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Notes from some long forgotten source. "Chaos (χαος) was used to define the most disperse and fluid state of matter, that in which no particular order could be observed. Interestingly enough, when van Helmont wanted to refer to steam-state of materials he was studying used the term chaos but in the particular Flemish accent, converting it to gas, by which the state is known to us today." To my mind it appears that the difficulty is only imaginary and not real. Rigidity and plasticity are not absolute terms but relative, and all solids are in fact both rigid and plastic. The apparent contrast between the two properties belongs to the laboratory and to those phenomena of nature involving small masses and small forces. When great masses and great forces are involved, as for example in the making of continents and mountain chains, the distinction loses value. The phenomena of mountain structure demonstrate that under sufficient strains great bodies of rock both bend and flow. Increased pressure increases the range of temperatures over which a substance can exist as a liquid. Reduced pressure reduces this range. At a certain special pressure the boiling and melting points will equal, and the substance can no longer exist as a liquid. Below this pressure, the only possible phase transition is from solid to gas (and vice versa). This phase change is called sublimation (the reverse process is called deposition or desublimation) and the temperature at which it occurs is called the sublimation point (or sublimation temperature). That's the essence of the upcoming discussion. If this is enough info for you, stop reading and jump to the next section. If you want to understand what I'm talking about then keep reading. Knowing why some phenomenon occurs is often more important than knowing that it occurs. (Of course, the reverse is also true, which is why I offer you the option to read on or jump ahead.) To a certain extent, liquids are like a minimum security prison. (Solids are like a maximum security prison in permanent lock down, but that's another matter.) The molecules within have limited freedom and can only leave infrequently or with great effort. As long as a liquid has some surface area exposed to the atmosphere, here and there a molecule within the liquid near the surface will be moving fast enough to escape the liquid prison and enjoy the freedom of a vapor molecule in the surrounding atmosphere. But rather unlike a a prison, the reverse process is also possible. From time to time, a molecule in the atmosphere will be traveling fast enough to plow its way through the tightly guarded walls of the liquid only to find itself trapped within. Both events are happening simultaneously, but not necessarily with equal probability. What is boiling and how is it different from evaporation? Both processes involve the same liquid to gas phase transition, but where evaporation can occur at any temperature boiling occurs only at a specific temperature. Let's return to the description of evaporation just discussed. Evaporation occurs whenever more molecules leave a liquid than enter. Condensation occurs whenever more enter than leave. These changes are driven by the concentration of liquid molecules in the atmosphere. When their concentration is low, it's more likely that molecules will leave the liquid phase than enter it, so evaporation rules. When their concentration is high, it's more likely that molecules will enter the liquid phase than leave it. When neither process dominates it must be because the atmosphere has just the right concentration of liquid molecules floating around within it — no more, no less than what it can handle. Under these circumstances the atmosphere is said to be saturated. The most energetic vapor molecules present in the atmosphere are fighting their way into the liquid. The most energetic liquid molecules are fighting their way out into the atmosphere. There's room in the atmosphere, but it has a limit. When