FOG IN THE SCOTTISH HIGHLANDS

Not only conductivity can be super. So can heated and cooled.

Water should turn to ice at 0 degrees C and to vapor (steam) at 100 degrees C. This is known to all who have studied elementary physics, but not to all water molecules. The temperature alone does not define the state (solid, liquid or gas) of a compound like water; pressure, the presence of drops or particles, and geometry are also factors. For example, the "saturated" pressure when water vapor should con-dense into liquid is precisely known when the vapor sits above a flat and continuous water surface. But when it fills the space be-tween tiny water droplets it becomes a headache to meteorolo-gists, cloud physicists, rain makers and others who deplore the fact that water knows so little physics.

A liquid above the temperature of its official boiling point is called superheated, and a gas below it is supercooled (with analogous expressions near the melting point). By expanding a volume of water vapor in a chamber with a piston one can super-cool (or "supersaturate") the vapor. But as soon as a "condensa-tion nucleus" appears in the vapor, the vapor near it will condense to water droplets. If, for example, an elementary particle such as an electron shoots through the space, it shock-ionizes the atoms it bumps into, and they become such points on which the vapor con-denses. Thus the path of the elementary particle becomes visible through a window in the chamber wall.

That is the principle of the cloud chamber, developed in 1911 by Charles Thomas R. Wilson (1869-1959), a Scotsman who had ob-served the early morning cloud effects in the valleys of the Scottish Highlands from the Ben Nevis observatory above them. The Wil-son cloud chamber was a marvel of physics in its day. It was first used to make visible the alpha and beta particles shooting out from radioactive sources, but later led to the discovery of new particles and the study of their behavior. The flourishing of nuclear physics in later decades could never have taken place without it. Wilson was awarded the Nobel Prize in 1927 for this and other discoveries. (He also noticed that the electric charge of the leaves of a gold-leaf electroscope would mysteriously disappear, and correctly specu-lated that an extraterrestrial source of radiation was responsible, thus anticipating cosmic rays.)