=Importance of Pure Air.=--Pure air means pure blood. The air of the mountain tops or by the sea fills us with life, while that of narrow streets, crowded rooms, unventilated dwellings, schools, churches, and theatres is depressing, weakening, and death-dealing. So far from the aristocracy having a monopoly of blue blood, it flows through the veins of high and low alike. It goes out from the lungs bright and rich with oxygen; it comes back to the heart dark with the waste and poisonous matters which it has gathered in its course. Atmospheric air is composed of several gases, the principal elements being oxygen, nitrogen, and watery vapor. All animal life requires oxygen for the combustion of the material supplied through the blood. The blood makes its circuit through the body three times a minute. It comes to the lungs laden with poisonous matter. Nearly one-third of the excretions of the body are eliminated through the lungs. The average adult contaminates about five thousand cubic inches of air with every breath. The importance of having an abundant supply of pure air at all times is obvious. In ordinary respiration an adult abstracts sixteen cubic feet of oxygen from the atmosphere every twenty-four hours, and adds to it fourteen cubic feet of carbonic acid in the same time. If the individual were confined in a close apartment, in which the air could not mingle with the atmosphere without, the processes of life could not long be maintained. History furnishes many instances of the direful effects of crowding a number of human beings into a limited space without ventilation. One hundred and fifty passengers were confined in the small cabin of a steamship one stormy night, and when morning came only eighty were found alive. Three hundred prisoners, after the battle of Austerlitz, were crowded into a close prison, and within a few hours two hundred and sixty of them had died. The effects of foul air are not usually so sudden nor so striking. More frequently they consist of a general deficiency of nutrition, loss of vigor of body and mind, and of the power of resistance to disease. Consumptive patients, in a large majority of cases, come from the classes whose occupations confine them to ill-ventilated rooms. A cramped position of the body while at work, and want of good wholesome food, contribute to the mortality from this cause. Absolutely pure air is rarely found in nature. Even in the open country there are three parts of carbonic acid in ten thousand parts of air. In cities and towns, the out-door air contains from four to five parts of carbonic acid. When, in dwellings and churches and halls, it reaches six to seven parts, its impurity is detected by the nose, the lungs suffer from a lack of oxygen, and the room feels close and stuffy.