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Virtually every school child is familiar with the story of Edward Jenner (1749-1823) and his conquest of smallpox. Jenner, who practised medicine in the English countryside, which was continually ravaged by smallpox, observed that milkmaids and cowmen did not fall victim to the disease. Furthermore, he established that these individuals, in the course of their work, had all suffered from cowpox, a relatively mild, self-limited disorder. Based on these epidemiologic observations, Jenner concluded that cowpox somehow prevented the more serious disease, smallpox. He then set about to test his hypothesis by inoculating lymph derived from a dairymaid into a boy; 1 month later he inoculated the boy with smallpox. The child did not develop smallpox. Thus began the science of immunology.
Somewhat less familiar perhaps is the story of John Tyndall, the 19th century British physicist. In the course of experiments designed to investigate why the sky is blue, Tyndall demonstrated that a beam of light can be perceived only when the light waves strike particles in their pathway and are scattered. This phenomenon accounts for the fact that the light beam from a movie projector is visible in a dark theatre because it hits myriads of dust particles in the air and for the fact that the beam of an automobile headlight is usually visible only in foggy conditions when particles of moisture fill the air. A corollary of this concept is that air containing only pure gases and no particles behaves like a vacuum (such as outer space) and remains pitch black when traversed by a beam of light.
Soon after making his basic observations on the physics of light, Tyndall applied his discovery to prove that Louis Pasteur was correct in disclaiming the phenomenon of spontaneous generation. Pasteur claimed that germs did not generate spontaneously, although they could appear in liquid solutions in which...