190 



seem that the river water might under more favorable condi- 

 tions develop a more abundant plankton. These favorable 

 conditions are to be found in the quiet backwaters of river-fed 

 lakes, where time for breeding is afforded. 



The loss which the residues of total solids suffer upon igni- 

 tion (heating to redness) includes the organic matters which 

 are burned away and such constituents of the mineral matters 

 as are volatile or are decomposed by heat into volatile sub- 

 stances. In stream waters the suspended portion of this mate- 

 rial may be a rough index of the quantity of plankton and silt 

 of organic origin, all of which on decay add to the water sub- 

 stances available for plant food. From the data in the table it 

 may be ascertained that the four localities yield respectively, 

 in the order of the the table, 7.7, 17.5, 1.9, and 8.2 parts per 

 million of such material. The excess in Spoon River (17.7) is 

 doubtless due to silt of organic origin, while the plankton pre- 

 sumably forms a larger proportion in Thompson's Lake and in 

 the Illinois River. The poverty alike of plankton and of silt in 

 Quiver Lake is reflected in the small amount (1.9) lost on igni- 

 tion in its waters. The loss, on ignition, of substances held in 

 solution shows no differences at all commensurate with the 

 relative production of plankton, though the trend of the differ- 

 ences is similar in three instances of the four. 



The chlorine is contained in surface waters in combination 

 with various basic elements, but chiefly in the form of common 

 salt. Its principal source is animal matter, sewage, or drain- 

 age from refuse animal matter. In our river and lake waters 

 it is largely an index of their relative contamination with sew- 

 age from cities within the drainage basin. Since its combina- 

 tions are not utilized by plants as food in any considerable quan- 

 tity, at least as compared with other constituents of the sew- 

 age, such as the nitrates, the chlorine becomes the best crite- 

 rion of the amount of sewage and thus of the principal adventi- 

 tious fertilizer which the waters examined by us contain. The 

 differences in the four localities are striking and significant. 

 The average chlorine in the Illinois River (21.6) is more than 



