Heat considered as a mode of motion;

Front matter

HEAT

CONSIDERED A3

A MODE OF MOTION:

Contents

LECTURE L

Introduction— Description of Instruments— The Thermo-electric Pile and the Galvanometer—Heat and Cold indicated by the Deflection of a Magnetic Needle —Heat generated by Friction, Compression, and Percussion— Waterfalls generate Heat— Friction of Railway Axles— The Force necessary to heat the Axles is withdrawn from the urging Force of the Engine— Meteorites probably rendered incandescent by Friction against Air— Rumford's Experiments on the Heat excited by Friction— Water boiled by Friction— Consumption of Heat when compressed Air is suffered to expand— Action of a Current of Air when urged by a bellows against the Face of the Thermo-electric Pile . page 13

Appendix To Lecture I

Mode of constructing a Thermo-electric Pile— Mode of constructing a Galvanometer—Mode of rendering Needles astatic— Experiments on the Magnetism of Galvanometer Coils, and Mode of avoiding this Magnetism ... SO

LECTURE II.

The Nature of Heat— the Material Theory supposes it to bo a subtle Fluid stored up in the inter-atomic Spaces of Bodies— The idea of 'Capacity' for Heat originated in this way — The Dynamical Theory supposes Heat to be a Motion of the ultimate Particles of Bodies — Rumford's and Davy's Views— Davy's Fusion of Ice by Friction— Bearing of the Experiment on the Material Theory — Heat and Light developed by the Compression of Air— Ignition of Bisulphide of Carbon Vapour in Fire Syringe — Thermal Effects of Air in Motion — Condensation of Aqueous Vapour by the Rarefaction of Air— Machine of Schemnitz — Deportment of a Conductor between the Poles of a Magnet— Apparent - Viscosity of the Magnetic Field — The Conductor encounters Resistance to its Motion — A Conductor swiftly rotating is struck motionless when the Magnet is excited— When the Conductor is compelled to rotate Heat is generated— Fusion of an Alloy by this Heat— Measurement of the Amount of Heat generated by a given Expenditure of Force — Dr. Mayer and Mr. Joule — ^The Mechanical Equivalent of Heat— Definition of the Term 'Foot-pound' — Heat developed increases as the height of the fall, and is proportional to the Square of the Velocity— Calculation of Heat generated by the impact of Projectiles—

Vlll CONTENTS.

Heat equivalent to the Stoppage of the Earth in its Orbit— Heat equivalent to the Falling of the Earth into the Sun— Preliminary Statement of Meteoric Theory of the Sun's Heat— Analysis of Combustion— Ignition of Diamond- Its Combustion in Oxygen due to the Showering of the Atoms of the Gas against the Surface of the Diamond— Structure of Flame— Candle and Gas Flames— Combustion on Mont Blanc— The Light of Flames is materially diminished by the Rarefaction of the- Air, though the Quantity of Combustible Matter consumed remains the same— Frankland's Experiments— All Cases of Combustion are due to the Collision of Atoms which have been tfrged together by their mutual Attractions . pagk 37

Appendix To Lecture II

Extracts from the Twentieth Aphorism of the 'Novum Organum '—Abstract of Count Kumford's Essay entitled ' An Enquiry concerning the Source of the Heat which is excited by Friction '—Note on the Compression of Bisulphide of Carbon Vapour , . 67

LECTURE III.

Expansion of Bodies by Heat -Liberation of Particles from the thrall of Cohesion —The Liquid and Gaseous States of Matter defined— Illustrations of the Expansion of Air by Heat-Ascent of Fire Balloon— Gases expand by a constant Increment for every Degree above 32° Fahr.— Coefacient of Expansion— Heating of Gas at a constant Pressure— Heating of Gas at a constant Voluma— In the former case Work is done by the Gas— In the latter case no Work is done — In the former case an Excess of Heat equivalent to the Work done must be Imparted — Calculation of the Mechanical Equivalent of Heat— Mayer and Joule's Determinations— Absolute Zero of Temperature— Expansion without Refrigeration— Expansion of Liquids— Exceptional Deportment of Water and Bismuth— Energy of Atomic Forces— Pyrometers — Strains and Pressures superinduced by sudden Cooling— Chilling of Metallic Wires by Stretching — Heating of India-rubber by Stretching— Contraction of stretched India-rubber by Heat 74

Appendix To Lecture III

Further Remarks on Dilatation— Linear, Superficial, and Cubic Coeflacients of Expansion— The Thermometer — Extracts from Sir Humphry Davy's First Memoir, entitled * Heat, Light, and the Combinations of Light ' . . 108

LECTURE IV.

Vibrations and Tones produced by the contact of Bodies of diflbrent Temperatures —The Trevelyan Instrument— Rotation of hollow Spheres by Electricity — Efiect of Pressure on Fusing Point— The Fusing Point of Bodies which contract on solidifying is raised by Pressure— The Fusing Point of Bodies which expand on solidifying is lowered by Pressure— Liquefaction of Ice by Pressure — Dissection of Ice by Calorific Beam— Negative Crystallisation, Ice reduced internally to Liquid Flowers, having six Petals each— Central Spot a Vacuum —Sound heard when the Spot is formed— Physical Properties of Water from which Air has been removed— Its Cohesion enormously augmented— It can bo

CONTENTS. IX

heated far above its boiling Point— Its Ebullition becomes Explosion— Applicalion of this Property to explain the Sound heard, when the central Spot is formed in Ice— Possible bearing of this Property of Water on Boiler explosions —The Boiling Point of Liquids— Resistance to Ebullition— Cohesion of Particles, adhesion to Vessel, external Pressure— Boiling Points on various Alpine Summits—The law of Conservation illustrated in the Steam Engine— The Geysers of Iceland— Description of the Geysers and their Phenomena— Bunsen's Theory —Experimental Illustration page 114

Appendix To Lecture IV

Abstract of a Lecture on the Vibrations and Tones produced by the contact of Bodies having different Temperatures— Extracts from a Paper on the Physical Properties of Ice 144

LECTURE V.

Application of the Dynamical Theory to the Phenomena of Specific and Latent Heat— Definition of Energy— Potential and Dynamic Energy, illustrated by the Raising and Falling of a Weight— Convertibility of Potential into Dynamic Energy and the reverse— Constancy of the Sum of both Energies— Application of the Ideas of Potential and Dynamic Energy to Atoms and Molecules —Magnitude of Molecular Forces— The separation of a Body' s Particles by Heat is an Act the same in kind as the separation of a Weight from the Earth- Work is here done within the heated Body— Interior Work— The Heat communicated to a Body divides itself into Potential and Dynamic Energy— All Single Atoms, whatever be their weight, possess the same amount of Dynamic Energy— Specific Heat or Capacity for Heat explained by Reference to Interior Work and to Atomic Number— Experimental Illustrations of Specific Heat — Table of Specific Heats— Influence of high Specific Heat of Water on Climate —Heat consumed in Change of Aggregation— Latent Heat of Liquids and Vapours—It is Heat consumed in conferring Potential Energy on the ultimate Particles— By Condensation and Liquefaction this Potential Energy is converted into Heat— Mechanical Value of the Union of Oxygen and Hydrogen- Mechanical Value of the Change from Steam to liquid Water— Mechanical Value of the Change from liquid Water to solid Ice— Experimental Illustrations-Consumption and Generation of Heat by Changes of Aggregation- Water frozen by its own Evaporation— The Cryophorus— Solid Carbonic Acid —The Spheroidal State of Liquids— In this State the Liquid is supported on a Bed of its own Vapour— Proofs that the Spheroidal Drop is not in Contact with the hot Surface underneath it— Experimental Illustrations of the Spheroidal Condition— Possible Bearing of these Facts on the Fiery Ordeal, and on Boiler Explosions— Freezing of Water and Mercury in red-hot Vessels . . 153

LECTURE VI.

Convection in Air- Larger physical Phenomena— Winds caused by the heating Action of the Sun— The upper and lower Trade Winds— Effect of the Earth's Rotation in changing the apparent Direction of Winds— The Existence of the upper Current proved by the Discharge of Ashes into it by Volcanoes— Effects which accompany the apparent Motion of the Sun from side to side of the Equator— Aqueous Vapour— Tropical Rains— Region of Calms— Europe, for the most Part in the upper Trade— Europe the Condenser of the Western At- 1*

■^ CONTENTS.

lantic—Thia is the Cause of the Mildness of European Temperature— Rainfall in Ireland— Effect of Mountain Ranges on Rainfall— Convection in Liquids- Experimental Illustrations— The Gulf Stream : its influence on the Climate of Britain— FormaUon of Snow— The Molecules aggregate to form Frozen Stars with Rays sixty degrees apart— Figures of Snow Crystals— Collection of Snow on Mountains— The Snow Line— Squeezing of this Snow to Ice— Formation of Glaciers— The Motion of a Glacier resembles that of a River— Theories of Glacier Motion— The Regelation of Ice— The Moulding of Ice by Pressure- Ancient Glaciers— Their Traces in Switzerland, England, Ireland, and Wales —The Cedars of Lebanon grow on Glacier Moraines— Theories of the Glacial Epoch— Not due to a Diminution of Solar Power, or to the passage of the Solar System through cold Regions of Space ...... page 185

Appendix To Lecture VI

Abstract of a Lecture on the Mer de Glaco 212

LECTURE YH.

The Conduction of Heat a Transmission of Molecular Motion— Different Bodies possess different Powers of Transmission— Good Conductors and bad Conductors—Experimental Illustrations— Experiments of Ingenhausz, Despretz, Wiedemann, and Franz— Table of Conductivities— Parallelism of Conduction of Heat and Conduction of Electricity— Good Conductors of the one are also good Conductors of the other, and vice versa- Influence of Heat on Electric Conduction—The Motion of Heat interferes with the Motion of Electricity— Conduction of Cold— Constancy of Temperature of Animal Body— Capacity to bear high Temperatures— Diversion of Heat from the Purposes of Temperature to the Performance of Work— Influence of Molecular Structure— Some Bodies conduct differently in different Directions— Conduction in Crystals and in Wood— Feeble Conductivity of Organic Substances— This secures them from sudden Alternations of Temperature-Influence of Specific Heat on tho Speed of Conduction— Anomalous Case of Bismuth as compared with Iron- Bismuth, though the worst Conductor, apparently transmits Heat most speedily— Action of Clothing— Rumford's Experiments— Influence of mechanical Texture on Conduction— A Powder conducts ill, on account of the incessant Break in the Continuity of the Mass along which the Motion of Heat is transmitted—Non-conductivity of Gypsum— Effect of Boiler encrustations— Withdrawal of Heat from Flames by good Conductors -The Motion of Flame, though intense, is much lowered by being transferred from so light a Body to a heavy one-Eflect of Wire Gauze-The Safety Lamp-Conduction of Liquids denied by Rumford, but proved by M. Despretz-Conduction of Gases denied by Rumford, but affirmed in the case of Hydrogen by Prof. Magnus— Cooling Effect of Air and Hydrogen-Experiments on Gaseous Conduction doubt- ^«1 222

LECTURE VIIL

Radiant Heat— Cooling a loss of Motion— To what is this Motion imparted ?-Pre. liminary Experiments on Sound— Communication of Vibrations through the Air to Membranes and to Flames-The Vibrations of a Body propagated through the Air and striking on the Drum of the Ear produce Sound- Analo. gous Phenomena of Light-Theories of Emission and Undulation— Discussion*

CONTENTS. XI

on the Babject—Newton—Huygbens—Euler—Young—Frefinel— Space fiUed with an elastic Medium called Ether— The Motion of a hot or of a luminous body communicated to this Ether is propagated through it in waves— In this form Heat is called Radiant Heat— The Thermo-electric Pile in relation to Radiant Heat— Distribution of Heat in the Electric Spectrum examined experimentally—Low Calorific Power of blue End of Spectrum— The most luminous Part not the hottest Part— Of the visible rays Red is the hottest— The maximum Calorific Action is beyond the Red, and is due to Rays which are incompetent to excite the Sense of Vision— Extra Violet Rays— Physical Cause of Colour— The Spectrum is to the Eye what the Gamut is to the Ear— The Colour of Light corresponds to the Pitch of Sound— Number of Impulses involved in the Perception of Light— Theory of Exchanges— Reflection of Radiant Heat from Plane Surfaces— Angle of Incidence equal to the Angle of Reflection—Experimental Proof— The obscure Rays of the Electric Lamp pursue the same Track as the luminous ones— Angular Velocity of reflected Ray twice that of rotating Mirror— Experiments with radiant Heat of Fire and of obscure Bodies— Reflection from curved Surfaces— Parabolic Mirrors— Explosion of Chlorine and Hydrogen in Focus of Mirror by Light— Explosion of Oxygen and Hydrogen by Radiant Heat— Reflection of Cold . . . page 261

Appendix To Lecture VIII

On the Sounds produced by the Combustion of Gases in Tubes . , .288

LECTURE IX.

The Intensity of Radiant Heat diminishes as the Square of the Distance from the radiant Point increases— Experimental Proof— Undulations of Sound longitudinal, of Light transversal— The ultimate Particles of diflferent Bodies possess diflferent Powers of Communicating Motion to the Ether— Experimental Illustrations of good and bad Radiators— Reciprocity of Radiation and Absorption —Protection by Gilding against Radiant Heat— Transmission of Radiant Heat through Solids and Liquids— Diathermancy— Absorption occurs within the Body— Absorptio;i of Light by Water— Radiant Heat passes through Diathermic Bodies without heating them— Athermic Bodies are heated— Concentration of Beam on bulb of Air Thermometer— Penetrative Power of Sunbeams- — Sifting of Radiant Heat— Ratio of Obscure to Luminous Radiation in various Flames ^^'

Appendix To Lecture IX

On some physical Properties of Ice ^^'

LECTURE X.

Absorption of Heat by Gases and Vapours— First Apparatus— Rocksalt Plates- PecuUarities of the Galvariometer— The higher Degrees of greater Value thai the lower ones— Improved Apparatus— Principle of Compensation permits th use of a powerful Source of Heat while it preserves the Needle in a seneitiv Position— Air, Oxygen, Hydrogen, and Nitrogen are practical Vacua to Radian Heat— Opacity of defiant Gas and Sulphuric Ether— Radiation through othe Gases and Vapours— Great difference of Absorbing Power— Radiation of Hea by Gases— The atom which Absorbs powerfully Radiates powerfully— Absorj tion by Gases at a Tension of an Atmosphere- Absorptions at smaller Tension

Xll CONTENTS.

—Comparison of Elementary and Compound Gases and Vapours— Radiation through Lampblack, &c page 341

Appendix To Lecture X

Calibration of the Galvanometer .376

LECTURE XL

Action of Odorous Substances on Radiant Heat — List of Perfumes examined— Action of Ozone on Radiant Heat— Influence of the Size of the Electrodes on the Quantity of Ozone generated— Constitution of Ozone— Radiation and Absorption of Gases and Vapours determined without any Source of Heat external to the gaseous Body itself— Dynamic Radiation and Absorption — Varnishing a metal Surface by a Gas — Varnishing of a Gas by a Vapour— Tenuity of Boracic Ether shown in Experiments on Dynamic Radiation— Influence of Length of radiating Column— In a long Tube, a Vapour at a small Tension may exceed a Gas at a high Tension, while in a short Tube the Gas exceeds the Vapour— Radiation through Humid Air — Action of the Vapour of the Atmosphere on Terrestial and Solar Radiation— Objections answered— Applicability of Rocksalt Plates— Experiments in Tube without Plates— Experiments without either Tube or Plates— Examination of Air from various localities— Influence of the Results on the Science of Meteorology— Application to tropical Rain Torrents— To the Formation of Cumuli— To the Condensation of a Mountainous Region— To Radiation Experiments at high and low Elevations— To the Cold of Central Asia— To the Thermometrio Range in Australia— To the Meteorology of Sahara— To Leslie's Experiments— To Wells's Theory of Iceformation in India— To Mellonrs Theory of Serein j . .. , . 379

APPENDIX TO LECTtTEE XI

Extracts from a Discourse on Radiation through the Earth's Atmosphere— Thermometric range in Asia, Africa, and Australia . . . . . . 415

LECTURE Xn.

Examination of the Diathermancy of Volatile Liquids and their Vapours— Apparatus for this purpose— Eocksalt Cell— Platinum Lamp— Experimental arrangement of Apparatus for determining Absorption of Heat by Liquids— Table of the Absorption of Heat by Liquids at various thicknesses— Table of the Absorption of Heat of the same quality by the Vapours of those Liquids — ^Absorption of Heat by th« same Vapours when the quantities of Vapour are proportional to the quantities of Liquid— Comparative View of the Action of Liquids and their Vapours on Radiant Heat— Predominance of Liquid "Water as an Absorbent fixes the predominance of its Vapour— Physical cause of Transparency and Opacity— Transparency defined as the Discord, and Opacity as the Accord, between the Vibrating Periods of the Source, and thelnterposed Substance— Influence of Temperature on the Transmission of Radiant Heat— Changes of Position through changes of Temperature — Radiation from various Flames through Vapours— Periods of Vi bration of a Hydrogen Flame proved to coincide with those of cool Aqueous Ya. pour- The same true of a Carbonic Oxide Flame and cool Carbonic Acid — Physical Analysis of the Human Breath— Radiation through Liquids, with a Hydrogen

CONTENTS. Xlll

Flame and a Platinum Spiral in a Hydrogen Flame as Sources of Heat— Explanation of certain results obtained by Melloni and Knoblauch . . page 423

Appendix To Lecttjee XII

On Luminous and Obscure Eadiation 456

LECTURE Xin.

Dew : a clear Sky and calm but deep Atmosphere necessary for its copious Formation—Dewed Substances colder than undewed ones— Dewed Substances better Eadiators than undewed ones — Theory of Wells — Dew is the Condensation of the Atmospheric Yapour on Substances which have been chilled by Eadiation — ^Lunar Eadiation— Constitution of the Sun— The bright Lines in the Spectra of the Metals— An incandescent Vapour absorbs the Eays which it can itself emit — Kirchhoff's Generalisation— Fraunhofer's Lines, caused by the Absorption of such Eays by the luminous Solar Atmosphere as that Atmosphere itself could emit — Solar Chemistry— Emission by the Sun— Horschel and Pouillet's Experiments— Mayer's Meteoric Theory— Effect of the Tides on the Earth's Eotation— Energies of the Solar System— Helmholtz, Herschel, Thomson, Waterston— Eelation of the Sun to Vegetable and Animal Life — Form of Solar Energy in Plants and Animals 410

Appendix To Lectuee XIII

Extract from a Lecture on the Physical Basis of Solar Chemistry— Extract from a Paper by Dr. Joule— Extracts from Dr. Mayer's Paper on Organic Motion and Nutrition 616

HEAT

CONSIDERED AS

A MODE OF MOTION".

LECTURE I.

INSTRUMENTS — GENERATION OP HEAT BY MECHANICAL ACTION- CONSUMPTION OP HEAT IN WORK.

appendix: — NOTES ON THE THERMO-ELECTRIC PILE AND GALVANOMETER.

THE aspects of nature provoke in man the spirit of enquiry. As the eye is made for seeing, and the ear for hearing, so the human mind is formed for understanding the phenomena of the material universe. The natural philosophy of our day results from the irrepressible exercise of this endowment. One great characteristic of Natural Science is its growth ; all its facts are fruitful, every new discovery becoming instantly the germ of fresh investigation. But no nobler example of this growth could be adduced than the expansion and development which men's thoughts and knowledge have undergone within the last two-and-twenty years, with reference to the subject which is now to occupy our attention. In scientific manuals, only scanty reference has, as yet, been made to the modern philosophy of Heat, and thus the public knowledge regarding it remains below the attain-

14 LECTUEE I.

able level. But the reserve is natural, for the subject is still an entangled one, and, in entering upon it, we must be prepared to encounter difficulties. In the whole range of ISTatural Science, however, there are none more worthy of being overcome, — none whose subjugation secures a greater reward to the worker. For by mastering the laws and relations of Heat, we make clear to our minds the interdependence of natural forces generally. Let us, then, commence our labours with heart and hope ; let us familiarise ourselves with the latest facts and conceptions regarding this all-pervading agent, and seek diligently the links of law which underlie the facts and give unity to their most diverse appearances. If we succeed here we shall satisfy, to an extent unknown before, that love of order and of beauty which, I am persuaded, is implanted in the mind of every person here present. From the heights at which we aim, v,^e shall have nobler glimpses of the system of Nature than could possibly be obtained, if I, while acting as your guide in the region which we are now about to enter, were to confine myself to its lower levels and already trodden roads.

It is my first duty to make you acquainted mth some of the instruments which I intend to employ in the examination of this question. I must devise some means of making the indications of heat and cold visible to you all, and for this purpose an ordinary thermometer would be useless. You could not see its action ; and I am anxious that you should see, with your own eyes, the facts on which our subsequent philosophy is to be based. I wish to give you the material on which an independent judgment may be founded ; to enable you to reason as I reason if you deem me right, to correct me if I go astray, and to censure me if you find me dealing unfairly with my subject. To secure these ends, I have been obliged to abandon the use of a common thermometer, and to resort to the little in-

THE THEBMO-ELECTEIO PILE AND GALVANOMETEE. 15

strument a b {fig. 1), whicli you see before me on the table, and wMcli is called a thermo-electric pileJ^

By means of this instrmnent I cause the heat which it receives to generate an electric current. You know, or ought to know, that such a current has the power of deflecting a freely suspended magnetic needle, to which it flows parallel. Before you I have placed such a needle m n (fig. 1), surrounded by a covered copper wire, the free

Fig. 1.

ends of which, w w are connected with the thermo-electric pile. The needle is suspended by a fibre, s 5, of unspun silk, and protected by a glass shade, g, from any disturbance by currents of air. To one end of the needle I have fixed a piece of red paper, and to the other end a piece of blue. All of you see these pieces of paper, and when the needle moves, its motion will be clearly visible to the most distant person in this room.f

* A brief description of the thermo-electric pile is given in the Appendix to this Lecture.

f In the actual arrangement the galvanometer here described stood on. a stool in front of the lecture table, the wires w w, being sufficiently long

16 LECTUKE I.

At the present moment the needle is quite at rest, and points to the zero mark on the graduated disc underneath it. This shows that there is no current passmg. I now breathe for an instant against the naked face a of the pile

a single puff of breath is sufficient for my purpose —

observe the effect. The needle starts off and passes through an arc of 90°. It would go further, did I not limit its swing by fixing, edgewise, a thin plate of mica at 90°. Take notice of the direction of the deflection ; the red end of the needle moved from me towards you, as if it disliked me, and had been inspired by a sudden affection for you. This action of the needle is produced by the small amount of warmth communicated by my breath to the face of the pile, and no ordinary thermometer could give so large and prompt an indication. We will let the heat thus communicated waste itself; it will do so in a very short time, and you notice, as the pile cools, that the needle returns to its first position. Observe, now, the effect of cold on the face of the pile. I have here, some ice, but I do not wish to wet my instrument by touching it with ice. Instead of doing so, I will cool this plate of metal by placing it on the ice ; then wipe the chilled metal, and touch with it the face of the pile. You see the effect ; a moment's contact suffices to produce a prompt and energetic deflection of the needle. But mark the direction of the deflection. When the pile was warmed, the red end of the needle moved from me towards you ; now its likings are reversed, and the red end moves from you towards me. Thus you see that cold and heat cause the needle to move in opposite directions. The important point here established is, that from the direction in which the needle moves, we cau, with certainty, infer whether cold or heat has 1been communicated to the pile ;

to reach from the table to the stool ; for a further description of the galvanometer, see the Appendix to this Lecture.

HEAT OF FRICTION. 17

and the energy with which the needle moves— the promptness with which it is driven aside from its position of rest

gives us some idea of the comparative quantity of heat

or cold imparted to it in different cases. In a future lecture I shall explain how we may express the relative quantities of heat with numerical accuracy ; but for the present a general knowledge of the action of our instruments will be sufficient.

My desire now is to connect heat with the more familiar forms of force, and I will, therefore, in the first place, try to furnish you with a store of facts illustrative of the generation of heat by mechanical processes. I have placed some pieces of wood in the next room, which my assistant will now hand to me. Why have I placed them there ? Simply that I may perform my experiments with that sincerity of mind and act which science demands from her cultivators. I know that the temperature of that room is slightly lower than the temperature of this one, and that hence the wood which is now before me must be slightly colder than the face of the pile with which I intend to test the temperature of the wood. Let us prove this. I place the face of the pile against this piece of wood ; the red end of the needle moves from you towards me, thus showing that the contact has chilled the pile. I now carefully rub the face of the pile along the surface of the wood ; I say ' carefully,' because the pile is a brittle instrument, and rough usage would destroy it ; — ^mark what occurs. The prompt and energetic motion of the needle towards you declares that the face of the pile has been heated by this small amount of friction. The needle, you observe, goes quite up to 90° on the side opposite to that towards which it moved before the friction was applied.

ISTow these experiments, which illustrate the developement of heat by mechanical means, must be to us what a boy's school exercises are to him. In order to ^x them on

18 LECTUKE I.

our minds, and obtain due mastery over them, we must repeat and vary them in many ways. In this task I ask you to accompany me. Here is a flat piece of brass with a stem attached to it ; I take the stem in my fingers, preserving the brass from all contact with my warm hand, by enveloping the stem in cold flannel. I place the brass in contact with the face of my pile ; the needle moves, showing that the brass is cold. I now rub the brass against the surface of this cold piece of wood, and lay it once more against my pile. I withdraw it instantly, for it is so hot that if I allowed it to remain in contact with the instrument, the current generated would dash my needle violently against its stops, and probably derange its magnetism. You see the strong deflection which even an instant's contact can produce. Indeed, when a boy at school, I have often blistered my hand by the contact of a brass button, which I had rubbed energetically against a form. Here, also, is a razor, cooled by contact with ice ; and here is a hone, without oil, along which I rub my cool razor, «s if to sharpen it. I now place the razor against the face of the pile, and you see that the steel, which a minute ago was cold, is now hot. Similarly, I take this knife and knife-board, which are both cold, and rub the knife along the board. I place the knife against the pile, and you observe the result ; a powerful deflection, which declares the knife to be hot. I pass this cold saw through this cold piece of wood, and place, in the first instance, the surface of the wood against which the saw has rubbed, ia contact with the pile. The needle instantly moves in a direction which shows the wood to be heated. I allow the needle to return to zero, and now apply the saw to the pile. It also is hot. These are the simplest and most common-placQ examples of the generation of heat by friction, and I choose them for this reason. Mean as they appear, they will lead us by degrees

HEAT OF COMPEESSION AND PEECUSSION. 19

into the secret recesses of ISTature, and lay open to our view the policy of the material universe.

Let me now make an experiment to illustrate the developement of heat by compression. I have here a piece of deal, cooled below the temperature of the room, and giving, when placed in contact with our pile, the deflection which indicates cold. I place this wood between the plates of a small hydraulic press, and squeeze it forcibly. The plates of the press are also, you will observe, cooler than the air of the room. After compression, I bring the wood into contact with the pile ; see the effect. The galvanometer declares that heat has been developed by the act of compression. Precisely the same occurs when I place this lead bullet between the plates of the press and squeeze it thus to flatness.