64 PHARMACEUTICAL BACTERIOLOGY ' 



and is, absolutely clean. The following solution will be found useful 

 as a cleansing agent for old as well as new glassware: 



Potassium Dichromate, 6 parts. 



Sulphuric Acid, 3° parts. 



Water, 4° parts. 



Of course, the sulphuric acid must be added Uttle by little with constant 

 stirring, in order to avoid excessive heat development. Soak the glassware 



Fig. 12. — Wire baskets for holding test-tubes. Cylindrical form and square form. 

 Each basket holds about fifty test-tubes. The wire is galvanized to prevent rusting 

 The round wire baskets should be used. 



in this solution for some time, several hours or more, and rinse, wash, drain 

 and wipe thoroughly afterward. The sole object to be attained is cleanli- 

 ness in the true sense of the word. The glassware must be clean bacterio- 

 logicaUy and chemically; that is, it must be free from microbes and 

 chemical substances. 



2. Plugging Containers with Cotton 



After the thorough cleansing above outlined, the test-tubes and flasks 

 are plugged with a good quality of non-absorbent commercial cotton. 

 The dry cotton plug forms most eflScient germ filter. All microbes are 

 caught and held in the meshes of the cotton, and yet the air is permitted to 

 pass through into the tube or flask. 



Open a roll of cotton, find the free end, and lay it out on the work table. 

 Take the test-tube in the left hand; remove a goodly tuft of cotton with 

 right hand, using thumb and first and second fingers. Place this over 

 the mouth of the tube or flask, and push it down to a distance of J^ to % 

 inch by means of a sohd glass rod rounded (by heat) at the ends. The 

 rod must not be too thick, as it will then not permit enough cotton to enter 

 the opening nor yet too thin, as it will then be forced through the cotton. 

 The plug must not be too tight, as that would interfere with subsequent 

 manipulations nor, yet too loose, for obvious reasons. Enough cotton 

 should project above the opening to permit of ready grasping between the 

 fingers in the later operations. 



