26 BACTERIOLOGY 



may be used either in the form of boiling or of steaming. 



Various special sterilisers have been designed for use in labora- 

 tories, but the amateur 

 bacteriologist, if he does 

 not possess these, can al- 

 ways use any convenient 

 domestic utensil, such as a 

 kitchen pot, fish - kettle, 

 vegetable-steamer, or the 

 like. Surgical instruments 

 are usually boiled up thor- 

 oughly before use, a lump 

 of soda being added to the 

 water to hinder corrosion 

 of the steel. In this method 



FIG. 4.-Hearson's Milk Steriliser. 



of applying heat, the objects steri- 

 lised are exposed to a temperature 

 of 100 C., i.e. the boiling point of 

 water; or if a somewhat higher 

 temperature is desired, e.g. in order 

 to destroy certain bacterial spores 

 which are not killed by boiling even 

 for a considerable time, super- 

 heated ^tearn under pressure may 

 be employed, and is obtained by 

 using a specially strongly con- 

 structed piece of apparatus known 

 as an autoclave, the lid or door of 

 which can be firmly screwed or 

 clamped down so as to tightly - 

 close the heating chamber. A 

 special safety valve is fitted to the 



autoclave, and can be so regulated that the steam " blows off" 

 above a required pressure, the additional temperature above 

 the boiling point being calculated from the pressure indicated 

 on the gauge as in an ordinary steam-boiler. 



FlO. 6. Chamberbmd Autoclave. 



