274 CLEANSING THE TANK. 



fectly well. The time and quantity of these additions 

 ought to be regulated by a hygrometer, the specific 

 gravity of the sea-water being maintained at about 

 1027, which is the average density of the waters 

 of the Atlantic. A tolerable approximation to accu- 

 racy, however, may be made, by marking on the 

 vessel the surface-level at first, and always maintaining 

 the same level. A glass cover greatly prevents loss 

 from evaporation, as will be manifest by the condensed 

 moisture on it, especially after a cold night. 



Cleansing the sides. — Though a few Periwinkles, as 

 already observed, will keep down the accumulation of 

 green confervoid growth on the sides of the Aquarium, 

 they will not do their work so regularly as to prevent 

 an unsightly appearance. Hence, about once in a 

 month, it will be well to take a stick with a bit of 

 soft, clean rag tied on the end, and rub off the green- 

 ness. It is easily accomplished, as the adhesion to 

 the glass is very slight ; but care must be taken, not 

 to disturb the animals or plants more than can be 

 helped ; and also not to remove any spawn that may 

 have been laid on the sides of the vessel. 



The accumulation of the green deposit, however, 

 on the rocks and stones in the Tank, must not be 

 cleared away, but be cordially welcomed. The spores 

 of the Green Algse, thus profusely scattered, soon 

 form, all over the bottom and on all projections, a 

 tender growth, which gives off oxygen-bubbles in 

 astonishing numbers, conducing immensely to the 

 health of the animals. As soon as this begins to as- 

 sume a woolly or downy appearance, the success of 

 the Aquarium may be considered as no longer proble- 



