Human Physiology. 95 



to keep us warm. All life, and light, and power, are from the 

 sun. Its force is also stored up in earth, water, and air, as 

 electricity. When oxygen in water parts with its hydrogen to 

 unite with zinc, a force is set free that shines as a spark, 

 explodes, gives a sensible shock, develops magnetic attraction 

 in a piece of iron, propagates its force along the atoms of 

 thousands of miles of wire, spells out a message on the other 

 side of the world. 



The forces that hold the untouching atoms of matter in their 

 places are incomprehensible as to their nature, but their action 

 may be seen in a thousand familiar examples. A fine silken 

 thread will sustain the weight of many pounds. Its atoms do 

 not touch; therefore the weight is sustained by the attraction 

 >of atom for atom, and is the measure of that force. A small 

 steel wire will hold up a thousand pounds. Warm the wire 

 and it lengthens, and allows the weight to descend; cool it 

 and the weight rises, by the diminished repulsion or increased 

 attraction of the atoms for each other. The walls of buildings 

 are drawn together by the cooling of iron rods. So of press- 

 ure. The weight of a column rests upon the repulsive force 

 of its lowest atoms. This force, in slender iron pillars, sustains 

 immense weights of masonry, which rise and fall hour by hour 

 as heat increases or cold lessens this repulsive force. 



The action of matter upon matter without contact is at every 

 distance equally incomprehensible. Newton thought that atoms 

 were as distant from each other, in proportion to their size, as 

 the stars; and that if the atoms of our earth could be brought 

 into actual contact, it might perhaps be compressed into the 

 space of a cubic inch; yet we have seen that the attractions, 

 repulsions, and polarity of these atoms give its point to the 

 needle or the sting of a bee, and its hardness to the diamond. 



The attraction of gravitation or weight acts through incon- 

 ceivable distances. Another attraction, that of cohesion, acts 

 only at distances immeasurably minute. By this, molecules 



