EVAPORATION — CLEANSING THE SIDES. 285 



rally maintains the most crystalline transparency and 

 purity. 



Evaporation. — If the Tank remain habitually 

 uncovered, or protected only by a coverlid of muslin, 

 daily evaporation will soon reduce the volume of the 

 water, and increase its specific gravity. The pure 

 water alone rises in vapour, the various salts held in 

 solution remaining the same in quantity, though the 

 water should be reduced to half its original bulk. It 

 is therefore needful that additions of pure fresh water 

 (not sea-water) be made from time to time, to replace 

 the loss by evaporation. Distilled water is of course 

 the best, but, practically, river-water will answer per- 

 fectly well. The time and quantity of these additions 

 ought to be regTilated 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 fii'st, and always maintaining the same 

 level. A glass cover greatly prevents loss from eva- 

 poration, as will be manifested by the condensed 

 moisture on it, especially after a cold night. 



Cleansing the Sides. — Though a few Peri- 

 winkles, 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 but that a rough coating of greenness will 

 still dim the transparency of the glass. The unsightly 

 appearance thus left must be got rid of by mechanical 

 means. A stick with a bit of rag tied around its end, 

 or, what is better still, a brush made on pm-pose, like 



