A Brief History of a Marine Tank. 265 



had much, attention, and was kept to a regular standard of 

 density. By degrees I slackened in my attentions, and this 

 did not result in disaster. Once, during the summer of 1863, 

 after long absence from home, and much absolute neglect of 

 the tank, I observed that the water had sunk to a low level, 

 and I inserted a hydrometer. What was my horror to find it 

 register a specific gravity of 1,416 ! Yet even then the 

 mesembry anthemums appeared as lively as usual, and dianthus 

 was slowly imbibing and contracting, puffing itself out to 

 enormous dimensions, and then becoming constricted, as if 

 tied with a cord, as is the wont of this lovely conjuror of the 

 deep. I found after a time that instead of adding fresh water 

 by slow degrees, as I did at first, I might add sufficient at once 

 to reduce the specific gravity to 1,028 without any harm to 

 the inmates. They would, perhaps, close for a few minutes, 

 and then shine out with greater splendour than before. In 

 every case when the water acquired an unnatural degree of 

 density, the periwinkles attached themselves firmly, shrunk 

 down deep into their shells, and drew their shutters down as 

 tight as possible, and so remained till things came right again, 

 when they would make up for lost time by increased activity 

 and voracity. Since then the tank has frequently lost as much 

 as two inches depth of water by evaporation, and I once found 

 the specific gravity to be 1,529 ! Yet nothing worse hap- 

 pened than a general torpor and constriction, and a few hours 

 after the reduction of the density to a proper standard, the 

 creatures were all as lively as ever. 



I have referred to cloudiness caused by deaths, but it is right 

 I should say, that with the exception of those few occurrences, the 

 tank has always been delightfully bright, and there never occurred 

 any growth on the front glass. When unstocked on the 24th 

 of June last, the water had the same bright appearance as when 

 it was first dipped in the English Channel ; but there were 

 floating on the surface some small patches of brown flocculent 

 matter, a sort of scum which I thought resulted from a gradual 

 accumulation of a linty sort of dust, the result of the tank 

 being amongst books, and which had crept in by the small 

 slit of the sixteenth of an inch which intervenes between the 

 edge of the glass cover and the side of the tank at each end. 

 The water was always rich in microscopic life, and its capacity 

 for sustaining a mixed collection of marine animals appears to 

 be in the proportion of not more than one to a gallon. For so 

 large a creature as a dianthus I should think two gallons none 

 too much, with a shallow vessel, and vegetation produced 

 in situ as essential elements of success ; whereas with similar 

 advantages two mesembry anthermims or daisies might be kept 

 in a gallon of water. Even a periwinkle needs nearly a gallon 



