The air which surrounds us on every side, though invisible, is not inactive. To the casual observer its existence might be doubted if it did not manifest itself in the gentle breezes which bear the faded leaves of the widowed trees to their resting place; or in the wintry blast which uproots the sturdy oak of the forest, or lashes the white-crested billows of the ocean into a storm. In this way does the air we breathe make itself known to us. The composition of atmospheric air, the laws which govern it, as illustrating the theory of ventilation, the action of air on the animal economy, on the vegetable and mineral kingdoms, we must pass by. Suffice it to say that atmospheric air is a compound; that oxygen is the life-sustaining element; and that carbonic acid gas has an opposite effect; also the nitrogen; and that when we breathe we take in oxygen and give out carbonic acid gas. There is a law which governs cases called gaseous diffusion, by which gases pass equally into one another, and thus become equally diffused. If it were not for the operation of this law, there would be no variation of climate, no winds to waft the mariner over the briny deep, no provision for the scattering of the sun’s golden rays of light. The beauty of a summer’s sky would have been lost. No clouds would have hung around the portals of the setting sun tinged with crimson and with gold. No verdure in the meadow would have greeted our eye. No ark of hope would have spanned the clear vault of heaven. But, by the combination of oxygen and hydrogen, and the laws which govern them, provision is made for all these blessings. Light and heat are freely scattered. Animal and vegetable life rejoice. The priests of the morning chant their matins when the sun arises, and the flowers scatter their evening incense when the sun descends to glorify the western world. In a sanitary point of view the law of gaseous diffusion is of importance, because noxious gases are thereby diluted by the atmosphere, and are thus rendered comparatively harmless; hence, by free ventilation, we may do much to lessen the predisposition to epidemics, and the virulence of poisonous malaria.