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 c with the burner by means of rubber tubing. A glass parti- 

 tion extending downward as a tube, g, makes an enclosed space in 



