Karl Herzfeld

Herzfeld:

I better first give you my timetable. I started at the University of Vienna already in the fall of 1910, stayed there until the spring of 1912. I spent the year ‘12-‘13 in Zurich, both at the University and at the Institute of Technology, the Technische Hochschule.... In the fall of ‘13 I went for the first semester to Gottingen, and then in February ‘14 I returned to Vienna to finish my degree... The degree was done with Hasenohrl.

Kuhn:

I’ve been uncertain which part of your work the degree represented. Was it the electron theory?

Herzfeld:

Yes, it was the electron theory had been published before, but it was possible to use a published paper as a dissertation. From July ’14 to November ‘18 I was in the army. The year from November ‘18 to February ‘20 I spent in Vienna. This was a politically very uncertain time and I wasn’t quite sure of my future career. I went to Munich in February. I spent three months in Munich early in ‘19. This came about so, as I say, in ‘18 it was unclear that an academic career would be possible. There was at that time no possibility for a physicist to get an industrial job. So I thought I might get an industrial job as a chemist, but it was then thought that I needed more analytical chemistry. And so I spent the time from January to March, ‘19, in Munich learning quantitative analysis. This was a very important time for my future, because it was at that time that Fajans and Sommerfeld got to know me personally. So as a result of that then, I got an offer in February 1920 to come to Munich and become Privatdozent there. Among other things, the reason I accepted that was that I also got an offer as an assistant in the laboratory of physical chemistry. You know in Germany at that time you were, not paid for being a Privatdozent, but were paid for being an assistant.

Kuhn:

Had you been a Privatdozent at Vienna at all?

Herzfeld:

In the fall of 1919, or in the spring of 1919, I had started the moves going, and they were coming to fruition when I got that offer from Munich. So I never really taught in Vienna.

Kuhn:

.1 asked that only because I know one of the biographies, either in Poggendorff or American Men of Science indicates that you were Privatdozent in Vienna.

Herzfeld:

The formalities, which were a long affair, had come near their end, and I probably would have.—if I hadn’t gotten that offer from Munich—I would have started teaching in Vienna. But I actually never taught in Vienna. I received— in January ‘26 I think it was— that offer from Johns Hopkins.... I was at Munich then from February 1920 to February 1926.

Kuhn:

If you’d be willing to do so, I’d simply now like to drop back to the beginning at Vienna and have you tell me about it. I’m interested in anything you could tell us, but particularly the educational situation and the relations between physics, theoretical physics, mathematics, and chemistry....

Herzfeld:

So far, this is easy to tell. Both in Germany and surely at that tine in Austria, the majority of the people who studied science were going to be high school teachers. In both these countries the high school teaching certificate was, as a matter of fact, stiffer than the Ph.D. This answers the question, the curriculum for most of these people was designed to prepare them as high school teachers and this automatically did not allow them to become experimental or theoretical physicists separately. In this country as you know it was the custom that each full professor had one institute. Everywhere in Germany and in Austria that I know—in Vienna, for example, the theoretical institute was in the physics building. Chemistry was separate. However, for example in Vienna there was something called the Physical Chemical Society, which corresponds to—I was going to say to the Philosophical Society in Washington. That is to say, there was a monthly colloquium in which both chemist and the physicists attended, so the personal relationships were relatively good. And it meant that in practice all the people who studied physics had at least some chemistry.

Kuhn:

Did this group then also include the other sciences, the biological sciences also?

Herzfeld:

No.

Kuhn:

Would it include astronomy and mathematics?

Herzfeld:

It might include astronomy and mathematics. Yes. I never took courses in astronomy. Of course I took courses in mathematics. In chemistry I just took general chemistry and analytical chemistry.

Kuhn:

You were at this point definitely intending to be a physicist? There was no thought of chemistry?

Herzfeld:

I definitely intended to be a physicist, yes.

Kuhn:

How far did mathematics then go? How much mathematics would you take as a matter of course in general?

Herzfeld:

At that time it was so that there were three full professors of mathematics, each of which taught a three-year course starting from calculus. You see there were then ... interlocking. In one year Professor Escherich started the beginning course and. carried it out. In the second year Professor Wirtinger started the beginning course. Now Escherich was a Weierstrass man; Wirtinger was a Riemann man, and so you got up to function theory, one as seen from the Weierstrass point and the other from the Riemann point. The third man was a number theorist, and him I never took. So I got up to what is now called function theory and spherical harmonics and things like that. Of course I did not at that time get matrices, which were thought to be unnecessary for a physicist. The theoretical physics was arranged in a four-year course, which was taught by Hasenohrl in a four-year course; one year mechanics, one year electricity and magnetism, one year optics, and thermodynamics and statistics…

Kuhn:

So you could start in, depending on when you arrived, you could start in on any one of these?

Herzfeld:

There was a single course introduction to theoretical physics corresponding to one which is taught in this country very often. So I took that first, and then I went on and took the other three.

Kuhn:

Who else was lecturing in physics then?

Herzfeld:

There were two experimental institutes at that time. One was under—when I started out— a man named (Lang), who was originally in crystal optics. And when he retired it was taken over by Lecher, you know the man of the Lecher wires. The second institute was Exner. He was interested, at that time, mainly in spectroscopy. Exner and Haschek:—Haschek was an associate professor— published the most complete tables of spectral lines. Then there were a number of younger men. Phillip Frank was a Privatdozent at that time. Hess was a Privatdozent at that time. Then a man named Przibmam, who has worked on (coloration) of minerals by radioactive rays. Kohlrausch. Then there was independent of that the radium institute of which Stefan Meyer was head. Among the other assistants was Schweidler, who was a radioactive man. In the group of students— yes, there were a number of people who were just finishing their degree.

Schrodinger had just finished his degree when I entered and was doing his military year. And the year after that he was an assistant.... I knew him quite well. I knew him when I entered the University. Then there was Thirring, who is the father of Walter Thirring, and then became professor of physics. Then there was a man named (Kottler), I don’t know whether you are familiar with him.... But he came to this country to an optical company after ‘38 and worked mainly on the refinement of Kirchhoff’s principle in diffraction. He was more of a mathematical physicist. Then, there was a man named (Wolf), who became professor of mechanical engineering at the college in Vienna, and a man named Flamm…

The people I am talking about now were students with me. One who had not yet become Privatdozent, but had his Doctor’s degree, was Arthur Erich Haas. Now as far at the interests were concerned, they were rather varied, but at that time really only Arthur Erich Haas was interested in the structure of the atoms. He tried to calculate equilibrium positions of electrons in the Thomson atom, and did a very good bit of work there. About this paper of Hasenohrl’s— Hasenohrl had been to the 1911 Solvay Congress at Brussels, and I think this paper was the result of the discussion in Brussels.

Kuhn:

But do you think that this was really a departure from his interests, that he had not himself been much interested in the subject of the quantum at all before the Solvay?

Herzfeld:

He had been very much interested in the problem of black-body radiation, but not in the atomic structure part.

Kuhn:

I wanted really—I hope this won’t derail us later, I’ve been very much puzzled to know, or to try to understand, why the research in radioactivity was often as local as it was. That is, Cambridge is a great center, and Vienna is a second great center, and Paris is a third great center. There is almost none of it to be found in Germany, or at least very, very minor work on it.

Herzfeld:

Very little in Germany, yes.... Well, in Vienna it was part often due to the fact, I am sure, that the source of radium was in Austrian possession. I mean, of course, the (Reichenstaal) was at that time Austria. And therefore it was easy, in a sense, to get relatively large amounts. Otherwise I am not quite sure. Then in 1912 there was a rich industrialist who gave a large amount of money to build an institute for radium.

Kuhn:

Later on, when you were at other places, were you struck by the lack of concern with radioactivity.

Herzfeld:

No, I wasn’t at that time. I think it is really so, partly, that, compared with what we are accustomed to here, the number of staff members in any particular field was relatively small. I mean there were a few large places. But let me say, a university like Wurzburg had two physicists. What is perhaps of interest here—to come back— is that the interest, of course, in statistics goes back to Boltzmann. And Schweidler was the first one to count the individual gamma processes by measuring fluctuations. The idea that you could count. Of course there was no Geiger counter, so what he did is measure fluctuations of ionization chambers. This was at that time. Thirring was also concerned—this is of course partly quantum theory— with calculating the specific heat of diamond, actually solving the vibration in the diamond lattice.... That must have been 1912 or something like that.... He must have used the Bohr-Karman theory, but it had not been possible at this time to actually solve the equations exactly, and Thirring developed a method— high temperature— where you could actually make the calculations without the complete solution. Otherwise I think the interest was rather more captured by relativity than by atomic theory.

Kuhn:

How aware were people at Vienna of the difficulties presented by black-body radiation? To what extent did they see really fundamental problems existing in physics and atomic theory?

Herzfeld:

My memory is very bad in that respect. I cannot really remember, except that Ehrenfest visited in 1910 or ’11 and he gave two talks. One was on the relativity theory, on the comparison between emission theory, that is the Ritz theory and relativity theory. But the other was in the blackbody field, in which he showed that you cannot avoid a discontinuity. That was a mathematical investigation which went backwards from a black-body radiation formula to the statistics. This is published.... I remember this talk, and this interesting discussion, but I cannot remember otherwise. Now at that time we had discussion about the problem— but this is of course again not quantum theory—whether isotopes ought to be counted as separate elements or not. That is to say, if different isotope classes diffuse, is there an entropy change, or is there not an entropy change.

Kuhn:

Was there a colloquium, a physics colloquium, at the University, in addition to the monthly meeting of the “Philosophical Society?”

Herzfeld:

I don’t think so. I can’t remember.... Yes, there must have been in the fall of ‘15, before I went to Gottingen, (Paneth) and Hevesy had just gotten back from Manchester....

Kuhn:

Can I ask you a few more questions about Vienna first? In this period, to what extent there was still doubt about the atom, particularly, what influence if any Mach’ s presence there may have had?

Herzfeld:

Not in physics, but the professor of physical chemistry was a man named Wegscheider. He was perhaps not under the influence of Mach but under the influence of Ostwald very strongly.... I don’t know whether he spoke against belief in the atom. But among the physicists there was no doubt. Except there used to be in (Brunn) a man named Jaumann, and he believed in a continuum theory.

Kuhn:

But this would be so rare as to be quite striking?

Herzfeld:

Yes.

Kuhn:

What about aether theory? Had Einstein’s relativity taken sufficiently strong hold so that physicists, at least in this group, were no longer worrying about the aether?

Herzfeld:

Yes. Hasenohrl once thought this must have been one or two years later — that one can always introduce a coordinate system which will not be inertial, which would produce arbitrary fictitious forces.

Kuhn:

I take it that you had a good strong training in basic thermodynamics and statistical mechanics?

Herzfeld:

Yes.... This I think must have been Boltzmann’ remaining influence.

Kuhn:

Did Hasenohrl teach that course?

Herzfeld:

Yes, all the last theoretical courses I had from Hasenohrl.

Kuhn:

Did he give both a phenomenological and a statistical approach, or was it statistical for him from the beginning?

Herzfeld:

My memory is not good enough about that, but my guess would be that it would have been first phenomenological. However, I can’t say that I really understood it. In the fall of ‘12 I went to Zurich. And Zurich was a very ideal place at that time because Laue had just come to the University and Einstein was at the Polytechnic. The experimenta1 physicist at the University was an old Swiss gentleman named Kleiner, who did not do any research. At the Polytechnic however there was Pierre Weiss, and he was surrounded mainly by Frenchmen, French-speaking people, and everything was magnetism.... Of course in Laue’ s group the interest was completely X-ray diffraction. And what we worried most about at that time is the relationship between the Laue theory and the Bragg theory.

Kuhn:

You had heard about X-ray diffraction already in Vienna? You see, the word of that comes out only fairly late in 1912, in the late summer or early fall. Would you have known much about von Laue before that announcement?

Herzfeld:

Oh yes. He had published a lot of papers on thermodynamics of radiation.

Kuhn:

And it was perhaps largely your interest in that that had taken you to him in the first place?

Herzfeld:

I can’ t remember. I can’t remember.

Kuhn:

Do you remember anything about the reception of the X-ray diffraction work in Vienna?

Herzfeld:

No, I can’t remember there having been a discussion. And then there was Einstein, as I say. And he was at that time very interested of course in quantum theory. He was mainly starting on the general theory of relativity. For me a strong influence was the fact that Otto Stern had come at the same time to Zurich for the purpose of becoming Privatdozent there. And from him is when I really understood thermodynamics—from the conversations with him.... I saw a great deal of him. He was interested in investigating the effect of the relationship between quantum theory and the kinetic theory of dissociation. So here quantum theory came in again strongly.

Kuhn:

Did people condemn themselves in Vienna with the Nernst theorem, the heat theorem? Or did you pick that up from him in Zurich?

Herzfeld:

Well I don’t think Wegscheider would, but I never took courses from him. How far it came into the thermodynamics lecture of Hasenohrl I can’t remember. No, I would say probably not much. I can’t remember much.

Kuhn:

You were still in Vienna when you did this paper on the atom, that builds on the work of Hasenohrl’s. Now you use the Thomson atom basically there and try to quantize it in accordance with the Hasenohrl theory. How standard itself was the Thomson atom? Was this the one model that anybody worried about?

Herzfeld:

This was the one model, yes.

Kuhn:

Had anybody heard of the Rutherford atom up to the time that you hear of the Bohr atom?

Herzfeld:

I don’t remember any mention of this.

Kuhn:

You would think that whole planetary atom first comes to you when you hear of the Bohr atom?

Herzfeld:

I am so sorry, I do not remember. I might have heard about it a year before.

Kuhn:

But the Thomson atom is pretty much the dominant model?

Herzfeld:

Yes.

Kuhn:

Is it thought of as terribly hypothetical, or did people think that probably the atom is like that?

Herzfeld:

No, I think, yes it is, insofar as you talk classical physics. You have the following situation: you have electrons, and you have a positive charge about which you don’t know anything. But then, you arc impressed by the fact that the reaction of the electrons to light is linear. That is to say, the refractive index and the absorption coefficient are independent of the intensity of light. And therefore your problem is what kind of charge distribution can you make so that in the atoms the electrons are subject to a Hooke’s law force.

Kuhn:

This works out very well. How did you feel about the results of that paper? You remember, that for the last Balmer lines you need something like sixty electronic charges. Does this seem to be on the right track?

Herzfeld:

I couldn’t remember anymore. After all, I was a young student and was glad to have [laughter] and was probably not very critical at that time.

Kuhn:

Do you remember any reactions to it? Was there much interest in doing this sort of thing? Did people talk to you about it?

Herzfeld:

Well, you see this was published in the Vienna Academy, which we informally called the Tomb of Honor, and not many people read it, not too many people read it.

Kuhn:

But you’ re not aware of other people at this same time who were trying to do the same sort of thing?

Herzfeld:

No, oh no. You see, Vienna was not like Munich or let me say Washington now. There you have constant traffic. Vienna is somewhat out of the beaten track. Now Hasenohrl always had his private correspondence, but this did not penetrate too deeply. The radium institute was somewhat more in touch with other work. And one of the most impressive changes for me was that Zurich was a really international place.

Kuhn:

Do you think it’s the lack of international contact that makes Vienna as little productive in the development of quantum theory as it then proves to be? It never really becomes mach of a center for it, does it? Or is that the effect of the war?

Herzfeld:

No, no. This is much more a question of personalities. Then Thirring became professor, his lectures were arranged so that they were very good in classical physics but practically no quantum theory. I mean, there are a number of his students in this country here, who had to leave. And they were very well trained in classical physics. Now this is going to change. Walter Thirring will not talk classical physics at all. But this is a question of personalities.

Kuhn:

What would you normally have read for your physics in particular?

Herzfeld:

Abraham and (Toeplitz)—wasn’t it sometime? In mechanics I think I used Helmholtz’ lectures. This is all I can remember.... How in Zurich, as I say, Einstein was himself at that time mainly interested in the first development of the general theory. That of course he had such a broad interest to listen to everybody’s interest. That in Zurich I am sure that no mention of Bohr was made. The first tine I heard about Bohr was when I cane back after the Zurich time and was in Vienna before going to Gottingen. There I met again Paneth and Hevesy who had been in Manchester and were full of Bohr.

Kuhn:

How did other people in Vienna feel about it?

Herzfeld:

I didn’t talk about it. This was before I left for Gottingen.

Kuhn:

You were, I take it then, very much impressed with the idea?

Herzfeld:

Yes.... Let me see. A few more things about Zurich. As I mentioned, we had a number of visitors. There was a Swede I think named Nordstrom; and then Ehrenfest came for a month. And then there was a Japanese named (Ishiwara) I think, but they were all interested in theory of relativity.... I was formally at the University but went to lectures at the Technical Hochschule. I mean, I took Laue’s lectures at the University. Zermelo was then there, and. I took from him a seminar on point set theory. And then I had Einstein’ s lectures at the Technical Hochschule. And then there was a mathematician named Hurwitz from whom I took a lecture at the Technical Hochschule. And then I took a laboratory in physical chemistry from a man named Bauer at the Technical Hochschule. And the colloquium was in common.... There were three of us. Myself, Stern, and a man named Kern, who was an Austrian— a somewhat older man who was working on an experimental dissertation in Kleiner’s laboratory. We three were all close.

Kuhn:

And it was really this, I take it, then, that started your interest in the dissociation theory. Was this what got you into the, electron theory?

Herzfeld:

No. The electron theory of metals—this did not.

Kuhn:

When did that interest really start?

Herzfeld:

In electron theory of metals? I couldn’t tell. I’m sure that what really interested me at that time was the temperature dependence of the electrical phenomena, and I just tried to see whether one could unify that. So the interest there was really an attempt at explanation experimentally.

Kuhn:

What particularly attracted you to Gottingen?

Herzfeld:

Well, actually I was disappointed in Gottingen. You see, the year before there had been a group in Gottingen which consisted of Born, Karman, Ewald; I had been interested in crystal physics. Now this group when I got to Gottingen was dissolved. Born was still there but he was no longer interested in that—mainly again in theory of relativity. And Ewald and Karman had left. Hilbert lectured on radiation, but his interest wasn’t really one which was very congenial to me. And so what I profited from in Gottingen were the following things: I had a very good course in function theory from Caratheodory; I had—to me—a very interesting seminar from Voigt and Runge on (grating) theory; and then I worked in the laboratory of Professor Simon, that is not Franz Simon, but that was a laboratory for applied physics. And I had an idea which I know now couldn’t work…I thought that you might get fluctuations in the number of electrons in a metal and therefore get fluctuations of conductivity. And therefore I took a thin sputtered film, put a D.C. voltage on it, and tried to see whether there is an A.C. fluctuation current. At that time all we had were crystal rectifiers, and from that we know now, that couldn’t work. But this was an interesting laboratory; Oldenberg was just taking his Doctor’s dissertation, and Grotrian was the assistant. And there was a nice social life there. But again, it wasn’t quantum theory.

Kuhn:

You 1a’otc, in that letter to Sam Goudsmit which he sent on to me, about the colloquium at which there was the discussion of the Bohr atom, and said in particular that Runge had been extraordinarily skeptical. That can’t have been the last discussion of the Bohr atom.

Herzfeld:

I think during that half year it was the last discussion of the Bohr atom.

Kuhn:

So that people did not necessarily take it so seriously that they went on talking about it? It remained you think probably just a crazy idea for most people there?

Herzfeld:

Yes. I at that time tried to—as I mentioned tried to see what the Zeeman effect to the Bohr atom was. But then Bohr had done it about the same time.... I’m fairly sure that Bohr wrote me a letter— and this unfortunately I don’t have— saying that he had also done it—when I sent him a copy. My paper hasn’t been cited anywhere, so this is—…Well you see the situation we are now talking abut—let me say— is early in l9l4, and within five months I was in the army.

Kuhn:

Ja. That of course does make a difference. Were other people around Gottingen interested in the attempt to do this sort of—

Herzfeld:

As far as I know, no.... There was for example Hilbert, who was very much interested in the mathematics of the story. And what worried people like Hilbert was the fact that the distribution of eigen values which the Balmer series shows is completely unmechanical; that the mechanics lets you expect an asymptotic distribution proportional to the square of the frequency.... How Hilbert was interested in the very problem, whether it was possible to build a mechanical model by all kinds of devices, which would have a distribution of eigen values like the Balmer series. There was a dissertation done under him which showed that this was possible provided you permitted action at a distance... I don’t remember who did it. It was done at the time I was there... But I remember that there was in Gottingen a so-called Mathematical Society which was a seminar. I remember this was presented at the time I was there in the Mathematical Society.

Kuhn:

Did you hear Hilbert himself talk about this problem?

Herzfeld:

Only in commenting on that. I don’t remember him lecturing.

Kuhn:

The quantum conditions for your treatment of the Zeeman effect of the Bohr atom I find very difficult to follow. Because it certainly looks as though the statement that’ s being made is that the change of energy in ialling from outside to presumably the ground state is n quanta, where hv is the quantum. A little further on one then proceeds to talk about transitions from one state to another, and the n suddenly becomes a subscript..

Herzfeld:

No, I am sorry. I understand what you say, but I don’t remember it. I was myself interested at that time mainly in other questions. For example, I was interested in the question of the absolute value of the entropy. I was interested in the question of how much the average and the most probable valves can be identified.... You see, generally, at that time very few people worried about statistical mechanics. This is perhaps a point which I may add to the story, which might not be in the later work. I am going now much farther back. Perhaps I should have said this before. When the Planck paper came out, I think the majority of people did not trust statistical mechanics and therefore thought it was really due not to a real atomic phenomenon, but due to a misuse, untrustworthiness of statistical mechanics. Very few people had any confidence in statistical mechanics, and in that aspect perhaps Vienna was in front so far as people in Vienna did trust statistical mechanics. Here or course the Einstein photo-electric paper had a very great influence because it showed the application of h to the individual atom. It was a case where no statistics was involved.

Kuhn:

Clearly in retrospect it should have done that. Yet in certain places at the time, it was not taken very seriously. I wonder, in Vienna during your student days, was that an important paper, the photo-electric paper?

Herzfeld:

I couldn’t tell....

Kuhn:

Did you yourself as a student read the first paper of Planck? Or would you have read the subsequent books? The Warmestrahlung.

Herzfeld:

I don’ t—One would read the Warmestrahlung, yes.

Kuhn:

Let me ask you one thing that occurs to me. In Planck’s own derivations of the black-body formula, he has a standard way of introducing the probability which I would say now makes no sense. Now I raise this with you at this point because I wondered whether there was any chance that it was worrying about this, about the nature of this Planck derivation, which might have had anything to do with bringing you to your problem about the substitution of total number for the number of most probable distributions.

Herzfeld:

No, I don’t think so. Going back to the influence of Stern and my interest in the electrons in metals. I gave a paper in the fall of 1913 in which I tried to develop the formula for the electron emission in metals. Implicitly this paper contained what is now known as the (Saha) formula. But I didn’t see that myself.

Kuhn:

At this point you go back to Vienna.

Herzfeld:

Ja, I know, but I don’t think I have any scientific recollections of this semester. And in July 1914 I went in the army, but I had the luck that I could do some work even during the war, being a theoretical physicist. [faint] I was an observer in the mountains; sitting in a hut. You can go out six or eight tines a day for twenty minutes. And either you play cards or you are luckier and write a paper.

Kuhn:

I take it your own work during the war was largely a continuation of work that you had started before?

Herzfeld:

No. I mean in a sense, of course. The papers I wrote during the war were first of all, as I wrote in my letter, the first systematic attempt at calculating the dissociation, the ionization of an atom. But at that time I did not properly understand the meaning of h in phase space.... For the ground state I did not automatically assume that the appropriate value was h. But I calculated that by choosing the limits improperly and got a quantity which was not exactly h but 2/3 of h or something like that. This was corrected in another paper. Then there was a paper on reaction rates, which in a sense of course goes back to Zurich. Partly of course my physical chemistry interest goes to the radium institute. I don’t really know why I got so interested in the reaction rates at that time. Then there was a paper in which I attempted to calculate the entropy of solutions.

Kuhn:

That again had been an interest, or related to an interest, of Stern’s.

Herzfeld:

Yes. [Discussion of the relation of the Nernst theorem to quantum theory here omitted.) My next interest went over again, deepening my understanding of statistics. I spent the semester in Munich; and then a year later got that offer to Munich.

Kuhn:

Now when you were at Munich immediately after the war, you were really there as a student?

Herzfeld:

I was a student, yes. I did not attend lectures.... I just learned analytical chemistry and quantitative analysis. I worked in Hoenigschmidt’s laboratory. This was the spring of 1919.... And then younger people in the labs of Sommerfeld, Lenz, and Ewald were both there. I went to physics colloquium and had contact with Sommerfeld.

Kuhn:

Did you sense a real difference in the position of the quantum in physics, as compared with the period immediately before the war?

Herzfeld:

I think very strongly—Well of course Sommerfeld had written on the quantization. In the years after then there was Kossel in Munich. And Kossel had written a big paper which has not been read much in the English-speaking world, which was a Bohr analogue to the Langmuir paper in chemistry. That is to say, he had, for the German-speaking world for the first time, punted out the details of, for example, sodium chloride formation; that sodium and chlorine take on rare gas structure. And he had written a big paper on chemistry from that standpoint.... He was at that time mainly working in Sommerfeld’s laboratory, but afterwards got a position in the physics department at the Institute of Technology, at the Technische Hochschule.

Kuhn:

But he rated pretty much throughout as a physicist?

Herzfeld:

Oh exclusively, yes. And there was a very funny story. Kossel’s father was a famous physiologist, editor of the journal of physiology. When Kossel later on had a call to Tubingen, somebody who supported him brought up the fact that he was so broad that he even published in physiology.

Kuhn:

When you yourself came back to Munich a year later, your assistantship is in chemistry.

Herzfeld:

Yes. But I was teaching both physics and chemistry.

Kuhn:

Was that usual, or were the chemists in the Institute for the most part more separated from the physicists?

Herzfeld:

Well, Fajans used to come to the physics colloquia.

Kuhn:

But otherwise the chemists were pretty much separate....

Herzfeld:

Physical chemistry was at that tine a division of the chemical institute.

Kuhn:

But it could also be done in physics? Kossel was doing physical chemistry in physics also, wasn’t he?

Herzfeld:

No, less than now here....

Kuhn:

Did you yourself get involved with the spectroscopy work at all? Because that tended to be a full-time occupation.

Herzfeld:

That tended to be a full-time occupation. The only thing in this respect was that I did an attempt to give a more physical interpretation of the Kerr and Faraday effects in terms of Zeeman effect and Stark effect.... Then, only at the end of the period after the Pauli paper on statistics had come out, did Sommerfeld intensely become interested in electron theory.

Kuhn:

You had meanwhile dropped your early interest in the electron theory entirely, hadn’t you?

Herzfeld:

Well, as I said before, when I went into it I was mainly interested in trying to explain temperature dependences. I didn’t think I could do much more in it. At that time I was mainly interested in reaction rates.

Kuhn:

I’d be terribly interested in any recollections you may have of those years now at Munich.

Herzfeld:

Well, this is a little out of place, but Sommerfeld had been in this country in ‘23. And when he came back then he reported on the Compton effect at that time. He brought back the proof that light was particles and proof that light was waves. Now the question of particles, that was the Compton effect. What was the proof that light was waves?...I think that at that time Compton had shown the diffraction of X-rays by artificial gratings, grating incidence. These were the two things.

Kuhn:

Was it your impression that you had not known of the Compton effect until Sommerfeld came back?

Herzfeld:

Yes.... Of course at that time it was impossible in Germany to buy American journals but somebody sent some of the American journals. But it is quite possible that we hadn’t looked at them, if they came delayed.

Kuhn:

Debye says that Sommerfeld used to say in the very beginning what he thought he wanted to do was to take formulas that somebody else had supplied, take the theory, and apply it, solve it. And to an amazing extent, this is of course what he does in quantum theory also. And I wondered to what extent this affects the tone at Munich of the seminars and of the colloquia?

Herzfeld:

Now what you say about Sommerfeld is I think quite true. You know his early career. He came from mathematics. But you see it in is books, where he declines to go into discussions of fundamental things.

Kuhn:

Do you have any feeling that this affects the subjects of the colloquia and the subjects that students are given to work on for their theses?

Herzfeld:

I hadn’t thought about it, but it is probably true.

Kuhn:

Do you remember any discussions of De Broglie’s work?

Herzfeld:

No.... Politics might have been in the background and have made people prefer Americans compared to French. This was after all the year after the war. At that time the French were still the enemy. I don’t know how much that influenced.

Kuhn:

The whole question of the status of the photon and at what point people were prepared to take it seriously. After the Compton effect it is very hard not to. But Einstein has been saying this for a long time. Do you remember when you became yourself conscious of the possible need to treat light as particles?

Herzfeld:

Well, I’m fairly sure that during the Munich period everybody was. Again talking about myself, Wentzel and I really had been worrying about the following problem: If you deal with light as particles and consider interference for example, if you have light emitted from a star, and after that quantum has been emitted you set up a spectrograph, how can you formulate a theory such that the equipment which has not yet been set up when the particle was emitted, determines the path of the light? So we had been worrying about that. I know that we wrote together a paper on dispersion. That must have been ‘24. . At Munich, with the experimental people, the following things were going on. When I came, the head of the department, the experimental physicist, was Roentgen. And when he died, Wien came. Now at that time there was Wagner in that laboratory, and he was mainly interested in proving and measuring the cut-off of the continuous X-ray spectrum. Then, they were quite interested in magnetism of gases. This is perhaps I don’t know, this has nothing to do with quantum theory—but there is again an amusing story. They thought at the time that they had discovered a new kind of magnetism, which they called meta-magnetism. And this was a process—now let me think a minute—according to which a gas would be dia-magnetic at low temperatures and para-magnetic at high temperatures. It is rather amusing what happened.

They measured the magnetic properties of the gases by having a little quartz rod deflected, and this reacted, as if the gas had been dia-magnetic at low temperatures and para-magnetic at high temperatures. Now what you measure is of course the difference between the magnetic properties of the surrounding gas and the sample. And it turned out that when you mounted your quartz rod by using a steel pincher, there remained enough steel on the quartz rod to falsify the matter. And of course the magnetism of that steel decreased at high temperature. This has nothing to do with quantum theory, but—

Kuhn:

No, though the concern with the magnetism of gases has a good deal to do with quantum investigations by this time. How did people feel when the new quantum theory papers came out? The Heisenberg paper, the Born-Jordan paper.

Herzfeld:

I can’t say that. I cannot remember... Of course Heisenberg was a Munich boy and if for no other reason—

Kuhn:

How about the Schrodinger equation? The 1926 papers of Schodinger.

Herzfeld:

I’m very sorry but I can’t remember details.

Kuhn:

You knew of course Schrodinger for a long time before this as well as then, and you had known Pauli at Munich. Are there things you remember about them? Of course one wishes so much that one could still talk to them on a project of this sort....

Herzfeld:

I met Schrodinger for the first time when I started at the University. He had just taken his degree and was doing his military year. He was at that time in uniform but was in Vienna only for a short time. He served elsewhere. And then I knew him when he came back.

Kuhn:

At that early stage in his career what sort of a reputation as a promising physicist did he have? Was he thought to be something quite special?

Herzfeld:

Ja, I think so. I mean, this was obviously a group of really promising people. In a sense Thirring did not work out as well as Schrodinger. Hasenohrl had, I am sure, a high opinion of him.

Kuhn:

Did you continue to see very much of him after your first meeting in Vienna?

Herzfeld:

Well, during the three years I did—after he came back from his services. Well, of course I was away, for a year and a half of that time. We corresponded during the war. After the war I think he was back in Vienna, but he soon, I think, he went to Zurich.... Then I rather lost touch with him.

Kuhn:

Pauli was at Munich until about 1922, wasn’t he?

Herzfeld:

Sommerfeld used to send his best people for a year to Gottingen. He had a great admiration for Hilbert and wanted them to be a year associated with Hilbert.... Pauli came as a student just about the same time when I came as a teacher. Or perhaps he had been there half a year earlier. Heisenberg’s father was professor of Byzantine Greek at the University... And I once walked home with old Professor Heisenberg. We lived in the sane direction. And he was talking about his son and was saying how in philology one hesitates for years before either publishing a theory or attacking somebody else’s theory; while he said, “My son says that he should write at the end of each of his papers ‘Six months guaranteed’.”

Kuhn:

Also, just before you left Heitler was your student.

Herzfeld:

Heitler was my own student, yes. His dissertation was on the theory of solutions, I think....

Kuhn:

Was he clearly again as a student a person of very special promise?

Herzfeld:

Well very intelligent. I don’t think that he showed his later originality.

Kuhn:

He was a physicist in these terms?

Herzfeld:

He was a physicist....

Kuhn:

When you decided at the end to come to the United States, was this because of the job situation?

Herzfeld:

No, not at all, not at all. The history of my coming to the United States was simply this. A New York banker named James (Speier) had given Hopkins $5O,OOO to invite a German professor every year for two months. And I got the first appointment. I came to this country only as a visiting professor. However, when that appointment was over, they asked me to stay; which of course I did. The man who had been teaching theoretical physics at Hopkins was Ames, and at that time he was made what was called at Hopkins— I forget it now— in effect Vice-President. And so they asked me to take his professorship of theoretical physics. I assume that Laporte had a lot to do with my being selected.

Kuhn:

How did you find American physics, as compared with German physics, when you arrived here? You were one of the relatively early people to come.

Herzfeld:

Yes. Well, of course Hopkins was an excellent place. There was not much in this country at that time; there was not much theoretical physics. Of course Kemble at Harvard and (Holliman). And there was very little statistical mechanics. Hopkins, of course, was at that time— how shall I put it—one of the most European universities; and so I felt very much at home. What surprised me most was really the complete lack of political interest among the students. I came from Munich in the early Hitler days.... And, I came from Vienna, where the University used to be closed two or three times a year because the students were fighting. Police, of course, from the old privileges, couldn’t enter the universities. The students (sent for the ram), and the police pushed on (the ram) until (the ram) fell down. The Yugoslavs were fighting the Germans, and the Catholics were fighting the liberals. The one thing I perhaps did not mention was Fajans’ interest. He was at that time to a large extent interested in chemical effects on color in inorganic substances. Systematic studies of the refractive index, effect of color change on absorption, and things like that. And he interpreted that in a—shall I say—primitive Bohr manner— the sense of tightening or loosening of bonds.

Kuhn:

I realize there is one thing I had meant to ask from this same period, and this is the dispersion problem.... That is, in terms of your own work and of your own feeling about it and Wentzel’s feeling about it.

Herzfeld:

As I say, what did worry us at the time, Wentzel and me, were the questions of the dispersion theory as an example of what happens when you put in an instrument late.

Kuhn:

And this is the photon versus the wave nature?

Herzfeld:

Yes.

Kuhn:

Bather than the special problem of how to do it with the Bohr atom?

Herzfeld:

Well, partly. But of course, the idea in the paper of the dispersion theory was that you could do it with the photon if you use the—