# Early history of the phase concept in the physical sciences

One of the first distinctions encountered in science education is that substances can assume different states of matter: for example, water can be found as a liquid, as a solid (ice), or as a gas (water vapor).

This concept gradually becomes more and more sophisticated: The state reflects the macroscopic variables of the problem (e.g. pressure, temperature), different substances have different phase diagrams, different phases can coexist under certain conditions, etc. It's thus not surprising that the phase concept is central to modern science and in particular our attempts to discover/create/understand novel forms of matter. (Most discussions of high-temperature superconductivity, for example, revolve around trying to obtain a theoretical understanding of the observed phase diagrams of various materials.)

For the purpose of this question, what I'm specifically interested in is the early history of the phase concept, and what references are available on the subject. For example, roughly when did human beings recognize that liquid water, solid ice, and water vapor could all be understood as different states of the same substance?

• The whole intellectual history seems too broad to me, I believe one could write a book about that. There will be especially large leap from the usual phases (solid, liquid, gas) to more exotic states (e.g., conducting/insulating phase in condensed matter physics). Could you maybe focus on some particular aspect of the whole thing? Oct 29 '14 at 14:28
• @OndřejČernotík: I was attempting to do so with the last sentence (indeed, that parting question was what got me thinking on this). But I can focus it more. Oct 29 '14 at 14:31
• @OndřejČernotík: I've edited the last paragraph and the title a bit to focus the question on the early history. Let me know if it's still too broad. Oct 29 '14 at 14:36
• It's just my opinion. Let's wait and see what others think. I'd certainly prefer current wording over the original one. Oct 29 '14 at 14:36

Presumably the recognition that water, ice and vapor are different states of the same substance goes back to prehistoric times: e.g. boiling water over a fire or observing the melting of ice.

The early history of the phase diagram in physics is connected to Gibbs, Maxwell and van der Waals: the van der Waals equation (1873) implies that there are coexisting liquid and gas phases below the critical temperature in real gases. Van der Waals received the 1910 Nobel Prize in physics for this work.

Maxwell, based on the work of Gibbs, constructed a three-dimensional plot of the variables volume, entropy and energy for a given pressure and temperature (1874). He did not exactly construct a phase diagram of water but "… modelled a fictitious substance, in which the volume is greater when solid than when liquid; and in which, as in water, the saturated vapour becomes superheated by compression."

• I wonder a bit about that first paragraph. Certainly observations like "if ice is exposed to warmth, then I'll see water" must go back quite far. But did they recognize them as yet the same substance? That seems like a greater conceptual hurdle. Oct 29 '14 at 19:50
• @Semiclassical I agree that 'substance' in the sense of a well-defined chemical composition is fairly recent. Perhaps 'matter' is the better term. The identification of water with 'two parts hydrogen, one part oxygen' dates from 1805 by J.L. Gay-Lussac and A. von Humboldt. Oct 29 '14 at 20:29
• It's that early conceptual murkiness that interests me, though. I also wonder about the experiments used to confirm it; for instance, evaporating a pot of water, capturing the steam, and watching it condense back into liquid. Similarly with ice: when they saw a lake freeze over and eventually thaw, was it recognized that the ice just a different form of the water? Though that history seems hard to pin down... Oct 29 '14 at 20:46
• @Semiclassical I can answer the first part of your question: distilled water is known at least since Alexander of Aphrodisias described the distillation process, around 200 AD. en.wikipedia.org/wiki/Distillation Oct 29 '14 at 21:06
• Ah, thanks. I imagine those 'technological' type of records are more relevant to this question than 'experimental' evidence. For instance, I think the practice of metallurgy and forging (with its melting and cooling of solid matter) probably would have given some impression, however confused, as to the state v. substance distinction. Oct 29 '14 at 21:11

The term phase' appears to have been coined by J. Willard Gibbs:

In considering the different homogeneous bodies which can be formed out of any set of component substances, it is convenient to have a term which shall refer solely to the composition and thermodynamic state of any such body without regard to its size or form. The word phase has been chosen for this purpose.’’

J. Willard Gibbs, `On the Equilibrium of Heterogeneous Substances,’’ Transactions of the Connecticut Academy of Arts and Sciences, 3, pp. 108-248 and 343-524.

Available on-line at https://archiv.ub.uni-heidelberg.de/volltextserver/13220/1/repertorium_gibbs2.pdf, where this statement appears on p. 306.