THE CHEMICAL ACTION OF LIGHT. 17 
In the foregoing sketch of the action of light upon plants, 
reference has been made solely to sunlight. But it is found, from 
experiments made with various kinds of lamps, that provided the 
light is sufficiently intense, carbon dioxide is decomposed and 
oxygen evolved under the influence of artificial light. In this 
connection, the striking experiments of Siemens with the electric 
light will be remembered. 
I have already mentioned the fact that the presence or absence 
of light has a great influence on the health of man, and that light 
is as necessary for his well-being as fresh air or pure water. 
Apart from the great stimulating action of light on the tissues of 
the animal body, it has quite recently been shown that light plays 
a most important part in the prevention of disease. Some years 
ago, Dr. Richardson said, “I once found by experiment that 
certain organic poisons, analogous to the poisons which propagate 
contagious diseases, are rendered innocuous by exposure to light.” 
This statement has quite recently received confirmation in the 
remarkable results which Buchner in Germany, and Marshall 
Ward in England, have obtained in experiments on the action of 
light on micro-organisms producing infectious diseases. It has 
been known for some time that sunlight exerts some retarding 
influence on the growth of micro-organisms, and some experi- 
ments leading Marshall Ward to suspect that ordinary daylight 
exercised similar effects, he examined this question with striking 
results, which leave little doubt that the most potent factor in the 
purification of the air and rivers of bacteria is sunlight. The fact 
that direct sunlight is efficacious as a bactericide had been long 
suspected, but it had never before been experimentally proved. 
Starting from the observation that a test-tube or small flask 
containing a little Thames water, with many hundreds of 
thousands of anthrax spores in it, may be entirely rid of living 
spores by continued exposure daily for a few days to the light of 
the sun, and that even a few weeks of bright summer daylight— 
not direct insolation—reduces the number of spores capable of 
development on gelatine, Marshall Ward tried the effect of 
direct insolation on plate cultures in order to see if the results 
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