although there are exceptions in this respect, some of them showing
shades of yellow, brown, olive, chocolate, and purplish red. This
variety of algae flourishes in the summer months, since a relatively
high temperature and shallow stagnant water favor its germination. If
the pond begins to dry up, the death of the organisms takes place, and
the result is a most disagreeable, persistent odor which renders the
water unfit for drinking purposes. This result is chemically due to
the breaking down of highly organized compounds of sulphur and
phosphorus in the presence of the large amount of nitrogen contained
in these plants. Decomposition is not necessary for some of the blue
greens to give off a bad odor, however. A number of them, on account
of their oil-content, produce an odor when in a healthy condition that
is sometimes likened to raw green corn or to nasturtiums, but usually
it cannot be so pleasantly described.

The Department of Agriculture has been able to solve the problem of
exterminating algae from water supplies.[1] The department has done
more; for it has succeeded in perfecting a method by which a reservoir
contaminated with typhoid or other pathogenic bacteria can be
purified. The work was begun with an inquiry into the extent of the
trouble from algal pollution. Letters were addressed to some five
hundred engineers and superintendents of water companies scattered all
over the United States. The replies, which came from almost every
State in the Union, were burdened with one complaint--"Algae are our
worst pest"; and with one prayer--"Come over into Macedonia, and help
us."

_A Cheap and Available Remedy for Algae_

Convinced of the need of earnest work, extensive laboratory
experiments were inaugurated. The problem presented was this: the
remedy must not only be readily available, but it must be cheap, that
advantage may be taken of it by the poorest communities, as well as by
those owning large reservoirs. Above all, the remedy must be
absolutely harmless to man; the poison used to exterminate algae must
not in any way affect the water drinkers. A large number of
substances were used in the experiments before the final decision
rested with copper sulphate. This salt is very poisonous to algae. On
the other hand, copper in solution just strong enough to destroy algal
growth could not possibly injure man; in fact, the temporary presence
of such a small amount of copper in drinking water could not be