CLEANSING THE SIDES. 31 



river-water will answer perfectly well. The time 

 and quantity of these additions ought to be regu- 

 lated 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 accuracy, 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. — The sea-water con- 

 stantly holds in suspension millions of the spores (or 

 seeds) of Alga?, ready to adhere and grow as soon 

 as they find a resting-place, and these are parti- 

 cularly abundant in the warm season. Whether 

 those of the green kinds, the Chlorosperms, such 

 as the Ulva, E)iteromorpha., and various kinds of 

 Confervce. be more plentiful than others, or whether 

 they are more easily satisfied with a place congenial 

 to their growth, I know not ; but these grow most 

 obviously, in the proportion of a thousand to one. 

 Before we have kept our tank stocked a fortnight, 

 its ti'ansparent sides begin to be sensibly dimmed, 

 and a green scurf is seen covering them from the 

 bottom to the water's surface, which constantly 

 accumulates, soon concealing the contents of the 

 vessel from distinct observation. On examining 

 this substance with a lens, we find it composed of 

 myriads of tiny plants, mostly consisting of a single 

 row of cells of a light green hue, forming minute 

 threads which increase in length at the extremity ; 

 others display small irregularly puckered leaves 

 of deeper green, which develop themselves into 

 Ulvce, or Enteromor2)h(B. 



