84 BACTERIOLOGY 



fluid hot. It should not come into contact with the hands or 

 with any large quantity of organic matter. 



CLEANING FLUID. 



Potassium or sodium bichromate 40 grams. 



Water 150 c.c. 



Dissolve the bichromate in water, with heat; 

 allow it to cool; then add, carefully, con- 

 centrated commercial sulphuric acid 230 c.c. 



Exact proportions are not necessary in making this fluid. Glass- 

 ware cleaned in it must be repeatedly rinsed subsequently. 



Plugs. The clean dry tubes or flasks are plugged with raw 

 cotton of a good grade which does not char too readily upon 

 heating. The cotton plugs may be carefully made by rolling 

 an oblong rectangular strip, of even thickness, into a firm cylinder 

 of proper size, rolled plugs, or more hastily made by stuffing the 

 cotton into the open end of the flask or tube, stuffed plugs. The 

 latter kind of plug serves very well for tubes in which media are 

 to be stored temporarily but is not so satisfactory for other 

 purposes. 



Sterilization. After plugging, the tubes are placed in a wire 

 basket and sterilized in the hot-air sterilizer or, sometimes, to 

 avoid charring, in the autoclave. This not only renders the 

 glassware free from bacteria but also gives more permanent 

 form to the plugs. 



THE COMMON CULTURE MEDIA. 



Broth. Broth, bouillon or beef- tea, is best made from fresh 

 meat, either beef, veal or chicken. Finely chopped lean meat, 450 

 to 500 grams, is mixed with 1000 c.c. of distilled water and either 

 allowed to stand over night in the refrigerator or else digested 

 for half an hour at temperature of 50 to 55 C. It is then strained 

 through muslin, yielding a filtrate of deep red color. Any ex- 

 cessive amount of fat should be skimmed off. To the filtrate, 

 which should measure 1000 c.c., are added: 



