If any pipes are required in connection with the water supply of the aquarium, according to either of the suggestions in a later portion of this chapter, such pipes may be fixed in their proper places as the cement sides are being built up. The next type of aquarium we have to describe is of low cost as far as the materials are concerned, and one that may be made by any one who has had a little experience in woodwork; and although the tank itself is of a simple rectangular form, yet it may be made to look very pretty with a suitable adjustment of rocks and weeds. It consists of a rectangular box, the bottom, ends, and back of which are of hard wood, firmly dovetailed together, and the front of plate glass let into grooves in the bottom and ends. All the joints and grooves are caulked with marine glue, but no paint should be used in the interior. This form of tank may be vastly improved by the substitution of slabs of slate for the wood, though, of course, this change entails a much greater expenditure of both time and cash; but supposing the work to be well done, the result is everything that could be desired as far as strength and durability are concerned. [Illustration: FIG. 36.--AQUARIUM OF WOOD WITH GLASS FRONT] In either of the rectangular tanks just described glass may be used for two sides instead of one only; and since this is not a matter of very great importance, the choice may well be left to the fancy of the one who constructs it. Some prefer an aquarium with glass on all sides, and where this is the case the framework may be made of angle zinc with all the joints strongly soldered. Such an aquarium may be made in the form of any regular polygon, for it is no more difficult to construct one of six or eight sides than of four. It is more difficult, however, to make such an aquarium perfectly watertight, for the glass, instead of being in grooves, has to be securely fastened to the metal frame by means of a cement on one side only, and this cement has to serve the double purpose of holding the glass and keeping in the water. Various mixtures have been suggested for this purpose, and among them the following are perfectly satisfactory:-- 1. Litharge 2 parts Fine sand 2 " Plaster of Paris 2 " Powdered resin 1 part Mix into a very thick paste with boiled linseed oil and a little driers. 2. Red lead 3 parts Fine sand 3 ” Powdered resin 1 part Mix with boiled linseed oil as above. Both these cements should be applied very liberally, and the aquarium then allowed to remain quite undisturbed for at least two weeks before any water is introduced. [Illustration: FIG. 37.--HEXAGONAL AQUARIUM CONSTRUCTED OF ANGLE ZINC, WITH GLASS SIDES]