The Scottish Naturalist. 355 



The presence of a little vegetation in the tanks promotes very- 

 much the purifying process, such vegetation absorbing the car- 

 bonic acid given off by the animals, and eliminating the oxygen. 

 It is to be observed that the purifying process is essentially and 

 purely a chemical one. . 



As in summer the heat causes the water to absorb a diminished 

 quantity of air, this loss is compensated for by making the 

 machinery work much quicker, thereby causing the water to 

 circulate much more quickly from the reseivoirs into the tanks. 



In summer there is a tendency amongst both animal and 

 vegetable substances to undergo a process of decomposition, 

 called eremacausis. When this occurs they require to be 

 removed. This may be accomplished in two ways — viz., either 

 by increasing the speed of the circulating machinery, or if any 

 of the tanks should be very bad, the whole of the water in these 

 can be readily emptied by means of a syphon, into the reservoir 

 below, such a large body of water not being contaminated in 

 any sensible degree thereby. 



The temperature of the building is always kept very cool 

 during summer ; being on an average 62 Fahr. This average 

 temperature is sustained all the year round. In winter hot 

 water-pipes are used to keep up an equable temperature of from 

 6o° to 6 5 Fahr., and the water at 55 Fahr. 



The lighting of the aquarium is a matter of the utmost im- 

 portance, as it greatly depends upon this, whether the water 

 in the aquariums can be kept pure, and those animals which prefer 

 the darkness, or those which prefer light, can be comfortably 

 lodged. 



The animals in the tanks maybe either viewed through the plate 

 glass forming the fronts, just as if you were looking in at a 

 a shop window; or, as in the case of a table or flat tank, 

 from above, much the same as one would view specimens in 

 the flat cases in a museum. The amount of light admitted should 

 be the smallest amount possible, and just sufficient enough to 

 allow the visitors to see the animals. The jeason for this is, that in 

 the sea those animals which prefer the darkness can readily gratify 

 their natural instincts by descending to the bottom, or hiding 

 under sea-weeds, in the crevices of the rock, or under the sand ; 

 while those that prefer the light can easily obtain it. In cap- 

 tivity, all these little matters must therefore be attended to, in 

 order that the animals may live in a healthy state. 



The diminished light has also the influence of preventing 



