EXAMINATION OF FILTERS 405 



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 

 passed through the filter, and the filtrate was found to be 

 sterile. Further investigation showed that this ceased to be 

 the case when the filtration was continued for a few hours 

 or less instead of a few minutes. It was found that in such 

 case the filtrate contained the same organisms as the un- 

 filtered water ; and the sterility of the earlier filtrates was 

 accordingly due to the circumstance that they had been 

 examined before sufficient time had been allowed for the 

 organisms to be washed through the filter. It was also 

 found that the chemical matters arrested by the filter 

 temporarily arrested a portion of the organisms, and served 

 as a suitable culture-ground for such organisms, which 

 survived and multiplied for considerable periods in the filter 

 before being ultimately washed through. In consequence, 



