There are some recent scientific discoveries that relate to impure air which may properly be introduced here. It is shown by the microscope that _fermentation_ is a process which generates extremely minute plants, that gradually increase till the whole mass is pervaded by this vegetation. The microscope also has revealed the fact that, in certain diseases, these microscopic plants are generated in the blood and other fluids of the body, in a mode similar to the ordinary process of fermentation. And, what is very curious, each of these peculiar diseases generates diverse kinds of plants. Thus, in the typhoid fever, the microscope reveals in the fluids of the patient a plant that resembles in form some kinds of sea-weed. In chills and fever, the microscopic plant has another form, and in small-pox still another. A work has recently been published in Europe, in which representations of these various microscopic plants generated in the fluids of the diseased persons are exhibited, enlarged several hundred times by the microscope. All diseases that exhibit these microscopic plants are classed together, and are called _Zymotic_, from a Greek word signifying _to ferment_. It is now regarded as probable that most of these diseases are generated by the microscopic plants which float in an impure or miasmatic atmosphere, and are taken into the blood by breathing. Recent scientific investigations in Great Britain and other countries prove that the _power of resisting_ these diseases depends upon the purity of the air which has been _habitually_ inspired. The human body gradually accommodates itself to unhealthful circumstances, so that people can live a long time in bad air. But the “reserve power” of the body—that is, the power of resisting disease—is under such circumstances gradually destroyed, and then an epidemic easily sweeps away those thus enfeebled. The plague of London, that destroyed thousands every day, came immediately after a long period of damp, warm days, when there was no wind to carry off the miasma thus generated; while the people, by long breathing of bad air, were all prepared, from having sunk into a low vitality, to fall before the pestilence. Multitudes of public documents show that the fatality of epidemics is always proportioned to the degree in which impure air has previously been respired. Sickness and death are therefore regulated by the degree in which air is kept pure, especially in case of diseases in which medical treatment is most uncertain, as in cholera and malignant fevers.