Chemists understood the atomic structure of molecules in the 1800s, yet it is often said that Einstein established the existence of atoms in a paper on Brownian motion, “Die von der Molekularkinetischen Theorie der Wärme Gefordete Bewegung von in ruhenden Flüssigkeiten Suspendierten Teilchen”, published in 1905.
This is perverse, and has seemed strange to me ever since I began reading the history of organic chemistry. Chemists often don’t get the credit they deserve, and this provides an outstanding example.
For years, I’ve read statements like this:
[Einstein] offered an experimental test for the theory of heat and proof of the existence of atoms….
[“The Hundredth Anniversary of Einstein’s Annus Mirabilis”]
Perhaps this was so for physicists in thrall (or unsuccessful opposition) to the philosophical ideas of another physicist, Ernst Mach; he had odd convictions about the relationship between primate eyes and physical reality, and denied the reality of invisible atoms.
Confusion among physicists, however, gives reason for more (not less!) respect for the chemists who had gotten the facts right long before, and in more detail: that matter consists of atoms of distinct chemical elements, that the atoms of different elements have specific ratios of mass, and that molecules consist not only of groups of atoms, but of atoms linked by bonds (“Verwandtschaftseinheiten”) to form specific structures.
When say “more detail”, I mean a lot more detail than merely inferring that atoms exist. For example, organic chemists had deduced that carbon atoms form four bonds, typically (but not always) directed tetrahedrally, and that the resulting molecules can as a consequence have left- and right-handed forms.
The chemists’ understanding of bonding had many non-trivial consequences. For example, it made the atomic structure of benzene a problem, and made a six-membered ring of atoms with alternating single and double bonds a solution to that problem. Data regarding chemical derivatives of benzene indicated a further problem, leading to the inference that the six bonds are equivalent. Decades later, quantum mechanics provided the explanation.
The evidence for these detailed and interwoven facts about atoms included a range of properties of gases, the compositions of compounds, the symmetric and asymmetric shapes of crystals, the rotation of polarized light, and the specific numbers of chemically distinct members of classes of molecules of identical atomic composition.
These chemists not only understood many facts about atoms, they understood how to make new molecular structures, pioneering the subtle methods of organic synthesis that are today an integral part of the leading edge of atomically precise nanotechnology.
All this atom-based knowledge and capability was in place, as I said, before 1900, courtesy of chemical research by scientists including Dalton, Mendeleev, van ’t Hoff, Kekulé, and Pasteur.
But was it really knowledge?
By “knowledge”, I don’t mean to imply that universal consensus had been achieved at the time, or that knowledge can ever be philosophically and absolutely certain, but do I think the “knowledge” term fits:
A substantial community of scientists had a body of theory that explained a wide range of phenomena, including the many facets of the kinetic theory of gases and a host of chemical transformations, and more. That community of scientists grew, and progressively elaborated this body of atom-based theory and technology to up to the present day. Their understanding was confirmed, explained, corrected, and extended by physics along the way.
Should we deny that this constituted knowledge, brush it all aside, and credit 20th century physics with establishing the simple fact that atoms exist? As I said: perverse.
But what about quantitative knowledge?
There is a more modest claim for Einstein’s 1905 paper:
…the bridge between the microscopic and macroscopic world was built
by A. Einstein: his fundamental result expresses a macroscopic quantity — the coefficient of diffusion — in terms of microscopic data (elementary jumps of atoms or molecules).
[“One and a Half Centuries of Diffusion: Fick, Einstein, Before and Beyond”]
This claim for the primacy of physics also seem dubious. A German chemist, Johann Josef Loschmidt, had already used macroscopic data to deduce the approximate size of molecules in a gas. He built this quantitative bridge between the microscopic and macroscopic world in a paper, “Zur Grösse der Luftmoleküle”, published in 1865.
I had overlooked Loschmidt’s accomplishment before today. I knew of Einstein’s though, and of a phenomenon that the sociologists of science call the Matthew Effect: to those who have great credit, more shall be given; from those who have little credit, it shall be taken away.
See also:
- A Map of Science
- How to Learn About Everything
- Chemists deserve more credit (2):
The 150th anniversary
of the first international science conference
[Scattered edits, 9 Sept 2010]
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The problem with atomism in 19th century chemistry was that even the convinced atomists could not agree on what the theory meant. There wasn’t a pre-established “paradigm,” if you go for Kuhnian sorts of explanations. Some spoke of atoms, others spoke of what we now call molecules, but even then the hard-and-fast laws were often, in the eyes of chemists, distasteful things. Many still dismissed atomic theory outright.
When the Copernican theory began to gain popularity, many astronomers liked how the calculations worked out but didn’t agree with the implications of a moving earth. Subscribing to the math but not the theory behind it was called “saving the phenomena,” and many of the chemists felt the same way about atomic theory.
Four years after Loschmidt published that bridge, Alexander Williamson addressed the London Chemical Society in his annual presidential speech:
That being said, Loschmidt really did pioneer a method which Einstein would later embrace, and by 1875 it was pretty difficult to deny, at least for chemists, the reality of molecules. Further, by 1901 (as evidenced by the Nobel Prize in chemistry that year), the reality of molecules was pretty firmly established by both physicists and chemists.
Now we get to Einstein. Between 1905 and 1910, a dozen methods of finding Avogadro’s constant had been established, half of which published by Einstein himself. Importantly, the different methods from disparate disciplines all yielded nearly the same number.
What Einstein had done, then, was not posit the reality of atoms and give possible physical explanations of their interactions, as the early chemists had done (but disagreed wildly upon); he’d made their reality irrefutable by showing their evidence in everything from brownian motion to electromagnetism. He drove the last nail in the coffin.
-scott
p.s. Chapter 5 of the Pais biography of Einstein handles this subject much more thoroughly.
@ Scott — Thanks for giving the story more context. My brief arguments for the chemists’ side of the story strongly emphasized one side (and I should add, gave no credit to the genuine subtlety of Mach’s position).
Looking back, it’s not surprising that, among chemists, “a considerable number of them [viewed the atomic theory] with mistrust, some with positive dislike.” This was surely more true of the more senior and prominent scientists, that is, the scientists who would naturally speak with authority, dominate the written record, and strongly influence our views of the time.
In general, what counts as scientific knowledge at a particular time often depends on which scientists one considers. The views of the eminent leaders of the day may differ from those of a large community of practice, and both will differ, at first, from the views of an expanding group that eventually becomes dominant for reasons that were, when examined, already persuasive. Discounting the latter group would, in my view, give too much weight to inertia, and too little to the state of available knowledge.
I would be interested to learn more about how young chemists viewed the reality of atoms in, say, 1899.
@Eric No problem, thanks for bringing the subject up. Not enough scientists really question their history. The state of “available knowledge” is a tricky question. Galileo certainly knew the earth revolved around the sun, much as Copernicus did, much as Aristarchus claimed to about 2,000 years before, but none of them had any physical proof. Galileo thought he did, but his argument was pretty flawed.
By the time “proof” came around, heliocentrism was already universally assumed, until people realized everything was relative and it didn’t matter anyway. Similarly, Ida Noddack correctly suggested nuclear fission in response to a Fermi experiment in 1934, however she had no theory to back her claim and her paper was largely ignored. It took Meitner & friends to accurately show nuclear fission in 1939.
In the life of an idea, wide acceptance can come well before or after proof of concept. Saying that nuclear fission was part of “available knowledge” in 1934 might be technically correct, but without the backing of the social infrastructure of science, it’s nothing more than empty words on a page. Einstein’s STR before ~1909 also falls in this category.
That being said, you’re very right to point out that, among chemists, atomic reality was pretty much assumed by 1899. Not universally, of course, and with very loud skeptics, but there was still a growing body of research around that assumption.
I don’t know much about the views of young chemists at the turn of the century, so I decided to do some journal paper title & abstract analysis for that period using the “Burst Detection Algorithm”. Basically, it detects periods of sharp increase in word usage. I stemmed words to improve results. All 2,000 articles analyzed dealt with atomic theory in some way.
Word|Weight|Start|End
theori|7.08|1831|1881
determin|11.07|1887|1918
potassium|5.69|1893|1907
mass|4.49|1893|1899
contain|5.3|1894|1914
stabil|6.24|1894|1896
base|5.96|1896|1896
alkaloid|6.13|1897|1909
asymmetr|4.36|1905|1914
solut|4.78|1905|1911
analysi|6.76|1907|1912
You’ll notice that, by 1893, chemists began writing papers about atomic mass very frequently. As the years progressed, the sorts of analyses they performed on these still-controversial atoms became increasingly complex. Obviously atomic theory had a firm foothold, regardless of whether it sat well with the majority.
Thanks again for the steady stream of interesting and informative posts.
Ack, I obviously formatted my link incorrectly, mind fixing it?
Done, and thanks for the deeper information.
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