18 PRACTICAL LESSONS IN SCIENCE. 



to be indivisible, absolutely hard, inelastic, unchangeable as 

 bright, sharp-angled and perfect as when they left the creative 

 hand . They are the bricks of the material world . They have com- 

 bined with each other to form millions of compounds; these com 

 pounds have been broken up and new combinations formed over 

 and over again, and yet these particles of matter show no marks 

 of abrasion or signs of decay. 



In general the molecules composing bodies are arranged in 

 some definite way, as the crystals so common among minerals, 

 and the various structural forms in plants and animals, leaving 

 many spaces unoccupied, which may be called structural spaces; 

 and it is supposed that the atoms and molecules which make up 

 even the most compact bodies, as the metals, do not touch each 

 other, so that even in a piece of gold there is more vacant than 

 occupied space. These spaces between the particles which make 

 up a body are called pores, and the property of matter depend- 

 ing upon the existence of pores is called porosity. Porosity sug- 

 gests the idea and possibility of the properties of compressibility 

 and expansibility. The porosity of water may be shown by filling 

 a glass with water and then sprinkling into the water fine salt or 

 sugar. The particles of salt seem to be smaller than the parti- 

 cles of water, and to run down among them as wheat may be 

 shaken into a barrel of apples or potatoes. Again, water pene- 

 trates the pores of wood, cloth, paper, some kinds of rock, etc., 

 by a process called absorption, showing the existence of pores in 

 all such bodies. 



Rest is that condition of a body when it is not changing its 

 position in regard to other bodies around it, while motion is that 

 condition of a body when it is changing its position in regard 

 to other bodies around it. 



That which changes the condition of a body as regards rest 

 or motion is called force. 



Bodies have no control over their conditions of rest or mo- 

 tion. They can not move themselves if at rest, and can not stop 

 themselves if in motion. It requires force to put a body, as a 

 wheel, in motion, and it requires force to stop the wheel when in 



