Page:Popular Science Monthly Volume 49.djvu/802

778 Robert Boyle (1670), who found, by the use of his air pump, that if he deprived animals of air they died. He vivisected in this way kittens, birds, frogs, fish, snakes, and insects. Boyle also discovered that by keeping animals in a closed reservoir the air became unfit to sustain life.

Priestley, a century later (1772), continued Boyle's experiments by keeping mice in air-tight receivers until the air was vitiated and would no longer support life. He then tried to restore the air to its former condition: he rarefied and condensed it, heated it, exposed it to water and earth, and treated it in many other ways, each time testing it with living mice to ascertain whether it would again support life. All this was to no effect. In every case the mice died. Finally, he found that after plants grew for a while in the vitiated air, mice could again live in it. Thus was discovered the important relation between animal and vegetable respiration, and we now plant trees and lay out parks, and call them the "lungs of our cities." Two points must be emphasized here: first, that Priestley could not have done this with dead mice; and, second, that no one except Lawson Tait and Miss Cobbe would have the hardihood to claim that he ought to have used live men instead of live mice, on grounds of moral rights, and from the fact that the physiology of man is "so different" from the physiology of the mouse.

Turning to still another important line of scientific work, diseases of microbic origin are said to cause four fifths of the sickness in the world. As an example of researches in this field, we may cite the classical work of Edward Jenner.

Jenner began to study in earnest the disease cowpox, and its relation to smallpox, in 1775. For twenty-one years he patiently investigated the subject, and found that no one who had once suffered an attack of cowpox was taken with smallpox, although frequently exposed. "Legends of the dairymaids" had told for generations that an attack of cowpox conferred exemption from smallpox forever after. Jenner might have told the same story; but, if he had not proved the truth of his assertion by experiment, we might still have nothing but "legends of dairymaids" and no vaccination.

In May of 1796 Jenner began his experiments. He says (page 29): "The more accurately to observe the progress of the infection, I selected a healthy boy, about eight years of age, for the purpose of inoculation for the cowpox." This inoculation was followed by an attack of the disease. But Jenner does not