582 Introduction to the Study of Science 



carbon dioxid directly in sunlight as do the chlorophyll-bearing 

 plants. But most kinds like the darkness in some degree. The 

 pathogenic variety of bacteria may be killed by exposure to 

 direct sunlight, a fact of value in housekeeping, and in the 

 treatment of certain skin diseases due to fungi or bacteria. 

 Concentrated light from an electric arc has been employed 

 beneficially in the treatment of such diseases. What influence 

 the Roentgen ray and radium ray may have is not definitely 

 known, though their use has been claimed as helpful in certain 

 kinds of skin diseases. 



Certain chemical conditions affect the growth and multi- 

 plication of bacteria. A few varieties thrive well in acid solu- 

 tions, but most flourish in alkaline conditions. Lime, a common 

 alkali, has a destructive influence upon some varieties but favors 

 others. This accounts in part for the hygienic character of 

 fresh whitewash, mortar, and lime plaster (page 418). Lime is 

 used in agriculture to correct an acid condition of the soil, 

 which favors useless bacteria and throws the useful bacteria into 

 an inactive state. Acids are detrimental to many kinds. The 

 acid secretions of the stomach, for example, are germicidal for 

 many kinds of bacteria, and serve in this way to protect the 

 body from these invading enemies. Almost all fruits are acid 

 enough to prevent the action of bacteria ; their decay is 

 initiated by molds that flourish on acid solutions. 



Chemical conditions which either destroy or prevent the 

 growth of bacteria are of practical value. Although very little 

 is known at present of the way in which poisonous substances 

 used as germicides act upon the bacteria, that limited knowl- 

 edge has been utilized in obtaining suitable preservatives for 

 many kinds of foods. Experience has long since hit upon many 

 non-poisonous preservatives which now have scientific sanction. 

 Such preservatives are not only aseptic, checking bacterial 

 growth, but also free from danger to human health. The most 

 common is salt, a strong solution of which has long been used 

 as an aseptic and preservative. Others are acetic acid, or vine- 



