THE MARINE AQUARIUM 



67 



of the tides, but perhaps this is in itself of no great advantage in 

 the aquarium except from the fact that it allows those creatures 

 that prefer to be occasionally out of the water for a time a better 

 opportunity of indulging in such a habit. And further, with regard 

 to both the arrangements for aeration above described, it should 

 be noted that earthenware vessels are much to be preferred to those 

 made of metal for the holding of sea water, since the dissolved 

 salts corrode metallic substances rather rapidly, and often produce, 



FIG. 39. AQUABIUH FITTED WITH APPARATUS FOB PERIODIC OUTFLOW 



by their chemical action, soluble products that render the water 

 more or less poisonous. 



Other methods of aerating the water of aquaria are practised, 

 but these, as a rule, are only practicable in the case of the large 

 tanks of public aquaria and biological laboratories, as the mechanical 

 appliances necessary to carry them out successfully are beyond the 

 means of an ordinary amateur. 



In such large tanks as those referred to it is common to force 

 a fine jet of air into the water by machinery. Sometimes this air 



