On January 7, 1610, Galileo, turning his telescope towards Jupiter, was the first to see the beautiful system of that planet in which the universe is epitomized. He had already studied the variegated surface of the moon, and he had seen the spots upon the sun. A little later, in spite of the feeble power of his instrument, he had discovered that the sun rotates upon an axis, and something of the wonderful nature of the planet Saturn had been revealed to him. The overwhelming evidence thus afforded of the truth of the hypothesis of Copernicus made him its chief exponent. The time had come for man to know, as he had never known or even dreamed before, his true relation to the universe of which he was so insignificant a part. In a single year nearly all of these capital discoveries were made. It was truly an era of intellectual expansion; never before and never since has man’s intellectual horizon enlarged with such enormous rapidity. One needs little imagination to share with this ardent philosopher the enthusiasm of the moment when, because some, fearing the evidence of their senses, refused to look through the slender tube, he wrote to Kepler: “Oh, my dear Kepler, how I wish we could have one hearty laugh together!... Why are you not here? What shouts of laughter we should have at this glorious folly!”

Galileo died in 1642, and in the same year Newton was born. When twenty-four years old he “began to think of gravity extending to the orb of the moon,” and before the end of the century he had discovered and established the great law of universal gravitation. Thus, at the end of the seventeenth century, the foundations of modern physics were in place. During the eighteenth century they were much built upon, but it was the nineteenth that witnessed not only the greatest advance in detail, but the most important generalizations made since the time of Galileo and Newton.

In endeavoring to present to the intelligent but perhaps unscientific reader a brief review of the accomplishments of that “wonderful century” in the domain of physics, one must not attempt more than an outline of greater events, and it will be convenient to arrange them under the several principal subdivisions of the science, according to the usually accepted classification.

HEAT

Although more than one philosopher of the seventeenth and eighteenth centuries suggested the identity of heat and molecular motion, the impression made was not lasting, and up to very near the beginning of the nineteenth century the caloric theory was accepted almost without dispute. This theory implied that heat was a subtle fluid, definite quantities of which were added to or subtracted from material substances when they became hot or cold. As carefully conducted experiments seemed to show that a body weighed no more or no less when hot than when cold, it was necessary to attribute to this fluid called caloric the mysterious property of imponderability, that is, unlike all forms of ordinary matter, it possessed no weight. To avoid calling it matter, it was by many classed with light, electricity, and magnetism, as one of the imponderable agents. Various other properties were attributed to caloric, necessary to the reasonable explanation of a steadily increasing array of experimental facts. It was declared to be elastic, its particles being mutually self-repellent. It was thought to attract ordinary matter, and an ingenious theory of caloric was constructed, modelled upon Newton’s famous but erroneous corpuscular theory of light. During the latter part of the eighteenth century Joseph Black, professor in the Universities of Glasgow and Edinburgh, developed his theory of latent heat, which, although founded upon a false notion of the nature of heat, was a most important contribution to science. The downfall of the caloric theory must be largely credited to the work of a famous American who published the results of his experiments just at the close of the eighteenth century. Benjamin Thompson, generally known as Count Rumford, was born in the town of Woburn, Massachusetts, in 1753. His inclination towards physical experimentation was strong in his early youth, and he received much instruction and inspiration from the lectures of Professor John Winthrop, of Harvard College, some of which he was enabled to attend under trying conditions. Having received special official consideration by appointment to office under one of the colonial governors, he was accused at the breaking out of the Revolutionary War of a leaning towards Toryism, and was thus prevented from making his career among his own people. At the age of twenty-two years he fled to England, returning to America only for a brief period in command of a British regiment. In England he soon became eminent as an experimental philosopher, and in 1778 became a Fellow of the Royal Society. He afterwards entered the service of the Elector of Bavaria, by whom he was made a Count of the Holy Roman Empire. In 1799 he returned to London and founded the “Royal Institution,” which was destined during the next hundred years to surpass all other foundations in the richness and importance of its contributions to physical science. It was while at Munich that Rumford made his famous experiments on the nature of heat, to which he had been led by observing the great amount of heat generated in the boring of cannon. Finding that he was able to make a considerable quantity of water actually boil by the heat generated by a blunt boring tool, he concluded that the supply of heat from such a source was practically inexhaustible and that it could be generated continuously if only the motion of the tool under friction was kept up. He declared that anything which could thus be produced without limitation by an insulated body or system of bodies could not possibly be a material substance, and that under the circumstances of the experiment, the only thing that was or could be thus continuously communicated was motion.

Count Rumford’s conclusions were not for a long time accepted. Davy, the brilliant professor and eloquent lecturer at the newly established Royal Institution, espoused the mechanical theory of heat and made the striking experiment of melting two pieces of ice by rubbing them together remote from any source of heat. His contemporary, Thomas Young, who overturned Newton’s corpuscular theory of light and showed that it was a wave phenomenon, also advocated Rumford’s notion of the nature of heat, but even among physicists of high rank it had made little headway as late as the middle of the nineteenth century. In the eighth edition of the EncyclopÆdia Britannica, published in 1856, the immediate predecessor of the current issue, heat is defined as “a material agent of a peculiar nature, highly attenuated.” And this, in spite of the fact that previous to that date the mechanical theory had been completely proved by the labors of Mayer, Joule, Helmholtz, and William Thomson (Lord Kelvin). By these men a solid foundation for the theory had been found in a great physical law of such importance that it is justly considered to be the most far-reaching generalization in natural philosophy since the time of Newton. Some account of this law and its discovery will be given later in this paper.

Among the most important of the century’s contributions to our knowledge of heat must be included the work of Fourier, as embodied in his Theorie Analytique de la Chaleur, published in 1822. Joseph Fourier was born in 1768, and died in 1830. He belonged to that splendid group of philosophers of which the French nation may always be proud, whose work constitutes a large part of the lustre of intellectual France during her most brilliant period, the later years of the eighteenth and the earlier years of the nineteenth century. His contemporaries included such men as Laplace, Arago, Lagrange, Fresnel, and Carnot. Fourier wrote especially of the movement of heat in solids, and as his thesis depended in no way on the nature of heat it will always be regarded as a classic. His assumption that conductivity was independent of temperature was shortly proved to be erroneous, but his general argument and conclusions were not greatly affected by this discovery. His work is one of the most beautiful examples yet produced of the application of mathematics to physical research, and mathematical and physical science were equally enriched by it. In its broader aspects his law of conduction includes the transfer of electricity in good conductors, and is the real basis of Ohm’s law.

One of the most skillful and successful experimenters in heat was also a Frenchman, Henri Victor Regnault (1810–78). He greatly improved the construction and use of the thermometer, and was the first to discover that the indications of an air thermometer and one of mercury did not exactly agree, because they did not expand in the same degree for equal increases of temperature. His most important work was on the expansion of gases, vapor pressure, specific heat of water, etc., and for careful, patient measuring he had a positive genius. Until he proved the contrary it had been assumed that all gases had the same coefficient of expansion, and Boyle’s law that the volume of a gas was inversely proportional to its pressure had not been questioned. His tables of the elastic force of steam have been of immense practical value, but his studies of the expansion of gases are of greater interest because they have pointed the way to one of the most important accomplishments of the century, the liquefaction of all known gases.

During the earlier years of this century it was the custom to consider vapors and gases as quite distinct forms of matter. Vapors always came, by evaporation, from liquids, and could always be “condensed” or reduced to the liquid form without difficulty, but it was not thought possible to liquefy the so-called “permanent” gases. The first man to attack the problem systematically was Michael Faraday, who, before the end of the first third of the century, had liquefied several gases, mostly by producing them by chemical reactions under pressure. Several of the more easily reducible gases or vapors, such as ammonia, sulphurous acid, and probably chlorine, had been previously liquefied by cold, but a quarter of a century elapsed after Faraday’s researches before the true relation of the liquid and gaseous states of matter was understood, and it was found that both increase of pressure and lowering of temperature were, in general, essential to the liquefaction of a gas. It was Thomas Andrews, of Belfast, who first showed, in a paper published in 1863, that there was a continuity in the liquid and gaseous states of matter, that for each substance there was a critical temperature at which it became a homogeneous fluid, neither a liquid nor a gas: that above this temperature great pressure would not liquefy, while below it the substance might exist as partly liquid and partly gas. He pointed out the fact that for the so-called permanent gases this critical temperature must be exceedingly low, and if such temperature could be reached liquefaction would follow.

Subsequent progress in the liquefaction of gases came about by following this suggestion. Very low temperatures were produced by subjecting the gas to great reduction in volume by pressure, removing the heat of compression by conduction and radiation, and then by sudden expansion its temperature was greatly lowered. As early as 1877 two Frenchmen, Pictet and Cailletet, had succeeded in liquefying oxygen, hydrogen, nitrogen, and air. During the past twenty years great improvements have been made in the methods of accomplishing these transformations, so that to-day it is easy to produce considerable quantities of all of the principal gases in a liquid form, and by carrying the reduction in temperature still further portions of the liquid may be changed to the solid state. The most important work along this line has been done by Wroblewski and Olszewski, of the University of Cracow, and Professor Dewar, of the Royal Institution in London. Temperatures as low as about two hundred and fifty degrees C. below the freezing-point of water have been produced, the “absolute zero” being only two hundred and seventy-three degrees C. below that point. These experiments promise to throw much light on the nature of matter, and they are especially interesting as revealing its extraordinary properties at extremely low temperatures. Among the most curious and suggestive is the fact that the electrical resistance of pure metals diminishes at a rate which indicates that at the absolute zero it would vanish, and these metals would become perfect conductors of electricity.

The dynamics of heat, or “thermo-dynamics,” was an important field of research in the early part of the century, on account of its practical application to the improvement of the steam-engine. The science was created by Carnot, who, in spite of the fact that his views regarding the nature of heat were erroneous, discovered some of the most interesting relations among the quantities involved, and discussed their applications to the heat engines with great skill. Subsequent contributors to the theory and practice of thermo-dynamics were Clausius, Rankine, Lord Kelvin, and Professor Tait.

The mechanical theory of heat naturally led up to what has already been referred to as the most important generalization in physical science since the time of Newton, the doctrine of

THE CONSERVATION OF ENERGY

This principle puts physics in its relation to energy where chemistry has long been in its relation to matter. If matter were not conservative, if it could be created or destroyed at will, chemistry would be an impossible science. Physics is put upon a solid foundation by the assumption of a like conservatism in energy; it can neither be created nor destroyed, although it may appear in many different forms which are, in general, mutually interconvertible.

Many men have contributed to the establishment of this great principle, but it was actually discovered and proved by the labors of three or four. Although it was practically all done before the middle of the nineteenth century, its general popular recognition did not come until a quarter of a century later. The doctrine was first distinctly formulated by Robert Mayer, a German physician, who published in 1842 a suggestive paper on “The Forces of Inorganic Nature,” which, however, attracted little or no attention. Mayer had not approached the problem from an experimental stand-point, but at about the same time it was attacked most successfully from this side by a young Englishman, James Prescott Joule, son of a wealthy brewer of Manchester, England. Joule made the first really accurate determination of the mechanical equivalent of a given quantity of heat, a physical constant which Rumford had tried to measure, reaching only a rough approximation. Substantially Joule’s result was that the heat energy necessary to raise the temperature of any given mass of water one degree Fahr. is the equivalent of the mechanical energy required to lift that mass through a height of seven hundred and seventy-two feet against the force of the earth’s attraction; and, conversely, if a mass of water be allowed to fall through a distance of seven hundred and seventy-two feet under the action of gravity, and at the end of its motion be instantly arrested, the heat generated will suffice to raise its temperature one degree Fahr. Of such vast importance is this numerical coefficient that it has been called the golden number of the nineteenth century. Since Joule’s time it has been redetermined by several physicists, notably by Professor Rowland, of Baltimore, the general conclusion being that Joule’s number was somewhat, but not greatly, too small.

The first clear and full exposition of the doctrine of the conservation of energy was given by Joule in a popular lecture in Manchester in 1847, but it attracted little attention until a few months later, when the author presented his theory at a meeting of the British Association for the Advancement of Science. Even among scientific men it would have passed without comment or consideration had it not been for the presence of another young Englishman, then as little known as Joule himself, who began a series of remarks, appreciative and critical, which resulted in making Joule’s paper the sensation of the meeting. This was William Thomson, who had been, only a year before, at the age of twenty-two years, appointed professor of natural philosophy at the University of Glasgow, now known as Lord Kelvin, the most versatile, brilliant, and profound student of physical science which the century has produced. From that day to the death of Joule (1889) these two men were closely associated in the demonstration and exploitation of a great principle of which they were at first almost the sole exponents among English-speaking people.

By an interesting coincidence, in the same year in which Joule announced the result of his experiments, the Physical Society of Berlin listened to a paper almost identical with Joule’s in character and conclusions, but prepared quite independently, by a young German physician, Herman von Helmholtz, destined to rank at the time of his death, in 1893, as one of the very first mathematicians of the age, doubtless the first physiologist of his time, and as a physicist with whom not more than one other of the nineteenth century may be compared. Helmholtz’s paper was rejected by the editor of the leading scientific journal of Germany, but his work was so important that he must always share with Joule and Kelvin in the glory of this epoch-making generalization.

Even a brief sketch of the history of the doctrine of the conservation of energy would be incomplete if mention were not made of the work of Tyndall. Although by original research he contributed in no small degree to the demonstration of the theory, it is mainly through his wonderful skill in popular presentation of the principles of physical science that he becomes related to the great movement of the middle of the century. His masterful exposition of the new theory in a course of lectures at the Royal Institution, given in 1862 and published in 1863 under the title Heat as a Mode of Motion, was the means of making the intelligent public acquainted with its beauty and profound significance, and the history of science affords no more admirable example of the possibilities and wisdom of popular scientific writing than this book. As for the principle of the conservation of energy itself it is not too much to say that during the last half of the century it has been the guiding and controlling spirit of all scientific discovery or of invention through the application of scientific principles.

LIGHT

The revival and final establishment of the undulatory or wave theory of light is one of the glories of the nineteenth century, and the credit for it is due to Thomas Young, an Englishman, and Fresnel, a Frenchman. Newton had conceived, espoused, and, owing to the great authority of his name, almost fixed upon the learned world the corpuscular or emission theory, which assumes that all luminous bodies emit streams of minute corpuscles, which are reflected, refracted, and produce vision. Many ordinary optical phenomena were explained by this hypothesis only with great difficulty, and some were quite unexplainable. The transmission of a disturbance or vibratory motion by means of waves, as in the case of sound, was a well-recognized principle, and Young and Fresnel applied it most successfully to the phenomena of light. Wave motion, in a general way, is only possible in a sensibly continuous medium, such as water, air, etc., and the theory that light was a vibratory disturbance transmitted by means of waves necessitated the assumption of the existence of such a medium throughout all space in which light travelled. What is known as the ethereal medium, at first a purely imaginary substance, but whose real existence is practically established, satisfies this demand, and the hypothesis that light is transmitted by waves in such a medium, originating in a vibratory disturbance at the source, has been of inestimable value to physical science. The work of Thomas Young was done in the very first years of the nineteenth century. He was for two years professor of Natural Philosophy in the Royal Institution just founded by Count Rumford, and he was the first to fill that chair. In 1801, in a paper presented to the Royal Society, he argued in favor of the undulatory theory, showing how the interference of waves would explain the color of thin plates. His papers were not, for several years, received favorably, and they were severely criticised by Lord Brougham. Augustus Fresnel followed Young, but quite independently, about ten years later, and by him the undulatory theory received elaborate experimental and mathematical treatment.

In the mean time another Frenchman had made a capital discovery in optics, which seemed at first to be quite incompatible with the wave theory. This was the discovery of what is known as polarization of light by Malus, a French engineer, who hit upon it while investigating double refraction of crystals, for a study of which the French Institute had offered a prize in 1808. Malus found that when light fell upon a surface of glass at a certain angle a portion of the reflected light appeared to have acquired entirely new properties in regard to further reflection, and the same was true of that part of the beam which was transmitted through the glass. The light thus affected was incapable of further reflection under certain conditions, and as the beam seemed to behave differently according to how it was presented to the reflecting surface, the term polarization was applied to the phenomenon. It was found that the two rays into which a single beam of light was split by a doubly refracting crystal (a phenomenon which had long been known) were affected in this way, and that light was polarized by refraction as well as by reflection. Malus was a believer in the corpuscular theory of light, but it was shortly proved, first by Thomas Young, that the phenomenon of polarization was not only not opposed to the wave theory, but that that theory furnished a rational explanation of it. This explanation, in brief, assumes that ordinary light is a wave produced by a vibratory motion confined to no particular plane, the direction of vibration being at right angles to the direction of the wave, and in any, or, in rapid succession, in all azimuths. When light is polarized the vibratory motion in the ether is restricted to one particular form, a line if plane polarized, a circle or an ellipse if circularly or elliptically polarized. This simple hypothesis has been found quite adequate, and through its application to the various phenomena of polarization, together with the application of Young’s theory of the interference of waves to the production of color, the undulatory theory of light was firmly established before the middle of the century. There were many noted philosophers, however, who stood out long against it, notably Brewster, the most famous English student of optics of the early part of the century, who declared that his “chief objection to the undulatory theory was that he could not think the Creator guilty of so clumsy a contrivance as the filling of space with ether in order to produce light.” In studying the nature of light it became very important to know how fast a light wave travelled. A tolerably good measure of the velocity of light had been made long before by means of the eclipses of Jupiter’s moons and by observations upon the positions of the stars as influenced by the motion of the earth in its orbit. It was found to be approximately one hundred and eighty thousand miles per second, a speed so great that it seemed impossible that it should ever be measured by using only terrestrial distances.

This extremely difficult problem has been solved, however, in a most satisfactory manner by nineteenth-century physicists. Everybody knows that in a uniform motion velocity is equal to space or distance divided by time. If, then, the time occupied in passing through a given distance can be measured, the velocity is at once known. As the velocity of light is very large, unless the distance is enormously great, the time will be extremely small, and if moderate distances are to be used the problem is to measure very small intervals of time very accurately. Light will travel one mile in about the one hundred and eighty-sixth thousandth part of a second, and if by using a mile as the distance the velocity of light is to be determined within one per cent., it is necessary to be able to detect differences of time as small as about one twenty-millionth of a second. This has been made possible by the use of two distinct methods. Foucault, on the suggestion of Arago, used a rapidly revolving mirror, a method introduced by Wheatstone, the English electrician, who used it in finding the duration of an electric spark. The essential principle is that a mirror may be made to revolve so rapidly that it will change its position by a measurable angle, while light which has been reflected from it passes to a somewhat distant fixed mirror and returns to the moving reflector. In the other method a toothed wheel is revolved so rapidly that a beam of light passing between two consecutive teeth to a distant fixed mirror is cut off on its return to the wheel by the tooth, which has moved forward while the light has made its journey. This method was first used by Fizeau. In either method, if the speed of rotation is known, the time is readily found. In point of time, Fizeau was the first to attack the problem, which he did about 1849. Foucault was perhaps a year later in getting results, but his method is generally considered the best. Both methods have been used by other experimenters, and very important improvements in Foucault’s method were made in the United States by Michelson about 1878. Michelson’s method increased enormously the precision of the measurements, and it has been applied by him and by Newcomb, not only for the better determination of the velocity of light in air, but for the solution of many other related problems of first importance. Michelson’s final determination of the absolute velocity of light (in the ether) is everywhere accepted as authoritative.

Another discovery in optics entirely accomplished during the nineteenth century and of the very first importance is generally known as “Spectrum Analysis.” This discovery has not yet ceased to excite admiration and even amazement, and especially among those who best understand it. By its use hitherto unknown substances have become known; to the physicist it is an instrument of research of the greatest power, and perhaps more than anything else it promises to throw light on the ultimate nature of matter; to the astronomer it has revealed the composition, physical condition, and even the motions of the most distant heavenly bodies, all of which the philosophy of a hundred years ago would have pronounced absolutely impossible.

The beginning of spectrum analysis was in 1802, when an Englishman, Dr. Wollaston, observed dark lines interrupting the solar spectrum when produced by a good prism upon which the sunlight fell after passing through a narrow slit. About ten years later, Fraunhofer, at Munich, a skilful worker in glass and a keen observer, discovered in the spectrum of light from a lamp two yellow bands, now known as the sodium, or “D” lines. Combining the three essential elements of the modern spectroscope, the slit, the prism, and the observing telescope, he saw in the spectrum of sunlight “an almost countless number of dark lines.” He was the first to use a grating for the production of the spectrum, using at first fine wire gratings and afterwards ruling fine lines upon glass, and with these he made the first accurate measures of the length of light waves. He did not, however, comprehend the full import of the problem which he thus brought to the attention of physicists. About twenty years later Sir John Herschell studied the bright line spectra of different substances and found that they might be used to detect the presence of minute quantities of a substance whose spectrum was known. Wheatstone studied the spectrum of the electric arc passing between metals, and in 1874 Dr. J.W. Draper published a very important paper on the spectra of solids with increasing temperature. Although quite in the dark as to the real nature of the phenomena with which they were dealing, these observers paved the way for the splendid work of the two Germans, Kirchoff and Bunsen, who, about 1860, found the key to this wonderful problem and made the science of spectrum analysis substantially what it is to-day. Its fundamental principles may be considered as few and comparatively simple.

Waves of light and radiant heat originate in ether disturbances produced by molecular vibration, and have impressed upon them all of the important qualities of that vibration. Molecules of different substances differ in their modes of vibration, each producing a wave peculiar to and characteristic of itself. A useful analogy may be found in the fact that when one listens to the music of an orchestra without seeing it it is easy to recognize the tones that come from each of the several instruments, the characteristic vibrations of each being impressed upon the waves in air which carry the sound to the ear. So delicate and so sure is this impression of vibration peculiarities that it is even possible to know the maker of a violin, for instance, by a characteristic timbre which must have its physical expression in the sound wave. The ear, more perfect than the eye, analyzes the resultant disturbance into its component parts so that each element may be attributed to its proper source. Unaided, the eye cannot do this with light, but the spectroscope separates the various modes of vibration which make up the confused whole, so that varieties of molecular activity are recognizable. The speed at which a source of sound is approaching or receding from the ear can be ascertained by noting the rise or fall in pitch due to the crowding together or stretching out of the sound waves, and in the same way the motion of a luminous body is known from the increase or decrease of the refrangibility of the elements of its spectrum.

Indeed, had nineteenth-century science accomplished nothing else than the discovery of spectrum analysis, it would have marked the beginning of a new epoch. By this device man is put in communication with every considerable body in the universe, including even the invisible. The “goings on” of Sirius and Algol, of Orion and the Pleiads are reported to him across enormous stretches of millions of millions of miles of space, empty save of the ethereal medium itself, by this most wonderful “wireless telegraphy.” And it is by the vibratory motion of the invisibly small that all of this is revealed; the infinitely little has enabled us to conquer the inconceivably big.

Many important contributions to the theory and practice of spectrum analysis have been made since the time of Kirchoff and Bunsen, only two or three of which can be referred to here. Instrumental methods by which spectra are produced and examined have been greatly perfected, and this is especially true of what is known as the “diffraction grating” first used by Fraunhofer. A quarter of a century ago Rutherford, of New York, constructed a ruling engine by means of which gratings on glass and spectrum metal were ruled with a precision greatly exceeding what had before been possible. A few years later Rowland, of Baltimore, made a notable advance in the construction of a screw far more perfect than any before made, producing gratings of a fineness and regularity of spacing far ahead of any others, and especially by the capital discovery of the concave grating, by means of which the most beautiful results have been obtained. Very recently Michelson, of Chicago, has invented the echelon spectroscope, which, although greatly restricted in range, exceeds all others in power of analysis of spectral lines. In his hands this instrument has been most effective in the study of the influence of a strong magnetic field upon the character of the spectrum from light produced therein, a most interesting phenomenon first observed by Zeeman and one which promises to reveal much concerning the relation of molecular activity to light and to magnetic force.

The development of spectrum analysis was necessarily accompanied by a recognition of the identity of radiant heat and light. The study of radiant heat, which was carried on during the earlier years of the century by Leslie, and later by Melloni and Tyndall, by what might be called thermal methods, has been industriously pursued during the last two decades by processes similar to those adopted for visual radiation. The most notable contribution to this work is the invention of the bolometer, by Langley, who, at Allegheny, and later at Washington, has made exhaustive studies of solar radiation in invisible regions of the spectrum, especially among the waves of greater length than those of red light, where he has found absorption lines and bands similar in character to those observed in the visible spectrum. He has also studied the absorption of the earth’s atmosphere, the relation of energy to visual effect, and many other interesting problems, the solution of which was made possible by the use of the bolometer. Mention must also be made of the invention by Michelson of an interference comparator, by means of which linear measurements by optical methods can be accomplished with a degree of accuracy hitherto unheard of. With this instrument Michelson has determined the length of the international prototype metre in terms of the wave length of the light of a particular spectral line, thus furnishing for the first time a satisfactory natural unit of length.

By far the most important contribution to the theory of light made during the last half of the century is that of Maxwell, who, in 1873, announced the proposition that electro-magnetic phenomena and light phenomena have their origin in the same medium, and that they are identical in nature. This far-reaching conclusion has been generally accepted and formed the basis of much of the most important work in physical research in process of elaboration as the century closed. To some of this reference will presently be made.

ELECTRICITY AND MAGNETISM

In no other department of physical science have such remarkable developments occurred during the past century as in electricity and magnetism, for in no other department have the practical applications of scientific discovery been so numerous and so far reaching in their effect upon social conditions. In a brief review of the contributions of the nineteenth century to the evolution of the telegraph, telephone, trolley-car, electric lighting, and other means of utilizing electricity, it will be possible to consider only a very few of the fundamental discoveries upon which the enormous and rather complex superstructure of to-day rests. Happily these are few in number, and their presentation is all the more important because of the fact that in the popular mind they are not accorded that significance to which they are entitled, if, indeed, they are remembered at all.

The first great step in advance of the electricity of Franklin and his contemporaries (and his predecessors for two thousand years) was taken very near the end of the eighteenth century, but it must be regarded as the beginning of nineteenth-century electricity. Two Italian philosophers, Galvani and Volta, contributed to the invention of what is known as the galvanic or voltaic battery, the output of which was not at first distinctly recognized as the electricity of the older schools. By this beautiful discovery electricity was for the first time enslaved to man, who was now able to generate and control it at times and in such quantities as he desired. Although the voltaic battery is now nearly obsolete as a source of electricity, its invention must always be regarded as one of the three epoch-making events in the history of the science during the past one hundred and twenty years. For three-quarters of a century it was practically the only source of electricity, and during this time and by its use nearly all of the most important discoveries were made. Even in the first decade of the century many brilliant results were reached. Among the most notable were the researches of Sir Humphry Davy, who, by the use of the most powerful battery then constructed, resolved the hitherto unyielding alkalies, discovering sodium and potassium, and at the same time exhibited in his lectures in the Royal Institution in London the first electric arc light, the ancestor of the millions that now turn night into day.

The cost of generating electricity by means of a voltaic battery is relatively very great, and this fact stood in the way of the early development of its applications, although their feasibility was perfectly well understood. Without any other important invention or discovery than that of the voltaic battery much would have been possible, including both electric lighting and the electric telegraph. Indeed, electric telegraphy had long been a possibility, even before the time of Galvani and Volta, but its actual construction and use was almost necessarily postponed until a second capital discovery came to remove most of the difficulties.

This was the discovery of a relation between electricity and magnetism, the existence of which had long been suspected and earnestly sought. A Danish professor, Hans Christian Oersted, was fortunate in hitting upon an experiment which demonstrated this relation and opened up an entirely new field of investigation and invention. What Oersted found was that when a conductor, as a copper wire, carrying an electric current, was brought near a freely suspended magnet, like a compass needle, the latter would take up a definite position with reference to the current. Thus an electric current moved a magnet, acted like a magnet in producing a “magnetic field.” The subject was quickly taken up by almost every physicist in Europe and America. Arago found that iron filings would cling to a wire through which a current was passing, and he was able to magnetize steel needles by means of the current. AmpÈre, another French physicist, studied Oersted’s wonderful discovery both experimentally and mathematically, and in an incredibly short time so developed it as to deserve the title of creator of the science of electro-dynamics.

The first to make what is known as an electro-magnet was an Englishman named Sturgeon, who used a bar of soft iron bent in a horseshoe form (as had long been common in making permanent steel magnets), and, after varnishing the iron for insulation, a single coil of copper wire was wrapped about it, through which the current from a battery was passed. There were thus two ways of producing visible motion by means of an electric current: that of Oersted’s simple experiment, in which a suspended magnetic needle was deflected by a current, and that made possible by the production, at will, of an electro-magnet. The application of both of these ideas to the construction of an electric telegraph was quickly attempted, and two different systems of telegraphy grew out of them. One, depending on Oersted’s experiment, was developed in England first and afterwards in Europe; the other, that involving the use of signals produced by an electric magnet, was developed in America, and was generally known as the American method. It has long ago superseded the first method in actual practice. Its possibility depended on perfecting the electro-magnet and especially on an understanding of the principles on which that perfecting depended. For the complete and satisfactory solution of this problem we are indebted to the most famous student of electricity America has produced during the century, Joseph Henry. In 1829, while a teacher in the academy at Albany, New York, Henry exhibited an electro-magnet of enormously greater power than any before made, involving all of the essential features of the magnet of to-day. The wire was insulated by silk wrapping, and many coils were placed upon the iron core, the intensity of magnetization being thus multiplied. Henry studied, also, the best form and arrangement of the battery under varying conditions of the conductor. An electro-magnetic telegraph had been declared impossible in 1825, by Barlow, an Englishman, who pointed out the apparently fatal fact that the resistance offered to the current was proportional to the length of the conducting wire and that the strength of the current would be thus so much reduced for even short distances as to become too feeble to be detected. Henry showed that what is known as an “intensity battery” would overcome this difficulty, discovering experimentally and independently the beautifully simple law showing the relation of current to electro-motive force which Ohm had announced in 1827. He also invented the principle of the relay, by which the action of a very feeble current controls the operation of a more powerful local system. It will thus be seen that the essential features of the so-called American system of telegraphy are to be credited to Henry, who had a working line in his laboratory as early as 1832.

Morse made use of the scientific discoveries and inventions of Henry, and by his indefatigable labors and persistent faith the commercial value of the enterprise was really established. In the mean time considerable progress was made in Europe. Baron Schilling, a Russian Councillor of State, devised and exhibited a needle telegraph. The two illustrious German physicists, Gauss and Weber, established a successfully working line two or three miles long in 1833, and this system was commercially developed by Steinheil in 1837. In England, Sir Charles Wheatstone made many important contributions, although using the needle system, which was afterwards abandoned. Before the middle of the century the commercial success of the electro-magnetic telegraph was assured, and in the matter of the transmission of messages distance was practically annihilated.

Oersted, Arago, AmpÈre, Sturgeon, and Henry had made it possible to convert electricity into mechanical energy. Motors of various types had been invented, and the possibility of using the new source of power for running machinery, cars, boats, etc., was fully recognized. Several attempts had been made to do these things, but the great cost of producing the current by means of a battery stood in the way of success. Another epoch-making discovery was necessary, namely, a method of reversing the process and converting mechanical energy into electricity. This was supplied by the genius of Michael Faraday, who had succeeded Davy in the Royal Institution at London. In 1831 Faraday discovered induction, the key to the modern development of electricity. He showed that while Oersted had proved that a current of electricity would generate a magnetic field and set a magnet in motion, this process was reversible. A magnet set in motion in a magnetic field by a steam-engine or any other source of power would produce, in a conductor properly arranged, a current of electricity, and thus the dynamo came into existence. In this brilliant investigation he was almost anticipated by Henry, who was working at Albany along the same lines, but under much less favorable conditions. Indeed, in several of the most important points, the American actually did anticipate the Englishman. Nearly half a century elapsed before this most important discovery was sufficiently developed to become commercially valuable, and it is impossible in this place to trace the steps by which, during the last quarter of a century, the production and utilization of electricity as existing to-day was accomplished, as a result of which the century closed, as one might say, in a blaze of light; and it is unnecessary, because most people have witnessed the spread of the fire which Faraday and Henry kindled.

Faraday’s discovery of induction furnished the basis of that marvellous improvement upon the telegraph by which actual speech is transmitted over hundreds and even thousands of miles. In connection with the invention of the telephone the names of Philip Reiss, Graham Bell, Elisha Gray, and Dolbear will always be mentioned, each of whom, doubtless independently, hit upon a way of accomplishing the result with more or less success. To Bell, however, belongs the honor of having first practically solved the problem and of devising a system which, with numerous modifications and improvements, has come into extensive use in all parts of the world. No other application of electricity has come into such universal use, and none has contributed more to the comfort of life.

While it is doubtless true that since Faraday’s time no discovery comparable with his in real importance has been made, the past twenty-five years have not lacked in results of scientific research, some of which may, in the not distant future, eclipse even that in the value of their practical applications. Among these must be ranked Clerk Maxwell’s theory of electric waves and its beautiful verification in 1888 by the young German physicist, Hertz. This brilliant student of electricity succeeded in actually producing, detecting, and controlling these waves, and out of this discovery has come the “wireless telegraphy” which has been so rapidly developed within the last few years. Many other discoveries in electricity of great scientific interest and practical promise have been recorded in the closing years of the century, but the necessary limits of this article forbid their consideration.

No account of the progress of physical science during the nineteenth century would be even approximately complete without mention of other investigations of profound significance. For instance, the study of the phenomena of sound has yielded results of great scientific and some practical value. The application of the theory of interference by Thomas Young; the publication of Helmholtz’s great work, the Tonempfindungen, in which his theory of harmony was first fully presented; the publication of Lord Rayleigh’s treatise; the invention and construction by KÖnig of acoustic apparatus, the best example yet furnished of scientific handicraft; all of these mark important advances, not only in acoustics but in general physics as well. The phonautograph of Scott and KÖnig, by which a graphic record of the vibrations of the vocal chords was made possible, was ingeniously converted by Edison into a speech recording and reproducing machine, the phonograph, by which the most marvellous results are accomplished in the simplest possible manner.

The century is also to be credited with the discovery and development of the art of photography, which, although not of the first importance, has contributed much to the pleasure of life, and as an aid to scientific investigation has become quite indispensable.

The wonderfully beautiful experiments of Sir William Crookes, on the passage of an electric discharge through a high vacuum, and other phenomena connected with what has been called “radiant matter,” begun about a quarter of a century ago and continued by him and others up to the present time, laid the foundation for the brilliant work of RÖntgen in the discovery and study of the so-called “X”-rays, the real nature of which is not yet understood. Their further investigation by J.J. Thomson, Becquerel, and others, seems to have revealed new forms and phases of radiation, a fuller knowledge of which is likely to throw much light on obscure problems relating to the nature of matter.

Concerning the “Nature of Matter,” the ablest physicists of the century have thought and written much, and doubtless our present knowledge of the subject is much more nearly the truth than that of a hundred years ago. The molecular theory of gases has met with such complete experimental verification, and is so in accord with all observed phenomena, that it must be accepted as essentially correct. As to the ultimate nature of what is called matter, as distinguished from the ethereal medium, what is known as the “vortex theory of atoms” has received the most consideration. This theory was developed by Lord Kelvin out of Helmholtz’s mathematical demonstration of the indestructibility of a vortex ring when once formed in a medium possessing the properties which are generally attributed to the ether.

Perhaps the most remarkable as well as the most promising fact relating to physical science at the close of the nineteenth century is the great and rapidly increasing number of well-organized and splendidly equipped laboratories in which original research is systematically planned and carried out. When one reflects that for the most part during the century just ended the advance of science was more or less of the nature of a guerrilla warfare against ignorance, it seems safe to predict for that just beginning victories more glorious than any yet won.

T.C. Mendenhall.

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