90 WRITINGS OF JOSEPH HENRY. [1855- 



The laws of force and motion, to which we have alluded 

 may be expressed as follows : 



LAWS OF FORCE. 



1. Every particle of matter, at a sensible distance, attracts 

 every other particle with a force varying inversely as the 

 square of the distance. In the phenomena of electricity and 

 magnetism, repulsion is also exhibited, acting in accordance 

 with the same law. 



2. Particles of matter at insensible distances, attract and 

 repel each other with great energy, the attractions and re- 

 pulsions appearing to alternate with minute changes of dis- 

 tance. 



LAWS OF MOTION. 



1. The law of inertia. — A body at rest tends to remain at 

 rest, and when put in motion by the application of any force 

 tends to move forever in a straight line with a uniform ve- 

 locity. 



2. The law of the co-existence of ^notions. — A body impelled 

 at the same moment by several forces in different directions, 

 will at the end of a given time be in the same position as 

 if the forces had each acted separately. 



3. The law of action and re-action. — When a force acts be- 

 tween two bodies of different masses, their momenta will be 

 equal and opposite. 



These laws were first given to the world in a definite form 

 by Sir Isaac Newton in his Principia. They are ultimate 

 facts of science, of which no satisfactory explanation is given ; 

 but by adopting them, as we do the axioms of geometry, 

 and reasoning downward from them, all the great truths of 

 modern astronomy have been evolved, as well as many of 

 the facts of the molecular action of bodies. 



Atomic Theory. 



In connection with the laws of the forces and motion of 

 matter, given above, we shall venture in this essay to express 

 some of the widest generalizations of the present day in 

 the form of what is called the atomic theory. This was the 

 original conception of an imaginative Greek philosopher, 



