1897.] Biological Studies in Massachusetts. 1025 
on the Nessler scale, and none occur in water where the aver- 
age color is above 1.00. The same fact is noticed in the case 
of the other organisms, but not as strongly emphasized as with 
the Diatomaceæ. The reason for this is doubtless on account 
of the difference in specific gravity between the diatoms and 
the other organisms. The diatoms, by reason of their silicous 
cell walls are heavy, while the other organisms not only are 
much lighter, but some of them liberate gas, causing them to 
keep near the surface. Being thus kept near the surface, the 
depth to which light penetrates in a body of water makes less 
difference with the growth of the Cyanophycee, for example, 
than it does with the diatoms, which constantly tend to sink, 
and which are kept near the surface chiefly by the vertical 
currents in the water. 
The “excess of chlorine ” means the difference between the 
amount of chlorine found in a sample of water and that found 
in the unpolluted water of the same region. To x certain 
extent it represents the amount of pollution which the water 
has received. It is important to know whether this element 
of the analysis bears any relation to the organisms and whether 
one may rightly infer that a large growth of organisms in a 
reservoir is any indication of the pollution of a water supply. 
A study of the tables shows that only to a small extent does 
the excess of chlorine influence the number of organisms ob- 
served, though there is a slight tendency for heavy growths of 
organisms to accompany high excess of chlorine. This fact 
corresponds with the common observation that vigorous 
growths of organisms are often observed in ponds far removed 
from any possible contamination. 
The hardness of a water, i. e., the abundance of carbonates 
of calcium and magnesium, appears to have some influence 
upon the organisms. This is noticed in all four classes, though 
it is most marked in the case of the Diatomacez and Infusoria. 
For example, of the 10 ponds low in hardness not one ever has 
the Infusoria as high as 1000 per c. c., while of the 11 ponds 
high in hardness not one but that has Infusoria above 100 
per c. c, and 4 of them commonly have them above 1000 
per c. c. 
70 
