VARIATION IN NACREOUS COLOR OF NAIADES 237 



the stream." Humic acid" seems to be an indefinite chemical term 

 applied to several acid compounds having their origin in the decay 

 of vegetable matter. It is commonly supposed to impart its straw 

 colored to deeper hues to the streams in which it is found. Such a 

 stream is the Shenango R. fed by Pymatuning Creek which in 

 turn" drains a swamp. For the reason that forests with their residual 

 humus are, under present conditions at least, most abundant near 

 the headwaters of streams, it may be reasonably supposed that 

 Humic Acid, if any, is most abundant there. As an acid, it must 

 tend to be neutralized later in those streams having an abundance 

 of lime, (CaC03), and consequently disappear, for analyses of the 

 water in various parts of the Upper Drainage where the shells were 

 collected (6,io), show that there is an increase in the amount of 

 lime present and alkalinity in general going down stream, with a 

 converse reduction, it may be assumed, of any acidity. Faussek, (2), 

 in studies with marine pelecypods came to the conclusion that water 

 containing acid promoted the formation of pigment, while he 

 believed that light played no part in this process. List, (11), 

 another observer was inclined to credit the importance of light as a 

 factor in pigment formation. As the headwaters are freer from silt 

 than those of the lower stretches, we have according to these in- 

 vestigators, more ideal conditions for the production of pigment 

 there than further down stream, where as a matter of fact it is less 

 abundant. In this behaviour of nacreous color of Najades in up- 

 stream regions we have a rough analogue to that of the reaction 

 of litmus to acids snd alkalies. Further, the water of L. Erie differs 

 from that of the Upper Ohio Drainage and its lower regions in 

 possessing a greater amount of lime and general alkalinity, especially 

 to note after any humic acid entering the lake has been neutralized. 

 L. Erie waters also contain certain chemicals such as magnesium 

 sulfate and chloride, which are not found, at least in similar quantity, 

 in the Upper Ohio Drainage. Now L. Erie shells have been found to 

 have greater percentages of Blue. This corresponds with the re- 

 action of litmus toward alkaline solutions. 



A second analogous example and one more closely related to the 

 factors concerned in the case is the behavior of iron compounds 

 toAvard CO2 (13). It is known that Humic Acid attacks the iron 

 oxides, (Fe2 03) which color soil red or yellow, and reduces these 

 compounds to ferrous oxide. Ferrous oxide then unites with the 

 CO2 omnipresent in soil water, forming ferrous carbonate, a colorless 



