66 PURE CULTURES OF 



This fact has long been known, and the cotton-wool filters 

 employed by Schroder and Dusch (1854) in their famous 

 experiments in connection with spontaneous generation have, 

 as is well known, led to an extended and varied application 

 in practice. 



In order to determine the most practical form of cotton- 

 wool filter for the propagating apparatus already described, I 

 requested my assistant, Mr. Poulsen, to carry out a series of 

 experiments (1887), and these gave the following results: 

 When it was found that metal tubes like those shown in 

 Figs. 8 and 10 (g and in) were suitable in form and size, the 

 experiments were made with them. As previously stated, 

 these tubes will hold a column of cotton-wool 22 cm. long 

 and 3 cm. in diameter. At the lower end there is a short 

 tube about f cm. in diameter. The other end is open, but is 

 provided with a mouth-piece, which screws on to it and 

 terminates above in a short tube. The cotton-wool is intro- 

 duced in small portions, and is rammed tight with the help 

 of a cylindrical rod. Before the mouth-piece is screwed on, 

 some cotton-wool is put into it to catch the coarser impurities. 

 It is important to note that no cotton-wool must be pressed 

 into the tube of the mouth-piece. The air is led through this 

 into the filter ; the opposite end, through which it passes out, 

 is closed with a tight plug of cotton-wool before the filter is 

 sterilised. The sterilisation is readily effected by heating the 

 filter for two hours at about 150 C. in an ordinary sterilising 

 oven. As there might be some doubt, from statements made by 

 Klein, whether such cotton-wool filters become sterilised right 

 through to the middle, experiments were also made to decide 

 this point. They proved that the middle portions of the 

 filter were sterile. In the trials with a filter sterilised as 

 above, a considerable volume of air was forced through under 

 a pressure of 3-4 atmospheres, and then passed into flasks 

 containing sterilised yeast-water, a liquid very favourable to 

 the development of bacteria. The flasks were exposed to a 



