The idea of standardizing disinfectants was first seriously proposed by Rideal and Walker. I Previous to that .time, despite the fact that the germicidal properties of a great many chemical substances had been thoroughly investigated, no scientific attempt had ever been made to establish a common basis of comparison. The results of any disinfection experiments are fundamentally influenced by such conditions as temperature, character of the organism employed, number of organisms in unit volume, and character of the medium. In the absence of complete data covering these points results are practically worthless, at least for purposes of comparison. Even with such data given it is still impossible, owing to the variable conditions obtaining in practice, to establish any relationship, or order of excellence, among the various disinfectants. At best we can only hope to establish such relationship under specified experimental conditions. Rideal and Walker proposed to establish phenol as a standard. This selection was made because phenol is a substance readily obtainable anywhere in a comparatively pure condition. The effect of phenol and that of any other disinfectant may be determined simultaneously using the same test organism. The germicidal power of the disinfectant under investigation is then expressed as the "carbolic-acid coefficient," this being the ratio of the concentrations of the two disinfectants which will kill all the test organisms in the same time. Rideal and Walker recognized also that the time required to disinfect was dependent upon the number of organisms initially present as well as upon the temperature, a fact previously pointed out by Kronig and Paul. 2 These variable factors were controlled as closely •

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By Earle B. P hel p ' , \.. ' i stan t S ' 0 1' o f l t e� ' a rc h i n C he m ical B iology a.t the M a 'sa-h u ' tt ' I n 't i t u tc o f ' l'e-h n o l o O 'y Filter beds of the Massachusetts Institute of Technology. Three kinds of distribution of sewage to the surface are shown-the small fountain to the • left, the spray to the right, and still farther to the right, the fluted cylinder of the Fidian distributor. 'filter chieflY. in the fact that crushed stone is employed for a filter medium. In this • agai Ii we are dealing with biological oxidation of the sewage, an action due to the numerous mjnute organisms inhabiting the filter. Con tact beds need breathing periods during which they

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