_Asepsis_ is a term which is restricted almost entirely to surgical operations and implies the taking of such precautions that foreign organisms are _kept out_ of the field of operation. Such an operation is an _aseptic_ one, or performed _aseptically_. A “deodorant or deodorizer” is used to destroy or remove an odor and does not necessarily have either antiseptic or disinfectant properties. The agents which are used for the above-described processes may be conveniently divided into _physical agents_ and _chemical agents_. PHYSICAL AGENTS. =1. Drying.=--This is doubtless the oldest method for _preventing the growth_ of organisms, and the one which is used on the greatest amount of material at the present time. A very large percentage of commercial products is preserved and transported intact because the substances are kept free from moisture. In the laboratory many materials which are used as food for bacteria (see Chapter XVI) “keep” because they are dry. Nevertheless, drying should be considered as an _antiseptic_ rather than as a _disinfectant_ process. While it is true that the _complete_ removal of water would result in the death of all organisms this necessitates a high temperature, in itself destructive, and does not occur in practice. Further, though many pathogenic bacteria are killed by drying, many more, including the spore formers, are not. Hence drying alone is not a practical method of _disinfecting_. [Illustration: FIG. 77.--A small laboratory hot-air sterilizer.] =2. Heat.=--The use of heat in some form is one of the very best means for destroying bacteria. It may be made use of by combustion, or burning, as direct exposure to the open flame, as dry heat (hot air), or as moist heat (boiling water or steam). Very frequently in veterinary practice, especially in the country, occasionally under other conditions, the infected material is best burned. This method is thoroughly effective and frequently the cheapest in the end. Wherever there are no valid objections it should be used. Exposure to the open flame is largely a laboratory procedure to sterilize small metallic instruments and even small pieces of glassware. It is an excellent procedure in postmortem examinations to burn off the surface of the body or of an organ when it is desired to obtain bacteria from the interior free from contamination with surface organisms.