1899] Moore,— Pollution of water-supplies by algae : 99 
study of the various organisms found in water-supplies, and it is through 
the efforts of the Massachusetts State Board of Health, that most of 
the information we have on the subject has been acquired. Up to the 
present time, the work has consisted largely of a determination of the 
occurrence, quantity, and effect of the algae, while the question as to 
the best way of preventing their appearance is one still to be answered. 
.— As a rule, the larger and more easily recognized algae do not pro- 
duce any pollution. Such forms as Spirogyra, Zygnema, Cladophora, 
Conferva, etc., while they may occur in ponds and reservoirs to a con- 
siderable extent are not likely to cause trouble unless it be a mechani- 
cal one. The case is different, however, when we come to some of the 
more minute species. Here the inconvenience and damage caused is 
frequently great, and while they do not actually poison the water it is 
rendered so disagreeable and unfit for use that, temporarily, the effect 
is much the same. Perhaps the best known forms responsible for this 
contamination are the members of the group popularly called the 
“blue-green algae." These plants, the Cyanophyceae, are considered 
bysome to be outside the limits of the true algae, and are called 
Schizophyceae, which are classed, together with the bacteria, under the 
Schizophyta. They certainly greatly resemble the bacteria both as to 
their morphology and general mode of life, and one reason the bacteria 
are now considered plants is because of the similarity between them 
and the Cyanophyceae. 
The forms of the blue-green algae are extremely various. Some- 
times the plant consists of a single cell, less than 41; mm. in diameter, 
which may or may not be united into colonies of considerable size. 
Again the plants are made up of bead-like chains of cells such as are 
found in the nostocs; or the filament of cells may be of the same 
diameter throughout its length, as in the oscillatorias. During the 
summer, especially, these algae multiply very rapidly by the simple 
division of their cells, and under favorable conditions are apt to occur 
is such quantities as to produce the so-called * pig-pen" odor. This 
is essentially the result of decomposition and is due to the breaking up 
of compounds of sulphur and phosphorus in the presence of a high 
percentage of nitrogen. In addition to the trouble caused by the 
decay of these forms, there are some species which have what may be 
called a natural odor, which is generally due to an oily substance con- 
"tained within the cells of the plant. This oil may be liberated in 
various ways, and then the water becomes charged with the characteristic 
