00:00:00 Because they have very deliberately organized that we should be here,

00:00:29 in Ithaca, today, to commemorate, of course, the birth, a hundred years ago, of Peter Debye,

00:00:42 and to show our appreciation of the extraordinary achievements which he communicated to our science.

00:00:59 And I have the task, this morning, of very briefly reminding you a little of his background in Europe,

00:01:13 and of the achievements and contributions which he had already given us before he came to Cornell.

00:01:29 Now, there are two extremely good reasons why I stop at 1940.

00:01:35 You will find that the compression involved in trying to give an account of Debye's work up to 1940 in 50 minutes is already very painful.

00:01:47 And secondly, there are very many colleagues here who could tell you much more fully, much more accurately than I could,

00:01:55 what Debye did after he came to Cornell.

00:02:01 Now, let's start at the beginning. He was born on March the 24th, 1884.

00:02:09 The March the 24th, you might feel, is rather trivial.

00:02:13 Yes, it's rather interesting. It was actually Joseph Priestley's birthday, and more interestingly still,

00:02:24 of the first 67 Nobel Prize winners in chemistry, three had the birthday of March the 24th.

00:02:37 Now, not very significant. The birthplace, far more relevant.

00:02:46 Debye was born in Maastricht, and his roots were in Maastricht.

00:02:55 Many of you will know that, of course, he moved around very considerably in Europe,

00:03:02 and was frequently asked, well, Professor Debye, you are a Dutchman, aren't you? Or are you German now?

00:03:09 I am a Maastricht man, would be the final answer to that query.

00:03:15 And let me just remind you, Maastricht, if we have a slide, firstly, to remind you of...

00:03:36 Good, thank you.

00:03:40 That is a very well-known photo of Professor Debye, of course. Now, the next slide, please.

00:03:48 Maastricht, in this rough sketch, is down here in the extreme southeast corner of the Netherlands.

00:03:54 It is the principal, you would say, city of the province of Limburg.

00:04:01 It was, for some centuries, ruled jointly by the Archbishop of Liege and the Counts of Brabant.

00:04:08 And being in the Limburg province, which is a very strongly Catholic area,

00:04:16 it hasn't got an extraordinary close attachment to the Protestant North Holland.

00:04:24 And that gives both the city, and indeed Debye as a character, some independence of outlook.

00:04:33 Just notice, too, that Maastricht, it's about 25 miles from Liege, and a similar distance from the

00:04:43 historically important city, which we now call Aachen in Germany.

00:04:52 Also, on the same map, I've put in Delft. You'll see the relevance of that shortly.

00:04:57 Now, Debye, if we have the next slide, please, was born in

00:05:11 properties which have now been reconverted. They've put a plaque up at the spot where the apartment was.

00:05:19 And as a youngster, until he went to school at about six years of age, he did not even talk Dutch.

00:05:30 He talked the Maastricht patois. And his school friends in Maastricht, whom I was able to see

00:05:37 some years ago, assured me that he continued to talk to them in this patois, and even to write to them in it.

00:05:46 Now, I have said his roots were in Maastricht. One can have from the Burgermeister the Debye

00:06:04 family tree. Back four generations, they are nearly all Maastricht people. Three of his

00:06:11 grandparents were both born and died in Maastricht, as did his parents. So, we really have to

00:06:23 remember this in, you might say, one's appreciation of Debye as an individual.

00:06:31 Obviously, he went to school in Maastricht. I think there is a slide.

00:06:36 That's right. Now, with modern progress, of course, this has been demolished.

00:06:45 But more important than the school is the record which he achieved when he left.

00:06:58 And that record is

00:07:01 notable

00:07:08 in two respects. Firstly, obviously, the quality of the performance.

00:07:16 But even more remarkable, I think, by modern standards, is the range and the significance

00:07:24 of the subjects. There are no... Pardon me, I've knocked when I shouldn't have. Could you go back?

00:07:37 I would suggest, with all respect, that there are no soft options in this, unless it might be

00:07:45 government. Do notice that by some remarkable...

00:07:56 I can't understand how Debye got only five in his English, because subsequently, and indeed at the

00:08:06 age of 15, the family have a memory of him teaching his father English, because his father was coming

00:08:12 to London on, you might say, business. And his English, which I've discussed this with some of

00:08:20 his Dutch colleagues, which is so good, was always his own writing. Now, with this record, of course,

00:08:33 the question arose of what would he do next. And here I must tell you a little of his parents.

00:08:43 His father was the foreman in a factory which produced household implements, from

00:08:54 frying pans to decorated iron gates. Very much respected, not only by his co-workers, but by

00:09:03 the citizens of Maastricht. But as in many families, the mother was really the dominant

00:09:11 personality. She was a very bright, keen, intelligent, numerate person. And she was for

00:09:20 very many years the cashier in the Maastricht Theater, which doubled in the American sense

00:09:31 for a small opera house. And his old school friends can tell you that up to his last years,

00:09:42 Debye could whistle a tune from an opera which he had heard 60 years previously,

00:09:49 and challenge them to identify which opera it had come from.

00:09:53 And many of those operas are no longer being produced.

00:09:56 You must have, in Cornell, a recording of an address he gave in that very theater, about 1954,

00:10:11 in which he says that it was there, at a performance of Gunnar's Faust, that he first

00:10:17 entered paradise. Of course, it's only people who really appreciate music, well,

00:10:24 accept that he could have had that experience. Now, with this record, he abandoned the possibility

00:10:35 of taking up a post, which had been arranged with Juergens, the fax manufacturers.

00:10:45 His father said that, if I have to work night and day, the boy shall go on to further education.

00:10:56 The choice was, because this impressive, though the list is, does not include either Latin or

00:11:04 Greek, he could not, in 1901, enter a Dutch university. So he had to choose between the

00:11:12 Technical University at Delft, or the Technische Hochschule at Aachen. He was able, of course,

00:11:19 to commute to Aachen daily, rising at five in the morning, to be there for the eight o'clock lecture.

00:11:27 Now, at Aachen, if Debye ever depended on a stroke of luck,

00:11:35 the only one which was of real significance was the fact that, when he got to Aachen

00:11:44 in experimental physics, he had as a teacher Max Wien, and in theoretical physics,

00:11:53 Arnold Sommerfeld. And you could have gone virtually across Europe to have found a better pair.

00:11:59 It was Sommerfeld who recognised, almost immediately, Debye's extraordinary abilities.

00:12:07 And when he left, after Debye had spent five years in Aachen, he took Debye with him,

00:12:17 as his assistant. Now, at Aachen, Debye did a diploma course in electro-technology.

00:12:26 And what I find quite extraordinary about this, is that there was no special attention.

00:12:34 He was given no special course in mathematics, as such. Although, there is no question that he was

00:12:43 a first-rate mathematician. And by that, I don't mean a person who could solve differential

00:12:49 equations or do elaborate computations. He was a mathematician by Göttingen standards,

00:12:58 and I need say no more. It was Sommerfeld, of course, who, appreciating Debye, took him now

00:13:12 as an assistant to Munich, where Sommerfeld had been appointed Professor of Theoretical Physics.

00:13:22 But before we leave Maastricht, because this was Debye's, you might say, leaving home from

00:13:32 Munich, let us look, that is Debye going, as it were, to Munich as a young man. And in Maastricht,

00:13:46 there was eventually placed in the town hall this bust of Professor Debye,

00:13:59 for which it is reliably reported that nothing in his career gave him greater pleasure than to think

00:14:11 that this was on the shelf, you might say, in Maastricht, because that is where he belonged.

00:14:19 Now, I come to the problem of summarizing Debye's contributions to molecular and physical science

00:14:34 from 1906 to 1940.

00:14:40 And this I found very difficult, how best to do this. What I have actually done is to write out

00:14:48 headings, you might say, just memos of topics with their dates.

00:14:56 And I put these on a series of about a dozen slides, which I will proceed to go through.

00:15:02 But I will comment only on some of the items on these slides.

00:15:08 Furthermore, I have, of course, left off many items, and I leave it to your judgment whether I've been

00:15:23 adequately discriminating or not in the choice of what I am going to remind you of.

00:15:30 Now, what is all important in assessing Debye's career is to realize that he was unusually gifted

00:15:45 not only with physical insight, but with the power of a first-class mathematical technique.

00:15:54 First-class mathematical technique. Let's look, oh yes, well now, in leaving

00:16:08 Maastricht, which is there, these are the stopping points in Debye's career in Europe.

00:16:16 Aachen, Zurich to the university firstly, then up to Utrecht, then

00:16:29 from Utrecht to Göttingen, of course, from Göttingen back to Zurich to the ETH, as we say,

00:16:37 then from Zurich to Leipzig and Leipzig to Berlin. You see, he really did move very

00:16:49 frequently, and I think this was possible because of this fact that he didn't really feel he belonged

00:16:56 in any of these particular places. Now, the next slide will indicate where he started. In Munich,

00:17:09 he completed his PhD thesis with Sommerfeld, Ueber, then Regenbogen. Concerning the Rainbow,

00:17:17 a truly evocative title. In this,

00:17:24 he considers the interaction between radiation and spherical particles of different

00:17:34 refractive properties. He considers the refraction, the dispersion, the scattering,

00:17:42 the reflection, and the light pressure on the particles. And it really is, by those who have read

00:17:50 it, said to be a completely masterly exercise in mathematical physical analysis of this system.

00:18:00 It served Debye extremely well, because it gave him such an extraordinarily sound basis,

00:18:08 because the interaction between radiation and matter was essentially the principal theme

00:18:14 of his life's work. Not, of course, entirely. And the second line reminds one of

00:18:25 a very small publication he made, which was to deduce the Planck radiation formula in a

00:18:35 consistent way. Some of you might wonder, well, I mean, was that necessary? Most certainly it was

00:18:41 necessary. Planck, you may know, arrived at the radiation formula by a series of truly painful

00:18:48 empirical steps. He published the formula, showed that it fitted the Lummer-Pringscheid

00:18:53 data extremely well, and then went away. And six weeks later, published a second account

00:19:01 of this equation, which he justified the formula. One couldn't say that he established it

00:19:11 theoretically, because, as the physicists will all know, the presentation is in two parts,

00:19:19 in one of which radiation is infinitesimally divisible. And in the second part, the interchange

00:19:27 with the walls of the blackbody radiator, the radiation is not infinitely divisible,

00:19:35 but is in parcels of h nu. And that is where the deduction rested, until Debye,

00:19:44 in an exercise in probability of radiation interactions, he produced an entirely

00:19:52 self-consistent deduction, in which the radiation is in particles, h nu, in both parts of the

00:20:01 deduction. Planck was so pleased with this that he used it subsequently, of course, in his own

00:20:09 theory of heat, the volumes, the monograph, in which he expounded the quantum theory.

00:20:15 Now, before I move, let me remind you that, from Munich, Debye went to the

00:20:23 University of Zurich, where he succeeded Einstein, who had just gone off to Prague.

00:20:28 And he presumably must have just asked, well, what was Einstein doing here? Well, Einstein

00:20:34 had, amongst other things, of course, published his very simple, basic theory of specific heats.

00:20:44 Now, there's one difficulty in all that I'm telling you. All these items are now built in

00:20:52 as the elementary aspects of modern physics, or modern physical chemistry. And

00:21:01 this means that, in a degree, they have lost their novelty, which is why people who are doing

00:21:07 research normally can't be bothered about the history of the subject. It's the soft option,

00:21:13 and only suitable for retired professors. But you must remember that there was a time

00:21:21 when these items were entirely novel, and it needed a man of great vision to see the steps,

00:21:29 which we now accept as virtually obvious. What Debye did, of course, was to upgrade

00:21:37 the Einstein treatment, as you know, putting in a spectrum of frequency,

00:21:40 and arrived at the first of the Debye equations. There are so many of them,

00:21:45 but I've just written this one in, of course, about the specific heat of constant volume

00:21:49 being a function of the Debye temperature. At the same time, there is a four-page paper

00:21:59 written, obviously, I would suggest, even within the last weeks he was at Zurich, because it is the

00:22:09 only poorly written paper of Debye's, which I am aware of. It's called this, strange enough,

00:22:17 kinetic theory of insulators. He just refers casually to the Langevin treatment of paramagnetism,

00:22:25 and proceeds to deduce an equation which, in an updated form, but which in my slide unfortunately

00:22:36 lacks a square there on the mu. That's because the compression was so great on the slide, you can see.

00:22:44 He put out this paper. Now, this is the birth of the electric dipole moment. Before that, the concept

00:22:56 was scarcely available anywhere. And what is significant, you see, about this, is that Debye

00:23:04 didn't write a paper talking about the electric dipole moments, and how they must be in all sorts

00:23:10 of molecules, and this, and that, and so on. No, what he did was to spell out very clearly

00:23:18 how an electric dipole moment could be measured if there were real ones. And he proceeded to use

00:23:26 available data to show that particular molecules had particular values of this electric dipole

00:23:33 moment. Now, subsequently to this paper, indeed within 12 months of it, there is a second

00:23:46 dielectric paper, which is a model of Debye's writing and method. And that's the paper on

00:23:56 the dispersion of the dielectric properties with frequency. He writes down a simple molecular

00:24:11 rate expression for the relaxation of the permittivity, as I think we should call it,

00:24:18 of a medium when the field is switched off. p at t is equal to p at time 0 times an exponential

00:24:28 e to the minus kt. Now, he didn't merely write that down. He deduced that that was a correct,

00:24:37 or at least an acceptable form, on the basis of a consideration of the Brownian rotational

00:24:45 movement of a spherical particle in a viscous medium. From that, he deduced the well-known,

00:24:55 extraordinarily effective, simple Debye expressions for the dielectric loss as a

00:25:00 function of frequency, and so on. He also used some of Drude's data, and it's quite remarkable.

00:25:09 He had to pick a course, a little, because the data in the literature as a whole was very

00:25:15 uncertain. But it is a fact that Debye's relaxation time for water at room temperatures

00:25:27 was not, his value was not certainly improved upon until 1946.

00:25:36 It's only then that data really showed that we could get a little bit better value than Professor

00:25:42 Debye's of 1913. Good. Now,

00:25:54 Debye, of course, having been in Munchen, he kept in touch with his colleagues there.

00:25:59 As Professor Ewald could tell you very well, I'm sure. And, of course, he learned

00:26:08 of the discovery of x-ray diffraction in the Friedrich Klipping experiment.

00:26:16 Now, it did take some months before the physicists were quite clear as to exactly how this

00:26:27 observation was to be interpreted, and that the young Bragg had the idea, walking in the

00:26:34 backs in Cambridge, that we could treat it as reflection by successive layers of atoms and ions.

00:26:41 But what Debye did was to consider what was going to be the influence of the vibration

00:26:50 of the atoms or ions in the lattice on the intensity of the scattered diffraction beams.

00:27:00 Now, of course, he was in a peculiarly well-informed position to do this. He had just

00:27:05 been treating specific heats of the lattice anyhow, and he came up with a treatment which

00:27:11 you're all familiar with, at least in general terms, in which he indicates how the intensity

00:27:19 will vary with temperature. But he has two models for this, depending upon whether there is or there

00:27:29 is not zero-point energy for the vibrations in the lattice. Of course, it wasn't at all clear

00:27:37 at that stage. Einstein was pretty clear, pretty certain, that there would be zero-point energy,

00:27:43 but it was in no way established. Debye goes through the analysis on both suppositions,

00:27:51 and his general temperature factor was confirmed by the elder Bragg a year later. The fact that

00:27:58 the temperature variation in the intensities really does indicate explicitly that there is

00:28:04 a zero-point energy wasn't fully established until 1925. And we now know of this treatment

00:28:10 called Debye-Waller treatment, because Waller really had to correct. There was a numerical

00:28:15 factor, I think it's a factor of two, which had slipped in Debye's exposition.

00:28:21 Now, notice that we are in Utrecht all this time. The dielectric dispersion, that treatment

00:28:39 of the influence of the lattice vibrations on the intensity of the diffraction by the lattice,

00:28:47 that was in Utrecht. And incidentally, the last phrase I think I had on the preceding slide

00:28:54 was essentially a quotation from Professor Ewald's evaluation of this work as one of

00:29:00 the great achievements of crystal physics, to have done this within a matter of 12 months

00:29:07 of discovering x-ray diffraction by lattices. But here are some other items.

00:29:18 Possibly smaller interest, it points out that the expansion of a lattice really is dependent upon

00:29:26 the anharmonic terms, of course, in the vibration. And that heat conduction, he suggests, and as far as

00:29:32 I know, this was quite original, but I'm open to correction here, in terms of the scattering

00:29:39 and decay of phonon waves. Certainly nothing was done about the treatment of

00:29:46 conduction in terms of phonons, not seriously, until Peierls tackled this question many years later.

00:29:56 Now, the last item here is really based on a note in a Sommerfeld-Runge paper. It's not by Debye.

00:30:09 To indicate, I'd say, a double analogy. On the analytical side, the Jacobian equation of motion

00:30:21 can be transformed via, not exactly, but something approximating to a Riccati

00:30:29 transformation, to an equation which is entirely similar to that for linear waves.

00:30:35 And Debye says, well, this perhaps corresponds to the two pictures, the geometrical optics

00:30:47 in Huygens' form, and classical wave optics. This comment, one could say, but is not entirely novel

00:31:00 on Debye's part, because Hamilton himself had some appreciation of this interrelation.

00:31:08 But, of course, this was of great significance. Obviously, you can see I've put down the

00:31:23 Schrodinger's name there to remind you that this was very much what Schrodinger became

00:31:35 interested in later. Now, our friend from Zurich has already told us a little in this context. I

00:31:46 will read to you, if I may, what Felix Bloch has said about this. Because, obviously, what I am

00:31:55 implying is that Debye had the insight, back in 1912, of a situation which really only finally

00:32:09 became explicit in 1926 with Schrodinger. Now, Felix Bloch's account, and this all relates now,

00:32:17 I'm jumping, to Zurich in 1926. Once, at the end of a colloquium, I heard Debye saying something

00:32:27 like this. Schrodinger, you're not working right now on anything very important anyway.

00:32:33 Why don't you tell us sometime about that thesis of de Broglie's? Well, it seems to have attracted

00:32:39 some attention. So, in one of the next colloquia, Schrodinger gave a beautifully clear account of

00:32:46 how de Broglie associated wave with a particle, and how he could obtain the quantization rules

00:32:50 of Niels Bohr and Sommerfeld by demanding that an integer number of waves should be fitted along

00:32:56 a stationary orbit. When he had finished, Debye casually remarked that he thought this sort of

00:33:03 way of talking was rather childish. As a student of Sommerfeld, he had learned that to deal properly

00:33:10 with waves, one had to have a wave equation. It sounded quite a trivial remark, and did not seem

00:33:18 to make much of an impression, but Schrodinger evidently thought a bit more about the idea

00:33:24 afterwards. Just a few weeks later, he gave another talk in the colloquium, which he started

00:33:33 by saying, my colleague Debye suggested that we should have a wave equation. Well, I have found one.

00:33:42 Now, the next slide is really on the same theme. We are back in 1913, remember, and on February the 10th,

00:33:59 Debye presented a paper which was published, offering as the generalized form for quantization

00:34:08 rule, the circular integral pdq as an integral number times h. Now, in July of that year,

00:34:20 there appeared the first of the three fundamental papers of Bohr's on the hydrogen atom,

00:34:29 and these are papers which I have read and tried to understand, and certainly as far as the first

00:34:35 is concerned, almost completely failed, because I really do not know what Bohr is trying to say,

00:34:41 because what he is actually doing, of course, is arriving at the magic Rydberg formula.

00:34:50 It's almost a replica of Planck's arriving at the radiation formula, and he tries to produce

00:34:59 a justification for this, which you can translate into this simple form, which is what one uses in

00:35:09 teaching elementary students, that the second momentum is n times hoq. Now, it doesn't

00:35:17 justify that. Anyway, Debye had spelt out this version, which is sometimes associated with

00:35:25 Wilson and Sommerfeld, because they introduced it when they subsequently considered elliptic orbits,

00:35:33 you see, but it was there from 1913, but there's also a little very relevant remark that Debye made

00:35:41 on this relation. He said it arose from a remark of Einstein's, who clearly seen this, but couldn't

00:35:50 be bothered to publish it, you see. Now, we must move rather more quickly now. At Göttingen, of course,

00:36:04 we find he got interested in the X-ray diffraction by the powder method. He also expounded

00:36:13 the theory of hydrogen atom-like spectra in terms of the Hamiltonian, which he published in 1916.

00:36:22 Now, Bohr very politely followed this two years later in his major exposition in the Copenhagen

00:36:30 papers. In this context, Debye deduced the spatial quantization of the electron orbits

00:36:43 in the presence of a magnetic field. Now, he was really, Sommerfeld had already, almost

00:36:50 simultaneously, but a little ahead of Debye, had already published this result. It's a very interesting

00:36:56 difference between the two. Sommerfeld was really convinced that there were these electron

00:37:07 orbits, and that they were at different angles in space, and therefore he encouraged in 1922

00:37:15 Stern and Gerlach, when they were trying to set up the experiment, very difficult at that time,

00:37:21 to, you might say, reveal this spatial quantization. Debye, when he was talking to Stern and Gerlach,

00:37:28 it's recorded that he laughed. He said, my dear friends, you shouldn't think that these orbits

00:37:36 are real. This treatment, the analogy the term uses is, this analysis is merely the railway

00:37:48 timetable of the electrons. It was a set of rules which he did not necessarily believe were

00:37:59 physically, you might say, real. Now, it's a very nice metaphysical point. Exactly how do we

00:38:07 interpret this experimentally observed spatial quantization of the electrons today? I could

00:38:15 certainly leave the question without attempting to answer it. He came back to Zurich, now to the

00:38:27 ETH, as the director of the physics division, and at the same time became the editor of the

00:38:37 Physicology of Zeitschrift. Now, one could give a half hour's, very interesting, I think, account

00:38:44 of the advantages of being the editor of a principal scientific journal, because I have now

00:38:50 dated a number of items, and in particular, I'm recording what is in volume 24 of the Physicology

00:38:59 of Zeitschrift, because there appears on six pages this paper with the title, X-ray Scattering

00:39:08 and Quantum Theory. Now, that is, without question, one of the most brilliant of all

00:39:13 Debye's papers. It's a masterly effort, so clear, spelling out all the basic aspects

00:39:23 of the quantum effect, of course. Now, he had, there is no doubt, according to a colleague, Professor

00:39:35 Stoiwa, who was at Minneapolis, Debye had actually written out a good deal of this two years earlier,

00:39:46 but had not published it until he saw the abstract of Compton's work, and then he published it.

00:39:56 Well, I didn't pursue this because the Compton effect was the first real proof which physicists

00:40:09 were able to accept of the, not so much the quantum quantization of energy, as of the photon

00:40:21 character of radiation. Up until that point, apart from a few of people with, you know, profound

00:40:31 insight, and this is very specifically Einstein and Debye, because they were in no doubt about this

00:40:38 as real, the rest of the physics world was still doubtful whether the energy was actually in these packets.

00:40:46 Now, in the same volume, there are the two treatments, firstly on the equilibrium properties,

00:40:57 and secondly on the conductivity of electrolytes. This is where the physical chemist usually, or used

00:41:05 to come across, Debye's name in the first place, but it remained, of course, a long interest of

00:41:13 Professor Debye's. There's even one paper which he wrote, interestingly enough, with Professor Pauling

00:41:20 when he was on a visit to California, and there are many other interesting and important ones

00:41:27 on the dispersion of the conductivity, written largely with Falkenhagen. However, we know all

00:41:33 those things from, let's say, general physical chemistry. Now again, on Zurich, this is the

00:41:40 second period in Zurich, and the two major, you might say, periods in Debye's career in Europe were

00:41:50 the ones in Utrecht, and this second period in Zurich. When you see Debye titling something a

00:41:58 note, please pay attention, because this is one of these extraordinary notes which one finds in the

00:42:08 literature. He is treating here the scattering and

00:42:17 general results of interaction of radiation, firstly, of course, with, you might say, monatomic

00:42:25 gas molecules and diatomic gas molecules. Now, that treatment was basic to a great deal of what

00:42:34 he subsequently did. He elaborated it, of course, in various directions, in particular in handling

00:42:40 it in liquids and so on, but that was not in a mathematical journal, really.

00:42:51 Then he moved to Leipzig, where he got into experimental work and did some studies of

00:42:57 diffraction of x-rays, and I've written down, out of interest, some of the very first results he got

00:43:05 for the cloning chlorine distance, for instance, in carbon tetrachloride gas, and there's the

00:43:11 modern value. At the same time, Mark was doing exactly the same thing by electron diffraction,

00:43:16 and as you could take the electron diffraction negative in a matter of seconds, whereas the x-ray

00:43:23 diffraction needed hours, they very quickly forgot and abandoned the x-ray diffraction

00:43:28 study, at least at that time, in device interest. Now,

00:43:41 yeah, still at Zurich, there's another note with this title, Remarks on Magnetization at Low

00:43:48 Temperature, and you know what this involves. It gives the complete quantitative analysis of the

00:43:56 cooling effect on removal of a magnetic field applied to a paramagnetic salt. Now, Langevin,

00:44:05 in 1905, had pointed out that oxygen, gas being paramagnetic, would cool slightly when the field

00:44:13 was switched off. He didn't attempt, of course, to evaluate it. What Debye did was to go through the

00:44:19 whole of this in terms of physical parameters. He couldn't estimate what it was because he didn't

00:44:29 have the necessary physical data. What he says is that there is reason to believe that the cooling

00:44:40 could be quite substantial. That is essentially where he left this interest until he actually

00:44:49 got to Berlin in his last years in Europe, and as you know, of course, Joke published his own

00:44:58 quite independent treatment of this in 1927, but Joke also went and tackled a very difficult

00:45:04 problem, as it then was, of achieving cooling by demagnetization, and he fortunately beat the Leiden

00:45:16 workers to it by a matter of just a few weeks in 1933, where he reported cooling down to

00:45:23 a quarter of a degree Kelvin.

00:45:29 Now, finally, Debye went to Berlin,

00:45:37 where the previous directors of what was the Kaiser Wilhelm Gesellschaft had been

00:45:41 Einstein and von Laue. The first thing Debye did was to change the name of the institute to the

00:45:47 Max Planck Institute. Planck, of course, was still alive, and there I've written in what were the main interests

00:45:59 he showed during this period, but much of it, I suspect, was taken up by the

00:46:08 building of the new laboratories and facilities at Berlin, for which you really had to thank the

00:46:15 Rockefeller Foundation for a very substantial grant, and this is very possibly one reason which

00:46:22 kept Debye in Berlin until even the outbreak of war. However,

00:46:33 yes, I could just tell you that, of course, having taken a professorship in Leipzig and subsequently

00:46:41 in Berlin, Debye, without really being too well aware of it, had really made himself into a German

00:46:48 citizen, because professors in Germany then, or much of the continent now, university professors

00:46:54 are civil servants, whether they like it or not, and it was accepted now that Debye was a German

00:47:02 citizen. When he realized this, he got quite concerned, and he made special application

00:47:12 to the Dutch parliament at The Hague to be reinstated as a Dutch citizen, and you might

00:47:19 say a small bill. This really had to pass through the Dutch parliament. The approval for a Dutchman

00:47:27 who had changed his nationality to re-become, if I may coin a verb,

00:47:34 very frequently done in the States, of course, re-become a Dutchman.

00:47:42 Even so, he was badgered, obviously, to some degree, and eventually was told he should no longer go

00:47:52 into the institute, because they had decided that that is where they would carry on their atomic,

00:47:57 shall I say, atomic energy project in Germany, and he wasn't to go in, and when he asked, well, what

00:48:05 can I do? I am the director. He was told in a letter from the minister of education that he

00:48:09 should stay home and write a book, and now that, I think, was, you might say, the trigger which moved

00:48:17 Debye out of Europe. You couldn't imagine him staying at home writing a book. What he did, of

00:48:24 course, he arranged to give a lecture. I think it was in Switzerland. He, of course, came out of

00:48:31 Germany and just did not return, and to the best of my knowledge, he did lose all his papers and

00:48:40 much else, of course, when he left Berlin in 1940.

00:48:47 You can see that I've come to the end, certainly, of my time and to some degree of my story.

00:48:57 Fortunately, there's no need for me to try to attempt a characterization of Debye's abilities.

00:49:08 I mean, it's a question of by their fruits shall ye know them. Just please read his papers.

00:49:21 What I will remind you of, which I think we were told earlier, Sommerfeld said that Debye,

00:49:32 both his career and his character, could be so well summarized in the phrase which he was

00:49:42 constantly using in the colloquium, aber das ist ja so einfach. It is really so simple.

00:49:52 I can speak as a student of many years standing, but it is clearly a function of the clarity of

00:50:05 the grasp which a lecturer has, whether you really are able to follow. I have listened to

00:50:13 Debye's lectures and felt, well, I understand all that. I mean, it's so straightforward, so

00:50:19 nothing to worry you at all. Then you go away. By the next day, you're wondering.

00:50:28 Now, let me finally offer you the next slide, please.

00:50:43 Well,

00:50:54 two quotations which you will immediately accept, I'm sure, as quite characteristic

00:51:03 of Debye, and also another quotation, a remark you might say made about it. There is

00:51:12 another slide, please.

00:51:26 It was just the very last.

00:51:30 Taken them out. Fatal.

00:51:33 Well, perhaps I could read what is on the slide. Is it coming? No, okay. Let me see if I've got it.

00:51:54 No, I'm sorry. I've got the manuscript here. Oh, maybe, maybe. Yes, yes, here we are.

00:52:00 Please, just a moment. Confusion now. It's at this end.

00:52:19 The two quotations, you might say, from Debye. Firstly, such a distinction, that is,

00:52:27 between chemistry and physics, is a mere play upon words and shows ignorance rather than

00:52:36 understanding. Second, indeed, it is the great beauty of our science that advancement in it,

00:52:45 whether in a degree great or small, instead of exhausting the subjects of research,

00:52:52 opens the door to further and more abundant knowledge. You would accept those as coming

00:52:59 from Debye, and certainly the comment, he smells the truth. Now, all three belong to Michael Faraday,

00:53:14 and I can think of no better way of finishing this account than to suggest to you that,

00:53:21 in some respects, I won't attempt to qualify it or wrap it up in any way, but you can, if you wish,

00:53:29 surely think of Debye as the 20th century counterpart of 19th century Michael Faraday.

00:53:41 Now, I'm not sure whether one could say less about Debye, but one certainly couldn't say more.

00:53:50 Thank you.