The ventilating air-current is so adjusted that the air which leaves the chamber is caused to pass through purifiers, where the water-vapor and the carbon dioxide are removed, and then, after being replenished with fresh oxygen, it is returned to the chamber ready for use. The general scheme of the respiration apparatus is shown in fig. 27. The air leaving the chamber contains carbon dioxide and water-vapor and the original amount of nitrogen and is somewhat deficient in oxygen. In order to purify the air it must be passed through absorbents for carbonic acid and water-vapor and hence some pressure is necessary to force the gas through these purifying vessels. This pressure is obtained by a small positive rotary blower, which has been described previously in detail.[18] The air is thus forced successively through sulphuric acid, soda or potash-lime, and again sulphuric acid. Finally it is directed back to the respiration chamber free from carbon dioxide and water and deficient in oxygen. Pure oxygen is admitted to the chamber to make up the deficiency, and the air thus regenerated is breathed again by the subject. BLOWER. The rotary blower used in these experiments for maintaining the ventilating current of air has given the greatest satisfaction. It is a so-called positive blower and capable of producing at the outlet considerable pressure and at the inlet a vacuum of several inches of mercury. At a speed of 230 revolutions per minute it delivers the air at a pressure of 43 millimeters of mercury, forcing it through the purifying vessels at the rate of 75 liters per minute. This rate of ventilation has been established as being satisfactory for all experiments and is constant. Under the pressure of 43 millimeters of mercury there are possibilities of leakage of air from the blower connections and hence, to note this immediately, the blower system is immersed in a tank filled with heavy lubricating oil. The connections are so well made, however, that leakage rarely occurs, and, when it does, a slight tightening of the stuffing-box on the shaft makes the apparatus tight again. ABSORBERS FOR WATER-VAPOR. To absorb 25 to 40 grams of water-vapor in an hour from a current of air moving at the rate of 75 liters per minute and leaving the air essentially dry under these conditions has been met by the apparatus herewith described. The earlier attempts to secure this result involved the use of enameled-iron soup-stock pots, fitted with special enameled-iron covers and closed with rubber gaskets. For the preliminary experimenting and for a few experiments with man these proved satisfactory, but in spite of their resistance to the action of sulphuric acid, it was found that they were not as desirable as they should be for continued experimenting from year to year. Recourse was then had to a special form of chemical pottery, glazed, and a type that usually gives excellent satisfaction in manufacturing concerns was used.