16 THE AQUARIAN NATURALIST. 



ascertain that this point is not exceeded, especially 

 during the hot months of summer. The reduction of 

 this gravity can be readily effected by the addition of 

 rain- or distilled water. Many of the creatures will 

 of themselves afford indications of this increase of 

 density. Some of the Actiniae will remain closed, and 

 become coated with a white slimy covering, within 

 which they remain for a length of time ; when, if the 

 specific gravity be lowered, this is very soon ruptured 

 by their expansion, and thrown off by the movements 

 of the tentacula. 



The importance of the foregoing remarks will be at 

 once appreciated, when we remember, that while the 

 water of the marine aquarium is undergoing a continual 

 diminution by evaporation, the salt does not evaporate ; 

 so that the solution of salt, becoming progressively 

 stronger and stronger, would be at last converted 

 into brine so strong, that no animals could exist in 

 such a pickle. A " specific-gravity bubble/' exactly 

 adjusted for the purpose, and which may now be 

 purchased from any dealer in Aquaria, left to float in 

 the tank, will indicate, by rising to the top, when dilu- 

 tion is requisite, and its faithful monitions should be 

 at once attended to. 



Our tank being thus prepared and filled with pure 

 sea-water, or its equivalent, the artificial solution of 

 marine salts, we next proceed to place in it appropriate 

 vegetation, as a necessary preliminary to the introduc- 

 tion of animal life. The plants employed for this 

 purpose must of course be all natives of the sea, and 

 fortunately such is the abundance and variety to be 

 met with on every coast, that the difficulty lies in 



