102 INVISIBLE OBJECTS 



lies heavy and motionless on our woodpile is made up of 

 countless billions of molecules each in rapid incessant motion. 

 The molecules of solid bodies cannot escape so readily as 

 those of liquids and gases, and do not travel far. The log 

 lies year after year in an apparently motionless condition, 

 but if one's eyes were keen enough, the molecules would be 

 seen moving among themselves, even though they cannot 

 escape into the surrounding medium and make long journeys 

 as do the molecules of liquids and gases. 



96. The Companions of Molecules. Common sense tells us 

 that a molecule of water is not the same as a molecule of 

 vinegar; the molecules of each are infinitely small and in 

 rapid motion, but they differ essentially, otherwise one sub- 

 stance would be like every other substance. What is it that 

 makes a molecule of water differ from a molecule of vinegar, 

 and each differ from all other molecules ? Strange to say, a 

 molecule is not a simple object, but is quite complex, being 

 composed of one or more smaller particles, called atoms, and 

 the number and kind of atoms in a molecule determine the 

 type of the molecule, and the type of the molecule deter- 

 mines the substance. For example, a glass of water is com- 

 posed of untold millions of molecules, and each molecule is 

 a company of three still smaller particles, one of which is 

 called the oxygen atom and two of which are alike in every 

 particular and are called hydrogen atoms. 



97. Simple Molecules. Generally molecules are composed 

 of atoms which are different in kind. For example, the 

 molecule of water has two different atoms, the oxygen atom 

 and the hydrogen atoms; alcohol has three different kinds 

 of atoms, oxygen, hydrogen, and carbon. Sometimes, how- 

 ever, molecules are composed of a group of atoms all of 

 which are alike. Now there are but seventy or eighty differ- 

 ent kinds of atoms, and hence there can be but seventy or 



