CHAPTER IV 

 PHYSICAL FORCES 



THE most universal properties number, figure and 

 motion possessed by all those bodies which our senses 

 can take cognisance of whether such bodies are solid, 

 liquid, or aeriform have now been treated of in an 

 elementary manner. 



We may next proceed to consider those forces which 

 are commonly said to affect bodies forces which bodies, 

 at any rate, make manifest to us, more or less frequently. 

 Every one knows that water behaves differently at dif- 

 ferent temperatures, and that the air is greatly affected 

 by heat, as also that the same is true of solid substances, 

 though in very diverse degrees. We all know that some 

 bodies are, or can be rendered, luminous, as well as that 

 sounds are transmitted to us through the air. Some 

 readers may have seen sparks emitted from the hairs of 

 a cat when rubbed, while others have doubtless, while 

 children, amused themselves with magnets, and no one 

 can be ignorant that the cleanest iron will get rusty 

 when long exposed to the air. 



It is also a matter known to everybody, that one and 

 the same material substance will be now hotter, now 

 colder ; now brightly luminous, at another time dull (as 

 e.g., a coal) ; occasionally sonorous and other times silent 

 (as a piano) ; now tranquil and motionless, yet after- 

 wards conspicuously turbulent as two effervescent 



