1890. | RAFTER—BIOLOGICAL EXAMINATION OF WATER. 4! 
Mr. A. L. Kean first used a small cell, made to contain ¢ cubic 
millimetre, early in the winter of 1888—89, and attempted by the use of 
such a cell to arrive at a quantitative determination of the number of 
organisms present in a given sample. Such a cell was found to be 
altogether too small to furnish other than uncertain results, although it 
probably suggested the larger cell which has become of great value. 
The use of sand as a filtering medium for this purpose was suggested 
by Desmond FitzGerald, Resident Engineer, Western Division Boston 
Water Works, but it was to the ingenuity of Professor Sedgwick that 
Fic. 3.—CcmMpLeTE APPARATUS FOR BIOLOGICAL EXAMINATION OF WATER. 
we owe the working out of the really useful application of those various 
devices. My own subsequent improvements, which are in the nature 
of refinements of technique, are fully set forth in this paper. 
The practical value of a method of this character will be readily 
recognized by all who understand the limitations of chemical analysis 
as applied to the decision of questions relating to the sanitary value of 
potable water. The most useful of the various chemical methods 
recognizes only two classes of organic impurity, namely, free and 
albuminoid ammonia, and groups every organic substance occurring in 
