ISOLATION AND PURE-CULTURE METHODS 13 



chemistry must direct. Ordinarily it is not enough to depend upon 

 hot water and soap in cleaning glass vessels. Petri dishes, test 

 tubes, etc., may be boiled for a short time prior to cleaning, and if 

 grease is present, a small quantity of potash lye (about 30 grams 

 per liter) may be added. If the glassware is immersed in water, a 

 porcelain-lined vessel should be used, and the latter may be placed 

 over the flame or in the steam sterilizer. Commercial hydrochloric 

 acid is convenient in many cases for general use. A chromic acid 



FIG. 2. SOME CHIEF TYPES OF GLASSWARE REQUIRED IN STUDENT CULTURE 

 WORK. (Photograph by Geo. M. Reed) 



cleaning mixture has also become quite generally adopted and gives 

 excellent results. This mixture may be made sufficiently strong for 

 ordinary purposes by dissolving 100 grams of potassium dichromate 

 in 1000 cc. of hot water, then when the salt is all dissolved and 

 the liquid cool, pour into it about 500 cc. of strong sulfuric acid, 

 stirring constantly. This liquid should be stored in large-mouthed, 

 glass-stoppered bottles, and used with care. It may be used repeat- 

 edly. When employed, it may act for from ten minutes to twenty- 

 four hours, and it may be followed by water, or soap and water, 

 etc. This mixture is not convenient to handle but is very effective. 

 Test tubes. Ordinarily these should be cleaned with hot water, 

 soap and a test tube brush ; and this cleaning may be preceded or 



