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In the introductory chapter, Atoms in Motion, of the Feynman Lectures on Physics (Volume One), Feynman alludes to a certain "physical method", which can be used to determine the structure of α-iron (and presumably) many other complex organic compounds.

Excerpt from page 1-8 (Chapter one, page eight):

How does the chemist find what the arrangement is? He mixes bottles full of stuff together, and if it turns red, it tells him that it consists of one hydrogen and two carbons tied on here; if it turns blue, on the other hand, that is not the way it is at all.

This is one of the most fantastic pieces of detective work that has ever been done—organic chemistry.

To discover the arrangement of the atoms in these enormously complicated arrays the chemist looks at what happens when he mixes two different substances together. The physicist could never quite believe that the chemist knew what he was talking about when he described the arrangement of the atoms. For about twenty years it has been possible, in some cases, to look at such molecules (not quite as complicated as this one, but some which contain parts of it) by a physical method, and it has been possible to locate every atom, not by looking at colors, but by measuring where they are. And lo and behold!, the chemists are almost always correct.

Knowing that this lecture was originally delivered in the early 1960s, what is this "physical method" Feynman's referring to and how does it work?

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    $\begingroup$ X-ray crystallography I guess. $\endgroup$ – Mithoron May 6 '17 at 20:01
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    $\begingroup$ He actually refers to "x-ray analysis" later in the chapter. $\endgroup$ – airhuff May 6 '17 at 22:18
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    $\begingroup$ Crystallography had existed long before the 1940s, though. The timing seems suspiciously appropriate for NMR spectroscopy, even if the simplified description does seem to fit X-ray diffraction more. $\endgroup$ – Nicolau Saker Neto May 13 '17 at 8:28
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Physical method is an old school terminology for what we would call as instrumental method of chemical analysis. There is a pretty famous textbook on "Physical Methods for Chemists." The book by Drago is quite popular. These physical methods, as the name indicates were borrowed from physicists. This included NMR, Group Theoretical Applications for IR & Raman (this was the first chapter), X-ray and all types of applied optical spectroscopy with some mathematical treatment etc.

We must appreciate the insight of chemists of the previous century who with limited tools imagined certain arrangement of atoms, purely on the basis of functional group and chemical analysis. If you happen to have a copy of I. L. Finar Organic Chemistry (vol I, II, the section on natural product chemistry illustrates how structural analysis was done in the pre-instrumental era. Just chemical tests, synthesis, and functional group analysis by means of chemical reagents. Those were the wet chemical methods of analysis. Hence the distinction in my opinion.

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    $\begingroup$ You aren’t really answering the question, though. $\endgroup$ – orthocresol May 5 at 13:07

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