A CONSTANT LEVEL WATER-BATH
In many college laboratories where live steam is not available for evaporation purposes, it is difficult to maintain a water-bath at boiling temperature without its being watched more or less continuously. The accompanying diagrams outline a simple apparatus which nearly any student can set up and which, when properly adjusted, will eliminate personal attention almost entirely. At the time of writing the authors have had such a bath at boiling temperature heated continuously for 125 hours without any adjustment whatever. The operation of the apparatus is briefly as follows (Figure 1). Water
250
C.c.
FILTER FLASK
&
F'
is siphoned from the filter flask through a siphon tube ABC into the waterbath until the water level in both becomes the same. When this point is reached the siphon temporarily stops flowing, but as the water level in the water-bath goes down, due to evaporation of the water, more water siphons over to equalize the two levels. The water in the filter flask is kept a t the same level continuously by a constant hut very small stream of water from the water tap. Excess water passes through the outlet into the sink. The central tube in the three-hole stopper in the filter flask is an air inlet that maintains the same atmospheric pressure in the filter flask as is at the other end of the siphon Cin the water-bath. Closing this air vent creates back suction through ABC draining the water-bath. On the other hand, if the water outlet into the sink is temporarily closed, the water level in
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-
the filter flask is raised and consequently the water level in the water-bath is raised. The end of the siphon tube B'C' is curved for the purpose of preventing steam bubbles from passing up the tube and stopping the siphon. Tube B C in actual operation, is kept cool by the flow of water into the waterbath. By raising or lowering the filter flask, the water level in the waterbath may be set at any height. In practice it has been found that water from the water mains contains some dissolved gas which separates out when it reaches the part of tube B C
250 c.e. FILTER FLRSK
From 2
that is inside the water-bath, this part heing very hot. These gas bubbles pass up through BC and collect throughout the length of the siphon tube, thereby preventing its operation. For periods of operation of four to six hours this is not objectionable, since the water outlet into the sink may be closed momentarily once or twice during that time and the bubbles will he ejected at C. To make the apparatus capable of operating for periods of several days and nights continuously, an inverted collecting bottle filled with water is inserted in the siphon line A B C , as shown in Figure 2.
