28 MICRO-ORGANISMS AND DISEASE. [CHAP. 



by bad results, still I have had also many 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 slight 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 cotton-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 C. for several hours for several 

 successive days. The cotton-wool ought to be just brown, 

 i.e. just singed. Too much charring 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, say only for a day or two in the air-chamber 

 for three or four hours is not absolutely sterile ; nor is 

 cotton-wool that has been kept in a compressed state in the 

 air-chamber for any number of 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 

 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. 



As stated above, a plug of sterile cotton-wool tolerably 

 firm, of about one to two inches, or two plugs of about one 



