in.] VESSELS, ETC., USED IN CULTIVATIONS 19 







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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 130 150 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 very brittle, and it is 

 i hen 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 inch each, are used for 

 the plugging of the flasks and test-tubes. An 

 assertion such as that made by Dr. Williams 

 at the British Association (Biological Section, 

 September 1883), that cotton- wool plugs are not 

 reliable, because they do not protect the fluids in 

 the vessels plugged with them from accidental 

 air-contamination, is to be accepted only as ap- 

 plying to very loose plugs and to cotton-wool 

 not properly sterilised. To good firm plugs of 

 sterile cotton-wool it evidently cannot apply, 

 since all the results of all workers in this field 

 (Pasteur, Sanderson, Cohn, Koch, Klebs, Buchner, 

 and many others) are against it. 



Instruments, such as the points of needles, and forceps, used 

 in the processes of cultivation, lifting up cotton-wool plugs, 

 making cotton-wool plugs, inoculations, &c., must be heated 

 in the open flame of a Bunsen burner, if they are to be 

 absolutely relied on for cleanliness. Scissors and knives used 



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