Chapter 82
Physics and natural science of bodies: heat, light, electricity, magnetism and the like.
237 passages · 186 principal · Covers the 49 volumes of primary texts
The idea of physics, as these volumes present it, covers the natural science of bodies and their powers: weight and attraction, the cohesion of solids and liquids, the burning of fuel, heat and its passage into work, electricity and magnetism, light and sound, and the explanation of earthly events such as tides, glaciers and storms. It is a great idea for the Harvard Classics because the collection holds it in two very different states. In the earlier writers physics appears as a hope, a quarrel over method, or a source of images for moral and religious argument. In the nineteenth-century lectures of Faraday, Helmholtz and Kelvin it appears as a working science, shown on the lecture table and organised by a few general laws. Reading the two together lets us watch the questions sharpen. We see what a physical explanation is, how many forces nature contains, and whether those forces are at bottom one.
The first question is what kind of knowledge physics is. Hobbes, mapping the sciences, places it among the studies that draw consequences from the qualities of bodies, celestial and terrestrial alike1. Kant gives it a mixed character: it treats the laws by which everything in nature happens, and it has both an empirical and a rational part2. Pascal is blunter about where its principles come from. In physics experiments are the only principles, and the consequences multiply as the experiments do3. He also admires a science like hydrostatics, which draws fine conclusions from a few premises4. Newton joins the two emphases. He studies particular powers such as gravity, elasticity and the resistance of fluids5. Yet he suspects that all natural phenomena may depend on forces by which particles attract or repel one another, though the causes of those forces remain unknown6. Locke takes Newton’s account of the planets as proof that true and certain knowledge of great masses of matter is possible7.
That success did not end the dispute about what counts as explanation. Voltaire sets the Cartesian world of vortices and impulsion against Newtonian attraction. He follows the contrast through the tides, light, the shape of the earth and chemistry8, and he credits Bacon with having guessed at gravitational attraction between earth, moon and planets9. Rousseau’s vicar finds both systems wanting. Neither the vortices nor attraction joined to a projectile force explains why motion exists at all10. Berkeley presses harder still. He argues that mechanical explanation by bodies and laws of motion cannot account for animals, plants, sensations or the marks of contrivance in nature11. Pascal, however, clears away one older kind of explanation. He mocks the claim that bodies shun the void, asking whether they have arms and nerves for the purpose12. Elsewhere he dismisses the doctrine that nature abhors a vacuum as an absurd account of bodily motion13. The moderns may disagree about mechanism and attraction, but they agree that nature does not act out of desire or dread.
Faraday’s lectures on the forces of matter take up Newton’s suggestion and give it concrete form. He defines force by demonstration: pulling, pushing, rubbed shellac lifting paper, the explosion of gunpowder14. He insists that attraction and heat are distinct powers, and that only a few such powers govern all natural phenomena15. Gravitation comes first. He weighs water and balances it against platinum and aluminium16,17. He argues that the same power makes water fall, presses on the scale and swings the pendulum18. He weighs marble and then the gas driven out of it, to show that gravitation is universal and constant in quantity19. Gases gravitate like solids and liquids, so all things gravitate whatever their form20. In his lectures on the candle, carbonic acid is poured like a liquid and floats soap bubbles because it is heavier than air21,22.
Cohesion is a second power, the attraction between particles of the same body. Burke had already explained fluidity by the roundness and weak cohesion of particles23. Faraday makes cohesion a subject of experiment. Broken flint and powdered glass cannot be rebuilt, but clean lead joins and dissolved alum recrystallizes24,25. Charcoal and diamond are one substance differently coalesced, and Prince Rupert’s drops shatter from internal tension26,27. Glass breaks shapelessly while mica and salt cleave regularly, because particles attract unequally in different directions28,29. Polarized light shows these hidden differences, and strain or heat alters them30,31. The same power holds soap films together32. Through capillary action it lifts melted tallow up the wick33,34.
Chemical affinity is a third attraction, acting between unlike particles and distinct from both gravitation and cohesion35. Faraday shows that it can overcome cohesion violently36. It may wait or act at once37. Combustion is always this affinity, as with lead so finely divided that it ignites in air, or iron kept from burning by a coat of oxide38. Affinity is fixed in proportion, like gravitation39. Unlike gravitation, though, its heat and light vanish when the action ceases40. The candle lectures trace this power through the flame. Fuel must become vapour, and the flame is fed by a rising current of air41. It burns in a ring where vapour meets air42. Starved of fresh air it dies or smokes43,44. Charcoal, by contrast, burns without flame and passes wholly into gas45,46.
Heat, in Faraday’s account, works against cohesion. It melts ice and turns water into steam by weakening the attraction of particles47. Melting absorbs heat and solidifying releases it48. A cubic inch of water yields about 1,700 of steam49. Heat expands air and metal, and it can be produced by friction or by sudden compression50,51. Darwin meets the same laws in the field. At altitude water boils at a lower temperature, so his potatoes stayed hard52. Older writers had pointed toward the link between heat and motion without establishing it. Bacon’s imagined furnaces generate heat by motion53. Milton’s Adam imagines fire kindled by colliding bodies54. Descartes treated fire, heat and light within a single system55. Pascal found it astonishing that heat should be the motion of molecules56.
Helmholtz turns that intimation into a law. He contrasts the older view of heat as an indestructible substance with the mechanical theory57. He notes that Carnot drew work from heat while friction produced heat of no known origin58. He takes Davy’s rubbing of ice as proof that friction creates heat rather than releasing latent heat59. The vis viva lost in friction reappears as heat60. Joule’s experiments fix a definite mechanical equivalent61, and heat and work prove interconvertible, so that expanding gases cool by doing work62. Raised weights, gunpowder and chemical affinity all store a capacity for work that is spent but not lost63,64,65. Hence the law credited to Mayer and Joule: the quantity of force in nature is unchangeable66. All forces convert into one another in equivalent quantities67, and perpetual motion is impossible68. The work of rivers, engines and animals traces back to the sun’s heat69.
Electricity and magnetism enter the books first as marvel. In the *Thousand and One Nights* a loadstone mountain draws the nails from ships by a property God gave it70,71. Franklin recounts how he took up experiments with a glass tube72. He tells how his paper on the sameness of lightning and electricity spread73, and how lightning drawn from clouds by a pointed rod confirmed the theory74. Darwin studies fulgurites, sand fused by lightning, and compares them with tubes made by galvanic shocks75,76. He infers how the stroke branches77 and speculates about causes of electrical disturbance78. Dana explains why a ship’s many points seldom suffer from lightning79. Faraday turns marvel into classification. Electricity is a dual power, attracting and repelling80,81. Magnetism is a separate double power concentrated at the ends of a magnet, and each fragment of a broken bar is itself a magnet82,83,84. Electricity is a transferable power without substance85. It passes instantly through metals and is stopped by glass, unlike the slow conduction of heat86,87.
The voltaic battery joins these powers together. In Faraday’s demonstrations, chemical force passes from zinc to platinum by contact88 and is carried along wires to show electrical attraction89. It splits water into a combustible gas that a spark recombines90,91, an energy he compares to lightning92. It deposits copper and reverses when the poles are exchanged93. From this Faraday argues for the correlation of forces94. The spark is the burning of zinc in the cells95. The power remains chemical affinity whatever we call its source96. The current makes iron a magnet97,98. Chemical, electric, magnetic, heat and light forces convert mutually99. Helmholtz reaches the same conclusion by way of work. Current carries the force of burning zinc to decompose water100,101, and each force spends its capacity for work as it acts102. Two literary writers sensed a related unity. Goethe wondered whether colour, magnetism and electricity depend on polarity103, and Emerson saw polarity in magnetism, electricity, gravity and affinity alike104.
Light shows most clearly how a conception can change. Dante uses equal angles of reflection to illustrate his argument105. Bacon imagines houses for demonstrating light and colour106 and sound107. Descartes treated light’s transmission from the stars108. Berkeley’s Hylas describes light as a fluid of agitated particles striking the optic nerve109, while Pascal reports the view that it is pressure56. Voltaire presents Newton’s prism, splitting light into seven coloured rays110. He explains Newton’s account of reflection and transparency by pores and density, together with his “fits” of light111, and the limit refraction places on telescopes112. Faraday speaks of matter attracting and bending light113,114. Kelvin’s lectures change the vocabulary. Sound and light are both waves and differ in frequency115. Light, however, is a transverse vibration of the luminiferous ether116. Radiant heat is continuous with light at lower frequencies117. Polarisation and diffraction confirm the undulatory theory118,119, and give measured wave-lengths120. Even the blue sky is explained by small particles acting on the ether121. Yet Kelvin must give the ether strange properties, a solid that the earth passes through as a cork moves through wax122,123.
These principles are then turned on the earth. Helmholtz explains snow lines and the descent of glaciers124, and why air cools as it rises, which accounts for the Föhn125. Glacier ice flows like a viscous body126. He deduces from the mechanical theory that pressure lowers the freezing point127, so that pressed ice melts and refreezes128. He imitates glacier formation with a hydraulic press129,130. Here the moderns openly disagree. Helmholtz reports the dispute between Faraday and the Thomsons over regelation131, and he holds pressure sufficient against Faraday’s contact-action132,133. Kelvin derives the tide-generating force from differential lunar attraction134. He sets equilibrium against dynamic theories135 and separates wind effects from true tides136. He argues that tides prove the earth’s rigidity137,138. Faraday demonstrates the pressure of the atmosphere139. Darwin explains mountain clouds by condensation140, records volcanic and seismic forces141, and reasons about why mines suffer less in earthquakes142.
Physical facts also serve other ends. Job’s whirlwind speaks of rain and lightning beyond human understanding143. Marcus Aurelius draws an analogy from sunlight passing straight through an opening144, and Browne uses optics and the fire-resistance of gold in theology145. Burke compares the destruction of institutions to throwing away steam, electricity or magnetism, forces tamed for use146. Burke also finds excessive light sublime because it overpowers the eye147. Some questions stay open. Newton left the causes of attraction unknown. Kelvin’s ether remains a puzzle. Faraday and Helmholtz differ over ice. And Berkeley’s doubt whether mechanism can reach life and sensation is not answered anywhere in these pages.
Introductory essay written by Claude Opus 5.5 from the outline and the notes on every passage below; quotations are checked against this edition.
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