12 



THE STUDENT S AQUARIUM. 



element, and live 24 hours out of water ? Because it can live 

 on the air while out of its element, but as soon as it has ex- 

 hausted the air that is in the pail of water it must suffocate, 

 unless you devise some way of keeping the water charged with 

 oxygen, air. This must show us at once what is needed, if 

 we desire to keep fish alive without changing the water, when- 

 ever the oxygen is exhausted. It has been known for many 

 years that all true fresh water aquatic plants, and some marine 

 algaes [plants], are capable of supplying oxygen to the water in 

 which they grow or are grown. It is also an established fact that 

 all true aquatic plants take root from a joint. Having become 

 acquainted with these facts, we are prepared to go ahead and 

 see if we can keep fish alive on this principle. Now we will get 

 our vessel, whatever we may have concluded to put the water 

 into, and put at the bottom of one side sand, and the other 

 gravel. This sand and gravel for the fresh water must not be 

 from the sea-shore but such as builders use, and have it washed 

 well before putting it into your tank. When putting it in you 

 can suit your fancy, for it is only for the purpose of helping 

 your captives to keep themselves clean, and the scavengers to 

 retire and rest in. I have stated that I generally put sand at 

 one end and gravel at the other. I have seen it prettily ar- 

 ranged in alternate strips of gravel and sand, and again, with 

 a circle of sand surrounded with gravel, — but in this, as I have 

 said, you may devise some original and pretty way of doing it. 

 Some small sea shells that have been allowed to remain 24 

 hours in water before using them, may be used with perfect 

 safety, and can be mixed with the gravel and sand. You can 

 build very pretty rock work with larger shells, by mixing % 

 Portland cement with % sand ; or if you prefer it, very realistic 

 rocks can be built with pieces of pumice stone. After you 

 have the gravel in and rock work built, it is advisable to fill 

 the tank with water and let it remain 24 hours, and then draw 

 that water off and throw it away. Though arches are generally 



