12 MICRO-ORGANISMS AND FERMENTATION. 



are able in time to grow through the pores of the filter, 

 especially if the water contains .much organic matter and the 

 temperature is favourable. 1 



Liquids and solid nutritive substrata are in most cases 

 sterilised by heat. The way in which this must be done, as 

 well as the duration of the heating process, are dependent on 

 the nature of the substratum in question. Direct boiling 

 on the sand-bath may be employed for the purpose of sterilis- 

 ing, for example, brewers' wort in Pasteur flasks, or, again, the 

 water-bath may be used. An excellent means of sterilisation 

 is afforded by steam either at 100 C. or under pressure 

 (110120 C.) in a Papiris digestor (autoclave). During 

 cooling, care must be taken that only absolutely pure air comes 

 in contact with the sterilised substance, the air entering the 

 vessel being filtered through cotton-wool or simply passed 



1 In breweries the) filtration of beer has been resorted to during the last few 

 years, the filtering media commonly used being paper, cellulose, asbestos, etc. 

 By such filtration brewers sometimes succeed, it is true, in freeing a beer 

 originally sound from deposits of various kinds, and in rendering it bright ; 

 but, on the other hand, the fact must be emphasised, that an indiscriminate 

 employment of this method may occasion great dangers, as has been directly 

 proved by the experiments made by THAUSING, WICHMANN, REINKE, LAFAK, 

 and others. If the filters are not effective, it may happen that only the yeast 

 cells are retained, but not the bacteria, which are then enabled to act with 

 much greater energy upon the liquid. Another great danger lies in the fact that 

 a filter, owing to deficient cleansing, may become a seat for the development of 

 different kinds of germs, contaminating all the beer passing through it. If a 

 single cask of a store-room has become infected, and the filter is not effectually 

 sterilised after the filtration of this beer, the disease will be communicated to 

 the whole of the beer. 



The filtering of water on a large scale is commonly effected by means of sand 

 filters, which are supported by several layers of stones of different size. In 

 these filters a considerable portion of the bacteria are retained by the layer of 

 mud deposited from the water covering the uppermost sand-layer. The water 

 must therefore not move too quickly through the filter, otherwise the mud- 

 layer will be broken up. In time, however, the bacteria grow into the sand, 

 part of which must then be removed. The filter has been found to lose a con- 

 siderable part of its power if the sand-layer is not at least two feet thick. The 

 efficiency of such filters is always liable to fluctuations, particularly in conse- 

 quence of changes of temperature and of the composition of the water, and, 

 according to various investigations made in this direction, they cannot be 

 expected to retain even a portion of the germs of the water for more than two or 

 three weeks. 



