That dust is an important factor in the causation of colds may be inferred from the facts that they are more common in the city than in the country, and that in the city they are more frequent in the spring and fall, when the streets are not watered. =The Rôle Played by Bacteria.=—Bacteria are distributed nearly everywhere and in larger quantities than is generally believed. The air in open spaces in cities contains from 100 to 1000 bacteria per cubic meter, while the air of an inhabited room contains from 6000 to 10,000. It can now be definitely stated that microörganisms are the immediate or exciting cause of bronchitis observed in diphtheria, in influenza, measles, whooping-cough, pneumonia, etc. These microörganisms are conveyed both directly from the sick to the well, and from the inhalations of the germs floating in the atmosphere. Whenever dust is raised, we breathe in a great number of microörganisms. In influenza the bacilli are found in the secretions of the nose, throat, and in the expectorations from the lungs. The bacilli are not only the cause of the acute infections, but also of chronic bronchitis. The bacterial flora usually present in the throat and the respiratory passages is rich and varied. So long as the mucous membrane lining these passages remains in a healthy condition, an unfavorable condition is offered for their growth and development and these microörganisms are harmless. But just so soon as the general vitality is lowered, or there is an impairment of the normal condition of the epithelium lining the respiratory tract, a culture-medium is provided in which these germs flourish and grow. Anything which will cause an irritation or congestion of the mucous membrane of the throat and bronchial tubes furnishes the necessary conditions for the infection to take place. The germs themselves excite an acute inflammation, and the inflammation extends from the head or throat to the bronchial tubes, through the spread of the infective agent along the respiratory tract. =Ventilation.=—Very few people in cities spend more than one hour a day in the open air, which means that they are housed up for the other twenty-three hours, so that no pains should be spared to bring up the quality of the indoor air to approximate as nearly as possible that of the outdoor air. The air of houses contains many more microbes than that of the street. For dwelling-houses 3000 cubic feet of fresh air is needed every hour. It is said that in the country the only bad air is in farmers’ houses, whence it has no chance to escape. Direct sunlight kills the tubercle bacillus in thin layers of sputum in five or six hours, and diffused sunlight in several days, and proper ventilation greatly facilitates this bactericidal action. A large cubic space is of little avail if the ventilation is inadequate. The windows should be at least one-seventh of the floor space.