PHYSICAL AND CHEMICAL AGENTS 29 



j|ermicidal.j ffiect upon b acteria, many species being 

 killed within a few seconds upon exposure to the 

 direct rays of the sun, and all showing different 

 degrees of sensitiveness to light. The electric light 

 exerts ^a similar effect to sunlight, but is less powerful. 



Moisture. — The abs ence of m oisturejnay cause 

 the death of bacteria or cause a suspension of de- 

 velopment. Certain species are deprived of the 

 power of reproduction without moisture, which under 

 favorable conditions will again grow and multiply. 



Desiccation (drying) destroys nearly all of I ha 

 p athogenic bacteria, the tubercle baci llu s being 

 one of the most resjstajitjg drying and the cholera 

 spirillum one of the most sensitive . The spores, 

 however, are_ex tremely resistant to d rying. It is 

 said that the spores of the anthrax bacillus will 

 survive drying for ten or more years.. 



This sensitiveness to drying, shown by a large 

 majority of disease-germs, would indicate thatjnfec.- 

 tion_ t hroug h t he air, cannot be so common as had 

 been supposed. 



Oxygen. — In their relation to^oxy gen bacter ia 

 are divided into three classes : (Q.) those which re- 

 quire free oxygen for the maintenance of their 

 activities, called obliqatoru aerobes,; ^2/ those which 

 do no t growexcept in the alm ost entire abs ence of 



