[Illustration: FIG. 79.--Vertical gas-heated laboratory autoclave.] [Illustration: FIG. 80.--Horizontal gas-heated laboratory autoclave.] _Steam._--Steam is one of the most commonly employed agents for sterilization and disinfection. It is used either as “streaming steam” at air pressure or confined under pressure so that the temperature is raised. For almost all purposes where boiling is applicable streaming steam may be substituted. It is just as efficient and frequently more easily applied. The principle of the numerous forms of “steam sterilizers” (Fig. 78) is essentially the same. There is a receptacle for a relatively small quantity of water and means for conducting the steam generated by boiling this water to the objects to be treated, which are usually placed immediately above the water. Surgical instruments may be most conveniently sterilized by boiling or by steaming in especially constructed instrument sterilizers. If boiled, the addition of carbonate of soda, about 1 per cent., usually prevents injury. [Illustration: FIG. 81.--A battery of two horizontal autoclaves in one of the author’s student laboratories. Steam is furnished direct from the University central heating plant.] _Steam under pressure_ affords a much more rapid and certain method of destroying organisms. Fifteen to twenty pounds pressure corresponding to temperatures of 121° to 125° is commonly used. Variations depend on the bulk and nature of the material. Apparatus for this purpose may now be obtained from sizes as small as one or two gallons up to huge structures which will take one or two truckloads of material (Figs. 79-91). The latter type is in common use in canning factories, dairy plants, hospitals, public institutions, municipal and governmental disinfecting stations. Very frequently there is an apparatus attached for producing a vacuum, both to exhaust the air before sterilizing, so that the steam penetrates much more quickly and thoroughly and for removing the vapor after sterilizing, thus hastening the drying out of the material disinfected. [Illustration: FIG. 82.--A “process kettle” (steam-pressure sterilizer) used in canning. Diameter, 40 inches; height, 72 inches.] The smaller types of pressure sterilizers are called “autoclaves” and have become indispensable in laboratory work. Fifteen pounds pressure maintained for fifteen minutes is commonly sufficient for a few small objects. For larger masses much longer time is needed. The author found that in an autoclave of the type shown in Fig. 81 it required ten minutes for 500 cc. of water at 15 pounds pressure to reach a temperature of 100°, starting at room temperatures, 20° to 25°. Autoclaves may be used as simple steam sterilizers by leaving the escape valves open so that the steam is not confined, hence they have largely replaced the latter.[18] [Illustration: FIG. 83.--Horizontal steam chest used in canning. Height, 32 inches; width, 28 inches; length, 10 feet.]