In examining smooth and rough surfaces to determine the cause of their color, he made use of the microscope, and pointed out the very obvious example of the difference in color of a rough and a polished piece of the same block of stone. He used some striking ill.u.s.trations of the effect of light and the position of the eye upon colors. "Thus the color of plush or velvet will appear various if you stroke part of it one way and part another, the posture of the particular threads in regard to the light, or the eye, being thereby varied. And 'tis observable that in a field of ripe corn, blown upon by the wind, there will appear waves of a color different from that of the rest of the corn, because the wind, by depressing some of the ears more than others, causes one to reflect more light from the lateral and strawy parts than another."(7) His work upon color, however, as upon light, was entirely overshadowed by the work of his great fellow-countryman Newton.

Boyle's work on electricity was a continuation of Gilbert's, to which he added several new facts. He added several substances to Gilbert's list of "electrics," experimented on smooth and rough surfaces in exciting of electricity, and made the important discovery that amber retained its attractive virtue after the friction that excited it bad ceased. "For the attrition having caused an intestine motion in its parts," he says, "the heat thereby excited ought not to cease as soon as ever the rubbing is over, but to continue capable of emitting effluvia for some time afterwards, longer or shorter according to the goodness of the electric and the degree of the commotion made; all which, joined together, may sometimes make the effect considerable; and by this means, on a warm day, I, with a certain body not bigger than a pea, but very vigorously attractive, moved a steel needle, freely poised, about three minutes after I had left off rubbing it."(8)

MARIOTTE AND VON GUERICKE

Working contemporaneously with Boyle, and a man whose name is usually a.s.sociated with his as the propounder of the law of density of gases, was Edme Mariotte (died 1684), a native of Burgundy. Mariotte demonstrated that but for the resistance of the atmosphere, all bodies, whether light or heavy, dense or thin, would fall with equal rapidity, and he proved this by the well-known "guinea-and-feather" experiment.

Having exhausted the air from a long gla.s.s tube in which a guinea piece and a feather had been placed, he showed that in the vacuum thus formed they fell with equal rapidity as often as the tube was reversed. From his various experiments as to the pressure of the atmosphere he deduced the law that the density and elasticity of the atmosphere are precisely proportional to the compressing force (the law of Boyle and Mariotte).

He also ascertained that air existed in a state of mechanical mixture with liquids, "existing between their particles in a state of condensation." He made many other experiments, especially on the collision of bodies, but his most important work was upon the atmosphere.

But meanwhile another contemporary of Boyle and Mariotte was interesting himself in the study of the atmosphere, and had made a wonderful invention and a most striking demonstration. This was Otto von Guericke (1602-1686), Burgomaster of Magdeburg, and councillor to his "most serene and potent Highness" the elector of that place. When not engrossed with the duties of public office, he devoted his time to the study of the sciences, particularly pneumatics and electricity, both then in their infancy. The discoveries of Galileo, Pascal, and Torricelli incited him to solve the problem of the creation of a vacuum--a desideratum since before the days of Aristotle. His first experiments were with a wooden pump and a barrel of water, but he soon found that with such porous material as wood a vacuum could not be created or maintained. He therefore made use of a globe of copper, with pump and stop-c.o.c.k; and with this he was able to pump out air almost as easily as water. Thus, in 1650, the air-pump was invented. Continuing his experiments upon vacuums and atmospheric pressure with his newly discovered pump, he made some startling discoveries as to the enormous pressure exerted by the air.

It was not his intention, however, to demonstrate his newly acquired knowledge by words or theories alone, nor by mere laboratory experiments; but he chose instead an open field, to which were invited Emperor Ferdinand III., and all the princes of the Diet at Ratisbon.

When they were a.s.sembled he produced two hollow bra.s.s hemispheres about two feet in diameter, and placing their exactly fitting surfaces together, proceeded to pump out the air from their hollow interior, thus causing them to stick together firmly in a most remarkable way, apparently without anything holding them. This of itself was strange enough; but now the worthy burgomaster produced teams of horses, and harnessing them to either side of the hemispheres, attempted to pull the adhering bra.s.ses apart. Five, ten, fifteen teams--thirty horses, in all--were attached; but pull and tug as they would they could not separate the firmly clasped hemispheres. The enormous pressure of the atmosphere had been most strikingly demonstrated.

But it is one thing to demonstrate, another to convince; and many of the good people of Magdeburg shook their heads over this "devil's contrivance," and predicted that Heaven would punish the Herr Burgomaster, as indeed it had once by striking his house with lightning and injuring some of his infernal contrivances. They predicted his future punishment, but they did not molest him, for to his fellow-citizens, who talked and laughed, drank and smoked with him, and knew him for the honest citizen that he was, he did not seem bewitched at all. And so he lived and worked and added other facts to science, and his bra.s.s hemispheres were not destroyed by fanatical Inquisitors, but are still preserved in the royal library at Berlin.

In his experiments with his air-pump he discovered many things regarding the action of gases, among others, that animals cannot live in a vacuum.

He invented the anemoscope and the air-balance, and being thus enabled to weight the air and note the changes that preceded storms and calms, he was able still further to dumfound his wondering fellow-Magde-burgers by more or less accurate predictions about the weather.

Von Guericke did not accept Gilbert's theory that the earth was a great magnet, but in his experiments along lines similar to those pursued by Gilbert, he not only invented the first electrical machine, but discovered electrical attraction and repulsion. The electrical machine which he invented consisted of a sphere of sulphur mounted on an iron axis to imitate the rotation of the earth, and which, when rubbed, manifested electrical reactions. When this globe was revolved and stroked with the dry hand it was found that it attached to it "all sorts of little fragments, like leaves of gold, silver, paper, etc." "Thus this globe," he says, "when brought rather near drops of water causes them to swell and puff up. It likewise attracts air, smoke, etc."(9) Before the time of Guericke's demonstrations, Cabaeus had noted that chaff leaped back from an "electric," but he did not interpret the phenomenon as electrical repulsion. Von Guericke, however, recognized it as such, and refers to it as what he calls "expulsive virtue." "Even expulsive virtue is seen in this globe," he says, "for it not only attracts, but also REPELS again from itself little bodies of this sort, nor does it receive them until they have touched something else." It will be observed from this that he was very close to discovering the discharge of the electrification of attracted bodies by contact with some other object, after which they are reattracted by the electric.

He performed a most interesting experiment with his sulphur globe and a feather, and in doing so came near antic.i.p.ating Benjamin Franklin in his discovery of the effects of pointed conductors in drawing off the discharge. Having revolved and stroked his globe until it repelled a bit of down, he removed the globe from its rack and advancing it towards the now repellent down, drove it before him about the room. In this chase he observed that the down preferred to alight against "the points of any object whatsoever." He noticed that should the down chance to be driven within a few inches of a lighted candle, its att.i.tude towards the globe suddenly changed, and instead of running away from it, it now "flew to it for protection"--the charge on the down having been dissipated by the hot air. He also noted that if one face of a feather had been first attracted and then repelled by the sulphur ball, that the surface so affected was always turned towards the globe; so that if the positions of the two were reversed, the sides of the feather reversed also.

Still another important discovery, that of electrical conduction, was made by Von Guericke. Until his discovery no one had observed the transference of electricity from one body to another, although Gilbert had some time before noted that a rod rendered magnetic at one end became so at the other. Von Guericke's experiments were made upon a linen thread with his sulphur globe, which, he says, "having been previously excited by rubbing, can exercise likewise its virtue through a linen thread an ell or more long, and there attract something." But this discovery, and his equally important one that the sulphur ball becomes luminous when rubbed, were practically forgotten until again brought to notice by the discoveries of Francis Hauksbee and Stephen Gray early in the eighteenth century. From this we may gather that Von Guericke himself did not realize the import of his discoveries, for otherwise he would certainly have carried his investigations still further. But as it was he turned his attention to other fields of research.

ROBERT HOOKE

A slender, crooked, shrivelled-limbed, cantankerous little man, with dishevelled hair and haggard countenance, bad-tempered and irritable, penurious and dishonest, at least in his claims for priority in discoveries--this is the picture usually drawn, alike by friends and enemies, of Robert Hooke (1635-1703), a man with an almost unparalleled genius for scientific discoveries in almost all branches of science.

History gives few examples so striking of a man whose really great achievements in science would alone have made his name immortal, and yet who had the pusillanimous spirit of a charlatan--an almost insane mania, as it seems--for claiming the credit of discoveries made by others.

This att.i.tude of mind can hardly be explained except as a mania: it is certainly more charitable so to regard it. For his own discoveries and inventions were so numerous that a few more or less would hardly have added to his fame, as his reputation as a philosopher was well established. Admiration for his ability and his philosophical knowledge must always be marred by the recollection of his arrogant claims to the discoveries of other philosophers.

It seems pretty definitely determined that Hooke should be credited with the invention of the balance-spring for regulating watches; but for a long time a heated controversy was waged between Hooke and Huygens as to who was the real inventor. It appears that Hooke conceived the idea of the balance-spring, while to Huygens belongs the credit of having adapted the COILED spring in a working model. He thus made practical Hooke's conception, which is without value except as applied by the coiled spring; but, nevertheless, the inventor, as well as the perfector, should receive credit. In this controversy, unlike many others, the blame cannot be laid at Hooke's door.

Hooke was the first curator of the Royal Society, and when anything was to be investigated, usually invented the mechanical devices for doing so. Astronomical apparatus, instruments for measuring specific weights, clocks and chronometers, methods of measuring the velocity of falling bodies, freezing and boiling points, strength of gunpowder, magnetic instruments--in short, all kinds of ingenious mechanical devices in all branches of science and mechanics. It was he who made the famous air-pump of Robert Boyle, based on Boyle's plans. Incidentally, Hooke claimed to be the inventor of the first air-pump himself, although this claim is now entirely discredited.

Within a period of two years he devised no less than thirty different methods of flying, all of which, of course, came to nothing, but go to show the fertile imagination of the man, and his tireless energy. He experimented with electricity and made some novel suggestions upon the difference between the electric spark and the glow, although on the whole his contributions in this field are unimportant. He also first pointed out that the motions of the heavenly bodies must be looked upon as a mechanical problem, and was almost within grasping distance of the exact theory of gravitation, himself originating the idea of making use of the pendulum in measuring gravity. Likewise, he first proposed the wave theory of light; although it was Huygens who established it on its present foundation.

Hooke published, among other things, a book of plates and descriptions of his Microscopical Observations, which gives an idea of the advance that had already been made in microscopy in his time. Two of these plates are given here, which, even in this age of microscopy, are both interesting and instructive. These plates are made from prints of Hooke's original copper plates, and show that excellent lenses were made even at that time. They ill.u.s.trate, also, how much might have been accomplished in the field of medicine if more attention had been given to microscopy by physicians. Even a century later, had physicians made better use of their microscopes, they could hardly have overlooked such an easily found parasite as the itch mite, which is quite as easily detected as the cheese mite, pictured in Hooke's book.

In justice to Hooke, and in extenuation of his otherwise inexcusable peculiarities of mind, it should be remembered that for many years he suffered from a painful and wasting disease. This may have affected his mental equilibrium, without appreciably affecting his ingenuity. In his own time this condition would hardly have been considered a disease; but to-day, with our advanced ideas as to mental diseases, we should be more inclined to ascribe his unfortunate att.i.tude of mind to a pathological condition, rather than to any manifestation of normal mentality.

From this point of view his mental deformity seems not unlike that of Cavendish's, later, except that in the case of Cavendish it manifested itself as an abnormal sensitiveness instead of an abnormal irritability.

CHRISTIAN HUYGENS

If for nothing else, the world is indebted to the man who invented the pendulum clock, Christian Huygens (1629-1695), of the Hague, inventor, mathematician, mechanician, astronomer, and physicist. Huygens was the descendant of a n.o.ble and distinguished family, his father, Sir Constantine Huygens, being a well-known poet and diplomatist. Early in life young Huygens began his career in the legal profession, completing his education in the juridical school at Breda; but his taste for mathematics soon led him to neglect his legal studies, and his apt.i.tude for scientific researches was so marked that Descartes predicted great things of him even while he was a mere tyro in the field of scientific investigation.

One of his first endeavors in science was to attempt an improvement of the telescope. Reflecting upon the process of making lenses then in vogue, young Huygens and his brother Constantine attempted a new method of grinding and polishing, whereby they overcame a great deal of the spherical and chromatic aberration. With this new telescope a much clearer field of vision was obtained, so much so that Huygens was able to detect, among other things, a hitherto unknown satellite of Saturn.

It was these astronomical researches that led him to apply the pendulum to regulate the movements of clocks. The need for some more exact method of measuring time in his observations of the stars was keenly felt by the young astronomer, and after several experiments along different lines, Huygens. .h.i.t upon the use of a swinging weight; and in 1656 made his invention of the pendulum clock. The year following, his clock was presented to the states-general. Accuracy as to time is absolutely essential in astronomy, but until the invention of Huygens's clock there was no precise, nor even approximately precise, means of measuring short intervals.

Huygens was one of the first to adapt the micrometer to the telescope--a mechanical device on which all the nice determination of minute distances depends. He also took up the controversy against Hooke as to the superiority of telescopic over plain sights to quadrants, Hooke contending in favor of the plain. In this controversy, the subject of which attracted wide attention, Huygens was completely victorious; and Hooke, being unable to refute Huygens's arguments, exhibited such irritability that he increased his already general unpopularity. All of the arguments for and against the telescope sight are too numerous to be given here. In contending in its favor Huygens pointed out that the unaided eye is unable to appreciate an angular s.p.a.ce in the sky less than about thirty seconds. Even in the best quadrant with a plain sight, therefore, the alt.i.tude must be uncertain by that quant.i.ty. If in place of the plain sight a telescope is subst.i.tuted, even if it magnify only thirty times, it will enable the observer to fix the position to one second, with progressively increased accuracy as the magnifying power of the telescope is increased. This was only one of the many telling arguments advanced by Huygens.

In the field of optics, also, Huygens has added considerably to science, and his work, Dioptrics, is said to have been a favorite book with Newton. During the later part of his life, however, Huygens again devoted himself to inventing and constructing telescopes, grinding the lenses, and devising, if not actually making, the frame for holding them. These telescopes were of enormous lengths, three of his object-gla.s.ses, now in possession of the Royal Society, being of 123, 180, and 210 feet focal length respectively. Such instruments, if constructed in the ordinary form of the long tube, were very unmanageable, and to obviate this Huygens adopted the plan of dispensing with the tube altogether, mounting his lenses on long poles manipulated by machinery. Even these were unwieldy enough, but the difficulties of manipulation were fully compensated by the results obtained.

It had been discovered, among other things, that in oblique refraction light is separated into colors. Therefore, any small portion of the convex lens of the telescope, being a prism, the rays proceed to the focus, separated into prismatic colors, which make the image thus formed edged with a fringe of color and indistinct. But, fortunately for the early telescope makers, the degree of this aberration is independent of the focal length of the lens; so that, by increasing this focal length and using the appropriate eye-piece, the image can be greatly magnified, while the fringe of colors remains about the same as when a less powerful lens is used. Hence the advantage of Huygens's long telescope.

He did not confine his efforts to simply lengthening the focal length of his telescopes, however, but also added to their efficiency by inventing an almost perfect achromatic eye-piece.

In 1663 he was elected a fellow of the Royal Society of London, and in 1669 he gave to that body a concise statement of the laws governing the collision of elastic bodies. Although the same views had been given by Wallis and Wren a few weeks earlier, there is no doubt that Huygens's views were reached independently; and it is probable that he had arrived at his conclusions several years before. In the Philosophical Transactions for 1669 it is recorded that the society, being interested in the laws of the principles of motion, a request was made that M.

Huygens, Dr. Wallis, and Sir Christopher Wren submit their views on the subject. Wallis submitted his paper first, November 15, 1668. A month later, December 17th, Wren imparted to the society his laws as to the nature of the collision of bodies. And a few days later, January 5, 1669, Huygens sent in his "Rules Concerning the Motion of Bodies after Mutual Impulse." Although Huygens's report was received last, he was antic.i.p.ated by such a brief s.p.a.ce of time, and his views are so clearly stated--on the whole rather more so than those of the other two--that we give them in part here:

"1. If a hard body should strike against a body equally hard at rest, after contact the former will rest and the latter acquire a velocity equal to that of the moving body.

"2. But if that other equal body be likewise in motion, and moving in the same direction, after contact they will move with reciprocal velocities.

"3. A body, however great, is moved by a body however small impelled with any velocity whatsoever.

"5. The quant.i.ty of motion of two bodies may be either increased or diminished by their shock; but the same quant.i.ty towards the same part remains, after subtracting the quant.i.ty of the contrary motion.

"6. The sum of the products arising from multiplying the ma.s.s of any hard body into the squares of its velocity is the same both before and after the stroke.

"7. A hard body at rest will receive a greater quant.i.ty of motion from another hard body, either greater or less than itself, by the interposition of any third body of a mean quant.i.ty, than if it was immediately struck by the body itself; and if the interposing body be a mean proportional between the other two, its action upon the quiescent body will be the greatest of all."(10)

This was only one of several interesting and important communications sent to the Royal Society during his lifetime. One of these was a report on what he calls "Pneumatical Experiments." "Upon including in a vacuum an insect resembling a beetle, but somewhat larger," he says, "when it seemed to be dead, the air was readmitted, and soon after it revived; putting it again in the vacuum, and leaving it for an hour, after which the air was readmitted, it was observed that the insect required a longer time to recover; including it the third time for two days, after which the air was admitted, it was ten hours before it began to stir; but, putting it in a fourth time, for eight days, it never afterwards recovered.... Several birds, rats, mice, rabbits, and cats were killed in a vacuum, but if the air was admitted before the engine was quite exhausted some of them would recover; yet none revived that had been in a perfect vacuum.... Upon putting the weight of eighteen grains of powder with a gauge into a receiver that held several pounds of water, and firing the powder, it raised the mercury an inch and a half; from which it appears that there is one-fifth of air in gunpowder, upon the supposition that air is about one thousand times lighter than water; for in this experiment the mercury rose to the eighteenth part of the height at which the air commonly sustains it, and consequently the weight of eighteen grains of powder yielded air enough to fill the eighteenth part of a receiver that contained seven pounds of water; now this eighteenth part contains forty-nine drachms of water; wherefore the air, that takes up an equal s.p.a.ce, being a thousand times lighter, weighs one-thousandth part of forty-nine drachms, which is more than three grains and a half; it follows, therefore, that the weight of eighteen grains of powder contains more than three and a half of air, which is about one-fifth of eighteen grains...."

From 1665 to 1681, accepting the tempting offer made him through Colbert, by Louis XIV., Huygens pursued his studies at the Bibliotheque du Roi as a resident of France. Here he published his Horologium Oscillatorium, dedicated to the king, containing, among other things, his solution of the problem of the "centre of oscillation." This in itself was an important step in the history of mechanics. a.s.suming as true that the centre of gravity of any number of interdependent bodies cannot rise higher than the point from which it falls, he reached correct conclusions as to the general principle of the conservation of vis viva, although he did not actually prove his conclusions. This was the first attempt to deal with the dynamics of a system. In this work, also, was the true determination of the relation between the length of a pendulum and the time of its oscillation.

In 1681 he returned to Holland, influenced, it is believed, by the att.i.tude that was being taken in France against his religion. Here he continued his investigations, built his immense telescopes, and, among other things, discovered "polarization," which is recorded in Traite de la Lumiere, published at Leyden in 1690. Five years later he died, bequeathing his ma.n.u.scripts to the University of Leyden. It is interesting to note that he never accepted Newton's theory of gravitation as a universal property of matter.

XI. NEWTON AND THE COMPOSITION OF LIGHT

Galileo, that giant in physical science of the early seventeenth century, died in 1642. On Christmas day of the same year there was born in England another intellectual giant who was destined to carry forward the work of Copernicus, Kepler, and Galileo to a marvellous consummation through the discovery of the great unifying law in accordance with which the planetary motions are performed. We refer, of course, to the greatest of English physical scientists, Isaac Newton, the Shakespeare of the scientific world. Born thus before the middle of the seventeenth century, Newton lived beyond the first quarter of the eighteenth (1727). For the last forty years of that period his was the dominating scientific personality of the world. With full propriety that time has been spoken of as the "Age of Newton."

Yet the man who was to achieve such distinction gave no early premonition of future greatness. He was a sickly child from birth, and a boy of little seeming promise. He was an indifferent student, yet, on the other hand, he cared little for the common amus.e.m.e.nts of boyhood. He early exhibited, however, a taste for mechanical contrivances, and spent much time in devising windmills, water-clocks, sun-dials, and kites.

While other boys were interested only in having kites that would fly, Newton--at least so the stories of a later time would have us understand--cared more for the investigation of the seeming principles involved, or for testing the best methods of attaching the strings, or the best materials to be used in construction.

Meanwhile the future philosopher was acquiring a taste for reading and study, delving into old volumes whenever he found an opportunity. These habits convinced his relatives that it was useless to attempt to make a farmer of the youth, as had been their intention. He was therefore sent back to school, and in the summer of 1661 he matriculated at Trinity College, Cambridge. Even at college Newton seems to have shown no unusual mental capacity, and in 1664, when examined for a scholarship by Dr. Barrow, that gentleman is said to have formed a poor opinion of the applicant. It is said that the knowledge of the estimate placed upon his abilities by his instructor piqued Newton, and led him to take up in earnest the mathematical studies in which he afterwards attained such distinction. The study of Euclid and Descartes's "Geometry" roused in him a latent interest in mathematics, and from that time forward his investigations were carried on with enthusiasm. In 1667 he was elected Fellow of Trinity College, taking the degree of M.A. the following spring.

It will thus appear that Newton's boyhood and early manhood were pa.s.sed during that troublous time in British political annals which saw the overthrow of Charles I., the autocracy of Cromwell, and the eventual restoration of the Stuarts. His maturer years witnessed the overthrow of the last Stuart and the reign of the Dutchman, William of Orange. In his old age he saw the first of the Hanoverians mount the throne of England.

Within a decade of his death such scientific path-finders as Cavendish, Black, and Priestley were born--men who lived on to the close of the eighteenth century. In a full sense, then, the age of Newton bridges the gap from that early time of scientific awakening under Kepler and Galileo to the time which we of the twentieth century think of as essentially modern.