ISOLATION AND PURE-CULTURE METHODS 15 



work, it is necessary to have glassware which is not only clean with 

 relation to extraneous substances, but which is as far as possible 

 free from the soluble substances which may be contained in the 

 glass itself. In the first place, it is well to have vessels of Jena or 

 of the best Bohemian glass. Such glassware . may be first cleaned 

 by the ordinary process. This is followed by boiling in the potash 

 or other alkaline solution. The vessels are then rinsed and boiled 

 in weak hydrochloric acid, and rinsed again. Finally, they are filled 

 with distilled water and steamed for a number of hours. 



In this connection it may be said that cover glasses which have 

 been perfectly cleaned and dried will give more trouble in the 

 preparation of hanging drop cultures than those less perfectly 

 washed. On the former there is a tendency for the drop to spread 

 or to shift its position at the slightest movement. Loss of stability 

 in the drop should, however, be sacrificed to absolute cleanliness 

 if one is doing quantitative work. The drop will have even a 

 greater tendency to spread if the cover glasses are flamed imme- 

 diately before being used. To avoid this latter difficulty, if wiped 

 with a clean sterilized linen cloth and placed in a sterile Petri dish 

 just as they are taken from the alcohol, there will be practically no 

 danger of contamination. 



Cover glasses which are to be used in making preparations of 

 bacteria should be absolutely clean, and the method above mentioned, 

 namely, boiling in an alkali, in acid, and in distilled water is requi- 

 site. They should be air-dried from strong alcohol. Thus prepared, 

 the covers will permit the operator to spread uniformly over the 

 surface a drop containing bacteria. 



III. THE PRINCIPLES AND METHODS OF STERILIZATION 



Vessels and media. Sterilization, as the term is generally em- 

 ployed, is merely the process of killing all of the organisms or 

 spores which may be present in a medium or vessel or upon a 

 given object. In culture work sterilization, therefore, is more par- 

 ticularly employed when a substance or vessel is to be used for 

 the culture of a particular organism, or to preserve nutrient media 

 from decomposition. The common and most effective method of 

 sterilization is by means of heat. Some important uses of chem- 

 ical agents in sterilizing are not here considered. Steam heat or 



