I'm writing a fictional story with early industrial age technology where a chemist discovers not only that diamond and graphite are made of the same substance, but that they differ by crystalline structure (cubic vs hexagonal). However, I'd like this to be a realistic depiction if possible and the chemist does not have access to electron tunneling microscopy or advanced spectrographic methods. The farthest spectroscopy has gotten is Fraunhofer's spectroscope and John Herschel's evaporogram, with visible light and the beginnings of an understanding of infrared and ultraviolet. Would 1850's level spectroscopy or other technology of the time be able to demonstrate the crystal structures of carbon (specifically graphite and diamond) without a 1900's understanding of atomic structure?
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1$\begingroup$ Interesting question. X-rays were not identified/human made until 1895 (Rontgen), and x-ray diffraction was not shown until 1912 (Laue). The physics understanding and technological requirements to harness x-rays seems too far fetched (those were a busy 50 odd years in physics). Visible spectroscopy just isn't going to cut it. $\endgroup$– Jon CusterJul 7, 2015 at 15:19
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$\begingroup$ Right, but early infra-red spectroscopy does exist (the evaporogram) which uses the heat differences of the refraction bands to produce an image. Would that be enough? $\endgroup$– hatch22Jul 7, 2015 at 15:21
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$\begingroup$ Optical and infra-red techniques will show that crystals are different, and certainly can help sort crystals into symmetry groups. They cannot yield actual atomic positions in a lattice. Now, clearly diamond and graphite are different symmetry groups (from bulk crystal observations). To show they are the same substance, one could, say, burn them in a closed container and see that the resulting gas has the same properties, optical or otherwise? $\endgroup$– Jon CusterJul 7, 2015 at 15:37
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$\begingroup$ Sure, in fact I was planning on the discovery that they are the same substance would come first, and them because of their obvious differences (brittleness, different color, etc) would lead to the question: "If they are the same substance, why do they have different properties?" Would the optical data be enough to show that one is hexagonal and one is cubic (at least roughly) without needing lattice positions? $\endgroup$– hatch22Jul 7, 2015 at 15:48
1 Answer
Would 1850's level spectroscopy or other technology of the time be able to demonstrate the crystal structures of carbon (specifically graphite and diamond) without a 1900's understanding of atomic structure?
Yes, using visible light and petrographic microscopes. The field of optical mineralogy was mature enough by the mid-1800's for one to determine the crystal structure of minerals. In particular, the work of Sir William Nicol is worth investigating (thin but useful Wikipedia article).
I will add that there's a pretty simple way to determine whether or not the crystal structure of a mineral is the same or different than that of another mineral - the so-called crystal habit.
Finally, diamond exhibits conchoidal fracture, and graphite does not. This might be the easiest way to tell that the two minerals have different crystal structures.
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$\begingroup$ This is a promising avenue. I had no idea this method existed. Do you think the basic hexagonal/cubic distinction would be visible at 1850's level microscopic examination (e.g. via light transference of a very thin sample)? $\endgroup$– hatch22Jul 7, 2015 at 15:50
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1$\begingroup$ I am comfortable stating that one could tell that diamond and graphite were different crystal structures at that time, but as to the assignment to specific crystal systems, I believe that was not something that was possible until (at least) 1913, when Bragg and Bragg published their work. $\endgroup$ Jul 7, 2015 at 16:00