In the first place, in order to learn what we may do to lengthen the span of life we must learn something of the nature of disease. Doctors tell us that diseases are of two classes. The first are hereditary, or inherited; those which pass from parents to their children and often run through an entire family. It is more often the _tendency_ to disease that is inherited, rather than the disease itself, and so even these inherited diseases may often be prevented by careful living. Diseases which may be inherited include rheumatism, gout, scrofula, diabetes, cancer and insanity. This class of diseases is the most difficult to prevent and to cure. For some of them no cure has been found. The other class comprises the diseases of environment, or personal surroundings,--that is, our manner of living both as regards our private life and our relations to other people. These diseases are largely preventable and it is with them that most of the work of prevention is to be carried on. A disease is considered preventable if, by using the best known means of treatment, it might be prevented or cured, so that either the disease or the death usually resulting from it would be avoided. Of course, not all deaths from a given disease could be prevented even with the best known means. Infant diseases constitute one class which is considered most hopeful of betterment through a pure milk supply and better hygiene; and yet many authorities believe that not more than half the deaths could be prevented owing to the large part played by weather conditions, feeble constitutions, and other unchangeable conditions. Preventable diseases may be divided into six classes: (1) Diseases caused by lack of proper hygiene. (2) Diseases caused by bad habits. (3) Contagious diseases. (4) Diseases caused by insects. (5) Accidents, wounds, or operations and their resulting diseases. (6) Diseases remedied by slight means. We will treat each of these in turn. (1) By proper hygiene is meant the proper treatment of the body as to breathing, eating, drinking, sleeping, bathing and rest. This treatment includes plenty of fresh air, both day and night, keeping outdoors as much as possible, and in well-aired houses the rest of the time. Vigorous but not violent exercise, brisk walking, regular physical exercise, such as is practised in gymnasiums, will go far toward keeping the body in good condition. The question of fresh air in the home is one of the most important points to be considered. The bedrooms, the living-rooms, and the kitchen should have the air changed constantly, not once or twice a day. In order to prevent drafts, and that the house may not be kept at too low a temperature in winter, a board, eight to twelve inches in height, may be placed across the bottom of a window that is raised. Many diseases, not only of the throat and lungs, but of the other organs, may be prevented by the constant introduction of fresh air into our rooms day and night.