30 
WORLD OF INVISIBLE LIFE 
Does sunlight 
affect them? 
It has been found by various tests that direct 
sunlight kills many microbes almost at once, 
and that even ordinary daylight hinders their 
growth a great deal. They grow best in the 
dark, and so dust or dirt exposed to sunlight 
soon loses most of its living microbes. Scientists 
have found out that it is not the heat of the 
sun which kills them, but certain rays in the 
sunlight. These are called the ultra-violet rays, 
because they are beyond (ultra) the violet rays 
in the band of colors that we call the spectrum. 
We cannot see these rays, but they are powerful 
and health-giving. These ultra-violet rays are 
often used to purify water. 
The action of sunlight on microbes can be 
shown by covering part of a plate upon which 
microbes have been grown in the laboratory, 
and allowing the sun’s rays to beat down upon 
the rest of the plate. Large families of microbes 
will grow in the covered part of the plate, but 
there will be no new growth in the part that 
is open to the sunlight. It has been found that 
the electric light will produce the same result. 
