Human Physiology. yj 



CHAPTER II. 



FORCE. 



Wha: is Gravitation? Magnetism Atomic Attraction and Heat Cohesion 

 Repulsive Forces Attraction of Liquids and Solids for Gases 

 Solution and Crystallisation Capillary Attraction Electricity Gal- 

 vanism Is Heat a mode of Motion? What is Heat? Force of the 

 Sun Unused and Unrecognised forces. 



IF we know but little of matter, we know less, if possible, of 

 the nature of force and its mode of action. If I hold a stone 

 in my hand I feel it pressing downward with a certain weight 

 or force ; but why downward rather than upward or sideways ? 

 If I remove my hand it goes in a straight line with increasing 

 velocity toward the centre of the earth. My own body is 

 drawn toward the same centre so powerfully, that I can only 

 overcome it by the counteracting force of muscular contractions, 

 equal to 150 pounds. What draws the atom, my body, all 

 bodies, even the atmosphere and the lightest gases toward the 

 earth's centre? Newton called this force, gravity, another 

 name for weight, and not an explanation. " Every particle of 

 matter in the universe," he said, " attracts every other particle 

 with a force, direct as the mass, and inversely as the square of 

 the distance/' He showed that the same force, gravitation, 

 attraction, drawing- to, or pulling, that brings a stone to the 

 ground, holds the moon in its orbit round the earth, the earth 

 and all planets round the sun, and holds the sun to some 

 unknown centre of the universe. But Newton did not even 

 venture to guess at the nature of this force or its mode of 

 action. 



The earth has no visible hold upon the stone I throw, or the 

 bullet I shoot into the air, but it draws them down. Two 



