General Rules

Some years ago the writer called attention* to the fact that quite as much depends upon the manner in which a cement is used as upon the cement itself. The best cement that ever was compounded would prove entirely worthless if improperly applied. The following rules must be vigorously adhered to if success would be secured:

1. Bring the cement into intimate contact with the surfaces to be united. This is best done by heating the pieces to be joined in those cases where the cement is melted by heat, as in using resin, shellac, marine glue, etc. "Where solutions are used, the cement must be well rubbed into the surfaces either with a soft brush (as in the case of porcelain or glass), or by rubbing the two surfaces together (as in making a glue joint between two pieces of wood.)

2. As little cement as possible should be allowed to remain between the united surfaces. To secure this the cement should be as liquid as possible (thoroughly melted if used with heat), and the surfaces should be pressed closely into contact (by screws, weights, wedges or cords) until the cement has hardened.

Where the cement is ft solution (such as gum in water) and the surfaces are very absorbent (such as porous paper), the surfaces must be saturated with cement before they are brought together.

4. Plenty of time should be allowed for the cement to dry or harden, and this is particularly the case in oil cements such as copal varnish, boiled oil, white lead, etc. When two surfaces, each half an inch across, are joined by means of a layer of white lead placed between them, six months may elapse before the cement in the middle of the joint has become hard. In such cases a few days or weeks are of no account; at the end of a month the joint will be weak and easily separated, while at the end of two or three years it may be so firm that the material will part anywhere else than at the joint. Hence, where the article is to be used immediately, the only safe cements are those which are liquified by heat and which become hard when cold. A joint made with marine glue is firm an hour after it has been made. Next to cements that are liquified by heat, are those which consist of substances dissolved in water or alcohol. A glue joint set? firmly in twenty-four hours; a joint made with shellac varnish becomes dry in two or three days. Oil cements, which do not dry by evaporation, but harden by oxidation (boiled oil, white lead, red lead, etc.), are the slowest of all.

*Technologist, Vol. I (1870), page 188.

Aquarium Cement

Litharge; fine, white, dry sand and plaster of paris, each 1 gill; finely pulverized resin, 1/3 gill. Mix thoroughly and make into a paste with boiled linseed oil to which dryer has been added. Beat it well, and let it stand four or five hours before using it. After it has stood for 15 hours, however, it loses its strength. Glass cemented into its frame with this cement is good for either salt or fresh water. It has been used at the Zoological Gardens, London, with great success. It might be useful for con structing tanks for other purposes or for stopping leaks.