AIR AND WATER FOR MICRO-ORGANISMS. 121 



after three or four days. If a comparison is to be made, and 

 the method is, in fact, entirely one of comparison, more 

 account must be taken of these factors than has generally 

 been the case hitherto. A further difficulty with the gelatine 

 cultures is that growths of mould frequently spread to such 

 an extent at the beginning of the experiment that it becomes 

 impossible to complete the analysis. It is only when the 

 above conditions are taken into account that the gelatine 

 methods can be advantageously employed for testing the 

 effectiveness of water and air filters. 



If the above experiments are considered more closely, it 

 will be seen that they not only indicate the means to be 

 adopted for the rational analysis of brewing waters, but that 

 they also throw light on more generally interesting biological 

 conditions. Thus, we learn that of the numerous bacteria 

 present in the samples of water examined, only extremely few 

 were able to attack the wort, and none of them the beer. 

 It is evident that samples of other waters may give a different 

 result, but the above appear to furnish a general rule. 



Since each drop (half water and half nutrient) sowed 

 on the pure gelatine gave a vigorous growth, it can be 

 safely concluded that each of the flasks containing wort or 

 beer, and to which similar drops had been added, also received 

 living germs. In the case of the above two more fully de- 

 scribed series of experiments it was proved that there /were 

 in several flasks as many as 60 bacteria, in many 20, and in 

 none less than 4 bacteria. Most of these germs were there- 

 fore unable to develop in the two liquids. As some of the 

 flasks of wort showed a growth of bacteria, I assume that a 

 few of the very numerous bacteria present in the water 

 belong to species which are able to attack wort. This was 

 distinctly seen in cases where a growth in wort consisted only 

 of a single species of bacterium. When traces of such a 

 pure culture were introduced into other flasks of wort, they 

 rapidly produced bacterial turbidity in the latter, as was to 



