THE FILTRATION OF DRINKING WATER. 99 



guhr (diatomaceous earth) is employed by NORDTMEYER (I.) and 

 Berkefeld for making the candle, and GARROS (I.) uses asbestos of 

 fine fibre. The reliable working of this filter is, however, not 

 illimitable, and that for two reasons : first of all, the pores become 

 gradually obstructed by the fine, slimy deposits separated from the 

 liquid, which necessitates the cleansing of the filter from time to 

 time ; secondly, the bacteria grow by degrees through the pores of 

 the filter, a circumstance first observed by Bourquelot and Galippe. 

 In some cases the pores of the filtering candle are too large ; con- 

 sequently, a germ-free filtrate is unobtainable. For testing the 

 efficiency of the filter, the photo-bacteria can, according to BEYER- 

 INCK (IV.), be employed with advantage. A comparison of the 

 capacity and efficiency of the Chamberland and Berkefeld systems 

 was drawn up by (inter alia) DACHNJEWSKI (I.), and the columns of 

 the Centralblatt fur Balderiologie contain numerous articles respect- 

 ing the advantages and defects of the aforesaid apparatus. Mention 

 should be made of the filter constructed by Breyer, which, accord- 

 ing to an investigation made by WICHMANN (I.), acts satisfactorily. 

 PLAGGE (I.) instituted exhaustive experiments in respect of the 

 efficiency of all the known water-filters designed for use on the 

 small scale. 



74. The Bacterium Filter in the Service of 

 Enzymology. 



In many instances the filter affords the sole reliable means of 

 sterilising a given liquid ; as, for example, when a species of 

 bacterium is to be tested with regard to its capacity of producing 

 enzymes. For this purpose it is necessary to free the culture, 

 containing any such chemically active substance, from germs,, 

 since otherwise it would be impossible to determine whether the 

 chemical reaction obtained by means of the sample is effected by 

 the enzyme itself, or primarily by the vital energy of the bac- 

 terium. The sterilisation admittedly necessary in such case cannot 

 be effected by heat, since this agency would at the same time 

 destroy the readily decomposable enzyme. There remains, there- 

 fore, but one way open to us, viz., removing the germs by filtration ; 

 and of the above-named apparatus (filters), therefore, there is like- 

 wise only one that is reliable and suitable for use for the purpose 

 in view, namely, that of Chamberland. This is, however, un- 

 fortunately expensive, and consequently not accessible in every 

 laboratory. For this reason the pattern described by A. KOCH (II.), 

 which is both efficient and cheap, forms a welcome substitute. 



Whichever of the two appliances be employed, it must never 

 be forgotten that, in its passage through the filter, the bacterial 

 culture under examination is not only deprived of germs, but 

 may also, under certain circumstances, part with some of its 

 chemical constituents, so that the equation :. Filtrate = bacterial 



