260 THE BOTTOM. 



introduced into tlie water whicli contains animals. 

 The water in wliicli it is soaked should be frequently 

 changed ; and as long as any prismatic scum appears 

 on the surface, the cement is unfit for use. I have 

 known a whole consignment of animals killed in one 

 night from a neglect of this precaution. 



The Bottom. — As very many marine animals 

 buiTOW, and as the observation of their proceedings is 

 very interesting, they should be provided with the 

 means of gratifying their inclinations. For this pur- 

 pose a layer of sand should be put on the bottom of 

 the tank, which may vary in depth from one to three 

 inches. If sand from a sea-beach can be readily 

 obtained, it is the most suitable : but the next best is 

 coarse river-sand, such as the Thames sand com- 

 monly sold at the stone-wharves of London for 

 building purposes. It should be well washed, imtil 

 the water runs away clean : fresh water will do very 

 well for this, but it must be drained off before the 

 sand is put in. What is called silver-sand, and the 

 common yellow earthy sand, sold in the shops for 

 scouring, are not at all suitable, as they will tinge the 

 water after any amount of washing, the former with 

 lime, the latter with ochre. 



Small pebbles or fine gravel, likewise well washed, 

 may be used to vary the bottom with the sand. 



Masses of rock, of dimensions suitable to the Aqua- 

 rium, should be put in, to afford shelter and conceal- 

 ment to such animals as like the gloom. To afford 

 this in the highest degree, a flat piece may be set, 

 like a table, or cromlech^ upon two or three upright 

 blocks J or two tall pieces may lean against each 



