The Marine Aquarium. 
237 
are hurtful to life if over-crowded. In the measureless volume of the sea 
itself, there is a measureless and inexhaustible supply of oxygen for all the 
inmates of the depths. Every crest that forms on the top of a wave, every 
streak of froth and foam, consists of atmospheric air entangled in the water. 
There is no such agitation of the surface of the water in a tank to refresh it 
in this way unless artificially produced; nor does the fresh breeze play over 
it perpetually; nor can it nourish forests of vegetation as the sea does, serving 
as breweries of oxygen for its stores of animal life. It follows from these 
considerations that an over-crowded tank is likely to become a dead-house. 
The prudent practitioner will always keep fewer animals in a given space than 
might be kept, rather than proceed to the other extreme, and risk the whole 
through aiming at too much. 
The question is of the utmost importance. It is a matter of common 
experience and observation that small quantities of water are quickly affected 
by changes of temperature, while large bodies are affected slowly, and never 
to such an extent as the smaller quantities. A small river varies in temperature 
more quickly and extensively than a large one, and all rivers vary in temperature 
more than the sea in the same latitudes. The larger the aquarium, therefore, 
the safer is it against extremes of temperature. Moreover, if built of heavy 
materials that are comparatively non-conductive of heat, the safer will it be 
during periods of great heat in summer, as, before the heat of the day can 
penetrate the mass, the cool of the evening will have returned; and perhaps 
the opening of a window, and the sprinkling of water on the outside of the 
vessel, may suffice to keep the temperature to a safe standard. Slate is one of 
the best materials for marine tanks, because of its great power of resisting 
sudden changes of temperature. Metal is the most convenient material 
in which to made rectangular vessels, but it is not the best in 
other respects, though in many cases it may be used with safety and 
advantage. 
Between marine and fresh-water aquaria there are many distinctions inde¬ 
pendent of the radical distinctions indicated by their names. The aeration of 
a fresh-water vessel is a matter of comparatively small consequence as compared 
with the difficulties that arise through deficiency of oxygen in a marine aquarium. 
So again, variations of temperature are less hurtful to fresh-water than to 
marine animals, and metal tanks may be employed for river fishes, though most 
unfit for genuine mariners. Other distinctions will be pointed out as we 
proceed. It is sufficient for the present that we have regarded the aquarium 
