By obtaining water from the purest sources, and by filtering it before drinking, by which nearly all extraneous matters would be _separated from it_. CHAPTER XXXVI. 772. _What is attraction?_ Attraction is the tendency of bodies to _draw near to each other_. It is called _attraction_, from two Latin words signifying _drawing towards_. 773. _How many kinds of attraction are there?_ There are five principal kinds of _attraction_:-- 1. The attraction of _gravitation_. 2. The attraction of _cohesion_. 3. The attraction of _chemical affinity_. 4. The attraction of _electricity_. 5. And _capillary attraction_. [Verse: "Behold, the nations are as a drop of a bucket, and are counted as the small dust of the balance: behold, he taketh up the isles as a very little thing."--ISAIAH XL.] 774. _Why do all bodies heavier than the air fall to the earth?_ Because they are influenced by the _attraction of gravitation_, by which all bodies are drawn towards _the centre of the earth_. 775. _Why do bodies lighter than the air ascend?_ Because the air, being a denser body, _obeys the law of attraction_, and in doing so _displaces lighter bodies_ that interfere with its gravitation. 776. _Why do fragments of tea, and bubbles floating upon the surface of tea, draw towards each other, and attach themselves to the sides of the cup?_ Because they are influenced by the _attraction of cohesion_. _Cohesion._--The act of sticking together. 777. _Why will a drop of water upon the blade of a knife leave a dark spot?_ Because the _iron of the knife attracts the oxygen of the water_, by _chemical affinity_; and the two substances form a thin coating of _oxide of iron_. _Affinity._--Attraction between dissimilar particles through which they form new compounds. 778. _Why do clouds sometimes move towards each other from opposite directions? and_ 779. _Why do light particles of matter attach themselves to sealing wax, excited by friction?_ Because they are moved by the _attraction of electricity_. 780. _Why will a towel, the corner of which is dipped in water, become wet far above the water?_ Because the water is conveyed up through the towel, by _capillary attraction_. The atoms of the water are attracted by the _threads of the towel_, and drawn up into the _small spaces between the threads_. _Capillary._--Resembling a hair, small in diameter. [Verse: "He stretcheth out the north over the empty place, and hangeth the earth upon nothing."--JOB XXVI.] 781. _Why do small bodies floating upon water move towards larger ones?_ Because the attractive power of a _large body_ is greater than that of _a small one_. As each atom of matter has inherent power of attraction, it follows that a _large aggregation of particles_ must attract in proportion to the number of those particles. 782. _Why do clouds gather around mountain tops?_ Because they are _attracted by the mountains_.