74 THE BOOK OF NATURE STUDY 



cannot produce oxygen fast enough for its requirements. It 

 is clear, then, that in nature the purifying action of the plants 

 is assisted by other agents. Watch the water tumbling over a 

 waterfall, the waves dashing on the beach, the rapids in a stream- 

 let, the rise and fall of the tide, and it will become clear to you 

 that in nature water is for the most part in movement, and this 

 movement, which means mingling with air, means the through 

 oxygenation of the fluid apart from its plant inhabitants. You 

 may say, however, that the still ditches may be full of animal 

 life, that the stagnant ponds are often crowded, that the rain- 

 water barrel even, or the neglected horse-trough, contains its 

 quota. To which the reply is, that quite often the animals con- 

 tained in these receptacles, though they live in the water, yet 

 are true air-breathers, rising to the surface for the purpose of 

 breathing. Again, the pond or ditch usually exposes a large 

 surface of water in proportion to its bulk, its surface also is raised 

 into ripples by the wind, and a considerable diffusion of oxygen 

 takes place in this way. Many apparently stagnant ponds or 

 ditches have also affluents or effluents, or both, which cause 

 currents to flow through them and have an aerating effect. 



In the sea the rise and fall of the tides is of great importance 

 in purifying the water, and there is another factor of some im- 

 portance, which also acts, though to a less degree, in fresh water. 

 Everyone has heard of the coral reefs of warm climates, which 

 are built up of carbonate of lime, and even in our seas the curious 

 little seaweeds known as corallines contain much of this sub- 

 stance, and there are an enormous number of shell-forming 

 molluscs and crustaceans. Think of the shells in a mussel-bed 

 or an oyster-bed, or even of those of the periwinkles and whelks 

 of the shore, and you will realise that in the sea the formation 

 of carbonate of lime goes on with some rapidity. Now if a shell 

 be taken, and a few drops of weak acid be poured upon it, a rapid 

 effervescence of course takes place, and the test with lime-water 

 shows that carbonic acid gas is being given off. It is then, under 

 natural conditions, not only the green plants which remove the 

 carbonic acid from the water ; all shell-forming organisms are 

 playing their part in the same process. In England great masses 

 of limestone occur, which may build up hills and mountains, as 



