Persons in the enjoyment of complete health and vigor are frequently very sensitive to recognize the different agencies deleterious to their health. In the same way as animals they possess a certain instinct in this respect. In fact it is by this faculty alone that they can enjoy a normal and robust health, as they are thus enabled to avoid all kinds of dangers to their health, the integrity of their healthy mind also giving them the necessary will-power for this purpose. Against all sorts of dangers to their health such persons, as a rule, are prompt to act; thus, when they come into a close room the air will soon become offensive to them, and they will either leave or ask for the opening of a window or of a ventilator, closed through folly. On the other hand sickly persons, or people who do not otherwise enjoy perfect health or well-balanced minds, will remain in such a vicious air and contract all the dangers consequent to it, shortening their prospects for a healthy youth and long life. That living in the vitiated air of a close room is deleterious to health is proved by a simple observation of the faces of those stopping for a long time, or habitually doing so by reason of their professions, in close localities. They will present a pale, gray sickly appearance, and it is a fact that they very rapidly acquire all sorts of infections; especially is tuberculosis very prevalent in such cases. We observe precisely the same thing in the case of plants which, if kept in a close room, especially where little light reaches them, soon lose their color and are destroyed by parasites; and exactly the same happens in the case of man. Prisoners, unfortunate work-people, living and laboring in large numbers in small and close quarters, waiters and similar employees, are those in whom tuberculosis is most frequently found. In respect to workmen, this may be more truly the case in Europe, where they live under more miserable conditions, than in America, where their position is possibly the most enviable of all wage earners.