94 INCUBATING OVENS AND THERMO-REGULATORS. 
When the thermometer is immersed in a water bath the tem- 
perature of which it is desired to regulate, and the proper electric 
connections are made, it acts as a circuit breaker. When the de- 
sired temperature is reached the mercury in the tube of the ther- 
mometer touches the wire b (Fig. 66), an electric circuit is com- 
pleted, and the valve is closed, shutting off the gas supply and 
preventing the temperature from going any higher. When contact 
is broken in the thermometer tube the valve opens and permits the 
gas to flow again. A small opening, 0 (Fig. 67), permits the con- 
stant flow of a sufficient amount of gas to prevent the flame from 
being extinguished. In practice, however, it is better to have a 
small side jet of gas, quite independent of that which passes through 
the valve, which burns constantly and relights the principal jet when 
the valve is opened. This apparatus is very well adapted for regu- 
lating the temperature of a water bath with precision, but for gene- 
ral use in connection with incubating ovens the ordinary gas regu- 
lator is preferable, on account of the trouble connected with keeping 
a galvanic battery in order when it is required to act at frequent 
intervals ‘‘ on a closed circuit,” for weeks and months together. 
The incubating apparatus of D’Arsonval is shown in Fig. 68. It 
is a cylindrical vessel of copper having double walls, and is provided 
with the thermo-regulator of D’Arsonval, by which very accurate 
regulation is maintained at any desired temperature. In its form 
this apparatus is not asconvenient as are the brood ovens made 
in the form shown in Fig. 58, with a swinging door which gives 
easy access to the interior, which is provided with one or more 
shelves upon which the cultures are placed. Various modifications 
