C. T. R. Wilson, inventor of the cloud chamber, claimed that the earliest ideas for this device came to him in 1894. While still a young student, he stood at the summit of Ben Novis, highest peak in Scotland, and was amazed by the optical phenomena produced when sun shone through the clouds. He resolved to build a "cloud machine" of his own in the laboratory and to further study these phenomena. The principle for building such a machine was well known at the time. Imagine an air-tight container filled with a piston that can be moved up and down. Then suppose that the air inside this container is saturated with some vapor, such as that of water or alcohol. Then, if the piston is suddenly withdrawn, the air inside the container expands and is cooled. The cooler air is able to hold less vapor than could...