18 MICRO-ORGANISMS AND FERMENTATION. 



avoided by the use of the flask constructed by the author (Figs. 

 8 and 9). On the large scale in breweries, yeast factories, etc., 

 cotton wool packed in suitable vessels is also used for filtering 

 air, or else the air is led through a large number of layers of 

 cotton wadding (Holler's filter). The complete sterilisation of 

 the air on the large scale cannot always be attempted, and 

 could not always be justified from an economic standpoint. 



In the filtration of water on the large scale, the conditions 

 existing in nature are imitated, where water is allowed to sink 

 through successive layers of soil, and the organic residues 

 and micro-organisms are deposited on the finer layers, until, 

 at a given depth, the water is sterile. Artificial filters con- 

 structed of a number of layers of varying coarseness were 

 first applied in London, and are now used in every country. 

 Such a filter consists of a bed of large stones, covered with 

 several layers of flints successively reduced in size so that 

 the topmost layer is about the size of a pea, and on this is 

 laid a layer of sharp sand about 5 feet in thickness, which 

 has previously been washed. The water is first stored in a 

 reservoir, where the larger particles settle out. When the 

 filter is used for the first time water is led in slowly from 

 below, so that all the air is driven out of the filter. It is then 

 allowed to stand quietly for some hours before the true filtra- 

 tion begins. This must be carried out slowly at first, and 

 then more rapidly. It has been shown that the distance 

 between the separate particles of sand is greater than the 

 bacteria, and, therefore, the retention of the bacteria is not 

 due to the sand filter. While the water is standing quietly 

 over the filter, slimy matters in suspension settle down and 

 form a fine skin of slime on its surface. This retains a few 

 bacteria, and as it always contains organic residues, it supplies 

 nutriment for the bacteria, and as a consequence they multiply. 

 A few bacteria settle on the grains of sand in the upper part 

 of the layer, and these become slimy, and so arrest the bacteria 

 subsequently carried down with the current of water. In 

 this way the upper part of the layer of sand gradually fills up, 

 so that the pores between the slimy grains of sand are now 

 smaller than the bacteria, and then, for the first time, it can 

 act as a true filter. It is now " ripe," and the water ins the 



