98 METHODS OF STERILISATION. 



percentage of germs in the atmosphere of breweries was more par- 

 ticularly investigated by E. CH. HANSEN (II.) ; and PETEI (I.) has 

 summarised all the methods of examination proposed up to 1887. 



The method, originally performed by Th. Schwann, of purify- 

 ing air by exposure to a red heat, is at present used by fermentation 

 physiologists in one instance only, viz., when working with the 

 so-called Pasteur flasks. When liquid is poured out of the lateral 

 tube whether for the purpose of taking a sample or for inocu- 

 lating a similar flask with the contents the air coming in in its 

 place is purified by holding the aperture or the first bend of the 

 swan-neck tube in the flame, i.e. heating it to redness. 



73. The Filtration of Drinking Water. 



The methods of sterilising liquids are various, but are not all 

 equally suitable for any given case. For example, the employ- 

 ment of poisonous substances is precluded when the liquid to be 

 sterilised is intended for human consumption, and the use of heat 

 which next suggests itself is frequently inapplicable on account 

 of the expense entailed. Such, for instance, is the case with the 

 drinking water of towns deriving their supply from a river. Under 

 these circumstances a so-called sand-filter is employed, the true 

 filtering layer of which is not the strata of sand and gravel, but the 

 mud which is gradually deposited thereon. A fuller consideration 

 of this subject, which belongs to the domain of Practical Hygiene, 

 may be passed over the more readily since it has been treated in 

 Tiemann-Gartner's work already alluded to. This may be referred 

 to, as also a very practical investigation performed by A. REINSCH 

 (I.), bacteriological adviser to the Altona Waterworks. 



The filtrate obtained from such filters intended for use on a 

 large scale is, when the service is carefully regulated under bac- 

 teriological control, found to be very low in germs, though not 

 perfectly free therefrom. If it be desired to attain such perfec- 

 tion which is necessary in times of epidemic other filters, of 

 greater powers of retention, and correspondingly diminished de- 

 livery, must be resorted to, and employed solely for the water 

 intended for human consumption. The prototype of these is the 

 apparatus invented by TIEGEL (I.) in 1871, and subsequently 

 (1884) improved, especially by Chamberland. In the form devised 

 by the last-named, the effective constituent of the bacterium filter 

 consists of a candle-shaped hollow cylinder of hard-burnt, porous, 

 unglazed porcelain ("biscuit"), with an effluent aperture at one 

 end, which, before use, is sterilised by dry heat. This candle 

 (bougie) is enclosed in a somewhat wider metallic cylinder, the 

 liquid to be freed from germs (the suspected potable water) being 

 forced into the intervening space, and, finding its way through the 

 porous wall of the candle, collects in the interior of the latter and 

 escapes through the aforesaid aperture at the bottom. Fine kiesel- 



