METHODS OF CULTIVATION. 181 



A rectangular copper vessel, having double 

 walls to contain water, enclosing an air-chamber, 

 will be found most suitable for this purpose. When 

 the space between the double walls is filled, the 

 air-chamber is surrounded with water on all sides, 

 except that through which access to it is ob- 

 tained. This side is closed by a swinging or 

 sliding door. If the oven is of considerable size, 

 it is well to have one or more adjustable shelves 

 in the interior, upon which tubes and flasks may 

 be placed, as well as upon the floor. A suitable 

 aperture at the top admits the thermostat to the 

 water-bath, and another aperture serves for the 

 introduction of more water when required. A 

 third aperture, through the centre of the upper 

 side of the oven, leads to the air-chamber, and ad- 

 mits of the introduction of a thermometer, the in- 

 dex of which can be read outside while the bulb is 

 inside of the oven. In a well-equipped laboratory 

 several of these culture-ovens will be required, as 

 experiments conducted at different temperatures 

 will often be under way at the same time. 



The most convenient way of heating an oven 

 of this kind is by the use of gas and of a Bunsen 

 or other burner, which insures the complete com- 

 bustion of the carbon. When gas is used, the 

 thermostat described below, well known in chemi- 

 cal laboratories, may be employed. 



Thermostat for Gas (Fig. 7). The elongated 

 glass bulb a contains a certain quantity of mer- 



