No abstract

The aims of this article are to describe the operation of the drinking bird in quantitative terms, and to show the similarities between heat engines and the bird.

A demonstration that is often performed in a class (1,2) shows bending of a thin stream of water in an electric field. The static field is readily created by charging a comb, balloon, or plastic rod through friction. This experiment attracts the students' immediate attention. Like many striking demonstrations, the experiment illustrates an important property-the polar nature of water-but also raises many interesting questions for a thoughtful student. What is the strength of the electric field? Does the experiment work with every polar compound? Since nonpolar compounds acquire an induced dipole moment in an electric field, do they also deflect in an electric field? The statement found in most textbooks, that polar molecules do and nonpolar molecules do not, is too simplistic; it misses the most interesting aspects of the demonstration. We give a more complete explanation of this phenomenon here. We will also describe a simple experiment that answers the questions raised above.

No abstract

Two different models for chemical bond were developed almost simultaneously after the Schrödinger formulation of quantum theory. These are known as the valence bond (VB) and molecular orbital (MO) theories. Initially chemists preferred the VB theory and ignored the MO theory. Now the VB theory is almost dropped out of currency. The context of discovery and Linus Pauling's overpowering influence gave the VB theory its initial advantage. The current universal acceptance of the MO theory is due to its ability to provide direct interpretation of many different types of experiments now being pursued. In current research both localized bonds and delocalized charge distributions play important roles and the MO theory has been successful in giving a good account of both.Keywords Chemical bond Á Linus Pauling Á R. S. Mulliken Á Valence bond theory Á Molecular orbital theory How central is the concept of a bond between atoms to chemical theory and praxis? The phrase ''nature of the chemical bond,'' popularized by Pauling and emphasized in the 1954 Nobel award leads one to think that the idea of bonding is the very knot that holds the strands of chemistry together. Since Pauling's original work the character of chemical research has changed dramatically with revolutions in spectroscopic and computational procedures. Hence, it is worth examining the ''nature of the chemical bond'' in the present context.Following the advent of quantum mechanics, two different models for the chemical bond were developed. These are known as the valence bond (VB) and the molecular orbital (MO) theories. Chemists with a strong background in quantum mechanics knew from the beginning that both theories embody serious approximations to the correct Schrödinger equation. Since the two theories differ only in the set of approximations in generating the wave function, they may be thought of as two different models rather than theories. To be consistent with popular usage, however, I will refer to them as theories. It should be