PURIFYING INFLUENCE OF SEA-WEED. 489 



and which occupy so large a tract of the sea- 

 bed, possess this function in common with the 

 rest of the vegetable inhabitants of the waters. 

 Dr. Johnston performed an interesting expe- 

 riment upon these plants, which pleasingly 

 illustrates their utility, minute and feeble 

 though they appear in the great waters by 

 which they are surrounded. He placed in a 

 small glass jar, containing about six ounces of 

 pure sea-water, a tuft of living coralline, about 

 the branches of which several little mussels, and 

 other animals, and a star-fish were crawling. 

 The jar was placed on a table and was seldom 

 disturbed, though occasionally looked at, and at 

 the end of four weeks the water was still pure, 

 the little animals all alive and active, and the 

 plant had grown sensibly larger. At the expi- 

 ration of eight weeks the water continued pure, 

 and many of the animals were living. Had the 

 coralline not been there, a day would have 

 sufficed for the animals to have extracted all 

 the oxygen of the water, and in a week or two 

 the water itself would have commenced the 

 changes of putrefaction. Nothing could more 

 conclusively exhibit the effect of plants upon 

 the waters of the ocean, for here was a sea in 

 miniature, the animal producing carbonic acid, 

 and the coralline absorbing and decomposing it, 

 and then emitting its oxygen. 



