Lister determined the number of bacteria present in a drop of the liquid under examination by counting, and then diluted this with a sufficient quantity of sterilized water so that each drop of the mixture should contain, on an average, less than one bacterium.
Hansen counted the number of yeast cells suspended in a drop of liquid diluted with sterilized water.
A volume of the diluted yeast was introduced into flasks containing sterilized wort, the degree of dilution being such that only a small proportion of the flasks became infected.
But even filters of this type, if they are to be fully relied upon, must be frequently cleaned and sterilized, and great care must be taken that the joints and connexions are watertight, and that the candles are without cracks or flaws.
Schuppan has shown that it is possible to produce good butter from Pasteurized or even sterilized cream, and Weigmann introduced the plan of artificially souring cream by means of pure cultures of B.