Chapter XVI: Introduction (5)

An electric railway may exemplify a power-transmission system in which power is delivered to moving vehicles. But the distances so covered are not generally more than a few miles from the generating station. Where, however, abundant water-power exists, as at Niagara, or where fuel is very expensive and power is to be had only at great distances from the place at which it is to be used, electricity furnishes the most effective means for transmission and distribution. Between the years 1880 and 1890 the device called alternating current transformer was developed to a considerable degree of perfection. It is, in reality, a modified induction coil, consisting of copper wire and iron, whereby a current sent through one of its coils will induce similar currents in the other coils of apparatus. It has the great advantage of having no moving parts. Faraday, in 1831, discovered the fundamental principle of the modern transformer. Not only, however, will the current in one coil of the apparatus generate by induction a new current in an entirely separate coil or circuit, but by suitably proportioning the windings we may exchange, as it were, a large low-pressure current for a small but high-pressure current, or _vice versa_. This exchange may be made with a very small percentage of loss of energy. These valuable properties of the transformer have rendered it of supreme importance in recent electrical extension. The first use made of it, in 1885–86, was to transform a high-pressure current into one of low pressure in electric lighting, enabling a small wire to be used to convey electric energy at high pressure, and without much loss, to a long distance from the station. This energy at high pressure reaches the transformer placed within or close to the building to be lighted. A low-pressure safe current is conveyed from the transformer to the wires connected to the lamps. In this way a current of two thousand volts, an unsafe and unsuitable pressure for incandescent lighting, is exchanged for one of about one hundred volts, which is quite safe. In this way, also, the supply station is enabled to reach a customer too far away to be supplied directly with current at one hundred volts, without enormous expense for copper conductors.

The alternating current transformer not only greatly extended the radius of supply from a single station, but also enabled the station to be conveniently located where water and coal could be had without difficulty. It also permitted the distant water-powers to become sources of electric energy for lighting, power, or for other service. For example, a water-power located at a distance of fifty to one hundred miles or more from a city, or from a large manufacturing centre where cost of fuel is high, may be utilized as follows: A power-station will be located upon the site of the water-power, and the dynamos therein will generate electricity at, say, two thousand volts pressure. By means of step-up transformers this will be exchanged for a current of thirty thousand volts for transmission over a line of copper or aluminum wire to the distant consumption area. Here there will be a set of step-down transformers which will exchange the thirty-thousand-volt line current for one of so low a pressure as to be safe for local distribution to lamps, to motors, etc., either stationary or upon a railway. The same transmission plant may simultaneously supply energy for lighting, for power, for heat, and for charging storage batteries. It may, therefore, be employed both day and night.

These long-distance power transmission plants are generally spoken of as “two-phase,” “three-phase,” or “polyphase” systems. Before 1890 no such plants existed. A large number of such installations are now working over distances of a few miles up to one hundred miles. They differ from what are known as single-phase alternating systems in employing, instead of a single alternating current, two, three, or more, which are sent over separate lines, and in which the electric impulses are not simultaneous, but follow each other in regular succession, overlapping each other’s dead points, so to speak. Early suggestions of such a plan, about 1880, and thereafter, by Bailey, Deprez, and others, bore no fruit, and not until Tesla’s announcement of his polyphase system, in 1888, was much attention given to the subject. A widespread interest in Tesla’s work was invoked, but several years elapsed before engineering difficulties were overcome. This work was done mainly by the technical staffs of the large manufacturing companies, and it was necessary to be done before any notable power transmissions on the polyphase system could be established. After 1892 the growth became very rapid.

The falls of Niagara early attracted the attention of engineers to the possibility of utilizing at least a fraction of the power. It was seen that several hundred thousand horse-power might be drawn from it without materially affecting the fall, itself equivalent to several millions of horse-power. A gigantic power-station has lately been established at Niagara, taking water from a distance above the falls and delivering it below the falls through a long tunnel which forms the tail race. Ten water-wheels, located in an immense wheel-pit about two hundred feet deep, each wheel of a capacity of five thousand horse-power, drive large vertical shafts, at the upper end of which are located the large two-phase dynamos, each of five thousand horse-power. The electric energy from these machines is in part raised in pressure by huge transformers for transmission to distant points, such as the city of Buffalo, and a large portion is delivered to the numerous manufacturing plants located at moderate distances from the power-station. Besides the supply of energy for lighting, and for motors, including railways, other recent uses of electricity to which we have not yet alluded are splendidly exemplified at Niagara. Davy’s brilliant discovery of the alkali metals, sodium and potassium, at the opening of the century, showed the great chemical energy of the electric current. Its actions were afterwards carefully studied, notably by the illustrious Faraday, whose discoveries in connection with magnetism and magneto-electricity have been briefly described. The electric current was found to act as a most potent chemical force, decomposing and recomposing many chemical compounds, dissolving and depositing metals. Hence, early in the century arose the art of electroplating of metals, such as electro-gilding, silver-plating, nickel-plating, and copper deposition as in electrotyping. These arts are now practised on a very large scale, and naturally have affected the whole course of manufacturing methods during the century. Moreover, since the introduction of dynamo current, electrolysis has come to be employed in huge plants, not only for separating metals from each other, as in refining them, but in addition for separating them from their ores, for the manufacture of chemical compounds before unknown, and for the cheap production of numerous substances of use in the various arts on a large scale. Vast quantities of copper are refined, and silver and gold often obtained from residues in sufficient amount to pay well for the process.

At Niagara also are works for the production of the metal aluminum from its ores. Similar works exist at other places here and abroad where power is cheap. This metal, which competes in price with brass, bulk for bulk, was only obtainable before its electric reduction at $25 to $30 per pound. The metal sodium is also extracted from soda. A large plant at Niagara also uses the electric current for the manufacture of chlorine for bleach, and caustic soda, both from common salt. Chlorate of potassium is also made at Niagara by electrolysis. The field of electro-chemistry is, indeed, full of great future possibilities. Large furnaces heated by electricity, a single one of which will consume more than a thousand horse-power, exist at Niagara. In these furnaces is manufactured from coke and sand, by the Acheson process, an abrasive material called carborundum, which is almost as hard as diamond, but quite low in cost. It is made into slabs and into wheels for grinding hard substances. The electric furnace furnishes also the means for producing artificial plumbago, or graphite, almost perfectly pure, the raw material being coke powder.

A large amount of power from Niagara is also consumed for the production in special electric arc furnaces of carbide of calcium from coke and lime. This is the source of acetylene gas, the new illuminant, which is generated when water is brought into contact with the carbide. The high temperature of the electric furnace thus renders possible chemical actions which under ordinary furnace heat would not take place. Henri Moissan, a French scientist, well known for his brilliant researches in electric furnace work, has even shown that real diamonds can be made under special conditions in the electric furnace. He has, in fact, probably practised in a small way what has occurred on a grand scale in nature, resulting in diamond fields such as those at Kimberley. One problem less is thus left to be solved. The electro-chemical and kindred arts are practised not alone at Niagara, but at many other places where power is cheap. Extensive plants have grown up, mostly within the five years before the close of the century. All of the great developments in this field have come about within the last decade.

The use of electricity for heating is not confined to electric furnaces, in which the exceedingly high temperature obtainable is the factor giving rise to success. While it is not likely that electricity will soon be used for general heating, special instances, such as the warming of electric cars in winter by electric heaters, the operation of cooking appliances by electric current, the heating of sad-irons and the like, give evidence of the possibilities should there ever be found means for the generation of electric energy from fuel with such high efficiency as eighty per cent. or more. Present methods give, under most favorable conditions, barely ten per cent., ninety per cent. of the energy value of the fuel being unavoidably wasted.

Another application of the heating power of electric currents is found in the Thomson electric welding process, the development of which has practically taken place in the past ten years. In this process an exceedingly large current, at very low electric pressure, traverses a joint between two pieces of metal to be united. It heats the joint to fusion or softening; the pieces are pushed together and welded. Here the heat is generated in the solid metal, for at no time during the operation are the pieces separated. The current is usually obtained from a welding transformer, an example of an extreme type of step-down transformer. Current at several hundred volts passed into the primary winding is exchanged for an enormous current at only two or three volts in the welding circuit in which the work is done. The present uses of this electric welding process are numerous and varied. Pieces of most of the metals and alloys, before regarded as unweldable, are capable of being joined not only to pieces of the same metal, but also to different metals. Electric welding is applied on the large scale, making joints in wires or rods, for welding wagon and carriage wheel tires, for making barrel-hoops and bands for pails, for axles of vehicles, and for carriage framing. It has given rise to special manufactures, such as electrically welded steel pipe or tube, wire fencing, etc. It is used for welding together the joints of steel car-rails, for welding teeth in saws, for making many parts of bicycles, and in tool making. An instance of its peculiar adaptability to unusual conditions is the welding of the iron bands embedded within the body of a rubber vehicle tire for holding the tire in place. For this purpose the electric weld has been found almost essential.

Another branch of electric development concerns the storage of electricity. The storage battery is based upon principles discovered by Gaston Planté, and applied, since 1881, by Brush, by Faure, and others. Some of the larger lighting stations employ as reservoirs of electric energy large batteries charged by surplus dynamo current. This is afterwards drawn upon when the consumer’s load is heavy, as during the evening. The storage battery is, however, a heavy, cumbrous apparatus, of limited life, easily destroyed unless guarded with skill. If a form not possessing these faults be ever found, the field of possible application is almost limitless.

The above by no means complete account of the progress in electric applications during the century just closed should properly be supplemented by an account of the accompanying great advances regarded from the purely scientific aspect. It is, however, only possible to make a brief reference thereto within the limits of this article. The scientific study of electricity and the application of mathematical methods in its treatment has kept busy a host of workers and drawn upon the resources of the ablest minds the age has produced. Gauss, Weber, Ampère, Faraday, Maxwell, Helmholtz, are no longer with us. Of the early founders of the science we have yet such men as Lord Kelvin, formerly Sir William Thomson, Mascart, and others, still zealous in scientific work. Following them are a large number, notable for valuable contributions to the progress of electrical science, in discoveries, in research, and in mathematical treatment of the various problems presented. Modern magnetism took form in the hands of Rowland, Hopkinson, Ewing, and many other able workers. Maxwell’s electro-magnetic theory of light is confirmed by the brilliant researches of the late Dr. Hertz, too early lost to science. Hertz proved that all luminous phenomena are in essence electrical. The wireless telegraphy of to-day is a direct outcome of Hertz’s experiments on electric waves. It is but little more than ten years since Hertz announced his results to the world. His work, supplemented by that of Branly, Lodge, Marconi, and others, made wireless telegraphy a possibility.

The wonderful X-ray, and the rich scientific harvest which has followed the discovery by Röntgen of invisible radiation from a vacuum tube, was preceded by much investigation of the effects of electric discharges in vacuum tubes, and Hittorf, followed by Crookes, had given special study to these effects in very high or nearly perfect vacua. Crookes, though especially enriching science by his work, missed the peculiar X-ray, which, nevertheless, must have been emitted from many of his vacuum tubes, not only in his hands, but in those of subsequent students. It was as late as 1896 that Röntgen announced his discovery. Since that time several other sources of invisible radiation have been discovered, more or less similar in effect to the radiations from a vacuum tube, but emitted, singular as the fact is, from rare substances extracted from certain minerals. Leaving out of consideration the great value of the X-ray to physicians and surgeons, its effect in stimulating scientific inquiry has almost been incalculable. The renewed study of effects of electric discharge in vacuum tubes has already, in the work of such investigators as Lenard, J. J. Thomson, and others, apparently carried the subdivision of matter far beyond the time-honored chemical atom, and has gone far towards showing the essential unity of all the chemical elements. It is as unlikely that the mystery of the material universe will ever be completely solved as it is that we can gain an adequate conception of infinite space or time. But we can at least extend the range of our mental vision of the processes of nature as we do our real vision into space depths by the telescope and spectroscope. There can now be no question that electric conditions and actions are more fundamental than many hitherto so regarded.

The nineteenth century closed with many important problems in electrical science unsolved. What great or far-reaching discoveries are yet in store, who can tell? What valuable practical developments are to come, who can predict? The electrical progress has been great—very great—but after all only a part of that grander advance in so many other fields. The hands of man are strengthened by the control of mighty forces. His electric lines traverse the mountain passes as well as the plains. His electric railway scales the Jungfrau. But he still spends his best effort, and has always done so, in the construction and equipment of his engines of destruction, and now exhausts the mines of the world of valuable metals, for ships of war, whose ultimate goal is the bottom of the sea. In this also electricity is made to play an increasingly important part. It trains the guns, loads them, fires them. It works the signals and the search-lights. It ventilates the ship, blows the fires, and lights the dark spaces. Perhaps all this is necessary now, and, if so, well. But if a fraction of the vast expenditure entailed were turned to the encouragement of advance in the arts and employments of peace in the twentieth century, can it be doubted that, at the close, the nineteenth century might come to be regarded, in spite of its achievements, as a rather wasteful, semi-barbarous transition period?

ELIHU THOMSON.

PHYSICS

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.