SCIENCE OF COMMON THINGS* 271 



Origin of color in leaves of trees. Why is the sky blue ? 



the bare ground, the faint light that proceeds from the 

 sky. 



176O Why are the leaves of plants green f 



Because a peculiar chemical principle, called chloro- 

 phyl, is formed within their cells, which has the property 

 of absorbing the red rays, and of reflecting the blue and 

 yellow, which mixture produces green. 



17G1 Why are leaves a light green in spring f 



Because the chlorophyl is not fully formed. 



17(33 Why do leaves turn brown in autumn f 



Because the chlorophyl undergoes decay, and is not 

 replaced as it is in spring. 



1763 Why do all things appear black in the dark f 



In the dark there is no color, because there is no light 

 to be absorbed or reflected, and therefore none to be 

 decomposed. 



Of course, in certain degrees of darkness, all objects are actually invi- 

 sible. The question refers to that peculiar degree of darkness when the 

 forms of objects may be seen, but not their hues. 



1764 Why does the sky appear blue ? 



Because the atmosphere absorbs the red and yellow 

 rays, and transmits the blue. 



1765 Why does the sun most generally fade artificial colors f 



Generally the loss of color arises from the oxidation 

 of the substances used in dyeing ; as tarnish and rust are 

 an oxidation of metals. Sometimes, however, the 

 ingredients of the dye are otherwise decomposed by the 

 sun ; and the color (which is due to a combination of 

 ingredients) undergoes a change as soon as the sun 

 deranges or destroys that combination. 



1766 What remarkable correspondence is there between the geographical 

 position of a region, and the colors of its plants and animals f 



In the tropics, where the sun shines longest and 

 brightest, the darkest green prevails over the leaves of 

 plants, the flowers and fruits are colored brightly, and 

 the plumage of the birds is of the richest description. 



1767 What is the natural coloration exhibited -in temperate climates f 



In temperate climates everything is of a more sub- 



