X. 
INCUBATING OVENS AND THERMO-REGULATORS. 
THE saprophytic bacteria generally, and many of the pathogenic 
species, grow at the ordinary temperature of occupied apartments 
(20° to 25° C.); but some pathogenic species can only be cultivated 
at a higher temperature, and many of those which grow at the 
“‘room temperature” develop more rapidly and vigorously when 
kept in an incubating oven at a temperature of 35° to 38° C. Every 
bacteriological laboratory should therefore be provided with one or 
more brood ovens provided with thermo-regulators to maintain a - 
constant temperature. These incubating ovens are made with dou- 
ble walls surrounding an air chamber. The space between the dou- 
ble walls is filled with water, which is usually heated by a small gas 
flame. The gas passes through the thermo-regulator, and its flow 
is automatically controlled for any temperature to which this is ad- 
justed. The exterior of the incubating oven is covered with felt or 
asbestos to prevent the loss of heat by radiation. A simple and 
cheap form which answers every purpose is shown in Fig. 58. The 
quadrangular box with double walls should be made of zinc or cop- 
per. An outer metal door covered with non-conducting material, 
and an inner door of glass, give access to the interior space ; and a 
thermometer introduced through an aperture in the top (Fig. 58, b) 
shows the temperature of this space when the door is closed. The 
stopcock e permits the drawing off of the water from the space be- 
tween the double walls, and the glass tube d shows the height of 
the water, as it is connected with the space containing it. The 
thermo-regulator passes through an aperture at one side of the oven 
into the water, the temperature of which controls the flow of gas. 
The ordinary thermo-regulator is shown in Fig. 59 as manufac- 
tured by Rohrbeck. A glass receptacle, shaped like an ordinary 
test tube, has an arm, c, for the escape of the gas, which enters by 
the bent tube a, which passes through a perforated cork and is ad- 
justable up and down. Tube a is connected with the gas supply and 
tube ¢ with the burner by means of rubber tubing. <A glass parti- 
tion extending downward as a tube, g, makes an enclosed space in 
