EFFECT OF PHYSICAL ENVIRONMENT 119 



brganisms, and if too intense stops their growth and accel- 

 erates the death rate. 



Light sometimes affects the direction of growth of micro- 

 organisms, particularly molds. The fertile hyphae, that is, 

 the conidiophores or sporangiophores of certain species of 

 molds tend to bend in the direction of light. An effect of 

 any external agency upon the direction of growth is called 

 a tropism; this effect produced by light is therefore termed 

 phototropism. 



Living protoplasm in general is more or less injuriously 

 affected by light. Too intense light applied to the human 

 skin, for example, will cause sunburn. Green plants accus- 

 tomed to growing in the shade will likewise be sunburned 

 when exposed to direct sunlight. Certain light rays have 

 the power of causing . coagulation of proteins. It is not 

 surprising, therefore, that microorganisms which do not 

 require light for their development are particularly sensi- 

 tive to it, and may be rapidly destroyed by certain rays. 



It will be recalled that sunlight, that is, white light, may 

 be divided into various rays of different wave lengths. At 

 one end of the visible spectrum are the reds, at the other 

 end the blue and the violet. Beyond the violet with wave 

 lengths still shorter is the so-called ultraviolet. In general 

 the rays of light in the blue and violet and in the ultra- 

 violet are particularly destructive to bacteria. It may be 

 observed that these are the same rays which also affect most 

 intensely the photographic plate. 



Commercial use of ultra-violet light in sterilization has 

 been attempted. The most satisfactory source of such light 

 is the mercury vapor arc lamp, or so-called Cooper-Hewitt 

 lamp. This is a quartz, not glass (glass is opaque to ultra- 

 violet light) cylinder from which most of the air has been 

 exhausted and which contains a small amount of mercury. 

 This mercury is vaporized and an arc passes from one end 

 to the other, causing the entire tube to glow. The light 



