by Will Durant
The second half of the seventeenth century was probably the most brilliant epoch in the history of the physics of light. First, what was light itself? Hooke, always ready to delve into difficulties, hazarded the view that light is “nothing else but a peculiar motion of the parts of the luminous body” 42—i.e., light differs from heat only in the more rapid motion of the body’s constituent particles.* Second, how fast does it move? Scientists had heretofore assumed that the speed of light was infinite, and even the venturesome Hooke had regarded it as in any case too great for measurement. In 1675 Olaus Roemer, a Danish astronomer brought to Paris by Picard, proved the finite velocity of light by noting that the period of eclipse of Jupiter’s innermost satellite varied according as the earth was moving toward or away from that planet; by computations based on the time of the satellite’s revolution and the diameter of the earth’s orbit, he showed that the variation in the observed time of the eclipse was due to the time taken by light from the satellite to traverse the orbit of the earth; and on that slender basis he calculated the speed of light at some 120,000 miles per second. (The current estimate is 186,000 miles.)
But how was light transmitted? Did it move in straight lines? If so, how did it get around corners? Francesco Grimaldi, Jesuit professor at Bologna, discovered and named (1665) the phenomena of diffraction—that rays of light passing through a small opening into a dark room spread more widely on the opposite wall than straight lines from source to wall would warrant, and that rays of light are slightly deflected from a straight line when they pass by the edges of an opaque body; these and other findings led Grimaldi to accept Leonardo da Vinci’s suggestion that light moves in expanding waves. Hooke agreed, but it was Huygens who established the wave theory still popular among physicists. In another classic of modern science, Traité de la lumière (1690), Huygens reported the conclusions he had reached from studies begun twelve years before: that light is transmitted by a hypothetical substance which he called aether (from the Greek word for the sky), and which he conceived as made up of small, hard, elastic bodies transmitting light in successive spherical waves spreading out from the luminous source. On this theory he formulated laws of reflection, refraction, and double refraction; he ascribed to the enveloping motion of the waves the ability of light to move around corners and opaque objects; and he explained translucence by supposing the ether particles to be so minute that they can travel around and between the particles composing transparent liquids and solids. But he confessed himself unable to explain polarization; this was one of the reasons why Newton rejected the wave hypothesis and preferred the corpuscular theory of light.
The seventeenth century made only modest advances in the study of electricity after the work of Gilbert and Kircher on magnetism, and of Cabeo on electrical repulsion. Halley studied the influence of terrestrial magnetism on compass needles, and was the first to recognize a connection between the magnetism of the earth and the aurora borealis (1692). Guericke reported in 1672 some experiments in frictional electricity. A ball of sulphur, after being rotated against his hand, attracted paper, feathers, and other light objects, and carried them around with it in its rotation; he likened this to the action of the earth carrying along with it the objects on or near its surface. He verified electrical repulsion by showing that a feather, placed between the electrified ball and the floor, jumped up and down from one to the other. He pioneered in studying conduction by proving that an electric charge could travel along a linen thread, and that bodies could become electrified by being brought near to the electrified ball. Francis Hauksbee, of the Royal Society, developed (1705–9) a better method of generating electricity by rapidly rotating an exhausted glass ball, and then applying it to his hand; the contacts gave off sparks an inch long, providing light enough for reading. Another Englishman, Wall, having produced similar sparks, likened their sound and light to thunder and lightning (1708). Newton made the same comparison in 1716; Franklin confirmed the relation in 1749. So, year by year, and mind by mind, the impenetrable immensity surrenders some teasing, luring fragment of its mystery.
VI. CHEMISTRY
This remarkable century saw the science of chemistry evolve from the experiments and vagaries of alchemy. Industry had long been accumulating chemical knowledge through such operations as smelting iron, tanning leather, mixing dyes, brewing beer; but the investigation of substances in their composition, combination, and transformation had been for the most part left to alchemists seeking gold, or to pharmacologists concocting drugs, or to philosophers, from Democritus to Descartes, puzzling over the constitution of matter. Some approach to chemistry had been made by Andreas Libavius in 1597 and by Jan van Helmont in 1640; but both of these men shared the alchemist’s hope of transmuting “base” metal into gold. Boyle himself made experiments with this aim. In 1689 he secured repeal of an old English statute against “multiplying gold and silver,” 43 and at his death (1691) he left to his executors a quantity of red earth with instructions for trying to turn it into gold. 44 Now that the transmutation of metals is a cliché of chemistry we can applaud the science in alchemy while condemning and concealing the itch for gold.
The greatest blow to alchemy was the publication of Boyle’s Sceptical Chymist (1661)—the prime classic in the history of chemistry. He apologized for “suffering” his treatise “to pass abroad so maimed and imperfect,” 45 but, with his many ailments, he was never confident of much longer life. He was consoled to “observe that of late chymistry begins, as indeed it deserves, to be cultivated by learned men who before despised it.” 46 He called his chemistry skeptical because he proposed to reject all mystical explanations and occult qualities as the “sanctuary of ignorance,” and was resolved to rely upon “experiments rather than syllogisms.” 47 He abandoned the traditional division of matter into the four elements of air, fire, water, and earth; these, he argued, were compounds, not elements; the real elements were rather “certain primitive and simple, or perfectly unmingled bodies, which, not being made of any other bodies or of one another,” are the ingredients of all compounds, and into which all compounds may be resolved. He did not mean that the elements were the ultimate constituents of matter; these minima naturalia, he thought, were tiny particles, invisible to the eye, and differing in shape and size, like the atoms of Leucippus. From the diversity and motion of these particles, and their union in “corpuscles,” all bodies, and all their qualities and conditions, like color, magnetism, heat, and fire, arise by purely mechanical means and laws.
Fire was as fascinating to scientists as to dreamers at the hearth. What made a substance burn? How explain those ever-changing tongues of flame, beautiful, imperious, and terrible? In 1669 a German chemist, Johann Joachim Becher, reduced all “elements” to two—water and earth; one form of the latter he called “oily earth,” which he believed present in all combustible bodies; this it was that burned. In the eighteenth century Georg Stahl, following this false lead, was to set chemistry askew for decades with his similar theory of “phlogiston.” Boyle took another cue. Noting that various burning substances ceased to burn in a vacuum, he concluded that “there is in the air a little vital quintessence . . . which serves to the refreshment and restoration of our vital spirits.” 48 His younger contemporary John Mayow, also of the Royal Society, advanced (1647) toward our current theory of fire by positing among the constituents of air a substance that unites with metals when they are calcined (oxidized); and he believed that a similar substance, entering our bodies, changes venous into arterial blood. A hundred years had to pass before Scheele and Priestley would definitely discover oxygen.
About 1670 a German alchemist, Hennig Brand, discovered that he could obtain from human urine a chemical that glowed in the dark without preliminary exposure to light. A Dresden chemist, Kraft, exhibited the new product before Charles II at London in 1677. Boyle drew from the secretive Kraft only the admission that the luminous substance “was somewhat that belonged to the body of man.” 49 The hint proved enough: Boyle
soon obtained his own supply of phosphorus, and by a series of experiments he established all that is yet known about the glowing of that element. The new product cost its purchasers six guineas ($315?) per ounce, despite the abundance of the source.
VII. TECHNOLOGY
Until the nineteenth century more stimulus was given by industry to science than by science to industry; and until the twentieth century inventions were made less often in the laboratory than in the shop or field. In the most important case of all, the development of the steam engine, the two processes may have proceeded hand in hand.
Hero of Alexandria, in or before the third century A.D., made several steam engines, but, so far as we know, these were used as toys or marvels to amuse the multitude rather than as mechanisms replacing human energy. Early in the sixteenth century Leonardo da Vinci described a gun which by steam pressure could propel an iron bolt twelve hundred yards; but his scientific manuscripts remained unpublished till 1880. Some of Hero’s Greek writings were translated into Latin in 1589. Jerome Cardan (1550) and Giambattista della Porta (1601) pointed out that a vacuum could be produced by the condensation of steam, and Porta described a machine for using the pressure of steam to raise a column of water. Similar applications of expanding steam were proposed by Salomon de Caus at Paris in 1615 and by Branca at Rome in 1629; and in 1630 David Ramsay obtained from Charles I of England a patent for machines “to raise water from low pits by fire . . . to make any sort of mills to go on standing waters by continual motion, without help of wind, waite [weight?], or horse.” 50 In 1663 Edward Somerset, Marquis of Worcester, received from Parliament a ninety-nine-year monopoly on “the most stupendous work in the whole world”—a “water-commanding engine” that raised water to a height of forty feet; 51 by this mechanism he proposed to operate waterworks for a large part of London, but he died before he could put his plans into effect. About 1675 Samuel Morland, master mechanic to Charles II, invented the plunger pump, and in 1685 he published the first accurate description of the expansive power of steam. In 1680 Huygens made the first gas engine with cylinder and piston driven by the expansive force of exploding gunpowder.
Huygens’ French assistant, Denis Papin, went to England, worked with Boyle, and published in 1681 an account of a “digester”—a pressure cooker—to soften bones by water boiling in a closed vessel. To prevent explosion he attached to the top of the vessel a tube that could be opened when the pressure reached a certain point; this first “safety valve” played a saving role in the development of the steam engine. Papin went on to show that the power of expanding steam could be piped pneumatically from one place to another. Moving to Marburg in Germany, he demonstrated (1690) the first engine in which the condensation of steam, producing a vacuum, was used to drive a piston. He suggested the possibilities of this machine for throwing bombs, raising water from mines, and propelling ships by paddle wheels; and in 1707 (precisely a century before Fulton’s Clermont moved up the Hudson River) he used his steam engine to drive a paddlewheel boat on the River Fulda at Cassel. 52 This boat, however, was wrecked, and the German authorities, comfortable in the status quo, and perhaps fearing the spread of unemployment, discouraged the development of mechanical power. 53
A similar apparatus had been offered to the Navy Board in England about 1700 by Thomas Savery, but had been turned down with the alleged comment, “What have interloping people, that have no concern with us, to do to pretend to contrive or invent things for us?” 54 Savery demonstrated his device on the Thames, but the Navy again rejected it. In 1698 Savery patented the first steam engine actually employed to pump water out of mines. In 1699 he was awarded a patent granting him for fourteen years “the sole exercise of a new invention . . . for raising water and occasioning motion by the impellant force of fire; which will be of great use for draining mines, serving towns with water, and for the working of all sorts of mills.” 55 Savery’s engines, however, proved costly and dangerous: they had gauge cocks but no safety valves; they were subject to boiler explosions; and though they were used in some mines to pump out water, the mine owners soon returned to the employment of horses.
At this point in the story we again meet Robert Hooke. About 1702, according to a reliable contemporary, he corresponded with a Dartmouth ironmonger and blacksmith, Thomas Newcomen, on the possibility of using the air-pump principle to produce mechanical power. “Could you make a speedy vacuum under your second cylinder,” he wrote, “your work is done.” 56 Apparently Newcomen had been experimenting with a steam engine; here science and industry visibly touched. Hooke was skeptical, let the matter drop, and again missed an opportunity. Newcomen joined with a plumber, John Cawley, to build (1712) a steam engine—with rocking beam, piston, and safety valve—that could be trusted to do heavy work without danger of explosion, and with fully automatic control. Newcomen continued till his death (1729) to improve his engine; but we may date from Savery’s patent in 1699, and Newcomen’s engine of 1712, the beginnings of the Industrial Revolution that in the next two centuries would change the face and air of the world.
VIII. BIOLOGY
The remarkable group of investigators that made the glory of the Royal Society extended its researches into the sciences of life. The omnipresent Hooke demonstrated experimentally what Sir Kenelm Digby—that “arrant mountebank,” as Evelyn called him 57—had already pointed out: that plants need air for their life. He sowed lettuce seed in soil under the open sky, and, at the same time, similar seed in similar soil in a vacuum chamber; the first grew an inch and a half in eight days, the other not at all. Hooke identified the part of the air used up in combustion with the part used up in plant and animal respiration, and described this used part as nitrous in character (1665). He showed that animals which had stopped breathing could be kept alive by blowing air into their lungs with bellows. He discovered the cellular structure of living tissue, and invented the term cell for its organic constituents. Through his microscope the members of the Society saw with delight the cells of cork, whereof, Hooke estimated, one cubic inch contained 1,200,000,000 cells. He studied the histology of insects and plants, and gave novel drawings of them in his Micrographia. Hooke was always on the verge of ranking with Galileo and Newton.
Another member of the society, John Ray, shared in giving its modern form to the science of botany. He was the son of a blacksmith, but he made his way to Cambridge, became a fellow of Trinity College, and was ordained an Anglican priest. Like Boyle, he gave his devotion to religion as well as to science. Because he would not sign the Act of Uniformity (1662) pledging nonresistance to Charles II, he resigned his fellowship, and set out with his pupil Francis Willughby on a tour of Europe to gather data for a systematic description of the animal and plant kingdoms. Willughby undertook the zoology, but died after completing the sections on birds and fishes. In 1670 Ray issued a Catalogus Plantarum Angliae, which became the frame of English botany. Helped by the improved terminology and classification established in 1678 by Joachim Jungius, Ray proposed a Methodus Plantarum Nova (1682), which divided all flowering plants into dicotyledons and monocotyledons according to their having two seed leaves or only one. He completed his great task in one of the chefs-d’oeuvre of modern science, the massive three-volume Historia Generalis Plantarum (1682–1704), which described 18,625 species of plants. Ray was the first to use the word species in its biological sense, as a group of organisms derived from similar parents and capable of reproducing their kind. This definition, and the later classification by Linnaeus (1751), set the stage for the controversy over the origin and mutability of species. Meanwhile Ray edited Willughby’s manuscripts on ichthyology and ornithology, and added a Synopsis Methodica Animalium Quadrupedum (1693), providing for modern zoology the first really scientific classification of animals. 58 Order was Ray’s first law.
Even in antiquity botanists had recognized that some plants might be termed female because they bore fruit, and others male because they did not, and Theophrastus, in the third century before Christ
, had observed that the female date palm produces fruit only if the dust of the male date palm has been shaken over it; but these ideas had been almost forgotten. In 1682 Nehemiah Grew, of the Royal Society, gave a new charm to flowers by definitely affirming the sexuality of plants. Studying plant tissues under the microscope, he noted the pores in the upper surface of leaves, and suggested that leaves are organs of respiration. Flowers he described as organs of reproduction: the pistils as female, the stamens as male, the pollen as seed. He mistakenly assumed that all plants are hermaphrodites, uniting male and female structures in one organism. In 1691 Rudolf Camerarius, professor of botany at Tubingen, definitely proved the sexuality of plants by showing that they would bear no fruit after the removal of their anthers—the pollen-containing part of the stamens.
On the same day (December 7, 1671) that the Royal Society of London received Grew’s first essay (The Anatomy of Vegetables Begun) it received also a manuscript from Marcello Malpighi of Bologna. The Society published it (1675) as Anatomes Plantarum Idea; the use of Latin was still facilitating the international of science. Malpighi divided with Grew the honor of establishing the histology of plants, but his major contribution was to zoology. In 1676 Mariotte, by chemically analyzing the residue of plants and the soil in which they had grown, showed that they absorb nutritive elements in the water that they suck from the earth. Neither Mariotte nor Grew nor Malpighi recognized the power of plants to take nourishment from the air; but the processes of nutrition and reproduction now discovered were a vast advance upon Aristotle’s vague explanation of plant growth by the expansive ambitions of the “vegetable soul.”