(2) This inimical effect can be produced by light irrespective of rise in temperature.
(3) The ultra-violet rays are the most bactericidal, and the infra-red the least so, which indicates that the phenomenon is due to chemical action.
(4) The presence of oxygen and moisture greatly increases this action.
(5) The sunlight acts prejudicially upon the culture medium, and thereby complicates the investigation and after-growth.
(6) The time occupied in the bactericidal action depends upon the heat of the sun and the intrinsic vitality of the organism.
(7) With regard to the action of light upon pathogenic organisms, some results have recently been obtained with Bacillus typhosus. Janowski maintains that direct sunlight exerts a distinctly depressing effect on typhoid bacilli. At present more cannot be said than that sunlight and fresh air are two of the most powerful agents we possess with which to combat pathogenic germs.
Pasteur's Large Incubator for Cultivation at Room Temperature
A very simple method of demonstrating the influence of light is to grow a pure culture in a favourable medium, either in a test-tube or upon a glass plate, and then cover the whole with black paper or cloth. A little window may then be cut in the protective covering, and the whole exposed to the light. Where it reaches in direct rays it will be found that little or no growth has occurred; where, on the other hand, the culture has been in the dark, abundant growth occurs. In diffuse light the growth is merely somewhat inhibited. It has been found that the electric light has but little action upon bacteria, though that which it has is similar to sunlight. Recent experiments with the Röntgen rays have given negative results.
In 1890 Koch stated that tubercle bacilli were killed after an exposure to direct sunlight of from a few minutes to several hours. The influence of diffuse light would obviously be much less. Professor Marshall Ward has experimented with the resistant spores of Bacillus anthracis by growing these on agar plates and exposing to sunlight. From two to six hours' exposure had a germicidal effect.