THE MAEINE AQUARIUM 63 



triangular file will then enable the operator to sever the tube at 

 any desired point. 



Yet another method of maintaining the air supply of aquaria is 

 adopted in the case of some of the large tanks of public aquaria 

 and biological laboratories situated close to the sea, and this 

 consists in renewing the water at every high tide by means of 

 pumps. 



It must not be supposed that an indoor aquarium, even when 

 well established, and supplied with the best possible system of 

 aeration, requires no further care and attention. In the first place 

 there is a continual loss of water by evaporation, especially in 

 warm and dry weather, and this must be rectified occasionally. 

 Now, when water containing salts in solution evaporates, the 

 water passing away into the air is perfectly free from the saline 

 matter, and thus the percentage of salt in the residue becomes 

 higher than before. It is evident, therefore, that the loss by 

 evaporation in a marine aquarium must be replaced by the 

 addition of fresh water, which should either be distilled, or from 

 the domestic supply, providing it is soft and moderately free from 

 dissolved material. 



But the question may be asked, ' Do not the marine animals 

 and plants utilise a certain amount of the saline matter contained 

 in the salt water ? ' The answer to this is certainly in the 

 affirmative, for all sea-weeds require and abstract small proportions 

 of certain salts, the nature of which varies considerably in the 

 case of different species ; and, further, all the shelled crustaceans 

 and molluscs require the salts of lime for the development of their 

 external coverings, and fishes for the growth of their bony 

 skeletons. Hence the above suggestion as to the replenishment 

 of loss by evaporation with pure water is not perfectly satisfactory. 

 It will answer quite satisfactorily, however, providing the sea 

 water is occasionally changed for an entirely new supply. Again, 

 since carbonate of lime is removed from sea water more than any 

 other salt, being such an essential constituent of both the external 

 and internal skeletons of so many marine animals, as well as of 

 the calcareous framework of the coralline weeds, we suggest that 

 the aquarium may always contain a clean piece of some variety of 

 carbonate of lime, such as chalk, limestone, or marble, which will 

 slowly dissolve and replace that which has been absorbed. 



Water is rendered denser, and consequently more buoyant, by 

 the presence of dissolved salts; and, since the density increases 



