174 THE SEA-SHORE. 



service in purifying the water in which they grow. 

 This end they effect in two ways, first, by ap- 

 propriating for their own support the foul matter, 

 which would render the water putrid and incapable 

 of supporting animal life ; and, secondly, by evolv- 

 ing oxygen, or vital air, which renders the water 

 fit for the respiration of fishes. Their thread-like 

 stems may be frequently discovered, thickly in- 

 vested with brilliant globules, which, when the 

 water is disturbed, rise to the surface, and discharge 

 their contents into the air. Some of the smaller 

 kinds, the substance ; of which is gelatinous, exer- 

 cise a mechanical action, by arresting particles of 

 impure matter, which are carried against them by 

 the water, and retaining them at the bottom. 

 Hence it happens that the water of running streams 

 is often perfectly pure, though the smooth pebbles 

 over which it flows are thickly coated with slime 

 and mud. 



You have no doubt observed a green vegetable 

 substance, clothing the damp walls of cellars, &c. 

 This belongs also to the Algae, and is not without 

 its use. " It forms," says Sir James E. Smith, 

 " in the wet months of winter a verdant tapestry 

 on damp walls and stones, in confined areas, and 

 dark subterranean buildings in which the inhabit- 

 ants of crowded cities gasp for air, the effects of 

 which on the atmosphere, by rendering it some- 

 thing more respirable, must be as beneficial as those 

 observed to be produced by analogous species on 

 corrupted water." 



" But, were the Algae really serviceable, neither 

 in supplying the wants, nor administering to the 

 comforts of mankind, in any other respect, their 

 character would be redeemed by their usefulness 



