TROPICAL AQUARIUM FISHES 107 
pedient. After hole is cut in slate, pour in lead or tin. When the metal 
is poured, use a form made of putty so that the surface of metal will come 
as high as the sand. The object in using the inverted pan is to have 
the heating surface come just above the sand. Then the heat is the most 
efficient as well as fairly diffused and does not interfere with the roots of 
plants. The objections to this type are that the aquarium has to be raised 
and that there is an occasional drip of condensed water caused by com- 
bustion. The best flame to use is a small gas Bunsen burner. They may 
be had of some scientific apparatus concerns or dealers in German 
aquarium supplies. These dealers also handle a rather good all-glass 
aquarium for heating by lamp, and which does not need to be raised. 
There is, however, the eternal liability of cracking. All-glass aquaria at 
best are liable to crack, and particularly when unevenly heated. In the 
German catalogs and aquarium publications will be found numberless 
heating devices, but after trying many of them and inventing some de- 
fective ones himself, the author finds those described here the most 
practical. Smells are caused by chilling the flame before combustion is 
complete. No more than the tip of flame should be allowed to touch the 
heating surface. Even this is not necessary in system shown in Fig. 76. 
Heat Control. In a room where there are violent changes of tem- 
perature, particularly when these dip to the cold side, it is desirable to 
have some means of heat control. To start in the simplest way first, an 
aquarium may be kept noticeably warmer over night by covering with a 
thick blanket, quilt or any warm fabric. If near a window the curtain 
should be pulled completely down. Another help for any aquarium near 
a window is to have a sheet of glass standing on the base and leaning 
against the top of the aquarium. This sheds much of the cold air which 
constantly falls from a window in cold weather. 
Tropical aquaria may be kept at a satisfactory temperature standing 
on a hot-water radiator, the heating result not being so extreme as might 
be supposed. 
The most satisfactory means of controlling temperature is to heat by 
gas and use a gas thermostat to control gas flow. See Fig. 78. This is 
placed either in the water or tightly against the outside of the aquarium 
and insulated from the influence of surrounding air by plenty of wool or 
cotton batting. It contains a large body of mercury over which the gas 
passes through a small space. As the water rises in temperature, the 
mercury expands and so reduces the gas supply, and vice versa. It is a 
very ingenious and effective device and may be obtained from makers of 
scientific glassware at small cost. Those selling them are glad to give 
instructions regarding regulation, etc. The Arthur H. Thomas Company, 
of Philadelphia, are specialists in this line. With this equipment in oper- 
