10 Dairy Bacteriology. 



4 . LigM . The influence of light on bacterial growth has 

 not been generally appreciated until within a few years. 

 Exposed to the rays of direct sunlight, many forms are 

 killed in a few hours. Even diffused daylight often ex- 

 erts a powerful inhibiting effect, if the exposure covers 

 any considerable length of time. Bacterial spores are not 

 as readily destroyed as the vegetative forms. 



17. Chemical substances. Different chemical sub- 

 stances exert a powerful toxic action on bacterial life 

 with which they come in contact. Those that destroy or 

 kill bacterial forms are known as disinfectants; those 

 that merely inhibit, or retard growth are known as anti- 

 septics. All substances possessing disinfecting power 

 must of necessity be antiseptic in their action, but not 

 all antiseptics are disinfectants even when used in strong 

 doses. Most of the disinfectants, under ordinary cir- 

 cumstances, have no use in creamery practice except as 

 preservatives for samples. Of these corrosive sublimate 

 and potassium bichromate are most frequently used. In 

 some countries antiseptics, principally of boracic acid or 

 formalin, are employed to "keep" milk, but in general, 

 their use is prohibited by law. 



18. The role of bacteria in nature. The great ma- 

 jority of bacteria like animal life, belong to a class whose 

 function it is to disintegrate organic matter and resolve 

 it into its constituent elements. The value of this break- 

 ing down process is evident at a glance when we consider 

 what would be the result, if all decay, putrefaction, and 

 decomposition were at once arrested. Not only would 

 the supply of carbonic acid gas be very soon absorbed, 

 stopping the development of chlorophyll-bearing plants, 

 but all nature would be clogged and soon buried under 

 its own debris. Dead and dying vegetable and animal 



