20 THE ATMOSPHERE AND THE FORCES IN IT. 



Another is the force which binds the particles of a stone 

 or of any other thing together, and makes it hard or strong 

 or tough, which force we call the Attraction of Cohesion. 

 Another is the force by which different tilings stick to 

 each other, as mortar to a brick, or glue to wood, which 

 we call the Force of Adhesion. And there are doubtless 

 other forces which we do not so well understand. 



64. One of these unknown forces is the Force of Vegeta 

 ble Life, which draws into a growing plant the several 

 substances which are necessary to its growth, and out of 

 them forms all the parts of the plant. Another is the 

 Force of Animal Life, which turns its food into the flesh 

 and bones and other parts of the animal. 



A third is the Power which the Light of the Sun exerts 

 upon all vegetables and animals, upon all colors, perhaps 

 upon all things within its reach. 



A fourth is the power by which electricity draws light 

 bodies, and perhaps heavy ones, towards an electrified 

 surface, and again repels them. 



65. It is from the influence of the sunlight that the 

 carbonic acid and water in the sap of growing plants arc, 

 turned into the substance called woody fibre, which gives 

 them their hardness and strength. A woody plant, 

 growing in the dark, lengthens, but forms no true wood, 

 and so has no hardness. 



It is the influence of this light which causes the evapo 

 ration at the surface of the leaves, which thickens the 

 juices, and -changes them into nourishing sap. Without 

 the sunlight, the peculiar odors and tastes are not formed, 

 nor all the beautiful variety of colors. 



66. That the light of the sun has this great power 

 over plants, is shown by the fact that most of those plants 

 which naturally grow in places where the sunshine daily 



