RADIATION. GRAVITATION. 19 



space, it is said to spread by radiation. As it spreads, it 

 expands every thing ; and as the temperature is every 

 where continually changing, from winter to summer, 

 from day to night, and every hour of the day and night, 

 all solids must be constantly expanding and contracting, 

 and the particles of which they are composed must be 

 continually approaching to and receding from each 

 other. 



In liquids, the particles that are warmed expand and 

 rise, while those that are cooled contract and sink, thus 

 producing currents upwards and downwards in the liquid. 

 Particles of other substances, floating or suspended in 

 the liquid, as they become warmer, rise towards the 

 surface, and, as they cool again, sink towards the bottom; 

 or, if one side of a particle expands more rapidly than 

 another, it turns over, seeming as if it had life and volun 

 tary motion. 



The vapors &nd gases, expanded by heat, become lighter, 

 rise upwards towards the surface of the atmosphere, and 

 their place is taken by cooler ones from every side. 



62. Why does not this perpetual strife of forces produce 

 disorder and chaos? 



These forces are not lawless forces. They all have 

 their limits within which they are compelled to abide. 

 Besides, there are other mighty forces always acting 

 against them, and constraining them to keep within their 

 bounds. 



63. One of these forces is the Attraction of Gravitation, 

 which makes a stone fall to the ground, and draws every 

 particle, every atom, towards every other, and all. towards 

 the centre of the earth, and the earth itself towards the 

 sun. This gives them all their weight, and brings them 

 to rust, and keeps them in their places. 



3 



