160 THE SEA-SIDE AND AQUARIUM. 



and one of white sand. The kelp was substituted 

 for the ' fossil alkali/ which, according to a probable 

 account, was accidentally found to contribute to the 

 formation of glass. According to Pliny, ' a merchant 

 vessel, loaded with nitre or fossil alkali, having been 

 driven ashore on the coast of Palestine, near the river 

 Bolas, the crew went in search of provisions, and 

 accidentally supported the kettles on which they 

 dressed them upon pieces of the fossil alkali. The 

 river sand above which this operation was performed 

 was vitrified by its union with the alkali, and thus 

 produced glass. The important hint, thus accidentally 

 obtained, was soon adopted, and the art of making 

 glass was gradually improved. Though kelp, till 

 lately, was chiefly employed in Britain in the manu- 

 facture of soap and glass, it is now principally manu- 

 factured for the iodine it contains, and no sea-weed 

 is so rich in iodine as this great tangle, especially its 

 woody stems.' " 



As I have already stated, two or three of the red 

 and green Algae are all that are necessary for purify- 

 ing the water in an Aquarium. There are, however, 

 several others of each class, w r hich, when introduced 

 judiciously, and watched with care, and rejected when 

 they begin to fade or die, look very graceful and ele- 

 gant. Many of the latter sort are seen to the best 

 advantage when made to jut out from the crevices of 

 rocks, or anchored close to the sides of the glass 

 such as the common red Coralline, Comllina offi- 

 emails, the Ptilota plumosa, the rich green feathery 



