THE FILTRATION OF DRINKING WATER 77 



charged with sterilised glass powder or sterilised fine sand, the contents of the 

 filter being then intimately mixed with a gelatinised nutrient medium, and the 

 whole poured into flat glass basins. The separate germs then develop into multi- 

 cellular families (colonies). When counted their number referred to unit 

 volume gives the germ content of the air. The difficulty in the way of study- 

 ing the cultures, caused by the presence of the powdered glass and sand, can be 

 overcome by substituting a soluble filtering medium, such as coarsely powdered 

 crystals of sodium sulphate of about 0.5 mm. in diameter. This is specially 

 recommended by MIQUEL (III.), to whom (be it remarked en passant) we owe 

 the most comprehensive experiments on the percentage of germs in the air. 

 Regular reports of his researches appear in the Year- Book (published annually 

 since 1879) of the observatory established, under his direction, for studies of this 

 kind, in the southernmost district of Paris. Readers are hereby referred to this 

 "Annuaire de 1'Observatoire de Montsouris." The percentage of germs in the 

 atmosphere of breweries was more particularly investigated by E. CH. HANSEN 

 (II.) ; and PETRI (I.) has summarised all the methods of examination proposed 

 up to 1887. 



The method, originally performed by Th. Schwann, of purifying 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 inoculating 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 employment 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-G'artner'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 bacteriological control, found to be 

 very low in germs, though not perfectly free therefrom. If it be desired to 

 attain such perfection which is necessary in times of epidemic other filters, of 

 greater powers of retention and correspondingly diminished delivery, must be 

 resorted to, and employed solely for the water intended for human consumption. 

 The prototype of these is the apparatus invented by TIEUEL (I.) in 7871, 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 



