20 POPULAR HISTORY OF THE AQUARIUM. 



putrescence takes place ; but if many of them have escaped 

 detection by falling into hollows, or by their minuteness, 

 they will putrefy the whole body of water, to the great 

 danger of the remaining animals. This will be seen by a 

 milky appearance pervading the whole. In such a dilemma, 

 the only way of proceeding will be to draw off and filter all 

 the water, removing the animals, for the time, in temporary 

 vessels. The drawing-off can be effected by means of a 

 siphon, so as not to disturb the sediment. Then, taking out 

 the pebbles and sand, rinse them and wipe out the tank ; re- 

 turn the water through a filter, which may be made with a 

 piece of sponge placed at the top of the tube of a funnel. 

 The sponge must not be pressed so closely in as to prevent 

 the water running in a gentle stream. In passing through 

 the air in a very thin column, every drop is brought into 

 contact with it; and this will tend to destroy putrefaction, 

 by chemical combination of the animal matter with the 

 oxygen of the air. After this tlie probability is that the 

 water will remain clear, and care must still be taken to re- 

 move bodies of animals that occasionally die. An occasional 

 partial filtration is a good habit, and a floating piece or two 

 of charcoal may give still further security. 



If a green turbidity arises in the water, it is the result of 



