497 



author) working conjointly. The first trip was made July i8 to 

 August 4; the second, August 1017; and the third, August 21-29. 



The weather of July and August, 191 1, was unusually dry and 

 hot, with the river continuously at very low level. A rain of about 

 a quarter of an inch fell July 20, in the upper Illinois valley, but 

 this had no perceptible effect on the river. Upwards of an inch of 

 rainfall August 10, followed by a rise of about six inches in the 

 Kankakee, also brought the Illinois up in the Morris-Marseilles sec- 

 tion, and the August stage of water was throughout a little higher 

 than that of July. 



In collecting from the waters of the flowing stream, all samples 

 were taken within eighteen inches of the surface, by means of a liter- 

 bottle opening and closing mechanically. On the return from the 

 field the contents of each liter flask were thoroughly shaken together 

 and 50 cubic centimeters were drawn oft' with a pipette and reduced 

 to 10 centimeters by straining through No. 575 S. & S. hard-pressed 

 filter-paper. After this condensed sample was thoroughly shaken, 

 a Rafter cell holding one cubic centimeter was filled, and counts of 

 the liz'ing plankton were made under a microscope by the methods 

 usually employed in presented specimens. The above method of 

 collection is a slight modification of that recently used by Kolkwitz,* 

 dift'ering from it chiefly in the fact that the samples were condensed 

 before examination. The fundamental feature is the direct field 

 examination and enumeration of the living organisms in small sam- 

 ples, which in the practice of Kolkwitz were not strained or con- 

 densed in any manner. 



This concentration process seemed to us to be called for to in- 

 sure a number of organisms in the samples examined, suflicient to 

 give a dependable count. The correctness of the method used is, in 

 our opinion, fairly attested by the general consistency and reason- 

 ableness of the figures obtained, if proper account is taken of the 

 known variability of hydrographic and chemical factors. A few 

 special tests of it were nevertheless made, (a) Duplicate counts of 

 collections from the same place on the same day showed neglible 

 differences only, (b) A collection obtained at Chillicothe August 

 17, containing a total of 162 organisms per cubic centimeter (not 

 counting bacteria,) was filtered, and a centimeter of the filtrate was 

 foimd to contain no organisms except a few bacteria, (c) A collec- 

 tion from Depue Lake containing 40,000 chlamydomonads per ceriti- 

 meter was filtered, and the filtrate contained only 147 green monads 



*Mitth. aus der Kgl. Preuss.-Aiistalt fur Wasserversorg. und Abwasserbeseiti- 

 gung zLi Berlin, 1907. 



