SAPROPHYTES 415 



and other similar materials used in preserving meats prevent 

 the growth of bacteria but in ordinary concentrations do not 

 kill them. On the other hand, there are certain substances 

 like corrosive sublimate, carbolic acid, and formaldehyde which 

 in comparatively weak solutions are deadly to bacteria. A 

 substance which checks the growth or development of bacteria 

 but does not kill them is called an ANTISEPTIC, and one which 

 kills them is called a DISINFECTANT. 



476. Antiseptics. Antiseptics of many kinds are widely 

 used in the preservation of food. Common salt, vinegar, 

 spices, and sugar are the ones most commonly used in the 

 household. Sugar is a good food for some bacteria and par- 

 ticularly so for yeasts and molds, but in very strong concentra- 

 tion such as is found in jellies, preserves, and the like, neither 

 yeasts nor bacteria can grow. Molds, however, are more able 

 to endure these strong concentrations of sugar. Boric acid 

 is sometimes used in the preservation of meats and butter; 

 formalin, in the preservation of milk; and benzoate of soda, 

 in the preservation of jams, catsups, and the like, but most 

 authorities are agreed that these and several other substances 

 like them are, in the long run, injurious to the health of the 

 consumer and therefore their use is unwise. 



477. Disinfectants. Disinfectants are used, not only for the 

 destruction of bacteria in houses -after a case of bacterial dis- 

 ease, but also in the preservation of lumber, railroad ties, fence 

 posts, mine props, and the like. Among the disinfectants used 

 for these latter purposes are creosote and zinc chloride. The 

 material to be preserved is soaked in a solution of the disin- 

 fectant and this prevents, not only the action of bacteria, but 

 also the attacks of wood borers and other insects. 



478. Effect of Light upon Bacteria. Most bacteria are 

 readily killed by direct sunlight or other bright light. Even 

 strong diffused light is highly detrimental to their growth. 

 This fact, however, is of little value in the preservation of 

 materials, for the bacteria in the interior of the substance are 

 effectively shielded from the light. 



