=3. Cold.=--That _cold_ is an excellent _antiseptic_ is illustrated by the general use of refrigerators and “cold storage.” Numerous experiments have shown that although many pathogenic organisms of a given kind are killed by temperatures below freezing, not all of the same kind are, and many kinds are only slightly affected. Hence cold cannot be considered a practical means for _disinfection_. [Illustration: FIG. 99.--A “flash process” pasteurizing outfit, with holder. _A_, flash pasteurizer; _B_, holder; _C_, cooler.] =4. Light.=--It has been stated (p. 75) that light is destructive to bacteria, and the advisability of having well-lighted habitations for men and animals has been mentioned. The practice of “sunning” bedclothing, hangings and other large articles which can scarcely be disinfected in a more convenient way is the usual method of employing this agent. Drying and the action of the oxygen of the air assist the process to some extent. Undoubtedly large numbers of pathogenic organisms are destroyed under natural conditions by the combined effects of drying, direct sunlight and oxidation, but it should not be forgotten that a very slight protection will prevent the action of light (Figs. 100 and 101). [Illustration: FIG. 100.--Effect of light on bacteria. × 7/10. The plate was inoculated in the usual way. A letter _H_ of black paper was pasted on the bottom. The plate was then exposed for four hours to the sun in January outside the window and then incubated. The black paper protected the bacteria. Outside of it they were killed except where they happened to be in large masses. Hence the letter shows distinctly. (Student preparation.)] =5. Osmotic Pressure.=--Increase in the concentration of substances in solution is in practical use as an _antiseptic_ procedure. Various kinds of “sugar preserves,” salt meats and condensed milk are illustrations. It must be remembered that a similar increase in concentration occurs when many substances are dried, and is probably as valuable in the preservative action as the loss of water. That the process cannot be depended on to _kill_ even pathogenic organisms is shown by finding living tubercle bacilli in condensed milk. The placing of bacteria in water or in salt solution in order to have them die and disintegrate (greatly aided by vigorous shaking in a shaking machine) (“autolysis,” p. 126) is a laboratory procedure to obtain cell constituents. It is not a practical method of disinfection, however. [Illustration: FIG. 101.--Effect of light on bacteria. × 7/10. This plate was treated exactly as the plate in Fig. 100, except that the letter is _L_, and that it was exposed inside the window and wire screen. The window was plate glass. It is evident that few of the bacteria were killed, since the letter _L_ is barely outlined. The exposure was at the same time as the plate in Fig. 100. (Student preparation.)]