
T 2 = 40° Centigrade to T\ = 140° Centigrade. Then
394 ON THERMODYNAMICS.
Tl  K = T, + T = 140° + 272r = 4m° Centigrade.
r 2  k = T 2 + T = 40° + 272?,° = 3121° Centigrade.
The mean apparent specific heat of liquid water between those tem
peratures is
K L = K w (or Joule's equivalent) X l'OOG = 1398 feet per Cent, degree;
consequently, the heat expended is equivalent to 139,800 footpounds.
The other numerical data are, —
dP
r— at 40° Centigrade = S'2075 lbs. per square foot per Cent, degree;
(i 
v = mean volume of 1 lb. of liquid water = 0017 cubic foot nearly.
Let it be required to find, in the first place, V B , the volume to which
the water must be allowed to expand by partial evaporation under
pressure, in order that its temperature may fall to 40° Centigrade; and,
secondly, how much power will be developed in all, after the water has
been totally reliquefied by refrigeration at constant pressure, at the
temperature of 40°.
First, by the equation (G3),
A $ = ~ (V B  v) = 1398 x hyp. log. i = 402G24 ;
divide by  = 82075 ; then Y B v = 49055 cubic feet.
addv= 0017
Aggregate volume of water and steam at 40°, V B = 49072 .,
As the volume of one pound of steam at 40° Centigrade, according to
the fourth column of the table in Article 38, is 313  5G cubic feet, it
appears from this calculation that somewhat less than onesixth of the
water will evaporate.
Secondly, it appears, from equation (G5), that after the water has been
restored to the liquid state by refrigeration at 40° Centigrade, the whole
power developed — that is to say, the area A B a — will be
1398 footpounds X j 412°5  312°5(l + hyp. log. <— — J j
= 1398 ft. lbs. X 10° Centigrade = 13,980 ft. lbs.,
or onetenth of the equivalent of the heat expended. The other nine
tenths constitute the heat abstracted during the reliquefaction at 40°
Centigrade.
ON THERMODYNAMICS.
395
This calculation further shows, that in order that one pound of water
and steam at 40° C. may be raised to 140° C. solely by compressing it
into the liquid state, it must occupy at the commencement of the operation
the volume V B = 4 9  07 2 cubic feet; and that the power expended in the
compression will be as follows : —
Footpounds.
Area of the curvilinear triangle ABa, Fig. 21, as already calculated, 1 3,9S0
Area of the rectangle aBY B v — P 2 (V B — v) = . . . 7,522
Total,
21,502
42. Proposition XVIII. — Problem. Having the same data as in the
last proposition, it is required to draw a curve of no transmission through any
point on the diagram of energy for the aggregate of a liquid and its vapour.
(Solution.) In Fig. 22, through the given point B draw the straight
isothermal line AB corresponding to the absolute temperature r v and
cutting the ordinate corresponding to the volume of total liquefaction in A.
Through A, according to the last proposition, draw the curve of no trans
mission, A D M. Let E D C be any other isothermal line, corresponding
to the absolute temperature r 2 , and cutting the curve A M in D. Draw
isothermal lines a b, e cl c at indefinitelysmall distances from A B, EDO
v A B
a
c
fa 55 ^
\
\d "^^T£ J
\
C — jnt
d
NM
A r E V n
X
Fk
respectively, corresponding to the same indefinitely small difference of
temperature dr. Draw the ordinates V D dD, V B bB; then draw the
ordinate V c cC at such a distance from V D rfD, that the indefinitelysmall
rectangles DCcd, ABba shall be equal. Then, as the difference 8 r is
indefinitely diminished, C approximates indefinitely to a point on the
required curve of no transmission, B N.
This is Proposition III. applied to aggregates, mutatis mutandis.
39G ON THERMODYNAMICS.
The symbolical representation of this proposition is as follows : — let F l
and P 2 be the pressures of the aggregate of liquid and vapour corresponding
respectively to the temperatures r r and r 2 ; then the following expressions
for the difference between the thermodynamic functions $ of the curves
A M, B N are equal,
A$ = ^(V V D ) = ^(V B »). • (GG.)
43. Corollary. (Absolute Maximum Efficiency of VapourEngines.)
If the volume V B be that corresponding to complete evaporation at the
temperature t u that is to say, if
V B = •,
then the curve B C N will represent the mode of expansion under pressure,
of vapour of saturation in working an engine, and will be defined by the
equation
YV {v ~ v)
V _ y = — . . . . (G7.)
(It
If in this equation be substituted the value of v — v, in terms of the
latent heat of evaporation at the higher temperature, given by equation
(60), it becomes
h.
'up • ■ (° a >
In this case the diagram A B C D, Fig. 22, is evidently that of a vapour
engine working with the absolute maximum of efficiency between the
absolute temperatures r x and r 2 . The heat expended at each single stroke,
per unit of weight of fluid, is the latent heat of evaporation at the higher
temperature, or L x ; the area of the diagram is given by the following
equation : —
T, — T.,
E = (r 1 r 2 )A$ = ^ 3.1* . . (09.)
This is the mechanical power developed at each single stroke by a unit
of weight of the substance employed. The efficiency is represented by
r = T ^ Ll2 , .... (to.)
L l T l  «
being the expression for the maximum efficiency of thermodynamic
engines in general.
ON THERMODYNAMICS. 397
The conditions of obtaining this efficiency are the following : —
First, That the elevation of temperature from r 2 to t x , during the
operation represented by the curve D A on the diagram, shall be produced
entirely by compression. The volume at which this heating by compression
must commence is given, according to Proposition XVII., by the following
equation : —
V. = . + ^ . K, h n , log. *=$ . . (71.)
dr
Secondly, That the expansive working of the vapour shall be carried on
until the temperature falls, by expansion alone, to its lower limit ; that is
to say, until the volume reaches the following value, obtained by adding
together equations (68) and (71) : —
Ve = . + / p; • { K, h )T . log. ^ + h } . (72.)
dr
44. Numerical Example.
To exemplify this numerically, let the same data be employed as in
Article 41, the substance working being one pound avoirdupois of water.
These data, with some additional data deduced from them, are given in
the folio win sj table : —
Temperature in Centigrade Degrees : —
Above melting ice (T),
Above zero of gaseous tension (t),
Above absolute cold (r — k),
Pressure in pounds per square foot (P), .
„ „ per square inch,
Initial Volume of saturated steam, V B = v\, = 8*004 cubic ft. per lb.
Latent Heat of Evaporation : —
In degrees, applied to one pound of liquid water, 509 o, l Centigrade.
In footpounds (Lj), . . . . . 707,445  3G
At upper limit
At lower limit
of Actual Heat.
. 140°
40°
4146
3146
4125
3125
. 7557
15334
525
1065
From these data are deduced the following results : —
Absolute Maximum Efficiency ; =  2424
398 ON THERMODYNAMICS.
Duty of one pound of water; being the area
of the diagram ABCD, . . 1 71,48475 ft. lbs.
Volume at which the compression must
commence; calculated as in Art. 41, . V D = 49"1 cubic ft. per lb.
Volume to which the expansion must be car
ried; calculated by equation (72), . V c = 258*1 cubic ft. per lb.
V c 258*1
Ratio of Expansion, . . . . = y = 8 .qq^ = 3225.
45. Liquefaction of Vapour by Expansion under Pressure.
In Fig. 22, let the abscissa? of the curve BFE indicate the volumes
corresponding to complete evaporation at the pressures denoted by its
ordinates. For most known fluids, a curve of no transmission, B C N,
drawn from any point B of the curve of complete evaporation in the
direction of X, falls within that curve ; so that by expansion of saturated
vapour under pressure, a portion in most cases will be liquefied.
To ascertain whether this will take place in any particular case, and to
what extent, equation (60), which gives the volume of unity of weight of
saturated vapour at the temperature t,, is to be compared with equation
(72), which gives the volume at the same temperature of unity of weight
of an aggregate of liquid and vapour, which lias expanded under pressure
from a state of complete evaporation at the temperature r v The difference
1 letween the volumes given by these equations is as follows (neglecting, as
usual, the expansibility in the liquid state) : —
'■'•'  v < = tV • { — —  k* • h yp lo s T y^ } ( 73 )
d F 2 (. T 2 — K T 1 — K T 2 — K )
JV
That this quantity is almost always positive appears from the following
considerations. The latent heat of evaporation L, is in general capable of
being represented approximately by an expression of this form :
L = a &(r k), . . . . (74.)
(For water, a = 79G° Centigrade x K w = 1,106,122 ft. lbs. ; b = 0695
X K w = 965772 ft. lbs. per Centigrade degree).
Hence, the second factor in equation (73) is nearly equal to
»(r,rj K L . hyp. logA ' . (75.)
(r x — k) . (t 2 — k) r 2 — k
Now
hyp. log. — < .
ON THERMODYNAMICS. 399
Therefore, the expression (75) is positive so long as
exceeds K L , the specific heat of the liquid. . (75 A.)
For water this condition is fulfilled for all temperatures lower than
523^° Centigrade (at which r x — k = 796° Centigrade); and there is
reason to believe that it is fulfilled also for other fluids at those tempera
tures at which their vapours can be used for any practical purpose.
To determine the proportion of the fluid which is liquefied by a given
expansion under pressure, we have the following formula, deduced from
equation (58) : —
v — V
n = ]  c (76.)
v 2 — v
As a numerical example, we may take the case of Art. 44, where
saturated steam at 140° Centigrade is supposed to be expanded under
pressure until its temperature falls to 40° Centigrade. The volume of one
pound of water and steam at the end of the expansion has already been
found to be
V c = 2581 cubic feet.
While, according to the table in Art. 38, the volume of a pound of
steam at that temperature is
v'. 2 = 313'56 cubic feet.
Consequently, the fraction liquefied by the expansion is
31356  2581 5546
31356  0016 313544
= 0177.
This conclusion was arrived at contemporaneously and independently,
by M. Clausius and myself, about four years since. Its accuracy was
subsequently called in question, chiefly on the ground of experiments,
which show that steam, after being expanded by being " wiredrawn," that
is to say, by being allowed to escape through a narrow orifice, is super
heated, or at a higher temperature than that of liquefaction at the reduced
pressure. Soon afterwards, however, Professor William Thomson proved
that those experiments are not relevant against the conclusion in question^
by showing the difference between the free, expansion of an elastic fluid, in
which all the power due to the expansion is expended in agitating the
particles of the fluid, and is reconverted into heat, and the expansion of
the same fluid under a pressure equal to its own elasticity, when the power
developed is all communicated to external bodies, such, for example, as the
piston of an engine.
400
ON THERMODYNAMICS.
The free expansion of a vapour will be considered in the sequel.
46. Efficiency of a VapourEngine without heating by compression.
The numerical example of Art. 44 sufficiently illustrates the fact,
that the strict fulfilment of the condition specified in Art. 43, as
necessary to the attainment of the absolute maximum of efficiency of a
vapourengine, is impossible in practice.
Let us consider, in the first place, the effect of dispensing with the
process D A, during which the fluid is supposed to have its high tempera
ture restored solely by compression.
The effect of this modification is evidently to add to the heat expended
that which is necessary to elevate the temperature of the liquid from r 2 to
t v and to add to the power developed an amount represented by the area
AD E (Fig. 22).
To express this symbolically, we have —
The latent heat of evaporation at t v as
before, ......
The additional heat expended (K h being
the mean specific heat of the liquid
between t x and r 2 ),
Total heat expended, .
Then, for the power developed, we have
L,
K,.(r.T 2 )
I^ + K^r,) (77.)
The area ABCD, as in Art. 43, = *  2 . L,,
the area AD E, as in Art. 40, equation (G5),
= K t { ( Tl  K )  (r 2  k) (l + hyp. log. T f^j }
the sum of which quantities is the total power developed, .
The efficiency may be expressed in the following form : —
Power developed _ r l — r
Heat expended t x — k
\ + K L (r x  r 2 )
(78.)
y (79.)
an equation which shows at once how far the efficiency falls short of the
absolute maximum.
ON THERMODYNAMIC'S.
401
For a numerical example, the same data may be taken as in Arts.
41 and 44. Then the heat expended, per pound of steam, is thus
made up : —
Latent heat of evaporation, as in Art. 44, .
Heat required to raise the water 100° C, as in Art. 41,
Total heat expended per lb. of water,
The power developed consists of, —
The area A B C D, as in Art. 4 4,
The area ADE, as in Art. 41, .
Footpounds.
707,44530
139,80000
847,24530
Footpounds.
171,48475
13,98000
Total power developed per lb. of water, 185,464*75
. 185,48475
• Efficiency 847,24536 '
Absolute maximum efficiency, as in Art. 44,
02189
02424
Loss of efficiency by omitting the heating by compression, 00235
or about onetenth part of the absolute maximum.
47. Efficiency of a VapourEngine with incomplete expansion.
It is in general impossible in practice to continue the expansion of the
vapour down to the temperature of final liquefaction ; and from this cause
a further loss of efficiency is incurred.
Let it be supposed, for example, that while the pressure of evaporation
/
r
^ Pi 1
i
1
H
P*
^\
Cr
3A
E
P 3
K
^~~^C
D\
s
>iyt
r
i
l
A
">
c
Fi<r. 23.
Pi corresponds to the line AB, in Fig. 23, and the pressure of liquefaction,
P 3 , to the line E D C, the pressure at which the expansion terminates, P,,
2c
402 ON THERMODYNAMICS.
corresponds to an intermediate line H L G. Let v A, ?•/ B, as before, be
the ordinates corresponding to complete liquefaction and to complete
evaporation, at the pressure Pj.
Draw, as before, the curves of no transmission A M, B N, cutting H L G
in L and G, and EDO in D and C; draw also the ordinate V KG,
cutting E D C in K.
Then the expansion terminates at the volume V (; , and ABGKE is the
indicatordiagram of the engine.
To find the power represented by this diagram, the area A L H is to be
found as in Art. 40, the area A B G L as in Art. 43, and the rectangle
HGKE by multiplying its breadth V G — v (found as in Art. 43) by its
height H E, which is the excess of the pressure at the end of the expansion
P 2 , above the pressure of final liquefaction, P 3 .
Hence, we have the following formula for the indicated power developed,
per unit of weight of fluid evaporated : —
E = area A B G K E = K L j (r,  ic)  (r,  k)
1 +hvp. log. T "^l + L./ 1 ^
n 5 r 2  J J ' r x K y (80.)
+ (P 2 P3)^{^ + K thy p.log^
r,«J
,/■
The heat expended is of course L, + K, (rj — t 3 ).
To illustrate this numerically, let the fluid be water; let the temperature
of evaporation be 1 40° Centigrade, and that of liquefaction 40°, as in the
previous examples; and let the expansion terminate when the pressure
has fallen to 100° Centigrade.
The numerical data in this case are the following : —
1. 2. 3.
During the At the end of During the
evaporation, the expansion, final liquefaction.
Temperature in Cent, degrees :—
Above melting ice, . . 140° 100° 40°
Above zero of gaseous ten
sion, T — .
Above absolute cold, r — k,
Pressure, in lb. per square f oot, P = 7557
Pressure, in lb. per square inch, .
146
3746
31 46
125
3725
3125
57
21164
153 34
525
14 7
1065
ON THERMODYNAMICS. 403
1. 2. 3.
During the At the end of During the
evaporation, the expansion, linal liquefaction.
in lb. per square foot per
Centigrade degree, . . 21416 75617 82075
Initial Volume of steam in cubic
feet per lb., . . . 8004
Latent heat of evaporation, L t , in
footpounds per lb. of steam, 707,44536
Total heat expended, in footpounds
per lb. of steam, . . 847,245*36
Mean specific heat of liquid water —
Between 40° and 140° Cent., 1398 feet of fall.
Between 100° and 140° Cent., 1409
Applying equation (80) to these data, we obtain the following results : —
Footpounds.
AreaALH, 2,818
AreaABGL, 68,601
Area HGKE= (P,  P 3 ) . (V 6  v) = 1963 lbs. per
square foot x 24'58 cubic feet, . . . = 48,250
Total power developed by 1 lb. of water evaporated, 119,669
Efficiency = ^^ =01413
J 847,245
Efficiency computed in the last article, . . . 0"21S9
Difference = loss of efficiency by incomplete expansion, 00776
V 2460
Ratio of expansion — y = —— = 307 nearly.
1 v 8004 J
If the power of the same engine be now computed by the tables and
formulae published in the twentieth volume of the Transactions of the Roijol
Society of Edinburgh (See p. 278), which were calculated on the supposition
that steam is sensibly a perfect gas, the following results are obtained : —
Ratio of expansion, " = 2921 = s in tables.
8'4204
404
ON THERMODYNAMICS.
"Action at full pressure " (PiV, in tables),
" Coefficient of gross action " (Z in tables) for the ex
pansion 2 "9 2 1 ,
Gross action (P^ Z), .
Deduct for backpressure of liquefaction, P 3 V G = 1533 4
X 246, . ■
Footpounds.
63,633
Power developed per lb. of steam, . 122,221
This result is too large by about one fortyseventh part, a difference to
be ascribed chiefly to the error of treating steam as a perfect gas. This
difference, however, is not of material consecpience in computing theoreti
cally the power of a steamengine, being less than the amount of error
usually to be expected in such calculations.
48. My object in entering thus minutely into the theory of the efficiency
of vapourengines is, not so much to provide new formulae for practical use,
as to illustrate the details of the mechanical action of heat under varied
and complicated circumstances, and to show with precision the nature and
influence of the circumstances which prevent the production, by steam
engines, of the absolute maximum of efficiency corresponding to the tem
peratures between which they work.
To illustrate the results of these calculations with respect to the con
sumption of coal, let it De assumed, as in Art. 33, that each pound of
coal consumed in the furnace communicates to the water, or air, or other
elastic substance which performs the work, an amount of heat equivalent
to 6,000,000 footpounds, which corresponds to a power of evaporating, in
Absolute Theoretical Maximum, being
the same for every perfect thermo
dynamic engine working between
the same limits of temperature,
140°  40°
L40°+2724° '
Efficiency.
Effect per pound of coal
in footpounds.
2424
1,454,400
Deductions: —
For raising the temperature of the
feedwater from 40° to 140° Cent.,
For stopping the expansive working
at 3 "07 times the initial volume
instead of 32 times, .
Reduced Efficiency and Effect,
00235
00776
oioii
141,000
465,600
606,600
01413
847, SOO
ON THERMODYNAMICS. 405
round numbers, about seven times its weight of water. Then the following
calculation shows the theoretical indicated duty of one pound of such coal,
when the limits of working temperature are 140° and 40° Centigrade, at
the absolute maximum of theoretical efficiency, and at the reduced
efficiency, computed in the preceding article, on the supposition that the
expansive working ceases at the atmospheric pressure.
The last of these quantities corresponds to a consumption of about 2*34
lbs. of coal per indicated horsepower per hour.
The conditions of the preceding investigations are very nearly fulfilled
in steamengines with valves and steampassages so large, and a velocity of
piston so moderate, that the pressure in the cylinder during the admission
of the steam is nearly the same with that in the boiler.
In many steamengines, however, the steam is more or less "wire
drawn ; " that is to say, it has to rush through the passages with a velocity,
to produce which there is required a considerable excess of pressure in the
boiler above that in the cylinder. The power developed during the
expansion of the steam from the pressure in the boiler to that in the
cylinder is not altogether lost; for, as already stated in Art. 45, it is
expended in agitating the particles of the steam, and is ultimately con
verted into heat by friction, so that the steam begins its action on the
piston in a superheated state; and both its initial pressure and its
expansive action are greater than those of steam of saturation of the same
density. The numerical relations of the temperature, pressure, and density
of superheated steam are not yet known with sufficient precision to
constitute the groundwork of a system of exact formulae representing its
action. Some general theorems, however, will be proved in the sequel,
respecting superheated vapours, which may be found useful when the
necessary experimental data have been obtained.
Calculation and experiment concur to prove that in Cornish single
acting engines the initial pressure of the steam in the cylinders is very
much less than the maximum pressure in the boilers ; generally, indeed,
less than onehalf.* It is doubtful, however, whether this arises altogether
from wiredrawing in the steampassages and valves ; for when it is con
sidered that in such engines, even at their greatest speed, the steamvalve
remains shut nearly the whole of each stroke, being opened during a small
portion of the stroke only, it may be regarded as probable that the sudden
opening of this valve causes a temporary reduction of temperature and
pressure in the boiler, itself.
49. Composite VapourEngines.
The steamandether engine of M. du Trembley is an example of what
may be called a composite vapourengine, in which two fluids are era
" See Mr. Pole's work on the Cornish Engine, and Art. 36 of a paper on the
Mechanical Action of Heat, Trans. Roy. Soc. o/Edin., Vol. XX. {Vide p. 291.)
400 ON THERMODYNAMICS.
ployed, a less and a more volatile; the heat given out during the lique
faction of the less volatile fluid serving to evaporate the more volatile
fluid, which works an auxiliary engine, and is liquefied in its turn °y
refrigeration.
Let the efficiency of the engine worked by the less volatile fluid be
expressed in the form
»*
so that is the fraction of the whole heat expended which is given out to
n
the more volatile fluid. Let the efficiency of the engine worked by the
mure volatile fluid be
iii
n
then the efficiency of the combined engines will be
1 (81.)
nn
If both the engines are perfect thermodynamic engines, let Tj be the
absolute temperature at which the first fluid is evaporated; t, that at
which it is condensed, and the second fluid evaporated ; and r 3 that at
which the second fluid is condensed ; then,
lUl 1 :'*; 11 = 1^^; l±.= l_Il=^ (81 A.)
n t 1 — k n To — k nn t x — k
bein" equal to the theoretical maximum efficiency of a simple thermo
dynamic engine working between the limits of temperature r t and r 3 .
Composite vapourengines, therefore, have the same theoretical maximum
efficiency with simple vapourengines, and other engines moved by heat,
working between the same temperatures; but they may, nevertheless,
enable the same efficiency to be obtained with smaller engines.
50. Curves of free expansion for nascent vapour.
By nascent vapour is to be understood that which is in the act of
risin^ from a mass of liquid. If this vapour be at once conducted to a
condenser, without performing any work, and there liquefied at a tempera
ture lower than that at which it was evaporated, its expansion, from the
pressure of evaporation down to the pressure of liquefaction, will take
place according to a law defined by a curve analogous, in some respects, but
not in all, to the curve of free expansion for a homogeneous substance,
referred to in Proposition VI. To determine theoretically the form of this
curve, it is necessary to know the properties of the isothermal curves and
curves of no transmission for the fluid in question in the gaseous state,
OX THERMODYNAMICS.
407
when above the temperature of saturation for its pressure. Having these
data, we can solve numerically the following problem : —
Proposition XIX. — Problem. To draw the curve of free expansion for
vapour nascent under a given pressure.
(Solution.) In Fig. 24, let AB, parallel to OX, be the isothermal line
of an aggregate of liquid and vapour at the pressure of evaporation P x
corresponding to the temperature t x : let A v v B v^ be ordinates parallel to
OY; so that v 1 is the volume of unity of weight of the liquid at this
temperature, and i\ that of unity of weight of the vapour at saturation.
Let D F be a line drawn parallel to X, at a distance representing any
lower pressure P 2 corresponding to the temperature r . It is required to
find the point where the curve of free expansion drawn from B inter
sects DF.
Let v., be the volume of unity of weight of the liquid at the lower
pressure and temperature, ?;., D an ordinate parallel to Y, and D A a
curve representing the law of expansion of the liquid as the pressure and
temperature increase. Draw the curves of no transmission D N, B L
indefinitely prolonged towards X; ascertain the indefinitelyprolonged
area LBADN; draw a curve of no transmission MC, cutting DF in C,
such that the indefinitelyprolonged area MCDN shall be equal to the
indefinitelyprolonged area LBADN; then will C be the point required
where the curve of free expansion B C intersects the line D F.
(Demonstration.) Unity of weight of the fluid being raised in the
liquid state from the temperature r 2 and corresponding pressure P 2 , to the
temperature t x and corresponding pressure P x ; then evaporated com
pletely at the latter pressure and temperature; then expanded without
performing work, until it falls to the original pressure P 2 ; then cooled at
this pressure till it returns to the original temperature r 2 , at which it is
finally liquefied ; the area A B C D represents the expansive power
408 ON THERMODYNAMICS.
developed during this cycle of operations, which, as no work is performed,
must be wholly expended in agitating the thud, and reproducing by
friction the heat consumed by the free expansion represented by the
curve B C, which heat is measured by the indefinitelyprolonged area
MCBL, which area is therefore equal to the area A B C D. Subtracting
from each of these equal areas the common area B U C, and adding to
each of the equal remainders the indefinitelyprolonged area LUDN.
we form the areas MCDN, LBADN, which are consequently equal.
Q. E. D.
51. Of the total heat of evaporation.
The symbolical expression of the preceding proposition is formed in
the following manner. The area LBADN represents the total heat of
evaporation, at the temperature r v from the temperature r 2 , and is composed
of two parts, as follows : —
LBADN
f 1 K h dr + L 11 . • (82.)
of which the first is the heat necessary to raise the liquid, whose specific
heat is K L , from t 2 to r v and the second is the latent heat of evaporation
at t v
Let v£ be the volume of unity of weight of the vapour at the pressure
P 2 and temperature of saturation t 2 ; draw the ordinate P 2 'E, meeting DF
in E, through which point draw the indefinitelyprolonged curve of no
transmission Eli: then is the area MCDN divided into two parts, as
follows : —
MCDN = MCER + EEDX= K r dr + L 2 , . (83.)
in which equation r c denotes the temperature corresponding to the point
C on the curve of free expansion, and K p the specific heat of the vapour,
at the constant pressure P 2 when above the temperature of saturation;
so that the first term represents the heat abstracted in lowering the
temperature of the vapour from r c to the temperature of saturation r 2 ,
at the constant pressure P 2 ; and the second term, the latent heat of
evaporation at r 2 abstracted during the liquefaction.
By equating the formula? (82) and (83), the following equation is
obtained : —
f 1 K L dT + L 1 L. 2 = f TC K v dr, . . (84.)
7"., T.,
ON THERMODYNAMICS. 409
which is the symbolical solution of Proposition XIX., and shows a relation
between the total heat of evaporation of a fluid, the free expansion of its
vapour, and the specific heat of that vapour at constant pressure.
52. Approximate Jaw for a vapour which is a perfect gas.
If the vapour of the fluid in question be a perfect gas, and of very
great volume as compared with the fluid in the liquid state, the curve;
BC will be nearly a hyperbola, and will nearly coincide with the
isothermal curve of the higher temperature r v to which, consequently,
r c will be nearly equal; and the following equation will be approximately
true :
I 1 K l4 dr + L X L,= j 'ly/r, . . (85.)
which, when the difference between the higher and lower temperatures
diminishes indefinitely, is reduced to the following: —
K, + ^ = K„, .... (86.)
that is to say: —
Corollary. — Theorem. When a vapour is a perfect gas, and very hilly
as compared with its liquid, the rate of increase of the total heat of evaporation
with temperature is nearly equal to the specific heat of the vapour at constant
pressure.
This was demonstrated by a different process, in a paper read to the
Royal Society of Edinburgh in 1850. It has not yet been ascertained,
however, whether any vapour at saturation approaches sufficiently near to
the condition of perfect gas to render the theorem applicable.
53. Concluding Remarks.
In conclusion, it may be observed, that the theory of the expansive
action of heat embodied in the propositions of this paper contains but one
principle of hypothetical origin — viz., Proposition XII., according to which
the actual heat present in a substance is simply proportional to its
temperature measured from a certain point of absolute cold, and multiplied
by a specific constant ; and that although existing experimental data may
not be adequate to verify this principle precisely, they are still sufficient
to prove that it is near enough to the truth for all purposes connected
with thermodynamic engines, and to afford a strong probability that it is
an exact physical law.
410 MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS.
XXL— OX FORMULAE FOR THE MAXIMUM PRESSURE
AXD LATENT HEAT OF VAPOURS.*
1. It is natural to regard the pressure which a liquid or solid and its
vapour maintain when in contact with each other and in equiUbrio, as the
result of an expansive elasticity in the vapour, balanced by an attractive
force which tends to condense it on the surface of the liquid or solid, and
which is very intense at that surface, but inappreciable at all sensible
distances from it. According to this view, every solid or liquid substance
is enveloped by an atmosphere of its own vapour, whose density close to
the surface is very great, and diminishes at first very rapidly in receding
from the surface ; but at appreciable distances from the surface is sensibly
uniform, being a function of the temperature and of the attractive force in
question.
2. Many rears since I investigated mathematically the consequences
of this supposition, and arrived at the conclusion, that although it is
impossible to deduce from it, in the existing condition of our knowledge
of the laws of molecular forces, the exact nature of the relation between
the temperature and the maximum pressure of a vapour, yet that if the
hypothesis be true, it is probable that an approximate formula for the
logarithm of that pressure for any substance will be found, by subtracting
from a constant quantity, a converging series in terms of the powers of
the reciprocal of the absolute temperature, the constant and the coefficients
of the series being determined for each substance from experimental data.
Such a formula is represented by
Log. P = A    C ,  &a,
T T
where P denotes the pressure, r the absolute temperature, that is, the
temperature as measured from the absolute zero of a perfect gasther
mometer, A the constant term, and B, C, &c, the coefficients of the
converging series.
3. On applying this formula to M. Regnault's experiments on the
pressure of steam, it was found that the first three terms were sufficient to
* Read before the British Association at Liverpool, September, 1854, and published
in the Philosophical Magazine, December, 1854.
MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS. 411
represent the results of these experiments with minute accuracy through
out their whole extent; that is to say, between the temperatures of
 30° and + 230° Centigrade
=  22° and 44G° Fahrenheit,
and between the pressures of .,.} 0() of an atmosphere, and 82 atmospheres.
Formula? of three terms were also found to represent the results of
Dr. Ure's experiments on the vapours of alcohol and ether, and formulae
of two terms those of his experiments on the vapours of turpentine and
petroleum, as closely as could be expected from the degree of precision of
the experiments. A formula of two terms was found to represent accu
rately the results of M. Eegnault's experiments on the vapour of mercury.
4. These formula?, with a comparison between their results and those of
the experiments referred to, were published in the Edinburgh New Philo
sophical Journal for July, 1849, in a paper the substance of which is
summed up at its conclusion in the following proposition (See p. 1) : —
If the maximum elasticity of any vapour in contact with its liquid be ascer
tained for three points on the scale of the airthermometer, then the constants of
an equation of the form
Lo g.P=A B ^
T T
•may be determined, which equation will give, for that vapour, with an accuracy
limited only by the errors of observation, the relation behceen the temperature (t),
measured from the absolute zero, and the maximum elasticity (P), at all
temperatures between those three points, and for a considerable range beyond
them.
5. In the case of water and mercury, the precision of the experimental
data left nothing to be desired. I have, however, in the table of constants
at the end of this paper, so far modified the coefficients for Avater and
mercury as to adapt them to a position of the absolute zero (274° Centi
grade, or 493°2 Fahrenheit below the temperature of melting ice), which
is probably nearer the truth than that employed in the original paper,
which was sixtenths of a Centigrade degree lower. This modification,
however, produces no practically appreciable alteration in the numerical
results of the formula?.
6. It was otherwise with respect to the other fluids mentioned, for
which the experimental data were deficient in precision, so that the values
of the constants could only be regarded as provisional.
7. A summary published in the Comptes Bendus for the 14th of August,
1854,* of the extensive and accurate experiments of M. Eegnault on the
* See Phil. Mag., Series 4, Vol. VIII., p. 2G9.
412 MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS.
elasticities of the vapours of ether, sulphuret of carbon, alcohol, chloroform,
and essence of turpentine, has now supplied the means of obtaining
formula?, founded on data as precise as it is at present practicable to
obtain, for the maximum pressures of these vapours.
A synopsis of these formulae, and of the constants contained in them, is
annexed to this paper. The constants, as given in the table, are suited
for millimetres of mercury as the measure of pressures, and for the scale
of the Centigrade thermometer; but logarithms are given, by adding
which to them they can be easily adapted to other scales.
The limited time which has elapsed since the publication of M. Regnault's
experiments prevents my being yet able to bring the details of the
investigation of the formula?, and of the comparison of their results with
those of experiment, into a shape suited for publication ; but I shall here
add some remarks on their degree of accuracy and the extent of their
applicability.
8. M. Regnault explains, that his experiments were made by two
methods; at low temperatures, by determining the pressure of the vapour
in vacuo; at high temperatures, by determining the boilingpoint under
the pressure of an artificial atmosphere. For each fluid the pressures
determined by both those methyl were compared throughout a certain
.series of intermediate temperatures.
For all Huids in a state of absolute purity, the results of those two
methods agreed exactly (as M. Regnault had previously shown to be the
case for water).
The presence, however, of a very minute quantity of a foreign substance
in the liquid under experiment was sufficient to make the pressure of the
vapour in vacuo considerably greater than the pressure of ebullition at
a given temperature ; and it would appear, also, that a slight degree of
impurity affects the accuracy even of the latter method of observation,
although by far the more accurate of the two when they disagree.
9. The degree of precision with which it has been found possible to
represent the results of the experiments by means of the formula?, cor
responds in a remarkable manner Avith the degree of purity in which,
according to M. Regnault, the liquid can be obtained.
Sulphuret of Carbon, M. Regnault states, can easily be obtained perfectly
pure. For this fluid, the agreement of the pressures computed by the
formula with those determined by experiment throughout the whole range
of temperature from — 16° Centigrade to + 136°, is almost as close as
in the case of steam.
Ether and Alcohol are less easy to be obtained perfectly pure. The
discrepancies between calculation and experiment in these cases, though
still small, are greater than for sulphuret of carbon.
For ether the formula may be considered as practically correct through
MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS. 413
out the whole range of the experiments, from — 20° Centigrade to
+ 116°; but for alcohol below 0° Centigrade, the discrepancies, though
absolutely small quantities, are large relatively to the entire pressures;
and the formula can be considered applicable above this temperature only.
Essence of Turpentine has been discovered by M. Eegnault to undergo a
molecular change by continued boiling. For this fluid the agreement
between the formula and the experiments is satisfactory above 40°
Centigrade, and up to the limit of the experiments, 222°, but not
below 40°.
It is impossible to obtain Chloroform free from an admixture of foreign
substances. Accordingly, M. Eegnault has found that the two methods
of determining the pressure of the vapour of this fluid give widely different
results, neither of which can be represented accurately by the formula
now proposed below the temperature of 70° Cent. From this temperature,
however, up to 130° Cent., the limit of the experiments, the agreement
is close.
10. In the cases of alcohol and turpentine, the discrepancies between
the formulae and the experiments at very low temperatures are such
as to indicate that they might be removed by introducing a fourth term
into the formulae, inversely proportional to the cube of the absolute
temperature; but the trifling and uncertain advantage to be thus obtained
would be outweighed by the inconvenience in calculation, and especially by
the necessity for solving a cubic equation in computing the temperature
from the pressure; whereas, with formula? of three terms, it is only
necessary to extract a square root, as the formula No. 2 shows.
1 1 . Although, for the mere determination of the maximum pressure of
a vapour at a given temperature, or its temperature at a given pressure,
a table, or a curve drawn on a diagram may be sufficient, still there are
many questions of thermodynamics respecting vapours for the solution
of which a formula is essential.
Amongst these is the computation of the latent heat of evaporation,
which is equivalent to the potential energy or work exerted by the
vapour in overcoming external pressure, added to that exerted in over
coming molecular attraction. For unity of iceight of a given substance,
this is a function of the pressure, temperature, and density; but for a
quantity of the substance such that its volume when evaporated exceeds
its volume in the liquid or solid state by unity of cubic space, the latent
heat of evaporation is simply the differential coefficient of the pressure
with respect to the hyperbolic logarithm of the absolute temperature, as
shown in the formula No. 3 ; so that, although the densities of the
vapours of the seven fluids referred to in this paper are yet known by
conjecture only, and not by direct experiment, we can, from the relation
between the pressure and the temperature, determine accurately how much
414 MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS.
heat must be expended in the evaporation of so much of each of them as
is necessary in order to propel a piston through a given space under a
given constant pressure, and thus to solve many problems connected with
engines driven by vapours of different kinds.
12. It is somewhat remarkable, that the coefficients of the reciprocal of
the temperature (B) in the formulae for ether, sulphuret of carbon, and
alcohol, are nearly equal; as also those of the square of the reciprocal of
the temperature (C) for ether and sulphuret of carbon.
In consequence of this, the pressure of the vapour of ether, and its
latent heat for unity of space, as above denned, at a given temperature,
exceed the corresponding quantities for sulphuret of carl ion at the same
temperature, in a ratio which is nearly, though not exactly, constant, and
whose average value is somewhat less than 1*5.
Synopsis of the Formula, &c.
Notation.
t = absolute temperature = temp. Cent. + 274 c C.
= temp. Fahr. + 4G1°2 F.
P = maximum pressure of vapour at the absolute tempera
ture T.
r — volume of unity of weight of the liquid.
V = volume of unity of weight of saturated vapour.
L = latent heat of evaporation of unity of weight of the fluid
expressed in units of work.
A, B, C, constants.
Formula.
1. To find the maximum pressure from the temperatu:c.
Com. \o r ' P = A — — ...
° T T 1
2. To find the temperature from the maximum pressi n
1 / f A  com, log. P B^ ) _ B
r ~ V t ~ C + 4 C i 2 G
MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS.
415
3. To find the latent heat of evaporation (expressed in units of work)
of so much of the fluid that its hulk when evaporated exceeds its bulk in
the liquid state by an unit of space, that is to say, of the weight ^ —
of fluid. In this formula the pressure must be expressed in units of
weight per square unit.
= r— =P( + ■:, X hyp. log. 10.
V  o
d;
(Hyp. log. 10 = 230258509,
the common logarithm of which is 03622157.)
Units of work are reduced to units of heat (degrees in unity of weight
of liquid water) by dividing by Joule's equivalent of the specific heat of
liquid water, which has the following values, according to the units of
temperature and length employed : —
Logarithms.
Centigrade scale, and metres, . . 423  54 26268969
Centigrade scale, and feet, . . 13896 3'1428898
Fahrenheit's scale, and feet, . . 772'0 28876173
Constants in the Formulae for Pressures in Millimetres of
Mercury, and Temperatuf.es in Centigrade Degrees.
Fluids.
A.
Log. B.
Log. C.
B
2 0"
B2
4C"
Ether,
7 1284
30596504
47065130
0011275
000012712
Sulphuret of carhon, .
GS990
30520049
47078426
0011044
000012197
Alcohol above 0° C, .
7 3259
30570610
52426805
0032610
00000 10634
Water,
78143
31S11430
50881S57
00061934
0000038358
Essence of turpentine I
above 40° C.,. . \
6 2522
2 9025209
53712157
00019511
0000038067
Chloroform above 70° C,
5S075
24007279
53919420
00051022
000000026032
Mercury up to 35S° C,
75243
34675637
To adapt the formula? to other scales of pressure, add the following
logarithms to the constants A : —
41 6 MAXIMUM PRESSURE AND LATENT HEAT OF VAPOURS.
For inches of mercury, . . . . . • 2 5951/
For kilogrammes on the square metre, . . . 1*1 3341
For pounds avoirdupois on the square foot, . . 0*44477
To adapt the formula' to the scale of Fahrenheit's thermometer
multiply B by 1*8, and C by (1*8) 2 = 324 ; that is to say,
Add to log. B, 02552725
Add to log. C • 05105450.
THE DENSITY OF STEAM. 417
XXIL— ON THE DENSITY OF STEAM.*
1 . The object of the present paper is to draw a comparison between the
results of the mechanical theory of heat, and those of the recent experi
ments of Messrs. Fairbairn and Tate on the density of steam, published in
the Philosophical Transactions, for 1SG0.
General Equation of Thermodynamics.
2. The equation which expresses the general law of the relations between
heat and mechanical energy in elastic substances was arrived at indepen
dently and contemporaneously by Professor Clausius and myself, having
been published by him in Poggendorff's Annalen for February, 1850, and
communicated by me to the Royal Society of Edinburgh in a paper which
was received in December, 1849, and read on the 4th of February, 1850
(See p. 23 Jf). The processes followed in the two investigations were very
different in detail, though identical in principle and in results ; Professor
Clausius having deduced the law in question from the equivalence of heat
and mechanical energy as proved experimentally by Mayer and Joule,
combined with a principle which had been previously applied to the theory
of substantial caloric by Sady Carnot, while by me the same law was
deduced from the " hypothesis of molecular vortices," otherwise called the
" centrifugal theory of elasticity."
3. Although, since the appearance of the paper to which I have referred,
the notation of the general equation of thermodynamics has been improved
and simplified in my own researches, as well as in those of others, I shall
here present it, in the first place, precisely in the form in which I first
communicated it to this Society, in order to show the connection between
that equation in its original form, and the law of the density of steam,
which has since been verified by the experiments of Messrs. Fairbairn and
Tate. The equation, then, as it originally appeared in the twentieth
volume of the Transactions of the Royal Society of Edinburgh (See p. 249)
is as follows : —
* Read before the Eoyal Society of Edinburgh on April 2S, 1S62, and published in
Vol. XXIII. of the Transactions of that Society.
2 D
418 THE DENSITY OF STEAM.
in which the symbols have the following meanings : —
r, the absolute temperature of an elastic substance as measured from
the zero of gaseous tension, a point which was then estimated to be
at 274 C G Centigrade below that of melting ice, but which is now
considered to be more nearly at 274° Centigrade, or 493 0, 2 Fahr.
below that temperature ;
k, A constant, expressing the height on the thermometric scale of the
temperature of total privation of heat above the zero of gaseous
tension. This constant was then only known to be very small ;
according to later experiments, it is either null or insensible ;
n M, The ideal or theoretical weight, in the perfectly gaseous state, of
an unit of volume of the substance, under unity of pressure, at the
temperature of melting ice ;
C, The absolute temperature of melting ice, measured from zero of
gaseous tension (that is to say, according to the best existing data
C = 274° Centigrade, or 493°2 Fahr.);
V, The actual volume of unity of weight of the substance ;
8 V, An indefinitely small increment of that volume;
<5 r, An indefinitely small increment of temperature ;
U, A certain function of the molecular forces acting in the substance ;
+ B Q', The quantity of heat which appears, or — B Q', the quantity
of heat which disappears, during the changes denoted by o V and
Bt, through the actions of molecular forces, independently of heat
employed in producing changes of temperature ; such quantity of
heat being expressed in equivalent units of mechanical energy.
The equation having been given in the above form, it is next shown
(See p. 252), that the differential coefficients of the function U have the
following values : —
 = C«M/cZV.^. . (3.)
4. The physical law of which the general equation just cited is the
symbolical expression, may be thus stated in words : — The mutual trans
THE DENSITY OF STEAM. 419
formation of heat and mechanical energy during any indefinitely small change in
the density and temperature of an elastic substance, is equal to the temperature,
reckoned from the zero of absolute cold, multiplied by the complete differential of
a certain function of the pressure, density, and temperature; which function is
either nearly or exactly equal to the rate of variation with temperature of the
work performed by indefinite expansion at a constant temperature.
5. It may be remarked that the quantity, —
cp = il hyp. log. r + ^^ (hyp. log. V  U) j
r ■ (±0
= ft hyp. log. r + / — d V
J a t J
(ft being the real specific heat of the substance in units of mechanical
energy), is what, in later investigations, I have called the " thermo
dynamic function;" and that by its use, and by making k = 0,,
equation (1) is reduced to the simplified form,
8Q' = rS0feSr; . . . (5.)
but the following notation is more convenient : Let $ h denote the
whole heat absorbed by the substance, not in units of mechanical
energy, but in ordinary thermal units, and J the value of an
ordinary thermal unit in units of mechanical energy, commonly
called "Joule's equivalent," so that
JSh = k$T SQ';
then the general equation of thermodynamics takes the form
J3/i = rS<£ (6.)
G. For the purposes of the present paper, the most convenient form of
the thermodynamic function is that given in the second line of equation
(i) ; but it may nevertheless be stated, that in a paper read to this Society
in 1855, and which now lies unpublished in their archives, it was shown
that another form of that function, via. : —
P„V„\, , fdV
fc+^)hyp.log.r/^P, . . (7.)
P V
was useful in solving certain questions ; —  ° denoting the same thing
u
with  — — in equation (1).
C n JM
420 THE DENSITY OF STEAM.
Application of the General Equation of Thermodynamics to
the Latent Heat and Density of Steam.
7. At the time when the general equation (1) was first published,
sufficient experimental data did not exist to warrant its application to the
computation of the density of a vapour from its latent heat. But very
soon afterwards, various points, which had previously been doubtful, were
settled by the experiments of Mr. Joule and Professor William Thomson ;
and in particular Mr. Joule, by his experiments, published in the Philoso
phical Transactions for 1850, finally determined the exact value of the
mechanical equivalent of a British unit of heat, to which he had been
gradually approximating since 1843, viz.: —
J = 772 footpounds;
and Messrs. Joule and Thomson in 1851, 1852, and 1853, made experiments
on the free expansion of gases, especially dry air and carbonic acid, which
established the very near, if not exact, coincidence of the true scale of
absolute temperature with that of the perfect gasthermometer ; that is to
say, those experiments proved that k in the equation (1) is sensibly = 0.
When, with a knowledge of these farts, equation (1) is applied to the
phenomenon of the evaporation of a liquid under a constant pressure, and
at a constant temperature, it takes the following form : —
Jh = r^(Yv), . . . (8.)
where
J denotes Joule's equivalent, or 772 footpounds per British unit of
heat (a degree of Fahrenheit in a pound of liquid water) ;
h, The heat which disappears during the evaporation of 1 lb. of the
liquid; that is, its latent heat of evaporation in British units;
r, The absolute temperature (= temperature on Fahrenheit's scale
+ 461°'2 Fahr.);
P, The pressure under which the evaporation takes place ;
V, The volume of 1 lb. of the vapour ;
v, The volume of 1 lb. of the liquid.
As the latent heat of evaporation of various fluids is much more
accurately known by direct experiment than the volume or density of their
vapours, the most useful form in which the equation (8) can be put, is
THE DENSITY OF STEAM. 421
that of a formula for computing the volume of a vapour from its latent
heat, viz.: —
v. + H .... (0.)
r
dr
8. Results of this formula were calculated by Messrs. Joule and Thomson, .
and by Professor Clausius for steam, and showed, as had been expected, a
greater density and less volume than the law of the perfectly gaseous
condition would give. Some results of the same kind, and leading to the
same conclusion, were also computed by me, and published in the Philoso
phical Transactions for 185354. But for some years no attempt was made
by any one to make a table for practical use of the volumes of steam from
equation (9); because the scientific world were in daily expectation of
the publication of direct experimental researches on that subject by
M. Eegnault.
9. At length, in the spring of 1855, having occasion to deliver to the
class of my predecessor, Professor Gordon, a course of lectures on the
mechanical action of heat, and finding it necessary to provide the students
with a practical table of densities of steam founded on a more trustworthy
basis than the assumption of the laws of the perfectly gaseous condition, I
ventured upon the step of preparing a table of the densities of steam for
every eighteenth degree of Fahrenheit's scale, from 86° to 410° inclusive,
with the logarithms of those densities and their differences, arranged so as
to enable the densities for intermediate temperatures to be calculated by
interpolation. Those tables were published in a lithographed abstract of
the course of lectures before mentioned, which is now out of print. The
same tables, however, have since been revised, and extended to every ninth
degree of Fahrenheit, from 32° to 428°, and have been printed at the end
of a work On Prime Movers. An account of the original tables was read
to the British Association in 1855.*
10. In the unpublished paper before mentioned as having been read
to this Society in 1854, the densities of the vapours of ether and bisulphuret
of carbon, under the pressure of one atmosphere, as computed by equation
(9), are shown to agree exactly with those calculated from the chemical
composition of those vapours.
11. The method of using equation (9) to calculate the volume of one
pound of steam, is as follows : —
I. Calculate the total heat of evaporation of steam from 32°, at a given
temperature T° on Fahrenheit's scale, by Eegnault's wellknown formula,
* The reason for using 9° Fahr. as the interval of temperature is, that it is equal to
5° Centigrade and to 4° Reaumur, so that the tables can be applied with ease to any
one of those three scales.
422 THE DENSITY OF STEAM.
10917 + 0305 (T° 32°) . . . (10.)
II. From that total heat subtract the heat required to raise 1 lb.
of water from 32° to T° Fahr., viz.,
f cc/T;
J 32'
c being the specific heat of water, the remainder will be the latent heat
of evaporation of 1 lb. of steam at T°; that is to say,
h = 10917 + 0305 (T  32°)  P cdT. . (11.)
J 32°
In computing the value of the integral in this formula, use has been
made of an empirical formula founded on M. Kegnault's experiments on
the specific heat of water, as to which, see the Transactions of this
Society for 1851, viz. : —
P c d T = T  T' + 0000000103 {(T  39°l) 3  (T  39°l) 3 } (11 A.)
III. The absolute temperature is given by the formula,
r = T + 461°2 Fahr. . . . (12.)
IV The value of r  is deduced from the following formula, first
a t
published in the Edinburgh Philosophical Journal for July, 1849 (See p. 1) :—
Com. log. P = A »; . . ■ (13.)
r t
from which it follows that
/' P = 2302GP( B + ^); . . (14.)
dr t V t /
the values of the constants for steam being, —
A, for pounds of pressure on the square foot, . 8'2591
log. B for Fahrenheit's scale . . . =343642
log. C „ „ . . . =559873.
V. The volume of a pound of liquid water at the temperature T may
be computed with sufficient accuracy for the present purpose by the
following formula : —
v nearly = 000801 Q^ + ^). . . (15.)
VI. These preliminary calculations having been made, the formula 9
can now be applied to the calculation of the volume of one pound of
THE DENSITY OF STEAM.
423
steam (making J = 772); and by this process the tables already mentioned
were computed.
Comparison of the Eesults of Theory with those of Messrs.
Fairbairn and Tate's Experiments.
12. The experiments of Messrs. Fairbairn and Tate on the density of
steam are described in a paper which was read to the Eoyal Society of
London, as the Bakerian Lecture, on the 10th of May, 1860, and published
in the Philosophical Transactions for ^hat year. The results of those
experiments give what is called the " relative volume " of steam : that is,
the ratio which its volume bears to that of an equal weight of water at
the temperature of greatest density, 39°  l Fahr.; but in the following
table of comparison, each of those relative volumes is divided by 62"425,
the weight of a cubic foot of water at 39 0, 1 in lbs., so as to give the
volume of 1 lb. of steam in cubic feet. The numbers of the experiments
are the same as in the original paper; those made at temperatures below
212° being numbered from 1 to 9, and those made at temperatures above
212° from 1' to 14'.
Comparison of the Theory with the Experiments of Messrs.
Fairbairn and Tate.
Volume of One Pound of Steam
in Cubic Feet
Difference
Number of
Experiment.
Temperature
Fahrenheit.
Difference.
Exper. Vol.
By Theory.
By Exper.
1
13677
18220
13260
0 40
i
2
155 33
8510
8544
034
i
3
15936
7764
7886
 1'22
_ 1
6 5
4
17092
6016
59 62
+ 054
+ TT7
5
17148
5943
5951
008
1
~TTT
6
174 92
5518
55 07
+ 011
+ T5T
7
1S230
4728
4887
159
_ 1
S
18830
4181
42 03
0 22
i
T5T
9
19878
33 94
34 43
049
1
?T7
1'
242 90
1561
15 23
+ 012
+ Tf^
2'
24482
1477
14 55
+ 022
+ CTT
3'
24522
1467
1430
+ 037
+ &
4'
25550
1239
1217
+ 022
+ eV
5'
26314
1096
1040
+ 056
+ tV
6'
267 21
1029
1018
+ 011
+ A
7
26920
9 977
9703
+ 0274
+ A
8'
274 76
9158
9361
0 203
i
9'
27330
9 367
8702
+ 0665
+ A
10'
27942
8539
8249
+ 0290
+ Tg
11'
2S258
8145
7964
+ 0181
+ A
12'
287*25
7603
7 340
+ 0263
+ T5
13'
29253
7 041
693S
+ 0103
+ BIT
14'
28825
7494
7201
+ 0293
+ 3fV
424 THE DENSITY OF STEAM.
Remarks on the Differences between the Theoretical and
Experimental Results.
13. The differences between the theory and the experiments as to the
volumes of steam at temperatures below 212° are, with few exceptions,
of very small relative amount; and they are at the same time so irregular
as to show that they must have mainly arisen from causes foreign to the
data used in the theoretical computations.
11. Above 212° also, the differences show irregularity, especially in the
case of experiments 8' and 9', where a fall of temperature is accompanied
by a diminution instead of an increase in the volume of one pound of
saturated steam, as determined by experiment. But still those differences,
presenting as they do, in every case but one, an excess of the theoretical
above the experimental volume, show that some permanent cause of
discrepancy must have been at work ; although they may not be regular
enough to determine its nature and amount, nor large enough to constitute
errors of importance in practical calculations relating to steamengines.
15. So far as it is possible to represent those differences by anything
like a formula, they agree, in a rough way, with a constant excess of about
021: of a cubic foot in the theoretical volume of one pound of steam
above the experimental volume; and this also represents, in a rough way,
the difference between the curves whose ordinates express respectively
the results of the theoretical formula and those of an empirical formula
deduced from the experiments, so far as those curves, as shown in a plate
annexed to the paper referred to (See Plate II.), extend through the limits
of actual experiment on steam, above 212°.
1G. As the principles of the mechanical theory of heat may now be
considered to be established beyond question, it is only in the data of the
formula that we can look for causes of error in the theoretical calculation.
I shall now consider those data, with reference to the probability of their
containing numerical errors.
I. Total Heat of Evaporation. — It is very improbable that this quantity,
as computed by M. Regnault's formula, involves any material error.
II. Sensible Heat of the Liquid Water. — The empirical formula from
which this quantity is computed was determined from experiments by
M. Regnault which agree extremely well amongst themselves. (For the
investigation of the formula, see Trans. Boy. Soc. Edin., Vol. XX., p. 441).
The subtraction of the sensible heat from the total heat leaves the latent
heat, upon which the increase of volume depends; hence, to account for
an error in excess of the formula for the volume by means of an error in
THE DENSITY OF STEAM. 425
the computation of the sensible heat, it must be supposed that the specific
heat of liquid water above 212° increases much more rapidly than M.
Eegnault's experiments show, so as to produce a correspondingly more
rapid diminution in the latent heat of evaporation. It is easily computed,
for example, that to account for an error in excess of 0*24 of a cubic foot
in the volume of one pound of steam at 266°, by an error in defect in
the sensible heat, we must suppose that error to amount to about 2 1
British thermal units per pound of water ; but such an error is altogether
improbable.
III. Absolute Temperature. — The position of the absolute zero may be
considered as established with a degree of accuracy which leaves no room
for any error sufficient to account for the differences now in question.
d P
IV. Function r . — The same may unquestionably be said of this
(It
function ; which represents the mechanical equivalent of the latent heat of
evaporation of so much water as fills one cubic foot more in the vaporous
than in the liquid state.
V. The volume of one pound of the liquid water is itself too small to
affect the question.
VI. The received value of the mechanical equivalent of a unit of heat
cannot err by so much as ^ part of its amount.
Conclusions.
1 7. It appears, then, that none of the data from which the theoretical
calculations are made are liable to errors of a magnitude sufficient to
account for the differences between the results of those calculations and
the results of Messrs. Fairbaim and Tate's experiments, small as those
differences are in a practical point of view. Neither does there appear
to have been any cause of error in the mode of making the experiments.
There remains only to account for those differences, the supposition that
there was some small difference of molecular condition in the steam whose
density was measured in the experiments of Messrs Fairbairn and Tate,
above 212°, and the steam whose total heat of evaporation, as measured
by M. Eegnault, is the most important of the data of the theoretical
formula, — a difference of such a nature as to make a given weight of
steam in Messrs. Fairbairn and Tate's experiments occupy somewhat less
space, and therefore require somewhat less heat for its production, than
42G THE DENSITY OF STEAM.
the same weight of steam in M. Regnault's experiments at the same
temperature. That difference in molecular condition, of what nature
soever it may have been, was in all probability connected with the fact,
that in the experiments of Messrs. Fairbairn and Tate the steam was at
rest, whereas in those of M. Eegnault it was in rapid motion from the
boiler towards the condenser. It is obvious, however, that in order to
arrive at a definite conclusion on this subject, further experimental
researches are required.
THE SECOND LAW OF THERMODYNAMICS. 427
XXIII— ON THE SECOND LAW OF THERMODYNAMICS.*
1. It has long been established that all the known relations between heat
and mechanical energy are summed up in two laws, called respectively
the first law and the second law of thermodynamics : viz. —
First Law. — Quantities of heat and of mechanical energy are con
vertible at the rate very nearly of 772 footpounds to the British (or
Fahrenheit avoirdupois) unit, or 424 kilogrammetres to the French (or
Centigrademetrical) unit of heat.
Second Law. — The quantity of energy which is converted from one
of those forms to the other during a given change of dimensions and
condition in a given body, is the product of the absolute temperature into
a function of that change, and of the kind and condition of the matter of
the body.
By absolute temperature is here to be understood temperature measured
according to a scale so graduated that the temperature of a homogeneous
body shall vary in the simple proportion of the quantity of energy it
possesses in the form of sensible or thermometric heat.
2. The laws of thermodynamics, as here stated, are simply the
condensed expression of the facts of experiment. But they are also
capable of being viewed as the consequence of the supposition, that the
condition of bodies which accompanies the phenomena of sensible heat
consists in some kind of motion amongst their particles.
3. The first law would obviously follow from the supposition of any
kind of molecular motion whatsoever, and it therefore affords of itself no
reason for preferring one supposition as to the kind of molecular motion
which constitutes sensible heat to another.
4. But if there be molecular motions in bodies, it is certain that,
although all such motions are capable of conversion into that which
constitutes sensible heat, some of them are not accompanied by sensible
heat. For example, the motion (supposed to be vibratory and wavelike)
which constitutes radiance, whether visible or invisible, is not accompanied
by sensible heat, and only produces sensible heat by its absorption; that
* Read before the British Association at Birmingham, September, 1S65, and
published in the Philosophical Magazine, October, 1805.
428 THE SECOND LAW OF THERMODYNAMICS.
is, in the language of hypothesis, by its conversion into some other hind
of motion; while, on the other hand, in the production of radiance
sensible heat disappears.
5. The object of the present paper is to give an elementary proof of
the proposition, that the second law of thermodynamics folloivs from the
supposition that sensible hint consists in any land of steady molecular motion
villi in limited spaces.
G. The term " steady motion " is here used in the same sense as in
hydrodynamics, to denote motion, whether of a continuous fluid or of a
system of detached molecules, in which the velocity and direction of
motion of a particle depend on its position only; so that each particle of
the series of particles which successively pass through a given position
assumes in its turn the velocity and direction proper to that position.
In other words, steady motion may be denned as motion in a set of
streams of invariable figure.
"When steady motion takes place in matter that is confined within a
limited space, the streams in which the particles move must necessarily
return into themselves, and be circulating streams, being in that respect
of the nature of whirls, eddies, or vortices.
7. Steady motion keeps unaltered the distribution of the density of
the moving matter; and it therefore keeps unaltered the forces depending
on such distribution, whether of the nature of pressure or of attraction.
In that respect it differs from unsteady motion, such as vibratory and
wavelike motion.
8. Conceive a limited space of any figure whatsoever to be filled with
matter in a state of steady motion. The actual energy of any particle of
that matter is the product of its mass into the halfsquare of its velocity;
and the actual energy of the whole mass of matter is the sum of all those
products; and because of the steadiness of the motion, the actual energy
of the particle which at any instant whatsoever occupies a given position
is some definite fraction of the whole actual energy, depending upon that
position, and upon the distribution of matter within the space; but the
scale of absolute temperature is defined as being so graduated that the
whole actual energy of the matter within the space is the product of
the absolute temperature, the mass of matter, and some function of the
sort and distribution of the matter ; therefore, the halfsquare of the velocity
of the particle which at any instant occupies a given position in the space con
sidered, is equal to the absolute temperature multiplied by some function of that
position, and of the sort and distribution of the matter.
9. Suppose now that the dimensions of the limited space in which the
moving matter is enclosed, and the distribution of that matter, undergo
an indefinitely small change by the application of suitable forces, and
that after that process the motion becomes steady as it was before.
THE SECOND LAW OF THERMODYNAMICS. 429
Then the dimensions and position of each circulating stream will have
been altered; and the work done in effecting that alteration will consist
of energy converted between the forms of potential energy of the applied
forces, and actual energy of the molecular motions — that is, between the
forms of mechanical energy and of heat. Consider now a point in one
of the circulating streams before the^ change, and let fall from it a
perpendicular upon the same stream after the change. The work done
in shifting the path of the particle which at any instant occupies that
point, is the product of the perpendicular displacement of the stream
into the force exerted along that perpendicular. But the perpendicular
displacement of the stream is a function of the position of the point
shifted, the distribution of matter in the space, and the change of
dimensions and distribution; and the force is equal and opposite either
to the centrifugal force of the particle or to one of its components, and
is therefore proportional to the square of the velocity of the particle, and
to some function of its position, and of the sort and distribution of
matter in the body. Therefore, the energy transformed in shifting the path
of any particle is proportional to the square of its velocity, and to some function
of its position, of the sort and distribution of matter in the space considered, and
of the change in dimensions of that space and in the distribution of the matter.
10. But the square of the velocity of the particle which at any instant
occupies a given position has already been shown to be proportional to
the absolute temperature, and to some function of that position and of
the sort and distribution of the matter ; therefore, if sensible heat consists
in any kind of steady molecular motion within limited spaces, the conversion of
energy during any change in the dimensions of such spaces, and in the distri
bution of matter in them, is the product of the absolute temperature into some
function of that change and of the sort and distribution of the matter.
11. In a paper "On the Mechanical Action of Heat," published in the
Transactions of the Royal Society of Edinburgh for 1850 (See p. %3Jj), the
author deduced the second law of thermodynamics, in the form above
stated, from the hypothesis of a particular sort of steady molecular motion
viz., revolution in circular streams or vortices. In a paper " On the
Centrifugal Theory of Elasticity," published in the same Transactions for
1851 (Seep. 4.9), he deduced the same law from the hypothesis of steady
molecular motion in circulating streams of any figure whatsoever, being a
proposition substantially identical with that set forth in the present paper;
but as the demonstration in the paper of 1851 involved tedious and intri
cate symbolical processes, he has written the present paper in order to
show that the demonstration can be effected very simply.
12. It is obvious that the steadiness of the supposed molecular motions
is the essential condition which makes the second law of thermodynamics
deducible from a mechanical hypothesis; and that no kind of unsteady
430 THE SECOND LAW OF THERMODYNAMICS.
motion, such as vibratory or wavelike motion, would lead to the same
results. If, then, it be admitted as probable, that the phenomena of heat
are due to unseen molecular motions, it must also be admitted, that
while the motions which constitute radiance may be vibratory and wave
like, the motions which constitute sensible or thermometric heat must be
steady and like those of circulating streams.
13. The function by which the absolute temperature is multiplied in
calculating the conversion of energy between the mechanical and the
thermic forms, is the variation of what the author has called the mcta
morphic function* being one term of the thermodynamic function,} which
corresponds to what Professor Clausius calls cntrop'tv.%
A P P E X D I X.
The following is the symbolical expression uf the demonstration given
in the paper.
Let m stand for the specific properties of the sort of matter which is
in a state of steady motion within a limited space ;
f for the figures and dimensions of that space, and of the paths
described by the particles contained in it ; and 8/ for any indefinitely
small change of such figures and dimensions ;
p for the position, relatively to the centre of the matter contained in
the space, of a point which is fixed so long as S/ = 0. Because the
motion is steady, each particle of matter which successively arrives at the
point p assumes the velocity, direction, and curvature of motion proper
to that point. Let v be that velocity, and r the radius of that curvature ;
then for a particle of mass unity, in the act of traversing p,
•>
Actual energy of mass 1 = — = kr, . . (1.)
where r is a quantity upon whose uniformity throughout the space the
steadiness of the motion depends, and k a function of (m, f, p) ; and
r 2 2 k T
Centrifugal force of mass 1 = = — — ; . . (2.)
r r v
2 Tt
in which r, and consequently — , are functions of (m, f, p).
* See " On the Science of Energetics," page 209.
+ Philosophical Transactions, 1S54.
X Ueber verschiedene fur die Anwendung beqneme Formen der Hauptghkhungen der
mechanischen Warmetheorie, April, 1865.
THE SECOND LAW OF THERMODYNAMICS. 431
Now, let the change denoted by 8/ take place, and let the steadiness of
motion be restored : let 8 n be the length of a line drawn through the
original position of the point p, so as to be perpendicular to the path of
A
the stream of particles which formerly traversed p; and let rn be the
angle made by 8 n with r. Then 8 n and r n are both functions of
(to, f, p, bf). Also, the work done, or energy converted, for a unit of
mass at the point p, while the path of the particles that traverse p is
shifted through 8 n, is as follows : —
A
v 2 ~ A 2 k t. 8 n. cos rn »« ,,»
. c n. cos rn = = t X function of (m,f,p, bf). (3.)
The energy converted during the change Sf, throughout the whole space
considered, is the sum of the quantities of energy converted for each unit
of mass within the space. But r by definition is uniform ; and the sum
of a set of functions of p is a function of / and m ; therefore, the whole
energy converted is
A
_ 2 h. Sn. cos r n . ,, , <* fX ,.,
r.2. = X functions of (?'/?,/, bj); . (4.)
r
and because bf is indefinitely small, the preceding expression is equivalent
to the following :
Energy converted = r . function (m,f) .Bf= t . 8F (m,f). (5.)
Let t be called absolute temperature, and this is the second law of thermo
dynamics. It is to be observed that / may be, and often is, a function of r.
432 THE SECOND LAW OF THERMODYNAMICS.
XXIV.— OX THE WANT OF POPULAR ILLUSTRATIONS OF
THE SECOND LAW OF THERMODYNAMICS *
1. The science of thermodynamics is based on two laws, the first of
which states the fact of the mutual convertibility of heat and mechanical
energy, while the second shows to what extent the mutual conversion of
those two forms of energy takes place under given circumstances. In the
course of the last few years the first law has been completely "popularised;"
it has been amply explained in books and lectures,! composed in a clear
and captivating style, and illustrated by examples at once familiar and
interesting, so as to make it easily understood by those who do not make
science a professional pursuit.
2. The second law, on the other hand, although it is not less important
than the first, and although it has been recognised as a scientific principle
for nearly as long a time, has been much neglected by the authors of
I id] mlar (as distinguished from elementary) works;! and the consequence
is that most of those who depend altogether on such works for their
scientific information remain in ignorance, not only of the . second law,
but of the fact that there is a second law ; and knowing the first law
only, imagine that they know the whole principles of thermodynamics.
The latter is the worst evil of the two : " a little learning " is not " a
dangerous thing " in itself, but becomes so when its possessor is ignorant
of its littleness.
3. In the present paper I do not pretend to supply that want, but
rather to point out its existence to authors who possess the faculty of
popularising ; in order that they, by means of lectures, writings, and
lectureroom experiments, may convey a general knowledge of the nature
and results of the second law of thermodynamics to those who feel an
interest in science without making it a regular study.
4. Before considering how the second law can best be stated and
explained, it may be well to point out how far it is possible to proceed
* From The Engineer of June 28, 1867.
f Such, for example, as Dr. Tyndall's Heat Considered as a Mode of Motion.
X The explanation of the second law of thermodynamics in Dr. Balfour Stewart's
excellent treatise on heat is elementary ; but it is not, nor does it profess to be, popular.
THE SECOND LAW OF THERMODYNAMICS. 433
with the solution of questions in thermodynamics by means of the first
law alone, without the aid of the second law. The first law informs us
that when mechanical work is done by means of heat, a quantity of heat
disappears, bearing a constant ratio to the quantity of work done — viz.,
that of one British unit of heat (or one degree of Fahrenheit in a pound
of water) for every 722 footpounds of mechanical work done. In order,
therefore, to calculate how much heat will disappear during a given
change in the dimensions of a substance under the action of given forces,
it is necessary to know the quantity of work done during such change ;
and the cases in which the expenditure of heat can be calculated by
means of the first law alone, are those and those only, in which the work
done can be directly measured ; that is to say, in which the work
is sensibly altogether external, or done against forces exerted between the
body under consideration and other bodies, and in which no part or no
sensible part of the work is internal, or done against forces exerted upon
each other by the particles of the body, and therefore incapable of direct
measurement.
5. The only phenomenon which fulfils that condition is the expansion
of a perfect or sensibly perfect gas ; that is, of a substance in a condition
such that its pressure at a given temperature is proportional, or sensibly
proportional, to its density simply. To illustrate this by an example : let
us suppose one pound of atmospheric air (which, though not absolutely
a perfect gas, may be treated as such for practical purposes) at the
temperature of melting ice (32° Fahr.), to be contained in a cylinder
whose sectional area is equal to one square foot, being confined in that
cylinder by means of a piston loaded Avith a pressure amounting to
4233 pounds. Then, from the experiments of Regnault on the pressure
and density of air, it is known that the length of the cylindrical space
occupied by the air is 6*1 93 ft.
Next, let the load on the piston be gradually diminished until it is
reduced to 2116 lbs., being onehalf of its original amount, and let the
question to be solved be — what quantity of heat must be communicated to the
air, in order that its temperature may remain constant during the expansion
which accompanies the diminution of pressure ? The solution is as follows: —
As the temperature is constant, and the air is treated as perfectly gaseous,
the product of the pressure on the piston into the volume of the air • that
is to say, 4233 x 6193 = 26215, remains constant during the expansion;
and the external work done in driving the piston against the gradually
diminishing load is found by multiplying that product by the hyperbolic
logarithm of 2, the rate of expansion, that is to say —
Work done = 26215 x '69315
= 18171 footpounds.
2 E
434 THE SECOND LAW OF THERMODYNAMICS.
That is the external work, and the internal work is practically inappreci
able ; therefore, that also is the mechanical equivalent of the quantity of
heat required in order to keep the temperature of the air constant; and
dividing 18171 by 772, that quantity of heat in ordinary British thermal
units is found to be 23*54 j that is, as much heat as would raise the
temperature of 23'54 lbs. of water one degree of Fahrenheit; and such is
the value of the latent heat of expansion of 1 lb. of air in doubling its
volume at the constant temperature of 32° Fain. Here, then, the solution
has been obtained by means of the first law of thermodynamics only,
without the aid of the second.
6. It is otherwise Avhen the external work is accompanied by the
performance of internal work to a practically important extent; in other
words, when we deal with substances which cannot be treated as perfectly
gaseous, such as fluids in the act of evaporating. For example, let
there be, as before, a cylindrical vessel, whose sectional area is one square
foot, and let it contain 1 lb. of water in the liquid state, at the temperature
of 212° Fahr., which will occupy a length of the cylinder equal to "017
of a foot. Let the water be confined by means of a piston, the load upon
which, in order to be just sufficient to confine the water, must be equal to
the mean atmospheric pressure on a square foot, that is to say, 211G3 lbs.
If additional heat be now communicated to the water, without altering the
load on the pistan, it is well known that its temperature does not rise, but
that it passes by degrees into the state of steam, driving the piston before
it. By the time the water has entirely assumed the state of steam, it
occupies 2G"3G ft. in length of the cylinder; so that the piston has been
driven through 26*343 ft. against the constant load of 21 16*3 lbs. Let
the question proposed be, to calculate from those data the expenditure of
heat. The external work has the following value : —
211G3 x 2G343 = 55750 footpounds,
and this is equivalent to 7 2" 2 units of heat nearly. But besides the
external work done in driving the piston, there is internal work done in
overcoming the cohesion of the particles of water ; and that internal work
is incapable of direct measurement.
7. Here it is that the second law of thermodynamics becomes useful ;
for it informs us how to deduce the whole amount of work done — internal
and external — from the knowledge which Ave have of the external work.
That law is capable of being stated in a variety of forms, expressed in
different words, although virtually equivalent to each other. The most
convenient form for the present purpose appears to be the following: —
To find the whole work, internal and. external, multiply the absolute tempera
ture at which the change of dimensions takes place, oy the rate per degree at
which the external work is varied ly a small variation of the temperature.
THE SECOND LAW OF THERMODYNAMICS. 435
By absolute temperature is meant temperature measured from the absolute
zero, or point of total privation of heat, which is known by theoretical
deduction from experimental data to he about 461 0, 2 below the ordinary
zero of Fahrenheit's scale.
8. To apply the second law to the present problem, suppose the tempera
ture at which the given increase of volume (viz., 26  343 cubic feet) takes
place, to be lowered by one degree of Fahrenheit. Then, from tables and
formulas founded on Regnault's experiments, we know that the pressure
of the steam is diminished by 42*05 lbs. on the square foot. Hence, the
external work is diminished by reduction of temperature at the following
rate : —
F = 42"05 x 26'343 = 1 107'7 footpounds per degree of Fahrenheit.
The absolute temperature is
t = 212° x 461°2 = G73°2 Fahr.;
and, therefore, the whole work, internal and external, done during the
evaporation of 1 lb. of water at the temperature of 212° Fahr. is,
t F = 673°2 x 11077 = 745,800 footpounds very nearly.
To reduce this quantity to British thermal units, divide by 772; the
result is
745,800 r 772 = 966,
being the latent heat of evaporation of steam at 212° Fahr.
The total work just calculated is made up as follows : —
Footpounds.
External Avork, computed in Article 6, . . 55,750
Internal work, 690,050
Total work, 745,800
from which it appears that the external work done in evaporating water
under the mean atmospheric pressure is less than *l\ per cent, of the
whole work; the remainder, or 92^ per cent., being internal work.
9. The second law may also be applied to solve the inverse problem,
of deducing from the expenditure of heat in a given process, and from
the relations between pressure and temperature, the change of dimensions
with which that process is accompanied; and such has been the use
chiefly made of that law in the actual history of thermodynamics.
Previous to the publication, in September, 1859, of the experiments of
43G THE SECOND LAW OF THERMODYNAMICS.
Messrs. Fairbairn and Tate, there did not exist any accurate determinations
of the density of saturated steam at different temperatures ; and, therefore,
some of the writers on thermodynamics found it necessary to calculate
that density theoretically, by the help of the second law, from the latent
heat of evaporation, which was very accurately known through the experi
ments of Regnault. The following is an example of such calculations : —
Latent heat of evaporation of 1 lb. of water at 212° Fahr.,
as known by experiment, in ordinary British thermal units, 9GG
Applying the first law, that is, multiplying by Joule's
equivalent, . . . . . . . . . 772
The value of that latent heat in footpounds of work is
found to be ........ 7 1 "» 752
Dividing that quantity of work by the absolute temperature,
G73°  2 Fahr., the work per degree of absolute temperature
is found to be, in footpounds, . . . . .1107*7
But the variation of pressure per degree, in pounds on the
square foot, is ....... . 42'0r>
Therefore, the increase of volume of 1 lb. of water in the act of evapo
rating, at 212° Fahr., is
11077 = 4205 = 26343 cubic feet ;
To which, adding the volume of the water in the
liquid state .......  017 „
There is found the volume of 1 lb. of atmospheric
steam, 26360 cubic feet.
10. The example by which the second law of thermodynamics has
here been illustrated, has been puposely chosen of a very simple kind ■
but that law enables the most complex questions respecting the expenditure
of heat required to produce a given mechanical effect to be solved ; and
the solution is always effected in the same manner : that is to say, by
deducing the total work, internal and external, from the manner in which
a small variation of temperature affects the external work.
11. The law of the efficiency of a perfect heat engine, may be stated thus :
If the substance — for example, air or water — which does the work in a
perfect heat engine receives all the heat expended at one fixed temperature,
and gives out all the heat which remains unconverted into work at a
THE SECOND LAW OF THERMODYNAMICS. 437
lower fixed temperature, the fraction of the whole heat expended which
is converted into external work is expressed by dividing the difference
between those temperatures by the higher of them, reckoned from the
absolute zero. Now this is, in fact, the second law of thermodynamics
expressed in other words ; but whether the demonstration of that fact,
that is, of the substantial identity of the second law, as stated in Article 7,
with the law stated in the preceding sentence, is capable of being put
in a popular form is doubtful, seeing that it involves the notion of limiting
ratios. The applications, however, of the law of the efficiency of a
perfect heat engine are very simple and easy. For example, it informs
us that if the steam which drives an engine receives all the heat expended
upon it at temperatures not exceeding 248° Fahr. (corresponding to the
absolute temperature 248° + 461°2 = 709°2), and if all the heat not
converted into external work is given out by that steam at temperatures
not below 104° Fahr. (being an ordinary temperature of condensation),
the efficiency of that engine, being the fraction of the whole heat expended
that is converted into external work, cannot possibly exceed the following
value —
248  104 144
248 + 4612 7092
•203.
The same law informs us that in order that the whole heat expended in a
heat engine may be converted into external work, it is necessary that the
temperature of the condenser or refrigerator should be the absolute zero —
a temperature unattainable by human means. Thus, a knowledge of the
second law of thermodynamics, as applied to the efficiency of heat engines,
is a safeguard against the formation of projects for increasing the perfor
mance of such engines beyond the highest possible limits.
1 2. There seems, then, to be no difficulty in explaining and illustrating,
in a popular way, the applications of the second law to various scientific
and practical questions, and the agreement of its results with those of
experiment, which agreement is the real proof of its being true. But
it appears by no means so easy to demonstrate popularly the connection
between the second law of thermodynamics with the idea of " heat as a
mode of motion." That connection consists in the fact that the second
law of thermodynamics necessarily follows from the established laws of
dynamics when they are applied to the supposition that the sort of motion
which constitutes heat is a whirling or circulating motion of the jwticles of
bodies— & supposition otherwise called the " hypothesis of molecular rortkes."
The original demonstrations of that fact, which appeared in February,
1850, and December, 1851 {Seep. 49), involve algebraical processes that
are quite beyond the range of a popular explanation ; and to understand
even the elementary proof without algebra, which was read to the British
438 THE SECOND LAW OF THERMODYNAMICS.
Association in 18G5 (See p. 427), requires the habit of scientific reasoning.
It is much to be wished that some means could be devised for making that
proposition as widely understood as the first law of thermodynamics
now is; for as matters now stand, the popular knowledge of thermodynamics
is, as regards the second law, eighteen years behind the actual state of
that science.
THE SECOND LAW OF THERMODYNAMICS. 43*)
XXV.— EXAMPLES OF THE APPLICATION OF THE SECOND LAW
OF THERMODYNAMICS TO A PERFECT STEAM
ENGINE AND A PERFECT AIRENGINE*
1 . The following examples are intended to illustrate the application of the
second law of thermodynamics to a perfect steamengine and a perfect air
engine, expending the same quantity of heat, and working between the
same limits of temperature, viz. : — Quantity of heat expended per stroke,
reduced to an equivalent quantity of mechanical work, in footpounds,
68420.
Degrees of Fahrenheit.
Temperatures. Ordinary scale. Absolute scale.
Upper limit, . . . .266° 727 0, 2
Lower limit, .... 104° 565°'2
Difference, or range, . . 1G2° 162 O
2. The phrase " perfect engine " is here used to denote an engine which
realises the greatest quantity of mechanical work possible with the given
expenditure of heat and between the given limits of temperature, being
the limit towards which actual engines may be made to approach through
the progress of practical improvements. Such an engine must fulfil the
following conditions : — There must be no waste of heat through conduction
or radiation, or of work through friction; and the whole heat expended
must be communicated to the working substance at the higher limit of
temperature. In other words, the elevation of the temperature of the
working substance from the lower to the higher limit must be effected
without any expenditure of heat ; for whatever heat is expended in pro
ducing elevation of temperature, is either wholly or partially lost as regards
the performance of mechanical work.
3. There are two ways of effecting the elevation of temperature without
expenditure of heat. One is to raise the temperature by compression of
the working substance in a nonconducting cylinder ; the mechanical work
necessary for that purpose being obtained by means of the expansion of
the working substance in a nonconducting cylinder while its temperature
is falling back from the higher to the lower limit. The other way is by
conduction, viz. : — to make the working substance, while its temperature
is falling, communicate its heat to a set of metal tubes, or to a network,
* From The Engineer of August 2, 1S67
440
THE SECOND LAW OF THERMODYNAMICS.
called a " regenerator " or " economiser," from which the heat is given out
again at the proper time to the working substance in order to raise its
k, temperature. Supposing those two
processes to be carried out in a
theoretically perfect manner, the re
sults as regards the economy of heat
are exactly the same.* In the follow
ing examples the steamengine will
be supposed to act by compression,
and the airengine by the aid of a
perfect regenerator.
4. According to the second law of
thermodynamics, the efficiency of each
of those engines is the same, viz. : —
162
727"2
= 02228;
and the work realised per
also the same, viz. : —
stroke is
68420x02228 = 15244 footpounds.
The object of the following calcula
tions is to show in detail what
becomes of the difference between
the whole energy expended in the
form of heat and that obtained in the
form of mechanical work, viz. ; —
G8420— 15244 = 531 7G footpounds.
1 ieing the rejected or necessarily lost
energy.
5. The diagrams of both engines
are represented in the figure. Abso
lute pressures are supposed to be re
presented in pounds on the square
foot by ordinates measured parallel
to OP; volumes occupied by the
working substance, in cubic feet, by
distances measured parallel to V.
• The fact that those results are the same is illustrated in the case of airengines by
numerical examples, which may be found in A Manual of the SteamEiufine and other
Prime Movers, pages 347 to 369. In practice the regenerator answers best for an air
engine, because of the very large space required for the other process.
THE SECOND LAW OF THERMODYNAMICS. 441
The diagram of the steamengine is marked by full lines and capital
letters ; that of the airengine by dotted lines and italic letters. In the
case of the steamengine the whole work done by the steam in driving the
piston is represented by the area FCDE; the work expended in com
pressing part of the steam and feeding the boiler by the area FBAE;
and the indicated work by the area BCDA.
In the case of the airengine the whole work done by the air in driving
the piston is represented by the area, f bed; the work expended in raising
the pressure of the air and feeding the engine by the area fade ; and the
indicated work by the area b e d a.
6. To begin in detail with the steamengine. From the limits of
temperature given in Article 1, it is found, by means of the proper table
or formula, that the limits of pressure are as follows : —
Lb. on the Lb. on the
Sq. In. Sq. Ft.
Absolute pressure of admission, F, 39  25 5652
Absolute back pressure, E, . . 106 152  G
7. The volume of water in the liquid state which is used per stroke is
represented by F B, a distance too small to be seen in the diagram ; the
volume of the same water when in the state of steam at the higher limit
of pressure by F C ; and, consequently, the increase of volume of the water
in the act of evaporating by B C. That increase of volume is produced
by communicating to the water the whole heat expended. Now 68420
footpounds is the mechanical value of the latent heat of evaporation,
under the higher of the given pressures, of so much Avater as fills one cubic
foot more in the state of steam than it does in the liquid state ; * so that
B C represents one cubic foot. The steam then expands in a nonconduct
ing cylinder, without receiving or giving out heat, until its pressure and
temperature fall to their lower limits. C D represents the expansion
curve, and E D the volume occupied at the end of the expansion, partly
by steam and partly by a small quantity of water which spontaneously
liquefies during the expansion. Part of the steam represented in volume
by D A, is then condensed by conduction of heat, at the lower limit of
temperature ; and a volume of steam represented by A E, less the volume
of the water in the liquid state, is left uncondensed, in order that by its
compression into the liquid state heat enough may be produced to raise
* Formula for that latent heat, in units of work : —
■where p is the absolute pressure and t the absolute temperature.
442 THE SECOND LAW OF THERMODYNAMICS.
the temperature of the feedwater to the higher limit. The curve of com
pression, showing how the pressure increases as the volume is diminished,
is represented by A B, and this completes the double stroke or revolution
of the engine. The following are the successive volumes occupied by the
steam at different periods, neglecting the volume of the liquid water, as
being so small compared with that of the steam that it is unnecessary to
take it into account for the present purpose.*
Cubic Feet.
Volume on admission, F C, taken as sensibly equal to
BC, 100
Volume at end of expansion, E D, .... 24*68
Volume at beginning of compression, E A, . . . t"08
The weight of water expended per stroke is 0'095G lb.
8. The areas shown in the diagram, as computed by the proper formula 1 ,
given in the footnote to the preceding article, are as follows: —
* The following are formulae for these volumes, and for the work represented by the
areas in the diagram, first demonstrated by the author in the Philosophical Trans
actions for December, 1853, and given also in A Manual of the SteamEngine and other
Prime Mov< rs. Let t and t' denote the higher and lower limits of absolute tempera
ture ; p and \> the corresponding pressures ; L and L' the corresponding values of
l> ; J, Joule's equivalent of the specific heat of liquid water (772 footpounds per
degree of Fahrenheit in a pound of water) ; H, the value in footpounds of the latent
heat of evaporation of 1 lb. of water at the higher limit (— 745,S00   7 J (T  A)
nearly, where A is the boiling point under one atmosphere) ; then —
E A J t' L . . t
bc= wn' ******'*
E D _ E A t' L
BC~BC + fL' ;
„ ,, _ „ JL \ . ./, . . t\) L(7  t')
F C D E = H \t  t ( 1 + hyp. log. t ,J { + ^— ^ ;
FBAE=^j«*'(l + hyp.log.^j;
B C D A = L£_Z_*1>.
All these formula; were independently demonstrated by Clausius in 1S55. (See his
Abhandlungen iiber die mechanische Wdrmetheorie.) If the water at the lower limit of
pressure in the example Mere all in the state of steam, it would occupy 29 "9 cubic feet.
The difference between this and 24  6S — viz., 5  22 cubic feet, shows what proportion of
the steam is spontaneously liquefied during the given expansion in a nonconducting
cylinder.
THE SECOND LAW OF THERMODYNAMICS. 4i3
Footpounds.
Total work obtained through the action of the steam
in driving the piston, allowing for back pressure,
FCDE = 16690
Work expended through compressing part of the steam
into the liquid state, F B A E = . . . .1446
Difference, being the indicated work per stroke,
BCDA = 15244
as already calculated in Article 4, by the second law of thermodynamics.
The lost or rejected heat,
68420  15244 = 53176 footpounds,
is the heat abstracted during the condensation of the volume of steam
represented by A D ; and it is impossible to get back any part of that
heat, because it is all abstracted at the lower limit of temperature, and
heat will not pass from a colder to a hotter body.
9. To proceed now to the case of the airengine. The limits of pressure
do not, as in the case of steam, depend on the limits of temperature, but
may be fixed according to convenience. In general, the lower limit of
absolute pressure is the atmospheric pressure, which we will estimate in
the present example at 21163 lbs. on the square foot, or 14*7 lbs. on the
square inch; and this is represented by Oe in the diagram. The upper limit
of absolute pressure being arbitrary, we will assume it in the present case
to be three atmospheres, or 63489 lbs. on the square foot, or 44*1 lbs. on
the square inch. It will presently be seen that the weight of air to be
used per stroke depends on the proportion borne by the upper limit of
pressure to the lower. That weight of air — to be afterwards determined
— is drawn into the airpump at the atmospheric pressure and at the
lower limit of temperature ; and it occupies a volume represented in the
diagram by e d. It is then compressed into a smaller volume, represented by
fa, so that it rises to the higher limit of pressure, and transferred from
the pump to the working cylinder ; and, in order that this compression
and transfer may cause the smallest possible expenditure of work, the air
must be kept during the compression at the lower limit of temperature by
means of a proper refrigerating apparatus for abstracting all the heat that
the compression generates. None of that heat can be got back, for it is
all abstracted at the lower limit of temperature. The temperature, then,
being uniform, the volume varies inversely as the absolute pressure ; fa,
in the present example, is onethird of e d, and the curve d a is a common
hyperbola. The work expended in the airpump, which is the exact
444 THE SECOND LAW OF THERMODYNAMICS.
equivalent of the heat generated there and abstracted by the refrigerating
apparatus, is represented by the area fade, and is computed in footpounds
by multiplying together the following factors: — The constant 53  15; the
hyperbolic logarithm of the ratio compression (hyp. log 3 = 1'0986); the
absolute temperature at which the compression takes place in degrees of
Fahrenheit (in this case 565 0, 2 Fahr.); and the weight of air used, in
pounds.
10. The air on its way from the pump to the working cylinder passes,
without change of pressure, through a perfect regenerator, in which all the
heat given out by the previous supply of air is stored up; and it thus
rises to the higher limit of absolute temperature, and at the same time
undergoes dilatation from the volume fa to the volume///, which volumes
are to each other in the ratio of the limits of absolute temperature; viz.: —
fa : fh : : 5G52 : 727*2 : : 1 : 128G7.
11. The next process is that the air continues to expand and drive the
piston until its pressure falls to the lower limit, its volume at the same
time increasing to that represented by c c; and it is then finally expelled,
giving out to the regenerator to be used over again the heat correspond
ing to the difference between the upper and lower limits of temperature.
In order that the greatest quantity of work possible may be obtained from
the expansion represented by the curve be, and in order also that the
air during its expulsion may give out to the regenerator a quantity of heat
sufficient to raise the temperature of the next supply of air to the higher
limit, the temperature must, by the supply of a sufficient quantity of heat,
be maintained uniform during the expansion represented by be; and that
quantity of heat constitutes the whole expenditure in a perfect airengine.
Such being the case, the volume varies inversely as the pressure ; e c, in the
present example, is three times fb, and the curve be is a common hyper
bola. It is evident, moreover, that the volumes represented by e d and e c
are to each other in the ratio of the limits of absolute temperature, already
given. The whole work obtained by the action of the air in the cylinder,
which is the exact equivalent of the whole heat expended, is represented
by the area/Jce, and is computed in footpounds by multiplying together
the following factors. The constant 53  15 ; the hyperbolic logarithm of the
ratio of expansion (hyp. log. 3 = 1  098G); the absolute temperature at
which the expansion takes place (in this case 727°  2 Fahr.) : and the weight
of air used, in pounds.
1 2. The area bed a represents the indicated work per stroke, being the
excess of the work obtained in the cylinder above the work expended in
the airpump ; and the proportion which it bears to j 'bee (representing the
whole expenditure of heat) is obviously that of the range of temperature
THE SECOND LAW OF THERMODYNAMIC'S. 445
to the higher absolute temperature, as already stated in Article 3, viz. :
162
— —  = 0'2228. In other words, the areas shown in the diagram repre
7272 ° l
sent the following quantities : — ■
Footpounds.
Total expenditure of heat per stroke, / b c e = . . 68420
Heat produced by compression and abstracted by the
refrigerator, fa de = . . . . . .53176
Indicated work per stroke, b c d a = . . . .1524 +
The indicated work per stroke may be calculated independently by multi
plying together the following factors: — The constant 53  15; the hyperbolic
logarithm of the ratio of expansion (hyp. 3 = 10986); the range of
absolute temperature (162° Fahr.); and the weight of air used, in pounds.
1 3. The weight of air used per stroke is determined by the considera
tion, that it is to be sufficient to absorb when expanding to three times
its initial volume at the absolute temperature 7 2 7° '2 Fahr., a quantity of
heat equivalent to 68420 footpounds. The calculation is as follows : —
Constant factor depending on the nature of the gas in
footpounds per degree of Fahrenheit, . . 53 15
Multiply by the absolute temperature,
727 3 2F.
Product in footpounds, being equal to the product of
the absolute pressure in pounds on the square
foot, and the volume of 1 lb. of air in cubic feet, 38651
Multiply by the hyperbolic logarithm of the rate of
expansion, hyp. log. 3 = . . ■ • 1*0986
Product being the heat absorbed by each pound of
air in footpounds, . . . . • • 42516
Heat expended 68420 _ 16113 lb
dividedby  42510 "
of air used per stroke. With a different ratio of expansion the weight
of air required per stroke, would be different, varying inversely as the
logarithm of that ratio. Had any other gas been employed instead of air
the only difference in this quantity would have been that arising from a
410
THE SECOND LAW OF THERMODYNAMICS.
different value of the constant factor. The following are the volumes
occupied by the air at different periods : —
ed
J"
J h
Cubic Feet.
^287
762
981
2943
14. The following is a summary of the comparison between the steam
engine and the airengine in the example given: —
Per Stroke.
SteamEngine.
POO) in >umls.
Air Engine.
Footpounds.
Difference.
Heat expended, .....
Heat rejected
Heat transformed into mechanical work,
Work obtained by expansion,
Work expended in compression. .
Indicated work,
( 16420
53176
• 120
53176
l :.•_• 1 1
15244
02228
02228
16690
1440
GS420
53176
51730
51730
15244
15244
The quantities in the column headed " difference," represent a diminu
tion of the work obtained by expansion, and an exactly equal saving in
the work expended in compression, produced by the mutual attraction of
the particles of water.
15. It may be useful to point out what effects Avould be produced by
departing from that condition of maximum efficiency which requires that
the elevation of temperature shall be produced without the expenditure of
heat. In the case of the steamengine there would be a gain of indicated
work, represented by the area FBAE = 144G footpounds, so that the
indicated work per stroke would be 16690 footpounds. At the same
time there would be an additional expenditure of heat, calculated as
follows : —
THE SECOND LAW OF THERMODYNAMICS. 447
Joule's equivalent of the specific heat of liquid water, 772
Number of degrees of Fahr. by which the temperature
of the feedwater has to be raised, . . . 162°
Weight of water used per stroke (lb.), . . . 00956
Product of those three factors, being the additional
expenditure of heat per stroke in footpounds. . 11956
Expenditure of heat as formerly calculated, . . 68420
Total, 80376
Efficiency, as diminished by omitting the compression,
16690 _
80376 ~
0208
being about fifteensixteenths of the greatest possible efficiency with the
given limits of temperature.
16. In the case of the airengine, suppose the regenerator omitted, so
that the whole elevation of temperature of the air has to be produced by
heat supplied from the furnace ; then, in the present example, there would
be an additional expenditure of heat, calculated as follows : —
Dynamical equivalent of the specific heat of air under
constant pressure, in footpounds per degree of
Fahr., 18345
Elevation of temperature, . . . . 162° F.
Weight of air used per stroke, the rate of expansion
being 3, 16113 lb.
Product of those three factors, being the additional
expenditure of heat in footpounds, . . . 47886
Expenditure of heat as formerly calculated, . . 68420
Total, 116306
Efficiency, as diminished by altogether omitting the
1 5944
regenerator . '"  = . . . . .0131 nearly,
116300
448 THE SECOND LAW OF THERMODYNAMICS.
or about sixtenths of the greatest possible efficiency with the given limits
of temperature.
17. It is evident that the regenerator, or some equivalent apparatus,
cannot be omitted consistently with economy of heat in an airengine.
An actual regenerator, however, will never succeed in storing and giving
out the whole supply of heat required for the elevation of temperature.
Suppose, for the sake of illustration, that in the present example the
regenerator stores and gives out ninetenths of the heat required, leaving
onetenth to be supplied from the furnace, then we have the following
expenditure of heat per stroke : —
Footpounds.
Expenditure a* originally calculated, . . . G8420
Additional expenditure through imperfect action of
the regenerator, ...... 4789
Total 73209
Efficiency, as diminished by imperfect action of the
15244
regenerator = 0208 nearly,
or about fifteensixteenths of the greatest possible efficiency with the given
limits of temperature.
18. The law which the preceding examples illustrate leads to the con
clusion that if by means of airengines greater economy of fuel than in
steamengines is to be attained, it must be by the following means : —
Working with a greater range of absolute temperature than is practicable
or safe in steamengines, and using the products of combustion directly
to drive the piston, so as to save nearly the whole of the heat that is
wasted in a steam boiler, or in an airengine in which the products of
combustion are not so used ; and it is probable that the latter is the more
easily practicable of the two means of economising heat.
19. The following examples illustrate the relation between the efficiency
of a heat engine and the consumption of fuel per indicated horsepower
per hour. The fuel is supposed to be nearly pure carbon, the mechanical
equivalent of the whole heat produced by the combustion of one pound of
it being 11,000,000 footpound'.
THE SECOND LAW OF THERMODYNAMICS.
449
Efficiency.
Indicated Work
per
Pound of Carbon.
•
Pound Carbon per
Indicated Horsepower
per Hour.
1
11,000,000
018
05
5,500,000
036
025
2,750,000
072
020
2,200,000
090
015
1,650,000
120
012:.
1,375,000
144
oioo
1,100,000
180
0075
825,000
240
0050
550,000
360
0025
275,000
720
SUPPLEMEX T.
20. I think it desirable to add the following explanations and illustra
tions of the principle, that in an airengine, under all circumstances
whatsoever, the heat produced hj the compression of the air is wholly and
unavoidably lost — a principle which is a necessary consequence of the fact
that heat never passes directly from a colder to a hotter body.
21. The heat produced by the compression must either be abstracted
from the air as fast as it is produced, so as to keep the temperature of the
air constant during the compression, or allow r ed to accumulate and raise
the temperature of the air. In the former case (which is that described
in Article 9) it is at once evident that the heat, being abstracted by
means of an external substance, such as water or air, that is colder than
the lowest temperature of the working substance, can never be transmitted
to the working substance again.
22. In the latter case, when the heat produced by the compression is
not abstracted, but allowed to accumulate and raise the temperature of the
air, the air is passed through the regenerator or econo miser on its way
2 F
450 THE SECOND LAW OF THERMODYNAMICS.
towards the working cylinder at an increased temperature higher than the
lower limit. Therefore, the air which has done its work and is escaping
through the regenerator in the opposite direction is not reduced below the
same increased temperature, for it is only through the entering air being
at a lower temperature that any heat can be abstracted from the escaping
air. Therefore, the escaping air carries off to waste a quantity of heat
equal to the quantity of heat produced by compression and allowed to
accumulate in the compressed air, so that the effect of such accumulation
is neutralised, and the total expenditure of heat remains unaltered.
23. Moreover, the increase of temperature of the air undergoing com
pression causes an increase in the quantity of work expended in
compressing it above the quantity of work which would be required if
the temperature were kept constant ; and thus the indicated work is
diminished, and the engine ceases to be one of maximum efficiency
between the given limits of temperature.
24. To illustrate this by an example, I will suppose that the compres
sion of the air takes place in a nonconducting cylinder, and is carried
to an extent such that the temperature of the air is raised from the lower
limit to tin higher limit by means of the compression alone. Then it is
known that the pressure of the air, instead of varying inversely as the
volume simply, varies inversely as that power of the volume whose
index is l  408, while the absolute temperature varies inversely
as that power of the volume whose index is 0'408. In Fig. 2 (as
in the former figure) let volumes in cubic feet be represented by
distances parallel to OV, and pressures in pounds on the square foot
by ordinates parallel to OP, so that the areas of diagrams represent
quantities of work in footpounds. Let O E represent the original
Fit
absolute pressure, E D the original volume . of the air, and F the
increased pressure at the end of the compression. Then, if the tempera
ture were kept uniform, the compression curve of the diagram woidd be a
common hyperbola D A, such that OF xFA= OE x ED. But the
THE SECOND LAW OF THERMODYNAMICS. 451
rise of temperature causes the compression curve to assume a steeper
figure, D B, of which the law has already been stated. The absolute
FB
temperature at the end of the compression is increased in the ratio ^rr',
and the work done in driving the compressing pump is represented by the
area of the diagram EFBD, and is equivalent to the quantity of heat
required in order to produce the rise of temperature from the lower to the
higher limit under constant pressure. Now, taking the same data as
before, viz. : —
Degrees of Fahrenheit.
Temperatures. Ordinary scale. Absolute scale.
Upper limit, . . . .266° 727°*2
Lower limit, .... 104° 565°*2
Difference, . . . .162° 162°*0
Ratio in which the absolute temperature is to be increased,
FB 727°*2
FA 565°*2
= 12867;
Dynamical value of the specific heat of air under constant pressure,
183*45 footpounds per degree of Fahrenheit.
Original pressure, OE = 2116*3 lbs. on the square foot ;
Original volume of one pound of air, 14*19 cubic feet;
We obtain the following results : —
ED
Ratio of volumes, :== — 1*8546 ;
Jb x>
■p.* * 0F ED ED FB i^±r
Ratio of pressures, ^ = ^ = ^ ' YJ. =
X 1*2867 = 2*3862;
Pressure at end of compression OF = 5050 pounds on the square
foot;
Volume of one pound of air at end of compression, 7*65 cubic feet;
Work done in compressing pump per pound of air,
162° x 183*45 = 29719 footpounds.
25. The air being now at the upper limit of temperature, let it be
transferred to the working cylinder, and there expanded without allowiug
432 THE SECOND LAW OF THERMODYNAMICS.
the temperature to fall below that limit, until the pressure falls to the
atmospheric pressure E, and then let it be discharged. The expansion
curve B C will, as in Article 11, be a common hyperbola, so that the ratio
of expansion will be
^^23862
BF  OE Jd8bJ '
and the volume of one pound of air at the end of the expansion will be
18 '2 6 cubic feet. The area of the diagram FBCE will represent at once
the work done in driving the piston, and the whole expenditure of heat.
The method of computing that area has already been stated in Article 11,
and the calculation in the present case is as follows, for each pound of un
used : —
Constant factor, . . . . . . . 5315
Absolute temperature at which the expansion takes
place, 727°2 F.
Hyperbolic logarithm of the ratio of expansion;
hyp. log. 2*3862 = 0*8697
Product of those three factors, being the work done
and heat expended in the working cylinder, per
pound of air used, in footpounds, . . . 336 K>
26. The air finally escapes at the higher limit of temperature, and
therefore carries to waste the whole of the heat which was employed in
raising its temperature, having been produced by compression. The value
of that heat in footpounds per pound of air has been already found, in
Article 24, to be 29719.
27. The indicated work represented by the area B C D is consequently,
in footpounds per pound of air,
33615  29719 = 389G ;
and the efficiency of the engine,
3896
33615 = ' 11Gnear1 ^'
or about onehalf of the greatest possible efficiency with the given limits
of temperature.
28. In order that the present example may be the more easily com
pared with the former examples, we will suppose that the heat to be
expended per stroke is, as before, equivalent to 68420 footpounds.
THE SECOND LAW OF THERMODYNAMICS. 453
Then the expenditure of air per stroke required in order to take up
that heat during its expansion is found to be
^= 20354 lb,;
whence the following results are obtained : —
Footlbs.
Waste heat per stroke equivalent to the work of
driving the compressing pump, 162° x 183'45
X 20354 = G0490
Indicated work per stroke, 3896 X 20354 = . . 7930
7930
Efficiency,— — = 0T16, as already found.
Volumes successively occupied by the air in cubic feet per
stroke :
ED = 2889; FB = 15'58; EC = 3717.
29. The case of Article 9 — where the whole of the heat generated in
the compressing pump is abstracted as fast as it is produced — and that of
the example just described — in which the whole of that heat is at first
employed in raising the temperature of the air while an equal quantity of
heat goes to waste with the escaping air that has done its Avork — form
two extremes, and between those extreme cases there may lie an indefinite
number of intermediate cases, in which part of the heat generated by com
pression is abstracted at once, and part employed in raising the tempera
ture of the air. It will be readily understood that in all those intermediate
cases the result is the same as in the two extreme cases — the whole of the
heat generated by the compression of the air goes to waste either at once
or with the escaping air, and none of it is available for conversion into
indicated work; nor would it be possible that it should become so
available, unless it were possible for heat to be directly transferred from
a colder body to a hotter body.
454 THE WORKING OF STEAM IN COMPOUND ENGINES.
XXVI.— OX THE WORKING OF STEAM IN COMPOUND
ENGINES*
1. Principal Kinds of Compomd Engines. — By a compound steamengine is
meant one in which the mechanical action of the steam commences in a
smaller cylinder and is completed in a larger cylinder. Those cylinders
are respectively called, for convenience, the highpressure cylinder and the
lowpressure cylinder. Two classes of compound engines will be con
sidered — first, those in which the steam passes directly or almost directly
from the highpressure to the lowpressure cylinder, the forward stroke of
the latter cylinder taking place either exactly or nearly at the same time
with the return stroke of the former cylinder ; and, secondly, those in
which the steam, on its way from the highpressure to the lowpressure
cylinder, is stored in a reservoir, so that any convenient fraction of a
revolution (such, for example, as a quarter revolution) may intervene
between the ends of the strokes of the cylinders. As to the latter class
of engines, reference may be made to a paper by Mr. E. A. Cowper in
the Transactions of the Institution of Naval Architects for 18G4, page 24S.
Sometimes, especially in the first class of compound engines (those without
reservoirs), there are a pair of lowpressure cylinders whose pistons move
together, and which act like one cylinder divided into two parts.
2. AilrtmtiKjcs of Compound Engines. — As regards the theoretical effi
ciency of the steam, the compound engine possesses no advantage over
an engine with a single cylinder of the dimensions of the lowpressure
cylinder, working with the same pressure of steam and the same rate of
expansion. The advantages which it does possess are the following : —
First, in point of strength, the action of the steam when at its highest
pressure takes place, in the compound engine, upon a comparatively small
piston, thus diminishing the amount of the greatest straining force exerted
on the mechanism and framing ; secondly, in point of economy of heat
and steam, in a singlecylindered engine it is necessary, in order to prevent
liquefaction and reevaporation of the steam, and consequent waste of
heat, that the whole metal of the cylinder should be kept, by means of a
steam jacket, at a temperature equal to that of the steam when first
* From The Engineer of March 11, 1870.
THE WORKING OF STEAM IN COMPOUND ENGINES.
455
admitted; whereas, in a compound engine, it is the smaller or high
pressure cylinder only which has to be kept at so high a temperature, it
being sufficient W'keep the larger or lowpressure cylinder at the tem
perature corresponding to the pressure at which the steam passes from
the highpressure to the lowpressure cylinder. Thirdly, in point of
economy of work : the whole of the force exerted by the piston rod upon
the crank in a singlecylindered engine takes effect in producing friction
at the bearings ; whereas, in compound engines, the mechanism can be
so arranged that the forces exerted by the piston rods on the bearings
shall, to a certain extent, balance each other, thus diminishing the friction.
When there are a pair c_
of lowpressure cylinders
with a high  pressure
cylinder between them
(as in the engines of
H.M.S. "Constance," by
Messrs. Randolph, Elder,
and Co.) the balance can
be made almost perfect.
These remarks apply not
only to the forces due
to the pressure of the
steam, but to those pro
duced by the reaction or inertia of the pistons and of the masses which,
move along with them. The advantages which have been stated are
obviously greatest with high rates of expansion.
3. Combination of Diagrams. — When the diagrams of the high and low
pressure cylinders of a compound engine are taken by means of one
indicator they have the same length of base; and when arranged in
the customary way for inspection they present appearances which are
represented in Fig. 1 for engines without reservoirs, and in Fig. 2 for
engines with reservoirs. In each Fig. A A is the atmospheric line, B
the zero line of absolute pressure, and the length P on that line is the
common length of the diagrams of both cylinders, as originally drawn.
The diagram of the highpressure cylinder is represented in Fig. 1 by
CDRH, and in Fig. 2 by CDKL; that of the lowpressure cylinder,
as drawn by the indicator, is represented in Fig. 1 by k i h, and in Fig. 2
by I i g h. In combining the diagrams of the two cylinders into one
diagram, it is to be borne in mind that when the area of a diagram is
considered as representing the work done by the steam on the piston at
one stroke, the length of the base of the diagram is to be considered as
representing the effective capacity of the cylinder : that is, the space swept
through by the piston at one stroke. Hence, in order to prepare the
Fi& 1.
456
THE WORKING OF STEAM IN COMPOUND ENGINES.
diagram of the lowpressure cylinder for combination "with that of the
highpressure cylinder, the lengths of its base, and of every line in it
OP li
Fig. 2.
parallel to its base, are to be increased in the ratio in which the effective
capacity of the lowpressure cylinder is greater than that of the high
pressure cylinder.* (When there
are a pair of lowpressure cylinders
combined with one highpressure
cylinder, they are equivalent to
one lowpressure cylinder of double
the capacity.) In each of the Figs.
1 and 2, then, the base P is, in
the first place, to be produced to
B, making B greater than P
in the proportion abovementioned.
To complete the preparation of
the lowpressure diagram draw, in
each case, a series of lines across it
parallel to the base B, such as
the dotted lines in each Fig., of
which one is marked h i e. Let c denote the ratio B 4 P. Then,
in the case of an engine without a reservoir (Fig. 1) draw P£ perpendi
cular to B, cutting all the parallel dotted lines, and on each of those
lines (such as s rq) lay off sq = c'rs. A curve kqej, drawn through
* Manual of the SteamEngine and other Prime Movers, page 334.
THE WORKING OF STEAM IN COMPOUND ENGINES.
457
the points, such as q, thus found, will be the required boundary of the
enlarged lowpressure diagram, kqejksJc, which, being joined on to the
C
D
T
R
Sfl
V
A
T'
H
Ssft'
A
A*
R'
I
G
»
Lb
Fig. 4.
highpressure diagram C D K H, makes the combined diagram. When
the engine has a reservoir, draw I (Fig. 2) perpendicular to B, and
crossing all the parallel dotted lines, and on each of those lines (such as
srk) lay off sk = c'sr. A curve Ikefg, drawn through the points,
such as h, thus found, will be the required boundary of the enlarged low
pressure diagram, which, being joined on to the highpressure diagram
C D K L, makes the combined diagram. In a theoretically perfect
458 THE WORKING OF STEAM IX COMPOUND ENGINES.
engine, in which the steam passed from the highpressure to the low
pressure cylinder without change of pressure or temperature, the two
diagrams would join exactly at the boundaries K L and k I in Fig. 2, or
KH and h h in Fig. 1, so as to form one diagram identical with that
produced by the same quantity of steam working between the same limits
of pressure in the larger cylinder only. But in actual engines there is
sometimes a gap between the high and lowpressure diagrams, as in the
Figs. 1 and 2 ; and sometimes, when the steam reservoir is heated, they
overlap each other.
4. llatcs of Expansion. — In Figs. 1 and 2 let D be the point where the
cutoff takes place in the highpressure cylinder; draw D c parallel to B 0;
OP O P>
then = is the rate of expansion in the highpressure cylinder; py 3 is the
total increase of volume in passing from the highpressure to the low
pressure cylinder ; and the product of those quantities,
OP OB _ OB
ri) ' or ~ 7d'
is the total rate of expansion.
5. Construction of Theoretical ExpansionDiagrams for Proposed Engines. —
In constructing the theoretical diagram of a proposed steamengine, certain
wellknown assumptions are made in order to simplify the figure and the
calculations founded upon it. In the first place, the pressure of the steam
during its admission is assumed to be constant, so that the uppermost
boundary of the diagram, as in Figs. 4 and 5, is a straight line, C D,
parallel to the zero line, B, the height C representing the absolute
pressure of admission. .Secondly, the back pressure is assumed to be
constant ; so that the lower boundary of the diagram also is a straight
line F G parallel to B, the height G representing the mean absolute
back pressure, as estimated from the results of experience. Thirdly, it is
commonly assumed that at the beginning of the forward stroke the pressure
rises suddenly from the back pressure to the pressure of admission, so that
the first endboundary of the diagram is a straight line G C perpendicular
to B. Fourthly, it is assumed that at the end of the forward stroke the
pressure falls suddenly from the pressure at the end of the expansion (or
final pressure) to the back pressure, so that the second endboundary of the
diagram is a straight line, E F, perpendicular to B. Fifthly, for the
expansion curve (D E in Figs. 4 and 5), which completes the boundaries
of the diagram, there is assumed a line of the hyperbolic class. Thus the
area of an assumed theoretical diagram of the work of the steam in a pro
posed engine is made up of a hyperbolic area C D E H, and a rectangular
area E F G H. The form of the expansion curve depends on a number of
circumstances, such as the initial pressure and temperature of the steam,
THE WORKING OF STEAM IN COMPOUND ENGINES. 459
the proportion of water (if any) admitted along with it in the liquid
state, the communication of heat between the steam and the metal
of the cylinder, the communication of additional heat to the steam during
its expansion by the help of a steam jacket. Writers on thermodynamics
have determined the exact form of that curve in various cases, such as
that of steam originally dry, expanding in a nonconducting cylinder;
that of steam originally containing a given proportion of moisture expand
ing in a nonconducting cylinder ; that of steam originally dry supplied
during the expansion with heat just enough to keep any part of it from
condensing; that of steam supplied during the expansion with heat
sufficient to keep it at a constant temperature. For elementary methods
of approximating to the results of the exact methods in such cases, see
The Engineer of the 5th January, 1866. For most practical purposes the
common hyperbola forms a good approximation to the true expansion
curve, and it is convenient because of the simplicity of the processes for
finding its figure, whether by calculation or by construction. To find by
calculation the series of absolute pressures corresponding to a given series
of volumes assumed by the steam, on the supposition that the expansion
curve is a common hyperbola, multiply the initial absolute pressure by the
initial volume; divide the product by any one of the given series of
volumes ; the quotient will be the corresponding absolute pressure. To
find a series of points in the common hyperbola, in Fig. 3, draw the two
axes X and Y perpendicular to each other; X to form a scale of
volumes, and to represent the zero line of absolute pressure; Y to form
a scale of absolute pressures. On X lay of A to represent the initial
volume of the steam; also OB', OB", &c, to represent a given series of
volumes occupied by the same steam during its expansion. On Y lay
off C to represent the initial absolute pressure of the steam. Through
C draw the straight line C a V b" , &c, parallel to X, and through the
points A, B', B", &c, draw the series of straight lines A a, B' b', B" b", &c,
parallel and equal to C. From draw the series of diverging straight
lines b' , b", &c, and mark the series of points C, C" &c, where they
cut A a. From these points, and parallel to X, draw the series of
straight lines C D', C" D", &c, and mark the series of points D', D", &c,
where they cut the series of straight lines B' b', B" b", &c. These points,
together with the point a, will be points in the required hyperbola,
a D' D", &c, which is taken as an approximation to the expansion curve.
6. Calculation of Mean Absolute Pressure and of Indicated Work in a
Theoretical Diagram. — Suppose that in Fig. 3 B"" represents the final
volume of the steam, so that D"" is the end of the expansion curve, and
that B"" D"" represents the final absolute pressure. The intermediate
volumes B', B", &c, are to be so chosen that the points B', B", &c,
shall divide A B"" into an even number of equal intervals. Multiply the
400
THE WORKING OF STEAM IN COMPOUND ENGINES.
series of absolute pressures represented by A a, B' V, &c., by " Simpson's
multipliers," which are, for the initial and final pressures, 1 ; and for the
intermediate pressures, 4 and 2 alternately; so that, for example, for four
intervals and five absolute pressures, the multipliers are 1, 4, 2, 4, 1; for
six intervals and seven absolute pressures, 1, 4, 2, 4, 2, 4, 1, and so on.
Add the products together; divide the sum by three times the number of
intervals. Multiply the quotient 1 >y the rate of expansion less one ; to
the product add the initial absolute pressure ; divide the sum by the rate
of expansion ; the quotient will be the required mean absolute pressure
nearly. — Example : Kate of expansion, 5 ; expansion divided into 8 equal
intervals; initial absolute pressure, 37*8 lbs. on the square inch.
Absolute Pressures. Multipliers.  Products.
Initial 37"M) 1 3780
r 37S x \ = 2520
378 X \ = 1890
378 X  = 1512
378 x I = 1260
378 X f = 1080
378 x ] = 945
. 378 X = 840
Final 37*8 X I = 7"56
Inter
mediate
Divide by 8 intervals x 3 :
Quotient
Multiply by rate of expansion 5 — 1
Product
Add initial absolute pressure .
Divide by rate of expansion .
Mean absolute pressure nearly
4
10080
o
3780
4
G048
2
2520
4
4320
•>
1890
I
3360
1
756
1\
) 3G534 sum
152225
4
6089
3780
9869 sum
19738
The remainder left after subtracting the back pressure from the mean absolute
pressure is the mean effective pressure, which, being multiplied by the area
of the piston, and by the distance moved through by the piston in a given
time, gives the indicated work of the steam in that time.*
* A wellknown formula for the ratio of the mean to the initial absolute pressure is
— — — y P °" r ; r being the rate of expansion. For a graphic approximate solution of
v
the same question, see The Engineer for April 13, 1SG6.
THE WORKING OF STEAM IX COMPOUND ENGINES. 4G1
7. Theoretical Combined Diagrams. — By the process described in the
preceding section there may be constructed the approximate theoretical
diagram of steam working with a given initial pressure and a given rate
of expansion, and against a given mean back pressure. In the case of a
proposed compound engine, that theoretical diagram is to be regarded as
the combined diagram of the two cylinders, and it is to be divided into
two parts, representing the parts of the indicated work done in the two
cylinders respectively. In each of the two Figs. 4 and 5, the theoretical
combined diagram is represented by C D E F G, C being the initial, and
B E the final absolute pressure, G = B F the mean back pressure, A A
the atmospheric line, B the zero line of absolute pressure, H E = F G =
O B the effective capacity of the large cylinder, C D the initial volume of
"F TT
steam admitted per stroke, and r—  the total rate of expansion. The
dividing line which marks the boundary between the highpressure and
lowpressure theoretical diagrams is represented in Fig. 4 by K H, and in
Fig. 5 either by k L, or by Jc I, or by some line near those lines, as will
afterwards be more fully explained.
8. Theoretical Diagrams of a Compound Engine without a Reservoir. —
When the steam passes directly, without loss of pressure or of heat, from the
highpressure to the lowpressure cylinder, the dividing line of the theoreti
cal compound diagram is found by the following process. In B (Fig. 4)
lay off P to represent the effective capacity of the highpressure cylinder.
Through P, parallel to C, draw the straight hue P J K, cutting the back
pressure line in J, and the expansion curve in K ; then K will be one end
of the dividing line. Through the lower end E of the expansion curve,
and parallel to B 0, draw E H, cutting C in H ; then H will be the
other end of the dividing line. To find intermediate points, draw, parallel
to B, a series of straight lines, such as T B Q, T' R' Q', across the part
of the diagram which lies below the point K, and in each of those lines,
for example, in Q R T, lay off R S, bearing the same proportion to R Q
that P bears to P B ; the points thus marked, such as S and S', will be
in the required dividing line K H. The areas of the two parts of the
theoretical diagram, CDKH, and K E F G H, being measured by ordinary
methods, wall show the comparative quantities of work done in the high
pressure and lowpressure cylinders respectively. The advantages of the
compound engine in point of diminution of stress and friction are most
fully realised when those quantities of work are equal; that is, when the
line KH divides the area CDEFG into two equal parts; for then the
mean values of the forces exerted through the two pistonrods are equal ;
hence the proportion borne by the effective capacity of the highpressure
cylinder to that of the lowpressure cylinder ought to be chosen so as to
realise that condition as nearly as possible. An exact rule for that pur
4G2 THE WORKING OF STEAM IN COMPOUND ENGINES.
pose would be too complex to be useful for practical purposes. The
following empirical rule has been found by trial to give a good rough
approximation to the required result in ordinary cases of compound
engines without reservoirs : Make the ratio in which the lowpressure
cylinder is larger than the highpressure cylinder, equal to the square of
the cube root of the total rate of expansion ; for example, if the total rate
of expansion is to be 8, let the lowpressure cylinder be four times the
capacity of the highpressure cylinder (this rule was tirst given in Ship
building, Theoretical and Practiced, page 275). When a table of squares
and cubes is at hand, look for the total rate of expansion in the column of
cubes, the required ratio will be found in the column of squares.
9. Theoretical Diagrams of a Compound Engine with a Reservoir. — To
realise theoretical perfection in the working of an engine with an inter
mediate steam reservoir, that reservoir should be absolutely nonconduct
ing, so that the steam may pass from it into the lowpressure cylinder at
exactly the same pressure and volume at which it is received from the
highpressure cylinder. Supposing this condition to be realised, let Op
in Fig. 5, represent the volume of steam admitted into the lowpressure
O P
cylinder at each stroke, so that ^ — is the rate of expansion in that
cylinder; then Op will also represent the effective capacity of the high
Op . ...
pressure cylinder, and will be the rate of expansion in it; and if^Z;
be drawn parallel to OC, so as to cut the expansion curve in /,•, this point
will be one end of the required dividing line. To find other points on
that line under the same theoretical conditions, combined with the sup
position that the forcing of the steam into and its delivery out of the
reservoir take place at certain times, produce B 0, making ONof a length
representing the capacity of the reservoir : then in C lay off L greater
than pi; in the proportion in which TSp is greater than NO; L will be
the other end of the dividing line Tc L, which line will be an expansion
curve for steam of the initial volume represented by N = M L, and
initial absolute pressure represented by OL = NM, and may be con
structed by the method of Sec. 5. The highpressure diagram will be
CDiL, and its lower boundary, h L, will represent the increase of pressure
during the process of forcing the steam from the highpressure cylinder
into the reservoir ; the lowpressure diagram will be L h E F G, and its
upper boundary L k will represent the diminution of pressure during the
process of delivering the steam from the reservoir into the lowpressure
cylinder. But, in reality, the entrance of the steam into and its delivery
from the reservoir take place partly at the same time, and the metal of
the reservoir abstracts heat from the entering steam, and gives heat back
to the escaping steam; the practical result, as shown by the diagrams
THE WOKKING OF STEAM IN COMPOUND ENGINES. 4G3
published in Mr. Cowper's paper already referred to. being that the
pressure of the steam in the reservoir is nearly constant, so that the upper
boundary of the lowpressure diagram nearly coincides with a straight
line, I 1; parallel to B. The same straight line also coincides nearly with
the lower boundary of the highpressure diagram.
In the engines experimented on by Mr. Cowper, the effective capacity
of the highpressure cylinder was somewhat smaller than the volume of
steam admitted at each stroke into the lowpressure cylinder, being repre
sented, for example, by P instead of by Op ; and the final pressure
P K in the highpressure cylinder was greater than the pressur