30 METHODS OF STERILISATION 



are easily replaced when broken or worn out. The 

 top of the oven is provided with a perforated ventilator 

 slide and two tubulures, the one for the reception of a 

 centigrade thermometer graduated to 200 or 250 C., 

 the other for a thermo-regulator. An ordinary mer- 

 curial thermo-regulator may be used but it is prefer- 

 able to employ a regulating capsule of the Hearson 

 type (see p. 219) with a spring arm adjusted to the lever 

 so that when the boiling-point of the capsule (e.g., 



FIG. 26. Hot-air oven. 



175 C.) is reached the gas supply is absolutely cut off 

 and the jet cannot again be lighted until the spring 

 arm has been readjusted by hand. The thermo- 

 regulator is by no means a necessity, and may be 

 replaced by a large bore thermometer with a sliding 

 platinum point, connected with an electric bell, which 

 can be easily adjusted to ring at any given temperature. 

 Even if the steriliser is provided with the capsule 

 regulator above described the contact thermometer 

 should also be fitted. 



