The dressings arranged in the large tray are placed in the sterilizer and the supply of steam is maintained through the constant and steady flow of water from the reservoir, which compensates the evaporation in the sterilizer. In about twenty minutes the formation of steam in the top of the reservoir exerts sufficient pressure to force all the boiling water from the reservoir into the sterilizer to the depth of about 1½ inches. The tray of instruments is now inserted and the process continued for another ten minutes. Much less heat is required with this apparatus than with those of ordinary type, while sterilization can be continued uninterruptedly for one and one half hours, if need be. =Sprague Sterilizer.=—The most perfect sterilizer is that of the Sprague type, in which a dry chamber is surrounded by steam under pressure. The apparatus is shown in Fig. 15. [Illustration: FIG. 15.—SPRAGUE TYPE OF STERILIZER.] Its cylindrical chamber is surrounded by two heavy copper shells, the space between which is occupied by the water. This compartment is entirely shut off from the sterilizing chamber, and as the steam is generated, the inner, or sterilizing, chamber becomes heated to a degree nearly equal to that of the steam in the surrounding cylinder; this prevents any condensation of steam taking place in the dressings. By opening the lever-handled valve at the bottom of the sterilizer in the rear, and the valve to the right, on top of the sterilizer, and allowing them to remain open for a space of four or five minutes, a vacuum is formed in the sterilizing chamber. These two valves are then closed, the lower one first, and the steam from the outer cylinder is allowed to enter the chamber, by opening the left valve on top. The contents should be allowed to sterilize for twenty or twenty-five minutes under a pressure of fifteen pounds. Then close the steam-supply valve; open the vacuum valve (right) and the lever-handled valve at the bottom; leave these open about the same time as in creating a vacuum at the beginning of the process; close both valves, then open the air-filter valve on the door, in order to break the vacuum; the door can then be opened and the dressings be taken out dry and absolutely sterile.