22 MATTER AND ITS PHYSICAL PROPERTIES. 



and a straight line may have the same length, although they have different 

 figures ; and, on the other hand, two arcs of different circles may have the same 

 figure, but very unequal lengths. The surface of a ball is curved, that of the 

 table plane ; and yet the area of the surface of the ball may be equal to that of 

 the table. 



ATOMS MOLECULES. 



Impenetrability must not be confounded with inseparability. Every body 

 which has been brought under human observation is separable into parts ; \ ; 

 and these parts, however small, are separable into others still more minute. 

 To this process of division no practical limit has ever been found. Neverthe- , ; 

 less, many of the phenomena which the researches of those who have success- 

 fully examined the laws of nature have developed, render it highly probable 

 that all bodies are composed of elementary parts which are indivisible and un- 

 alterable. The component parts, which may be called atoms, are so minute as 

 altogether to elude the senses, even when improved by the most powerful aids 

 of art. The word molecule is often used to signify component parts of a body 

 so small as to escape sensible observation, but not ultimate atoms, each mole- 

 cule being supposed to be formed of several atoms, arranged according to some 

 determinate figure. Particle is used also to express small component parts, 

 but more generally is applied to those which are not too minute to be discover- 

 able by observation. 



FORCE. 



If the particles of matter were endued with no property in relation to one 

 another, except their mutual impenetrability, the universe would be like a mass 

 of sand, without variety of state or form. Atoms, when placed in juxtaposition, 

 would neither cohere, as in solid bodies, nor repel each other, as in aeriform 

 substances. We find, on the other hand, that, in some cases, the atoms which 

 compose bodies are not simply placed together, but a certain effect is mani- 

 fested in their strong coherence. If they were merely placed in juxtaposition, 

 their separation would be effected as easily as any component particle could 

 be removed from one place to another. Take a piece of iron, and attempt to 

 separate its parts : the effort will be strongly resisted, and it will be a matter 

 of much greater facility to remove the whole mass. It appears, therefore, that 

 in such cases the parts which are in juxtaposition cohere, and resist their mutual 

 separation. This effect is denominated force ; and the constituent atoms are 

 said to cohere with a greater or less degree of force, according as they oppose 

 a greater or less resistance to their mutual separation. 



The coherence of particles in juxtaposition is an effect of the same class as 

 the mutual approach of particles placed at a distance from each other. It is 

 not difficult to perceive that the same influence which causes the bodies A and 

 B to approach each other, when placed at some distance asunder, will, when 

 they unite, retain them together, and oppose a resistance to their separation. 

 Hence this effect of the mutual approximation of bodies toward each other is 

 also called force. 



Force is generally defined to be " whatever produces or opposes the produc- 

 tion of motion in matter." In this sense, it is a name for the unknown cause of 

 a known effect. It would, however, be more philosophical to give the name, 

 not to the cause, of which we are ignorant, but to the effect, of which we have 

 sensible evidence. To observe and to classify is the whole business of the 

 natural philosopher. When causes are referred to, it is implied that effects of 



