HYDROGEN BONDS IN WATER HAVE COVALENT PROPERTIES, new experiments have shown directly for the first time, confirming a controversial prediction of great importance to understanding water and the many other structures such as DNA which contain hydrogen bonds. Within a single water molecule, hydrogen and oxygen are held together by "sigma" bonds, which are strongly covalent, meaning that electrons are shared between atoms. Between groups of water molecules, however, are much weaker "hydrogen bonds." These bonds are principally electrostatic attractions between positively charged hydrogen--which readily gives up its electron in water--and negatively charged oxygen-- which receives these electrons--in a neighboring molecule. In the 1930s, after quantum mechanics had forever changed the world view of physics, famous chemist and Nobel Laureate Linus Pauling proposed that electron clouds associated with hydrogen and sigma bonds would somewhat overlap with one another, affecting each other's properties. However, the extent of this effect has been contentious and experimentally untested--until now. Shining intense synchrotron x rays on a single crystal of ice from several different directions, and plotting the energy spectrum of the scattered x rays, a US-France-Canada team (Eric Isaacs, Lucent Technologies) observed wavelike interference fringes. The presence of these fringes means that the electrons participating in the hydrogen bond are at least in part quantum mechanically shared (covalently) between neighbors just as Pauling had predicted. Since hydrogen bonds play a significant role in determining water's properties, this experiment is likely to shed light on the mysteries of water (such as the fact that water expands upon freezing) which have been so important to the advent and evolution of life on this planet. (Isaacs et al., Physical Review Letters, 18 January 1999; additional information at www.aip.org/physnews/preview/1999/h-bond/h-bond.htm)