EXAMINATION OP FILTERS 311 



reaction, and Koch is of opinion that the presence of the 

 cholera bacillus in the water is proved if comma-shaped 

 organisms are found which exhibit the indol reaction, and 

 which give rise to the characteristic symptoms on inocula- 

 tion into the peritoneum of guinea-pigs. 



BACTERIOLOGICAL EXAMINATION OF FILTERS. 



When chemical analysis was the only means at com- 

 mand for examining water, it was found that in a majority 

 of cases those waters which had been statistically con- 

 victed of spreading disease contained an excess of organic 

 matter. Hence it was inferred that the organic matter 

 was the cause of the disease ; and filters were constructed 

 of carbon, asbestos, natural stone, spongy iron, and similar 

 materials, for the purpose of removing this excess of 

 organic matter. It was found that they all did so in a 

 greater or less degree, but that their efficiency in this 

 respect decreased on use, and ultimately disappeared until 

 the filtering medium had been renewed or cleansed. With 

 precisely similar results, preparations of these materials, 

 such as silicated carbon, manganous carbon, magnetic iron, 

 and the like, were tried for the same purpose, and many 

 filters composed of successive strata of several of these were 

 constructed. It ultimately became known that the diseases 

 caused by water were due to micro-organisms, and that the 

 presence of excess of organic matter in most waters which 

 were dangerous was due to the fact that the microbe was 

 generally either conveyed through excreta containing 

 soluble organic substances, or best nourished in waters of 

 such composition. The filters already in use were there- 

 upon assumed to act by arresting the microbes contained in 

 the water. This assumption was after a time supported by 

 experiment, in which a small quantity of infected water was 



