PHYSICAL AGENTS 135 



oxidation, but it should not be forgotten that a very sHght 

 protection will prevent the action of light (Figs. 94 and 95). 

 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 remem- 

 bered that a similar increase in concentration occurs when 



Fig. 94. — Effect of light on bacteria. X to- 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. Out- 

 side of it they were killed except where they happened to be in large masses. 

 Hence the letter shows distinctly. (Student preparation.) 



many substances are dried, and is probably as valuable in 

 the preservative action as the loss of water. That the pro- 

 cess cannot be depended on to kill even pathogenic organ- 

 isms 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," 



