_Dry Heat._--Dry heat is not nearly so effective as moist heat as a sterilizing agent. The temperature must be higher and continued longer to accomplish the same result. Thus a dry heat of 150° for thirty minutes is no more efficient than steam under pressure at 115° for fifteen minutes. Various forms of hot-air sterilizers are made for laboratory purposes (Fig. 77). On account of the greater length of time required for sterilization their use is more and more restricted to objects which must be used dry, as in blood and serum work, for example. In practice the use of hot air in disinfecting plants is now largely restricted to objects which might be injured by steam, as leather goods, furs, and certain articles of furniture, but even here chemical agents are more frequently used. _Moist Heat._--Moist heat may be applied either by boiling in water or by the use of steam at air pressure, or, for rapid work and on substances that would not be injured, by steam under pressure. Boiling is perhaps the best household method for disinfecting all material which can be so treated. The method is simple, can always be made use of, and is universally understood. It must be remembered that all pathogenic organisms, even their spores, are destroyed by a few minutes’ boiling. The process may be applied to more resistant organisms, such as are met with in canning vegetables, though the boiling must be continued for several hours, or what is better, repeated on several different days. This latter process, known as “_discontinuous sterilization_,” or “_tyndallization_,” must also be applied to substances which would be injured or changed in composition by too long-continued heating, such as gelatin, milk, and certain sugars. In the laboratory such materials are boiled or subjected to steaming steam for half an hour on each of three successive days. In canning vegetables the boiling should be from one to two hours each day. The principle involved is that the first boiling destroys the growing cells, but not all spores. Some of the latter germinate by the next day and are then killed by the second boiling and the remainder develop and are killed on the third day. Occasionally a fourth boiling is necessary. It is also true that repeated heating and cooling is more destructive to bacteria than continuous heating for the same length of time, but the development of the spores is the more important factor. Discontinuous heating may also be used at temperatures below the boiling-point for the sterilization of fluids like blood serum which would be coagulated by boiling. In this case the material is heated at 55° to 56° for one hour, but on each of seven to ten successive days. The intermittent heating and cooling is of the same importance as the development of the spores in this case. (Better results are secured with such substances by collecting them aseptically in the first place.) [Illustration: FIG. 78.--The Arnold steam sterilizer for laboratory use.]