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Oral History Transcript — Dr. Joel Henry Hildebrand

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Interview with Dr. Joel Henry Hildebrand
By Thomas S. Kuhn and John L. Heilbron
In Berkeley, CA,
August 6, 1962

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Joel Henry Hildebrand; August 6, 1962

ABSTRACT: This interview was conducted as part of the Archives for the History of Quantum Physics project, which includes tapes and transcripts of oral history interviews conducted with ca. 100 atomic and quantum physicists. Subjects discuss their family backgrounds, how they became interested in physics, their educations, people who influenced them, their careers including social influences on the conditions of research, and the state of atomic, nuclear, and quantum physics during the period in which they worked. Discussions of scientific matters relate to work that was done between approximately 1900 and 1930, with an emphasis on the discovery and interpretations of quantum mechanics in the 1920s. Also prominently mentioned are: E. Q. Adams, Josiah Cook, Peter Josef William Debye, Walter Kossel, Irving Langmuir, E. P. Lewis, Gilbert Newton Lewis, Walther Nernst, Wilhelm Ostwald, Eddie Slate, Edgar Smith, J. J. van Laar, Jacobus Henricus van't Hoff, Bill Williams; American Chemical Society meeting (World War I), University of California at Berkeley, Massachusetts Institute of Technology, and University of Pennsylvania.

Transcript

Hildebrand:

Let me go right back to the beginning and why I became a chemist. I was born with a good deal of natural curiosity. Then at a private elementary school I attended in Camden, New Jersey, I was fortunate enough to be confronted with science by a man who came perhaps once in a fortnight to lecture on science to the kids. And he was very good. He was enthusiastic and he had good demonstrations for the period. He made hydrogen blow soap bubbles, and then ignite them. He had a Hero's fountain, and picked out electrified bits of sealing wax and so on. Well, I was entranced, and went home and repeated everything for which I could possibly gather the materials. I discovered in my grandfather's library in a little Pennsylvania Dutch town, East Berlin, a number of very good books.

He had a better library than many college graduates today although he had had only a common school education. I found such things as (Queasy's) Fifteen Decisive Battles of History, Plutarch's Lives, and my first book on chemistry, written by (Youmans),published in 1857. Well, I soaked it up from cover to cover. I had Newton's Astronomy, and Dana's Geology, well-illustrated. I had Nature Readers at a still earlier age. When we went out into the woods and fields with my associates, I could answer their questions because I was such an avid reader. I could tell them about the tadpoles and the dragon-flies and so on. And I discovered what fun it is to explain something, at least for me. I was a teacher from then on.

My next major encounter with chemistry occurred in my senior year in high school. I had an excellent high school in Wayne, Pennsylvania, a suburb of Philadelphia. ... That was a public school. I had as a teacher of mathematics one of the best teachers of the subject who ever lived. I had good instruction in mathematics and in history. I had three foreign languages, in varying amounts — Latin, French, and German. The educationists tell us nowadays how terrible the old schools used to be. Why, I could, get into any college today, I think, on the basis of the education I had. And if the schools were then as poor as some of them now say they are, I'm not an educated man at all.

The principal of the school was a liberal arts graduate of Harvard, a cultivated man, very musical. I was very musical. He and I went together to hear symphony concerts in Philadelphia. He taught chemistry, and had had one year of chemistry at Harvard. He discovered that I knew rather more chemistry than he did. Very wisely, instead of trying to cover up his ignorance by making formal assignments and directions about making the right notes, he gave me the key to the laboratory and his books, along with the little laboratory manual that I was supposed to follow.

He didn't insist that I repeat any experiment that I already completely understood. But his principal book, as a text at Harvard, was Chemical philosophy by Josiah Cook. Cook is mentioned in Katherine Drinker Bowen's Life of Oliver Wendell Holmes as the first man to introduce laboratory or in Harvard chemistry, which is rather interesting. It was' called Chemical Philosophy. You see it's a relic of the time when all the science was natural philosophy. And it was philosophy of a sort, because evidently he didn't do very much laboratory work, but he philosophized. It was before the day of Mendelejeff, or at least before he had understood anything about Mendelejeff, and he had a theory of the elements according to which an element must have even valences or odd valences but it couldn't have both. To make nitrogen always odd, he had to double up NO and write it N2O2. But I had seen it written NO also. You see until sometime after (Canizarro) in 1858, there was still confusion about the formulas of substances.

In this (Youmans') book water was HO, sulphuric acid was HOSO and the atomic weight of carbon was six, oxygen was eight, and so on. I decided to find out which was correct, NO or N2O2. I devised an experinent as I would devise today. I made and collected over water in a graduated tube some NO and in another tube oxygen. I poured the NO into excessive oxygen to be sure to get N204 rather than N203. I didn't know how little N2O3 would be formed. And the volume relations were two NO to one 02 and not one to one.

I think that's when I really began to be a scientist because I discovered that the authority of science rests in a well-reasoned experiment and not in a book or even a Harvard professor. And to hell with the Harvard professor. It did me a great deal of good. It probably was the most important experiment I ever, performed for my own development, because it provided me with confident skepticism about the things that I heard as a Freshman at the University of Pennsylvania, or rather as a sophomore.

I didn't take chemistry until my sophomore year. I selected a combination major, chemistry and physics. Or no, perhaps I did take chemistry my freshman year, I forget. ... I didn't have any formal course in physics at school, but I took physics in the University.

Kuhn:

What level had the Mathematics gotten to by the time you got there?

Hildebrand:

Oh, I won the entrance prize in mathematics to the Univ. of Pennsylvania for the best entrance examination. I could have gone in by certificate. The plan had just been introduced, but it seemed to me like going in by the back door. Furthermore, I wanted to have a crack at this fifty dollar prize. I had had mathematics through solid geometry and trigonometry. I understood pretty well that which I had taken. I didn't just go through and learn the rules and formulas. But I won the prize. Edgar S. Smith was the head of the department — when I say "head" I don't mean "chairman." He had been educated in Germany, in Göttingen, and had imported the German system, where there was one full professor in an institute, and the rest of them were all his (???).

And he was the czar; he made pronouncements. He was very nice about it; he was a gentleman; very courteous. Nevertheless, he was the authority. I'll tell you later on about my experience as an instructor there. But as generally in those days, every professor who taught freshmen seemed to think it necessary to distinguish chemistry from physics. And he defined chemistry as the science dealing with the deep-seated, permanent changes in matter. And as a dutiful freshman should, all took it down and recited it. I remembered it, and recited it when I had to, but I was pretty skeptical about the adequacy of that definition.

One of his demonstrations of the distinctions was performed with an electro- magnet. He picked up a nail, and then when he broke the current the nail dropped. That wasn't permanent, that was physics. I wanted to ask him, had I dared , "Suppose the core of a magnet happened to be made of steel, and the nail remained sticking there would that make it chemistry?" Again he made nitrogen chloride, which immediately exploded as it rose to the turpentine which was on top. That wasn't very permanent, but that was still the chemistry.

A good deal of his reasoning was of that sort. He asserted that hydrogen is like the metals. Now, of course, hydrogen can replace a metal in the formula of sodium chloride and so on, but he pushed it to the physical properties; he arranged a platinum wire to glow in the air with an electric current. Then he arranged to surround it with hydrogen, and the same voltage would make it glow. He said that shows that the hydrogen is like a metal. It conducts heat and electricity. Well, that is just terrible logic. I've always been glad that I had gotten over the illusion that you must accept what your professor says. Well, of course, elements were the ultimate indivisible particles. I think somebody once asked one of the leading British scientists whether an atom might conceivably be broken up. He said, "No, the name atom means indivisible." That's a sample of the kind, of reasoning we sometimes got.

My undergraduate chemistry was long on contact with the elements. I had to spend a whole year on mineral analysis, which taught me a good deal about behaviors, especially of some of the rarer elements. I had no physical chemistry. It wasn't given until I gave it, after coning back from a year in Germany. The order of courses followed the historical order, general chemistry, qualitative analysis, quantitative analysis, organic chemistry. Some of the teaching was pretty poor, which was probably a good thing, because I think a person should have a few poor teachers to help put him on his own, and help wean him away from authoritarianism. I was slated to get some more training in physical chemistry, and come back and teach a course in physical chemistry.

Kuhn:

How much physics and math did you have?

Hildebrand:

Oh, I had calculus, of course, in University, and I had several years of physics, as a graduate. ... I didn't take the course in chemistry in the Towne scientific school, which is a more technical course to prepare industrial chemists. I took a liberal arts course in the college of arts and sciences, with a double major in chemistry and physics. As a graduate student I had calculus and differential equations. I had an excellent course in analytical mechanics which was very good practice in translating English into mathematics and back again. That gave me a good deal of facility in handling calculus. The physics department then was not much of a research department, but the men were fairly good teachers. There was one man, A.N. Richards, who was very good at teaching mechanics and such things. Well, I went abroad after I got my degree, and spent a year in Nernst's laboratory. I began by taking his Practicum, because I was going to have to give a laboratory course. And each exercise was the occasion of my studying all I could learn about a given subject. And then I did a little Arbeit with Nernst. I came back then as an instructor in chemistry.

Kuhn:

Let me interrupt. I take it that at that point in this country, prior to your going to Germany, physical chemistry as a field was practically unknown.

Hildebrand:

Yes, the only places where there was any physical chemistry to speak of was first of all, the physical-chemical laboratory under the direction of A.A. Noyes at M.I.T. Lewis had gotten his degree at Harvard, and had gone abroad for a year. But at the M.I.T. laboratory were engaged I might say a majority of the young fellows who later were the teachers of physical chemistry throughout the country.

Harry C. Jones, at Johns Hopkins had studied abroad in Ostwald's laboratory and came back as one of the prophets of the new order. But he overreached himself a good deal. He didn't have the brains and scientific ability to become one of the leading lights. For example, he accepted the physical chemistry of van't Hoff and Ostwald not very critically. He used the freezing point lowering of solutions — the limiting law for freezing point lowering — and applied it to concentrated solutions of such things as calcium chloride. From the lowering he calculated how much of the water must be water hydration, and the rest of it was water of solvation. Well, some unkind person pointed out that his water of hydration as calculated was more than the water in the container. He didn't have any idea you see, what van't Hoff made perfectly clear; that this is a limiting law. And he applied it to concentrated solutions.

Kuhn:

Do you know at what point you yourself had decided to get over into physical chemistry?

Hildebrand:

Just naturally. The questions I thought of were questions that had to do with physical chemistry. For example, Smith was very much interested in discovering an element. And he had a notion that an element might be discovered in the neighborhood of titanium. We called it columbium, now niobium. He went to great pains to purify sodium columbiate — it's very hard to free it from titanium. And I offered to test the purity of his columbium spectroscopically. And I did an arc spectrum of it, a very complicated spectrum, so I could tell whether or not the titanium lines are represented; Furthermore, he had two other topics... One was what he called complex inorganic acids. He would take sodium tungstate, and add a little phosphate, some (vanidate) and so on, and cook it up. And then he'd let it cool and he'd get several different colored crops of crystals and pick them out and, analyze them, without any application of phase rule.

I couldn't tell to what extent some of these might have been solid solutions, although probably not many of them were. Then he also did an electroanalysis, rapid analysis, with a platinum dish cathode and a rotating flat platinum spiral anode. And by rotating the anode, he would stir it, and get out his stuff very rapidly. My thesis consisted of doing this, of electrolyzing alkali halides with what is essentially a (Kastner- Kellner) cell, using a silver plated platinum anode. This industrial cell for getting sodium hydroxide consisted of an inner vessel. You electrolyze the sodium into mercury; you decompose it in an outer compartment; and then the chlorine goes off the anode. I caught the chlorine and weighed it. I'd get a little oxide, which I could decompose by heating it gently in an oven, and then I'd weigh the increase, which was the (halide). I also put a nickel wire outside in electrical contact with the mercury on account of the over-voltage of hydrogen.

You see, I read a lot of these things, although Smith didn't teach them. I could then (titrate) the alkali, and weigh the halogen. I could make a determination of two halogens together that way, with the two unknowns. The things that interested me about that, you see, were the physical- chemical questions. And I'm pretty sure I must have acquired Nernst's Theoretische Chemie and I read physical chemistry, because the theoretical questions were the ones that interested me. Making up a recipe for a quantitative analysis wouldn't appeal to me particularly.

Kuhn:

I take it you got no thermodynamics from the chemists in this period?

Hildebrand:

No, not a bit.

Kuhn:

Did you get any from the physicists?

Hildebrand:

Yes, I had a graduate course in thermodynamics from the chairman of the department, I can't think of his name just this minute. It was a course given from his notes, which he had taken at Harvard or Yale as a student. And I don't think he'd ever added anything to it. And I got a D, which is "distinguished" in the course, but what it meant physically I had very little notion of and neither did he. It was just deriving formulas, with virtually no application to physical systems.

Kuhn:

The phenomenological or statistical approach?

Hildebrand:

No statistics at all at that time. In fact, statistics was not thought of very much. I have Nernst's Theoretische Chemie, the 1909 or '10 edition, here in the library, and it mentions entropy once in fine print with a statement as to why you don't need to use it. He just used the temperature derivative of total energy, and he wasn't very clear about Helmholtz' and Gibbs' free energy. Van der Waals, and especially van Laar, had a completely analytical type of thermodynamics, but van Laar mixed it up inextricably with the Van der Weals equation. And he scorned the kind of Carnot cycle derivations which van't Hoff and Nernst had used. He didn't understand at all the distinction between (non) polar and polar substances. He derived results which were all right formally but didn't meet the physical tests.He had a polemic style better suited to making enemies than friends; and he was not accepted by Nernst. And Nernst was the Pope, you see, at that time. Unless Nernst smiled upon you, you didn't get very far, so van Laar was an embittered man.

I took lectures by van't Hoff also over there. It was Ausgewählte Kapitel der physikalischen Chemie. It was just practically a rehash of his lectures, which, of course, is quite an historical book. I was a not infrequent guest at van't Hoff's home. I think one motive was that he wanted his children — he had a son and a daughter, and the daughter later married a Johns Hopkins physiologist — to have practice in English. But it was a very interesting experience to sit down and, talk with him as frequently as I did.

Nernst was not an engaging personality. He was a pompous man, not genial. But, of course, terrifically stimulating, because his book was the Bible of all physical chemistry. He was (Vorsitzender) of the (Holtzen) Gesellschaft when I was a student. At the end of a discussion, he would give the Schlusswort, and that settled it. I was fortunate in having my doctor's degree when I was in Germany, so I didn't have to bone up to pass examinations, but I could make my own selection of what was important and what was not. Nernst, in my second semester, gave a course entitled Atomistik. Up until shortly before that time Ostwald had maintained that it was immoral, scientifically immoral to even mention the word atom, because nobody had seen them, and they couldn't ever be seen. He tried to explain such things as the law of multiple proportions on the thermodynamic basis.

The discovery of radioactivity, and especially such phenomena as the Spinthariscope indicated the particle nature of radiation. ... In these lectures on Atomistik he began by announcing — I think it was seven — laws or theories that he felt had been so well-established, that they were no longer open to question. Well, it was the first and the second law of thermodynamics, and then his heat theorem. Also, the law of conservation of mass, which at best at that time had been very accurately tested with a balance by (Hernandolt), who found no detectable difference in weight before and after reaction. And also, the law of conservation of energy. Well, now here today you see these are true collectively but not individually.

Another was the aether theory. ... And then another was the law of action at a distance, which included gravitation and Coulomb's law. Well, of course, when you get down to sub-atomic distances you have to do something about that too. And all of this had not yet had any effect on Nernst. After I came to California in 1913, I reviewed a book on the atom by J.J. Thomson, the man who had done more than anybody else to elucidate the structure of matter. And in it there was no refernce to Bohr, to the Bohr atom. He had a formula which allowed him to keep the electrons at a distance from the nucleus, just picked out of the sky.

Kuhn:

Had you been yourself taught to take atoms pretty seriously before you went to Germany?

Hildebrand:

Well, yes, although (Cook's) discoveries, the discovery of the electron, showed that there must be something smaller than atoms. And I used to use that in my early teaching at Pennsylvania, which began in 1907, as evidence that atoms were composed of something or other. But there was, of course,... Prout's hypothesis. That, of course, gave a hint that there must be something structural about atoms. And I was much impressed by that. That together with the electron indicated that atoms were not indestructible, that they had a structure.

Kuhn:

But you had not been brought up to take a point of view like Ostwald's?

Hildebrand:

No, oh no, no. No, that was too theoretical, as a matter of fact, for Smith. Smith was more than anything else an analyst and a — well, an analyst. He had a vast store of practical knowledge and feeling for chemical reactions. He'd say, "Well, if you add a little more hydrochloric acid and boil it a little longer it'll come out right."

Well, organic chemistry of the day consisted largely of this sort of lore, and a leading professor was a man who had, more of it than the younger fellows had. But he had, very little theory. We were rarely asked, a theoretical question by Smith, and Smith pooh-poohed some of the theory, especially the ionic theory.

Well, even when I came back as an instructor, if I mentioned ions, he would allow me to do it, but he had a sort of cynical smile on his face implying that he thought I'd get that nonsense knocked out of my head as I got a little older. He didn't understand — he and (Conberg) at Wisconsin and Armstrong in England, did not understand that when you put salt in your soup you're not adding metallic sodium and chlorine, that the electrons made a difference, and the chloride ion is a totally different substance from chlorine gas. But I was tremendously impressed by all those things. I wanted to understand them. And they weren't the kind, of questions that seemed important to Smith. He said, "Can you analyze sulphuric acid? I don't care if you do it by mass action or ionic theory but can you analyze it?"

That was his attitude.

Heilbron:

Was Nernst already interested in the quantum theory when you arrived? And talking about it?

Hildebrand:

No, I don't recall any mention of it.

* * *

I started in as a young instructor at the University of Pennsylvania. I was penurious. There were no salary scales. I was an instructor at $1000 per annum for three years, and at $1200 for another three years. I was publishing fairly good papers all the time. I taught the first course in physical chemistry, and I had a laboratory. The only piece of apparatus I had was acquired after several years. I was allowed to buy a (bolt potentiometer), which I could use with a thermocouple to make emf measurements and so on. I had some old sulfated cells. If I wanted a piece of glass apparatus, I blew it myself. If I wanted a screw-driver, I borrowed it from the personal property of the janitor. I decided, however, that each of my studies for the early years would be on a new topic. I had to educate myself. That has some advantages, because you don't accept a glib explanation. Nobody can give it to you. You work it out for yourself, and you finally get it.

Well, one of my earlier researches was measuring the vapor pressures of amalgams. Richards and Forbes, and again Hewlett and Cranshaw, had measured the emf very carefully of dilute zinc and cadmium amalgams. And I saw that it was possible thermodynamically to relate the vapor pressures of the mercury to the emf of the concentration cell. I derived the Gibbs-(Duhem) equation for the purpose. I had never heard of it. ... And that was really part of what started me off on theory of solutions. I also investigated the non-violet solutions of iodine. ... That was the beginning of the investigations of iodine solutions which I've continued to the present, because by doing systematic work with iodine I've been able to first of all distinguish physical from chemical solutions. The violet ones are physical, 'and the others are not.

They have a different temperature coefficient. And, also, you can measure them very conveniently, because you can titrate iodine, so nicely, or you can get it by absorption, by the extinction coefficient. I'm still finding things out by iodine solutions. I could stop and give you a lecture on that, but that isn't what you're here for. Well, I measured the equilibrium between the dissociation of barium peroxide. ... I also have done work with the hydrogen electrode before I came here. I followed the course of reactions between acids and bases, and ions and OH and so on. ... I showed how you titrate carbonate solution with acid. ... That was a very important paper, at least very popular.

When I came out here in 1913 to see whether I would like the University and they would like me, on my way back I attended the American Chemical Society meeting in Milwaukee. They put this paper of mine on the general program. And I gave it out here as one of my lectures. Well, it made an impression in both places. I should have ordered a thousand reprints of that paper, because it gave a practical method for following all sorts of interesting — for knowing what's going on.

Kuhn:

At what point in this line of development, or was it not until you actually got out here, did you begin to become conscious of the existence of this thing going on in left field, quantum theory?

Hildebrand:

Oh, it wasn't until I got out here. Because the department of physics in Pennsylvania was even more backward than the department of physics here. Lewis was very much interested in relativity at first. He had met Einstein and talked with him and was much impressed with him, so he gave several seminars on relativity. Ed Tolman was here, who had also been interested in it. It displeased Eddy Slate, who was chairman of the department of physics, and was an old-line physicist. Lewis had no right to discuss these matters, which were physics, in the department of chemistry. Slate would die, you know, before he ever — before relativity even made any impression on him, because it's contrary to reason. And even more so quantum theory would be.

Kuhn:

You think you'd scarcely heard of these things yourself until you got out here?

Hildebrand:

I'm pretty sure I'd never heard of — I have heard of relativity, but it is a good deal outside of my ken, you see. I'd probably heard of the Michelson-Morley experiment, but that it had any connection with chemistry of course I wouldn't have thought of. And quantum theory, that wasn't until I got out here.

Lewis, of course, was interested in thermodynamics, and he had talked about this heat capacity matter at a rather early date. There was a good deal of coolness on the part of Nernst toward Lewis because Lewis did not give Nernst all the full accord which he desired for the exclusive development of the Nernst heat theorem. I think you'll find evidences of that in Lewis' writings somewhere or other. And that may have been a reason why Lewis never got a Nobel prize, because Nernst was influential undoubtedly for a long time in the selection of the Nobel prize winners in physics and chemistry.

Heilbron:

Was there any interest in these photo-chemical reactions by 1913?

Hildebrand:

I don't think so; not in the reactions. Lewis was very much interested in the Wien-Stephan radiation studies, and several of his papers show that. Any of these that you don't have, you're welcome to take. I got them all out for your benefit. ... Now this is my biographical memoir on Lewis, which I originally wrote for the Royal Society. Then the National Academy decided they ought to publish it meanwhile, although Lewis had thrown his note at them by resigning.

Lewis was an enfant terrible at times. He liked to shock people. During the period when Millikan was pretty much dominating the Academy, Lewis was rather disgusted, and lost patience with the refusal of the Academy to elect certain people whom he thought ought to be in it, and so he just resigned. Some of us tried to persuade him to rejoin, Campbell did, and (Lorsch) and. E.B. Wilson, but he didn't do it.

Well, the Academy council decided after Lewis' death, that because he had been a member, they ought to publish the memoir, so they asked me to write it. Well, I said, "I've already written as good a one as I can," so we got permission from the Royal Society to reprint it. I made a few editorial changes, but they are not substantially different from this version, so you may take this. It won't give you directly what you want, but it will give you a little bit of atmosphere, the way things went on here in the early days.

I came here and found an altogether fresh atmosphere. I illustrated in my one incident or two, how Lewis liked, to make challenging statements, sounding as if he was committed to them, to stir people up. At one of the earliest seminars I attended he made such a statement, and E.Q. Adams, a very brilliant Graduate student who has never amounted to very much because he hasn't had the judgement or the drive and originality, but very critical, said, "No, that isn't so." I turned around in astonishment. If I'd said that to Smith, I'd have been treated with scorn. Lewis turned to him almost eagerly and said, "No, why not?"

Another time a student contradicted Lewis and Lewis said, "Well, that's an impertinent remark, but it's also pertinent!" I had, about an 18 hour teaching load at Pennsylvania. Out here, Lewis protected me from a large teaching load. I had the big freshman chemistry lectures, so I had only about six or eight hours. Also, at Pennsylvania all the thesis work had to be done under Smith, the way it was in a German institute.

Smith explained to me that at Columbia, where they didn't have a head, the men were jealous of each other, whereas we were safe from jealousy, we underlings, because he did it all. So I came out here and Lewis sent graduate students to me to see if I might have something to offer which would appeal to them. So you can hardly imagine the sense of emancipation that I felt.

There was a new department. The oldsters who were left over accepted gracefully the new regime, and the new men that Lewis brought with him in 1912, I came in '13, and the new ones that we acquired, gave a totally new atmosphere. Well, now, you've got quite a reel, haven't you? It's been a thrilling thing to live through this period where perhaps 95 per cent of what I use today wasn't known when I got my doctor's degree.

Kuhn:

How was Lewis' reception of the various quantum ideas, his considerable antipathy to the Bohr atom?

Hildebrand:

That's an interesting question. You see, Lewis had, done this remarkable study, published in The Atom and the Molecule to explain valences. This was preceded by at least a year and a half, perhaps two years, of discussion of these matters in a special colloquia, seminar, that he gave, and we all butted in and gave our views. This was an inductive approach to the chemical bond. The physicists of the day, of course, didn't understand it, because the evidence was not the kind of evidence they were used to handling — the different formulas of compounds — the fact that the sum of the positive and negative valence numbers is eight. Well, Lewis explained that by a hypothesis which I think was merely a numerical affair rather than very geometrical, but he talked about the cubic atom, and he drew actually pictures, showing its relation to the octet rule. Then he made a great deal, of course, of the fact that very few compounds have other than an even number of electrons. So that's an indication, you see, of electron pairing.

Now to reconcile the octet with the pair, he distorted the cube into a tetrahedron more or less. So you could have a pair, a bond consisting of a pair, a double bond consisting of an edge of a tetrahedron, and a triple bond a face.

Kuhn:

The distortion to a tetrahedron comes after that first paper on the atom and the molecule.

Hildebrand:

Yes, it does. Lewis wrote this just before he went to France in World War I. I went with him at that time. And while he was gone, Langmuir had read this paper and was very much impressed by it, and he wrote a much longer paper based pretty largely upon the octet rule. Well, Lewis was a little bit peeved that Langmuir should take advantage of his absence in the war to exploit Lewis' ideas. Langmuir, I think, didn't do that with the idea of piracy so much as the fact that Langmuir was not very conscious of what went on in other people's minds. He probably just didn't bother to think about how Lewis would take this. Well, it made a tremendous impression because he presented it at a big Chemical Society meeting, got a prize for it and so on. Then Lewis — I think on that account partly — emphasized the pair, which Langmuir had under-emphasized, under-appreciated, and Lewis preached from then on that this was his major contribution.

Now Kossel at about the same time had similar ideas which I think Lewis gives fair account of. Their ideas of the gain and loss of electrons led to the same conclusions. Now you see before the discovery of the spin and so forth, physicists couldn't conceive of a pair of electrons doing anything, and yet this mass of inductive evidence was really very powerful. The physicists had to wait until they could discover electron pairs in their own way.

Kuhn:

What about the Bohr atom in this?

Hildebrand:

Yes, well you see the Bohr atom did not give an indication of electron pairs. ... We heard about it, because of its spectroscopic significance, but it didn't seem to be significant for the understanding of chemical bonding.

Heilbron:

What about the hydrogen molecule, where the binding is in the two electrons held in the plane of symmetry?

Hildebrand:

I don't remember how the Bohr atom at that time explained double ownership, the sharing of electrons. ...

Kuhn:

Were these things much discussed? In the 1916 paper, Professor Lewis does talk a bit about the Bohr atom and what seems to be the matter with it. He doesn't see how chemistry can use a dynamical model. Do you remember colloquia or lectures or discussion with him in which this sort of thing went on?

Hildebrand:

I don't remember anything beyond what the paper says about it.

Heilbron:

Lewis in 1916, or '17, offered a model for a possible spectroscopic series. He was the force changing from positive to negative. He has a picture.

Hildebrand:

I don't remember that, no. ... It's not the kind of a thing that would have made any —.

Heilbron:

Chemists didn't much care then about the spectroscopy, except to use it, was that it?

Hildebrand:

Yes. We were interested, of course, in using it. Well, I remember referring to Kayser's handbook when I was doing spectra in Pennsylvania. But we were interested in the fact, of course, that somebody had said at the time that spectrum lines show that it is much more complicated than a grand piano. ... I think that was one of the principal figures of the day, I don't know who said it.

Kuhn:

Were you here when professor Stern visited?

Hildebrand:

Oh yes. ... We enjoyed Stern very much. He was a genial person. He knew his thermodynamics. He and Lewis argued things out indefinitely with great glee.

Kuhn:

I take it there was a good deal of discussion then also about the validity of the Nernst heat theorem? Between those two.

Hildebrand:

Oh yes. Well, that had to do with what I said earlier about the premonitions of it, and about Theodore William Richards and Lewis, and the rather artificial treatment of the temperature dependence of the thermodynamic quantities. They used to call Nernst's integration constants the chemical constants. But, of course, the study of heat capacities was needed in order to put the thing on the right track.

Kuhn:

Well, Lewis' problem you feel, with Nernst's heat theorem, was the feeling that there had been some other people involved with it, and there was more to it than Nernst was seeing.

Hildebrand:

Yes, yes.

Kuhn:

I'm interested in that, because I had before rather the impression that Lewis might have thought that it wasn't so general a rule, or might have questioned its validity.

Hildebrand:

No, it's not so much that. Lewis didn't call it the Nernst heat theorem. He called it the third law of thermodynamics in his writings, you notice. It's the third law of thermodynamics. See the treatment of Nernst was a formal treatment, based upon his algebraic expansions. Well, chemists were using it just as a rule. Lewis was much more interested in the actual significance of these things, as you can see from the work which he helped to encourage at least on the heat capacity of glasses and other things at low temperatures. Of course, the Debye temperature was an important development that impressed us all.

* * *

Kuhn:

I take it the department under Lewis was largely a physical chemistry department?

Hildebrand:

Yes, it was, although we always kept pretty close to the ground. We didn't go off just discussing thermodynamics as a metaphysical subject, you see. Lewis was very alert to chemical and physical phenomena, and he would discus an organic paper. In his discussion of valence he used an immense range of chemical phenomena, organic and inorganic. He was a very different type of a person from a person who gets obsessed with the beauty of his mathematics. In this respect I agree with him.

[Comments by Hildebrand on scientific methodology here omitted.]

Hildebrand:

Now Lewis is like Debye in this respect. Debye is and Lewis was a superb theoretician. And, yet they keep their feet close to the ground. I heard Debye in a symposium state in a few words what somebody had put on the board in a complex mathematical form. The young colleagues gave me a very fine birthday celebration last September, a two day symposium here. Debye was one of those who attended. And it was a delight then as always before to hear the extremely concise, clear way in which he stated a thing in physical terms.

I'm just writing a little book on introduction to kinetic theory for high school age and college freshmen that don't have a good freshman text. I'm approaching it by trying to get the concepts straight first of all, not by deducing an equation and saying here it is. "Damn you now, substitute nunbers and get the answers." I'm pointing out the equation is simply a very concise way of saying something, of relating quantities, that ought to be clearly in your mind as concepts.

Kuhn:

What sort of relations did the physical chemists have with the physicists here?

Hildebrand:

Oh very friendly. In the very early days, Slate and Lewis didn't have any. One fellow, Bill Williams, had been a West Point graduate. He became a General in World War I. He was teaching school. He left the army and was teaching school in Oakland. He used to come out to Lewis' seminars. And although he never produced anything, he was a very keen critic and contributed in that way a good deal to the development of Lewis' ideas. E.P. Lewis' ideas. E.P. Lewis became chairman of the physics department. He was a spectroscopist. He discovered the after-glow of nitrogen and explained it and so on. And we got along very well with him.

I don't remember others we had any hob-nob with particularly, but with the advent of younger men, especially Lawrence and the people he brought in, relations became very close and cordial. Jenkins and Brewer. Birge, of course, laid the foundation for Giaque's discovery of the isotopes of oxygen. We all had all our graduate students take physics, especially a course in quantum theory which Williams Gave; and Birge's spectroscopy. And then they'd take vector analysis in mathematics. And our students became better at physics than the physics students did at chemistry. Of course, now the distinction between chemical physics and physical chemistry is a continuum.

Heilbron:

Do you recall how Lewis felt about the Schrödinger mechanics? Did he think that was the solution to these difficulties he raised about Bohr?

Hildebrand:

Oh, I think he did.

[Hildebrand, Kuhn, and Heilbron consult the index of the 1923 edition of Lewis and Randall's textbook in order to ascertain Lewis' views on the quantum.]

Hildebrand:

Well, now that's all there is on quanta.

Kuhn:

Good, I'm very grateful to you, sir.

Hildebrand:

It's been very interesting. The older I get, I fell a sense of superiority over you youngsters because you've never been through the war! It's been a tremendously exciting thing intellectually to live through the development of these things from nothing.

Kuhn:

There's been no other period quite like it, the rapid fundamental change.

Hildebrand:

And not many people 80 years old who have been even as much concerned with it as I have with things. I've been recording a number of interviews for the University archives on the history of the University and to some extent, touching upon these questions, although I haven't gone into it for the lady who took it the way I have for you, because she couldn't ask the same questions. But here we established a department, and I was in it when it was only one year old, a department that has furnished more of the physical chemists to the universities of the country of any distinction than any other. Now we take a kind of pride in the other institutions that —.