CLEANING GLASSWARE. 9 



and boil for at least one hour, not using a preliminary sterilization in 

 the autoclave. The tubes are now cleaned with a test-tube brush, 

 thoroughly rinsed with tap water and placed in a i% solution of 

 hydrochloric acid for a few minutes; then rinsed thoroughly in 

 water and placed in test-tube baskets, mouth downward, and allowed 

 to drain over night. When thoroughly dry they may be plugged and 

 sterilized. To plug a test-tube, pick out a little pledget of plain 

 absorbent cotton about two inches in diameter from a roll. Place 

 it over the center of the tube and with a glass rod push the cotton down 

 the tube about an inch. The cleaning fluid commonly used in labora- 

 tories consists of one part each of potassium bichromate and com- 

 mercial sulphuric acid with ten parts of water. This is an excellent 

 mixture for cleaning old slides, etc., especially when grease or balsam 

 is to be gotten rid of. It is very corrosive, however. An efficient and 

 less corrosive methcd for cleaning slides and covei>glasses is to leave 

 them over night in an acetic acid alcohol mixture (two parts of glacial 

 acetic acid to one hundred parts of alcohol). After drying and 

 polishing out of this mixture, it is well to pass the slides and cover- 

 glasses through the flame of a Bunsen burner or alcohol lamp to 

 remove every vestige of grease. Ordinarily, rubbing between the 

 thumb and forefinger with soap and water, then drying with an old 

 piece of linen, and finally flaming will yield a perfect surface for 

 making a bacterial preparation. 



CONCAVE SLIDES, FERMENTATION TUBES. 



The concave slide is ordinarily used for making hanging-drop prepara- 

 tions. A substitute which is equally good may be made by spreading 



FIG. 3. Hanging drop, over hollow ground slide. (Williams.} 



a ring or square of vaselin smaller than the cover-glass to be used 

 in the middle of the slide. Then putting a loopful of salt solution 

 in the center of the space, and inoculating with the culture to be 

 studied, we finally cover it with a cover-glass, gently pressing the mar- 

 gins down on the vaselin. This gives a preparation for the study 



