CHEMICAL CHANGES 



205 



present in the vessels or materials which are to contain those to 

 be studied, and to guard as much as possible against contamina- 

 tion from outside sources, such as the air. Bacteria may be pres- 



Fig. 48. Colonies of bacteria growing in a gelatine plate. Kansas Station. 



ent as spores which are a resting, or "seed" stage of bacteria, and 

 are much more difficult to destroy than the growing bacteria. 



Methods. Forceps, cover slides, etc., are sterilized by heating 

 in a bunsen burner. Glassware, such as flasks, beakers, pipettes, 

 etc., and other articles which can be subjected to it, may be 

 sterilized by dry heat one hour in an air bath or oven at a tem- 

 perature of 170 C. In order to prevent contamination after 

 sterilization, they are placed in closed vessels until needed. Flasks 

 and pipettes may be plugged with cotton wool, which allows the 

 entrance of air, but excludes bacteria. 



