COMPOSITION OF MATTER 49 



So thoroughly have mathematicians mastered these un- 

 varying laws that they can tell just where in their orbits 

 the earth or any of the planets will be at any future time, 

 or were at any past time. The exact date of any eclipse 

 in the future or in the past can be determined, and even the 

 path of the moon's shadow across the earth. Disputed 

 dates of events in ancient history which occurred during 

 eclipses of the moon have been determined to the exact 

 hour in this way. 



One hundred years ago Uranus was thought to be the 

 farthest planet in the solar system. But years of patient 

 observation revealed the fact that its movement was not 

 in exact accord with the schedule astronomers had mapped 

 out for it. Two mathematicians, one in France and the 

 other in England, working separately without each other's 

 knowledge, concluded that this must be owing to the at- 

 traction of a more distant planet, as yet undiscovered. They 

 calculated what must be the exact position of this planet. 

 When on the night of September 23, 1846, a telescope was 

 directed to this point, a half hour's search revealed the 

 planet Neptune. 



Composition of Matter. It is the work of chemists to 

 find out of what matter is composed. They tell us that all 

 matter consists of minute particles, called molecules. These 

 molecules are constantly moving about in the spaces that 

 exist between them, hitting and bumping against one 

 another. 



The fact that minute invisible particles may be given off by 

 a substance is readily shown by opening a bottle of ammonia 

 or exposing a piece of musk in a room. Soon in every part 

 of the room the presence of these substances may be recog- 



