While it is important for every hospital to know something of the quality of the food and water supplied to its inmates, it is of still greater importance to know that in every department of the hospital there is provision for the entrance of pure air and the exit of impure air. We may live without food for days or weeks, but the breath of life is the first great essential of existence. Deprived of it and death results. Poison it and deterioration in health results. “Bountiful nature has supplied an inexhaustible supply of this essential, and the means for its purification. Only when man’s ingenuity or stupidity thwarts her efforts, confines the air, prevents its free circulation, does it retain the poison he has contributed to it.” A large part of the science of sanitation can be summed up in one word: cleanliness; clean air, clean food, clean water, clean rooms, clean beds and bed linen, clean occupants of the beds, and clean appliances. [Sidenote: Dirt] Dirt has been defined as “matter out of place.” To have the ability to recognize when matter becomes dirt, and get it into its proper place and proper form, is to have the key to hospital sanitation. It means a strict attention to what elsewhere might be considered small details. It is said that under the microscope the dust of an ordinary house resolves itself into soot, minute particles of cotton and wool fibres, spores of bacteria, starch grains, pulverized straw, epithelial and epidermic debris, and fragments of food. It consists to a great extent of organic substance, capable of decomposition. When added to this we have some of the constituents of dust common to a hospital, such as shreds of linen or wool from soiled bedding, pieces of hair, dried particles of pus, blood, and human tissue, crystals of urine, etc., the consideration of how to get dust into its proper place and form, how to deal with it, so that it is not a menace to both the healthy and sick inmates, is no small or unimportant part of hospital management. [Sidenote: Flies] Flies—the common, innocent-looking house flies, that formerly were regarded simply as annoyances to be endured, have been found to be among the most active agents in the spread of disease. Now that the germ theory is so well understood, it is not difficult to see how this takes place. Victor Vaughan, a member of the Army Medical Commission during the Spanish-American war, states as follows his observations regarding the fly as an agent in spreading typhoid fever in the camps: “They swarmed over fecal matter in the latrines. They visited and fed on food prepared for soldiers in the mess tents. In some instances when lime had been spread over the contents of latrines, flies, with their feet white with lime, were seen walking over the food. Officers whose mess tents were protected by means of screens suffered less proportionately than those whose tents were not so protected.”