THE NEED OF PURE AIR.--Outdoor life has been so urgently advocated of late that the public has come to appreciate its benefits almost as fully as do physicians. The existence of pregnancy does not lessen, but rather enhances, the value of fresh air; in order to enjoy the best health during this period one should spend at least two hours out of doors every day. Neither the season of the year nor the state of weather should modify this obligation. If the sun is shining the "airing" is more delightful, but it should be taken in bad weather also, on a protected porch or in a room with the windows wide open. Even when the injunction to be regularly out of doors is observed women are accustomed to spend the greater portion of the day in the house, and on that account special attention must be given to keeping the air of the house pure. Ventilation takes care of itself in summer, when the windows are open, but in cold weather, when in our anxiety to keep the temperature comfortable we may overlook the need of fresh air, it demands close attention. The necessity of ventilation at all times is due, of course, to the composition of the atmosphere and to the changes produced in it as we breathe. The air about us is a mixture of gases, of which oxygen and nitrogen are the most important. Although nitrogen, which constitutes four- fifths of the atmosphere, is taken into our lungs in breathing, we make no use of it, but breathe it out in precisely the same condition as we take it in. As chemically combined in the food-stuff known as protein, nitrogen is indispensable to animal life; but our bodies make no use of the gaseous form of nitrogen. Oxygen, on the other hand, supports life; and though it forms less than one-fifth of the atmospheric air, it is present in ample amount for our needs. After we draw air into our lungs, the oxygen it contains is absorbed by the blood and used by the tissues. In return our tissues give up a waste product, carbonic acid gas, which is thrown off by the lungs. It is interesting to observe that the carbonic acid gas which animals exhale supports the life of plants, and that the plants, under the influence of sunlight, give back pure oxygen to the atmosphere. Obviously, the complementary relation exhibited here is of mutual benefit.