78 Ml'THODS OF STKK1IJSATION 



of the caudle, collects in the interior of the latter and escapes through the 

 afores.iid apiM-ture at the bottom. Fine kieselyuhr (diatomaceous earth) is 

 employed by NOKDTMKVKK (I.) and lijrkefeld for making the candle, and GARROS 

 (I.) uses asbestos of tine fil>ro. The reliable working of this filter is, however, 

 not illimitable, and that for two reasons: lir.^t of all, the pores become gradually 

 obstructed by the fine, slimy deposits separated from the liquid, which necessi- 

 tates 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 

 Boturquelot and Galippe. In some cases the pores of the filtering candle are too 

 large; consequently, a germ-free filtrate is unobtainable. For testing the 

 efficiency of the filter the photo-bacteria can. according to BEYERINCK (IV.), be 

 employed with advantage. A comparison of the capacity and efficiency of the 

 Chamberlain! and IVrkefeld systems was drawn up by (int<'r <ilin) I >A< -n \.IK\\M\ i 

 (I.), and the columns of the Centralblnft J'Hr Bakterioloyie contain numerous 

 articles respecting the advantages and defects of the aforesaid apparatus. 

 Mention should be made of the filter constructed by Breyer, which, according 

 t > 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 Enzymolog-y. 



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 bacterium. 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, therefore, 

 but one way open to us, viz., removing the germs by filtration ; and of the above- 

 named apparatus (filters), therefore, there is likewise only one that is reliable 

 and suitable for use for the purpose in view, namely, that of Chamberland. 

 This is, however, unfortunately 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 

 M>nie of its chemical constituents, so that the equation, Filtrate = bacterial 

 culture - bacteria, does not always hold good. The filtering cylinder, especially 

 when used for the first time, retains varying amounts of the individual consti- 

 tuents of the liquid passing through it, a fact that was first recorded by FLI 

 and SIKOTININ (I.) in 1888, and more closely examined by ARLOING (I.) in 1892. 

 We will, in this place, merely refer to the oxidising influence of the air, observed 

 more particularly by Miquel in the separation of urase from cultures of uric- 

 bacteria. It is therefore advisable to perform such filtering operations in an 

 atmosphere of pure hydrogen. 



75. The Beer Filters 



used in the biewery must also be briefly considered here. The object of these 

 appliances is to render the beer bright, i.e. perfectly clear and transparent, when 



