IV] PREPARATION OF CULTURE-MEDIA 47 



neck of the stock flask and the beaker covered with the 

 glass plate. Of course the quantity poured into the beaker 

 should be large enough to supply the required number of 

 test-tubes or small flasks. The stock flask containing still 

 some fluid, having been opened for however short a time, 

 has of course been exposed to air-contamination, and there- 

 fore must be treated accordingly, if the fluid left in it is to 

 serve as sterile nourishing material on a future occasion. 

 Consequently it is subjected to boiling for from fifteen to 

 thirty minutes. 



Next, the fluid that has been poured into the beaker 

 (covered with the glass plate) is poured as quickly as pos- 

 sible into the test-tubes, one after the other, by lifting with 

 sterile forceps the plug and pouring in the fluid to a depth 

 of one and a half to two and a half inches, and the plug 

 replaced. 



During this procedure contamination with air-organisms, 

 if there be any about, becomes inevitable. To lessen this 

 chance as much as possible, it is necessary to lift the plug 

 with sterile forceps, to pour the fluid as rapidly as is prac- 

 ticable into the test-tube or flask, and to replace immediately 

 the cotton-wool plug. Further, it is necessary to bear in 

 mind that the atmosphere is not at all times and everywhere 

 equally contaminated (see Prof. Tyndall's observations). I 

 generally avoid undertaking this process on windy days, and 

 when I do it, I generally close windows and doors and keep 

 the air in the room as still as possible. I do not do it in a 

 room in which recently (say an hour or two previously) the 

 floor, walls, or tables have been swept. 



I have opened under these conditions the plugs of test- 

 tubes containing sterile material, and purposely exposed them 

 for a time varying from one to ten seconds, and I have not 

 seen more than from i to 2 per cent, contaminated. 



