_The Chemical Properties of Air._—It is of more vital importance that the air which we breathe should be pure than the food which we eat should be, although the latter is universally conceded to be a matter of prime importance. The reason is that the poisons in the air, inspired by the lungs, pass directly into the blood, whereas, taken into the stomach, the action is much slower, and there is at least the possibility of their passing through the digestive tract unassimilated. The olfactory nerves are the normal guides as to the purity of the air, and, if they have not been dulled by long usage in breathing impure air, they are extremely sensitive to impurities in the atmosphere. _Country Air._—In the open air there is a constant, even though insensible, movement of the currents of air; the result is a constant renewal or ventilation of the air. There is, in addition, the evaporation from brooks, rivers, lakes, the dew and rain, which aid in cleansing the air from dust; the peculiar freshness of the air in the country after a heavy fall of rain is familiar to all. The “bouquet” of the air, most noticeable and delightful in the early morning, especially in the spring of the year, is due to the fragrance given off from the flowers, plants, and trees, and imparts a feeling of exhilaration and a sense of the joy of living. Sunshine increases the respiratory movements. Wind clears the air of impurities, and is only harmful when it carries dust with it, or when it is so strong that it impedes the respiration or bodily movements. Very weak persons get out of breath easily when battling against the wind. _Town Air._—Even the outdoor air of towns has its full quota of oxygen,—21 per cent.,—and so is healthier than indoor air. The carbonic acid in the air varies from 0.2 to 0.6 per cent. Among the impurities of the air are smoke, fog, and dust. =Dust and Its Relation to Disease.=—Dust, consisting of particles of all kinds of organic and inorganic matter, is a nuisance of indoor as well as outdoor life. That the dust in cities is the intolerable nuisance and menace to public health that it is, is due to the filthy condition of the streets. The specific cause of tuberculosis is the tubercle bacillus. Considering the prevalence of the custom of spitting on the pavements, streets, floors of public conveyances, and public halls, that the dust from the streets is carried into the houses on the shoes and the trailing skirts of women, it is self-evident that anything which stirs up the dust, as sweeping, stamping on the floor with the feet, dancing, and on the streets strong currents of air and high winds are a most serious menace to the health and lives of the community. In addition to the fact that dust is the great carrier of the tubercle bacillus, the particles of dust cause a direct irritation of the mucous membranes lining the nose, throat, larynx, and bronchial tubes.