QUICKSANDS AND FIRE-STONES 65 



its particles, but when the sand-particles are wetted they 

 adhere closely to each other ; the air is driven out, and 

 the water does not exactly take an equivalent space, but 

 occupies less room than the air did, owing to the close 

 clinging together of the wet particles. If you add a 

 little water to some dry sand under the microscope, you 

 will see the sand-particles move and cling closely to one 

 another. " Capillary attraction " — the ascent of liquid 

 in very fine tubes or spaces — is a result of the same sort 

 of adhesive action. If you walk on the firm, damp sand 

 exposed at low tide on many parts of the seashore when 

 it is just free from water on the surface, you will see 

 that when you put your foot down the sand becomes 

 suddenly pale for some seven inches or so all round your 

 foot. The reason is that the water has left the pale-looking 

 sand (dry sand looks paler than wet sand), and has gone 

 into the sand under your foot, which is being squeezed 

 by your weight. The water passing into that squeezed 

 sand enables its particles to sit tighter or closer together, 

 and so to yield to the pressure caused by your weight. 

 You actually squeeze water " into " the sand, instead of 

 squeezing water " out " of it, as is usually the case when 

 you squeeze part of a wet substance — say a cloth or a 

 sponge. When you lift your foot up, you find that your 

 footmark is covered with water — the water you had 

 drawn to that particular spot by squeezing it. It 

 separates as soon as the pressure is removed. 



Quartz and quartzite pebbles occur on the 

 South as well as the East Coast. They are sometimes 

 called " fire-stones," because they can be made to 

 produce flashes of flame. If you take a couple of these 

 pebbles, each about as big as the bowl of a dessert-spoon 

 (a couple of flint peebles will serve, but not so well), and 

 holding one in each hand in a dark room, or at night, 



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