SYSTEMATICALLY SELECTED YEASTS. 67 



temperature of 30 C. for at least fourteen days. In order to 

 test for mould and yeast fungi, similar experiments were 

 made with sterilised beer-wort. In all cases the experiments 

 were so conducted that had any germs been present in the 

 air they must have been taken up by the liquid. In addition 

 to this, portions of the cotton-wool from the upper end of the 

 tube were afterwards introduced as quickly as possible, and 

 with the necessary precautions, into flasks containing the 

 sterilised liquids mentioned. As was to be expected, it was 

 found that the latter always contained living organisms. 

 This was also the case with the air passing through the 

 filter if this were too loosely packed and contained only 

 25 grams or less cotton-wool ; when, on the other hand, the 

 filter was more tightly packed, and contained 35 grams or 

 more cotton-wool, the air after being forced through, even 

 under great pressure, was always found to have been freed 

 from all germs and to be perfectly sterile. With force, 50 

 grams of cotton-wool can be pressed into the tube with 

 moderate ease, but this quantity is not necessary In the 

 above experiments, 16 liters of air were on the average passed 

 through the liquid in each flask. 



For its employment in practice it is important to ascertain 

 how often the filter must be sterilised. It might, for instance, 

 be assumed that the micro-organisms retained by the cotton- 

 wool at the upper end might be able to multiply, and if the 

 filter became moist, to penetrate through the cotton-wool and 

 thus render the filter useless. In order to determine whether 

 this were so, some of the filters used in the earlier experi- 

 ments, and which contained micro-organisms at their upper 

 ends, were set aside in the laboratory for six months. They 

 were then tested in the same manner as formerly, with the 

 result that they were always found to yield sterile air. Tests 

 of a more severe character were then made as follows : The 

 cotton-wool in the cover of the filter was dipped in nutrient 

 liquids containing vigorous growths of bacteria, yeasts and 



F 2 



