It is, however, a variety of algae not easily detected that contaminates the water. So long as they are in a live, healthy condition they benefit drinking water by purifying it. Indeed, some scientists have attributed the so-called self-purification of a stream entirely to the activities of these plants. Of such, one form, _Chlamydomonas_, is bright grass green in appearance. But the largest group--the plants which have the worst reputation as polluters of drinking water--are popularly known as the "blue-green algae" (_Schizophyceae_). The common name tells the color of these plants, 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_