284 APPLIED BIOLOGY 



ing), will kill all bacteria of typhoid and tuberculosis. Spores 

 of other bacteria may survive the boiling point for hours; 

 but boiling for a short time on successive days (known as 

 discontinuous sterilization) will cause the spores to germi- 

 nate, and then they are easily killed by heat. Fortunately, 

 the disease-producing (pathogenic} bacteria likely to be in 

 drinking water and milk do not form spores. 



Sterilization of substances containing bacteria is usually 

 accomplished, as already described for molds ( 245), by dis- 

 continuous steaming or boiling on two or three days. The 

 first boiling (100 C.) swells the spores, and the later heating 

 kills them. Steam under about 30 Ib. pressure and having 

 a temperature of about 130 C. will kill even resistant spores 

 in half an hour. Dry hot air at about 350 C. is also effi- 

 cient as a sterilizer of such articles as clothing. 



Pasteurization (discovered by Pasteur) means heating 

 milk to a temperature of 60 to 70 C., and keeping at this 

 temperature for 10 to 20 minutes. Its advantage is that it 

 does not change milk as does heating to boiling point (100 C.), 

 while it kills the disease bacteria likely to be present in im- 

 pure milk. In some cities much milk is now pasteurized. 



(e) Light. Strong sunlight kills all bacteria which are 

 directly exposed, as on the surface of soil. Some species are 

 killed in a few minutes, and few can withstand hours of 

 exposure. This is important, for it suggests the value of sun- 

 light for killing bacteria in sunny rooms, and especially on 

 clothing, carpets, etc., which can be exposed out-of-doors. 

 It also suggests the importance of building houses so as to 

 get the maximum of sunlight ; for example, shade trees should 

 not stand too near houses. The value of sunlight in disin- 

 fecting streets cannot be too strongly emphasized. Sprin- 

 kling with water increases the action, for wet bacteria stand 

 less light. It is unfortunate that all streets in cities cannot 

 be in the north-south direction so that all their surfaces will 

 get the full force of sunlight at mid-day. 



