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THE CHEMICAL ACTION OF LIGHT 57
gigantic plants shot forth from the warm ground under the influence of the sunlight. They flourished luxuriantly in an atmosphere so rich in carbonic acid, the carbon of the carbonic acid passed over into the form of wood, and thus for thousands of years the carbonic acid in the atmosphere was continuously diminished. Revolutions of the earth's surface succeeded ; whole tracts were buried under sand and clay ; their forests decomposed, and were changed into coal. A fresh vegetation sprouted forth from the newly formed soil, and again absorbed under the influence of light the carbonic acid of the atmosphere, to be once more engulfed. Thus the carbon from the carbonic acid of the atmosphere was stored as coal in the depths of the earth ; and thus the atmosphere, by the chemical effect of light, became continually richer in oxygen, until at length, after countless ages, it attained that wealth of oxygen which made the existence of man possible.
We see, therefore, that the chemical influence of light has played an important part in the development of our planet, and that it continues to do so in the economy of nature.
We will now enter the domain of chemistry, in order to explain the phenomena which occur when substances sensitive to light are exposed to its action.
The distinction between a physical and chemical change has been alluded to on p. 2.
Physical Changes produced by Light. — Now light is able to produce both chemical and physical changes. We have already stated that the red mineral, realgar, falls into a yellow powder when exposed to light. This is a physical change, for the yellow powder is still realgar, and if it is fused it forms, on cooling, compact red masses, which are again changed on exposure to the light. The number of physical changes of this kind occasioned by light is not great, but the phenomena are in themselves remarkable.