![Portrait of P. N. Lebedev]

P. N. Lebedev

ON THE TWENTY-FIFTH ANNIVERSARY OF THE DEATH OF P. N. LEBEDEV

P. P. Lazarev, Moscow

On March 14 of this year, 25 years had passed since the death of the greatest Russian physicist, the creator of the first physical school in Russia—Petr Nikolaevich Lebedev. It is therefore necessary to recall here the life and scientific activity of this outstanding scholar.*

P. N. Lebedev was born in Moscow in 1866 and received his early education at the Petropavlovsk German School.

While still a boy, he became interested in physical phenomena, and with the years these interests appeared ever more vividly and clearly, so that by the age of 16 Lebedev had formed quite definite ideas about the goal in life that could satisfy him. This goal was scientific research, and in order to achieve it Lebedev, after a two-year stay at Khainovsky’s private real school, entered the Moscow Imperial Technical School as a student. Access to the university, where scientific studies would have been freer, was closed to him as a non-classicist.

Although, given the mass of obligatory work required by the special technical school, he had little free time, he nevertheless managed to read a great deal and already in his first year at the school took up his first scientific work. The result of this investigation, as of many subsequent ones begun at the school, was negative, but this did not weaken Lebedev’s energy, and he became more and more drawn into research.

Several attempts at engineering design clearly showed him that not the activity of a technician, but science, was his vocation, and he resolved to obtain the possibility of devoting himself to science.

For this purpose, in October 1887 he moved to Strasbourg, where he enrolled as a student. Strasbourg University was at that time one of the largest scientific centers in Germany, to which students flocked from all over the world. It is enough merely to mention such scholars as the physiologist Goltz, the pathologist-anatomist Recklinghausen, and the first rector of Strasbourg University, the well-known botanist

* The biographical data given below are reproduced from the biography of P. N. Lebedev compiled by the author of this article for the collected works of P. N. Lebedev (Moscow, 1913).

de Bary, to understand the enormous significance of Strasbourg at that time; but even against this brilliant background there stood out the director of the physics institute, Prof. A. Kundt, into whose laboratory the young Lebedev entered from the very beginning.

Kundt was not only a remarkable scholar who enriched science with a series of brilliant investigations; he was the head of a large international school of physics, which gave us such first-rate investigators as Röntgen, Warburg, Braun, Galvaks, Wien, Rubens, du Bois, and others.* Specialists working under Kundt flocked to his laboratory from all over the globe, and at the weekly colloquia, in P. N.’s own words, “all nationalities filed past”; German speech was heard not only with the various local shades of united Germany, but also with all those national echoes and irregularities with which foreigners speak it, from San Francisco to Tokyo.

P. N. describes his first acquaintance with Kundt as follows: “With inward trembling I set off for the physics institute to Kundt. The porter very politely invited me to sit down in the ‘professor’s office’—a laboratory quite Faustian in character. . . . Kundt was working in another laboratory, and therefore I had to wait about five minutes until, at last, ‘he himself’ appeared. He was not handsome; chestnut, bristling hair, a high ‘intelligent’ forehead, deep-set blue eyes, an aquiline nose, an energetic mouth and a light-red beard, a face all pitted with smallpox—all this ought to have made an unpleasant impression, but with him, on the contrary, a penetrating, terribly intelligent gaze and at the same time an expression of the utmost good nature produced a strong opposite effect; he was not tall and was rather broad-shouldered. He received me remarkably kindly; I had never expected kindness to such a degree. After talking with me about what I wanted to do, and learning that I intended to take the doctoral examination, he promised to guide me in choosing lectures.” The good impressions received at the first acquaintance with Kundt’s laboratory grew still stronger with time; in Lebedev’s letters we meet the following characteristic thoughts:

“With each day I fall in love with physics more and more. . . . Soon, it seems to me, I shall lose the human form; already now I have ceased to understand how one can exist without physics.”

“The colloquium, which only recently seemed to me less congenial than an apocalyptic beast, has now turned into a source of delight.” Lebedev then describes in great detail the work at the institute and enthusiastically recounts the successes achieved by his senior comrades in the laboratory; but, of course, the central figure, to which all Lebedev’s attention was drawn, was Kundt; of him he wrote as follows:

* Among the Russian scholars who were Kundt’s pupils, besides Lebedev, were also B. B. Golitsyn, D. A. Goldhammer, G. G. de-Metz, D. P. Konovalov, V. A. Mikhelson, V. Natanson, S. Ya. Tereshin, V. A. Ulyanin, and V. S. Shcheglyaev.

“I am surprised, I am amazed, I am delighted, I am enchanted, of course, by Kundt.” And further: “The pleasure (obtained from reading) turns into enjoyment, especially when Kundt is involved (lectures or conversations with him). I never thought that a man could affect me so astonishingly, so bewitchingly, as my patron does, often not even by words, but simply by his genius.”

The working atmosphere of the small German town embraced Lebedev, and from the very first month he plunged into the study of the literature. “For me,” he writes at this time, “every page read contains more pleasure than the labor spent on mastering it; thus from morning till evening I am occupied with what I have wanted to occupy myself with since the age of 12, and I have only one sorrow—the day is short.” To help with this sorrow, P. N. later made an arrangement with one of his institute comrades * to read the literature in parts and to tell each other over dinner what had been read. Already in his earliest days in Strasbourg, Lebedev also took up independent work, and the thick notebook of his protocols and projects contains a mass of interesting material. Despite the fact that many of the works were well conceived, not one was brought to completion: everything seemed to P. N. insignificant and uninteresting; in these first investigations he seemed to be testing his strength.

In 1888 Kundt moved as professor to Berlin, where he took Helmholtz’s chair **. Naturally, Lebedev, who valued Kundt so highly, followed him there, and the year spent with him in Berlin left indelible impressions on Lebedev for his whole life. In addition to working in Kundt’s institute, during this time Lebedev attended Helmholtz’s lectures on theoretical physics, and these lectures, just like the reports of the great physicist-physiologist in the Physical Society, remained perhaps among the brightest memories of his entire life. Helmholtz’s very personality made an altogether exceptional impression on Lebedev, and he later often recalled how he could not utter a single word to Helmholtz because of embarrassment when P. N. had to enroll in his lectures.

Relations with Kundt remained good, and Kundt held his talented pupil in very high esteem. How much Lebedev was working at that time, and what an enormous number of topics were being born in his head, can be seen from a poem written by Kundt in 1889. The beginning of this poem is as follows:

Ideen hat Herr Lebedew
Per Tag wohl zwanzig Stück,
Und für des Institutes Chef,
Ist’s wahrlich noch ein Glück,

* Later Academician B. B. Golitsyn.

** In 1888 Helmholtz was appointed president of the Physikalisch-Technische Reichsanstalt and remained professor of theoretical physics at the university.

Dass er die Hälfte schon verliert
Eh’er sie überhaupt probirt*.

Valuing his pupil highly, Kundt, naturally, demanded much more of him than of others. I had occasion to hear from P. N. Lebedev himself about one characteristic episode. Working in Berlin, in the physics laboratory, Lebedev often had to use, for one of his investigations, an old mercury pump that required constant topping up with mercury. Prizing his time, P. N. designed and himself made a device for the automatic feeding of mercury. Using it, he could freely leave his laboratory during pumping. During one such absence, when P. N. was at a lecture by Helmholtz, Kundt entered his room; noticing Lebedev’s new apparatus, he at once called several of his assistants and began, with enthusiasm, to praise P. N. to them as a designer. Just at that moment P. N. Lebedev himself appeared, having already partly overheard what Kundt had said. No sooner had Lebedev crossed the threshold than Kundt pounced on him and began scolding him for wasting time in vain on technical problems and being distracted from science. Defending himself against such attacks, Lebedev wrote in one of his letters at that time: “Kundt often scolds me on account of the work, but often also because I have my own train of thought and he has his—and we do not agree.” These disputes, however, only strengthened still more the relations between teacher and pupil, and Lebedev was one of the few practical workers in Kundt’s laboratory in whose work Kundt did not interfere.

One of Lebedev’s last investigations during this Berlin period was his attempt to show that, with sufficiently thin conducting layers, the latter can lose the ability to shield bodies from an external electric field. “Making use of the remains of the semester,” P. N. writes, “I threw myself into the work and made experiment after experiment. From the very beginning of January I worked every day, and not once did I have a successful experiment.” Further, after describing his state of mind, Lebedev writes: “I went to Kundt to consult with him; he said outright that the question was too important for science to abandon it without having tried everything possible, without arriving at irrefutable experimental proof that it is impossible, and I do not have that, since my experiments give nothing decisive; but that, on the other hand, he advises me for the time being to leave this topic, take up a simple doctoral work and pass the examination—and immediately afterward to take up that work again.”

Kundt’s advice played a significant role in Lebedev’s fate. He firmly decided to leave his more complex investigations for a time and sit down to his doctoral dissertation.

* Translation: “Lebedev has up to twenty ideas a day, and for the head of the institute it is a true blessing that he loses half of them before setting about carrying them out.”

Accidental circumstances did not allow him to do this in Berlin. A realist by education, Lebedev did not know the ancient languages that were necessary for him in order to pass the doctoral examination in Berlin, and after long hesitation Lebedev decided to move to Strasbourg, where Latin was not required for obtaining the doctoral degree. Leaving Berlin and leaving behind the laboratory with which so many memories were connected for Lebedev, he naturally recalled with warm feeling his first brilliant teachers, Kundt and Helmholtz, and in one of his last letters from Berlin there are the following lines: “Kundt is an artist and a poet, ardent, responsive—he stirs and sustains an excited state of mind, while Helmholtz speaks clearly and simply of eternal truth, of infinite beauty.”

In Strasbourg, at the institute of Prof. Kohlrausch, Lebedev plunged headlong into work and soon began to obtain the first results of his proposed investigation. His letters at this time constantly contain notes: “My work is moving ahead very successfully.”

Finally, after the doctoral examination and the presentation of his dissertation, Lebedev received the doctoral degree in 1891.

Preparation for the examination took quite a long time, and the examination itself was accompanied by a comic episode that Lebedev often recalled later. P. N. intended to choose, for his answers in the examination, in addition to physics, two branches of mathematics, but two weeks before the examination Prof. Kohlrausch suggested that, instead of geometry, he take chemistry, and, despite Lebedev’s protest, compelled him to agree to the proposal. As a result, in a very short time Lebedev had to acquaint himself with Fittig’s enormous course of organic chemistry—a course he had never studied before; and although the examination was passed magna cum laude, nevertheless the rapid and superficial preparation left in his soul, for his whole life, a prejudice against chemistry, especially organic chemistry, which he subsequently expressed in a very figurative way.

In order to understand the significance of the first experimental work and to clarify for oneself the whole subsequent course of Lebedev’s ideas, one must know that shortly before this time Hertz’s brilliant investigations had appeared, showing that electromagnetic disturbances can propagate in the form of oscillations in the ether, and that these rays of “electric force,” as Hertz called them, are thus entirely analogous to light oscillations. These works, which for the first time confirmed Maxwell’s idea of the electromagnetic nature of the light ray, required a number of further consequences. One of them was the following: if light is an electromagnetic phenomenon, then, falling upon a molecule, it can produce in it the same phenomena that an electromagnetic ray produces in surrounding bodies; and since these effects will depend on the nature of the body on which the ray falls (on its conductivity, etc.), it seemed necessary first of all to study the electrical properties of the individual molecules of which the body is composed. If these molecules

will represent perfect conductors, and if their dimensions are small in comparison with the dimensions of the intermolecular intervals, then, as Clausius showed, it is possible easily to calculate the dielectric constant of the medium. A series of experiments was carried out with artificial media consisting of a dielectric and a conductor (mercury ointment), and the experiments showed agreement with theory. Lebedev sets himself the task of investigating the electrical properties of molecules, in other words—the task of determining whether a gas, consisting of separate free molecules not united into one compact whole, obeys the laws discovered by Clausius or not. But before carrying out this task, Lebedev has to develop the method itself, and his method, which was a further development of Hopkinson’s method, enabled him to perform the necessary measurements with astonishing accuracy.

The verification of Clausius’s conclusions concerning the relation between the dielectric constant and the filling of space proved unexpectedly good. Only for alcohol did P. N. obtain divergent results, and these discrepancies could easily be explained by the variability of the very value of the dielectric constant precisely for alcohol. The change in the degree to which space is filled by molecules was achieved either by changing the pressure or by changing the temperature of the gas. Finally, using the equation of state of matter in the form given by van der Waals, Lebedev shows that the order of magnitude of the quantities theoretically calculated from the theory for the filling of space and directly observed in experiment is one and the same. Lebedev formulates the result of the entire work as follows:

“a) Faraday’s assumptions that molecules are electrically conducting bodies, or, as Mascart and Joubert put it, that molecules have an extremely high dielectric constant, do not lead to any contradictions with the observed phenomena and are explained in a very simple manner.

b) The Lorentz relation may be usefully applied as an empirical formula connecting the density of a body with its dielectric constant.”

Thus, in the simplest case it was shown that an electric field can act on molecules and that the latter may then be regarded as absolutely conducting bodies of definite dimensions—as resonators.

Simultaneously with the study of the dielectric constant of gases, Lebedev undertakes the study of theories of comet tails and rather soon arrives at a correct conception of the forces responsible for this phenomenon. On this subject he writes:

“I seem to have made a very important discovery in the theory of the motion of heavenly bodies, especially comets,” and further: “The law found extends to all celestial bodies. I communicated it to Wiener,* at first he declared that I had gone mad, and the next day, having understood what the matter was,

* Later professor at the University of Leipzig.

I congratulate you very much. At first I was in a state of great nervous tension, but now that the law has been proved I am not at all worried; partly, perhaps, because of this—I will not conceal it—that I am bewildered, even stunned, by its generality, which at first I had not anticipated. The law I have derived is not the work of a momentary discovery: for about two years I have carried its beginnings within me. The question with which I have long been occupied I love with all my soul as, I imagine, parents love their children.”

The study of the theory of cometary tails and the establishment of the cause of the repulsion of a tail from the sun compelled Lebedev to draw further conclusions from these ideas—conclusions already entirely belonging to molecular physics.

Inside a body that consists of separate molecules-resonators, these latter may produce electrical oscillations of a definite period, acting on neighboring molecules, and this may cause either attraction or repulsion of individual particles. Molecular forces, which determine dissolution, diffusion, osmotic pressure, and so forth, may thus be reduced to the mutual action of molecules-resonators, if only it can be shown that resonators can indeed exert a mechanical influence upon one another. But here an enormous difficulty is encountered, since, despite every desire to discover the forces of interaction of resonators, even the most experienced investigators could not discover them. However, an accidental circumstance led Lebedev to undertake the difficult investigation of mechanical forces in resonators, namely: the study of the theory of cometary tails. Bessel and Olbers showed that the form of a cometary tail can be explained by assuming a repulsive force emanating from the sun and acting inversely proportional to the square of the distance from the sun. Bessel sought the nature of this force in electrical repulsions; Lebedev, following Maxwell’s theory, supposed that light can exert pressure on a molecule, and that the law of this pressure was sufficient to explain the existing forms of cometary tails. And since the molecule itself is a resonator and light is an electromagnetic alternating field, it is clear that such a field must exert a mechanical action on a resonator. Therefore the solution of the question of ponderomotive forces in resonators is closely connected, in Lebedev’s works, with his conception of the forces in the tails of comets.

These important and interesting questions were developed by Lebedev in parallel with his dissertation work, and already on January 4, 1891, he noted in his diary: “For the essay on the forces of pressure in wave-like motion a plan is beginning to take shape for me,” and further he set out, on four pages, a project for future work, which contained all those consequences that were proved in his later investigations. At the last colloquium in Strasbourg Lebedev reported on his investigations into the pressure of radiant energy, and on this occasion in one of his letters he wrote:

“Today (July 20, 1891) is a very important day in my life: today I spoke for the last time in the colloquium

on the question that for three years now has occupied me continuously: on the essence of molecular forces. For two hours I spoke and demonstrated experiments, which succeeded for me in a way that is rarely achieved.” Further, describing the general impression of the report, Lebedev adds: “Kohlrausch concluded: I note this as a very fruitful idea, but the conclusions should be drawn with great caution—and above all it is necessary to do everything experimentally.”

Soon after this colloquium, where Lebedev set forth, as it were, the program of all his future activity, he leaves Strasbourg and moves to Moscow, where A. G. Stoletov offers him the position of assistant. Taking leave of Strasbourg, Lebedev writes:

“The happiest time of my life was my stay in Strasbourg, in such an ideal physical environment. What my further fate will be—I see only a misty patch with a large question mark. One thing I know: I shall work as long as my eyes see and my head is fresh.”

Upon returning to Moscow, it was first of all necessary to secure for himself the possibility of continuing scientific work. The old physical laboratory of the university, where Lebedev had to settle at first, was very small and not adapted to the tasks he set himself. It was located on the upper floor of a small two-story house in the courtyard of the university’s old building, and the hall for the general practicum was adjacent to the rooms for scientific work. These inconveniences, however, did not disconcert Lebedev, and he set about organizing the laboratory with ardor; above all, he established a good workshop where the necessary instruments could be made. Little money was required for this, but even these modest funds were often lacking, and Lebedev later recounted how he had terrified A. G. Stoletov by ordering various supplies for the workshop for 300 rubles. Having finished equipping the laboratory in this way, Lebedev plunged headlong into scientific research. At the same time he took a very ardent and active part in the scientific work of the Moscow learned societies and, together with B. B. Golitsyn (at that time a laboratory assistant in the physical laboratory), organized at the Society of Lovers of Natural Science, Anthropology, and Ethnography a series of reports and surveys on the latest advances in physics. Finally, a series of public lectures on the most varied, topical subjects in the physical sciences rounded out Lebedev’s multifaceted activity. In general, this period of his Moscow life was hardly not the most active period in P. N.’s life. The works undertaken by P. N. at this time concerned the development of his basic ideas on the interaction of resonators.

In 1894 appeared Lebedev’s first work devoted to the action of waves on resonators, and in three papers that appeared one after another Lebedev clarified the law of these actions. The method he used consisted in the fact that electrical, acoustic, and hydrodynamic oscillations, excited by the corresponding sources of oscillations, fell upon a movable resonator—

nator and deflected it from its original position. By counteracting this deflection and measuring the force arising thereby, one can measure the influence of the period of the incident wave on the magnitude of the interaction. The results of the investigation may be formulated as follows:

a) The mechanical action of the exciting wave on the resonator is proportional to the incident energy and depends only on the ratio of the numbers of oscillations, and not on their absolute magnitude.

b) If the resonator is tuned higher than the exciting source of oscillations, then it is attracted to it, and this attraction has a maximum near resonance itself; upon passing through resonance it turns into repulsion, which diminishes as the detuning of the resonators increases.

With respect to acoustic resonators, which, together with hydrodynamic ones, yielded completely identical laws, certain special features were also observed: “Here,” Lebedev writes, “there appeared one feature of the phenomenon which was quite unexpected for me: whereas in the immediate vicinity of the source the law of the ponderomotive action of an acoustic wave on its resonator is identical with the corresponding law for electromagnetic, and also for hydrodynamic, oscillations, with a constant increase of the distance this identity gradually becomes effaced, and its place is taken by a new law, entirely different from the preceding one: thus, at small distances, near resonance, upon passing through it, there is observed a change from maximum attraction through zero (at complete resonance) to maximum repulsion of the resonator by the source, whereas, as the distance from the latter is increased, the attraction, gradually smoothing out, finally disappears altogether, and its place is taken by repulsion.” Lebedev succeeds in showing theoretically that far from the source (in the case of a plane wave) such a case must also occur for electromagnetic waves; and in his Russian doctoral dissertation he adds: “In nature a similar phenomenon (in a more complicated form) is represented by the repulsive action of the sun’s rays on the gaseous molecules of comet tails.” Lebedev’s works on ponderomotive forces in resonators earned him the Russian degree of doctor, which, at the petition of Moscow University, he received without the preliminary master’s examination and without submitting a master’s dissertation.

Having finished his experiments on models, Lebedev incidentally carried out an elegant piece of work from the technical point of view on the double refraction of “rays of electric force,” in which he discovered the shortest electromagnetic waves that had so far been obtained.*

* The apparatus for short waves was demonstrated by Prof. Augusto Righi at the Bologna Academy, and he wrote to P. N. on this occasion: “At the same time as you receive this letter, you will receive also your small apparatus, which I demonstrated at the Academy of Sciences in Bologna. The physicists who took part in the meeting were greatly delighted with these apparatuses.”

After this he takes up the main task of his life—the proof that light exerts pressure on molecules and that, consequently, the latter can attract or repel one another under the influence of mutual radiation; and first of all he undertakes a simpler technical problem—a problem that more surely promised success: the pressure of light on solid bodies.

In his treatise on electricity and magnetism Maxwell points to a theoretically imagined experiment that could show the pressure of light on solid bodies. P. N. Lebedev’s classic work on the mechanical action of light waves, reported by him at the Paris congress of 1900 and then printed in Annalen der Physik in 1901, is the realization of Maxwell’s idea. In P. N. Lebedev’s experiments a light beam from a voltaic arc fell upon a light vane enclosed in a vessel and, by twisting the thread to which the latter was attached, made it possible to measure the mechanical action of the pressure of light. On the other hand, knowing from calorimetric observations the energy incident upon the vane and the reflecting power of the vane, the same pressure could be calculated by Maxwell’s formula. The experiments gave a result in excellent agreement with theory and showed for the first time that light really does exert pressure on bodies. The chief difficulty in Lebedev’s experiments was the radiometric forces, which P. N. Lebedev managed to eliminate by an extremely simple and convenient method.

How complex and difficult these experiments were is evident from the fact that a number of brilliant experimentalists, for example Bartoli and Crookes, despite enormous efforts and skill, were unable to solve this problem, and the question, although theoretically developed, remained open from the experimental point of view. Its solution, however, was not only in keeping with the general course of Lebedev’s ideas, but was also closely connected, as P. N. then imagined it, with the substantiation of electromagnetic theory itself; and therefore Lebedev set to work with all the greater zeal, since success for him was beyond doubt. Yet between the beginning of the work and its completion there passed a vast interval of time, during which it was necessary to study all aspects of the phenomenon and, chiefly, the additional radiometric forces, to find ways by which they could be considerably reduced; and only after this were the investigations brilliantly completed and the existence of light pressure proved beyond doubt. The end of these investigations is not without some interest. Exhausted by his preceding teaching work and examinations, P. N. during the summer of 1900 felt so ill that he could not himself carry out those mechanical manipulations that were connected with the investigation, and in the observations he was assisted by an attendant at the physical laboratory. Once during an experiment, because of an accidental jolt during pumping, the fine thread with the device that had detected the pressure broke off, and the instrument was damaged; it did not seem possible for the ailing P. N. to build his apparatus again, and although he did not consider

considered his research completely finished in the sense of accuracy, he found it necessary to publish it in the form in which it was at that moment.

What impression this remarkable work made on the scientific world is best seen from the words of Lord Kelvin, spoken to K. A. Timiryazev: “You may know,” said Kelvin, “that all my life I fought with Maxwell, not recognizing his light pressure, and now your Lebedev has forced me to surrender before his experiments.”

W. Crookes, in a letter to Lebedev concerning this work, wrote, among other things: “You have succeeded in finding a method for detecting and measuring extremely small forces of the direct pressure of light, and moreover at a time when they are masked and complicated by much larger radiometric forces.”

Already known for his first works, which had placed him among the ranks of good experimentalists, by this work Lebedev at once won himself an honorable name: from a good scholar he immediately became a first-class physicist, and at the present time there is no textbook in which his work on light pressure is not cited.

For his work on the pressure of light on solid bodies, the Academy of Sciences awarded P. N. a prize.

Having brilliantly substantiated the doctrine of the pressure of light on solid bodies, P. N. immediately turns to the question of the action of radiant energy on molecules as on resonators, to the pressure of light on gases. In addition to the purely physical significance of this work, it also had enormous cosmic significance. As was indicated above, P. N. was responsible for the idea of explaining comet tails by the pressure of light rays, and at the present time the doctrine of the pressure of light on gases is of colossal significance in all branches of astrophysics. If the first work on light pressure was difficult, then the second problem at first glance seemed simply impossible. Schwarzschild’s calculations, made by him, it is true, under quite definite assumptions (for reflecting bodies), showed that if the dimensions of metallic particles are reduced to the size of molecules, then the pressure of radiant energy becomes so small in relation to the attraction experienced by every ponderable particle from the surrounding bodies that the effect of pressure cannot be observed; and Arrhenius even based on these calculations a theory of a number of meteorological phenomena, considering the role of cosmic dust in interplanetary space. True, as later became clear, Schwarzschild’s reasoning could not apply to molecules, and P. N., armed with his preceding experiments with resonators, saw this clearly; nevertheless, enormous theoretical foresight, enormous confidence in the results, was needed in order to undertake an investigation of the pressure of light on gases, in order to carry it through to the end. It is impossible to enumerate all those variants of experiments that were made in order to discover the phenomenon; it is enough to say that up to twenty final apparatuses—apparatuses with which measurements were made—were constructed. Many times it seemed that

the investigation gives a completely negative result, that it is impossible to eliminate the incidental perturbing forces, that the phenomenon cannot be observed; and each time Lebedev found some catch that allowed him to make a new version of the experiment, so as to be able to carry through to the end everything that had been so brilliantly conceived by him. Finally, in 1910, the investigation was completed and appeared in print.

The method by which Lebedev worked consisted in this: rays of light, passing through a gas enclosed in a box divided by a partition into two compartments, set it into circulation. If one excludes those motions of the gas which depend on the nonuniform heating of the various layers, then the whole effect should depend only on the light pressure, and this pressure can be measured by placing in the dark part of the box a small piston, not touching the walls, upon which the gas will press in its motion. An extremely important circumstance that could ensure the result of the experiment was the elimination of thermal motions of the gas depending on nonuniform heating, and this circumstance was resolved by Lebedev in a very simple and elegant way. Gas in general is a poor conductor of heat, and only hydrogen conducts heat very well; therefore, by mixing hydrogen with all the gases under investigation, one can equalize the temperature in the various layers so well that convection, depending on heating, will not produce any appreciable effect. By illuminating the layer now from one side, now from the other, and at the same time observing the deflections of the piston, one can measure the magnitude of the pressure of light rays on a gas.

One of the first scientists to congratulate Lebedev on his remarkable discovery was Schwarzschild, who wrote to Lebedev on February 9, 1910: “I well remember with what doubt I heard in 1902 about your proposal to measure the pressure of light on a gas, and I am filled with all the greater amazement when I read how you removed all obstacles.” For his work on the pressure of radiant energy, the Royal Institution of Great Britain elected Lebedev in 1911 to its honorary membership.

Here it may not be uninteresting to point out that the very idea of pressure on a gas arose almost simultaneously with the general idea of the pressure of light. As early as 1894 we find in P. N.’s diary the following entry: “The pressure of a light ray on a gas should conveniently be investigated for gases in the visible spectrum (NO₂, J, Br, Cl) in an apparatus resembling a convection thermometer, and perhaps a radiometer.”

There follows a detailed description of the apparatus. Whether the apparatus was constructed and tested—of this no data remained either in the notes or in personal conversations with P. N.; but what is interesting is that the very idea of the possibility of the phenomenon was already at that time perfectly clear to Lebedev.

During the period of his work on the light pressure on gases, P. N. also worked a great deal on the question of the motion of the Earth in the ether; of these works only one small experiment was published, which is a modified

toward Roland’s experiment; all the rest, as not having yielded any positive results, was left unpublished by Lebedev. But if one examines all the apparatus designed by him for these experiments, one can marvel both at his striking talent as a constructor and at his remarkable ability to overcome the experimental difficulties encountered; and these unpublished investigations, scarcely less than all that was printed, justify Prof. W. Wien’s words that Lebedev “possessed the art of experimentation to a degree scarcely matched by anyone else in our time.”* These experiments of Lebedev, together with all the numerous unsuccessful experiments that attempted to establish the motion of the earth in the ether, led to the development of the special principle of relativity.

Finally, the last works, begun in 1909, were devoted to the magnetism of rotation and were prompted by Hale’s remarkable discovery, which revealed magnetic phenomena around sunspots. Already on April 2, 1909, just after the colloquium at the university where Hale’s work had been reported, P. N. writes in his diary: “I am working on terrestrial magnetism in connection with Hale’s discovery of the magnetism of sunspots.” There follows an exposition of his views, which amount to this: that every rotating body, owing to its motion, must produce a separation of positive and negative charges in the molecules and, as a consequence of this, a magnetic field. A series of experiments, completed in 1911, however, gave no positive results in this respect and even showed that all assumptions concerning such separability of charges were incorrect. These experiments were cut short by Lebedev’s death while still in progress.

In 1911 P. N. left the university together with a number of other professors. The reason for his departure was the events that took place at the university as a consequence of the order of the Minister of Public Education dismissing the rector (Prof. A. Manuilov), his assistant (Prof. M. A. Menzbir), and the prorector (Prof. P. A. Minakov). His last works, from the autumn of 1911, were carried out in a small laboratory arranged in private premises.** This last period of P. N.’s work is the most tragic in his life. The awareness of a severe heart disease, which since 1901 had already begun to manifest itself in attacks that often kept him from sleeping, and the awareness of his own and his family’s lack of security, greatly distressed P. N.; the only consolation he found at that time was in the successes achieved by his pupils.

At the same time as Shanyavsky University, two institutions—namely, the Chief Chamber of Weights and Measures and the Nobel Institute in Stockholm—made attempts to attract P. N. into the number of their employees—

* From a letter of Prof. W. Wien to Prof. V. A. Mikhelson on the occasion of Lebedev’s death.

** Funds for the establishment of the laboratory were provided by the Society for the Promotion of the Advancement of Experimental Sciences and Their Practical Applications named after Kh. S. Ledentsov, by A. L. Shanyavsky University, and by donations from private individuals. Shanyavsky University, from March 1911, invited P. N. to be one of its lecturers.

bots. The director of the Chamber, Prof. N. G. Egorov, proposed inviting P. N. as a research associate after the proposed reorganization of the Chamber. At the same time, the director of the physico-chemical laboratory of the Nobel Institute, Prof. S. Arrhenius, wrote to Lebedev: “Naturally, it would be a great honor for the Nobel Institute if you wished to settle there and work, and we, without doubt, would provide you with all the necessary means so that you might have the opportunity to continue working.” Passing on further details of the possible arrangement at the institute, Arrhenius wrote: “You, of course, would receive an entirely free position, as befits your rank in science.”

These proposals, which made it possible to work scientifically without any teaching duties, gave P. N. enormous satisfaction. However, none of this was destined to be realized: already in January 1912 he felt unwell; in February he took to his bed, and on March 1 (14) he was no more.

Not only did the scholarly world of old Russia, headed by the Academy of Sciences, respond to Lebedev’s death, but Western scholars as well, and above all the Royal Institution in London, sent expressions of their condolences. Among the telegrams and letters received by the Moscow Physical Society founded by P. N. (about 100 in number), there are, among others, letters from Arrhenius, Röntgen, Nernst, Rubens, Warburg, Thomson, Crux, Lenard, Righi, Korn, and a whole series of other scholars; and this shows that “our” Lebedev was no stranger in the West either.

We now turn to another aspect of P. N.’s scientific activity, namely to the work of his students. The awareness of the necessity of such work arose in Lebedev very early—in this respect Lebedev is a vivid illustration of that type of scholar whom V. Ostwald calls romantics. Having developed mentally early, richly gifted by nature, and with an extraordinarily rapid reaction to his surroundings, Lebedev, even in his youth, felt that the mass of questions and ideas that came into his head oppressed him, did not give him the necessary peace for his work, and he began to strive consciously to free himself from these ideas by handing them over to his students. As early as 1893, not long before the time when he took several practical students under his supervision, he wrote in his diary: “The abundance of thoughts and projects does not leave me quiet time for work.” At first, not having an independent laboratory, Lebedev organized special investigations in the laboratory of Prof. A. P. Sokolov, and at that time 6 specialists worked under his supervision. Soon after moving to the new physical institute of the university, when Lebedev was placed in charge of the laboratory, their number doubled.

All the work carried out by P. N.’s students stood in the closest relation to his own work, often serving as supplements to the main theme pursued by P. N. himself.

Thus, during the period of work on light pressure, a whole series of studies on the pressure of radiant energy arose in Lebedev’s laboratory.

Using analogies, Lebedev assumed the existence of such pressure also in sound vibrations and in water waves. The works of Altberg and Kaptsov resolved this question, and subsequently Zernov’s work made it possible to apply the pressure method to absolute measurements of the force of sound, alongside Rayleigh’s disk. This same pressure method later made it possible to detect the shortest sound waves in air and to discover the damping of acoustic waves in air as a function of wavelength (Neklepaev).

Another series of works was closely connected with electrical oscillations and was a continuation of P. N.’s own work in this field; thus there appeared works on the absorption of electrical oscillations (Romanov), on the study of undamped waves (Shchodro), and a work on the magnetic properties of bodies for high-frequency oscillations (Arkadev). Finally, these works for the visible spectrum culminated in the construction of a special spectrograph for infrared rays, which made it possible automatically to record absorption in various rays of the spectrum (Yakovlev). The idea of such an instrument had arisen as early as the end of the 1890s and changed several times in P. N.’s conception, until at last it acquired the form described in the Physikalische Zeitschrift.

A mass of ancillary questions also received resolution (for example, Esmarch’s work on magnetic shielding). The laboratory thus worked according to a definite plan, according to a definite program outlined by P. N. The very topics of the works, before they were assigned to the practicum students, were thought over many times by P. N. himself, and he often said the following to those of his pupils who had to direct scientific investigations independently: “Never propose to a beginner a question that would not give a clear answer and for the solution of which the path would not be determined.”

P. N. applied this requirement in an even more categorical form to his own topics as well, and therefore among the works he proposed there were no failures. In them there was no experimenting at random; there was always a definite aim and a clearly developed method for solving the fundamental question.

In order to keep abreast of all the work of his specialists, P. N. had to think over the details of these investigations for a long time, and I often had occasion to find P. N. far past midnight discussing the work of his practicum students. When going abroad for rest, P. N. often could not restrain himself and sent one letter after another, in which he indicated the necessary additions to the work or some changes in it.

As interesting as his conversations in the laboratory were, equally lively and instructive were his remarks at the weekly colloquia, which arose in 1901 in the old laboratory and from which the Moscow Physical Society later grew, whose founder was P. N. At these colloquia everyone, from the beginning student to their leader, felt themselves members of one large family, and in this way it became possible

that union of workers which is so necessary in scientific work.

In concluding this brief and incomplete sketch of the life of P. N., so rich in inner content, one cannot help recalling the words he spoke in his address dedicated to the memory of Kundt:

“In Kundt, science lost a physicist in the broadest sense of that title: not only a first-rate scholar, who labored so much for contemporary physics, but also an incomparable teacher, who cared for the future of his beloved science by educating and training its future workers. If Kundt, throughout his entire life, working tirelessly, strove to expand as much as possible the domain of our knowledge by his investigations, then, perhaps to an even greater degree, he strove to transmit to each of his pupils a particle of his soul, a particle of that selfless love of knowledge which leads humanity toward truth.” These same words, with the deepest gratitude toward P. N., will be repeated by all those who had occasion to work in his laboratory.