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THE CHEMICAL ACTION OF LIGHT 55
chemical intensity of light is much greater than in spring, perhaps in consequence of the greater transparency of the air. According to Roscoe, it is in August and September more than one and a half times as great as in March or April.
These variations in the chemical intensity of light are very important to the life of plants. The green leaves of plants inhale carbonic acid and exhale oxygen under the influence of light. This breathing process does not take place without the presence of light. The green colour of leaves and the variegated colours of flowers only exist under the operation of light. In the dark, plants only develop sickly blossoms, like the well-known white sprouts of potatoes kept in cellars.
The necessity of light for the life of plants is also seen in the effort made by plants kept in darkened rooms to reach the apertures which admit light. Hence a plant develops with an energy proportional to the intensity of the light. The greater fruitfulness of the tropics is to be ascribed, not only to the higher temperature, but also to the greater chemical intensity of the light. Recent observations have established that the yellow and red rays, and not the blue and violet, produce the greatest chemical effect on the leaves of plants.
The Part played by Animal and Vegetable Life in determining the Constitution of the Atmosphere. — We have now arrived at the knowledge of the importance of light for the economy of nature. Atmospheric air is a mixture of two gases, oxygen and nitrogen. Nitrogen is a perfectly innocuous gas, serving to dilute the oxygen ; for the latter, though essential to life, is injurious if undiluted.
In breathing, part of the oxygen is absorbed in the lungs ; it forms, with the organic constituent parts of the body, carbonic acid and water. The carbonic acid and water are exhaled by us and dispersed again in the air,