16 Dairy Bacteriology. 



stand effects of desiccation without serious injury, and 

 many of the non-spore-prod uciirg types retain their 

 vitality for some months. The bacteria found in the air 

 are practically all derived from the soil, and exist in the 

 air in a dried condition, in which they are able to re- 

 main alive for considerable periods of time. In a dried 

 condition, active cell growth is not possible, but when 

 other conditions, such as moisture and food supply are 

 present, resumption of growth quickly begins. This 

 property is also of importance in the dairy as in the 

 preparation of dry starters for creameries and cheese 

 factories. 



Effect of light. Bright sunlight exerts a markedly in- 

 jurious effect on bacterial life, both in a spore and in a 

 growing condition. Where the direct sunlight strikes,, 

 more or less complete disinfection results in the course 

 of a few hours, the effect being produced by the chem- 

 ical or violet rays, and not by the heat or red rays of 

 the spectrum. This action, however, does not penetrate 

 opaque objects,, and is therefore confined to the surface, 

 In diffused light, the effect is much lessened, although 

 it is exerted to some extent. Sunlight exerts a beneficial 

 effect on the general health and well-being of animal 

 life, and is a matter of importance to be taken into con- 

 sideration 'in the erection of buildings for animals as 

 well as for people. 



Effect of chemicals. A great many chemical sub- 

 stances exert a more or less powerful toxic action on 

 various kinds of life. Many of these are of great service 

 in destroying bacteria or holding them in check. Those 

 that are toxic and result in the death of the cell are 

 known as disinfectants; those that merely inhibit, or re- 

 tard growth are known as antiseptics. All disinfectants 



