Air is purest over the sea and over wind-swept heights of land. It, however, always contains some foreign substances, and always micro-organisms except over mid-ocean. Even the upper strata of atmosphere are not free from microscopic forms of life, as has been shown in experiments made with hail at the Johns Hopkins Hospital in 1890 by Dr. Abbott. Large hailstones were washed in distilled and sterilized water, and then melted, and cultures made from different layers; in all of these organisms were found, showing that they extend into the air a long distance from the earth.[12] Impurities of various kinds are constantly passing into the air, but so vast is the expanse of the atmosphere as compared with the impurities daily thrown into it from the lungs of man and the lower animals, from fires, manufactories, and decomposing matter, that they quickly disappear. Air is the greatest or, as one writer says, the most immediate necessity of life. We could live without it only a few seconds. We constantly use it, whether sleeping or waking, and perhaps this accounts in part for the utter carelessness and indifference which most people have for the quality of that which they breathe. Even those persons who know something of the nature of air, make but little effort to provide themselves with a constantly pure supply. =Effects of Breathing Bad Air.= If the effects of breathing bad air were immediate, there would then be an immediate remedy for the present total lack of any systematic means of ventilation in most houses. But the effects of breathing bad air are, like those of some slow and insidious poison, not noticeable at once, and often manifested under the name of some disease which gives no clue to the true cause. Dr. Van Rensselaer, in the Orton Prize Essay on Impure Air and Ventilation, makes the statement that statistics show that of the causes of mortality the most important and farthest-reaching is impure air. =Amount of Air Required for one Person.= Sanitarians have agreed that each individual requires at least 3000 cubic feet of air every hour. A room 10 × 15 × 20 holds 3000 cubic feet of air, which should be changed once every hour in order that one individual shall have the required amount. If three persons are in the room, it must be changed three times. The effect of bad ventilation is well illustrated by the condition of the horses in the French army some years ago. With small close stables the mortality was 197 in every 1000 annually. The simple enlargement of the stables, and consequent increase of breathing-space, reduced the number in the course of time to 68 in every 1000, and later, from 1862 to 1866, with some attention paid to the air-supply, the number fell to 28½ per 1000.[13]