EQUIPMENT, APPARATUS, AND LIBRARY 15 



prism of cement behind it, strengthens the angle and also protects the 

 water from a large surface of cement, which might yield up enough of 

 its lead to injure delicate animals. 



Lids may be made either of perforated tin, with the edges turned over 

 to stiffen them, or of wire screen fastened to either wooden or tin frames. 

 The lids will be needed when the aquaria are used as insect>breeding 

 cages and for feeding tests with toads, frogs, or other insectivorous ani- 

 mals. The hinges of brass or tin should be soldered to the frames when 

 they are made. These points are sufficiently illustrated in Fig. 7. 



Aquarium cement. The formula used by the United States Fish Com- 

 mission is : by weight 6 parts whiting, 1 part red lead, 1 part litharge ; 

 mix thoroughly while dry, and as required for use make into a stiff 

 putty with pure linseed oil. The oil must be free from any trace of 

 adulteration with iish oil, or the cement is likely to remain soft and 

 sticky. Do not expect the cement to " set " hard. If it did this, the 

 expansion and contraction, under changes of temperature of metal and 

 glass with which it is associated, would be likely to crack either the 

 glass or the cement. It should remain as a tough gum, solid enough to 

 support the pressure of water and pliable enough to accommodate 

 itself to changes of temperature. 



Lay the right amount of cement in all the angles of the frame with 

 a putty or case knife, and press the glass for bottom, sides, and ends 

 into place in the order named ; smooth all joints on the inside, removing 

 any extra cement ; spring pliable green twigs against the sides and ends 

 to hold them in place, and smooth up the outside joints. It is prefer- 

 able to paint the frame, letting the paint dry well, before setting the 

 glass. A coat of spar varnish along the angles on the inside will protect 

 the cement from contact with the water and also insure against leaks. 

 Leaks seldom occur, but if they do, mark them, dry the aquarium 

 thoroughly, press a little cement into the cracks, and give another 

 coat of spar varnish to the inside corners. After drying for a week, the 

 aquarium is ready to use. 



NOTE. It is easier to avoid daubs of cement than to clean them off the 

 glass. The same applies to hands, clothes, tables, floors, and everything else. 

 A class in aquarium-making can easily daub themselves and the laboratory 

 so as to disgust all concerned with the work ; or, by the use of a few news- 

 papers, can avoid all this to the satisfaction of everybody. Since the cement 

 contains lead, it is better not to let it come into contact with the skin more 

 than is necessary. Stir, mix, and lay it with proper tools. 



