BACTERIA IN WATER 79 



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

 reproduced again and again if such was necessary ; but these 

 will suffice 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 here be 

 said, 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., 

 boiling. As we have seen, moist heat at the boiling point 

 maintained for five 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. Never- 

 theless in epidemics due to bad water it is desirable to revert 

 to this simple and effectual purification. 



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

 many cases, but little modification from each other. The 

 materials out of which they are made are chiefly the follow- 

 ing: 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 Dr. Sims Woodhead 

 and Dr. Cartwright Wood our knowledge of the quality of 

 these substances as protectives against bacteria has been 

 largely increased. They concluded that a filter failed to act 

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

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

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

 through of the micro-organisms. The conditions which 

 chiefly influence the growth of bacteria through a filter ap- 

 pear to be the temperature, the intermittent 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 the 



