40 MICRO-ORGANISMS AND DISEASE [chap. 



labour and time. Long experience in these matters has 

 taught me that, although in some instances less scrupulous 

 care has not been followed by bad results, still I have seen 

 occasionally unpleasant failures owing to slight laxity in 

 these matters. 



Several weeks' work may be annihilated by a single omis- 

 sion. Sometimes one is perhaps in a sHght hurry, and does 

 not think the want of an additional heating of the test-tube 

 or cotton-wool or an additional boiling of the fluid will be 

 followed by any bad consequences. But, alas, nature does 

 not take into account our convenience, and failure is our 

 reward. If in any kind of experiments " overdoing " is an 

 error in the right direction, it is in these very experiments in 

 the cultivation of micro-organisms. 



The cotto7i-wool used for plugging flasks and test-tubes is 

 prepared by pulling up loosely a quantity of good cotton- 

 wool and exposing it in a loose state in the air-chamber to a 

 temperature of 130° to 150° Q,. for several hours on two suc- 

 cessive days. The cotton-wool ought to be just slightly 

 brownish — i.e. just faintly singed. Too much singeing makes 

 it brittle, and it is then difficult to make of it a satisfactory 

 plug. The plug used should not be too firm and not too 

 loose : in the former case it is not easy to lift it up quickly, 

 and in the latter it does not close sufficiently well. Cotton- 

 wool that has been kept in the air-chamber for an hour or 

 two is not absolutely sterile ; nor is cotton-wool that has been 

 kept in a compressed state in the air-chamber for several 

 days. The central portions remain under these conditions 

 quite white and are not sterile. No cotton-wool that is 

 not just brown — i.e. just faintly singed — is safe from risk of 

 impurity. No cotton-wool steeped in absolute alcohol, 

 strong carbolic acid, or any other disinfecting fluid, for ever 

 so many days or weeks, can be absolutely relied on. 



