ARTIFICIAL SALT WATER. 157 



specially prepared for aquarium keepers, and may be 

 purchased at the natural history dealers. We confess, 

 however, to a disinclination to its use, although we have 

 seen some healthy and beautiful marine tanks, full of 

 life and vigour, supported by water prepared in this 

 manner. At Berlin and Hanover, the marine tanks 

 have their sea water manufactured artificially. The 

 best way to use it, after mixing carefully in an earthen 

 jar, is to allow it to stand in a quiet place, with a few 

 sea-weeds thrown into it, and afterwards expose it 

 to the sunlight. Strangely enough, spores will then 

 develop in it, having come from the atmosphere ; or 

 been given off by the sea-weeds. Owing, however, to 

 the conveniences which railways and railway excur- 

 sions now afford, almost every intending aquarium 

 keeper can get his own sea water from the sea itself. 



Sea water can easily be made to indicate the re- 

 lative saltness or otherwise by using the " specific 

 gravity beads," sold by London dealers. There are 

 two kinds of them, both in the shape of pretty thin 

 glass balls. One of these floats when the water is 

 of the right strength, and the other sinks. Directly 

 the floating ball begins to sink it indicates that the 

 water is weak in saline matter ; whilst if the sinking 

 bead rises, it is time to add a little fresh water until it 

 falls again. Both these kinds of balls, or floats, are 

 used in small tanks, and they are usually differently 

 coloured, so as to be soon recognised. A good 



