584 Introduction to the Study of Science 



hydrogen peroxid . This in ordinary strength acts as an oxidizing 

 agent, and in a sufficiently concentrated form is a good dis- 

 infectant. Cuts and abrasions of the skin are advantageously 

 bathed with hydrogen peroxid. In dilute solutions it is an 

 excellent gargle. 



Tooth powders and mouth washes are sometimes supposed 

 or claimed to be germicidal. These usually contain some of the 

 essential oils, such as peppermint, eucalyptus, thyme, and some 

 contain hydrogen peroxid. The solutions are necessarily weak 

 and possibly only slightly disturbing to bacteria. The mechani- 

 cal action of the toothbrush with a plentiful supply of water 

 performs the really effective work of prophylaxis. 



All soaps are prophylactic, that is, preventive of bacterial 

 growth. They are so because they promote cleanliness, and 

 cleanliness is prophylactic. But soaps are not disinfectant 

 unless they are kept for a long time in contact with the body. 

 The chief value of soap is detergent, or to remove dirt, which 

 property is associated with the mechanical action involved in 

 its use. Other hygienic values claimed for a soap are usually 

 fictitious or, if present, are at the expense of its cleansing 

 qualities. In general a good vegetable oil soap, vigorously 

 applied, is, next to a strong salt solution, one of the most con- 

 venient and cleansing aseptics. 



The biological conditions of bacterial life and growth are many 

 and complex. Some varieties are interdependent or coopera- 

 tive, and some in conflict with others. Just what may deter- 

 mine such relations is not known. Bacteria have been studied 

 mostly in the laboratory, in pure cultures, while in nature 

 varieties are ordinarily mixed, except in some diseases of plants 

 and animals. 



A few examples of relations between different groups of 

 bacteria are taken from familiar substances. In milk one 

 species of bacteria produces lactic acid. If no interference 

 occurs, the acid soon becomes excessive for the acid-producing 

 bacteria, which cease to develop in the medium they created. 



