16 MICRO-ORGANISMS AND FERMENTATION. 



evidently perforated, and is, therefore, useless. The first 

 runnings of a filter, even a perfect filter, are not always sterile, 

 and after a filter has been in use for a short time germs always 

 pass through it. This happens because the germs at length 

 grow through the pores, since it is almost unavoidable that 

 substances which supply nutriment to the bacteria should not 

 penetrate into the filtering medium. The surface of the filter 

 must, therefore, be frequently cleaned, and the filter sterilised, 

 which is most easily done by boiling it in water. 



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 of 

 sound origin from deposits of various kinds, and in rendering 

 it bright ; but, on the other hand, it has been directly proved 

 by Thausing, Wichmann, Reinke, Lafar, and others that an 

 indiscriminate employment of this method may prove ex- 

 tremely dangerous. If the filters are not effective, it may 

 happen that only yeast cells are retained and not bacteria, 

 which can then react with much greater energy upon the liquid. 

 Another great danger lies in the fact that a filter, when it is 

 imperfectly cleansed, may harbour colonies of different kinds 

 of germs, causing the contamination of all beer passing through 

 it. If a single cask in a cellar has become infected, and the 

 filter is not effectually sterilised after the filtration of its con- 

 tents, the disease will be communicated to the whole of the 

 beer. It is, of course, a great mistake to use a filter which 

 has been allowed to stand for a day without previous steri- 

 lisation ; the different species will have rapidly propagated in 

 the favourable substratum, and will be swept off by the fol- 

 lowing filtration. In this stage of development the cells of wild 

 yeasts are much more vigorous than those of the cultured 

 yeasts, so that the disease organisms will multiply rapidly, 

 and cause serious infection. A warning must be given against 

 treating the filter with water at a temperature below the 

 boiling point ; a thorough cleansing can only take place by 

 prolonged boiling. By careful handling of the excellent filters 

 now manufactured by several makers a more stable product 

 can be obtained than that before filtration, as the experience 



