IV.] 



PREPARATION OF CULTURE-MEDIA. 



23 



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 atmo- 

 sphere is not at all times and everywhere equally contaminated 

 (see Prof. Tyndall's observations). I generally avoid under* 

 taking 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 



Fia. 6. A BEAKER CONTAINING A NUMBER OF CULTURE-TUBES PLUGGED 



WITH COTTON-WOOL. 



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 kept them so for a 

 time varying from one to ten seconds, and in some instances 

 I have not seen more than from 10 to 20 per cent, con- 

 taminated. 



Now, having filled the required number of test-tubes and 

 flasks with the desired quantity of fluid, I subject these 

 seriatim to boiling. By means of an ordinary test-tube 



