70 BACTERIA IN WATER 



filter removed 98-7 per cent, of the total bacteria, 99'8 per cent, of 

 B. coli, and 99'9 of the typhoid bacillus.* 



The teaching of these figures could, with great ease, be emphasised 

 again and again if such was necessary ; but sufficient has been said 

 to show that sand filtration, when carefully carried out, offers a more 

 or less absolute barrier to the passage of bacteria, whether non- 

 pathogenic or pathogenic. 



Domestic Purification of Water 



Something may, however, be added, from a bacteriological point 

 of view, relative to what is called domestic purification. There is but 

 one perfectly reliable method of sterilising water for household use, 

 viz., hoiling. As we have seen, moist heat at the boiling-point main- 

 tained for a few minutes will kill all bacteria and their spores. The 

 only disadvantages to this process are the labour entailed and the 

 "flat" taste of the water. Nevertheless, in epidemics due to bad 

 water, it is desirable to revert to this simple and effectual purification. 



There are a large number of domestic filters on the market with, 

 in many cases, but little difference between them. The materials 

 out of which they are made are chiefly the following: carbon and 

 charcoal, iron (spongy iron or magnetic oxide), asbestos, porcelain 

 and other clays, natural porous stone, and compressed siliceous and 

 diatomaceous earths. From an extended research in 1894 by Prof. 

 Sims Woodhead and Dr Cartwright Wood, who repeated and 

 extended experiments by Freudenreich, Schofer, and others, our 

 knowledge of the quality of these substances as protectives against 

 bacteria has been largely increased.f They concluded that a filter 

 failed to act in one of two ways. It was either pervious to micro- 

 organisms, or its power of filtering became modified owing to (a) 

 structural alteration of its composition, or (&) to the growing through 

 of the micro-organisms, which had been demonstrated by previous 

 workers. The conditions which chiefly influence the growth of 

 bacteria through a filter appear to be the temperature, the inter- 

 mittent use of the filter, and the species of bacteria. The higher the 

 temperature and the longer the organisms are retained in the filter 

 the more likely is it that they will grow through, and in the next 

 usage of the filter appear in the filtrate. As to this species, those 

 multiplying rapidly and possessing the power of free motility will 

 naturally appear earlier in a filtrate than others. Woodhead and 

 Wood concluded that out of 18 different kinds of domestic filter, each 

 of which had its supporter, the Pasteur-Chamberland candle filters 



* Thirty-fourth Ann. Rep. State Bd. of Health, Massachusetts, 1903, pp. 224 

 and 269. ^ 



t Brit. Med. Jour., 1894, i., pp. 10f)3, 1118, 1182, 1375, 1486. 



