BULLETIN OF THE UNITED STATES FISH COMMISSION. 275 
of care and responsibility as well as to interest and take the mind from 
grosser things, nothing can have a higher value. 
THE AQUARIUM TANK. 
Aquarium tanks are made wholly of glass, or with a frame-work of 
some kind of metal, as of iron or brass, and of wood, slate, and marble, 
or any of these materials in combination with metal. 
Glass, slate, and marble being substances devoid of properties hav- 
ing an injurious chemical action in connection with water, or where 
there is, as in the aquarium, more or less of decomposition of organic 
matter, need no further mention in this connection. 
The iron tank, however, demands especial attention. By reason of 
the cheapness of the material, by far the greater number of rectangular 
tanks in use are made of cast-iron. Without entering specifically into 
the chemical actions involved in the introduction of iron rust (oxide of 
iron), or of the decomposition of the various varnishes or mineral paints 
used to prevent oxidation, in connection with decaying organic matter, 
it need only be stated that they are in the highest degree injurious. 
Galvanizing is often resorted to to overcome this difficulty, but it is not 
a satisfactory protection. In the French navy galvanized-iron tanks 
have been abandoned as receptacles for drinking-water for the reason 
that poisonous oxides are given off. For the same reason the U. S. Fish 
Commission has refused to use galvanized tanks on its ships. Undoubt- 
edly the best material for preventing oxidation is a coating of the best 
grade of Portland cement, which will form, if properly applied, a solid, 
adhesive, protective surface, perfectly pure and practically indestruct- 
ible. It is mixed with water and applied like mortar ; first, however, 
scraping from the iron any paint or other substance of smooth and oily 
nature, to which it will not adhere well. After standing twenty-four 
hours to set, it should be soaked for two or three days before using, to 
remove all the lime, the water being changed several times in that in- 
terval. At any time that the tank is not in use it should be allowed to 
stand with water in it, for fear that the cement may crack or become 
loose from the unequal expansion and contraction of the two substances. 
Otherwise it is practically indestructible. The cement should not be 
allowed to touch the glass. It can be bevelled off where it approaches 
the glass, after it has set, but before it becomes perfectly hard. Port- 
land cement is used exactly as is plaster of Paris, but becomes much 
harder ; in fact, becomes an artificial stone. 
THE PRINCIPLES INVOLVED IN THE ESTABLISHMENT OF AN AQUA- 
RIUM. 
The vessel used is called an aquarium tank. The term aquarium 
implies such an adjustment of the relations between the animal and 
vegetable life in the aquarium tank that the one will sustain the other. 
