34 MICRO-ORGANISMS AND DISEASE. [CHAP. 



plug is replaced in the 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 therefore 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, then placed in the incubator, 

 boiled again the next day and put back in the incubator, 

 where it is left at a temperature of from 32 to 38 C. for 

 several days. If after a week or so the fluid remains limpid, 

 it is of course to be considered sterile. 



Next, the fluid that has been poured into the beaker 

 (covered with the glass plate) is poured as quickly as possible 

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

 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 clean forceps, to pour the fluid as rapidly as is practicable 

 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. 



