Professor Einstein's Happiest Thought
One of the subjects that Einstein worked on during the miracle year had do with the following problem in chemistry. Suppose you dissolve a small amount of sugar in a solution of water. The sugar molecules will then diffuse through the water until they come into a state of equilibrium, or balance, with the water. The sugar and water will be uniformly mixed. The diffusion of the sugar molecules in the water will produce a pressure that can be measured by putting a thin sheet into the solution and letting the sugar molecules hit it as they diffuse.
Einstein produced a theory about this process that enabled him to determine both the size and the number of the sugar molecules in the solution. The number of molecules in what is known as the [mole] of a substance—an amount equal to the molecular weight of the substance in grams—is called Avogadro's number. This number is named for Amedeo Avogadro, an Italian scientist who first conjectured in 1811 that a given volume of any gas at a fixed temperature and pressure would contain the same number of molecules as the same volume of any other gas under the same conditions. Avogadro's number, usually simply called