THE MARINE AQUARIUM. 



41 



injury to the inhabitants. For the purposes of aeration 

 some prefer an arrangement of pipes along the floor of 

 the tank, connected with a pair of bellows, by means of 

 which fine jets of air are forced through holes in the pipe, 

 whence they rise through the water, purifying it as they 

 go. But this plan necessitates constant attention, which 

 is out of the question for business men. On the other 

 hand, the fountain cisterns can be made large enough to 

 hold sufficient water to supply the fountain for a day, and 

 need no looking after. But the great agent to be relied 

 upon for purification is the growths of confervoid algae, 

 of which the spores are certain to be introduced in the 

 sea water with which we fill our tank. It is not advisable 

 to introduce any grown specimens of sea-weeds, as in 

 nearly every case such introduction would be attended 

 with failure. 



Having taken the trouble to procure a supply of water 

 from the sea, or from one of the railway companies that 

 now supply the larger towns with sea-water, we must 

 mind that it is kept up to the natural density. It will 

 be understood that from a large surface of water exposed 

 to the air there is great evaporation, and the water thus 

 lost flies off perfectly pure, leaving all its saline particles 

 behind. Consequently the water in the tank, whilst 

 decreasing in bulk, increases in density, so that if left 

 alone it soon becomes a strong brine in which the 

 animals will be pickled. To obviate this difficulty soft 

 fresh water must be added, but in doing so care must be 

 taken not to make the water too fresh. But the dealers 

 in aquarium requisites supply what are known as "gravity- 

 bulbs," of which there are two kinds. One is so weighted 

 that it remains on the floor of the tank whilst the water 

 is of the proper density, but should the latter get too salt 



