34 LIGHT. 



The effect of light upon vegetation is well 

 known. Many flowers follow the course of the 

 sun, and most flowers turn themselves more or less 

 towards the light. Plants that grow in darkness 

 are pale and without colour, and when this is the 

 case they are said to be etiolated, or blanched. 

 Gardeners avail themselves of this fact to render 

 some vegetables, as celery and endive, white and 

 tender. The more plants are exposed to the light, 

 the more colour they acquire. Vegetables are 

 not only indebted to light for their colour, but 

 their taste and odour are derived from the same 

 source. From this it happens, that hot climates 

 are the native countries of perfumes, odori- 

 ferous fruits, and aromatic resins. The action of 

 light on the organs of vegetables causes them to 

 pour out streams of oxygen gas from the surfaces 

 of their leaves, while exposed to the sun, whereas, 

 on the contrai'y, when in the dark, they emit air 

 of a noxious quality. 



Animal life seems also to be no less influenced 

 by light. Birds that inhabit tropical countries 

 have much brighter plumage than those of the 

 north. Animals in general seem to droop when 

 deprived of light ; and no doubt it is \Qry essential 

 to the health of human beings. 



The colour of metallic oxides is changed by the 

 action of light : the yellow oxide of tungsten be- 

 comes blue by exposure to hght ; the white salts 

 of silver become black, and green precipitate of 

 iron becomes red. Some oxides of metals lose 

 w^eight by exposure to light, as the red oxide of 

 mercury ; others lose their oxygen entirely, or 

 become reduced, as the oxide of gold. Light, 

 tlien, has the property of separating oxygen from 

 several of the oxides. 



