A History of Science - v.3 (Williams)

distances of the surfaces.

"It is not improbable that the colors of the integuments of some insects, and of some other natural bodies, exhibiting in different lights the most beautiful versatility, may be found to be of this description, and not to be derived from thin plates. In some cases a single scratch or furrow may produce similar effects,

by the reflection of its opposite edges."[3]

This doctrine of interference of undulations was the absolutely novel part of Young's theory. The allcompassing genius of Robert Hooke had, indeed, very nearly apprehended it more than a century before, as Young himself points out, but no one else bad so much as vaguely conceived it; and even with the sagacious

Hooke it was only a happy guess, never distinctly outlined in his own mind, and utterly ignored by all others.

Young did not know of Hooke's guess until he himself had fully formulated the theory, but he hastened then

to give his predecessor all the credit that could possibly be adjudged his due by the most disinterested observer.

To Hooke's contemporary, Huygens, who was the originator of the general doctrine of undulation as the explanation of light, Young renders full justice also.

For himself he claims only the merit of having demonstrated the theory which these and a few others of his

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predecessors had advocated without full proof.

The following year Dr. Young detailed before the

Royal Society other experiments, which threw additional light on the doctrine of interference; and in 1803

he cited still others, which, he affirmed, brought the doctrine to complete demonstration. In applying this demonstration to the general theory of light, he made the striking suggestion that "the luminiferous ether

pervades the substance of all material bodies with little or no resistance, as freely, perhaps, as the wind passes through a grove of trees." He asserted his belief also that the chemical rays which Ritter had discovered

beyond the violet end of the visible spectrum are but still more rapid undulations of the same character as those which produce light. In his earlier lecture he had affirmed a like affinity between the light rays and the rays of radiant heat which Herschel detected below the red end of the spectrum, suggesting that "light

differs from heat only in the frequency of its undulations or vibrations--those undulations which are

within certain limits with respect to frequency affecting the optic nerve and constituting light, and those

which are slower and probably stronger constituting heat only." From the very outset he had recognized the affinity between sound and light; indeed, it had been this affinity that led him on to an appreciation of the undulatory theory of light.

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But while all these affinities seemed so clear to the great co-ordinating brain of Young, they made no such impression on the minds of his contemporaries. The

immateriality of light had been substantially demonstrated, but practically no one save its author accepted

the demonstration. Newton's doctrine of the emission

of corpuscles was too firmly rooted to be readily dislodged, and Dr. Young had too many other interests to

continue the assault unceasingly. He occasionally wrote something touching on his theory, mostly papers

contributed to the Quarterly Review and similar periodicals, anonymously or under pseudonym, for he had

conceived the notion that too great conspicuousness in fields outside of medicine would injure his practice as a physician. His views regarding light (including the original papers from the Philosophical Transactions of the Royal Society) were again given publicity in full in his celebrated volume on natural philosophy, consisting

in part of his lectures before the Royal Institution, published in 1807; but even then they failed to bring conviction

to the philosophic world. Indeed, they did not

even arouse a controversial spirit, as his first papers had done.

ARAGO AND FRESNEL CHAMPION THE WAVE THEORY

So it chanced that when, in 1815, a young French

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military engineer, named Augustin Jean Fresnel, returning from the Napoleonic wars, became interested

in the phenomena of light, and made some experiments concerning diffraction which seemed to him to controvert the accepted notions of the materiality of light,

he was quite unaware that his experiments had been anticipated by a philosopher across the Channel. He communicated his experiments and results to the French Institute, supposing them to be absolutely novel. That body referred them to a committee, of which, as good fortune would have it, the dominating

member was Dominique Francois Arago, a man as versatile as Young himself, and hardly less profound, if

perhaps not quite so original. Arago at once recognized the merit of Fresnel's work, and soon became a

convert to the theory. He told Fresnel that Young

had anticipated him as regards the general theory, but that much remained to be done, and he offered to associate himself with Fresnel in prosecuting the investigation. Fresnel was not a little dashed to learn that

his original ideas had been worked out by another while he was a lad, but he bowed gracefully to the situation and went ahead with unabated zeal.

The championship of Arago insured the undulatory

theory a hearing before the French Institute, but by no means sufficed to bring about its general acceptance. On the contrary, a bitter feud ensued, in which Arago was opposed by the "Jupiter Olympus of the Academy,"

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Laplace, by the only less famous Poisson, and by the younger but hardly less able Biot. So bitterly raged the feud that a life-long friendship between Arago and Biot was ruptured forever. The opposition

managed to delay the publication of Fresnel's papers,

but Arago continued to fight with his customary enthusiasm and pertinacity, and at last, in 1823, the

Academy yielded, and voted Fresnel into its ranks, thus implicitly admitting the value of his work.

It is a humiliating thought that such controversies as

this must mar the progress of scientific truth; but fortunately the story of the introduction of the undulatory

theory has a more pleasant side. Three men, great both

in character and in intellect, were concerned in pressing its claims--Young, Fresnel, and Arago--and the relations of these men form a picture unmarred by any

of those petty jealousies that so often dim the lustre of great names. Fresnel freely acknowledged Young's priority so soon as his attention was called to it; and Young applauded the work of the Frenchman, and

aided with his counsel in the application of the undulatory theory to the problems of polarization of light,

which still demanded explanation, and which Fresnel's fertility of experimental resource and profundity

of mathematical insight sufficed in the end to conquer.

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After Fresnel's admission to the Institute in 1823

the opposition weakened, and gradually the philosophers came to realize the merits of a theory which

Young had vainly called to their attention a full quartercentury before. Now, thanks largely to Arago, both

Young and Fresnel received their full meed of appreciation.

Fresnel was given the Rumford medal of the

Royal Society of England in 1825, and chosen one of the foreign members of the society two years later,

while Young in turn was elected one of the eight foreign members of the French Academy. As a fitting culmination of the chapter of felicities between the three

friends, it fell to the lot of Young, as Foreign Secretary of the Royal Society, to notify Fresnel of the honors shown him by England's representative body of scientists; while Arago, as Perpetual Secretary of the French

Institute, conveyed to Young in the same year the notification that he had been similarly honored by the

savants of France.

A few months later Fresnel was dead, and Young survived him only two years. Both died prematurely,

but their great work was done, and the world will remember always and link together these two names in

connection with a theory which in its implications and importance ranks little below the theory of universal gravitation.

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VII. THE MODERN DEVELOPMENT OF ELECTRICITY AND MAGNETISM

GALVANI AND VOLTA

The full importance of Young's studies of light might perhaps have gained earlier recognition

had it not chanced that, at the time when they were made, the attention of the philosophic world was turned with the fixity and fascination of a hypnotic stare upon another field, which for a time brooked no rival. How could the old, familiar phenomenon, light, interest any one when the new agent, galvanism, was in view?

As well ask one to fix attention on a star while a meteorite blazes across the sky.

Galvanism was so called precisely as the Roentgen

ray was christened at a later day--as a safe means of begging the question as to the nature of the phenomena involved. The initial fact in galvanism was the discovery of Luigi Galvani (1737-1798), a physician of

Bologna, in 1791, that by bringing metals in contact

with the nerves of a frog's leg violent muscular contractions are produced. As this simple little experiment

led eventually to the discovery of galvanic electricity and the invention of the galvanic battery, it

may be regarded as the beginning of modern electricity.

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The story is told that Galvani was led to his discovery while preparing frogs' legs to make a broth for his invalid wife. As the story runs, he had removed the skins from several frogs' legs, when, happening to touch the exposed muscles with a scalpel which had lain in

close proximity to an electrical machine, violent muscular action was produced. Impressed with this phenomenon,

he began a series of experiments which finally

resulted in his great discovery. But be this story authentic or not, it is certain that Galvani experimented

for several years upon frogs' legs suspended upon wires and hooks, until he finally constructed his arc of two different metals, which, when arranged so that one was placed in contact with a nerve and the other with a muscle, produced violent contractions.

These two pieces of metal form the basic principle of

the modern galvanic battery, and led directly to Alessandro Volta's invention of his "voltaic pile," the immediate ancestor of the modern galvanic battery.

Volta's experiments were carried on at the same time

as those of Galvani, and his invention of his pile followed close upon Galvani's discovery of the new form

of electricity. From these facts the new form of electricity was sometimes called "galvanic" and sometimes

"voltaic" electricity, but in recent years the

term "galvanism" and "galvanic current" have almost entirely supplanted the use of the term voltaic.

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It was Volta who made the report of Galvani's wonderful discovery to the Royal Society of London, read

on January 31, 1793. In this letter he describes Galvani's experiments in detail and refers to them in

glowing terms of praise. He calls it one of the "most beautiful and important discoveries," and regarded it as the germ or foundation upon which other discoveries

were to be made. The prediction proved entirely correct, Volta himself being the chief discoverer.

Working along lines suggested by Galvani's discovery,

Volta constructed an apparatus made up of a

number of disks of two different kinds of metal, such

as tin and silver, arranged alternately, a piece of some moist, porous substance, like paper or felt, being interposed between each pair of disks. With this "pile,"

as it was called, electricity was generated, and by linking together several such piles an electric battery could

be formed.

This invention took the world by storm. Nothing

like the enthusiasm it created in the philosophic world had been known since the invention of the Leyden jar, more than half a century before. Within a few weeks after Volta's announcement, batteries made according

to his plan were being experimented with in every important laboratory in Europe.

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As the century closed, half the philosophic world was speculating as to whether "galvanic influence"

were a new imponderable, or only a form of electricity; and the other half was eagerly seeking to discover

what new marvels the battery might reveal. The least imaginative man could see that here was an invention that would be epoch-making, but the most visionary dreamer could not even vaguely adumbrate the real measure of its importance.

It was evident at once that almost any form of galvanic battery, despite imperfections, was a more satisfactory instrument for generating electricity than the frictional machine hitherto in use, the advantage lying in the fact that the current from the galvanic battery could be controlled practically at will, and that the apparatus itself was inexpensive and required comparatively little attention. These advantages were soon made apparent by the practical application of the electric current in several fields.

It will be recalled that despite the energetic endeavors of such philosophers as Watson, Franklin, Galvani,

and many others, the field of practical application of electricity was very limited at the close of the eighteenth century. The lightning-rod had come into general use, to be sure, and its value as an invention can hardly be overestimated. But while it was the

result of extensive electrical discoveries, and is a most

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